Mucoid degeneration of the anterior cruciate ligament (ACL) is a rare cause of knee pain. We report a case of a patient with mucoid degeneration of the ACL, presenting with posterior knee pain and no history of a major knee trauma. On clinical examination, the active range of motion showed a flexion deficit. The posterior knee pain was induced by passive hyperflexion of the knee. There was no evidence of ligamentary instability. MRI showed a diffuse thickening of the ACL with a nodular mass on the femoral insertion occupying the intercondylar notch, with increased signal intensity on both T1- and T2-weighted images. Arthroscopic evaluation showed a diffuse hypertrophy of the ACL, throughout the entire length of the posterolateral bundle (PLB). A yellowish homogenous mass on the femoral insertion of the ACL impinged on the posterior cruciate ligament (PCL) in flexion and occupied the entire intercondylar notch. We performed an arthroscopic debridement of the hypertrophied tissues as precisely as possible. This resulted in a nearly complete removal of the PLB and immediate relief of symptoms. Examination of knee stability after debridement showed a stable ACL. Arthroscopic debridement of the mucoid degeneration of the ACL proved to be a safe and effective method, without causing ligamentary instability in daily activities.

No abstract