Partial-thickness ACL tear with tibial traction cysts — the subtle intrasubstance signal that changes management
Key images
Clinical context
An adult presented with two months of atraumatic left knee pain. Multiplanar, multisequence MRI of the left knee was performed without intravenous contrast.
Key finding
The ACL demonstrated increased intrasubstance signal with partial fibre disruption and traction-related cystic change at its tibial attachment, consistent with a partial-thickness tear with preserved continuity. A focal full-thickness chondral defect (Outerbridge grade IV) was present in the intercondylar notch region of the distal femur with underlying subchondral cystic change. There was extensive grade II mucoid degeneration of the posterior horn of the medial meniscus without a surface-reaching tear, and a mild-to-moderate joint effusion. The PCL, collateral complexes, posterolateral corner, extensor mechanism and lateral meniscus were intact.
Teaching point
A partial ACL tear can be subtle: preserved continuity with only increased intrasubstance signal and partial fibre disruption. Traction-related cystic change at the tibial insertion is a useful corroborating sign of ligament strain and should prompt careful scrutiny of fibre integrity rather than being dismissed as an incidental ganglion. Distinguishing a partial from a complete tear directly informs conservative versus surgical management.
Teaching case shown for educational purposes. Anonymized; no identifiable patient data. A radiology report is a specialist opinion and must be interpreted with the full clinical context by the treating physician.