Biomechanical analysis of scapular neck malunion––a simulation study

Objective. To explain loss of shoulder function following scapular neck malunion in terms of biomechanical changes around the gleno-humeral joint. Design. Biomechanical modelling study. Background. Residual rotation of the scapular neck after fracture can lead to pain and loss of function, and the indications for surgical intervention are contested. Methods. A 3D, large-scale, musculo-skeletal model of the upper limb was used to compare shoulder biomechanics in the case of scapular neck malunion with normal anatomy. Abduction of the humerus was simulated with three models: normal anatomy, 24° and 40° inferior scapular neck rotation. Results. Predicted muscle activation differed greatly between the control and the fracture cases. The motion required additional muscle effort for the maintenance of gleno-humeral stability in the fracture cases. Higher moments in the plane of abduction were generated by the teres major, pectoralis major and biceps brachii muscles with high humeral elevation angles. The rotator cuff muscles were severely shortened in the post-fracture cases and the forces in these muscles were greatly reduced in a test of loaded abduction with the humerus at 90°. Conclusions. Given the function of the rotator cuff muscles as stabilisers of the gleno-humeral joint, it is concluded that the loss of force in these muscles, together with other changes in muscle activation, will lead to loss of arm function in patients with scapular neck malunion.