Tensile stress of the lateral patellofemoral ligament during knee motion.

Abnormal patellar retinacular tension has been considered to result in patellar lateral subluxation/dislocation. In this study, an in vitro experimental model was developed to determine the tensile stress of the lateral patellofemoral ligament, the thickest portion of the retinaculum, under loading conditions simulating two knee motion patterns: passive knee motion and unweighted active knee extension. The results indicate that the tensile stress increased significantly before 30 degrees of knee flexion, which strongly supports the clinical findings that patellar subluxation/dislocation occurs at approximately 20 degrees to 30 degrees knee flexion angles.