Mucoid degeneration of the anterior cruciate ligament corrected with autograft reconstruction

The anterior cruciate ligament (ACL) is the most frequently injured knee ligament, typically in non-contact athletic injuries in young adults. Mucoid degeneration of the ACL (ACL-MD) is an uncommon ACL pathology in which glycosaminoglycans deposition within the collagenous bundles leads to hypertrophy, loss of full knee flexion and knee pain without instability. ACL-MD usually presents in individuals over 40 years with sudden-onset knee pain after minimal trauma or as an incidental MRI finding. ACL-MD is rarely described in young adults. We present a case of ACL-MD in a previously healthy adult in his early 20s who presented with 3-month recalcitrant dull left knee pain and limited range of motion after 'stepping funny' with slight twisting. This case highlights the need to critically reflect on the anatomical structures when presented with musculoskeletal pathologies and to consider the unique presentation of musculoskeletal disease in atypical age groups.