Types of Shoulder Arthroplasties
Generally speaking there are two main categories of shoulder arthroplasty:
- Total shoulder replacement
A hemiarthroplasty replaces the head of the humerus with an intramedullary stemmed implant. It consists of a humeral component only. This is used when the native glenoid surface is in good condition. Indications include:
- Osteonecrosis of the humeral head without associated secondary degenerative arthritis
- Recent four-part and head-splitting fractures of the proximal humerus
- Recent three-part fractures of the humeral head in the elderly
- Certain neoplasms of the proximal humerus
- insufficient glenoid bone stock to support a glenoid component
- Glenohumeral osteoarthritis with massive rotator cuff tear. In this scenario, the goal of treatment is predominantly pain relief, as functional improvements after hemiarthroplasty are modest. Total shoulder arthroplasties have a high rate of glenoid component loosening. Hemiarthroplasty surgery offers excellent pain relief for those patients without glenoid component complications.
Bipolar hemiarthroplasty. The humeral component is interlocked with a larger metal-backed polyethylene shell that articulates with the native glenoid.
Total shoulder replacements (TSR) employ both humeral and glenoid components. They are used in patients with arthritis where the humeral head and glenoid surfaces are damaged. TSR designs are classified as:
- Non-constrained: These prostheses have no physical link between the humeral and glenoid components and rely on the surrounding musculotendinous cuff for stability. The glenoid component conforms to the humeral head. The components attempt to recreate normal anatomy and relationships. This minimizes stresses at interfaces and allows for early rehabilitation. Precise surgical technique is critical for success, with proper tension of capsular, muscular, and tendinous surfaces needed for stability.
- Constrained: Initial first-generation designs for total shoulder replacements were constrained, meaning the humeral and glenoid components were mechanically coupled around a fixed center of rotation. These were used in patients with rotator cuff deficiencies and helped prevent superior migration of the the humeral component. This design causes most forces to be borne by the prosthesis and interfaces instead of the surrounding soft tissues, leading to higher rates of fracturing of components and loosening. Today, this design type is rarely employed.
Stanmore constrained shoulder prosthesis. Appearance is very similar to hip replacement. The humeral head snap fits into the glenoid component. Forces are transmitted away from the joint into the surrounding bone leading to high rates of loosening.
- Semi-constrained: These prostheses are similar to non-constrained designs except that the superior glenoid component has a superior extension that prevents superior migration of the humeral component.