Monday, April 16, 2012

Reverse total shoulder mechanics

Reverse shoulder
Reverse total shoulder

JSES recently published an in vitro study of the effect of deltoid tension and humeral version in reverse total shoulder arthroplasty.

In this cadavaric study, the authors examined the effect of increased deltoid tension (achieved by increasing implant thickness) and humeral version on the in vitro mechanics of the shoulder. They found that the force necessary for dislocation was not significantly affected by implant thickness or version.

In a previous posts we discussed the relationship between increased humeral length and acromion/spine fractures and nerve injuries, suggesting that maximizing deltoid tension may not be the safest way to achieve stability in a reverse total shoulder.

The authors mention briefly the issue of 'inferior impingement' as a mechanism for instability of reverse total shoulders (this is the same mechanism that results in notching as discussed before).  In this study, the authors used the Tornier Aequalis Reverse prosthesis. With respect to 'inferior impingement' it is of interest to examine the radiograph on the prosthesis website, reproduced below, which shows a slightly abducted arm with the prosthesis in place. We've added an arrow to show the 'inferior impingement'. It seems possible that this 'inferior impingement' may have at least three undesirable effects: (1) limitation of adduction, (2) notching and (3) instability due to levering apart of the two joint surfaces with adduction.


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