Posteromedial Corner Knee Injury

Posteromedial corner injury is frequently under-recognised in acute knee trauma. Up to 70 percent of PMC injuries occur as part of a multi-ligament knee injury, and an isolated grade III sMCL tear should always prompt a careful search for an associated ACL tear (most common), PCL, or meniscal lesion. Missed PMC injury leads to persistent anteromedial rotatory instability, valgus collapse, and accelerated medial compartment osteoarthritis.

Isolated PMC injury is uncommon. In any patient with a grade III sMCL tear, always perform Lachman, posterior drawer, pivot shift, dial test, and reverse pivot shift to exclude associated cruciate or posterolateral injury. Up to 70 percent of PMC injuries have another ligament involved, and missing a concurrent ACL rupture is the most common reason for failed conservative treatment of an apparent isolated MCL injury.

Q: Name the four core static stabilisers of the posteromedial corner. A: Superficial MCL (sMCL), deep MCL (dMCL), posterior oblique ligament (POL), and posteromedial capsule, with the semimembranosus expansion acting as the dynamic stabiliser. The sMCL is the primary restraint to valgus at 30 degrees flexion, while the POL is the primary restraint to internal rotation of the tibia.

Q: What examination finding confirms anteromedial rotatory instability (AMRI)? A: The Slocum test (or Hughston anterior drawer in external rotation). With the knee flexed to 90 degrees and the foot fixed in 15 degrees of external rotation, the tibia is allowed to rotate back into neutral. The lateral tibial plateau reduces from an anteriorly subluxed position - this reduction is pathognomonic for AMRI from PMC deficiency.

Q: A patient has valgus opening at 0 degrees (full extension). What does this tell you? A: It is not isolated sMCL injury. Valgus opening in full extension implies the PMC plus the cruciate ligaments (typically ACL) are both injured, because the cruciates and posterior capsule act as secondary valgus restraints in extension. Always examine the ACL, PCL, and posterolateral corner when there is medial opening in full extension.

Q: What is the optimal management of an isolated grade II sMCL injury? A: Hinged knee brace for 4-6 weeks with early protected range of motion. The sMCL has excellent healing potential. Weight-bearing as tolerated, progressive ROM, and a structured rehabilitation programme. Return to sport at 8-12 weeks for non-pivoting activities and 12-16 weeks for pivoting. Surgery is rarely required.

Q: What is the optimal window for acute primary repair of the PMC? A: Within 3 weeks of injury. After this window, the sMCL and POL retract, become scarred, and the tissue quality is too poor for primary repair - staged or chronic reconstruction with autograft or allograft is then required. Outcomes after acute repair are consistently superior to outcomes after delayed reconstruction.

Q: A chronic PMC deficiency with AMRI is being reconstructed. What two structures must be addressed? A: The sMCL and the POL. The sMCL limb controls valgus; a separate POL limb (often a second limb of the same hamstring graft) controls internal rotation and AMRI. Reconstructing the sMCL alone leaves residual AMRI - this is the most common technical error in PMC reconstruction.