Hypertrophic Mucoid Degeneration of the Anterior Cruciate Ligament Mimicking a Tear: A Case Report and Diagnostic Pitfall

Mucoid degeneration (MD) of the anterior cruciate ligament (ACL) is an underdiagnosed condition marked by glycosaminoglycan infiltration within the ligament fibers. Its appearance on magnetic resonance imaging (MRI) can closely mimic an ACL tear, leading to diagnostic errors with major clinical consequences. We report a 54-year-old woman with a 6-month history of chronic knee pain and a sensation of heaviness, without any trauma. Physical examination showed a mildly positive anterior drawer test, interpreted as pain-limited rather than indicative of true instability, with a negative Lachman test. MRI revealed a thickened ligament with diffuse high signal intensity on proton density fat-saturated sequences, a celery stalk appearance on sagittal images, and a markedly thickened, crescent-shaped axial appearance of the hypertrophied ligament filling the intercondylar notch. The fibers appeared preserved and parallel to the Blumensaat line. Additional findings included near-complete filling of the intercondylar notch, intraosseous cysts at both the femoral and tibial ACL footprints, a Stoller grade III signal in the anterior horn of the lateral meniscus consistent with a meniscal tear, and a small joint effusion. The patient was managed conservatively with non-steroidal anti-inflammatory drugs and physiotherapy, with significant pain reduction and near-complete recovery of knee extension at six to eight weeks of follow-up. This case illustrates the imaging features that distinguish mucoid degeneration from ligamentous rupture on MRI, since misdiagnosis can lead to unnecessary surgery.