Introduction

Most studies on the medial knee structures focused on valgus or posteromedial rotatory instability, while there is a lack of data regarding anteromedial rotatory knee instability (AMRI). Clinically, a residual AMRI is, however, often observed. Thus, the aim of this study was to investigate the biomechanical contribution of medial sided anatomical structures in response to external loads. It was hypothesized, that the superficial medial collateral ligament (sMCL) is the primary restraint to anterior tibial translation (ATT) performed in external tibial rotation (ER).

Methods

8 fresh-frozen human cadaveric knees were used for this study. Knees were mounted onto a 6 degrees of freedom robotic testing system(KUKA KR60/3). According to the convention of Grood and Suntay a coordinate system was established. Knee kinematics and forces were recorded with a force/moment sensor (ATI-Theta FT-sensor; Schunk). The following loads were applied: 1) 134N ATT performed at 5Nm ER to simulate AMRI, 2) 5Nm ER, 3) internal tibial rotation, and 4) 10Nm valgus rotation. These loading conditions were tested in the native knee and after sectioning sequentially the following structures: 1) medial retinaculum and fascia, 2) anteromedial capsule, 3) sMCL, 4) deep MCL (dMCL), 5) posterior oblique ligament (POL), 6) anterior cruciate ligament (ACL), 7) lateral collateral ligament (LCL), popliteus tendon, popliteofibular ligament. The contribution of each structure to the resultant forces was calculated and presented as percent from the native knee state. A repeated-measures ANOVA with a post-hoc Bonferroni correction was performed (p<0.05).

Results

ATT performed in ER was primarily restraint by the ACL between 0° and 60° knee flexion with a contribution between 50.2% at 30° and 31.2% at 60°. The sMCL also had a significant contribution to restrain these loads with 27.1% (p<0.05) at 60° and 36.8% (p<0.05) at 90°. At lower flexion angles, the contribution of the sMCL did not exceed 15% (NS). While the POL had only a negligible role, the anteromedial fascia and retinaculum showed a contribution of 8.6% at 0° to 14.9% at 90°.
ER was primarily restraint by the medial retinaculum and medial fascia at 0° with 19.2%. At 30°, the major contribution (25.2%; p<0.05) was provided by the sMCL followed by the retinaculum and fascia with 23.6% (p<0.05). At 60° and 90° similar results could be observed. Interestingly, the anteromedial capsule had a significant role only at 120°.
Valgus rotation was primarily restraint by the sMCL (p<0.05), while the POL was the primary restraint to internal tibial rotation at 0° and 30° (p<0.05).

Conclusion

Data of this study suggests that ATT performed in ER was primarily restraint by the ACL followed by the sMCL at deeper flexion angles. Thus, when evaluating AMRI with the anterior drawer performed in ER at 90° knee flexion, excess laxity may be due to an insufficiency of the sMCL. The medial retinaculum and fascia as well as the sMCL were the main contributors to isolated ER at early flexion angles.
When treating patients with high grade AMRI, clinicians should consider that an isolated sMCL may not fully address this combined instability pattern.