The relationship between shoulder osteology and glenohumeral axial rotation.

The range of motion of the Glenohumeral joint (GHJ) varies between individuals and depends on the position of the humerus. Currently there is limited understanding of the causes of this variation. An improved understanding of the constraints of the GHJ and their influence on the range of motion of the GHJ can be used in the clinical assessment of the shoulder, shoulder rehabilitation and shoulder implant design. This thesis aims to investigate the relationship between the bone geometry of the GHJ and the axial rotational range of the joint. A protocol to measure the maximum internal-external rotation of the GHJ was developed to quantify the axial rotational range of the GHJ during active motion and during passive motion in response to a measured torque. The protocol was used to investigate the effect of plane and elevation angle on the axial rotational range of the GHJ in 30 asymptomatic subjects. The intra-subject, inter-subject, intra-session and inter-session reliabilities of the protocol were quantified. Two-dimensional and three-dimensional bone geometrical parameters acquired from MRI scans of the shoulder were used to describe normal variation in the shape of the GHJ. A linear regression expression was formulated at each humeral position using three geometrical parameters to predict the maximum active and passive internal and external rotations. Segmented MRI scans were used to create a bone model of the subject’s shoulder and collision detection investigated points of bony collision which were likely to limit the subject’s maximum internal and external rotation. Two-dimensional geometrical parameters of the humerus, articular cartilage, glenoid, acromion and coracoid provide an accurate estimate of the maximum internal and external rotation of the GHJ. These parameters and the observed bony constraints from collision detection improve understanding of how the shape of the humerus and scapula at the GHJ affect the range of motion that can be achieved. Different bony constraints at the GHJ lead to changes in the range of motion with humero-thoracic elevation angle and elevation plane; whilst variation in the geometrical parameters between subjects result in differences in the maximum internal and external rotation between individuals