Lateralization and Distalization Shoulder Angles in Reverse Shoulder Arthroplasty: Are They Still Reliable and Accurate in All Patients and for All Prosthetic Designs?

Background: Recently, the lateralization shoulder angle (LSA) and distalization shoulder angle (DSA) have been proposed to assess lateralization and distalization in reverse shoulder arthroplasty (RSA). However, there is insufficient evidence about the influence of patient anatomy and prosthesis design on these measurements. This study aims to investigate the impact of patient anatomy and implant design on LSA and DSA measurements and to assess the validity of the previously reported "optimal" ranges for these parameters. Methods: Patients who underwent the RSA procedure using four different prosthetic designs between April 2014 and June 2023 were retrospectively evaluated. Postoperative LSA and DSA measurements were compared according to implant design, preoperative glenoid morphology (Favard classification), and the Hamada grade. The correlation of LSA and DSA with preoperative shoulder anatomy (critical shoulder angle, CSA, and acromial index, AI) was also assessed. Results: In total, 135 shoulders were included in the study, with a mean age of 71.7 +/- 7.9 years. The mean LSA was 88.4 +/- 11.8 degrees and mean DSA was 40.6 +/- 12.5 degrees. According to prosthetic design, both mean LSA and DSA values differed significantly (p < 0.05). Lateralized designs (Groups I and IV) had significantly higher mean LSA values. The Favard classification and Hamada grade of shoulders did not show a significant influence on LSA and DSA measurements (p > 0.05). DSA was observed to be significantly correlated with CSA and AI (p < 0.05; r = -0.27 and -0.189, respectively). Conclusions: Prosthetic design and preoperative shoulder anatomy had a significant influence on LSA and DSA measurements in RSA. Optimal LSA and DSA values may lack validity and reliability and should not be applied to all patients.