2017 ISAKOS Biennial Congress ePoster #1089
Biomechanical Function of the Anterolateral Capsule of the Knee
Daniel Guenther, MD, PhD, Hannover GERMANY
Sebastián Irarrázaval, MD, Santiago CHILE
Amir Ata Rahnemai Azar, MD, Pittsburgh, PA UNITED STATES
Stephanie Laurel Sexton, BS, Pittsburgh, PA UNITED STATES
Kevin M Bell, Pittsburgh, PA UNITED STATES
Freddie H. Fu, MD, Pittsburgh, PA UNITED STATES
Richard Debski, PhD, Pittsburgh, PA UNITED STATES
Volker Musahl, MD, Pittsburgh, PA UNITED STATES
University of Pittsburgh, Pittsburgh, Pennsylvania, UNITED STATES
FDA Status Not Applicable
The biomechanical properties of the anterolateral capsule show that it functions as a sheet of tissue rather than a true knee ligament. Therefore, when considering surgery for the anterolateral complex, the amount of rotatory knee instability should be carefully quantified to avoid overconstraining the knee.
Secondary restraints need to be evaluated, particularly when anterior cruciate ligament (ACL) reconstruction fails. The purpose of this study was to determine the function of the anterolateral capsule in terms of tissue strain and in situ forces in the intact and ACL-deficient knee, and to evaluate kinematics of the knee after reconstructions were performed. The hypothesis was that the anterolateral capsule would not function like a traditional ligament and extra-articular reconstruction would improve knee stability in a combined ACL/anterolateral capsule injury model.
Four loading conditions were applied to fresh-frozen cadaveric knees (n=7) at 30°, 60°, and 90° knee flexion using a robotic testing system. Maximum principal strain in the anterolateral capsule was determined by tracking the motion of a grid of markers attached to the capsule during application of the loading conditions. The force distribution in the anterolateral capsule, ACL and lateral collateral ligament (LCL) was determined using the principle of superposition. Kinematics were determined for multiple knee states including ACL-reconstructed and anterolateral capsule-deficient knee, and ACL-reconstructed with extra-articular tenodesis knee. After statistics were performed, significance was set at p<0.05.
The ACL-deficient state was always found to have higher peak maximum principal strain in the anterolateral capsule than the intact knee, and strain was increasing with knee flexion angle (p<0.0167). The magnitude of maximum principal strain in the anterolateral capsule was double compared to the failure strain of traditional ligaments. The average angles calculated between the direction vectors of the maximum principal strain and a proposed anterolateral ligament’s direction ranged between 38° and 129.9° and were significantly different than 0°.
For the ACL-deficient knee in response to an anterior tibial load, the force transmitted between each region of the capsule was 434% greater than the force in the anterolateral capsule (p<0.001) at 30° of flexion. In response to an internal tibial torque at 30°, 60° or 90° of knee flexion, the force in the anterolateral capsule was significantly smaller than the other structures at all knee flexion angles for both loading conditions (p=0.004; p=0.04).
In a combined injury to the ACL and the anterolateral capsule, an additional extra-articular tenodesis was necessary to restore kinematics of the intact knee. Compared to the intact knee, two out of seven reconstructed specimens showed overconstrained internal tibial rotation (p>0.05).
The findings of this study show that with respect to biomechanical function of the ACL-deficient knee, the anterolateral capsule carries negligible forces in the longitudinal direction and much larger strains than knee ligaments that are not distributed along the longitudinal axis. Thus, the anterolateral capsule does not function as a traditional ligament and should be considered a sheet of tissue, thus supporting the hypothesis of this study. However, in the ACL-deficient knee, an extra-articular procedure might be necessary to reduce intact kinematics in combined injuries. The contribution of the anterolateral capsule to rotatory knee instability should be carefully assessed and quantified to avoid over-constraint of the knee in combined knee ligament reconstruction procedures.