Mucoid hypertrophic mass of the anterior cruciate ligament causing knee extension block

We report on the case of a patient with a knee extension block caused by a mucoid hypertrophic mass on the anterior cruciate ligament (ACL). Magnetic resonance imaging showed diffuse thickening of the ACL with a mass-like lesion anterior to the tibial attachment. Arthroscopy revealed a huge amorphous mass arising from ACL fibers at the tibial attachment that impinged on the femoral intercondylar notch on knee extension. The treatment included excision of the mass and accompanying abnormal ACL fibers and notchplasty, and resulted in complete recovery of knee motion and symptom relief without instability. This case shows that a degenerative mass in the form of a mucoid hypertrophied ACL can cause a knee extension block.