Glenohumeral Mismatch in Anatomic Total Shoulder Arthroplasty
Anatomic shoulder arthroplasty was originally introduced in 1952 by Neer1. The original Neer anatomic total shoulder prosthesis was designed with a glenohumeral radial mismatch of zero, where both the humeral-head and glenoid components had equivalent radii of curvature2. A conforming articulation was chosen on the basis of the theoretical advantages of lowering contact stresses and linear polyethylene wear. However, the native humerus and glenoid do not form a perfectly congruent articulation in the non-diseased state3,4. This incongruency allows both surface translation and rotation about 5 axes of freedom. Measurable translations exhibited by the normal shoulder also occur within all shoulder prostheses, regardless of the radial mismatch5,6. Although surface translation may better replicate native shoulder joint mechanics, these translations also lead to shear forces, which may have undesired effects on polyethylene. These trade-offs have led surgeons to debate the role of radial mismatch between the humerus and the glenoid7,8. Conformity is defined as the relationship of the humeral head and glenoid surface radii of curvature (radial mismatch)9. A conforming joint would have the same radii of curvature, and a mismatched articulation would have different radii of curvature between the humeral and glenoid surfaces. This should not be confused with constraint, which is related to the height of the glenoid rim and the perimeter of the humeral head covered by the glenoid articulation9.