The Role of the Indirect Femoral Insertion of the Anterior Cruciate Ligament in Restraining Tibial Translation and Rotation: Implications for Anatomic Femoral Tunnel Placement

OBJECTIVES: Despite a trend toward lowering the femoral tunnel and thus positioning ACL grafts partially within the indirect insertion of the ACL attachment, the biomechanical role of the indirect insertion has not been described. The purpose of this study was to determine the effect of debridement of the indirect femoral ACL insertion on tibiofemoral translation and rotation. It was hypothesized that debridement would have a negligible affect on tibiofemoral kinematics. METHODS: Knee kinematics in six degrees of freedom were measured on a robotic device in nine cadaveric knees. Measurements were made with the ACL intact, following debridement of the indirect insertion, and following complete sectioning of the ACL. Three loading conditions were used: (1) a 134 N anterior tibial load, (2) a combined 10 Nm valgus and 5 Nm internal rotation torque, and (3) a simulated, robotic pivot shift. Debridement involved exposing 5-6 mm of bone from the central, inferior aspect of the direct ACL attachment to the inferior cartilage margin. Anterior tibial translation (ATT) was recorded in response to an anterior tibial load and combined rotatory loads at 0°, 15°, 30°, 45°, 60°, and 90° of knee flexion. Additionally, posterior tibial translation and external tibial rotation were recorded during a simulated, robotic pivot shift. Based on pilot data, it was estimated that 9 specimens would be required to detect a 1.4 mm difference in ATT between different experimental conditions with 80% power at an alpha level of 0.05. Data were analyzed using repeated measures ANOVA. RESULTS: For the anterior tibial loading condition, debridement of the indirect insertion increased tibial translation by 0.37±0.24 mm at 0º (P<.01) and by 0.16±0.19 mm at 15º (P<.05; increases were < 1 mm in all specimens). ACL deficiency increased ATT in response to an anterior tibial load (P<.0001) with maximum effect at 15º (11.26±1.15 mm vs. ACL intact; 11.04±1.08 mm vs. indirect insertion debridement). For the combined rotatory loading condition, debridement increased tibial translation by 0.17±0.11 mm at 0º (P<.01; increases were < 0.3 mm in all specimens) with no effect at other flexion angles. ACL deficiency increased ATT in response to a combined rotatory load (P<.01) with maximum effect at 15º (4.45±0.85 mm vs. ACL intact; 4.44±0.84 mm vs. indirect insertion debridement). The contribution of the indirect insertion to restraining ATT in response to an anterior tibial load or a combined rotatory load was ≤ 5.5% in all specimens, at all flexion angles. During the simulated pivot shift, posterior tibial translation (12.79±3.22 mm) and external tibial rotation (17.60±4.30º) were greater in the ACL deficient condition (P<.0001) compared with the ACL intact (1.29±1.34 mm and 1.54±1.61º) and indirect insertion debridement conditions (1.28±1.34 mm and 1.54±1.47º). Posterior translation and external tibial rotation were not significantly different between the ACL intact and debridement conditions (P=.68, P=.99). CONCLUSION: Debridement of the indirect insertion resulted in a less than 1 mm increase in tibial translation and a less than 0.5º increase in tibial rotation in all specimens and testing conditions. Thus, the indirect femoral insertion of the ACL contributed minimally to restraint of tibial translation and rotation. These results support placement of the soft tissue aspect of a BPTB graft within the area of the direct insertion of the ACL and superior to the biomechanically insignificant indirect insertion.