Tuesday, March 11, 2014

Reverse total shoulder - effect of center of rotation on muscle function

Reverse total shoulder arthroplasty component center of rotation affects muscle function

These authors point out that medialization of the glenohumeral center of rotation in reverse total shoulder arthroplasty alters the moment arm of the deltoid, slackens the remaining external rotators, and increases the risk for scapular notching due to unwanted contact between the humeral component and the glenoid.

They used scapulohumeral bone models reconstructed from computed tomography scans and virtually implanted with primary or reverse total shoulder arthroplasty implants. The placement of the glenosphere was varied to simulate differing degrees of “medialization” and inferior placement relative to the glenoid. Muscle and joint forces were computed during shoulder abduction using musculoskeletal modeling software.

This study quantified some tradeoffs in selecting optimum placement of the glenosphere in reverse total shoulder.  Medialization reduces the bending moment at the implant-bone interface but requires inferior placement to reduce the risk for impingement and scapular notching. Lateralization with a bone graft can reduce the risk for impingement but carries the risk of higher bending moment at the host-graft interface.

Comment: We have posted before on the relationship of glenosphere center of rotation on the kinematics of reverse total shoulder:

Lateralization in reverse total shoulder

Reverse shoulder arthroplasty leads to significant biomechanical changes in the remaining rotator cuff.

The change in center of rotation with reverse total shoulder arthroplasty and its application to a consecutive series of 68 shoulders having reconstruction with one of two different reverse prostheses.

Effects of glenosphere positioning on impingement-free internal and external rotation after reverse total shoulder arthroplasty

We prefer the technique shown here because the prosthesis offers lateral offset while also offering secure fixation of the glenoid component to bone. Lateralization is desirable not only because it reduces the risk of notching and instability from unwanted humeroscapular contact, but also because it restores the tension in any residual external rotators (which are slackened by medialization). Using modern designs, a bone graft is not needed to achieve lateralization.

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