Non-contact ACL injuries in female athletes: an International Olympic Committee current concepts statement

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1 1 IOC Medial Commission and Karolinska Institutet, Stokholm, Sweden; 2 IOC Medial Commission, Lausanne, Switzerland; 3 Department of Orthopedis, University of Minnesota, Minnesota, USA; 4 University of Vermont College of Mediine, Vermont, USA; 5 Faulty of Sports Sienes, University of Waseda, Tokyo, Japan; 6 Sports Mediine Centre, Duke University, Durham, North Carolina, USA; 7 Department of Orthopedi Surgery, University of Ioannina, Ioannina, Greee; 8 Cininnati Children s Sports Mediine Biomehanis Centre, Human Performane Laboratory, University of Cininnati College of Mediine, Cininnati, Ohio, USA; 9 Oslo Sports Trauma Researh Center, Norwegian Shool of Sports Sienes, Oslo, Norway; 10 Chivas USA and LA Galaxy, Pepperdine University, FIFA Medial Committee, FMARC Member, Malibu, California, USA; 11 Harvard Medial Shool Division of Sports Mediine, Children s Hospital, Boston, USA; 12 Institute of Sports Siene and Clinial Biomehanis, University of Southern Denmark, Denmark; 13 Department of Orthopedis, Lund University, Lund, Sweden; 14 IOC Medial and Sientifi Department, Lausanne, Switzerland; 15 Department of Exerise and Sports Siene, University of North Carolina at Greensboro, North Carolina, USA; 16 Stokholm Sports Trauma Researh Center, Karolinska Institutet, Stokholm, Sweden; 17 Medsport, University of Mihigan, Mihigan, USA; 18 Sientifi Ativities IOC and Orthopaedi Center, Ullevaal University Hospital and Oslo Sports Trauma Researh Center, Oslo, Norway Correspondene to: Professor Emeritus Per Renström, Flötviksvägen 51, Hässelby, Sweden; per.renstrom@telia.om Aepted 14 April 2008 Non-ontat ACL injuries in female athletes: an International Olympi Committee urrent onepts statement P Renstrom, 1 A Ljungqvist, 2 E Arendt, 3 B Beynnon, 4 T Fukubayashi, 5 W Garrett, 6 T Georgoulis, 7 T E Hewett, 8 R Johnson, 4 T Krosshaug, 9 B Mandelbaum, 10 L Miheli, 11 G Myklebust, 9 E Roos, 12 H Roos, 13 P Shamash, 14 S Shultz, 15 S Werner, 16 E Wojtys, 17 L Engebretsen 18 ABSTRACT The inidene of anterior ruiate ligament (ACL) injury remains high in young athletes. Beause female athletes have a muh higher inidene of ACL injuries in sports suh as basketball and team handball than male athletes, the IOC Medial Commission invited a multidisiplinary group of ACL expert liniians and sientists to (1) review urrent evidene inluding data from the new Sandinavian ACL registries; (2) ritially evaluate highquality studies of injury mehanis; (3) onsider the key elements of suessful prevention programmes; (4) summarise linial management inluding surgery and onservative management; and (5) identify areas for further researh. Risk fators for female athletes suffering ACL injury inlude: (1) being in the preovulatory phase of the menstrual yle ompared with the postovulatory phase; (2) having dereased interondylar noth width on plain radiography; and (3) developing inreased knee abdution moment (a valgus intersegmental torque) during impat on landing. Well-designed injury prevention programmes redue the risk of ACL for athletes, partiularly women. These programmes attempt to alter dynami loading of the tibiofemoral joint through neuromusular and proprioeptive training. They emphasise proper landing and utting tehniques. This inludes landing softly on the forefoot and rolling bak to the rearfoot, engaging knee and hip flexion and, where possible, landing on two feet. Players are trained to avoid exessive dynami valgus of the knee and to fous on the knee over toe position when utting. The inidene of anterior ruiate ligament (ACL) injury remains high, espeially in young athletes aged years. In spite of the fat that some suessful prevention programmes have been introdued, ACL injury ontinues to be the largest single problem in orthopaedi sports mediine, with the inidene of non-ontat ACL tears being muh higher in female athletes in sports suh as basketball and team handball than in male athletes. As ACL injury remains a signifiant problem, espeially in young female athletes, proedures for improved prevention and management are needed. The mehanism of ACL injury is an important fous of disussion, as an ACL tear is more often a non-ontat event with a deeleration or a hange of diretion manoeuvre than a ontat or diret blow injury. A prophylati neuromusular and proprioeptive training programme may redue the number of ACL injuries in female athletes. The President of the International Olympi Committee (IOC) Jaques Rogge stated in 2001 that the most important goal of the IOC Medial Commission is to protet the health of the athlete. The IOC Medial Commission therefore invited a group of physiians, physial therapists, biomehanists and sientists ative in ACL researh to review urrent evidene relating to risk fators, prevention programmes and the need for further researh onerning non-ontat ACL injury in young female athletes. EPIDEMIOLOGY OF ANTERIOR CRUCIATE LIGAMENT INJURIES The inidene of ACL injuries in the sporting population has been estimated from a variety of soures inluding data on surgial reonstrutions. Three national ACL surgial registries have been established (Norway 2004, Denmark 2005 and Sweden 2006) 1 to gather information on the details of ACL surgery and to monitor the outomes of this surgery. Information is gathered through a registration form ompleted by the surgeon postoperatively. From these registries we an estimate an inidene of ACL injury, although this number under-represents the true inidene as non-operative ACL injuries are not aptured. From the Norwegian data, a total of 2793 primary ACL ligament reonstrution operations were registered by 57 hospitals during 18 months. This orresponds to an annual population inidene of primary ACL reonstrution surgeries of 34 per itizens (85 per itizens in the main at-risk age group of years). The number of ACL operations differs between the sexes in the seond deade of life, with females having the most ACL reonstrutions in the year age group (fig 1). 1 In looking at a possible differene between the sexes, in the Swedish Registry found a higher proportion of both primary ACL reonstrutions and revisions in men than in women (59% vs 41% and 55% vs 45%, respetively). In an ative German population the overall inidene was 70 per itizens in the more physially ative proportion of the population. 2 In Sweden the inidene of ACL injury in the population aged years was 81 per itizens Br J Sports Med 2008;42: doi: /bjsm

2 Figure 1 Distribution of patients in the Norwegian National Knee Ligament Registry by age and sex. ACL injuries: selet sports Looking at the ACL injury rates produed in selet sports, the National Collegiate Athletis Assoiation (NCAA) Injury Surveillane System (ISS) provides us with a unique 16-year sample of 15 sports aross a ollege age group (typially years). 3 The ISS is a olletion of data from representative olleges and universities (15% on average) and is not a registry. Approximately 5000 ACL injuries were reported over 16 years, produing an average of 313 injuries per year in the sample. Assuming the sample represents 15% of the total population, this equates to an average of more than 2000 ACL injuries in these 15 ativities per year. If the number of reported injuries is onsidered, Amerian football produed the greatest number of ACL injuries. However, if ACL injuries are ranked as a perentage of ACL injuries on a team ompared with all injuries on that team, female sports dominate the list (female football/soer, female larosse, female gymnastis and female basketball; fig 2A). If the ACL injury rate per 1000 exposures is onsidered, female gymnastis rate first with men s spring Amerian football seond, losely followed by female football/soer, female basketball and men s in-season Amerian football (fig 2B). Taken as a whole, the most ommon mehanism of ACL injuries for the sports ommonly assoiated with this injury was non-ontat in nature. The exeptions were men s Amerian football, men s ie hokey and men s wrestling. The NCAA data report exposures as one session of sport (either pratie or play) and do not reord exposure in terms of hours. In this data set, the largest differene in injury rate (injury/athleti exposure 61000) between the sexes was in football/soer (females twie that of men) and basketball (females three times that of men). Despite signifiant hanges in the pae of the game of football/soer and basketball and the improvement in sport-speifi skills of females in general, the rate of non-ontat ACL injury has remained stable over the 16- year period of NCAA data olletion. 4 For the age group slightly younger than ollege level (14 18 years), the rate of non-ontat ACL injuries in soer was twie as high in females as in males. For basketball the rate of injury in the younger female age group is the highest nearly four times that of males. The ratio of male to female injuries dereases slightly at the ollege level and approximates 1 at the professional level (table 1). Alpine skiing There are no onsistent data regarding differenes between the sexes in the inidene of ACL injuries in rereational skiing, but female ompetitive alpine skiers have an inidene rate twie Figure 2 Ourrene of anterior ruiate ligament (ACL) injury expressed as (A) a perentage of all injuries and (B) the rate per 1000 exposures (games and praties ombined, through ). that of male ompetitive alpine raers. 5 Other studies have reported that rereational skiers have the highest inidene of injuries while expert skiers have the lowest inidene. 6 The best longitudinal data in this sport are from a 34-year ase-ontrol study in a single ski area in northern Vermont. During this time there were skier visits and these were assoiated with injuries, of whih 2539 (14%) were ACL tears. Sine 1991 there has been a 40% derease in the rate of ACL injuries, with the inidene at the present time being one ACL injury per 3101 skier visits. Women onstituted 40% of the ontrol population but suffered 63% of all ACL injuries. The risk ratio for women sustaining ACL injuries averaged 2.5 times that of men in the time period of this study. 5 Team handball Several studies have reported the inidene of ACL injuries in team handball. A retrospetive study found that the inidene of ACL injury was highest in women playing at the top level (0.82 ACL injuries/1000 playing hours) ompared with 0.31 injuries/1000 playing hours in men. The relatively high inidene of ACL injuries in female players partiularly elite Br J Sports Med 2008;42: doi: /bjsm

3 Table 1 Rate of anterior ruiate ligament (ACL) injuries relative to age Exposures Rate in females Rate in males High shool Collegiate Professional players has been onfirmed by later prospetive studies. 6 7 The highest inidene of ACL injuries ourred in female elite handball in Norway with 2.29 ACL injuries/1000 math hours. The inidene of ACL injury is high in team handball ompared with other team sports suh as football/soer, basketball and volleyball. However, a diret omparison between the studies is diffiult beause different methods were used to ollet and analyse the data and the way in whih the inidene of ACL injuries was reported was inonsistent. Some report the number of injuries per 1000 playing hours while others report the number of ACL injuries/1000 athlete exposures. Nevertheless, all studies have found that women have a higher inidene of ACL injuries than men. The inidene rate of ACL injuries in all sports studied appears to be signifiantly greater during ompetition than during training and this finding is onsistent among sports. 6 RISK FACTORS FOR ACL INJURY The fundamental basis for knee trauma prevention researh is that injuries are not random events; instead they our in patterns that reflet underlying auses. Understanding the underlying auses or risk fators for one of the more severe sports-related knee injuries an ACL disruption is important for the development of intervention strategies and for identifying those at inreased risk of injury. This provides a target group for intervention. The risk fators for ACL injury have been onsidered as either internal or external to an individual. External risk fators inlude type of ompetition, footwear and surfae, and environmental onditions. Internal risk fators inlude anatomial, hormonal and neuromusular risk fators. External risk fators Competition in games versus pratie Very little is known about the effet of type of ompetition on the risk of an athlete suffering ACL injury. Myklebust et al 8 reported that athletes are at a higher risk of suffering an ACL injury during a game than during pratie. This finding introdues the hypothesis that the level of ompetition, the way in whih an athlete ompetes, or some ombination of the two inreases an athlete s risk of suffering an ACL injury. Footwear and playing surfae Although inreasing the oeffiient of frition between the sports shoe and playing surfae may improve tration and sports performane, it also has the potential to inrease the risk of injury to the ACL. Lambson et al 9 found that the risk of suffering an ACL injury is greater in football athletes who have boots with a higher number of leats and an assoiated higher torsional resistane at the foot-turf interfae. Olsen et al 10 reported that the risk of suffering an ACL injury is greater in female team handball athletes who ompete on artifiial floors that have a higher torsional resistane at the foot-floor interfae Ratio of females to males These data suggest that the rate of ACL injuries dereases as female athletes mature and the level of play inreases. than in those who ompete on wood floors. This relationship did not exist for male athletes. Protetive equipment Funtional braing appears to protet the ACL-defiient knee of alpine skiers from repeated injury; however, the effet of these braes on an ACL graft is inonlusive and requires more study. Koher et al 11 studied professional skiers with ACL-defiient knees and found a greater risk of knee injury in those who did not wear a funtional brae than in those who did use a brae (risk ratio 6.4). MDevitt et al 12 performed a randomised ontrolled study of the use of funtional braes in adets attending the US military aademies who underwent ACL reonstrution. At the 1-year follow-up the use of funtional braing did not affet the rate of ACL graft re-injury. It is important to point out that there were only three injuries among those in the unbraed group and two injuries in the braed group. This was an impressive study that required onsiderable work, and a larger sample size that produes more re-injuries may arry with it the apability to determine whether or not funtional braing an redue the likelihood of ACL graft injury. Meteorologial onditions For sports that are played on natural or artifiial turf, the mehanial interfae between the foot and playing surfae is highly dependent on the meteorologial onditions. However, very little is known about the effet of these variables on an athlete s risk of suffering an ACL injury. Orhard et al 13 reported that non-ontat ACL injuries sustained during Australian football were more ommon during periods of low rainfall and high evaporation. This work introdues the hypothesis that meteorologial onditions have a diret effet on the mehanial interfae (or tration) between the shoe and playing surfae, and this in turn has a diret effet on the likelihood of an athlete suffering an ACL injury. Summary and future diretions for external risk fators The evidene regarding an athlete s omplete external and internal risk fator profile for ACL injury is unlear beause most of the investigations have studied isolated variables. Only the investigation by Uhorhak et al 14 used a multivariate approah to establish a seletion of risk fators that are assoiated with an athlete s risk of suffering an ACL tear. Very little is known about the effet of sport-speifi fators (eg, rules, referees, oahing), meteorologial onditions (eg, tration at the shoe-surfae interfae), playing surfaes and protetive equipment on the risk of suffering an ACL injury. These potential risk fators merit further investigation. Little is known about the effet of age, athletiism, skill level, psyhologial harateristis and prior knee injury as risk fators for ACL injury. For example, almost everything that is known about the inidene rate of ACL injuries in speifi sports has ome from studies performed in 396 Br J Sports Med 2008;42: doi: /bjsm

4 preollegiate (high shool) and ollegiate athletes. There are very few data on the inidene of ACL disruptions in subjets that are younger or older then this narrow age group and, onsequently, the effet of age on the likelihood of suffering an ACL tear is not well understood. Internal risk fators Anatomial risk fators Abnormal posture and lower extremity alignment (eg, hip, knee and ankle) may predispose an individual to ACL injury by ontributing to inreased ACL strain values; alignment of the entire lower extremity should therefore be onsidered when assessing risk fators for ACL injury. Unfortunately, very few studies have studied alignment of the entire lower extremity and determined how it is related to the risk of ACL injury. Most of what is known has ome from investigations of speifi anatomial measures. Noth size and ACL geometry The dimensions of the interondylar noth have been the most disussed anatomial feature in the published literature in relation to aute ACL injuries. Geometri differenes in the size and shape of the ACL have not been well haraterised. In general, studies on ACL geometry and noth dimensions are diffiult to interpret beause of the lak of standardised methods to obtain the data. Despite the number of methods for measuring the noth, noth width measurement of bilateral knees with ACL injury is smaller than that of unilateral knees with ACL injury, and noth widths of bilateral and unilateral knees with injury to the ACL is smaller than noth widths of normal ontrols. This implies a strong assoiation between noth width and ACL injury. 15 In trying to understand the relationship between a small noth and the risk of ACL injury, the size of the ACL has been reviewed. Methods to determine both noth size and ACL dimensions inlude radiographi, MRI and photographi tehniques. Despite these differenes, reent reports have onluded that the ACL is geometrially smaller in women than in men when normalised by body mass index. The properties of the ACL material may differ between the sexes, and there might be an additional link to the assoiation between noth width/acl size and ACL injury. Posterior tibial slope A highly signifiant orrelation has been reported between the posterior inferior tibial slope and anterior tibial translation. 15 It is well known in the veterinary literature that tibial plateau levelling osteotomies are of value for the treatment of ranial ruiate ligament ruptures. 16 Although two small studies did not find a relationship between non-ontat ACL injuries and the audal slope of the tibia, there is new evidene to suggest that more speifi measurement of the lateral tibial slope angle by MRI might be of value. One reent ase-ontrol study 18 suggested that subjets with ACL-defiient knees had a signifiantly greater slope of the lateral tibial plateau and a lower slope of the medial tibial plateau than a ontrol group. This paper suggests that the tibial slope of the medial and lateral ondyle should be ompared separately. 18 The ausal relationship between the posterior slope of the tibial plateau and ACL injury, pivot shift grade, risk of re-injury and potential risk of arthritis remain areas for future researh. Summary and future diretions for anatomial risk fators Although there is good evidene to suggest an assoiation between a smaller width of the intraondylar noth and an inreased risk of ACL injury and, on average, females have smaller nothes than males, the ausal relationship between the two are probably related to a smaller noth housing a smaller ACL. The internal-external rotatory laxity of the tibia relative to the femur is diffiult to measure. There is a spetrum of biologial variation and the threshold between normal and pathologial behaviour is not known. Its role in ACL injury and its onsequenes remain unknown. Anatomial risk fators may not be easy to orret; however, it is important to understand them if we are to be able to identify those at inreased risk of an ACL injury. Hormonal risk fators Struture and mehanial properties of the ACL Studies haraterising the struture and mehanial properties of the ACL indiate that women have smaller ACLs that may have lower tensile linear stiffness, are haraterised by less elongation at failure and lower energy absorption and load at failure than men. These differenes are not explained entirely by dimensional harateristis (ie, differene in size between men and women). This is supported by similar sex differenes in the struture and mehanial properties (greater elongation, greater strain and lower stiffness in women) of the medial gastronemius 21 and patellar 22 tendons, whih are also not explained entirely by anatomial differenes and tendon size. At the more marosopi level, women also have greater tibiofemoral joint laxity (ie, anterior knee laxity, genu reurvatum) and lower joint resistane to translation and rotation These differenes are not limited to the sagittal plane but are also refleted in frontal and transverse plane motion Together, these findings indiate that a broad physiologial mehanism (eg, hormone ontrol, differenes in artiular geometry, ollagen turnover) may explain these sex differenes, whih have been impliated in ACL injury. Although the auses/mehanisms of these sex differenes are not well understood, the identifiation of sex hormone reeptors (eg, oestrogen, testosterone, relaxin) on the human ACL has prompted studies of the potential of sex hormones to affet the struture, metabolism and mehanial properties of the ACL. To date, no speifi mehanism has been identified by whih hormones influene the biology and physiology of the ACL, but there is enough evidene to suggest that hormones are signifiant fators in the normal biology and physiology of ollagen, musle and bone. The onfliting findings to date are in large part due to our poor understanding of this omplex system, and unfortunately the various models used to examine these relationships are often not diretly omparable. To understand the full effets of hormones on soft tissue and ACL injury, researhers must: Consider the effets of oestrogen, progesterone and testosterone (at a minimum) on knee injuries in regularly menstruating, physially ative women. The relevane of various animal studies (beause of their physiologial differenes, oestrus yles) are of unknown signifiane to the human ACL and other soft tissues. 15 Aount for the individual variability in hormone profiles and thus their effets (eg, injury, laxity and other soft tissue differenes). This individual variability is signifiant and methods (espeially sampling tehniques) used to examine Br J Sports Med 2008;42: doi: /bjsm

5 suh parameters of menstrual yle phase and hormone levels must reflet this omplexity. Although the questionnaire-based method of haraterising phasing of the menstrual yle is onvenient, it does not provide an aurate representation of a female s hormone milieu. 35 Further, studies that examine joint laxity, musle properties and biomehanis at a single time point per yle phase are not adequate to identify the parameters of interest, and this approah does not reflet the omplexity of the variations in the magnitude, timing and relative phasing of hormone hanges aross the female menstrual yle, even among women who have a normal 28-day yle. 36 In addition, the effets of hormones are not immediate but our with a time delay. It is therefore important to apture the hormone profile on days leading up to injury or testing. Understand how normal training, ompetition and stress (injury, environment, et) alter the female hormone milieu and their ultimate effets on musuloskeletal tissue. Realise that the effets of hormones and their ultimate effets on soft tissue strutures are on a ontinuum. Oestrogen, progesterone and testosterone are present in both men and women, with onentrations of individual hormones varying widely both within and between sexes. Similarly, men and women vary widely in joint behaviour (joint laxity and stiffness) and neuromusular ontrol. Although women are, on average, more prone to these extremes, men also experiene large variations. It is lear that there are sex differenes in musle performane. It is well aepted that mehanial loading will affet the biology and physiology of tendons and ligaments. However, the extent to whih absolute hormone onentrations or variations aross the yle augment or inhibit tissue remodelling in response to mehanial stress is yet to be understood. ACL injury and the menstrual yle There appears to be a onsensus emerging from the literature that the likelihood of inurring an ACL injury does not remain onstant during the menstrual yle, with a signifiantly greater risk during the preovulatory phase than during the postovulatory phase. Initial work by Wojtys et al 35 used self-reported menstrual history data to haraterise the menstrual status of a subjet at the time of injury and demonstrated a signifiantly greater prevalene of non-ontat ACL injuries among women athletes during the preovulatory phase of the menstrual yle. In a subsequent study by the same group, 37 urine levels of oestrogen, progesterone and luteinising hormone metabolites were used to haraterise a subjet s menstrual status at the time of injury. This onfirmed that signifiantly more ACL injuries ourred during days 9 14 of a 28-day yle with fewer injuries than expeted during the postovulatory phase (defined as day 15 to the end of the yle). Arendt et al 38 found that female athletes were at inreased risk of suffering an ACL injury during the preovulatory phase of their menstrual yle ompared with the postovulatory phase. Slauterbek et al 39 also reported a disproportionally greater number of ACL injuries during the preovulatory phase of the menstrual yle, with fewer injuries ourring as the yle progressed. In a study of rereational alpine skiers, serum onentrations of progesterone and oestradiol were used to stage the phase of a skier s menstrual yle at the time of ACL injury. 40 Skiers in the preovulatory phase of their menstrual yle were signifiantly more likely to suffer an ACL tear than those in the postovulatory phase (odds ratio 3.22). A omparison between this investigation and the previous study by Wojtys et al revealed a striking similarity. In the study of rereational alpine skiers, 74% of the women with ACL injuries were in the preovulatory phase of their menstrual yle and 26% were in the postovulatory phase. Likewise, Wojtys et al 37 found that 72.5% of the women not using oral ontraeption experiened ACL injuries during the preovulatory phase of their menstrual yle ompared with 27.5% during the postovulatory phase. In ontrast, Myklebust et al 8 studied ompetitive European team handball players over 3 years and found an inreased risk of ACL injury during the week before or just after the onset of menstruation. The stabilising effet of oral ontraeptives on the female hormone profile and our understanding of their ultimate impat (both aute and hroni) on soft tissue behaviour, injury or performane is yet to be defined. It is important to realise that the type (ie, progesterone only versus ombined oestrogen and progesterone ompounds) and dosage varies widely among oral ontraeptives, and both endogenous and exogenous levels must be aounted for. There is no onlusive evidene that oral ontraeptives have a protetive effet speifially against ACL injury. Summary of hormonal risk fators Women have smaller ACLs that may have lower linear stiffness, less elongation at failure and lower energy absorption and load at failure ompared with men. Women also have greater joint laxity and dereased musular stiffness. While the speifi auses/mehanisms for these sex differenes are not well understood, hormonal involvement is impliated as they extend beyond pure anatomial differenes. Researh to date suggests that oestrogen alone is probably not responsible for hanges in the struture, metabolism and mehanial properties of the ACL as interations with yli loading and other hormones appear to alter the effets of oestradiol. However, the role of other sex hormones (eg, relaxin, progesterone, testosterone) in the biology and pathology of the ACL are poorly understood. Although animal studies have improved our understanding of the effets of hormones on the mehanial and metaboli properties of the ACL, their linial relevane to the human ACL is questionable. Hormone profiles vary widely among women with regard to the timing, phasing and amplitude of hormone hanges aross the yle. This variability suggests that some women may experiene greater effets of sex hormones on ligament biology than others, potentially exposing those individuals to greater hanges in strutural integrity and risk of injury. There is a pauity of linial researh examining these effets in women who are oligomenorrhoei or use ontraeptive hormones, and who represent a large perentage of the physially ative female population. There appears to be mounting evidene that women are at a signifiantly greater risk of ACL injury during the preovulatory phase of the menstrual yle than during the postovulatory phase. Future diretions Future basi siene studies investigating the effets of hormones on the struture, metabolism and mehanial properties of ollagen should examine all relevant hormones at their normal physiologial onentrations using models relevant to the physially ative female. 398 Br J Sports Med 2008;42: doi: /bjsm

6 Table 2 Use of video analysis to study mehanisms of non-ontat ACL injury in sport Referene Total No. analysed Methods and materials Boden et al Visual inspetion and questionnaires. Videos obtained from professional and ollegiate teams: football (56%), basketball (30%), soer (9%), volleyball (4%). 7 women, 16 men Ebstrup et al Visual inspetion. Prospetive olletion of videos from Danish indoor ball games. Two representative handball injuries and one basketball injury analysed. All women Teitz Visual inspetion. Retrospetive multientre video analysis: 20 basketball, 18 football, 9 soer injuries. Only basketball injuries analysed. 3 men, 11 women Olsen et al Visual inspetion and questionnaires. Retrospetive and prospetive video olletion of women s Norwegian or international handball ompetition Krosshaug et al Visual inspetion. Retrospetive video olletion from high shool, ollege and NBA, WNBA basketball. 13 men, 17 women Krosshaug et al D model-based image mathing. One male NBA basketball player (4 amera views), one female Norwegian elite team handball player (3 amera views) It is ritial that study designs address the omplexity of the female menstrual yle, inluding the potential time delay effets and individual variations in hormone profiles. Sampling at multiple time points within a phase is reommended. Future studies should extend beyond normal menstruating females and examine the effets of hormone variations (amenorrhoea, oligomenorrhoea, oral ontraeptive use) on both aute and hroni soft tissue hanges and the potential for injury. Limited evidene suggests that hormones may modify the normal remodelling effets of ollagen seondary to mehanial loading. Future studies should examine these effets in ombination using models relevant to the physially ative female. Future studies should aim to examine how normal training, ompetition and stress (injury, environment) alter the female hormone milieu and their ultimate effets on musuloskeletal tissue. MECHANISMS FOR NON-CONTACT ACL INJURIES To develop speifi methods for preventing sports injuries, it is important to understand the ausative event or mehanism of injury, as outlined by Bahr and Krosshaug. 41 A number of different methodologial approahes have been used to study the mehanisms of injury in sports. 42 These inlude interviews with injured athletes, analysis of video reordings of atual injuries, linial studies (where the linial joint damage is studied to understand the mehanism of the injury), in vivo studies (measuring ligament strain or fores to understand ligament loading patterns), adaver studies, mathematial modelling and simulation of injury situations, or measurements/estimation from lose to injury situations. Video analysis Video analysis is essential as it is usually the only way to obtain kinemati information from the atual event. In rare ases, injuries have even ourred during biomehanial experiments, but for obvious ethial reasons we annot base our researh on this approah. Six studies have used video analysis to study nonontat ACL injury mehanisms in sports (table 2) These studies were in general agreement that injuries ourred in utting or landing situations. The knee was reported to be relatively straight at the point of injury. All the studies agreed that knee valgus was seen frequently. Boden et al 43 found that the amount of internal/external rotation at the time of rupture was minimal. This agrees with the findings of Olsen et al 46 where the amount of internal/external knee rotation was 10u or less in 90% of the ases. However, the interpretation of the findings varied onsiderably. Olsen et al 46 stated that valgus loading in ombination with external or internal knee rotation aused the injury and proposed noth impingement as a plausible ause of the exessive ACL loading. Boden et al 43 and Teitz, 45 on the other hand, hypothesised that a vigorous eentri quadrieps ontration was the main ause. Unfortunately, the reported variables among the studies using the visual inspetion approah are non-standardised, making it diffiult to ompare them. Another major limitation of these studies is that all exept from one are based on visual inspetion. Krosshaug et al 49 validated the visual inspetion method and found the auray and preision among six experiened ACL researhers to be poor. For example, it was found that the true knee flexion angle was generally twie as high as the estimate. When the analysts estimated 30u knee flexion, the true angle was 50 60u. Results from studies based on this method must therefore be interpreted with great aution. To overome the inherent diffiulties of visual inspetion of human motion from video, Krosshaug et al 50 developed a novel and versatile tehnique using model-based image mathing from one or more amera views. In this method, 3D models of the surroundings as well as a ustomised skeleton model are manually mathed to the bakground video footage. This method proved to be muh more aurate with a differene in root mean square from traditional marker-based motion analysis of,12u for hip and knee flexion/extension. In a new study, Krosshaug et al 48 demonstrated the feasibility of this method to atual injury videos. Detailed time ourses for joint kinematis and ground reation fore were obtained for a fouramera basketball video and a three-amera European team handball video. The valgus angle inreased abruptly in both ases, from 4u to 15u within 30 ms and from 3u to 16u within 40 ms for the basketball and handball injury, respetively. However, in order to make general statements on typial injury kinematis, a systemati approah to olleting and analysing more injury videos is needed. Biomehanial studies Reent studies have onsistently shown that the predominant fores that affet strain in the ACL are anterior-direted shear fores applied to the tibia (either from external soures suh as an anterior-direted fore applied to the bak of the lower leg or Br J Sports Med 2008;42: doi: /bjsm

7 through internal mehanisms suh as ontration of the dominant quadrieps musles with the knee near extension). Important ontributions to ACL strain values ome from fores applied in the oronal and transverse planes of the knee. Biomehanial data have ome from adaveri studies of knees, strain gauges plaed in vivo at the time of surgery and from analytial modelling. Cadaveri studies show that anteriordireted shear fores reate most strain in the ACL with some added inrease from varus, valgus and internal rotation moments External torque applied to the knee produes relatively low ACL strain values. Valgus torque alone reates ACL strain only after signifiant injury to the medial ollateral ligament. Interestingly, omplete injury to the medial ollateral ligament was neessary before signifiant injury to the ACL due to valgus torques applied in isolation. 54 These adaveri studies emphasise the importane of anterior shear fores in ACL injury. With the knee near full extension, ontration of the quadrieps musle and the resulting fore developed in the patellar tendon produes an anterior-direted shear fore on the proximal aspet of the tibia that strains the ACL. The ACL strain values are proportional to the magnitude of fore in the patellar tendon and the angle of flexion of the knee. The patella tendon forms an angle with the tibia and its geometri orientation is able to produe ACL strain when the knee is near extension. An impulsive load applied to the patellar tendon with the knee in slight flexion has the potential to reate an injury to the ACL. 55 Appliation of patellar tendon fore alone ould ause enough ACL strain to alter it grossly and to inrease anterior knee laxity. Appliation of impulse to a adaveri knee flexed at 25u inreased strain in the ACL. When the impulse was applied 15u out of the knee flexion plane to reate a valgus fore, the amount of strain was inreased. 56 In vivo studies have produed similar onlusions. Strain gauges were plaed on the ACL after arthrosopi surgery in whih the ACL was normal. The gauges were left in plae long enough for the subjets to reover and perform rehabilitation exerises with moderate ontration of the leg musulature. 57 Strain in the ACL is affeted by anterior-direted shear loads applied to the tibia (relative to the femur) and a little by varus/ valgus torques. Rehabilitation exerises an also produe signifiant strain in the ACL. Exerises that inlude ontrafotion of the quadrieps in isolation with the knee near full extension produe high ACL strain values, while o-ontration of the hamstring musles in ombination with ontration of the dominant quadrieps musles redues ACL strain values in omparison to ontration of the quadrieps musle group in isolation. Strain gauges have been implanted in subjets undergoing a high speed stop jump. The ACL was strained before heel strike and reahed a maximum value very lose to the maximal ground reation fore at landing. 58 Cadaveri and in vivo studies have not learly eluidated the biomehanial fators seen in the motion analysis studies of ACL injury. These studies often show a prominent motion in the oronal plane that involves femoral addution and internal rotation, knee flexion and valgus, tibial rotation, and foot and ankle valgus. Motion analysis studies show a omplex onneted ombination of movements whih appear as an apparent valgus in the oronal plane but probably have only a relatively small ontribution from atual medial opening of the knee, whih would injure the medial ollateral ligament if all the apparent valgus was isolated to oronal plane knee motion. Kinemati analysis Kinemati analysis of athleti ollege students revealed several differenes in motion during running, side-step utting and ross-over utting. Women landed with less knee flexion and maintained a straighter knee during the entire stane phase. In addition to flexion, women displayed more valgus through the entire stane phase. It should be pointed out that the flexion ours at the knee itself, while the valgus seen in the oronal plane ould not have ome entirely from rotation of the tibia around a stationary femur in the oronal plane but inludes hip internal and external rotation ombined with knee flexion. The musle firing was assessed by eletromyography (EMG). The quadrieps EMG was higher in women, with ativation levels nearly twie those of the maximum EMG levels in musle under maximum loading onditions in a dynamometer. Hamstring ativity was less in women and the EMG values were only about half the level of a maximum measured by a dynamometer. ACL injuries appear to our with hard and awkward landings on the knee when it is positioned near extension. The female knee is in more extension and has higher quadrieps ativation at initial ontat with the ground. The straighter knee and the higher quadrieps ativation an ombine to produe more strain in the ACL. 59 A kineti and kinemati analysis was performed for men and women performing a forward jump with a stop on a fore plate followed by a jump slightly behind, straight above, or slightly ahead of the initial landing loation. 60 Inverse dynamis allowed omputation of anterior shear fores whih would at to produe strain on the ACL. Women had greater anteriordireted shear fores on the tibia for all jump onditions. Maximum ACL strains were observed near the initial ontat period. 61 Fatigue studies using a similar kineti analysis showed that fatigue inreased anterior-direted shear fores in both men and women. 62 A kinemati study of young male and female football/soer players showed that boys and girls land from a stop jump with a similar amount of knee flexion and valgus at ages 11 and 12. However, women begin to land with straighter knees progressively up to age A mehanism seen frequently in ACL injuries involves a hard and awkward landing with the knee near extension (eg, straight). The hard landing implies high fores that are not dampened by hip, knee and foot motions. The awkward part of the landing may involve landing outside the antiipated landing manner with higher than usual abdution or valgus moments applied about the hip and knee joints. 64 Summary and future diretions for mehanisms of ACL injury Whole body motions related to mehanisms of injury: almost 80% of ACL injuries are non-ontat in nature. Injuries often our when landing from a jump, utting or deelerating. A ombination of anterior tibial translation and lower extremity valgus are probably important omponents of the mehanism of injury in these athletes. Likely omponents to the injury mehanism inlude: anterior translation, dynami valgus of the lower extremity with the joint near extension, low flexion probably due to inreased quadrieps ativity (and this ould also inlude inreased gastronemius ativity), most or all of the fore on a single leg or foot with the foot displaed away from the body s entre of mass and inreased trunk motion. Potential neuromusular imbalanes may be related to omponents of the injury mehanism: women have more quadrieps dominant neuromusular patterns than men. 400 Br J Sports Med 2008;42: doi: /bjsm

8 Hamstring reruitment has been shown to be signifiantly higher in men than in women. The hamstring to quadrieps peak torque ratio tends to be greater in men than in women. Beause of the likely injury mehanism, it is reommended that athletes avoid knee valgus and land with more knee flexion. Lower extremity valgus (knee abdution) loading and anterior tibial translation are likely to be involved in the mehanism. Future researh should ombine several researh approahes to validate the findings suh as video analysis, linial studies, laboratory motion analysis, adaver simulation and mathematial simulation. EVALUATION OF ACL INJURY Little is known about differenes between the sexes in the evaluation of ACL injury. We therefore suggest that future studies should inlude large enough sample sizes to allow detetion of possible sex differenes in the outomes of interest. Natural history There is a little knowledge about the natural history of an ACL injury. The true inidene is probably not known. With MRI as a diagnosti tool the inidene is 8 per , whih has been verified by register studies However, it an be assumed that many patients with ACL injuries are not diagnosed at the time of the injury. A proportion of these athletes will present with knee symptoms or after re-injury. Others will never be diagnosed and are either without symptoms or have adapted to the altered knee funtion. The proportion of these ategories is unknown. In a US ommunity-based ohort there was a prevalene of ACL injuries of 4.8% in individuals aged years unseleted for problems. 65 The ourse of an ACL injury depends on many different fators suh as trauma mehanism, assoiated injuries, anatomial fators and ativity level after the injury. There is no evidene of a sex differene. Independent of treatment, some patients will ope with their desired ativity level ( opers ), some will adapt to an ativity level adjusted for the knee funtion ( adaptors ) and others will be unable to ope with being ACL-defiient ( non-opers ). Earlier reports show that eah of these groups onstitutes about one-third of the population. 66 This is supported by a reent study of primarily non-operatively treated ACLs. 67 Summary and future diretions for evaluation of ACL injury The diagnosis is made by a history of sudden knee pain during strenuous ativity, an inability to ontinue, a pop, haemarthrosis. The ativity level is diffiult to determine beause there are no solid data on the normal derease in ativity level without injury. In the longer perspetive, the level of ativity does not seem to be differ with different treatments. The ourse is determined by whether they are opers, adaptors or non-opers. There is no aurate way to define the ategories in the aute phase. No good data exist on the natural history of untreated lesions ombined with a ompliant training programme and various levels of post-injury ativity levels. Prospetive studies are needed to study the natural history of the ohort, but these are diffiult to arry out. Assoiated injuries As evaluated by MRI, an isolated ACL is an infrequent phenomenon although assoiated ligament injuries may not be very ommon. 65 In the Norwegian National Knee Ligament Registry (NLKR), a total of 2793 primary ruiate ligament reonstrution surgeries were registered by 57 hospitals. 1 In 27 ases (1%) a lateral ollateral ligament injury was reported, while a medial ollateral ligament injury was reported in 129 ases (5%). Non-operative treatment of medial ollateral ligament injury is effetive if ombined with reonstrution of the ACL. 68 Knees with aute ACL injuries should be evaluated for menisus tears as these are identified in approximately 50% of ases. 69 Lateral menisus tears are more frequent in the aute setting while medial menisus tears are assoiated with hroni ACL defiieny. 70 In the NLKR there were a total of 1287 (47%) assoiated menisus tears, 90% of whih were treated surgially. 1 In a study of ACL injury in 151 patients (36% women), 38% had an assoiated medial menisus tear. 65 ACL injury is a major long-term risk fator for the development of osteoarthritis, and no urrently available treatment has been shown to redue this risk fator. 71 It is not lear if it is the assoiated menisus tear at baseline, the development of a seondary menisus injury, the menisetomy or all of these that onstitute risk fators for the development of osteoarthritis of the knee. 72 Artiular artilage lesions were reported in 712 knees (26%) in the NKLR, 59% of whih were treated surgially. 1 When grading the artilage lesions aording to the International Cartilage Repair Soiety lassifiation, (31%) ases were lassified as grade 1, 283 (40%) as grade 2, 151 (21%) as grade 3 and 49 (7%) as grade 4 (grading was missing in 7 ases). MRI is useful in assessing assoiated injuries suh as bone ontusions, intra-artiular fratures and assoiated ligament injuries. One reent study has mapped out onomitant fratures and menisus injuries and traumati bone marrow lesions in the aute ACL injured knee by MRI and quantitative MRI (qmri) in 121 subjets (26% women). Most of the ACL injured knees had a ortial depression frature whih was assoiated with larger volumes of bone marrow lesions. 74 Clinial examination Physial examination Examinations appear to detet differenes in end point better than differenes in displaement. 75 Joint motion varies within a normal population but there is little left-to-right variation in normal subjets. The ACL funtions as a primary stabiliser to limit anterior tibial displaement and a seondary stabiliser to restrain tibial motion. The auray of diagnosti tests for ACL injury has been questioned. A positive pivot shift test result is best for ruling in an ACL rupture, whereas a negative Lahman test result is best for ruling out an ACL rupture. It is also onluded that, using sensitivity and speifiity values only, the Lahman test is better overall for ruling in and ruling out ACL ruptures. 76 The first linial examination after an aute knee trauma has a low diagnosti value. For isolated ACLs, physial examination ontinues to have high speifiity. Instrumented testing devies and stress radiography are used for objetive measures but their use seems to be delining. Further assessment with MRI improves the hanes of a orret diagnosis of intra-artiular pathology and is reommended in the early phase after rotational knee trauma. 65 The auray of MRI in prediting ACL injuries is 95% or more. Br J Sports Med 2008;42: doi: /bjsm

9 Outome sores and outome after ACL injury During the last deade, onsensus has been reahed within the field of sports mediine, orthopaedis and general mediine that the primary outome measure in linial trials should be patient-relevant. Thus patient-reported outomes with different fous have been developed, usually ategorised as generi, jointspeifi or disease-speifi. For patients with ACL injury, disease-speifi, joint-speifi and generi instruments are used. Books and literature reviews outlining and omparing the urrently available instruments for patients with knee omplaints have been published Validity, reliability and responsiveness The most important property of an outome measure is the responsiveness. For a measure to show high responsiveness in linial studies, good validity and reliability are neessary prerequisites. A safe hoie is therefore to hoose an outome measure with proven large effet sizes. Validity is not an absolute or dihotomous property, and validation is a ontinuously ongoing proess. The orrelation between knee pathology and the patient s pereption is generally weak, 81 indiating that, for patient-relevant outome measures, the more important aspet is that the measure is validated for patients of similar age, sex and physial ativity level rather than the speifi knee pathology. A further fat in support of this view is that knee pathologies suh as ACL tear, menisal tear or artilage injury often oinide and symptoms from speifi pathologies annot be separated. 82 Sore aggregation and sore ategorisation Strutural outomes suh as joint laxity and radiographi hanges do not orrelate well with patient-relevant aspets suh as pain and funtion. Aggregating these outomes into one total sore will jeopardise interpretation of the results, and outomes on different levels need to be reported separately. Tapper and Hoover 83 introdued a system for evaluation of symptoms and funtion following menisetomy in 1969 in whih they ategorised outome into four ategories: exellent, good, fair and poor. This approah is appealing and the raw sores of established knee soring sales are frequently ategorised into these same four ategories using arbitrarily hosen ut-off values. However, ategorisation an introdue bias and lak of preision. The Lysholm soring sale 78 is a ommonly used knee injury soring sale whih aggregates funtion and symptoms into one single sore from 0 to 100 (worst to best). A ut-off of 84 points is used to ategorise a good/exellent outome. But what does a Lysholm sore of 84 really represent? A patient ould have a slight limp, some problems with stairs and some pain and be ategorised as good/ exellent with a total sore of 89 out of 100. Alternatively, a patient ould experiene frequent instability that preludes sports ativity, have some pain and yet sore 85 and be ategorised as having a good/exellent outome. Most probably the patients themselves would not ategorise either of these senarios as a good/exellent outome. Categorising the raw sores of rating sales tends to inflate the result, and the interpretation of ategorial data depends on the ontent of the partiular rating sale and the relative weight given to eah omponent aggregated into the total sore. 84 Individuals rated as exellent or good on one sale may therefore be rated as only fair on another sale. Avoidane of data generalisation remains the optimal method for studying the outome of knee injury. 84 Administration mode of questionnaires Substantial bias may be introdued if the operating surgeon answers the questionnaire. 85 This also holds true for unbiased observers and applies to orthopaedi onditions other than ACL injury as well Patient-relevant questionnaires should thus be self-administered (ie, filled out by the patients themselves in a neutral setting). The administration mode needs to be aounted for when omparing the patient-relevant outomes between studies and over time within the same group of patients. The awareness of interviewer bias has inreased over the years and more reently developed outome measures are intended for self-administration. Summary and future diretions for outome sores and outomes It is important to evaluate the patient s perspetive of ACL injury. To minimise bias, patient-administered questionnaires should be used. Aggregating strutural outomes, symptoms and funtion into one total sore will jeopardise interpretation of the results, and outomes on different levels need to be reported separately. Categorising the result of a sore into good/exellent, fair and poor should be avoided as this will inflate the results, introdue bias and laks preision. Motion analysis ACL injury inreases pathologial anterior tibial translation and internal tibial rotation. Motion analysis enables detetion of subtle kineti and kinemati hanges that may influene the final linial outome. There has been a gradual development of funtional adaptations that protet the knee joint from exessive anterior translation episodes. Suh a mehanism is limited quadrieps funtion near extension (the quadrieps avoidane pattern) and inreased and prolonged hamstring ativity. Another possible mehanism of hondral damage is inreased internal tibial rotation whih an lead to exessive loads in artilage areas not ommonly loaded and thus ontribute to future osteoarthritis. A better understanding of kineti and kinemati hanges and their final effet on the menisi and artilage may provide useful information in the evaluation of an individual s natural ourse Summary of motion analysis ACL injury results in hanges in kinematis, kinetis and neuromusular ativity. Anterior tibial translation, internal rotation of the tibia and quadrieps avoidane are typial features seen following an ACL injury. These hanges most probably ontribute to the development of osteoarthritis. MANAGEMENT OF ACL INJURY All healthare units that servie patients with knee sprains should have the apability to reognise the presene of a torn ACL or should be able rapidly to refer the patient to a lini that an onfirm the diagnosis if it is suspeted. 65 The treatment goals are to restore knee funtion, to permit the patient to return to a desired level of ativity without experiening giving way and to minimise the risk of osteoarthritis. Treatment an be either surgial with a reonstrution of the ACL followed by rehabilitation or rehabilitation alone. The literature supports both treatment regimes, but a reent Cohrane review Br J Sports Med 2008;42: doi: /bjsm

10 onsiders that good quality randomised trials are required to determine whih is the preferred treatment in different situations. The deision regarding the treatment must therefore be individualised, and fators suh as oupation, sports ativities and assoiated injuries must be onsidered. A high ativity level, speifi demands and repairable menisus injury are relative indiations for subaute ACL reonstrution. If highrisk ativities are not antiipated, the primary treatment option for an ACL injury is rehabilitation training for 3 4 months followed by assessment of knee funtion and quality of life. If re-injury ours or the patient is not satisfied for any reason, a late reonstrution an be performed. The major tehnial priniples for an ACL reonstrution have beome less ontroversial in reent years with improved knowledge of the anatomy, biomehanis and the healing proess of the ACL following reonstrution. The hoie of grafts has reeived muh attention in the literature. In general there is little evidene to suggest signifiant differenes between hamstring and bone-tendon-bone (BTB) grafts regarding shortand mid-term outome. 92 Reports of more degenerative hanges in the tibiofemoral joint after ACL reonstrution with BTB grafts need to be verified in randomised linial trials. 93 An inreased failure rate in women ompared with men has been reported with the hamstring graft. 94 In another study with questionable power, greater anterior knee laxity over time was found in women ompared with men after reonstrution with the hamstring graft. 95 These data may support the use of a BTB graft as the first hoie in women but, importantly, it must be pointed out that there is no lear orrelation between laxity and funtional outome. 96 The double bundle tehnique for ACL reonstrution may have advantages regarding the long-term results sine it seems to have the potential for diminishing rotational laxity. 97 This may redue the risk of later osteoarthritis but no data are available to support this theory. There is urrently no onvining evidene that this tehnique results in better funtional results than the single bundle proedures. It appears that the inidene of ACL injuries among prepubesent hildren is similar in boys and girls. 98 At puberty the inidene in girls beomes higher than in boys and, aording to the Sandinavian ACL registries, more ACL reonstrutions are performed on girls at this age. 99 The priniples for treatment of ACL injuries in this age group are not gender-speifi. 100 The high risk of seondary injuries after the initial injury must be onsidered and, independent of the treatment hoie, these patients must be followed arefully. A speifi problem is the potential risk of growth disturbanes when reonstrution of the ACL is performed before puberty. 101 Rehabilitation after ACL reonstrution Motion and immediate weight bearing postoperatively have been standard sine the 1970s. 102 Pain inhibition is a ommon problem postoperatively (first days) and an result in inability to ativate the thigh musles, primarily the quadrieps, irrespetive of the graft used, and the hamstrings when the hamstring graft is used. Eletrial musle stimulation may therefore play an important role in the early phase of rehabilitation after ACL surgery in order to stimulate ativation of the musles and is more effetive in women than in men for preventing musle hypotrophy. 103 Failure to regain full knee extension after ACL reonstrution is the most ommon ompliation. 104 Range of motion exerises with partiular attention to regaining omplete knee extension are therefore important and should be started during the first postoperative days. Full knee extension is individualised, with some patients having normal hyperextension. Knee extension of the ontralateral leg should therefore be examined and the same degree of knee extension should be obtained in the ACL reonstruted knee. Both losed kineti hain (CKC) exerises and open kineti hain (OKC) exerises for the quadrieps musle should be used in the rehabilitation programme. 105 OKC training should, however, be introdued autiously, starting with a range of motion between 90u and 40u with a 10u inrease every week. CKC exerises are reommended from the beginning of rehabilitation. A reent study showed greater anterior knee laxity when OKC quadrieps exerises were introdued 4 weeks after hamstring ACL reonstrution than when OKC exerises were introdued after 12 weeks. 106 It is therefore reommended that OKC quadrieps training should not begin within 8 weeks of surgery. The same differene was, however, not found in patients with patellar tendon ACL reonstrutions. Strengthening of the hamstrings may be diffiult when one or more of the musle tendons has been removed for the graft. Quadrieps strengthening is as important in patients with hamstring grafts as it is in patients with patellar tendon grafts. More problems were experiened with gaining strength of the hamstring musles after ACL reonstrution with hamstring grafts than gaining strength of the quadrieps musle after patellar tendon ACL reonstrution. 107 A number of authors have emphasised the importane of neuromusular training to restore knee joint stability and regain a normal motion pattern. 108 When using funtional performane tests for evaluation, it has been found that a battery of at least three (or four) different tests should be used. 109 Patients should be tested during nonfatigued musle onditions as well as fatigued onditions. 110 There is as yet no evidene that rehabilitation after ACL reonstrution should be different for men and women. However, in 4890 patients with ACL reonstrutions, inreased laxity after reonstrution with a hamstring graft ourred independent of sex (Werner, unpublished 2008). No signifiant differene was seen between the sexes in improvement of quadrieps and hamstring musle torques, one-leg hop test for distane, linial outome as evaluated by the Knee Osteoarthritis Outome Sore (KOOS) and ativity level as evaluated by the Tegner Ativity Sale 6 8 months after reonstrution and rehabilitation. The findings of inreased laxity after ACL reonstrution with a hamstring graft may imply that rehabilitation should be slowed down after reonstrution with this graft. Return to sport Shelbourne and Gray 111 stated that: It has beome almost universally aepted that rehabilitation of the healing ACL graft with rapid return to sports that have the potential to generate high stresses in the graft and knee does not lead any deleterious effets. Early return to sports in reent years has been advoated beause of better surgial tehniques and understanding of ACL biomehanis and the healing proess. Most artiles published in reent years advoate a return to unrestrited sports 6 months or later following ACL reonstrution. Evidene-based evaluation has not proved that an early (2 4 months) return to sport is safe and effiaious, but it is possible to return to sports early (2 4 months) after ACL reonstrution. From the perspetive of the graft and rehabilitation, it is also possible to do so with autely injured Br J Sports Med 2008;42: doi: /bjsm

11 ACL-defiient knees without surgery A few artiles have been published that advoate return to sports between 4 6 months. These are ase series whih are unable to prove that suh an early return is more effetive or safer than a more onservative rehabilitation. High-quality evaluations have shown that early weight bearing, early mobilisation, rehabilitation without braes and early CKC exerises are safe. 118 Return to sports following ACL reonstrution should be based on regaining funtion, range of motion, strength and patient s desire (goal-based, not timebased) However, in the long term, hanges in artilage metabolism noted following ACL reonstrution must also be onsidered. This is important when disussing the timing of a return to demanding ativities after an ACL injury/reonstrution. Cartilage metaboli MRI and synovial fluid studies indiate that ACL-injured joints do not regain their normal glyosaminoglyan (GAG) struture until 2 years after the injury A low GAG ontent in the artilage matrix is probably a risk fator for osteoarthritis, sine the altered struture of the matrix may expose the ollagen network and make the matrix more suseptible to the development of osteoarthritis. Only two of the reently published randomised ontrolled trials and systemati reviews onerning rehabilitation following ACL reonstrution have dealt with return to sports. In a randomised ontrolled trial of rehabilitation following ACL reonstrution in soer athletes, Ekstrand et al 119 ompared a standard 6-month protool with an extended 9-month protool and found no differenes in outomes 1 year after surgery. In another randomised ontrolled trial, Beynnon et al 117 ompared a 19-week rehabilitation programme with one that lasted 31 weeks. Exerises were instituted over different time intervals based on the amount of strain produed within the normal ACL. No differenes were found in anteroposterior laxity, International Knee Doumentation Committee (IKDC) linial assessment, KOOS, one-leg hop tests, markers of artilage metabolism or Tegner Ativity Sale 2 years after surgery. Both aelerated and non-aelerated rehabilitation produed inreases in the synthesis of type II ollagen and proteoglyan. Both programmes appeared to affet artilage metabolism similarly. Changes in artilage metabolism following ACL reonstrution persist for up to 2 years. This warrants the attention of all liniians as it may have profound effets on the eventual development of osteoarthritis, no matter what rehabilitation is undertaken. Long-term studies have shown that about 50% of patients have radiographi osteoarthritis of the knee joint 15 years after an ACL injury, independent of treatment. However, some studies have shown a muh lower prevalene The study by Neuman et al 123 found no osteoarthritis after non-operative treatment if there was no seondary menisus injury. All these studies had slow and ontrolled rehabilitation after the injury or after surgery, whih may indiate the relevane of the GAG loss as desribed by Tiderius et al. 122 It therefore seems possible to redue the risk of osteoarthritis markedly, but it is unlear whether this is ompatible with return to high demanding sports ativities. Summary of management The priniples of treatment of ACL injuries in prepubesent hildren are not sex-speifi. A speifi problem is the potential risk of growth disturbanes when a reonstrution of the ACL is performed before puberty. The high risk of seondary injuries after the initial injury must be onsidered and, independent of the treatment hoie, these patients must be followed arefully. High quality evaluations have shown that early weight bearing, early mobilisation, rehabilitation without braes and early CKC exerises are safe. There is no evidene in the literature that rehabilitation should be different for men and women after ACL reonstrution. Return to sports following ACL reonstrution should be based on regaining funtion, range of motion, strength and patient s desire (goal-based, not time-based). Long-term studies have shown that about 50% of patients will have radiographi osteoarthritis 15 years after an ACL injury, independent of treatment. Data are available indiating that it is possible to redue the risk of osteoarthritis markedly. There is no evidene that the method for this redution ould be surgial, exept the fat that stabilisation may lower the risk of a seondary menisus tear. It is unlear whether this is ompatible with a return to high demanding sports ativities. Remaining questions Is it possible to redue the risk of menisus tears with speifi rehabilitation protools or is the only way a surgial stabilisation? Is the subtle inreased anterior knee laxity and oupled internal rotation often noted following ACL reonstrution safe for injured/healing artiular artilage? What is the effet of rehabilitation and return to sport on the menisus, whether injured or not? How does the prolonged alteration in artilage metabolism observed following ACL reonstrution affet the future status of the artilage surfaes? How fast an rehabilitation and return to sport be advaned without injuring the healing ACL graft? What are the effets of body weight and musle fores on graft healing? We need more studies with high levels of evidene to answer these questions. PREVENTION OF ACL INJURY A growing number of injury prevention programmes targeted at reduing the risk of ligamentous knee injury in general and ACL injury in partiular have been reported in the literature Although a number of risk fators for ACL injury have been proposed, only the biomehanial risk fators have been examined in suffiient depth to support the design and evaluation of preventive interventions. Most prevention programmes attempt to alter the dynami loading of the tibiofemoral joint through neuromusular and proprioeptive training. The studies to date fousing on biomehanial modifiations have resulted in the redution of lower extremity injuries in athletes. However, the studies vary widely both in their approah to injury prevention and the validity of the study design. Most studies to date have been non-randomised and very few have been onduted as randomised ontrolled trials. Despite this, we now have valuable information to assist us in preventing ACL injuries in female athletes. Suessful programmes share a number of ommon elements. Most inlude one or more of the following: traditional strething, strengthening, awareness of high-risk positions, 404 Br J Sports Med 2008;42: doi: /bjsm

12 Table 3 Relation of omponents of prevention programmes to speifi risk fators for anterior ruiate ligament (ACL) injury Position Intervention strategy How? Extended knee at initial ontat Knee flexion Conentri HS ontrol and soft landing Extended hip at initial ontat Hip flexion Iliopsoas and retus femoris ontrol and soft landing Knee valgus with tibiofemoral Address dynami ontrol; derease Lateral hip ontrol upon landing loading dynami valgus Balane defiits Proprioeption drills Dynami balane training Skill defiieny Improve agility Agility drills to address deeleration tehniques and ore stability HS, hamstring. tehnique modifiation, aerobi onditioning, sports-speifi agilities, proprioeptive and balane training and plyometris. The relation of these omponents to speifi risk fators for ACL injury is summarised in table 3. Eduation Ettlinger et al 125 used a relatively simple approah to prevention of ACL injury in downhill skiers, attempting to modify high-risk behaviour through eduation and inreased awareness. In this prospetive non-randomised trial, 4000 on-slope alpine ski instrutors and patrollers in 20 ski areas ompleted training and reporting requirements during the skiing season. The training kit inluded a 19 min ACL Awareness Training videotape showing 10 reorded ACL injuries sustained by alpine skiers of various levels and various written materials. The videotape used guided disovery, allowing viewers to visualise arefully seleted stimuli and inorporate this information into their skiing in order to avoid high-risk behaviour and manage high-risk situations to redue the risk of ACL injury. Partiipants also underwent an awareness training session whih inluded proper body positioning, understanding of the phantom foot ACL injury mehanism, and strategies to avoid high-risk positions as well as effetive reation strategies. The two seasons before the intervention season served as historial ontrols, during whih area employees had sustained an average of 31 serious ACL sprains per season. During the intervention season, employees sustained 16 serious ACL sprains (6 in the untrained group and 10 in the trained group), a redution of 62% ompared with the normalised expeted number of ACL injuries in the trained individuals of 26.6 (p,0.005). Isolated strengthening and onditioning Cahill and Griffith 126 looked at the effet of inorporating weight training into preseason onditioning for high shool Amerian football teams. Over the 4 years of the study they noted a redution in reported knee injuries and knee injuries requiring surgery in the intervention group. Isolated proprioeptive training In a study of the effet of isolated proprioeptive training on the risk of ACL injuries in soer players, Caraffa et al 127 onduted a non-randomised prospetive study of 600 semi-professional and amateur soer players in Umbria and Marhe in Italy; 20 teams (10 amateur and 10 semi-professional) underwent proprioeptive preseason training in addition to their regular training session (group A) and 20 teams (10 amateur and 10 semiprofessional) ontinued training in their usual fashion (group B). The intervention group (group A) was subjeted to a five phase progressive balane training programme onsisting of: no balane board; retangular balane board; round balane board; ombination (retangular/round); and a BAPS board (Camp Jakson, Mihigan, USA). The duration/frequeny was 20 min/ day for 2 6 days/week inluding a minimum of 3 times per week during the season. The groups were followed for 3 years and all players with a potential knee injury were evaluated; 10 arthrosopially onfirmed ACL injuries ourred over the three seasons (0.15 ACL injuries/team/season) in group A ompared with 70 suh injuries (1.15 ACL injuries/team/season) in group B (p,0.001). Unfortunately, no differentiation was made between ontat and non-ontat ACL injuries. Neuromusular training Tehnique Several studies have used a neuromusular training programme for injury prevention. 128 Henning 129 implemented a prevention study in two NCAA Division I female basketball programmes over the ourse of 8 years. He proposed that the inreased rate of ACL injury in female athletes was primarily funtional and was related to knee position and musle ation during dynami movement. In knee extension the quadrieps exerts a signifiant anterior translational fore on the tibia, thus imparting a sheer fore on the ACL. Conversely, as the knee moves into flexion, the anterior translational fore on the tibia is dereased, thereby dereasing the torque on the ACL seondary to the ontration of the hamstrings. To derease the risk of ACL injury, Henning proposed that the athletes ut, land and deelerate with knee and hip flexion. In addition, he proposed a rounded ut manoeuvre instead of a sharp or more aute angle during the ut yle. He also proposed that a one-step stop deeleration pattern should be avoided and replaed with a three-step quik stop. This intervention programme was geared at hanging player tehnique stressing knee flexion upon landing, using aelerated rounded turns and deeleration with a multi-step stop. This protool was ompleted on the basketball ourt without any additional equipment requirements. The intervention group had an 89% redution in the rate of ourrene of ACL injuries. 130 Sadly, Dr Henning s passing in 1991 prevented the publiation of this researh. However, it served as the ruial foundation to numerous prevention programmes that followed. Proprioeption and strengthening Henning s onept of athleti modulation has been widely aepted. Wedderkopp et al 131 tested a programme inluding funtional strengthening and balane training (use of an ankle dis for min at all pratie sessions). Teams were randomised into two groups with 11 teams in the intervention group (N = 111) and 11 in the ontrol group (N = 126). The group using the ankle dis inurred 14 injuries ompared with 66 injuries in the ontrol group (p,0.01). The intervention Br J Sports Med 2008;42: doi: /bjsm

13 group had a lower rate of injury during pratie (0.34/1000 h vs 1.17/1000 h, p,0.05) and games (4.68/1000 h vs 23.38/1000 h, p,0.01). The intervention group suffered two knee injuries while the ontrol group inurred eight knee injuries. No data speifi to ACL injury were provided. Irmisher et al 132 developed the Knee Ligament Injury Prevention (KLIP) programme involving 15 min of strengthening and plyometri ativities for female high shool soer, volleyball and basketball players. In the first season of a 2-year non-randomised prospetive study, 43 shools partiipated in the programme (17 basketball (N = 191); 11 soer (N = 189); 15 volleyball (N = 197)) and 69 shools served as the ontrol group (28 basketball (N = 319); 14 soer (N = 244); 27 volleyball (N = 299)). The study design inluded a training session for the oahes and athleti trainers and weekly ompliane heks for athlete partiipation for both games and praties. There were no signifiant differenes between the two groups after one season; three arthrosopially onfirmed ACL injuries in the intervention group (inidene rate 0.167) vs four in the ontrol group (inidene rate 0.078). Anedotally, there were no non-ontat ACL injuries in the soer and volleyball players in the intervention group, with all of the injuries in the intervention group ourring in basketball players. Possible explanations for the lak of impat inlude the abridged duration of this intervention programme (9 weeks) and the fat that the programme was onduted after training. Neuromusular fatigue at the end of training may diretly affet the biomehanial tehnique of the athlete and limit any potential protetive benefit of ACL injury prevention programmes. Varied training Other studies have inorporated additional dimensions of neuromusular training into ACL prevention programmes. The Cininnati Sportsmetri inludes flexibility, strengthening (through weight training) and plyometri ativities over min. Hewett et al 133 studied the effet of this programme on the inidene of knee injury in high shool aged soer, volleyball and basketball athletes. Forty-three teams (N = 1263 athletes) inluding 15 female teams (N = 366) implemented the programme; 15 additional female teams (N = 463) served as same-sex untrained ontrols and 13 male sports teams (N = 434) served as the male ontrol group. Coahes and trainers implemented the programme based on a videotape and manual. The programme was performed 3 days/week on alternate days. Seventy perent of the intervention athletes (248/366) ompleted the entire 6-week programme and the remainder ompleted at least 4 weeks of training. The inidene of serious knee injuries (N = 14) in the female ontrol group was 0.43/1000 player exposures ompared with 0.12 in the female intervention group (p = 0.05) and 0.09 in the male ontrol group. The intervention group also had a lower rate of non-ontat injuries (p = 0.01) and non-ontat ACL injuries (p = 0.05). The inidene of non-ontat knee injury was 0.35/1000 player exposures in the ontrol group ompared with 0 in the intervention group and 0.05 in the male ontrol group. ACL injuries have also been problemati for European team handball players. Myklebust et al 134 onduted a non-randomised prospetive study among 900 division I III ompetitive female handball players over a 3-year period in Norway. The ontrol group omprised 60 teams (N = 942 players in season), and 58 teams (N = 855 in ) and 52 teams (N = 850 in the season) formed the intervention group. The intervention onsisted of a 15 min programme foused on landing, utting and planting tehnique with 5 min spent on eah of three exerise omponents (floor, balane mat and wobble board). The programme lasted 5 7 weeks with different exerises introdued eah week. It was performed three times per week during the first 5 7 weeks and then one a week during the season. A physial therapist was designated to eah team to assess ompliane during the seond intervention season (2000 1). Speial equipment inluded an instrutional videotape, a poster delineating the tasks to be ompleted, six balane mats and six balane boards. Teams were required to ondut a minimum of 15 training sessions over the 5 7-week period with.75% player partiipation. Only 15 (26%) of the 58 teams from the seond season and 15 (29%) of the 52 teams from the third season ompleted the neessary number of sessions, although ompliane was higher among the elite division teams (42% and 50%, respetively). Overall, there were 29 ACL injuries during the ontrol season, 23 injuries during the first intervention season (odds ratio 0.87; CI 0.50 to 1.52; p = 0.62) and 17 injuries during the seond intervention season (odds ratio 0.64; CI 0.35 to 1.18; p = 0.15). However, during the seond intervention season, 14 ACL injuries (2.2%) ourred in players from the ontrol group ompared with 3 (1.1%) in players who ompleted the intervention (p = 0.31). In the elite division alone, 4 ACL injuries (8.9%) ourred in players from ontrol teams ompared with 1 ACL injury (0.6%) in those who ompleted the intervention (p = ). Thus, there was a redution in the total number of non-ontat ACL injuries from 18 in the ontrol season to 7 during the seond intervention season (p = 0.04). This intervention inluded elements of plyometri ativities, proprioeption and agilities, but did not inlude any elements of strength training. Limitations of the study inlude non-randomisation of the subjets, insuffiient power and ontrol data that were olleted during an earlier season. Strengths of the study inlude measures of ompliane by a medial liniian and the use of an eduational videotape and poster. The study suggests that the inlusion of a neuromusular balane-based training programme may impart some protetive benefit to the ACL. Comprehensive training Olsen et al 135 studied a programme designed to prevent lower limb injury in youth team handball. One hundred and twenty European team handball lubs (intervention group: 61 teams, 958 players; ontrol group: 59 teams, 879 players) partiipated in an 8-month intervention programme that onsisted of four sets of exerise lasting min. The training onsisted of warm-up exerises (jogging, bakward running, forward running, sideways running and speed work), tehnique (plant, ut and jump shot landing), balane (passing, squats, bouning, perturbation) and strength and power (squats, bounding, jumps, hamstrings). Eah lub was instruted on how to perform the programme and was issued a training handbook, five wobble boards and five balane mats. The programme foused on proper biomehanis during landing, ore stability and feedbak on tehnique between paired team members. The intervention teams onsisted of boys and girls aged years who ompleted 15 onseutive training sessions at the start of the season followed by one training session per week for the remainder of the season. Sixty six lower limb injuries (6.9% of players) were reported in the intervention group ompared with 115 (13.1%) in the ontrol group (relative risk 0.51; 95% CI 0.36 to 0.73; p,0.001); 19 aute knee injuries (2.0%) ourred in the intervention group ompared with 38 (4.3%) in the ontrol 406 Br J Sports Med 2008;42: doi: /bjsm

14 group (relative risk 0.45; 95% CI 0.25 to 0.81; p = 0.007); and 3 knee ligament injuries were reported in the intervention group ompared with 14 in the ontrol group (relative risk 0.20; 95% CI 0.06 to 0.70, p = 0.01). All 3 knee ligament injuries in the intervention group were ACL injuries while 10 of the 14 reported knee injuries in the ontrol group were ACL injuries; no data were provided on ACL injuries alone. A ompliane rate of 87% was reported. The strength of the team handball programme is that it is a warm-up programme whih makes it easier to be performed at every training session. This may be essential for long-term suess of suh a programme. The intervention inluded strength, flexibility, agility, plyometri and proprioeptive ativities to address the defiits most ommonly found in female athletes. Many of these intervention programmes require speial equipment, speialised training, or signifiant time ommitment. In 1999 the ACL PEP Program: Prevent injury and Enhane Performane was developed in Santa Monia, California. This prevention programme onsists of warm-up, strething, strengthening, plyometris and sport-speifi agilities to address potential defiits in the strength and oordination of the stabilising musles around the knee joint. It was designed as an alternative warm-up so that the desired ativities ould be performed on the field during pratie without speialised equipment for ease of implementation. The programme onsists of an eduational videotape/dvd that demonstrates the proper and improper biomehanial tehnique for eah presribed therapeuti exerise. An entire team an omplete the 19 omponents in less than 20 min. 136 An early non-randomised study among highly ompetitive year-old female lub soer players using the programme showed promising results. 137 During the first year of the study (2000) 1041 female lub soer players (52 teams) performed the PEP programme and 1902 players (95 teams) served as age- and skill-mathed ontrols. In the intervention group there were 2 Figure 3 Single leg squat. The athlete should maintain a straight line through the hip, knee and toe. She should keep a horizontal orientation of the hips and avoid a pelvi tilt during one-legged squat balane exerises. The athlete should be enouraged to reah deep knee flexion when performing this drill. ACL tears (0.2 ACL injuries/athleti exposure (AE)) ompared with 32 ACL tears (1.7 ACL injuries/ae) in the ontrol group, a derease of 88% in ACL ligament injury. In year 2 of the study (2001) 4 ACL tears were reported in the intervention group (inidene rate 0.47 injuries/ae) ompared with 35 in the ontrol group (inidene rate 1.8 injuries/ae). This orresponds to an overall redution of 74% in ACL tears in the intervention group ompared with an age- and skill-mathed ontrol group in year 2. The limitations of this study inlude non-randomisation of the subjets, no onsistent diret oversight of the intervention and ompliane measurements that were only ompleted in a small subset of intervention teams. The strengths of the PEP Program inlude the fat that it is an on-field warm-up programme that requires only traditional soer equipment (ones and soer ball). It is done 2 3 times a week over the ourse of the 12-week soer season and is 20 min in duration. It inludes progressive strength, flexibility, agility, plyometri and proprioeptive ativities to address the defiits most ommonly demonstrated in female athletes. Deeleration patterns are addressed, stressing the multi-step deeleration pattern, and proper landing tehnique is enouraged with knee and hip flexion while landing on the ball of the foot and avoiding genu valgum by using the abdutors and lateral hip musulature. In addition, sine the programme is designed as a warm-up, ompliane rates are higher and the element of neuromusular fatigue does not affet the performane of the therapeuti exerises. This study was followed by a randomised ontrolled trial using the PEP Program in Division I NCAA women s soer teams in the 2002 Fall season. 137 Sixty-one teams (N = 1429 athletes) ompleted the study (35 ontrol teams (N = 854 athletes) and 26 intervention teams (N = 575 athletes)). No signifiant differenes were seen between intervention and ontrol athletes with regard to age, height, weight or history of past ACL injuries. After using the PEP Program during one season there were 7 ACL injuries in the intervention athletes (rate 0.14) and 18 in ontrol athletes (rate 0.25; p = 0.15). No ACL injuries were reported in intervention athletes during praties ompared with 6 in ontrol athletes (rate 0.10; p = 0.01). During games the differene between the intervention and ontrol groups was non-signifiant (7 vs 12; p = 0.76). Non-ontat ACL injuries ourred at over three times the rate in ontrol athletes (n = 10; rate 0.14) ompared with intervention athletes (n = 2; rate 0.04; p = 0.06). Control athletes with a prior history of ACL injury suffered a reurrene five times more frequently than the intervention group (0.10 vs 0.02; p = 0.06); this differene reahed signifiane when limited to non-ontat ACL injuries during the season (0.06 vs 0.00; p,0.05). There was a signifiant differene in the rate of ACL injuries in the seond half of the season (weeks 6 11) between the intervention group (0.00) and the ontrol group (0.18; p,0.05). This supports the notion that it takes approximately 6 8 weeks for a biomehanial intervention programme to impart a neuromusular effet. Overall, these studies provide evidene that prevention training programmes an redue the risk of ACL injury. This has been shown in male and female athletes from various sports and aross different age groups. Nevertheless, a number of important issues remain. Programme speifis Pratially, there is a ost-benefit analysis that needs to be onsidered before initiating an injury prevention programme on a large sale. Br J Sports Med 2008;42: doi: /bjsm

15 Figure 4 Drop vertial jump test. To identify athletes at risk of severe knee injuries, the drop vertial jump test as desribed by Hewett et al 148 should be used. The athlete is instruted to drop off the box and immediately jump as high as you possibly an. Athletes who land with good valgus knee motion should perform neuromusular training before sports partiipation. Training whih emphasises the hip-knee-toe line position when landing (avoid kissing knees ) ould prevent future ACL injuries. Reprodued with permission from Hewett et al. 147 First, what equipment is neessary and at what is its ost? Extensive and more expensive equipment is neessary for programmes suh as the Frappier Aeleration Program, 138 the Cininnati Sportsmetri Program 133 and the various programmes using some form of balane board Other suessful programmes suh as PEP and the Henning programme 129 an be performed with minimal resoures. Seond, what is the minimal time ommitment needed to provide adequate protetion? What is the minimum duration of an injury prevention programme or does it need to be ontinued, perhaps at a lesser frequeny throughout the ourse of the season? When initiating a neuromusular intervention programme, it takes approximately 4 6 weeks to impart a benefit to the athlete. Most of the programmes studied to date have a relatively intense start-up period for 4 6 weeks followed by less frequent and, in some ases, no additional training. Maintenane of the programme in a routine manner as part of a warm-up and ongoing training is ruial to suessful redution of injury. Timing of intervention Ideally, these prevention programmes should be introdued as early as possible in the training period. In some sports this would be at the age of 6 10 years. Effet on performane One additional advantage of prevention programmes is that, properly exeuted, they an serve to enhane performane as well as prevent injury. These benefits inlude inreased vertial jump, improved ontrol of dynami load of the knee, improved balane and inreased hamstring strength, power and peak torque Summary of prevention programmes Based on these studies, we know that there an be a quantifiable redution in the risk of ACL for athletes, partiularly women, who omplete a well-designed injury prevention programme. Most of these programmes attempt to alter dynami loading of the tibiofemoral joint through neuromusular and proprioeptive training. An emphasis is plaed on proper landing tehnique; landing softly on the forefoot and rolling bak to the rearfoot, engaging knee and hip flexion upon landing. Two-feet landing is enouraged where possible. 133 When utting manoeuvres, athletes should avoid exessive dynami valgus of the knee upon landing and squatting; they should aim to ahieve the knee over toe position (fig 3). Intervention programmes have foused on inreasing hamstring, gluteus medius and hip abdutor strength, and addressing proper deeleration tehniques. Suessful implementation of these programmes requires the ollaboration of governing bodies, sports sientists, physiians, oahes, parents and athletes. Important fators for a suessful prevention programme The programme should inlude strength and power exerises, neuromusular training, plyometris and agility exerises. Design as a regular warm-up programme inreases adherene. Fous should be on performane of the hip-knee-foot line and kissing knees should be avoided (exessive valgus strain). 408 Br J Sports Med 2008;42: doi: /bjsm