Role of the Anterior Cruciate Ligament, Anterolateral Complex, and Lateral Meniscus Posterior Root in Anterolateral Rotatory Knee Instability: A Biomechanical Study

Background: Injuries to the anterior cruciate ligament (ACL), Kaplan fibers (KFs), anterolateral capsule/ligament (C/ALL), and lateral meniscus posterior root (LMPR) have been separately linked to anterolateral instability. Purpose: To investigate the contributions of the ACL, KFs, C/ALL, and LMPR to knee stability and to measure instabilities resulting from their injury. Study Design: Controlled laboratory study. Methods: Ten fresh-frozen human knees were tested robotically to determine restraints of knee laxity at 0 degrees to 90 degrees of flexion. An 88-N anterior-posterior force (anterior and posterior tibial translation), 5-N center dot m internal-external rotation, and 8-N center dot m valgus-varus torque were imposed and intact kinematics recorded. The kinematics were replayed after sequentially cutting the structures (order varied) to calculate their contributions to stability. Another 10 knees were tested in a kinematics rig with optical tracking to measure instabilities after sequentially cutting the structures across 0 degrees to 100 degrees of flexion. One- and 2-way repeated-measures analyses of variance with Bonferroni correction were used to find significance (P < .05) for the robotic and kinematics tests. Results: The ACL was the primary restraint for anterior tibial translation; other structures were insignificant (<10% contribution). The KFs and C/ALL resisted internal rotation, reaching 44% +/- 23% (mean +/- SD; P < .01) and 14% +/- 13% (P < .05) at 90 degrees. The LMPR resisted valgus but not internal rotation. Anterior tibial translation increased after ACL transection (P < .001) and after cutting the lateral structures from 70 degrees to 100 degrees (P < .05). Pivot-shift loading increased anterolateral rotational instability after ACL transection from 0 degrees to 40 degrees (P < .05) and further after cutting the lateral structures from 0 degrees to 100 degrees (P < .01). Conclusion: The anterolateral complex acts as a functional unit to provide rotatory stability. The ACL is the primary stabilizer for anterior tibial translation. The KFs are the most important internal rotation restraint >30 degrees of flexion. Combined KFs + C/ALL injury substantially increased anterolateral rotational instability while isolated injury of either did not. LMPR deficiency did not cause significant instability with the ACL intact.