Mucoid degeneration of the anterior cruciate ligament: a little-known cause of deep knee pain Mucoid degeneration of the anterior cruciate ligament (ACL) is a little-known cause of deep atraumatic knee pain that is relatively frequent. Recent studies have reported a prevalence of 1,8-9,2% on MRI of the knee. Mucoid lesions consist of interstitial deposits of glycosaminoglycan between the collagen bundles. This causes hypertrophy of the ACL, which leads to mechanical impingement at the femoral notch, typically resulting in deep or posterior knee pain in terminal flexion and/or extension. Unlike the often-occurring traumatic ACL ruptures, there are no instability complaints. The gold standard for diagnosing mucoid degeneration of the ACL is MRI. This shows an abnormally thickened and ill-defined ACL with an increased intraligamentary signal on all sequences, but the orientation and continuity are usually maintained. Arthroscopically, the mucoid degeneration can be recognized by a hypertrophic ACL with yellowish to brown substances between the fibers. In addition, an absence of the synovial outline of the ACL is seen regularly. An arthroscopic partial or total ACL resection is the treatment of choice for symptomatic mucoid degeneration of the ACL and results in good post-operative outcomes without development of instability.
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