Anterior tibial translation

Quadriceps exercises are used sparingly in the early rehabilitation of ACL reconstructions because of concern about prematurely stretching the ACL graft. The aim of this study was to determine if a maximum isometric quadriceps contraction significantly translates the tibia anteriorly at 15°, 30°, 45°, 60°, and 75° of flexion. Secondly, the role of the ACL in knee stability was analyzed by comparing the amount of tibial translation in normal ACL deficient, and reconstructed knees. Thirdly, the location in the motion arc where a quadriceps contraction produces anterior tibial translation was determined. Anterior tibial translation was measured using an arthrometer (KT-1000) during an 89 N and manual maximum translation applied to the knee at rest. The manual maximum translation test determines the magnitude of anterior tibial translation produced by a high anterior force applied directly to the proximal calf. These translations were compared to the tibial translation intrinsically induced by a quadriceps contraction. Testing was performed in normal (N 22), ACL deficient (N=1 0), and reconstructed (N = 10 ) knees. Anterior tibial translation produced by a maximum quadriceps contraction was measured at 15°, 30°, 45°, 60°, and 7 5° of flexion. The extension exercise resulted in less anterior tibial displacement than an 89 N drawer and half the translation produced by a manual maximum translation (P < 0.001). Instrumented laxity testing produced greater anterior translation of the tibia than a maximum isometric quadriceps contraction. Anterior tibial translation was the same during maximum isometric knee extension in all tested knees. Anterior translation in the isometrically loaded knee occurred during the terminal 60° of knee extension, and was controlled by the compression forces driving the congruent articular surfaces together, and not the ACL. Instrumented laxity testing is an accepted technique that has been safely used to serially measure knee stability after an ACL reconstruction. Early postoperative knee extension exercises; can be expected to induce less strain in the ACL than instrumented laxity testing. These data raise the question of whether the anterior tibial displacement produced by a maximum quadriceps contraction is detrimental to a recently reconstructed ACL. One of the concerns that surgeons have had about prescribing quadriceps exercises following ACL reconstructions is the possibility that the ACL graft may stretch. This concern is based on the fact that the quadriceps muscle is known to generate both an extensor torque and an anterior drawer. 14 The possibility that this intrinsically induced anterior shear force may produce plastic deformation of an immature ACL graft has been raised. Many authors have therefore recommended avoidance of extension exercises from 0° to 30°,70° to 45°, 0° to 60°,15 or 0° to 70° during the “early” (variable time intervals) rehabilitation period. These restrictions are inconsistent because they are based on studies using different experimental techniques and testing conditions. A question that remains is whether the tension produced in the ACL during active knee extension is clinically significant. There are two techniques for determining ACL tension that could be applied to answer this question. Direct measurement of ligament tension could be used. This technique requires that a measuring device be directly attached to the ligament. In vivo analysis in large numbers of patients can be difficult because of inherent problems in placing electrodes in human subjects, in addition to several other technical limitations. The inser*Presented at the American Academy of Orthopaedic Surgeons, New Orleans, Louisiana February 1990. The views expressed herein are those of the author and do not reflect the official policy or posiition of the United States Department of Defense or the United States Government †Address correspondence and reprint requests to: Stephen M. Howell, MD, 7601 Timberlake W ay Suite 103, Sacramento, CA 95823 tion of buckle transducers affects the resting tension of the ligament. Measurements made with buckle or Hall-effect transducers sample tensile changes in only a small portion of the ligament. Alternatively, a relative, indirect estimate of ACL tension could be made by comparing the measured amount of tibial translation produced under different loading conditions. Qualitatively, an increase in anterior tibial translation would be expected to correlate with increasing ACL tension; however, one must remember that this relationship is not linear. Although this indirect technique does not directly measure ACL tension, it is relatively easy to use in vivo. Many variables have been shown to affect the amount of anterior tibial translation during active knee extension. Large resistant loads, applied distally on the tibia, increase the anterior translation of the tibia and enlarge the motion arc where this translation occurs l4 Increasing joint compression decreases anterior tibial translation and the tension in the ACL. 12, 13, 18 Isometric knee extension theoretically increases ACL tension more than isokinetic exercise because it occurs without a hamstring cocontraction. The integrity of the ACL ligament has been observed to affect the magnitude of anterior tibial translation in vitro. These factors should be accounted for during the formulation of an experimental protocol designed to study anterior tibial translation during active knee extension. The purpose of this study was three-fold. First, the amount of tibial translation was measured by applying a known anterior drawer(extrinsic drawer), and comparing this translation to the displacement produced by an isometric quadriceps contraction (intrinsic drawer) at 15°, 30°, 45°, 60°, and 75° of flexion. This comparison was made to determine if the quadriceps-induced drawer significantly translates the knee. Second, the role of the ACL in knee stability was analyzed under these same extrinsic and intrinsic loading conditions by comparing the amount of tibial translation in normal, ACL deficient, and reconstructed knees. Finally, the flexion angles were determined where anterior translation of the tibia occurs due to a maximum quadriceps contraction. MATERIALS AND METHODS The control group consisted of 22 volunteers (20 males, 2 females; average age, 27 years). All had normal knees to clinical testing and no history of significant knee trauma or surgery. Ten patients admitted for elective ACL reconstruction for a chronic anterior knee instability comprised the ACL deficient study group (eight males, two females; average age, 25 years). None had had a previous meniscectomy. Ten patients were studied with stable, reconstructed knees (nine males, one female; average age, 29 years). The inclusion criteria for the reconstructed group was an absent pivot shift, stable Lachman test, and KT-1000 testing measurements of the involved knee at 89 N and manual maximum anterior translation (MMT) within 2 mm of the normal knee: All three groups underwent the same, standardized testing protocol consisting of measuring tibial translation with the knee at rest and during a maximum isometric extension contraction at five different angles of knee flexion. The subjects were seated in a position giving low back support and secured to the exercise table (Lumex Corp., Ronkonkoma, NY) by two large VELCRO straps (VELCRO USA Inc., Manchester, NH) placed across the chest and waist (Fig. 1). A third strap was applied across the thigh above the knee to be tested. The subjects’ arms were maintained alongside their trunk, and their hands gripped the sides of the table for stability. The rotation axis of the resistance arm (Cybex II Isokinetic Dynamometer, Lumex Corp.) was aligned to the flexion-extension axis of the knee. The distal end of the resistance pad was secured posterior to the tibia 29 cm distal from the joint line. This distal placement of the resistance pad increases the anterior tibial drawer and enlarges the motion arc where this translation occurs.7,9,14 An anterior VELCRO strap secured the distal tibia to the resistance arm. The dynamometer was attached to a dual-channel strip chart recorder that provided a simultaneous display of torque and flexion angle. The angle of knee flexion during testing was controlled at either 15°, 30 ̊, 45 ̊, 60 ̊, or 75 ̊ by adjusting the position of the resistance arm. The angle was kept constant by setting the angular velocity of the dynamometer at 0 deg/sec. A knee arthrometer (KT-1000, MEDmetric Ligament Arthrometer, San Diego, CA) was used to determine the laxity of the knee by measuring the translation of the tibia with 574 Howell American Journal of Sports Medicine Figure 1. Top and bottom: Patient positioning and the application of the KT-1000 knee arthrometer are depicted for the testing of tibial translation at 15° of knee flexion. Vol. 18, No. 6, 1990 Anterior Tibial Translation 575 respect to the femur. This instrument was selected because the technique has been shown to be reproducible when applied by experienced testers. 4, 11 Additionally, these laxity tests have been applied both preoperatively and postoperatively in ACL injured and in postreconstruction knees without inducing iatrogenic injury to a torn or reconstructed ACL. The arthrometer was placed on the anterior aspect of the tibia and secured with two VELCRO straps. The distal foot on the instrument case was placed immediately distal to the resistance pad with the proximal centering arrow properly positioned over the joint line. The two freely movable sensor pads were in contact with the tibial tubercle and the patella. The instrument is designed to detect relative anterior-posterior displacement motion between these two sensor pads. Anterior and posterior displacement loads were applied through a force-sensing handle that emitted an audiotone when a 67 N (15 pound) and 89 N (20 pound) push or pull force was applied through the sensing handle. The millimeters of tibial translation are indicated by a dial on the instrument case, and were read to the nearest 0.5 mm. Laxity testing began with the patient secured to the exercise table. The position of the resistance arm was adjusted so that the knee was fixed at 15 ̊ of flexion. A goniometer was used to ensure accurate positioning of the knee. Two laxity measurements (89 N and MMT) were made of the relaxed knee. The measurement reference position was obtained by applying an 89 N posterior load and then releasing it several times until a reproducible unloaded knee position was obtained. The instrument dial was then set at 0 mm. An 89 N anterior force, followed by an 89 N posterior force, was then applied to the relaxed knee and the displacement was read directly off the dial. The dial had to return to 0 ± 0.5 mm when the posterior force was released. The test cycle was repeated until three successive tests indicated the same excursion within 1 mm. The mean of the three tests was recorded to the nearest 0.5 mm. The MMT was determined using the same testing cycle. For the MMT test the anterior force was applied by the examiner’s hand exerting a strong (unmeasured) anterior force applied directly to the proximal calf instead of using the force handle. Two additional laxity measurements were made of the subject’s knee during a maximum isometric extension contraction produced by extending the knee against the immobile resistant arm. The third measurement determined the amount of tibial translation produced intrinsically by the quadriceps contraction (quadriceps active drawer-QAD). In this laxity test the subject was instructed to actively extend his or her knee to maximum contraction after the resting relationship of the tibia and femur was determined with the KT-1000. Displacement of the dial was noted as positive for anterior translation and negative for posterior translation. The amount of tibial translation was recorded when the assistant observed that the torque production had peaked and plateaued for 3 seconds on the strip recorder, indicating a sustained, maximum isometric contraction. This testing cycle was repeated until three successive tests indicated the same excursion within 1 mm. The fourth and final displacement measurement consisted of loading the knee extrinsically by applying an MMT test to the isometrically contracted knee. The examiner applied a strong (unmeasured) anterior force directly to the proximal calf during the QAD. The total extrinsic and intrinsic translation produced by the MMT and QAD was recorded after averaging three successive tests that were within 1 mm. The subjects were allowed to rest between each muscle contraction so that the contractions were performed without fatigue. The rotational alignment of the arthrometer case was kept constant at each testing angle to minimize fluctuations in anterior-posterior laxity due to inconsistent alignment of the instrument. The four measurements of tibial translation (two at rest, and two during maximum isometric extension) were re-peated at 30 ̊, 45 ̊, 60 ̊, and 75 ̊ of flexion. All measurements of tibial translation were performed by the author. Statistical analysis of the four measurements derived from each of the normal knees was performed using a twotailed, paired t-test. Comparisons between the normal and ACL deficient, and normal and reconstructed knees were based on a two-tailed, nonparametric Mann-Whitney test. The “z” statistic was used for comparison with tables of the normal distribution to determine statistical significance.