Saturday, October 21, 2017

Reverse total shoulder - failure by polyethylene dissociation from humeral stem

Polyethylene dissociation from humeral stem status after reverse total shoulder arthroplasty

These authors present 4 cases of polyethylene dissociation from the humeral component after a Zimmer Trabecular Metal reverse total shoulder.

The manufacturer's recommended method for securing the polyethylene insert is show in this link technique.

The authors suggest that surgeons should be aware of this possibility if a closed reduction of a dislocated reverse total shoulder is not possible. They point out that dissociation may be difficult to distinguish on radiographs; however, subtle clues can be present, including decreased spacer distance between the glenosphere and humeral stem as well as a soft tissue shadow caused by the radiolucent polyethylene.

All 4 patients were male; all patients underwent surgery with the Zimmer TM RSA (although this complication has been seen with other designs). There was a design change that affected the polyethylene humeral stem junction and the ease of placement for this specific product; however, the complications occurred both before and after this design change. 

Comment: The direction of forces applied to a reverse total shoulder frequently deviate from the what is desired for concavity compression. As a result, potentially displacing loads are applied to each element of the system: the baseplate, the glenosphere, the humeral polyethylene, and spacers. These loads can challenge each component junction: "each junction is a potential disjunction". 

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