Functional Loss With Displacement of Medial Epicondyle Humerus Fractures: A Computer Simulation Study.

BACKGROUND Assessment and management of the medial humeral epicondyle fracture remains controversial, with conflicting reports of displacement direction and consequent functional deficits unclear. The purpose of this study was to define biomechanically likely directions of medial epicondyle fracture displacement and to determine possible changes in muscle function related to that displacement. METHODS A 3-dimensional computer model of the upper extremity was used to simulate the consequences of medial epicondyle fracture displacements from 1 to 20 mm in the anterior, medial, and inferior directions relative to the humerus with the elbow at 90-degree flexion and neutral forearm rotation (a replication of accepted positions for clinical strength testing). Muscle length and force were calculated following displacement. Maximum isometric wrist flexion moments were calculated over the full range of wrist motion based on known force-generating properties of the muscles. RESULTS Anterior displacement resulted in shortened muscles and reduced wrist flexion moment, with a decrease in strength averaging 2% for every 1 mm of anterior fragment displacement at neutral wrist position (maximum decrease of 39% with 20 mm displacement). In contrast, displacement in the medial and inferior directions resulted in stretched muscles and increased wrist flexion moments and therefore are not biomechanically likely. CONCLUSIONS Computer simulation of a medial epicondyle fracture suggests that anterior displacement could result in a dramatic loss of initial muscle strength and function. Medial displacement is unlikely to occur in vivo due to consequential muscle lengthening, suggesting that alternatives to the historical use of AP radiographs to assess displacement of this fracture are needed. CLINICAL RELEVANCE Our work provides a biomechanical explanation for anterior displacement of medial epicondyle fractures observed radiographically and motivates alternative methods of fracture assessment. A functional basis for determining acceptable displacement of medial epicondyle fractures is suggested; however, all individual clinical factors should be considered.