Is preoperative 3D planning reliable for predicting postoperative clinical differences in range of motion between two stem designs in reverse shoulder arthroplasty

Background: We aim to predict a clinical difference in the postoperative range of motion (RoM) between 2 reverse shoulder arthroplasty (RSA) stem designs (Inlay-155 degrees and Onlay-145 degrees) using preoperative planning software. We hypothesized that preoperative 3D planning could anticipate the differences in postoperative clinical RoM between 2 humeral stem designs and by keeping the same glenoid implant. Methods: Thirty-seven patients (14 men and 23 women, 76 +/- 7 years) underwent a BIO-RSA (bony increased offset-RSA) with the use of preoperative planning and an intraoperative 3-dimensional-printed patient-specific guide for glenoid component implantation between January 2014 and September 2019 with a minimum follow-up of 2 years. Two types of humeral implants were used: Inlay with a 155 degrees inclination (Inlay-155 degrees) and Onlay with a 145 degrees inclination (Onlay-145 degrees). Glenoid implants remained unchanged. The postoperative RSA angle (inclination of the area in which the glenoid component of the RSA is implanted) and the lateralization shoulder angle were measured to confirm the good positioning of the glenoid implant and the global lateralization on postoperative X-rays. A correlation between simulated and clinical RoM was studied. Simulated and last follow-up active forward flexion (AFE), abduction, and external rotation (ER) were compared between the 2 types of implants. Results: No significant difference in RSA and lateralization shoulder angle was found between planned and postoperative radiological implants' position. Clinical RoM at the last follow-up was always significantly different from simulated preoperative RoM. A low-tomoderate but significant correlation existed for AFE, abduction, and ER (r = 0.45, r = 0.47, and r = 0.57, respectively; P < .01). AFE and abduction were systematically underestimated (126 degrees +/- 16 degrees and 95 degrees +/- 13 degrees simulated vs. 150 degrees +/- 24 degrees and 114 degrees +/- 13 degrees postoperatively; P < .001), whereas ER was systematically overestimated (50 degrees +/- 19 degrees simulated vs. 36 degrees +/- 19 degrees postoperatively; P < .001). Simulated abduction and ER highlighted a significant difference between Inlay-155 degrees and Onlay-145 degrees (12 degrees +/- 2 degrees, P = .01, and 23 degrees +/- 3 degrees, P < .001), and this was also retrieved clinically at the last follow-up (23 degrees +/- 2 degrees, P = .02, and 22 degrees +/- 2 degrees, P < .001). Conclusions: This study is the first to evaluate the clinical relevance of predicted RoM for RSA preoperative planning. Motion that involves the scapulothoracic joint (AFE and abduction) is underestimated, while ER is overestimated. However, preoperative planning provides clinically relevant RoM prediction with a significant correlation between both and brings reliable data when comparing 2different types of humeral implants (Inlay-155 degrees and Onlay-145 degrees ) for abduction and ER. Thus, RoM simulation is a valuable tool to optimize implant selection and choose RSA implants to reach the optimal RoM.