Friday, May 31, 2013

Positioning of the glenosphere in reverse total shoulder

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

These authors obtained computed tomography scans of the scapula and humerus from 7 cadaveric specimens, and simulated 3-dimensional reconstructions with reverse total shoulder models.
For each glenosphere position, the humerus was allowed to freely rotate at a fixed scaption angle (0°, 20°, 40°, and 60°) until encountering bone-to-bone or bone-to-implant impingement. Superior positioning blocked rotation. Inferior positioning and lateralization improved rotation. 

The authors concluded from this model that optimal glenosphere positioning was achieved with inferior translation, inferior tilt, and lateralization in all degrees of scaption. 

Comment: But range of motion is not the only consideration. As we've stated before, the concern about inferior tilt is that the base plate is no longer seated on quality subchondral bone as shown in this figure from the article.
Thus our favored positioning puts the inferior aspect of the glenosphere at the inferior margin of the glenoid bone and to use a component with lateral offset. We place the glenosphere flat on the best quality bone with minimal reaming to preserve the precious glenoid bone stock.

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