Altered Glenohumeral Biomechanics in Proximal Humeral Fracture Malunion.

Abstract

There is little biomechanical evidence to support the traditional guideline that 45° 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. 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°, 30°, and 45° deformities in varus, valgus, antecurvatum, retrocurvatum, and combined varus-antecurvatum and valgus-retrocurvatum. Specimens were then retested. 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° led to notable increases in GP compared with the intact state. Varus deformities of 30° and 45° caused significant increases in subacromial pressures and limited abduction markedly from 60° to 54.2° and 44.6° (P < 0.001). Varus and antecurvatum proximal humerus deformities as small as 15° were associated with notable alterations in glenohumeral joint mechanics. With valgus and retrocurvatum deformity, statistically significant joint alterations occurred only at higher deformity levels.