Anteroposterior tibiofemoral displacements during isometric extension efforts The roles of external load and knee flexion angle

winner of the rehabilitation award Anteroposterior tibiofemoral displacements during isometric extension efforts The roles of external load and knee flexion angle KENNETH A. JURIST, MD, AND JAMES C. OTIS,* PhD From the Pathokinesiology Laboratory, The Hospital for Special Surgery, affiliated with New York Hospital-Cornell University Medical College, New York, New York ABSTRACT Rehabilitation of knee extensors is a major consideration in patients with cruciate injuries and repairs. Extension exercises, however, can produce undesirable loads on the injured or replaced anterior cruciate ligament (ACL) resulting in permanent stretch of this restraint. Our study measures the tibiofemoral displacements associated with proximal, middle, and distal locations of the external resisting force and knee flexion angles of 30, 60, and 90. Our results show that tibiofemoral displacements are dependent on both location of the external resisting force and knee flexion angle, with anterior displacements being associated with distal displacement of the resistance pad. The results suggest that patients with ACL injuries or repairs conduct extension exercises with a proximal position of the resistance pad to minimize loads and prevent stretching of this constraint. * Address correspondence and repnnt requests to James C OtiS, PhD, Associate ScienUst, Department of Biomechanics, The Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021 254 Numerous studies of sagittal plane knee motion have been directed at correlating laxity with the contributions of the supporting soft tissue structures.1-3.5. 7,8,11,13,16 The more recent studies have measured anteroposterior displacement with an externally applied load simulating the clinical drawer test. 1,5,11,13,14,20 Markolf et al., 13,14 using their instrumented clinical testing apparatus, quantitatively measured anteroposterior displacement as a function of knee flexion angle and noted a decrease in laxity with increasing flexion angle. They also calculated a marked increase in joint stiffness with a voluntary contraction of the muscles about the joint. Using a more analytical approach, several investigators examined the forces across the knee joint under different loading conditions.12, 16,19 These authors have indicated that the ACL experiences tensile loading during quadriceps contraction at the terminal 30 of extension. To date, no study has measured displacement of the tibiofemoral joint in vivo during an active knee extension effort. The purpose was to measure this motion under active of this study quadriceps loading using noninvasive means. BACKGROUND Early anatomical studies qualitatively demonstrated, changes in laxity following sequential ligament sectioning. Through the development of sophisticated testing devices, quantification of knee laxity became available. 3,5,8,11,13,14,20 More recent reports have used a spring-loaded device with the patella and tibial tubercle as reference points for tibiofemoral displacement enabling measurements of laxity in vivo. These findings have contributed greatly to our present understanding of anteroposterior stability of the knee. Numerous mechanical studies have detailed the forces involved in knee motion, including Lindahl and Moving Morrison,16 and Smidt.l9 For their radiographic analysis Lindahl and Movin used a cable with a cuff applied about the ankle to resist active knee extension. Through a series of calculations they estimated the load on the ACL, the magnitude of the quadriceps force, and the joint reaction force. Their results indicate that the tensile load on the ACL increases as the knee is extended, is equal to the resisting force at 40 of flexion, and rises steadily to reach four times the resisting force as full extension is reached. Morrison, through gait analysis, calculated a peak force of approximately 155 N on the ACL coinciding temporally with the peak quadriceps contraction just after heel strike. Smidt analyzed the forces across the tibiofemoral joint during active knee extension and flexion. The maximum anterior

255 shear force resulting from quadriceps contraction occurred at 15 of flexion and steadily declined as flexion increased, becoming negative at 75 and 90 of flexion. Grood et al. have shown that adding 31 N to the foot during extension efforts against gravity results in an increased anterior tibial displacement throughout most of the extension range with a maximum average of 2 mm at 45 of flexion. This demonstrates that anterior displacement increases with a distally applied load. Furthermore, the additional anterior displacement following their removal of the ACL demonstrates that the ACL was a primary constraint. In summary, the above studies illustrate that there are conditions of geometry and load which determine whether or not the ACL will be placed under tension during active extension. The objective of our study will be to specify to some degree those geometrical and loading conditions which will minimize tension on the ACL during active extension. MATERIALS AND METHODS Subjects The left knees of 15 male volunteers with an age of 26.7 ± 5.4 (mean ± 1 SD) years, a weight of 77.3 ± 6.3 kg, and a height of 179 ± 7 cm participated in the study. Clinical histories revealed left knees which were normal and had never undergone surgery. Clinical examinations conducted by one of the authors revealed stable ligaments with a full range of knee motion symmetric with the other side, no effusions, well-developed thigh musculature, and no generalized ligamentous laxity in any of our subjects. One subject had an injury to the contralateral knee as a child and required ipsilateral epiphysiodesis, and another occasionally had symptoms suggesting patellofemoral pain when running, but experienced no pain during the test procedure. Test procedure The subject was placed in the right side down decubitus position with his hip in a neutral attitude. The thigh was secured in a V-shaped padded support with two large Velcro straps (Lumex Corporation, Ronkonkoma, NY), the knee joint was free in space, and the foot was supported on a table with the height adjusted to prevent varus-valgus stress on the knee. The contralateral leg was flexed in a comfortable position and rested on the testing bench. A handgrip was provided for the subject to grasp for added stability during the strenuous part of the test. For each subject the effective length of the tibia, do, was measured as the distance from the adductor tubercle to the distal tip of the medial malleolus. Markings were made on each subject s leg corresponding to the three positions where the external force would be applied. The distances from the tubercle were d1 0.95 do, d2 0.60 do, and d3 0.25 do and = = = were referred to as distal, middle, and proximal, respectively. A flexor torque of 33.9 N-m (25 ft-lb) was applied through a cable and cuff for each of the three positions by varying the applied force to satisfy the condition that d1f1 d2f2 = = d3f3, where f, is the applied force at the respective positions. The magnitude of the torque selected was determined from the maximum force that could be applied at the proximal location without causing discomfort, since loading at the proximal position requires the greatest applied force. A 100 pound load cell (model 3397, Lebow Associates, Troy, MI) mounted in series with the cable and a chart recorder (model 2200S, Gould, Inc., Cleveland, OH) were used to monitor the applied force. The Knee Laxity Tester (The Stryker Corporation, Kalamazoo, MI) was used to measure tibiofemoral displacement of the tibia. It references the dnterior surface of the tibial tubercle to the anterior surface of the patella, which is assumed to reflect the anteroposterior location of the femur. Its scale is calibrated in millimeters, and readings may be obtained in both the positive (anterior) and negative (posterior) directions. Certain precautions must be taken to assure consistent and reliable readings with this device. A false reading is produced with axial rotation of the tibia or a change in flexion angle of the knee. Therefore, an electrogoniometer, which measured knee flexion angle, was attached to the subject s leg throughout the test. Flexion angle was displayed continuously on the strip chart recorder and on a digital readout in clear view of the subject. The subject used the digital display for positioning the knee joint and the continuous display for maintaining flexion angle within 1. A change in angle of more than 1 in any direction voided the trial. The examiner closely observed the tibia and voided the trial if axial rotation was observed. For each of the three locations of the external force, displacement measurements were taken with the knee at 30, 60, and 90 of flexion. The sequence was randomly varied to eliminate bias due to testing order. The displacement measured was the difference between the loaded and the unloaded states and, thus, does not reflect total anteroposterior displacement. Displacement was measured from the maximally loaded to the fully relaxed position. With the subject in place, the technician manually applied a force perpendicular to the long axis of the tibia until the required force for that position was achieved. The subject was informed of an increasing force against his leg and was instructed to maintain a constant joint angle by monitoring the displays provided. When the desired force was attained, the Stryker device was zeroed and the subject was instructed to maintain a constant angle as the force was gradually reduced. When unloaded, the subject was asked to completely relax his muscles and a reading was taken to the nearest five-tenths of a millimeter. This was repeated twice for each configuration and the average of the readings was recorded as the displacement for that position. Initial practice trials were conducted until the subject became proficient at maintaining joint angle. The means and SDs of the tibiofemoral displacements for each of the external force locations and knee flexion angles were determined. One-way analyses of variance were employed to determine whether the mean displacements dif-

256 fered for the various external force locations and for the various knee flexion angles. RESULTS The means and SDs of the displacements for the 15 knees are shown in Figure 1 for the knee flexion angles of 30, 60, and 90, and the proximal, middle, and distal locations of the external force resisting extension. As the location of the external load was moved proximally on the tibia and the mean displacements became increasingly negative, the tibia displaced more posteriorly. Results of the one-way analyses of variance between tibiofemoral displacement and external load location at each of the flexion angles demonstrate statistically significant (P < 0.01) differences in displacement for the different external load locations at each of the flexion angles studied Results of the one-way analyses of variance between tibiofemoral displacement and knee flexion angle demonstrate statistically significant (P < 0.01) differences in displacement over the three flexion angles used for middle and distal positions of the external load; however, when the external force was applied at the proximal position, tibiofemoral displacement was independent of knee flexion angle, consistently resulting in posterior displacement of the tibia. DISCUSSION The sensitivity of anteroposterior tibiofemoral displacement to location of external load has been alluded to but never measured in vivo. This phenomenon can be explained by considering the three fundamental forces acting on the tibia (Fig. 2): the external force, R; the force of the patellar ligament, P; and the joint reaction force, J. The patellar ligament force and the external load with known points of application and directions define a concurrent point 0 Figure 1. Summary of the mean tibiofemoral displacements for the proximal (PROX), middle (MID), and distal (DIST) locations of the external resisting force at knee flexion angles of 30, 60, and 90. Figure 2. In (a), the distal external resisting force (R) and the patellar ligament force (P) determine the concurrent point (C) which the joint reaction force (J) must intersect. The force (J) reflects the need for a restraint to limit anterior displacement of the tibia. In (b), the proximal resisting force (R ) defines the concurrent point (C ). As a result, the joint reaction force (J ) reflects the need for a restraint to limit posterior displacement of the tibia. through which the joint reaction force (which passes through the point of tibiofemoral contact) must also pass in order to satisfy conditions for static equilibrium. From its orientation in Figure 2A, it can be seen that the joint reaction force, in addition to applying a compressive force to the tibia, also acts posteriorly. This posteriorly acting component reflects a load on the ACL, limiting anterior displacement of the tibia relative to the femur. When the external load is moved proximally (Fig. 2B), static equilibrium requires that the shear component of the joint reaction force revert from acting in a posterior direction to acting in an anterior direction, reflecting a load on the posterior cruciate ligament (PCL), limiting posterior displacement of the tibia. Thus, a distal external load will result in anterior displacement with ACL loading, and a proximal external force will result in a posterior displacement with PCL loading. At the present time there is considerable controversy as to the appropriate treatment of ACL injuries. The spectrum of opinions runs from primary repair in the acute injury to simply concentrating on muscle strengthening exercises in chronic injuries. If in the chronically unstable knee a course of muscle strengthening has failed, reconstructive surgery can be performed. The choice of procedures is extensive and involves intraarticular as well as extraarticular reconstruction. Common to all of these procedures and regimens is the need for muscle strengthening of the thigh. Whether surgery has been performed or conservative rehabilitation is implemented, extension exercises that use a distally placed resistance will cause shear loads that will result in anterior

257 tibiofemoral displacement. In addition, if the primary restraint to anterior displacement is torn and secondary restraints are used, such exercises may load these structures beyond their tolerable limits and compromise their effectiveness as the remaining restraint to anterior displacement. Johnson9 discusses substitution of a single resistance pad with a dual-pad attachment to reduce anterior shear loads at the knee during extension exercises. One of the pads is located proximally on the tibia and is rigidly attached by a bar to a second pad located distally on the tibia. The machine arm is connected to a fulcrum or pivot point on the connecting bar such that adjusting of the length of the machine arm will position the fulcrum over either the proximal pad, the distal pad, or somewhere in between. If the fulcrum is over the distal pad, all of the resistance will be transmitted across the distal pad, which is thus equivalent to a single distal pad. If the fulcrum is located between the two pads, then the resisting force will be distributed to both pads with the greater force occurring at the pad closest to the fulcrum. However, a single pad located the same distance from the knee joint as the fulcrum on the dual-pad attachment will produce the equivalent joint reaction load as the dual-pad apparatus. Consequently, the use of a dual-pad system provides no advantage over the use of a single pad in reducing shear loads at the knee during extension exercise. The joint reaction force will depend on the location of the center of force on the tibia, which is located at the pad for a singlepad system and at the fulcrum for the dual-pad system. An inverse relationship between anterior displacement of the tibia and flexion angle of the knee joint has been demonstrated. This relationship is in agreement with the analysis of Lindahl and Movin,12 which suggests that, during an extension effort against an external load applied perpendicular to the distal tibia, the magnitude of the load on the ACL will increase as flexion angle decreases. Grood et al. also found that anterior tibial displacement increased as flexion angle decreased; however, they used gravitational loads which did not apply a constant torque at the knee joint so that a strict comparison to our results is not possible. In addition, the increased anterior tibial displacement obtained when we applied our resistance load more distally is consistent with the increased anterior displacement measured by Grood et al. when a weight was applied at the ankles of their cadaver specimens. The magnitudes of the anterior and posterior tibial displacements measured during our study were less than those previously reported for passive knee joint laxity testing.8,l1, 13,20 In the present study, the magnitudes of the displacements were measured as the subjects went from a loaded to an unloaded state. The mean values for the displacements with the knee at 30 of flexion ranged from a posterior displacement of 2.1 mm with a proximal external force to an anterior displacement of 1.5 mm with a distal external force. Although the sum of these two displacements may not reflect the total anteroposterior excursion at 30 of flexion since we are not able to define an absolute zero position, the 3.6 mm range of these values is comparable to the 4.6 mm excursion obtained by Hsieh and Walker8 when measuring anteroposterior laxity with compressive load across the knee joint, and also comparable to the excursion of approximately 3 mm obtained by Markolf et au4 when measuring anteroposterior laxity when the knee joint was stabilized by voluntary contraction of the musculature about the knee. Paulos et al.,18 in their article on rehabilitation of the reconstructed ACL, present their own guidelines for beginning weightbearing and muscle strengthening, as well as the results of an international survey of &dquo;knee experts.&dquo; The authors, in their moderate protection period between the third and sixth month, introduce crutch weaning and muscle strengthening. They caution against the early permission to bear full weight and encourage muscle strengthening prior to full weightbearing. They refer to the high stresses experienced by the ACL in walking and on muscle strengthening from 30 to full extension. The results of the survey showed that the mean time to full weightbearing was 7.7 weeks, while 60% either did not restrict range of motion or actually encouraged full extension during exercise. From the results of the present study, it would be evident that there is a viable method to allow early exercise through a prescribed range of motion prior to full weightbearing that would not jeopardize the result of surgery. In a recent report by Johnson et al.,l the end result of cruciate ligament reconstruction could be correlated with laxity changes at followup. There was a statistically significant decrease in the number of good results in the patients who demonstrated increased laxity on the Lachman test, implying that the ligament stretched out since there was still a good end point. Only 11 of 27 failures could be explained by complete disruption of the graft. This finding underscores the need to do everything possible to protect the ligament or its replacement. In conclusion, our results suggest that quadriceps muscle strengthening can be safely started between 90 and 30 of flexion without jeopardizing the integrity of the cruciate ligament or its replacement by using a proximally placed resistance to active extension. Further work is needed to document tibiofemoral displacements near full extension. Most existing equipment used for muscle strengthening can be modified as needed to minimize loads on the ACL. ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the Clark Foundation and the valuable assistance of Mr. Richard Tiru. REFERENCES 1 Bargar WL, Moreland JR, Markolf KL, et al In-vivo stability testing of postmeniscectomy knees Clin Orthop 150 247-252, 1980 2 Brantigan O, Voshell AF The mechanics of the ligaments and menisci of the knee joint J Bone and Joint Surg 23 44-66, 1941 3 Butler DL, Noyes FR, Grood ES Ligamentous restraints to antero-postenor drawer in the human knee A biomechanical study J Bone and Joint Surg 62A. 259-270, 1980 4 France EP, Daniels AU, Goble EM, et al: Simultaneous quantitation of knee ligament forces J Biomechanics 16. 553-564, 1983

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