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

Objective. The first aim of this study was an approach tol quantify the 3D kinematics of the glenohumeral joint referred to the joint surfaces. The me thod was used to study the glenohumeral patho-arthrokinematics related to m inor anterior instability at the end of the late preparatory phase of throw ing. Study design. Using a finite helical axis approach, arthrokinematics focuse d 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 instability' can be validly termed as an instab ility. Methods. Helical CT-data of discrete shoulder positions were three-dimensio nally reconstructed. Based on humeral. and scapular sets of skeletal landma rks, rotation matrices and translation vectors were estimated and processed in glenohumeral finite helical axes. The finite helical axis parameters. o f rotation, shift and direction were related to a co-ordinate system embedd ed on the glenoid, whereas the position of the finite helical axis was rela ted to the articulating surface of the humeral head. Results. From 90 degrees abduction and 90 degrees external rotation to full cocking (90 degrees abduction with full external rotation and horizontal e xtension), the humeral head in the normal shoulders did not externally/inte rnally rotate on the glenoid. In contrast, a large external rotation compon ent was found in the minor unstable shoulders. The geometrical centre of th e humeral head of the normal shoulders translated into a posteriorized posi tion on the glenoid, whereas in minor anterior instability it translated ce ntrally on the glenoid. Conclusions. Compared with in vitro biomechanical research which states tha t towards full cocking the anterior part of the inferior glenohumeral ligam ent limits anterior translation and external rotation of the humeral head o n the glenoid, the results suggest in minor anterior instability, a dysfunc tion of the anterior part of the inferior glenohumeral ligament.