Journal of Orthopaedic Research. Bruce D. Beynnon *, Braden C. Fleming, Ryan Labovitch, Bradford Parsons. Introduction

Transcription

1 Journal of Orthopaedic Research 20 (2002) Journal of Orthopaedic Research Chronic anterior cruciate ligament deficiency is associated with increased anterior translation of the tibia during the transition from non-weightbearing to weightbearing Bruce D. Beynnon *, Braden C. Fleming, Ryan Labovitch, Bradford Parsons Dcpuvtment of Orthopucdics und Rehnhilitntion, McC lure MLlsL,ul~~skrletul Re.seurch Center, University of l ermont, Room 438.4, Robert T Stu$ord Hull, Burlingion, I T , LISA Received 7 November 2000; accepted 24 July 2001 Abstract Translation of the tibia relative to the femur was measured while a group of subjects with normal knees and group with anterior cruciate ligament (ACL) tears underwent transition from non-weightbearing to weightbearing stance. Subjects were positioned in the Vermont knee laxity device (VKLD) with muscles relaxed and the limb segment and compressive joint load offset (nonweightbearing). A lateral radiograph of the knee, with the posterior aspects of the femoral condyles superimposed, was obtained to document the position of the tibia relative to the femur. Immediately after, a compressive load equal to 40% of bodyweight was applied to each foot, and a second radiograph was obtained to document the change in position of the tibia relative to the femur. The transition from non-wcightbearing to weightbearing produced a significant increase of anterior translation of the tibia relative to the femur (mean; 3.4 mm) for the subjects with ACL tears compared with the contralateral normal knees (0.8 mm). Similarly, there was a significant increase in anterior translation of the tibia for the subjects with ACL tears compared to the group of subjects with normal knees (1.2 mm). The fourfold increase in anterior translation of the tibia for the knees with ACL tears compared to the contralateral side is a concern because it is substantially greater than the 95% confidence limits of the side-to-side differences in anterior-posterior knee laxity measured from subjects with normal knees. This observation could explain, at least in part, one of the mechanisms that initiates damage to the meniscus and articular cartilage in subjects that have suffered an ACL tear Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved. Introduction The anterior cruciate ligament (ACL) is the most common completely disrupted ligament in the knee and, once injured, it results in a detrimental increase in anterior translation of the tibia (relative to the femur) when the knee is non-weightbearing [1,8]; however, the effect of this injury on knee biomechanics during weightbearing is not well understood. For example, the transition of the knee from the non-weightbearing situation with muscles relaxed to a weightbearing posture creates an increase in knee stiffness and a decrease in anterior- posterior displacement of the tibia relative to the femur (A-P laxity) [18,19], but much less is known about how the position of the tibia changes relative to -~ * Corresponding author. Tel.: : Fdx: E-ind uddres.\: beynnonccusalus.med.uvm.edu (B.D. Beynnon). the femur during the transition between these conditions. This information is important because the knee undergoes transition between non-weightbearing and weightbearing conditions frequently. Understanding how the tibia translates relative to the femur for such a common activity may provide insight into one of the mechanisms that predisposes the meniscus, articular cartilage, and other ligaments to further damage such as the onset and progression of osteoarthritis [6,7,9,12,21]. Torzilli et al. [26] used a cadaver model to study the knee with and without the ACL sectioned when the joint was unloaded and when it was loaded by simulating quadriceps activity and bodyweight. They reported that knees with the ACL sectioned underwent a substantial anterior neutral-position shift of the tibia relative to the femur during the transition between unloaded and loaded conditions, and that a much smaller shift occurred for the normal knee with intact ligaments [26]. Li et al. [I71 reported a similar observation using the /02/$ - see front matter C) 2002 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved. PI1: S (01 )0U115-2

2 B. D. Beq'nnon et ul. I Jiiurnul of' Orthopwdic Rcwlirdi 20 i I porcine model. Our work in human subjects has shown that ACL strain increased when subjects transitioned from non-weightbearing to weightbearing conditions [5], a response that was attributed to anterior translation of the tibia relative to the femur. To our knowledge, no study has investigated the combined effect of compressive load and lower extremity muscle contraction on translation of the tibia relative to the femur for normal and ACL-injured knees in humans. The objective of this investigation was to measure the translation of the tibia relative to the femur during the transition from non-weightbearing to weightbearing in a group of subjects with ACL tears and a contralatera1 normal knee, and a control group with normal knees. We hypothesized that transition from nonweightbearing to weightbearing would produce a greater anterior translation of the tibia relative to the femur for knees with ACL tears compared to that of normal knees. Materials and methods Eleven subjects (9 males and 2 females, age range: years) with a chronic ACL tear in one knee and a normal contralateral knee, and a control group of five subjects with normal knees were studied. Subjects were identified from our University clinic and asked to participate if they had suffered an ACL tear, initially diagnosed as a grade 111 tear by clinical examination, at least one year prior to enrollment (range: 2-20 years), and had their tear confirmed by either arthroscopic visualization (9 subjects), or MRI (2 subjects). Subjects were excluded if they had a history of neurologic disease (Dementia, Parkinson's, or clinical evidence of neuropathy), history of metabolic disease, previous operations on their knee (other than diagnostic arthroscopy or partial meniscectomy), failed ACL reconstruction, history of a fractured tibia or femur on either side, current use of narcotic pain medication or clinically detectable instability of the posterior cruciate, medial collateral or lateral collateral ligaments. All subjects had similar range of knee motion compared with the contralateral normal knee, and normal hip and ankle joints. Immediately prior to testing, the subjects were reevaluated to document that no additional injuries, other than the original ACL tear, occurred to the index and contralateral normal knee. This included a complete lower extremity exam according to the international knee documentation committee (IKDC) [I 1 J protocol, and KT-1000 measurements of knee laxity. In an effort to characterize the subjects and assess the impact of the ACL tear on their symptoms, function, and activity level, each participant completed the knee injury and osteoarthritis outcome score (KOOS) [22], the ACL-quality of life (ACL-QOL) assessment [20], the Lysholm score, and the modified Tegner activity score [25]. The five control subjects were male and ranged between 23 and 10 years of age. Controls were excluded if they suffered from neurologic or metabolic disease, had previously undergone lower extremity surgery or suffered prior knee trauma as evidenced by instability of the primary knee ligaments. Subjects were evaluated while supine in the Vermont knee laxity device (VKLD). The VtiLD was developed to measure the translation of the tibia relative to the femur as the knee transitions from nonweightbearing to weightbearing positions, and to characterize the anterior-posterior load-displacement behavior of the knee (Figs. 1 and 2) [27]. In the current study, we were interested in studying the epiect of transition from non-weightbearing to weightbearing on displacement of the tibia relative to the femur because this occurs frequently during gait and therefore, only the former feature of the VKLD was used. Important features of VKLD include the capability to apply controlled loading to the tibiofemoral joints of each limb (an absolute zero shear load condition was created across the knee while it was unweighted to establish the neutral position of the tibia relative to the femur, and Fig. 1. The VKLD, seen from a lateral view. was used to load the legs independently, and create the non-weightbearing and weightbeai-ing conditions [27]. A 6 degree-of-freedom load sensor was attached to the bottom of each foot. A counterweight system was applied to the thigh and shank to create an anterior-posterior directed shear load at the knee that was zero, establishing a reproducible neutral position of the tibia relative to the femur, and to apply a compressive load to both feet that simulated weightbearing. The axes of rotation of the counterweight system were aligned with the hip and ankle axes of rotation. The interface between the counterweight system and its attachment to the thigh and shank incorporated linear bearings (see Fig. 2 for details) that allowed unrestricted movement of these limb segments in the medial-lateral direction. Once a subject was placed in the VtiLD, adjustment of the position of the leg involved unlocking the foot plate (allowing rotation about the long axis of the tibia without imparting internal-external torque to the tibiofemord joint), unlocking the foot cradles (allowing unrestricted displacement of the foot in the proximal- distal direction), unrestricted rotation about the hip (the subject relaxed the leg muscles) and unrestricted flexion- extension motion of the knee. With these degrees-of-freedom unrestricted. positioning of the limb was accomplished by the investigator grasping the thigh and shank and moving the leg to a position without imparting iiiternalexternal torque across the knee. Once the proper limb position was established, the foot plate and cradle were locked in place. The attachments of the counterweight system to the thigh and shank were allowed to remain free and translate in an unrestricted manner in the medial-lateral direction and therefore the counterweight system did not impart internal-external or varus-valgus torques to the knee during non-weightbeariny and weightbearing conditions. Fig. 2. The study subject is positioned in the VKLD with the X-ray source and film aligned to obtain a lateral view of the ACL-deficient knee.

3 D. Ber'mron ct al. I Jiwnal if' Ovthopirdic Re.veart (2002 J standardized compressive loads were applied through the ankle and hip axes of rotation of each limb during weightbearing) and the capacity to align the lower limb using the technique described by Staubli et al. [23]. This approach permitted planar radiographic measurement of the position of the tibia relative to the femur for unweighted and weightbearing conditions while checking for movement of the leg outside the plane of the radiograph. Previous work by our group has demonstrated that the VKLD provides reliable measurement of the anterior-posterior, load-displacement response of the knee, referred to by some as A--P laxity, that are similar to those made with the KT and planar radiography [27]. In addition, we have shown that measurement of the anterior-posterior position of the tibia relative to the femur using the planar radiographic technique described by Staubli et al. [23] produces measurements that are correlated with the highly accurate Roentgen stereophotogrammetric analysis [lo]. Each subject was placed in the VKLD with the axes of rotation of the ankle and hip. identified by palpating the most prominent aspects of the lateral malleolus and greater trochanter, respectively, aligned with the axes of rotation of the thigh and shank portions of the VKLD counterweight system (Fig. 1). Once positioned in the VKLD (Fig. 21, each foot was strapped to a foot cradle that could be locked in position (used to evaluate the knee while it was non-weightbearing) or unlocked and allowed to move in the horizontal plane (used to evaluate the knee during weightbearing). The ankles were fixed at a flexion angle of 90" and the knee at 20". After the subject's feet were strapped into the cradles, thigh and leg counterweights were applied to eliminate the posterior directed gravity forces acting on the thigh and shank segments and standardize the initial position of the tibia relative to the femur [27]. The thigh and shank counterweights, and their respective locations, were selected using the geometrical model of Zatsiorski et al. [30] to estimate segment masses and the respective location of the center of mass. When the subjects relaxed their leg muscles. this approach ailowed us to establish a resultant shear load across the tibiofemoral,joint that was zero, and create the same initial conditions for the measurement of translation of the tibia relative to the femur for all subjects. The six degree-of-freedom load sensor located at the foot (Fig. I) was monitored to ensure that the compressive load was zero. The weightbearing load was applied to each foot cradle through a system of pulleys such that after unlocking the foot cradles, each leg would experience a compressive force equal to 40Y;) of bodyweight that acted through the axes of rotation of the ankle and hip joints. This was chosen to produce 80% of bodyweight or the loading condition experienced during double leg stance (assuming that 20"O of bodyweight was distributed below the knee). Once the counterweights were applied, the subject was instructed to extendlflex their knees against the resistance of the foot cradles until a flexion angle of 70" was obtained. After this was established, the knee position indicator portion of the VKLD was aligned with a reference line on the anterior aspect of the patella to provide the subject with a reference that could be used to maintain the same knee position during weightbearing. The X-ray beam was directed from a lateral position, oriented 6" craniolateral to caudomedial and centered on the lateral epicondyle of the femur as recommended by Staubli et al. [33] for taking planar stress X-rays of the knee (Fig. 2). X-ray cassettes were placed against the medial aspect of the subjects knee in the sagittal plane, and fixed perpendicular to the VKLD. A scale was placed on the anterior aspect of the tibia in the mid-sagittal plane and used in the subsequent data analysis to account for magnification effects. Once the beam was properly oriented, a non-weightbearing radiograph was obtained. The radiograph was then evaluated to establihh whether the spherical profiles of the posterior aspects of the medial and lateral femoral condyles were superimposed using the approach described by Staubli et al. [23]. If the condyles were not superimposed, the foot plate was unlocked, and the investigator held the leg at the distal aspect of the thigh and proximal portion of the shank and rotated the limb about the hip joint and foot plate with the goal of aligning the posterior aspects of both femoral condyles. The VKLD was designed to accomplish this without producing torques (flexionextension, internal-external or varus-valgus) or loads (anterior-posterior, medial-lateral or proximal-distal) on the tibia relative to the femur. The X-ray procedure was repeated until the posterior aspects of the medial and lateral femoral condyles were superimposed on the radiograph according to the technique described by Staubli et al. [23]. This was done to allow identification of the medial and lateral con- dyles, and to create a reproducible reference for measuring the position of the tibia relative to the femur. If the posterior aspects of the femoral condyles remained in the same superimposed view on unweighted and weightbearing radiographs, this ensured us that rotation of the femur, and our measurement reference, did not occur. The subject was then placed in the weightbearing condition as described above and instructed to maintain their knee at 20' of flexion by utilizing the visual cue provided by the VKLD reference position indicator and contracting their muscles. A second radiograph was then obtained. The non-weightbearing and weightbearing X-rays were obtained consecutively within seconds of each other so direct comparison could be made between these conditions. The same protocol was utilized for the contralateral knee, and for both knees of the control group. Translation of the tibia relative to the femur produced by the transition from non-weightbearing to weightbearing conditions was determined from measurements made on the respective radiographs using an approach similar to that described by Staubli et al. [23]. This required identification of the femoral and tibial condyle surfaces on each X-ray film using a backlit digitizing tablet (Digi-pad, GTCO, Rockville, MD, LISA). The posterior aspects of the medial and lateral femoral condyles were superimposed on the X-ray film; this allowed us to identify the medial femoral condyle by its smaller diameter in the anterior -posterior plane compai-ed to the lateral side, and its convex profile was used to distinguish it from the distinct biconvex profile of the lateral condyle. The medial tibial condyle was distinguished by the concavity and larger anterior-posterior diameter compared to the lateral side. Once the appropriate condyles had been identified, each was marked at the most posterior aspect of the subchondral bone, and the position of these points was then digitized relative to a reference line that was constructed tangent to the posterior aspect of the middyaphysis of the tibia. From the non-weightbearing radiograph, the position of the medial and lateral condyles of the tibia relative to the femur were determined using reference lines based on the most posterior aspect of the femoral and tibial condyle cortices. Similarly, from the weightbearing radiographs, the position of the medial and lateral condyles were measured. Translation of the medial condyle of the knee produced by the transition from non-weightbearing to weightbearing conditions was then determined by calculating the difference in the position data from the corresponding non-weightbearing and weightbearing radiographs obtained from the medial condyles of the knee. Similarly, translation of the lateral condyles of the knee produced by the transition was evaluated by calculating the difference in the position data from radiographs obtained from the lateral portion of the knee. The average of the medial and lateral condyle translation values was then calculated to quantify the translation of the tibia relative to the femur produced by the transition from non-weightbearing to weightbearing conditions in the mid-sagittal plane. Staubli et al. [23] has reported that the accuracy of this technique is &0.5 mm. Comparison of the translation of the tibia relative to the femur produced by the transition between non-weightbearing and weightbearing postures was made between the knees with ACL tears and the contralateral normal knees with the students paired T-test, while comparison between the knees with ACL tears and the control knees was made with the non-paired T-test. Differences in translation values were considered statistically significant at p < Results For those subjects with ACL tears, the transition from non-weightbearing to weightbearing produced significantly greater anterior translation of the tibia, relative to the femur, as compared to the contralateral, normal knee (p = 0.007). On average, the knees with an ACL tear underwent an anterior translation of 3.4 mm (S.D. = 2.6 mm) compared to 0.8 mm (3.2 mm) for the contralateral normal knee (Fig. 3(a) and (b)). In contrast, for the control group, there was no difference in translation between = 0.61). The average an-

4 B D Beynnon el (sl I Journd of Orthop~i~& Rrrrurc h 20 f 2002 i and emotional status was The KOOS values were as follows; pain (mean 88), symptoms (90.6), activities of daily living (96.5), sport and recreation function (83), and knee-related quality of life (72.6). The mean Tegner and Lysholm scores were 6.2 (range: 1-9) and 83.9 (range: 65-94), correspondingly. Discussion ACL-d ACLi Left Right Test Group Control Group Test Group C'oiitrol Group Fig. 3. (a) Anterior tibial translation, relative to the femur, produced by transition from non-weightbearing to weightbearing postures for the test group (including the ACL-deficient (ACL-d) and ACL-intact (ACL-i knees), and the control group (left and right knees). (b) The difference in anterior tibial translation, with corresponding 95% confidence intervals, between ACL-deficient and contralateral knees (test group), and between left and right knees of the control group. Movement from non-weightbearing to weightbearing produced a significant increase in anterior translation of the tibia for the ACL-deficient knee, but no difference between knees for the control group. terior translations for the control knees were 1.4 mm (3.9 mm) and 1.0 mm (3.6 mm) for the left and right sides, respectively, values that were similar to those observed for the contralateral, normal knees of the subjects with ACL tears. The ACL tear limited subjects in terms of their ability to take part in recreational activities and sport, and most felt that this compromised their quality of life. The ACL-QOL outcome values were as follows; symptoms and physical complaints (mean = 79), work-related concerns (88.7), recreational activities and sport participation or competition (53), lifestyle (77.8), and social This investigation revealed that knees with chronic ACL tears undergo an anterior translation of the tibia relative to the femur that is 4 times greater than the contralateral normal knee, and 2.9 times that of normal, control knees when transitioning from non-weightbearing to weightbearing postures. Previous studies have shown that compressive loading of the knee produces a dramatic decrease in the anterior-posterior, load-displacement response of the tibiofemoral joint, in comparison to the unweighted condition, and this decreased motion between articular surfaces has been considered protective by some [18]. Our findings in subjects with chronic ACL tears indicates that the compressive load produced by weightbearing is associated with a substantial increase in anterior translation of the tibia relative to the femur compared to the unweighted condition. Our findings are consistent with the cadaver study performed by Torzilli et al. [26] that demonstrated ACLsectioned and ACL-intact knees underwent an anterior translation of the tibia of 14.3 and 5.5 mm, respectively, when exposed to a compressive joint load and simulated quadriceps activity, a loading condition similar to our study. Although these translations were much greater than those measured in our investigation, the relative increase was similar. The difference in translation magnitudes between the earlier work of Torzilli et al. and our findings can be explained by the fact that the cadaver model may not have re-created the exact sequence and magnitude of muscle activation that has been shown to effect knee kinematics and ACL strain behavior in vivo [3,4]. Further support of these findings come from our observation of increased ACL strain as a subject transitions from non-weightbearing to weightbearing [5], an increase that is produced by anterior translation of the tibia. Most subjects underwent an anterior translation of the tibia relative to the femur as a result of the transition from non-weightbearing to weightbearing; however, some underwent a posterior directed translation. Recognizing that application of the compressive load may have produced a change in the positon of the limb relative to the plane of the radiograph, and thereby introduced error, the non-weightbearing and weightbearing films were compared to determine the magnitude of change in knee flexion angle, to ensure that

5 336 B. D. Beynnon et 01. I Journal of Orthopucdic Restwrch 20 (2002 i the posterior aspects of the medial and lateral femoral condyles remained superimposed, and to estimate the change of internal-external rotation position of tibia relative to the femur. The former analysis was done because subtle change in the flexion position of the knee would be associated with an unwanted translation of the tibia relative to the femur. The latter comparison was made because internal-external rotation of the femur would change the measurement reference position, and internal-external rotation of the tibia relative to the femur would introduce error into the measurements of translation. For all subjects, the change in flexion angle was very small. Normal and ACL-injured knees underwent a small increase of extension during the transition (mean f S.D. = 1.9" 5 3.4", and 1.4" i 3.8", respectively). Similarly, the posterior aspects of the femoral condyles remained superimposed, and the change in internal-external rotation position of the tibia relative to the femur was very small for all subjects. For the normal and ACL-injured knees there was a very small difference in translation between the medial and lateral condyles (mean i S.D. = , and 0.7 f 0.6, respectively). Post-hoc regression analysis revealed that translation of the tibia relative to the femur for both the ACL-injured and normal knees could not be explained by the change of knee flexion or by change in position of the posterior aspects of the medial condyles relative to the lateral condyles (the Y' values for ACL-injured and ACL-intact knees were consistently less than 0.04). We confirmed that knee flexion angle was similar between left and right sides, internal-external rotaion of the tibia relative to the femur did not occur, and use of the VKLD ensured that the applied compressive load was standardized and consistently acted through the axes of rotation of the hip and ankle. The only difference between sides was absence of the ACL and therefore, we attribute the increased anterior translation to be associated with disruption of this structure. During activity, the knee transitions from nonweightbearing to weightbearing many times; for example, between the toe-off and heal-strike phases of gait. The forces studied here were equivalent to those produced when transitioning from a non-weightbearing to a weightbearing posture without inclusion of the inertial loads produced by the limb segments, and therefore they were slightly lower than those produced during level walking. Berchuck et al. [3] predicted that during level walking, foot strike produced a maximum anterior shear force, a force that acts to displace the tibia in an anterior direction, that was 13.9% of bodyweight multiplied by height for knees with ACL tears. During athletic activities that involve contraction of the dominant quadriceps muscles, the anterior directed shear forces are much greater, and when considered in combination with the low coefficient of friction at the articular interface, this may result in increased anterior translation of the tibia for knees with ACL tears that are considerably greater than the fourfold increase that was measured in this study. There was increased anterior translation of the tibial articular surface in the knees with ACL tears compared to knees with intact ACLs during the transition from non-weightbearing to weightbearing and this may have important clinical implications with regard to further damage to other structures such as the menisci. This is supported, at least in part, by cadaver studies of the unweighted, ACL-sectioned knee that have shown one mechanism of anterior tibial restraint to be the wedging apart of the tibiofemoral articular surfaces by the posterior horn of the meniscus [16,24]. Further, a recent cadaver study by Hollis et al. [I31 revealed that sectioning the ACL produces a substantial increase in meniscus strain values with the knee unloaded and loaded. Whether or not the posterior horn of the meniscus acts as restraint to the substantial anterior translation produced by the transition from nonweightbearing to weightbearing for knees with ACL tears in vivo remains unknown. However, when one considers that the incidence of concomitant meniscal tears associated with chronic ACL tears ranges between 53% and 100?4 [14,15,28,29] in combination with our observation of a fourfold increase of anterior translation of the tibia relative to the femur for knees with ACL tears compared to normal, it supports the hypothesis that the increased translation is resisted by the posterior horns of the menisci, exposing these structures to increased strain, and in turn, this could produce tears that alter their load bearing function. We studied a group of subjects with chronic ACL tears that were only slightly limited by joint pain, and were able to perform their activities of daily living relatively free of symptoms. However, the subjects felt that their ACL tear reduced their quality of life and limited their ability to take part in sports and recreation activity. From this perspective, these subjects appear to have reduced their activity level to compensate for their injury. Our observation that the transition from nonweightbearing to weightbearing produced a substantial anterior translation of the knee with an ACL tear is a concern because it is in excess of 2.7 mm or the 95% confidence limits based on the side-to-side differences in anterior-posterior translation measured from subjects with normal, unweighted knees; [8] however, it is important for us to point out that the magnitude of anterior translation of the tibia relative to the femur that is harmful to healing articular cartilage or meniscus or is part of a repetitive loading process that subsequently leads to tears of these structures remains unknown. Future studies of knees with ACL tears should determine the relation between increased anterior translation produced during activities that challenge the knee (e.g., during the transition between non-weightbearing and weightbearing as used in this study or a different "stress

6 B. D. Beq nnon et al. I Journal of Orthopriedic Research 20 ( test that challenges the knee) and the prevalence of meniscus damage. If such a relationship exists, it may be helpful for identifying those patients that would benefit from an ACL reconstruction procedure that decreases anterior translation of the tibia and reduces the likelihood of further injury. Acknowledgements Support for this research was provided by the Department of Orthopaedics and Rehabilitaiton at the University of Vermont, the McClure Endowment, and NIH grant #ROl-AR References [I] Andriacchi TP, Birac D. Functional testing in the anterior cruciate ligament-deficient knee. Clin Orthop 1993;288:40-7. [7] Berchuck M, Andriacchi TP, Bach BR, Reider B. Gait adaptations by patients who have a deficient anterior cruciate ligament. J Bone Jt Surg, Am 1990;72(6): Beynnon BD, Johnson RJ, Fleming BC, Pope MH, Renstroin PA, Nichols CE. Anterior cruciate ligament strain behavior during rehabilitation exercises in vivo. 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