BACKGROUND:
Various reverse total shoulder arthroplasty (rTSA) implant options are available for the humeral and glenosphere components. This study used a cadaveric biomechanical shoulder simulator to evaluate how hardware configurations in 2 common rTSA systems affect (1) abduction/adduction range of motion (ROM), (2) rotational ROM, and (3) forces to elevate the arm.

METHODS:
Seven pairs of shoulders were tested on a biomechanical shoulder simulator before and after rTSA implantation. The Aequalis Reverse Shoulder (Tornier, Edina, MN, USA) and the Reverse Shoulder Prosthesis (RSP; DJO Surgical, Austin, TX, USA) were implanted in opposing shoulders. Aequalis implant options included humeral polymer insert thickness and eccentricity and glenosphere tilt. RSP implant options included glenosphere diameter and lateralization, humeral shell offset, and polymer insert depth.

RESULTS:
Both the RSP and Aequalis shifted the center of rotation inferior and medially compared with native shoulders (P < .001). Increased Aequalis insert thickness reduced adduction (P < .003) and internal/external (P < .028) passive ROM. The 10° inferiorly tilted glenosphere increased deltoid abduction forces (P < .032). In the RSP, smaller glenosphere diameter (P < .012), a semiconstrained humeral insert (P < .023), and a neutral humeral shell offset (P < .002) all decreased adduction deficit, whereas lateral glenosphere offset increased passive abduction ROM (P < .028). Increased humeral shell offset decreased passive internal/external rotation ROM (P < .050).

DISCUSSION:
Hardware configurations in rTSA have different effects on passive ROM and deltoid forces required for abduction. Identifying these changes may guide surgical decision making during rTSA placement.





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