Altered Glenohumeral Biomechanics in Proximal Humeral Fracture Malunion

Introduction: There is little biomechanical evidence to support the traditional guideline that 45 degrees represents acceptable proximal humerus deformity. We evaluated glenohumeral contact pressure (GP) and area, subacromial contact pressure, and joint abduction to assess biomechanical changes with different proximal humerus deformities. Methods: Fifteen fresh-frozen cadaver shoulders were used. Intact specimens were tested on a custom dynamic shoulder frame. Subsequently, a surgical neck fracture was made in each specimen and fixed using a custom dual hinge plate for fixation of 15 degrees, 30 degrees, and 45 degrees deformities in varus, valgus, antecurvatum, retrocurvatum, and combined varus-antecurvatum and valgus-retrocurvatum. Specimens were then retested. Results: Compared with the intact state, GP was markedly lower with all levels of varus and varus-antecurvatum deformity. Valgus and combined valgus-retrocurvatum deformity of 45 degrees led to notable increases in GP compared with the intact state. Varus deformities of 30 degrees and 45 degrees caused significant increases in subacromial pressures and limited abduction markedly from 60 degrees to 54.2 degrees and 44.6 degrees (P < 0.001). Discussion: Varus and antecurvatum proximal humerus deformities as small as 15 degrees were associated with notable alterations in glenohumeral joint mechanics. With valgus and retrocurvatum deformity, statistically significant joint alterations occurred only at higher deformity levels.