Mechanism of limitation of pronation/supination of the forearm in geometric models of deformities of the forearm bones

Abstract Objective. To clarify the mechanism of limitation of pronation/supination of the forearm associated with the angular deformity of the forearm bones and narrowing of the interosseous space. Design. A three-dimensional geometric model of the forearm bones with the interosseous membrane and its axial section were used. Background. Limitation of pronation/supination associated with the deformity of the forearm bones is one of the significant problems encountered in the treatment of the forearm fracture. Elucidation of its mechanism is important for its prevention and treatment. Methods. In the axial section, the effects of the positional relationship between the axis of pronation/supination and the forearm bones on the range of pronation/supination was studied using analytic geometry in each model with non-narrowing or narrowing of the interosseous space. Subsequently, in the three-dimensional model each forearm bone with the same angular deformity, the direction and magnitude of the angular deformity which would lead to limitation of pronation/supination were calculated using analytic geometry. Each parameter of the models was obtained by the radiographic measurements of the normal forearms. Results. When the axis of the pronation/supination passed through the interosseous region (less than 2 cm radioulnarly and 0.8 cm anteroposteriorly) in the model of the axial section without narrowing of the interosseous space, more than 40° of pronation and supination were possible. When the axis deviated from this region, significant loss of pronation/supination was observed associated with restriction by the interosseous membrane rather than impingement. Furthermore, the area of this region decreased according to narrowing of the interosseous space with shortening of the interosseous membrane. In the three-dimensional model, the direction and magnitude of the angular deformity which would lead to significant loss of pronation/supination was more than 14° radially, 7° ulnarly, 5° anteriorly, 4° posteriorly. Conclusions. The positional relationship between the axis of pronation/supination and the forearm bones with the interosseous membrane may play an important role regarding pronation/supination of the forearm. Relevance Evaluation of the bone deformities based on understanding this mechanism of limitation of pronation/supination would lead to an appropriate treatment of malunion of the forearm bones.