The purpose of this study is to determine whether there is a difference between the result of treatment of shoulder osteoarthritis with two different shoulder prostheses.

In this randomized study, we wish to investigate with the use of clinical and radiological parameters whether there is a difference between the result of treatment of shoulder osteoarthritis with the Copeland resurfacing prosthesis and the Global CAP resurfacing prosthesis.

Clinical parameters with the help of the Constant Shoulder Score (CSS) and Western Ontario Osteoarthritis of the Shoulder Index (WOOS):

1. Pain
2. Activities of Daily Living (ADL)
3. Range of Motion (ROM)
4. Muscle strength

Radiologic parameters:

1. Migration of resurfacing prostheses with use of MB-RSA
2. Bone density around the prosthesis measured with Dual Energy X-ray Absorptiometry (DEXA)
3. Correlation between bone density and prosthesis migration

Inclusion Criteria:

1. Patients with shoulder osteoarthritis with cartilage defect of the caput humeri and not the glenoid cavity
2. Aged 18 years or older and fit
3. Informed, written consent -

Exclusion Criteria:

1. Patients found unsuitable preoperatively for a resurfacing prosthesis
2. Patients aged 85 or older
3. Patients with rheumatoid arthritis
4. Patients who previously had undergone shoulder alloplasty or other major shoulder surgery (more than just diagnostic arthroscopy)
5. Patients unable to avoid NSAID after surgery
6. Patients requiring regular systemic steroid treatment
7. Female patients taking hormone substitution
8. Patients with metabolic bone disease
9. Severe shoulder instability with large rotator cuff defect ( will often necessitate special prosthesis components or even major soft-tissue surgery)
10. Female patients in the fertile age range who do not use safe anti-conception (oral contraception, intrauterine devices, depot gestagens, vaginal hormone rings) -