Similar optimal distalization and lateralization angles can be achieved with different reverse shoulder arthroplasty implant designs

Background: Optimal postoperative distalization (DSA) and lateralization (LSA) shoulder angles have been described as radiological measurements correlated with function after reverse shoulder arthroplasty (RSA). The proposed optimal values are DSA between 40 degrees and 65 degrees and LSA between 75 degrees and 95 degrees; however, whether these values can be reached with different implant designs is unclear. Aim: To determine which RSA implant could achieve higher rates of optimal DSA and LSA, to determine any association between each implant and optimal DSA and LSA, and to assess the correlation of the preoperative critical shoulder angle (CSA) and acromial index (AI) with the DSA and LSA. Methods: This was a retrospective comparative study of all patients who underwent primary RSA for rotator cuff arthropathy. Three RSA implant designs were included, based on which patients were divided into three groups: group 1 (FH Arrow (TM); 16 patients) comprised onlay implants with a 135 degrees neck-shaft angle (NSA); group 2 (Biomet Comprehensive (TM); 20 patients) comprised onlay implants with a 147 degrees NSA; group 3 (Mathys Affinis (TM); 15 patients) comprised inlay implants with a 155 degrees NSA. The AI and CSA were measured on preoperative ra-diographs. The DSA and LSA were measured on true AP postoperative radiographs. Results: The mean DSAs were 44 degrees, 46 degrees, and 46 degrees (P = 0.671) and the mean LSAs were 92 degrees, 91 degrees, and 82 degrees for groups 1, 2, and 3, respectively (P = 0.003). Group 3 had lower LSA than groups 1 (- 10 degrees; P = 0.005) and 2 (-9 degrees; P = 0.002). Optimal DSA and LSA were achieved in 71% and 73% of all arthroplasties, respectively. No association between implant designs and achieving an optimal DSA or LSA was observed. None of the implant designs had DSA >65 degrees. The most common cause of failure to reach an optimal LSA in onlay implants was a combination of LSA >95 degrees and DSA <40 degrees. The most common cause of failure in group 3 was DSA <40 degrees. The DSA was negatively correlated with the AI (-0.384; P = 0.006) and CSA (-0.305; P = 0.033). No correlation was observed between the LSA and AI (P = 0.312) or CSA (P = 0.137). Conclusion: The LSA is lower with the Mathys Affinis (TM) implant than with the Biomet Comprehensive (TM) and FH Arrow (TM) implants; however, most LSAs are in the optimal ranges, and no association is observed between different implant designs and optimal DSA and LSA. Implant design, CSA, and AI should be considered during preoperative planning to achieve optimal DSA and LSA.