Effect of displacement of fractures of the greater tuberosity on the mechanics of the shoulder

Using a dynamic biomechanical model of malunion of the shoulder, we have de termined the change in deltoid force required for abduction with various co mbinations of superior and posterior displacement of fractures of the great er tuberosity of the humerus. We tested eight fresh human cadaver shoulders in a dynamic shoulder-testing apparatus during cycles of glenohumeral abdu ction from 0 degrees to 90 degrees. The greater tuberosities were osteotomi sed and stabilised to represent malunion with combinations of superior and posterior displacements of 1 cm and less. The peak force was measured for e ach displacement in each specimen and statistically compared with values of no displacement using a repeated-measures analysis of variance. The abduction force was significantly increased by 16% (P = 0.006) and 27% (p = 0.0001) by superior displacements of 0.5 cm and 1 cm, respectively, wh ile combined superior and posterior displacement of 1 cm gave an increase i n force of 29% (p = 0.001). While treatment criteria for acceptable residua l displacement of the greater tuberosity are widely used, there is little i nformation on the direct biomechanical effects of displacement on shoulder mechanics. Although the results of conservative treatment are influenced by a number of factors, including associated injuries, rehabilitation and the pre-existing function of the shoulder, our data suggest that small amounts of residual displacement may alter the balance of forces required to eleva te the arm at the glenohumeral joint.