Shoulder osteoarthritis (OA) is a frequent problem in our aging population and is believed to occur in up to 20% of the population. OA leads to pain, restriction of movement and functional disability. Furthermore, chronic instability, characterized by humeral head migration (HHM), and pathologic changes of the glenoid bone, i.e. glenoid erosions, is common in OA. Different types of glenoid morphology are associated with shoulder OA, depending on the amount and localization of the glenoid erosion as well as the amount and direction of HHM. On the basis of these factors glenoids are classified in A, B, C and D types 4. In this classification A glenoid is a centered or symmetric arthritis without posterior subluxation of the humeral head. The B glenoid is characterized by asymmetric bone erosion, leading to glenoid retroversion, combined with posterior HHM. The D glenoid is defined by glenoid anteversion or anterior humeral head subluxation, while C glenoid is a dysplastic glenoid with at least 25° of retroversion "not caused by erosion.

Total shoulder arthroplasty (TSA) has been shown to relieve the pain and improve joint function of patients with OA. However, several complications such as component loosening and polyethylene damage has been reported and it has been revealed that 7.3% of glenoids may show signs of asymptomatic radiographic loosening annually after primary anatomic TSA. Although, aberrant glenohumeral contact mechanics has been suggested to be one of the primary potential causes, the mechanism of such fixation failure is still unclear. Glenohumeral conformity, eccentric loads associated with shoulder instability, bone quality, cementing techniques, implant orientation and design are all confounding factors that indeed affect the mechanical environment of the glenoid component, and, more specifically, also in terms of the contact mechanics.

Knowledge of in vivo glenohumeral joint contact mechanics before and after total shoulder arthroplasty and its interplay with patient- and surgery-related parameters may unravel the high rate of glenoid component loosening in patients with osteoarthritic glenoids and provide insight for the improvement of patient function, implant designs, implant longevity, and surgical technique. Previous ex-vivo and computational modelling studies that focused on the glenohumeral contact area after TSA, suggested that arm elevation in the scapular plane results in aberrant posterior translation of the humeral head in TSA patients. However, an in-vivo study based on dual-plane fluoroscopy, demonstrated that this only occurs in 50% of TSA patients. These discrepancies could arise from analyzing an assorted population of TSA patients without taking into account any specific information with regards to their preoperative osseous and soft-tissue status such as type of glenoid erosion, degree of glenoid retroversion, and amount of rotator cuff degeneration. Postoperative factors such as degree of retroversion correction, amount of joint-line medialization and glenohumeral components' radial mismatch were also not considered in these studies.

The main goal of this study is evaluating in-vivo glenohumeral contact patterns in patients with osteoarthritic glenoids before and after TSA, to unravel the high rate of glenoid component loosening. To that aim, the project is divided into four sub-objectives. The first objective is to compare glenohumeral contact areas before and after surgery in patients with osteoarthritic glenoids, using EOS (micro-dose x-ray) stereo radiographic imaging, to evaluate whether osseous correction during surgery is able to correct glenohumeral joint kinematics. The second objective is to compare OA patient's glenohumeral contact areas before and after surgery with healthy subjects. The third objective is to identifying variations in glenohumeral contact patterns after surgery with respect to the pre-and postoperative state of the patients. The last objective is to evaluate the influence of the location of the glenohumeral contact area after surgery on the long-term outcome and survival of the glenoid component.