• American Shoulder And Elbow Surgeons Shoulder Index (ASES) [ Time Frame: 12 months post-operative ] [ Designated as safety issue: No ]
  

• Constant Score [ Time Frame: 12 months post-operative ] [ Designated as safety issue: No ]
  
• Western Ontario Osteoarthritis of the Shoulder Index (WOOS) [ Time Frame: 12 months post-operative ] [ Designated as safety issue: No ]
  
• Simple Shoulder Test (SST) [ Time Frame: 12 months post-operative ] [ Designated as safety issue: No ]
  
• Radiographic Evaluation [ Time Frame: 12 months post-operative ] [ Designated as safety issue: No ]
  
• Early post-operative pain [ Time Frame: 6 weeks post-operative ] [ Designated as safety issue: No ]
  

Resurfacing arthroplasty is gaining popularity for the reconstruction of advanced shoulder arthritis. New generation re-surfacing prostheses are engineered to anatomically restore a smooth articulating surface to the humeral head, while preserving bone stock, and potentially minimizing the risk for diaphyseal fracture. However, the current gold-standard for joint replacement is a modular prosthesis with a stem extending into the bony diaphysis of the humerus. Long-term fixation has proven very reliable with these implants. The results for Shoulder Resurfacing prostheses have largely been reported in retrospective reviews and have been very favorable. The purpose of this multi-centered, randomized, controlled trial will be to determine if patient self-assessed outcomes (American Shoulder and Elbow Society (ASES) Score) following humeral resurfacing are superior to those following a conventional, modular, stemmed, humeral prosthesis. The first phase of this study will be a one year pilot recruitment period. This is necessary in order to determine both a center by center, and national recruitment rate, and to determine a sample size necessary for a sufficiently powered full randomized trial. If a full randomized trial is not considered feasible following Phase 1, then a prospective cohort study will be completed.

Pre-Operative Inclusion Criteria:

• Failed standard non-surgical management (analgesics, nsaids, physiotherapy, activity modification)
• Persistent pain and disability for at least 6 months
• Imaging demonstrates advanced humeral head cartilage loss

Pre-Operative Exclusion Criteria:

• Active or systemic joint infection
• Significant muscle paralysis of shoulder girdle
• Charcot's Arthropathy
• Major medical illness (life expectancy <2years)
• Unable to speak and read English/French
• Psychiatric illness that precludes informed consent
• Unwilling/unable to remain in study for one year

Intra-Operative Inclusion Criteria:

• Advanced humeral head cartilage loss AND ONE of:
• minimal/no glenoid cartilage loss
• complete glenoid cartilage loss and no bony erosion
• complete glenoid cartilage loss and concentric bony erosion with no humeral anterior/posterior subluxation

Intra-Operative Exclusion Criteria:

• Eccentric glenoid erosion with biconcavity of glenoid and subluxation of humeral head
• Inability of humeral head to support the CAP surface replacement