Objective: Forearm position for immobilizing an isolated medial humeral epicondyle fracture has not yet been established. This study aimed to investigate the relationship between forearm rotation and medial humeral epicondyle displacement.

Materials and Methods: A cadaveric study was performed in which a medial epicondyle fracture was simulated by performing an osteotomy. Fracture displacement was measured using a digital Vernier caliper from full pronation to full supination in 10- degree increments. Displacement was measured at the point of maximum distance. After that, the fragment was reduced and stabilized with K-wire. Intra-observer reliability and prediction of displacement based on forearm rotation was analyzed.

Results: Five cadavers (ten medial epicondyles) were involved in the study. The mean displacement ranged from 14.39 mm at full pronation to the most reducible 1.60 mm at full supination, a mean difference of 12.79 mm (SD 2.39, 95% confidence interval: 11.31, 14.27). There was a significant
correlation between forearm rotation and actual medial epicondyle displacement (Pearson r = 0.91, p<0.001). Using a displacement of less than 5 mm as the criterion for conservative treatment, the best position for fracture stabilization was >20 degrees supination with a sensitivity,
specificity, PPV, and NPV of 93.3%, 91.3%, 87.5%, and 95.5%, respectively (area under the curve 0.92; p<0.001). No fragment was displaced during forearm rotation following fixation with two K-wires.

Conclusion: Forearm rotation significantly affects actual medial humeral epicondyle displacement. The more the forearm supinates, the less the medial epicondyle is displaced. A forearm rotation of at least 20 degrees of supination confines the fragment to <5 mm of displacement.

Keywords: Medial epicondyle fracture; Immobilization; Forearm rotation