MR-based 3D-analysis of glenohumeral joint translation in patients with shoulder instability.

Aim: Until now, pathological translation of the glenohumeral joint could no t be assessed three-dimensionally and in functionally important arm positio ns in the living. The objektive of this study was therefore to develop an M R-based technique for determining the three-dimensional glenohumeral transl ation in functionally relevant positions in vivo. Method: In an open MR sca nner both shoulder joints of 5 volunteers with an unilateral traumatic inst ability were examined in different positions of abduction and rotation. Aft er semiautomatic segmentation, 3D reconstruction of the bony structures of the shoulder girdle was performed and the center of mass of the glenoid cav ity was determined and used as reference point. In a virtual reality, the m idpoint of the humeral head was assessed and its position relative to the c enter of mass of the glenoid cavity was calculated. Results: At 30 degrees of abduction, in both shoulders, the humeral head was positioned inferior a nd posterior relative to the glenoid cavity (healthy: 0.42+/-1.1 inf., 0.75 +/-1.0 mm post.; unstable: 1,31+/-0.87 mm inf., 0.51+/-1.28 mm post.) The m aximal translation (to anterior and inferior) was observed both on the heal thy side (mean 1.0 mm, max. 1.8 mm) and in the unstable shoulders (mean 2.5 mm, max. 4.6 mm) with the arm in 90 degrees of abduction and external rota tion, thus being 1.7 to 2.5 times higher in the pathological shoulders. Con clusions: With this technique the glenohumeral translation can be quantifie d three-dimensionally in functionally important positions and without proje ctional artefacts. In the future, this method can be applied to patients wi th different entities of shoulder instability.