Distinct Patterns of ACL Engagement Explain Variations in Anterior Knee Laxity

Introduction: Anterior laxity is highly heterogeneous. Identify those patients is important since high laxity means increase risk of ligament injury and ACL graft failure. During a clinical laxity exam the ACL carries force to resist the applied load, yet how force builds in the ACL as the joint moves through its passive envelope of motion has not been fully characterized. Thus, we assessed whether variations in anterior laxity could be explained by variations in how the ACL builds force as the tibia translates anteriorly.

Methods: A robotic manipulator was used to load 20 cadaveric knees anteriorly at 30° flexion simulating a Lachman exam. ACL engagement was parameterized using three measures: 1) anatomic slack in mm, (tibial motion from the neutral position of the joint to the tibial position where the ACL began to carry force); 2) anatomic stiffness in Newtons (N)/mm, (the slope of the linear portion of the ACL force – tibial motion response after the ACL began to carry force); and 3) ligament force at the peak applied load in N. Anterior knee laxity was related to these parameters of ACL engagement using univariate and multivariate regression models (p < 0.05).

Results: Univariate analysis revealed that variations in anterior laxity are associated with variations in anatomic slack of the ACL (r²=0.83, p<0.001), but not the anatomic stiffness or ACL force at the peak applied load (r²=0.00, p>0.43). Multivariate modeling revealed that all three parameters of ACL engagement (p<0.001) were associated with variations in anterior knee laxity (r²=0.95).

Discussion: Anatomic slack of the ACL is a key driver of variations in anterior laxity. Thus, surgical parameters that impact anatomic slack of the ACL graft such as pretension and joint position when fixing the graft in place should be given close consideration to control anterior laxity.