¹Steadman Philippon Research Institute, Vail, USA
²Department for Shoulder and Elbow Surgery, Charité – Universitätsmedizin Berlin, Berlin, Deutschland
³The Steadman Clinic, Vail, USA

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Objectives and questions: The use of preoperative three-dimensional (3D) planning and patient-specific instrumentation for reverse total shoulder arthroplasty (rTSA) has gained in popularity. There exists a paucity of data on the influence of component selection on postoperative range of motion (ROM). The purpose of the present study was to evaluate the effect of glenosphere lateralization or inferiorization and humeral offset on postoperative ROM using a 6-degrees- of-freedom (6-DOF) robotic arm.

Material and methods: Twelve cadaveric shoulders underwent computed tomography. Reverse total shoulder arthroplasty implantation was planned using 3D software. Each shoulder was dissected of all soft tissue and rTSA (Univers ReversTM, Arthrex Inc.) implantation was performed using patient-specific instrumentation. Each specimen was mounted to a 6-DOF robotic arm and tested with 3 different glenospheres (standard, +4 mm lateralized or +2.5 mm inferiorized) and non-constrained humeral liners (+3, +6 or +9 mm) for a total of 9 configurations. Robotic assessment consisted of ROM testing in abduction, flexion and adduction under 5 Nm torque. To match the repeated measures design, a 2-factor linear mixed effect model was created for each test whereby the choice of the glenosphere was a categorical factor and humeral offset was a continuous factor.

Results: The statistical model showed no interaction between the choice of the glenosphere and humeral liner, so their effects were analyzed separately. Lateralizing the glenosphere by 4.0 mm or inferiorizing it by 2.5 mm significantly improved abduction ROM (+3.6° and +3.5°, p < 0.001, respectively), flexion ROM (+1.7° [p = 0.015] and +4.0° [p < 0.001], respectively) and adduction ROM (+4.0° and +4.9°, p < 0.001, respectively) when compared with the standard glenosphere. Glenosphere inferiorization was associated with an increase in flexion ROM by 2.3° (p < 0.001) when compared with the lateralized glenosphere. Increasing humeral offset significantly reduced abduction (-0.5°/mm, p < 0.001) and flexion ROM (-0.9°/mm, p < 0.001), but did not significantly affect adduction ROM.

Discussion and conclusions: The implant configuration that was associated with the most favorable ROM was a +2.5 mm inferiorized glenosphere, which led to significant improvements in all three ROM, and a +3 mm humeral liner, which was associated with improved abduction and flexion ROM.