Influence of anteromedial facet defects of the coronoids process on the posterior bundle of the medial collateral ligament in human cadaveric specimens

Anteromedial facet (AMF) fractures of the coronoid process are typically accompanied by tears of the lateral collateral ligament (LCL) and the posterior bundle of the medial collateral ligament (pMCL). Addressing the bone defect and repairing the LCL is essential in the acute setting to prevent persistent varus posteromedial rotatory instability (VPMRI) and subsequent progression to osteoarthritis. However, pMCL repair is not typically performed. Therefore, the present biomechanical study investigates the amount of stress experienced by the pMCL in cases of anteromedial facet defects and its role in contributing to VPMRI of the elbow. Nine fresh-frozen cadaveric elbows were tested under varus loading (3 Nm) with elbow flexion at 30°, 60°, and 90° in 3 distinct bone defect scenarios: (A) non-fractured coronoid; (B) anteromedial facet fracture, subtype II; and (C) anteromedial facet fracture, subtype III. The strain of the LCL and the ulnar collateral ligament (UCL) with its anterior (aMCL) and posterior bundles (pMCL) were measured quantitively via strain gauges (μm/m), as an estimate for their involvement in the instability pattern. Across all scenarios, the highest strain of the pMCL under varus force was observed with elbow flexion at 30° with a supinated forearm (mean = 171.72 μm/m, min 22.79 μm/m, max 348.9 μm/m, SD = 110.5 μm/m), while the aMCL remained relaxed. The degree of elbow flexion significantly influenced the strain on the pMCL under varus force (P = .01). Strain on the pMCL increased in scenario B (162.45 μm/m, SD 122.2 μm/m) and scenario C (185.1 μm/m, SD 117.8 μm/m). There was no statistically significant difference between both scenarios (P = .96). The pMCL is notably influenced by varus force, particularly with elbow flexion at 30°. AMF coronoid fractures resulted in a higher strain of the pMCL. Repairing the pMCL in cases of AMF defects could enhance elbow stability.