Congenital Absence of the Anterior Cruciate Ligament.

The incidence of congenital absence of the anterior cruciate ligament (ACL) is extremely low. Congenital ACL absence has most often been found in association with conditions such as knee dislocation, knee dysplasia, proximal focal femoral deficiency, and fibular hemimelia. We report on the incidental finding of ACL aplasia in a patient with a medial meniscal tear and history of leg-length discrepancy. As has been found in prior case studies, this patient had hypertrophy of the meniscofemoral ligament of Humphrey, which likely lent her stability. This case highlights the importance of differentiating between a stable and an unstable knee in congenital absence of the ACL.