Mechanics of Glenohumeral Arthroplasty.

Smoothness

Providing joint smoothness

Providing joint smoothness is a primary objective of shoulder arthroplasty. In the presence of an intact glenoid fossa covered with good articular cartilage, a humeral hemiarthroplasty should suffice. The articular cartilage may be assessed by preoperative radiographs and at surgery by observation, palpation, and by listening to the sound when it is struck with a small blunt elevator: thin cartilage or bare bone will cause the elevator to ring, while normal cartilage will yield only a dull "thunk."

In glenohumeral arthroplasty, joint smoothness is provided by the metal on polyethylene articulation. Care must be taken to ensure the absence of nonarticular contact between humeral bone and the prosthetic glenoid. Inferior or posterior humeral osteophytes can present a particular problem in this regard.

In hemiarthroplasty for cuff tear arthropathy, the undersurface of the "acetabularized" coracoacromial arch is usually polished smooth with a consistent diameter of curvature. The prosthetic humeral articular surface and the tuberosities must provide a smooth congruent surface to mate with this arch. Achieving this goal requires attention to the selection and positioning of the prosthetic humeral joint surface so that it replicates that of the joint surface that is excised. The tuberosities are sculpted so that they are congruent with the prosthetic joint surface. We hypothesize that the large smooth joint contact area achieved in this procedure decreases joint contact pressures and is thus responsible for its success in restoring comfort and function in the difficult problem of cuff tear arthropathy.

The arthroplasty must also establish smoothness at the nonarticular humeroscapular motion interface. Scar, adhesions, and hypertrophic bursa must be excised. The sites of reattachment of the rotator cuff, including the subscapularis, must slide smoothly against the outer aspect of the motion interface. Immediate postoperative motion may be helpful in preventing the reformation of scar and adhesions in this motion interface.

Disclaimer

This resource has been provided by the University of Washington Department of Orthopaedics and Sports Medicine as general information only. This information may not apply to a specific patient. Additional information may be found at http://www.orthop.washington.edu or by contacting the UW Department of Orthopaedics and Sports Medicine.

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