Varus posteromedial rotatory instability: a biomechanical analysis of posterior bundle of the medial ulnar collateral ligament reconstruction

Background Recently, there has been growing interest in the involvement of the posterior bundle of the medial ulnar collateral ligament (pMUCL) in varus posteromedial rotatory instability (PMRI). Varus PMRI has been observed clinically, but the degree of involvement of the pMUCL remains unclear. This study assessed the degree to which the pMUCL is involved in stabilizing the elbow and the feasibility of a pMUCL reconstruction to restore stability. Methods Movements simulating PMRI were performed in 8 cadaveric elbows. Joint gapping values were obtained by 3-dimensional motion capture for the proximal and distal aspects of the ulnohumeral joint. Specimens were assessed at “intact,” “cut coronoid + pMUCL,” “reconstruction,” and “cut anterior aspect MUCL + reconstruction” conditions with mechanical testing at 30°, 60°, and 90° of elbow flexion. Results Proximal joint gapping significantly increased from intact to cut coronoid + pMUCL at 60° and 90°, and distal joint gapping significantly increased at 90°. In the reconstruction condition, joint gapping across the proximal joint at 60° and 90° significantly recovered, as did distal joint gapping at 90°. In the cut anterior aspect MUCL + reconstruction condition, no significant increase occurred in proximal or distal joint gapping. Conclusions Transection of the pMUCL with a coronoid fracture leads to increased joint gapping, suggesting the presence of PMRI. PMRI can still occur with an intact lateral ligamentous complex. A pMUCL tendon graft reconstruction confers some elbow stability in this injury mechanism.