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Range of Impingement-Free Abduction and Adduction Deficit After Reverse Shoulder Arthroplasty

Hierarchy of Surgical and Implant-Design-Related Factors

¹ Florida Orthopaedic Institute Research Foundation, 13020 North Telecom Parkway, Tampa, FL 33637. E-mail address for M.A. Frankle: frankle{at}pol.net

Investigation performed at the Phillip Spiegel Orthopaedic Research Laboratory at the Florida Orthopaedic Institute Research Foundation, Tampa, Florida

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants of less than $10,000 from Encore Medical. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Encore Medical). Also, a commercial entity (Encore Medical) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

Methods: A computer model was developed to virtually simulate abduction/adduction motion and its dependence on five surgical and implant-related factors. Three conditions were tested for each factor, resulting in a total of 243 simulated combinations. The overall motion was determined from 0° of abduction until maximum abduction, which would be limited by impingement of the humerosocket on the scapula. In those combinations in which 0° of abduction could not be achieved, the adduction deficit was recorded.

Results: The largest average increase in the range of impingement-free abduction motion resulted from a more lateral center-of-rotation offset: the average increase was 31.9° with a change in the center-of-rotation offset from 0 to 10 mm, and this change resulted in an increase in abduction motion in eighty of the eighty-one combinations. The position of the glenosphere on the glenoid was associated with the second largest average increase in abduction motion (28.1° when the glenosphere position was changed from superior to inferior, with the change resulting in an increase in seventy-one of the eighty-one combinations). These factors were followed by glenosphere tilt, humeral neck-shaft angle, and prosthetic size in terms of their effects on abduction motion. The largest effect in terms of avoiding an adduction deficit was provided by a humeral neck-shaft angle of 130° (the deficit was avoided in forty-nine of the eighty-one combinations in which this angle was used), followed by an inferior glenosphere position on the glenoid (deficit avoided in forty-one combinations), a 10-mm lateral offset of the center of rotation, inferior tilt of the glenosphere, and a 42-mm-diameter prosthetic size.

Conclusions: An understanding of a hierarchy of prosthetic design and implantation factors may be important to maximize impingement-free abduction motion as well as to avoid inferior impingement.

Clinical Relevance: A better understanding of these factors may lead to a reduction in inferior scapular notching and improvements in impingement-free motion after reverse shoulder replacement.

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