Novel measure of articular instability based on contact stress confirms that the anterior cruciate ligament is a critical stabilizer of the lateral compartment.

Knee instability following anterior cruciate ligament (ACL) rupture is common, compromising function, and causing cartilage and meniscal damage. In this study, instability at the level of the articular surfaces was characterized with a new measure: articular instability. Articular instability was defined as the change in location of the center of contact stress per unit of applied load. The effect of ACL-deficiency on articular instability was quantified in response to combined abduction and internal rotation moments simulating the clinical pivot shift, which recreates the sensation of instability. Eleven cadaver knees were loaded using a robotic manipulator and tibiofemoral contact stress was measured using a stress transducer. Sectioning the ACL led to pronounced articular instability on the lateral compartment in 4 of 11 knees. In these 4 knees articular instability increased posteriorly up to 403% and increased laterally up to 754%. Factors driving inter-specimen variations in articular instability might include articular morphology, ligamentous laxity, and the applied loads. This novel description of contact mechanics confirms that the ACL prevents sudden changes in the relative position of the lateral articular surfaces. It is applicable to any loading conditions and provides a unique measure to quantify the effects of ACL injury and reconstruction.