Our finding that ACL sectioning increased the forces carried by the lateral meniscus and by cartilage-to-cartilage contact on the medial femoral condyle corroborates previous observations in the literature6,8,9. Musahl et al. found that deficiency of both the lateral meniscus and the ACL led to more anterior translation of the lateral compartment during a pivot-shift maneuver than did isolated ACL deficiency6. Ahmed et al. reported contact between the medial femoral condyle and the tibial spine with applied internal rotation and anterior load10. Our data also corroborate the finding by Li et al. of a lateral shift in contact position on the medial compartment of the tibia toward the medial tibial spine in ACL-sectioned knees13.
Sectioning the ACL increased cartilage-to-cartilage contact force carried by the medial femoral condyle in extension and by the lateral meniscus in flexion (Tables I and III). The congruency of the femoral notch and tibial spine in extension probably allowed the articular stabilizers to engage, thereby limiting tibial subluxation and preventing the increased lateral meniscus force measured in flexion.
We found that the increase in cartilage-to-cartilage contact force on the medial compartment due to ACL sectioning was 3 to 7 times greater than load differences due to condyle sectioning order. Therefore, the condyle sectioning order did not confound the conclusions of the study. Changes in force carried by cartilage-to-cartilage contact of the lateral femoral condyle were small (<5 N) and clinically irrelevant.
This study had limitations. Loads acting on the knee during high-impact sports are probably greater than those applied in this study. However, the compressive force that we applied approached half of a typical body weight and created direct cartilage-to-cartilage load transfer across the lateral compartment. Moreover, 300 N of compressive force was applied to only 5 of the tested knees. Nevertheless, sectioning of the ACL resulted in statistically significant differences in the forces carried by the lateral meniscus and via cartilage-to-cartilage contact on the medial femoral condyle. Additionally, since the pivot shift is performed in the absence of muscle activation, the role of the patellofemoral joint in stabilizing the knee is minimal; therefore, it was not loaded. Moreover, muscle forces were not included since instability episodes likely occur too quickly (∼85 ms) to be resisted by voluntary muscle loading9,34.
In conclusion, a distinct interplay of intra-articular forces occurs during the multiplanar rotatory loading that is characteristic of clinical and functional pivoting events8,9. Specifically, the lateral meniscus and medial femoral condyle impart, respectively, higher anterior and lateral shear forces on the tibia in an ACL-sectioned knee in response to applied torques. These intra-articular forces suggest a mechanism for the clinical patterns of intra-articular derangement observed in the setting of acute and chronic ACL insufficiency, including damage of the lateral meniscus and osseous remodeling of the medial compartment.
Investigation performed at the Biomechanics Department, Hospital for Special Surgery, New York, NY
Disclaimer: The views expressed in this publication are those of the authors and do not reflect official policy or position of the Department of the Navy, Department of Defense, or the United States Government.
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