Loading of shoulder girdle muscles in consequence of a glenohumeral arthrodesis

Sometimes a glenohumeral arthrodesis operation is performed at patients with a brachial plexus lesion. In the glenohumeral arthrodesis the humerus is fused to the scapula. The fusion angles between scapula and humerus determine the restoration of function after the operation. In this study the effect of fusion angles on the hand position and maximal force exertion has been simulated using a musculoskeletal model of the shoulder girdle. Mean scapular and clavicular positions (10 subjects) are used as input variables. Mean fusion angles (18 patients with a glenohumeral arthrodesis) are used as reference position. Output variables are the maximal force which can be exerted with the hand and muscle forces of the thoracoscapular muscles. In order to achieve a mobility area in the mid-sagittal plane, the humerus should be internally rotated 60° with a little abduction and forward flexion. Maximal force can be exerted when the scapula is laterally rotated and protracted beside the thorax. In that position m. serratus anterior has good moment arm to counterbalance the external moment. It is recommended to use an external fixator for the glenohumeral arthrodesis in order to be able to adjust the fusion angles outside the operation theatre.