Background: Several in vitro studies have investigated the biomechanics of reverse total shoulder arthroplasty (RTSA); however, few in vivo studies exist. The purpose of this study was to examine in vivo RTSA contact mechanics in clinically relevant arm positions. Our hypothesis was that contact would preferentially occur in the inferior region of the polyethylene liner.
Methods: Forty patients receiving a primary RTSA were recruited for a prospective cohort study. All patients received the same implant design with a non-retentive liner. Stereo radiographs were taken at maximal active range-of-motion. Model-based radiostereometric analysis was used to identify implant position. Contact area between the polyethylene and glenosphere was measured as the geometric intersection of the two components and compared between polyethylene size, arm position, and relative position within the liner.
Results: There were no differences in the proportion of contact area in any arm position between polyethylene liner sizes, ranging from 30±17 % to 38±23 % for 36 mm liners, and 32±21 % to 41±25 % for 42 mm liners. Contact was equally distributed between superior and inferior halves of the liner at each arm position (p=0.061-0.791), however, greater contact area was observed in the outer radius of the liner when the arm was flexed (p=0.002).
Conclusion: This study highlights that contact mechanics are similar between 36 and 42 mm liners. Contact area is generally equally distributed throughout the liner throughout the range of motion, and not preferentially in the inferior region as hypothesized.
Level of evidence: Basic Science Study; Kinesiology.
Keywords: BIO-RSA; biomechanics; contact mechanics; glenosphere lateralization; radiostereometric analysis; reverse total shoulder arthroplasty.
Copyright © 2020. Published by Elsevier Inc.