Isolated ligamentous injury can cause posteromedial elbow instability: a cadaveric study

Background Elbow posteromedial rotatory instability (PMRI) is known to occur with fracture of the anteromedial coronoid and injury to the posterior bundle of the medial ulnar collateral ligament (pMUCL). However, whether instability results from isolated pMUCL injury remains unclear. The purpose of this study was to quantify displacement about the ulnohumeral joint to evaluate whether isolated sectioning of the pMUCL results in elbow PMRI. Methods Nine cadaveric elbows underwent movements simulating PMRI by application of axial compression with varus and internal rotation moments. Gapping values at both the proximal and distal aspects of the medial ulnohumeral joint were then recorded for “intact” and “pMUCL-sectioned” elbows at positions of 30°, 60°, and 90° of flexion. Results After pMUCL transection, torsion increased by 2.6° ± 0.7° (P = .054) at 30° and 4.5° ± 1.2° (P = .039) at 60° of flexion. Proximal ulnohumeral joint gapping also increased at 30° (1.4 ± 0.4 mm; P = .039), 60° (1.5 ± 0.6 mm; P = .039), and 90° (1.5 ± 0.7 mm; P = .017), respectively. No increases in distal ulnohumeral gapping occurred at any angle of flexion. Discussion Sectioning of the pMUCL results in significant increases in torsion and displacement about the proximal ulnohumeral joint. Our findings demonstrate that elbow PMRI can occur secondary to isolated ligamentous injury. Clinicians mindful of this previously unrecognized role of the pMUCL as a stabilizer may wish to consider methods of restoring pMUCL integrity when treating medial elbow instability.