Lessons learned from the last 20 years of ACL-related in vivo-biomechanics research of the knee joint

Transcription

1 Knee Surg Sports Traumatol Arthrosc (2013) 21: DOI /s KNEE Lessons learned from the last 20 years of ACL-related in vivo-biomechanics research of the knee joint Evangelos Pappas Franceska Zampeli Sofia A. Xergia Anastasios D. Georgoulis Received: 7 December 2011 / Accepted: 28 February 2012 / Published online: 23 March 2012 Ó Springer-Verlag 2012 Abstract Purpose Technological advances in recent years have allowed the easy and accurate assessment of knee motion during athletic activities. Subsequently, thousands of studies have been published that greatly improved our understanding of the aetiology, surgical reconstruction techniques and prevention of anterior cruciate ligament (ACL) injuries. The purpose of this review is to summarize the evidence from biomechanical studies on ACL-related research. Methods High-impact articles that enhanced understanding of ACL injury aetiology, rehabilitation, prevention and adaptations after reconstruction were selected. Results The importance of restoring internal tibial rotation after ACL reconstruction has emerged in several studies. Criteria-based, individualized rehabilitation protocols have replaced the traditional time-based protocols. Excessive knee valgus, poor trunk control, excessive quadriceps forces and leg asymmetries have been identified as potential high risk biomechanical factors for ACL tear. Injury prevention programmes have emerged as low cost and effective means of preventing ACL injuries, particularly in female athletes. Conclusion As a result of biomechanical research, clinicians have a better understanding of ACL injury aetiology, prevention and rehabilitation. Athletes exhibiting E. Pappas Division of Physical Therapy, Long Island University-Brooklyn Campus, Brooklyn, NY 11201, USA evangelos.pappas@liu.edu F. Zampeli S. A. Xergia A. D. Georgoulis (&) Department of Orthopaedic Surgery, Orthopaedic Sports Medicine Center, University of Ioannina, PO Box 1042, Ioannina, Greece georgoulis@osmci.gr neuromuscular deficits predisposing them to ACL injury can be identified and enrolled into prevention programmes. Clinicians should assess ACL-reconstructed patients for excessive internal tibial rotation that may lead to poor outcomes. Level of evidence III. Keywords Internal rotation Knee injury prevention Rehabilitation Sex differences Variability Introduction The field of biomechanics is a multidisciplinary area of science that involves among other disciplines physicians, surgeons, physical therapists and bioengineers. Each specialty contributes its unique expertise in the development of new ideas, novel methodology and protocols with the ultimate goal of answering clinically important questions and improving patient care. Aristotle s On the Motion of Animals is considered the first organized effort to apply principles of physics and mathematics on the movement of living organisms. Major leaps in technology in the last 20 years and closer collaboration between disciplines have led to increases in the number of publications in the field of biomechanics. A PubMed search on knee biomechanics yielded 7,473 articles in the last 20 years ( ) which represents a more than fourfold increase in publications compared to the previous 20 years. The purpose of this review article is to describe the different methods used in in vivo knee biomechanics and summarize the new knowledge that has emerged from the large number of studies that have been published in the last two decades on the aetiology, prevention, surgical reconstruction and rehabilitation of anterior cruciate ligament (ACL) injuries.

2 756 Knee Surg Sports Traumatol Arthrosc (2013) 21: The ACL is the guide of the screw-home mechanism of the knee joint; an automatic axial rotation that is inevitably and involuntarily linked to flexion and extension. When the knee is flexing, the tibia internally rotates. As the knee extends, the femoral condyles roll and glide on the tibial plateau and the tibia externally rotates. At full extension, the knee joint locks in a position of maximal stability (close packed position). Although the principal movement of the knee is flexion extension, internal external rotation plays a very significant role especially in athletic activities that require pivoting [39]. In terms of arthrokinematics, the ACL primarily prevents anterior tibial translation which is clinically assessed with the Lachman-Noulis test, a reliable non-invasive diagnostic test for ACL rupture [90]. In addition, the ACL plays a crucial role in limiting axial rotation [3]. It has been suggested that an important goal of ACL reconstruction (ACLR) should be to restore rotational stability which may in turn result in better long-term functional outcomes [24]. Biomechanical tools for in vivo research Several tools have been developed for the objective measurement of motion. From Eadweard J. Muybridge s series of photographs in The Horse in Motion (Fig. 1) that challenged common beliefs by demonstrating that all hooves leave the ground when they are tacked under the running horse to current advanced, high-accuracy motion analysis systems, objective measures of motions have provided new insights and commonly disproved theories that were widely accepted. Kinematics (the study of moving bodies without regard to forces) data collection is primarily achieved with the use of high-speed cameras and skin markers in a laboratory environment. The markers can be either passive (reflecting light, e.g., Vicon or Motion Analysis systems) or active (generating light, e.g., Optotrak). The advantages of these systems are the ease of application, safety and quick data processing that frequently allow real-time viewing and immediate replay, while the main disadvantage is that there is relative motion between the markers and the skin. Reliability and validity studies with latest versions of these systems have shown that the errors generated are usually small enough to allow for accurate data interpretation [22, 51, 91]. Even better validity is achieved with the use of intracortical pins that directly insert into bony segments and are attached to clusters of markers [74]; however, the invasiveness of this methodology has made it less popular than skin markers. Other methods of collecting kinematic data Fig. 1 Early efforts in objective motion analysis

3 Knee Surg Sports Traumatol Arthrosc (2013) 21: include accelerometers, electrogoniometers, electromagnetic devices [106] and fluoroscopy [96]. Lately, important information on the mechanism of ACL injuries has been provided by data from injuries that were captured on video and subsequently analysed [43, 44]. In vivo biomechanics are nowadays used as a valuable tool for orthopaedic surgeons, physical therapists and other members of the rehabilitation team involved in the prevention and treatment of ACL injuries. In recent years, important, clinically relevant biomechanical studies have greatly enhanced our understanding of the ACL-deficient (ACLD) and ACLR knee and have influenced surgical and rehabilitation protocols. Kinetic data are collected with the use of force transducers; commonly, a force plate installed flush with the floor. As the subject performs a task on the force plate, ground reaction forces are calculated. With the use of inverse dynamics, kinetic and kinematic data can be combined to calculate joint forces and moments. In an intriguing series of experiments, force transducers were directly implanted on the ACL to measure strain during different activities [20, 29]. Finally, electromyography (EMG) and isokinetic devices have provided insight on muscle activity and torque production, respectively. EMG measures the electrical activity generated by muscle contractions; however, there are limitations with signal interpretation and clinical implications, motion artefacts and normalization. Isokinetic measurements provide useful information on muscle strength but as the tests are commonly single joint, single plane motions, there are questions on their clinical applicability. The future of in vivo biomechanical research is exciting as new technologies have emerged that allow collection of data outside the laboratory which opens up new avenues for research in more ecological environments such as the clinic or the sports field. These systems use either cameras that have been optimized for outdoor tracking of skin markers (e.g. Qualisys, Vicon) or markerless systems [16] and although promising currently very few studies have utilized them. Biomechanical adaptations in ACL-deficient and reconstructed patients Early biomechanical studies that assessed motion patterns of ACLD and ACLR patients demonstrated that many ACLD patients walk with a quadriceps avoidance gait characterized by decreased quadriceps activity and lower external knee flexion moment in an effort to control anterior translation of the tibia [11, 41]. However, Knoll et al. [41] found that in patients with chronic ACL deficiency the quadriceps avoidance gait is less common than previously thought. It was also demonstrated that it takes 8 months after ACLR to return to pre-injury gait patterns [41]. These studies introduced the use of gait analysis and in vivo biomechanics in ACL reconstruction surgery and shed light on the gait pattern of ACLD and ACLR patients. However, the fact that only low demanding activities such as walking were examined and that the focus was on sagittal- and transverse-plane biomechanics led to initial conclusions that an almost complete restoration of gait patterns is achieved after ACLR. However, studies showed there is no significant relationship between anterior tibial translation, sagittal-plane knee kinematics and knee function [27, 97]. In a study where volunteers with chronic ACLD and matched healthy controls were compared, it was found that both groups demonstrated similar sagittalplane knee kinetics and kinematics during gait, step activity and cross-over hopping despite of the ACLD group having significantly worse functional levels and significant strength deficits [99]. In summary, the literature suggests that focusing on sagittal-plane motion may not reveal the whole spectrum of neuromuscular adaptations in ACLD and ACLR athletes. Some patients with ACLD demonstrate a quadriceps avoidance adaptation which can be detected by sagittal-plane measures. ACLR results in restoration of most sagittal-plane pathological deficits although some adaptations remain. In the light of ACL s important role in controlling tibial rotation, there was a clear need to expand kinetic and kinematic investigation in the transverse plane. In their landmark study, Georgoulis et al. [26] examined ACLD patients, bone-patellar tendon-bone (BPTB) ACLR patients and healthy matched controls during walking. It was demonstrated that during this low-demand activity, ACLD patients demonstrated greater tibial internal rotation that was decreased to closer to normal levels after ACLR. This study spotlighted the importance of tibial rotation for ACLR patients. In a subsequent investigation with a higher demand activity (stair descend and pivoting), it was found that tibial rotation was significantly larger in the ACLR knee as compared to the contralateral intact leg and the healthy control group [76] (Fig. 2a). The tibial rotation was determined in terms of tibial rotation range of motion (TR ROM) during the pivoting period, and the results of this study indicated that ACLR did not restore TR ROM to normal levels even though anterior tibial translation was restored (Fig. 3). When the intensity of the task was increased by instructing volunteers to perform drop landing and subsequent pivoting that applies increased rotational loading at the knee and imitates a sports activity task, the knees of subjects with ACLR and ACLD demonstrated increased tibial internal rotation [77] (Fig. 2b). In a followup evaluation of the ACLD volunteers from the previous study utilizing a within-subject design, it was found that increased tibial rotation was not restored to normal limits

4 758 Knee Surg Sports Traumatol Arthrosc (2013) 21: Fig. 2 Stick figure from VICON motion analysis system demonstrating the high demanding tasks that set increased rotational load to the knee joint: a descending and pivoting task and b landing and pivoting task Fig. 3 A graph that indicates the tibial rotation range of motion (TR ROM) during the pivoting period for ACLreconstructed (ACLR), contralateral intact and healthy control knee after ACLR with BTB during pivoting after stair descend and drop landing [77]. Similarly, increased internal tibial rotation during pivoting was observed when ACLR was performed with a hamstrings autograft [25] suggesting that transverse plane kinematic deficits after ACLR are independent of graft choice. Finally, when pivoting was assessed in subjects with the two most commonly used ACLR grafts (hamstrings and BPTB) in the same study, it was confirmed that significantly increased tibial rotation persisted when compared with healthy controls, whereas no differences were found between the two ACLR groups [14]. In summary, when transverse plane biomechanics were assessed in a series of studies in the last 8 years, it was consistently found that ACLD patients demonstrate higher internal tibial rotation that is decreased but not restored to normal levels after ACLR. It was also found

5 Knee Surg Sports Traumatol Arthrosc (2013) 21: that the deficits are more apparent when high demand activities placing rotational stress on the knee are investigated (Fig. 4). Recently a new methodology has been developed, which offers a more holistic approach concerning the study of gait based on the fact that walking is a rhythmic activity. However, a closer examination reveals that each step is not identical to the previous or to the next one. These variations that exist among subsequent strides derive from the underlying mechanisms that produce gait [92]. Stergiou et al. [93] developed the optimal amount of movement variability proposition suggesting that under healthy conditions, each motor task is characterized by an optimal amount of variability which provides the human body with flexibility, adaptability and the capacity to respond to unpredictable stimuli and stresses and environmental demands. The achievement of this optimal variability is desired during human motor development and motor learning. On the contrary, ageing and disease are characterized by altered (increased or decreased) variability and are associated with diminished flexibility and capability to respond to stimuli. This approach has been recently used to examine gait after ACL rupture by assessing knee flexion extension variability over multiple gait cycles. It was observed that compared to healthy controls, the ACLD knee exhibits decreased gait variability [56], indicating that steps are more similar to each other than in healthy knees. This could be due to the loss of the mechanical restraint or of the proprioceptive input provided by the ACL. The decreased gait variability indicates that a patient with ACLD is more careful in the way he or she walks in order to eliminate any extra movements, exhibiting increased rigidity in movement patterns. According to the optimal amount of movement variability proposition, these changes are associated with decreased flexibility and responsiveness to environmental demands, leading possibly to injury and the development of future pathology. When backward (instead of forward walking) was assessed, the same finding was confirmed with ACLD subjects demonstrating less variability in their gait not only in the ACLD but also in the contralateral, uninjured leg [102], which could imply diminished functional responsiveness to the environmental demands and increased likelihood for injury for both knees of ACLD patients. Backward walking is a common rehabilitation exercise after ACLR and a frequent athletic activity during sports such as basketball and tennis; therefore, the clinical significance of these findings should be emphasized for clinicians working with ACLD patients. Subsequent studies have shown that after ACLR with either a BPTB or a hamstrings autograft, there is increased variability in knee flexion extension during gait which indicates decreased flexibility and adaptability to stimuli and stresses [57, 58]. This could be due to altered muscle activity in ACLR limbs that may derive from the altered proprioceptive input. Thus, ACLR patients exhibit greater divergence in the movement trajectories. This could signify that patients after ACLR feel secure to increase and add extra movement. However, because the innate proprioceptive channels are damaged, gait variability and knee function are not restored to normative levels. It is important to note that in the abovementioned studies, all ACLR subjects had normal function and stability as measured by traditional measures. This indicates that measures of gait variability may be more sensitive and subsequently more helpful in the assessment of various conditions as has already been done in other medical domains. Applications of in vivo biomechanical findings on ACL reconstruction surgical technique Fig. 4 The phases of the drop landing task that serves as a common model for investigating biomechanical behaviour in athletes The importance of the ACL in providing rotational stability and subsequent normal knee function cannot be overemphasized [24]. The pivot-shift test is the most widely used clinical test for examining rotational stability of the knee and it is predictive of subjective and objective outcome, patient discomfort, failure to return to previous sports activity level and development of osteoarthritis of the knee at long-term [38, 42]. The initiation and progression of knee joint OA in patients with ACL injury has been linked to abnormal knee joint biomechanics during dynamic in vivo activities [5, 94]. Current ACLR techniques do not seem to fully restore normal function of the knee as

6 760 Knee Surg Sports Traumatol Arthrosc (2013) 21: excessive tibial rotation is still present during highly demanding activities after ACLR [25, 26]. It has been suggested that pathologically increased tibial rotation loads specific regions of the articular cartilage that were not loaded prior to the ACL injury [5, 49, 94] and that this altered contact mechanics could produce local degenerative changes to the articular cartilage of the knee joint [4]. Additionally, some individuals with ACLD can stabilize their knees following ACL rupture even during activities involving cutting and pivoting (copers) while others have instability even with activities of daily living (non-copers). In a series of studies, it was demonstrated that copers exhibited motion patterns similar to uninjured controls, while non-copers had decreased knee motion and external knee flexion moments and achieved peak hamstring activity later in the weight acceptance phase by using a strategy involving more generalized muscle co-contraction [79, 80]. Although current ACLR techniques frequently result in good functional results, biomechanical studies document that rotational kinematics are not fully restored after ACLR [25, 26]. The biomechanical behaviour of traditional single-bundle grafts is similar to the anteromedial bundle of the ACL. Prior clinical and in vitro biomechanical studies have advocated for a more horizontal placement of the ACL graft in the coronal plane to better control abnormal transverse plane knee motion [47, 50, 84]. In two of these studies, knee joint kinematics were measured while applying forces that simulate the pivot-shift test in a biomechanical cadaver model and showed that a more oblique orientation of the ACL graft better restores internal tibial rotation limits [50, 84]. In the third study, it was demonstrated that ACLR patients with a negative clinical pivotshift test had a more oblique orientation of the ACL graft in the coronal plane MRI [47]. It is important to note, however, that a study failed to find a difference in subjective grading of the pivot-shift test between patients with a traditional and a more oblique graft [37]. This could be due to the difficulty with objectively grading the clinical pivotshift test as when a navigational system was used patients with an oblique graft had better rotational stability [85]. Similarly, when tibial rotation during pivoting was assessed in vivo during pivoting, it was demonstrated that both techniques restore anterior translation but none completely restores tibial rotation although placing the graft in a more oblique position is more successful in preventing excessive internal tibial rotation [78]. Zampeli et al. [103] also demonstrated that tibial rotation was better restored in ACLR patients with a more oblique graft in the coronal plane during pivoting. In summary, it appears that a more oblique placement of the ACL graft in the coronal plane results in better control of tibial rotation during athletic activities. Another surgical reconstruction technique that has been suggested to better restore internal tibial rotation limits is the double-bundle ACLR. Although the restoration of knee rotational stability after double-bundle ACLR has been demonstrated in the cadaveric model and with passive stress tests in humans [15, 104], limited data exist on dynamic functional biomechanical tests. A recent motion analysis study showed that anatomic double-bundle ACLR successfully restores knee rotational stability during pivoting within normative levels [46]. However, a study that assessed tibial rotation during a cutting task found that both single- and double-bundle ACLR equally limit tibial rotation [54]. Future biomechanical and clinical research will provide insight on whether the increased cost, technical complexity and possibly increased complication rate of the double-bundle ACLR is justified by better restoration of transverse plane kinematics and ultimately by superior clinical results. Rehabilitation after ACL reconstruction After ACLR, the rehabilitation team works with the patient on a safe and quick return to premorbid activity level without compromising the integrity of the graft. Several factors should be taken into consideration when designing an individualized, postoperative rehabilitation protocol for patients with ACLR such as the desired return activity level [2], time to surgery [7], surgical technique and graft choice [83], postoperative muscle strength [101] and proprioceptive deficits [10]. Current ACL rehabilitation protocols, trend on accelerated return to pre-injury activity level for young athletes by utilizing immediate mobilization, early weight bearing and muscle strengthening [98] and on the extensive use of criteria-based rehabilitation protocols that use specific objective and subjective parameters [18, 62]. Although ACLR is considered to be a successful procedure as it results in return to pre-injury activity levels for the majority of patients [87], it has been reported that deficits in function [60], muscle strength [101], neuromuscular response [72], kinematics [57, 77] and kinetics [82] persist even after a successful return to sports. Restoration of lower extremity muscle strength constitutes one of the most important rehabilitation goals after ACLR. A battery of tests that measure strength and neuromuscular performance such as isokinetic tests that measure torque and performance tests (hop tests for distance or timed) should be used for a comprehensive and objective evaluation of progression from one phase to the next. The consensus in the literature is that strength deficits depend on graft choice; the use of patellar tendon graft results in knee extensor muscles strength deficits while the use of hamstrings graft results in knee flexor strength deficits that

7 Knee Surg Sports Traumatol Arthrosc (2013) 21: persist for up to 24 months postoperatively [55, 101]. The use of a Limb Symmetry Index (LSI) where the strength of the reconstructed leg is expressed as percentage of the strength of the uninjured leg has been incorporated into criteria-based rehabilitation protocols [55, 95]. The athlete is cleared to return to sports from a strength performance perspective only when a minimum of 90 % LSI is achieved [6]. A controversial issue in ACL rehabilitation is the timing and safety of closed kinetic chain (CKC) and open kinetic chain (OKC) exercises. A series of research studies where a strain transducer was arthroscopically implanted on the ACL of volunteers demonstrated that weight bearing does not decrease ACL strain as it had been previously suggested [19, 20]. This finding suggests that prescribing weight-bearing exercises that require contraction of the quadriceps with the knee close to full extension is not safer than performing similar OKC exercises [29]. However, clinicians need to be cautioned to avoid the early introduction of OKC exercises in patients with hamstrings grafts as it can lead to laxity with anterior translation of the tibia [28]. Additionally, the early introduction of OKC exercises for strengthening of the quadriceps does not lead to improved strength in patients with either patellar tendon or hamstrings graft; it seems that quadriceps strength deficits depend primarily on the choice of graft and not the timing of introduction of OKC exercises [28]. The first rehabilitation goal is to prevent postoperative complications by selecting the optimal time between injury and surgery in order to decrease pain and swelling as well as to improve knee muscle strength and achieve full range of motion (ROM) [98]. It appears that knee function and muscle strength at the time of surgery [18, 86] are of great importance for the final outcomes after ACLR. Although the early stages of rehabilitation focus on the management of pain and swelling, the prevention of muscle atrophy and the gradual improvement of ROM, the later stages, emphasize recovery of muscle strength [101], functional outcome [95] and neuromuscular control [60]. The early restoration of full knee extension range of motion is crucial for the attainment of satisfactory short- and long-term outcomes [86]. As the physical therapist designs the later stages of the rehabilitation protocol and exercises become more demanding, the graft choice and desired activity level should be taken into consideration [65, 83]. The use of highly individualized protocols that are criteria-based should be emphasized as the patient progresses through the different phases of rehabilitation [63]. Achieving symmetrical gait as evidenced by audibly rhythmic foot strike patterns and acceptable balance is necessary for progressing to the 2nd stage of the protocol. Less than 15 % difference between the ACLR and the uninjured leg in isokinetic peak flexion and extension is necessary for progression to the 3rd stage. Less than 15 % difference between legs in the hop tests (single leg hop for distance, triple hop for distance, 6-m timed hop and vertical hop for height) is necessary for progression to the 4th stage [63]. Achieving symmetry in drop landing forces (within 15 %), the modified agility t test time (within 10 %), peak isokinetic torque and balance are necessary before clearing the patient for return to sports [6, 63] and to decrease the risk of re-injury [71]. As it has been described in the previous section, neuromuscular adaptations persist in the ACLR patient after surgery. The physical therapist should identify these deficits in a timely manner and address them with proper adjustments of the rehabilitation protocol. Biomechanical predictors and mechanisms of ACL injury In recent years, several research groups have performed prospective coupled biomechanical-epidemiological studies where a large number of athletes are measured in a biomechanical laboratory at baseline while performing an athletic task. The athletes are then followed prospectively to identify those who suffered ACL injury. The motion patterns of those who suffered ACL injury are compared to controls to identify predictors of injury. Hewett et al. [33] measured landing biomechanics at baseline for 205 female athletes participating in high school basketball and soccer and followed them for one to two seasons. They found that high knee valgus angle and moment and high side-to-side differences in knee valgus angle and moment during landing from a jump were strong predictors of future ACL injury. The authors described these two faulty landing strategies as ligament dominance and leg dominance, respectively [31]. A second epidemiological-biomechanical study measured trunk displacement with an electromagnetic device after a sudden force release and core proprioception in 277 collegiate athletes. It found that female athletes who suffered knee injury had higher trunk displacement and poorer trunk proprioception at baseline leading the authors to describe these deficits as trunk dominance [106, 107]. Myer et al. [59] measured 1692 high school athletes at baseline for isokinetic knee flexion and extension and by using a matched case control design they found that the female athletes who went on to suffer ACL injury had lower knee flexor strength and higher relative knee extensor strength compared to male athletes. In contrast, the female athletes who did not tear their ACL had lower knee extensor and higher knee flexor strength compared to male athletes [59]. The authors described these deficits as quadriceps dominance. The force exerted by the quadriceps can result in anterior translation of the tibia and subsequent strain of the ACL which can be

8 762 Knee Surg Sports Traumatol Arthrosc (2013) 21: potentially resisted by the hamstrings. Therefore, the female athletes who tore their ACL had a strength profile consistent with increased anterior tibial translation and increased ACL strain. In the final epidemiologic-biomechanical study, Zebis et al. [108] followed prospectively 55 female athletes who were measured for EMG pre-activation of the quadriceps and hamstrings during a side cutting manoeuver. They found that athletes who went on to tear their ACL had lower semitendinosus and higher vastus lateralis EMG pre-activity than athletes who did not tear their ACL. These findings are consistent with the quadriceps dominance deficits. It is important to note that all of the biomechanical-epidemiological studies focused on identifying predictors for ACL injury in female athletes only. Due to the higher incidence of ACL injuries among female (compared to male) athletes, it is harder to do similar studies for males as it will require testing a higher number of athletes at baseline. However, this creates a clear gap in the literature; without identifying predictors of ACL injury in male athletes, it is difficult to design evidence-based injury prevention programmes. Several research groups have systematically analysed ACL injuries that were captured on tape to forensically recreate the mechanism of injury. They found that the mechanism of ACL injury involves excessive trunk lateral displacement [35], knee valgus [9, 35, 43, 44, 66], low knee flexion angle [8, 66], tibial internal rotation [43], limited ankle plantarflexion at initial contact [9] and excessive hip flexion [9]. The described mechanisms are largely consistent with the four theories of neuromuscular deficits that were identified by the biomechanical-epidemiological studies. Lateral trunk displacement is part of the trunk dominance, knee valgus is part of the ligament dominance and low knee flexion angle is consistent with the quadriceps dominance as in this position, the knee extensors are more effective in producing anterior translation of the tibia. Biomechanical sex differences In an effort to identify biomechanical differences between male and female athletes, several studies have compared both groups during performance of standardized tasks. As demonstrated by several studies [21, 40, 53, 69, 70, 81] and one systematic review [12], females perform athletic tasks with higher knee valgus than males suggesting that ligament dominance may be contributing to the higher incidence of ACL injuries among females. This is further supported by a study that showed females to exhibit greater hip internal rotation and hip adduction moment than males [73]. Less consensus exists on whether females perform athletic manoeuvres with lower knee flexion than men as some studies showed that there is no difference [23, 40, 69, 70] while others showed that females land with straighter knees [36, 52]. Stronger support for the quadriceps dominance theory as a potential mechanism for the sex disparity in ACL epidemiology is provided by studies that found females to demonstrate preferential quadriceps activation compared to males [1, 52, 88, 105]. As the trunk dominance theory has been proposed more recently, there are fewer studies that investigated potential sex differences. Males preferentially activate the deep abdominal muscles compared to the superficial abdominal muscles during landing from a jump which may provide better trunk stability [45]. Similarly, few studies have examined whether female athletes exhibit more leg dominance during athletic manoeuvres. Ford et al. [21] and Pappas et al. [68] have found that kinematic asymmetries are higher among female athletes compared to their male counterparts when performing drop and forward jump-landing tasks. Unlike the clear differences between male and female athletes, there are no differences between male and female dancers landing from a jump [67] suggesting that the neuromuscular deficits that have been identified in female athletes are not inherent but instead they can be largely prevented with proper training. In summary, strong support currently exists for the notion that female athletes exhibit more profound ligament dominance deficits compared to male athletes. Less research exists on the other three neuromuscular deficits that have been proposed; however, it seems to be suggesting that sex differences do exist. Injury prevention programmes The ability of organized injury prevention programmes to decrease the number of ACL injuries in female athletes has been demonstrated over the last 20 years [32, 75]. As many of the predictors of ACL tear are biomechanical in nature, injury prevention programmes aim to change motion patterns in athletes at high risk for ACL injury. In addition to improving performance such as jump height [13, 61], sprint time [61] and hop for distance [61], these programmes have also been shown to decrease knee valgus [13, 61, 64], increase knee flexion at initial contact and at peak [13, 30, 48, 61], improve balance [17], decrease ground reaction force [30, 34] and enhance hamstring muscle activity [100, 109]. A recent meta-analysis [110] concluded that components of commonly used ACL injury prevention programmes are effective in improving postural sway and functional balance and that there are indications for these programmes in improving jump performance, agility and neuromuscular control. Hewett et al. [32] suggested that programmes combining plyometric training with correction

9 Knee Surg Sports Traumatol Arthrosc (2013) 21: of faulty biomechanical techniques and balance training are the most effective in preventing ACL injuries. Despite the evidence supporting the effectiveness of programmes in ACL injury prevention, they have not been widely implemented possibly due to time constraints during practice. FIFA has developed a quick and easy to implement injury prevention programme that can be used instead of a typical warm-up and has demonstrated effectiveness in reducing serious injury in a large randomized control trial [89]. Considering the low cost and evidence for effectively reducing ACL injury factors, the widespread use of injury prevention programmes is highly encouraged. Conclusions In the last 20 years, we have witnessed great increases in scientific knowledge on the understanding of the function of the ACL, its importance in limiting internal tibial rotation and applications on rehabilitation protocols. Although the optimal surgical technique remains elusive, placing the graft in a position that better controls tibial internal rotation seems promising. Biomechanical research has identified factors that predispose athletes to ACL injury such as excessive knee valgus, preferential quadriceps activity, lack of trunk control and leg-to-leg asymmetries. The introduction of injury prevention programmes seems to be effective by changing biomechanical risk factors and subsequently reducing ACL injuries. Athletes exhibiting neuromuscular asymmetries predisposing them to ACL injury can be identified and enrolled into prevention programmes. Clinicians should assess ACL-reconstructed patients for excessive internal tibial rotation that may lead to poor outcomes. References 1. Ahmad CS, Clark AM, Heilmann N, Schoeb JS, Gardner TR, Levine WN, Ahmad CS, Clark AM, Heilmann N, Schoeb JS, Gardner TR, Levine WN (2006) Effect of gender and maturity on quadriceps-to-hamstring strength ratio and anterior cruciate ligament laxity. Am J Sports Med 34(3): Alentorn-Geli E, Myer GD, Silvers HJ, Samitier G, Romero D, Lazaro-Haro C, Cugat R (2009) Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 2: a review of prevention programs aimed to modify risk factors and to reduce injury rates. Knee Surg Sports Traumatol Arthrosc 17(8): Andersen H, Dyhre-Poulsen P (1997) The anterior cruciate ligament does play a role in controlling axial rotation in the knee. Knee Surg Sports Traumatol Arthrosc 5(3): Andriacchi TP, Briant PL, Bevill SL, Koo S (2006) Rotational changes at the knee after ACL injury cause cartilage thinning. Clin Orthop Relat Res 442: Andriacchi TP, Mündermann A, Smith RL, Alexander EJ, Dyrby CO, Koo S (2004) A framework for the in vivo pathomechanics of osteoarthritis at the knee. Ann Biomed Eng 32(3): Ardern CLWK, Taylor NF, Feller JA (2011) Return to the preinjury level of competitive sport after anterior cruciate ligament reconstruction surgery: two-thirds of patients have not returned by 12 months after surgery. Am J Sports Med 39(3): Bernstein J (2011) Early versus delayed reconstruction of the anterior cruciate ligament: a decision analysis approach. J Bone Joint Surg Am 93(9):e48 8. Boden BP, Dean GS, Feagin JA Jr, Garrett WE Jr (2000) Mechanisms of anterior cruciate ligament injury. Orthopedics 23(6): Boden BP, Torg JS, Knowles SB, Hewett TE (2009) Video analysis of anterior cruciate ligament injury. Am J Sports Med 37(2): Bonfim TR, Jansen Paccola CA, Barela JA (2003) Proprioceptive and behavior impairments in individuals with anterior cruciate ligament reconstructed knees. Arch Phys Med Rehabil 84(8): Bulgheroni P, Bulgheroni MV, Andrini L, Guffanti P, Giughello A (1997) Gait patterns after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 5(1): Carson DW, Ford KR (2011) Sex differences in knee abduction during landing. Sports Health Multidiscip Approach 3(4): Chappell JD, Limpisvasti O (2008) Effect of a neuromuscular training program on the kinetics and kinematics of jumping tasks. Am J Sports Med 36(6): Chouliaras V, Ristanis S, Moraiti C, Stergiou N, Georgoulis AD (2007) Effectiveness of reconstruction of the anterior cruciate ligament with quadrupled hamstrings and bone-patellar tendonbone autografts. Am J Sports Med 35(2): Colombet PMD, Robinson JMSF, Christel PMD, Franceschi J-PMD, Djian PMD (2007) Using navigation to measure rotation kinematics during ACL reconstruction. Clin Orthop Relat Res 454: Corazza S, Mündermann L, Chaudhari A, Demattio T, Cobelli C, Andriacchi T (2006) A markerless motion capture system to study musculoskeletal biomechanics: visual hull and simulated annealing approach. Ann Biomed Eng 34(6): Couillandre A, Duque Ribeiro MJ, Thoumie P, Portero P (2008) Changes in balance and strength parameters induced by training on a motorised rotating platform: a study on healthy subjects. Annales de Réadaptation et de Médecine Physique 51(2): Eitzen I, Holm I, Risberg MA (2009) Preoperative quadriceps strength is a significant predictor of knee function two years after anterior cruciate ligament reconstruction. Br J Sports Med 43(5): Fleming BC, Ohlen G, Renstrom PA, Peura GD, Beynnon BD, Badger GJ (2003) The effects of compressive load and knee joint torque on peak anterior cruciate ligament strains. Am J Sports Med 31(5): Fleming BC, Renstrom PA, Beynnon BD, Engstrom B, Peura GD, Badger GJ, Johnson RJ (2001) The effect of weightbearing and external loading on anterior cruciate ligament strain. J Biomech 34(2): Ford K, Myer G, Hewett T (2003) Valgus knee motion during landing in high school female and male basketball players. Med Sci Sports Exerc 35(10): Ford KR, Myer GD, Hewett TE (2007) Reliability of landing 3D motion analysis: implications for longitudinal analyses. Med Sci Sports Exerc 39(11): Ford KR, Myer GD, Toms HE, Hewett TE (2005) Gender differences in the kinematics of unanticipated cutting in young athletes. Med Sci Sports Exerc 37(1):

10 764 Knee Surg Sports Traumatol Arthrosc (2013) 21: Fu FH, Zelle BA (2007) Rotational instability of the knee: editorial comment. Clin Orthop Relat Res 454: Georgoulis AD, Ristanis S, Chouliaras V, Moraiti C, Stergiou N (2007) Tibial rotation is not restored after ACL reconstruction with a hamstring graft. Clin Orthop Relat Res 454: Georgoulis AD, Papadonikolakis A, Papageorgiou CD, Mitsou A, Stergiou N (2003) Three-dimensional tibiofemoral kinematics of the anterior cruciate ligament-deficient and reconstructed knee during walking. Am J Sports Med 31(1): Gokeler A, Schmalz T, Knopf E, Freiwald J, Blumentritt S (2003) The relationship between isokinetic quadriceps strength and laxity on gait analysis parameters in anterior cruciate ligament reconstructed knees. Knee Surg Sports Traumatol Arthrosc 11(6): Heijne A, Werner S (2007) Early versus late start of open kinetic chain quadriceps exercises after ACL reconstruction with patellar tendon or hamstring grafts: a prospective randomized outcome study. Knee Surg Sports Traumatol Arthrosc 15(4): [Epub 2007 Jan 2012] 29. Heijne A, Fleming BC, Renstrom PA, Peura GD, Beynnon BD, Werner S (2004) Strain on the anterior cruciate ligament during closed kinetic chain exercises. Med Sci Sports Exerc 36(6): Herman DC, Oñate JA, Weinhold PS, Guskiewicz KM, Garrett WE, Yu B, Padua DA (2009) The effects of feedback with and without strength training on lower extremity biomechanics. Am J Sports Med 37(7): Hewett T, Ford K, Hoogenboom B, Myers G (2010) Understanding and preventing ACL injuries: current biomechanical and epidemiologic considerations update. IJSPT 5(4): Hewett T, Ford K, Myer G (2006) Anterior cruciate ligament injuries in female athletes: Part 2, a meta-analysis of neuromuscular interventions aimed at injury prevention. Am J Sports Med 34(3): Hewett T, Myer G, Ford K, Heidt R, Colosimo A, McLean S, Van Den Bogert A, Patterno M, Succop P (2005) Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes. Am J Sports Med 33(4): Hewett T, Stroupe A, Nance T, Noyes F (1996) Plyometric training in female athletes: decreased impact forces and increased hamstring torques. Am J Sports Med 24(6): Hewett TE, Torg JS, Boden BP (2009) Video analysis of trunk and knee motion during non-contact anterior cruciate ligament injury in female athletes: lateral trunk and knee abduction motion are combined components of the injury mechanism. Br J Sports Med 43(6): Huston LJ, Vibert B, Ashton-Miller JA, Wojtys EM (2001) Gender differences in knee angle when landing from a dropjump. Am J Knee Surg 14(4): Jepsen CF, Lundberg-Jensen AK, Faunoe P (2007) Does the position of the femoral tunnel affect the laxity or clinical outcome of the anterior cruciate ligament reconstructed knee? A clinical, prospective, randomized, double-blind study. Arthroscopy 23(12): Jonsson H, Riklund-Åhlström K, Lind J (2004) Positive pivot shift after ACL reconstruction predicts later osteoarthrosis 63 patients followed 5 9 years after surgery. Acta Orthop 75(5): Kapandji I (1987) The physiology of the joints, vol 2, 5th edn. Churchill Livingstone, Edinburgh 40. Kernozek T, Torry M, Van Hoof H, Cowley H (2005) Gender differences in frontal and sagittal plane biomechanics during drop landings. Med Sci Sports Exerc 37(6): Knoll Z, Kocsis L, Kiss RM (2004) Gait patterns before and after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 12(1): Kocher MS, Steadman JR, Briggs KK, Sterett WI, Hawkins RJ (2004) Relationships between objective assessment of ligament stability and subjective assessment of symptoms and function after anterior cruciate ligament reconstruction. Am J Sports Med 32(3): Koga H, Nakamae A, Shima Y, Iwasa J, Myklebust G, Engebretsen L, Bahr R, Krosshaug T (2010) Mechanisms for noncontact anterior cruciate ligament injuries. Am J Sports Med 38(11): Krosshaug T, Nakamae A, Boden B, Engebretsen L, Smith G, Slauterbeck J, Hewett T, Bahr R (2007) Mechanisms of anterior cruciate ligament injury in basketball: video analysis of 39 cases. Am J Sports Med 35: Kulas A, Schmitz R, Shultz S, Henning J, Perrin D (2006) Sexspecific abdominal activation strategies during landing. J Athl Train 41(4): Lam M-H, Fong DT-P, Yung PS-H, Ho EP-Y, Fung K-Y, Chan K-M (2011) Knee rotational stability during pivoting movement is restored after anatomic double-bundle anterior cruciate ligament reconstruction. Am J Sports Med 39(5): Lee MC, Seong SC, Lee S, Chang CB, Park YK, Jo H, Kim CH (2007) Vertical femoral tunnel placement results in rotational knee laxity after anterior cruciate ligament reconstruction. Arthroscopy 23(7): Lim B-O, Lee YS, Kim JG, An KO, Yoo J, Kwon YH (2009) Effects of sports injury prevention training on the biomechanical risk factors of anterior cruciate ligament injury in high school female basketball players. Am J Sports Med 37(9): Logan M, Dunstan E, Robinson J, Williams A, Gedroyc W, Freeman M (2004) Tibiofemoral kinematics of the anterior cruciate ligament (ACL)-deficient weightbearing, living knee employing vertical access open interventional multiple resonance imaging. Am J Sports Med 32(3): Loh J, Fukuda Y, Tsuda E, Steadman R, Fu F, Woo S (2003) Knee stability and graft function following anterior cruciate ligament reconstruction: comparison between 11 o clock and 10 o clock femoral tunnel placement. Arthroscopy 19(3): Maletsky LP, Sun J, Morton NA (2007) Accuracy of an optical active-marker system to track the relative motion of rigid bodies. J Biomech 40(3): Malinzak R, Colby S, Kirkendall D, Yu B, Garrett W (2001) A comparison of knee joint patterns between men and women in selected athletic activities. Clin Biomech 16(5): McLean S, Lipfert S, Van Den Bogert A (2004) Effect of gender and defensive opponent on the biomechanics of sidestep cutting. Med Sci Sports Exerc 36(6): Misonoo G, Kanamori A, Ida H, Miyakawa S, Ochiai N (2012) Evaluation of tibial rotational stability of single-bundle vs. anatomical double-bundle anterior cruciate ligament reconstruction during a high-demand activity a quasi-randomized trial. Knee 19(2): Mohtadi NG, Chan DS, Dainty KN, Whelan DB (2011) Patellar tendon versus hamstring tendon autograft for anterior cruciate ligament rupture in adults. Cochrane Database Syst Rev 9:CD Moraiti C, Stergiou N, Ristanis S, Georgoulis A (2007) ACL deficiency affects stride-to-stride variability as measured using nonlinear methodology. Knee Surg Sports Traumatol Arthrosc 15(12): Moraiti C, Stergiou N, Vasiliadis H, Motsis E, Georgoulis A (2010) Anterior cruciate ligament reconstruction results in alterations in gait variability. Gait Posture 32(2): Moraiti CO, Stergiou N, Ristanis S, Vasiliadis HS, Patras K, Lee C, Georgoulis AD (2009) The effect of anterior cruciate ligament reconstruction on stride-to-stride variability. Arthroscopy 25(7):

11 Knee Surg Sports Traumatol Arthrosc (2013) 21: Myer GD, Ford KR, Barber Foss KD, Liu C, Nick TG, Hewett TE (2009) The relationship of hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes. Clin J Sport Med 19(1): Myer GD, Ford KR, Khoury J, Succop P, Hewett TE (2011) Biomechanics laboratory-based prediction algorithm to identify female athletes with high knee loads that increase risk of ACL injury. Br J Sports Med 45(4): Myer GD, Ford KR, Palumbo JP, Hewett TE (2005) Neuromuscular training improves performance and lower-extremity biomechanics in female athletes. J Strength Cond Res 19(1): Myer GDPM, Ford KR, Hewett TE (2008) Neuromuscular training techniques to target deficits before return to sport after anterior cruciate ligament reconstruction. J Strength Cond Res 22(3): Myer GDPM, Ford KR, Quatman CE, Hewett TE (2006) Rehabilitation after anterior cruciate ligament reconstruction: criteria-based progression through the return-to-sport phase. J Orthop Sports Phys Ther 36(6): Noyes FR, Barber-Westin SD, Fleckenstein C, Walsh C, West J (2005) The drop-jump screening test. Am J Sports Med 33(2): Nyland J, Cottrell B, Harreld K, Caborn DNM (2006) Selfreported outcomes after anterior cruciate ligament reconstruction: an internal health locus of control score comparison. Arthroscopy 22(11): Olsen O, Myklebust G, Engebretsen L, Bahr R (2004) Injury mechanisms for anterior cruciate ligament injuries in team handball. Am J Sports Med 32(4): Orishimo KF, Kremenic IJ, Pappas E, Hagins M, Liederbach M (2009) Comparison of landing biomechanics between male and female professional dancers. Am J Sports Med 37(11): Pappas E, Carpes FP (2012) Lower extremity kinematic asymmetry in male and female athletes performing jump-landing tasks. J Sci Med Sport 15(1): Pappas E, Hagins M, Sheikhzadeh A, Nordin M, Rose D (2007) Biomechanical differences between unilateral and bilateral landings from a jump: gender differences. Clin J Sport Med 17(4): Pappas E, Sheikhzadeh A, Hagins M, Nordin M (2007) The effect of gender and fatigue on the biomechanics of bilateral landings from a jump: peak values. J Sports Sci Med 6(1): Paterno MVSL, Ford KR, Rauh MJ, Myer GD, Huang B, Hewett TE (2010) Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport. Am J Sports Med 38(10): Patras K, Ziogas G, Ristanis S, Tsepis E, Stergiou N, Geo AD (2009) High intensity running results in an impaired neuromuscular response in ACL reconstructed individuals. Knee Surg Sports Traumatol Arthrosc 17(8): Pollard CD, Sigward SM, Powers CM (2007) Gender differences in hip joint kinematics and kinetics during side-step cutting maneuver. Clin J Sport Med 17(1): Ramsey DK, Wretenberg PF, Benoit DL, Lamontagne M, Németh G (2003) Methodological concerns using intra-cortical pins to measure tibiofemoral kinematics. Knee Surg Sports Traumatol Arthrosc 11(5): Renstrom P, Ljungqvist A, Arendt E, Beynnon B, Fukubayashi T, Garrett W, Georgoulis T, Hewett TE, Johnson R, Krosshaug T, Mandelbaum B, Micheli L, Myklebust G, Roos E, Roos H, Schamasch P, Shultz S, Werner S, Wojtys E, Engebretsen L (2008) Non-contact ACL injuries in female athletes: an International Olympic Committee current concepts statement. Br J Sports Med 42(6): Ristanis S, Giakas G, Papageorgiou CD, Moraiti T, Stergiou N, Georgoulis AD (2003) The effects of anterior cruciate ligament reconstruction on tibial rotation during pivoting after descending stairs. Knee Surg Sports Traumatol Arthrosc 11(6): Ristanis S, Stergiou N, Patras K, Vasiliadis H, Giakas G, Georgoulis A (2005) Excessive tibial rotation during highdemand activities is not restored by anterior cruciate ligament reconstruction. Arthroscopy 21(11): Ristanis S, Stergiou N, Siarava E, Ntoulia A, Mitsionis G, Georgoulis A (2009) Effect of femoral tunnel placement for reconstruction of the anterior cruciate ligament on tibial rotation. J Bone Joint Surg Am 91(9): Rudolph K, Axe M, Snyder-Mackler L (2000) Dynamic stability after ACL injury: who can hop? Knee Surg Sports Traumatol Arthrosc 8(5): Rudolph KS, Axe MJ, Buchanan TS, Scholz JP, Snyder-Mackler L (2001) Dynamic stability in the anterior cruciate ligament deficient knee. Knee Surg Sports Traumatol Arthrosc 9(2): Russell K, Palmieri R, Zinder S, Ingersoll C (2006) Sex differences in valgus knee angle during a single-leg drop jump. J Athl Train 41(2): Salem GJ, Salinas R, Harding FV (2003) Bilateral kinematic and kinetic analysis of the squat exercise after anterior cruciate ligament reconstruction. Arch Phys Med Rehabil 84(8): Samuelsson K, Andersson D, Karlsson J (2009) Treatment of anterior cruciate ligament injuries with special reference to graft type and surgical technique: an assessment of randomized controlled trials. Arthroscopy 25(10): Scopp JM, Jasper LE, Belkoff SM, Moorman CT III (2004) The effect of oblique femoral tunnel placement on rotational constraint of the knee reconstructed using patellar tendon autografts. Arthroscopy 20(3): Seon JK, Park SJ, Lee KB, Seo HY, Kim MS, Song EK (2011) In vivo stability and clinical comparison of anterior cruciate ligament reconstruction using low or high femoral tunnel positions. Am J Sports Med 39(1): Shelbourne KD, Klotz C (2006) What I have learned about the ACL: utilizing a progressive rehabilitation scheme to achieve total knee symmetry after anterior cruciate ligament reconstruction. J Orthop Sci 11(3): Shelbourne KD, Gray T (1997) Anterior cruciate ligament reconstruction with autogenous patellar tendon graft followed by accelerated rehabilitation. Am J Sports Med 25(6): Sigward SM, Powers CM (2006) The influence of gender on knee kinematics, kinetics and muscle activation patterns during side-step cutting. Clin Biomech 21(1): Soligard T, Myklebust G, Steffen K, Holme I, Silvers H, Bizzini M, Junge A, Dvorak J, Bahr R, Andersen T (2008) Comprehensive warm-up programme to prevent injuries in young female footballers: cluster randomised controlled trial. BMJ 337:a Soucacos PN, Papadopoulou M, Georgoulis A (1998) The Noulis behind the Lachman test. Arthroscopy 14(1): States RA, Pappas E (2006) Precision and repeatability of the Optotrak 3020 motion measurement system. J Med Eng Tech 30(1): Stergiou N, Buzzi U, Kurz M, Heidel J (2004) Nonlinear Tools in Human Movement. In: Innovative analyses of human movement. Human Kinetics, Champaign 93. Stergiou N, Harbourne RT, Cavanaugh JT (2006) Optimal movement variability: a new theoretical perspective for neurologic physical therapy. J Neurol Phys Ther 30(3): Stergiou N, Ristanis S, Moraiti C, Georgoulis A (2007) Tibial rotation in anterior cruciate ligament (ACL)-deficient and ACLreconstructed knees: a theoretical proposition for the development of osteoarthritis. Sports Med 37(7):