Glenohumeral joint kinematics related to minor anterior instability of the shoulder at the end of the late preparatory phase of throwing

Objective. The first aim of this study was an approach to quantify the 3D kinematics of the glenohumeral joint referred to the joint surfaces. The method was used to study the glenohumeral patho-arthrokinematics related to minor anterior instability at the end of the late preparatory phase of throwing. Study design. Using a finite helical axis approach, arthrokinematics focused on: (i) the rotations and shift of the humeral head on the glenoid cavity, and (ii) the migration of contact of the articular surfaces. Background. Controversy still exists whether the clinical syndrome called ‘minor anterior glenohumeral instabilityʹ can be validly termed as an instability. Methods. Helical CT-data of discrete shoulder positions were three-dimensionally reconstructed. Based on humeral and scapular sets of skeletal landmarks, rotation matrices and translation vectors were estimated and processed in glenohumeral finite helical axes. The finite helical axis parameters of rotation, shift and direction were related to a co-ordinate system embedded on the glenoid, whereas the position of the finite helical axis was related to the articulating surface of the humeral head. Results. From 90° abduction and 90° external rotation to full cocking (90° abduction with full external rotation and horizontal extension), the humeral head in the normal shoulders did not externally/internally rotate on the glenoid. In contrast, a large external rotation component was found in the minor unstable shoulders. The geometrical centre of the humeral head of the normal shoulders translated into a posteriorized position on the glenoid, whereas in minor anterior instability it translated centrally on the glenoid. Conclusions. Compared with in vitro biomechanical research which states that towards full cocking the anterior part of the inferior glenohumeral ligament limits anterior translation and external rotation of the humeral head on the glenoid, the results suggest in minor anterior instability a dysfunction of the anterior part of the inferior glenohumeral ligament.