Sunday, November 2, 2014

Reverse total shoulder - function of two design types

Reverse total shoulder arthroplasty: a biomechanical evaluation of humeral and glenosphere hardware configuration

These authors used a cadaveric biomechanical shoulder simulator to evaluate options within the  Aequalis Reverse Shoulder and the Reverse Shoulder Prosthesis systems with respect to their effect on abduction/adduction range of motion, rotational ROM, and forces to elevate the arm.

Both designs resulted in major shifts of the center of rotation inferiorly and medially compared with native shoulders.
This result is demonstrated in a study we have published previously that included this figure showing the preoperative-postoperative change in the center of rotation with the two systems.

They found that hardware configurations in rTSA have different effects on passive ROM and deltoid forces required for abduction. 

Comment: There are many different designs of reverse total shoulder components and within each design system there are many prosthetic options. Each system has its unique features and a substantial learning curve. The wide range of shoulder problems and patients with these problems requires the surgeon to be thoughtful about the application of this technology and the surgical priorities.

While we are to a degree informed by cadaver model results such as those reported here, the surgical priorities are mostly governed by other considerations. Our practice is to use a system that provides excellent immediate fixation of the glenosphere (without having to wait for bone ingrowth), variable glenosphere diameters of curvature and neck length allowing for East-West tensioning and avoidance of scapular notching. At surgery, we use larger genosphere diameters when stability is in question and smaller diameters when stability is not an issue. Using East-West tensioning to achieve stability may help avoid the acromial and neurological consequences of excessive South tensioning. Before selecting the final components, the shoulder is checked in a full complement of positions to identify and resolve unwanted contact between the humeral component and the scapula.


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