Translating humeral posture into prosthetic planning: BMI, humeral abduction resting angle, and simulated range of motion in an Altivate 135° reverse shoulder arthroplasty model.

Background: Scapular notching in reverse total shoulder arthroplasty is more prevalent in slim patients and less frequent in obese patients. Current arthroplasty planning software often lacks the integration of patient-specific arm positioning when aiming to predict impingement-free motion. This study hypothesizes that obese patients exhibit a more abducted humeral resting position, measured as the humeral abduction resting angle (HARA) and reverse shoulder arthroplasty (RSA)-HARA, and that greater abduction increases impingement-free range of motion (ROM). Methods: Standing radiographs of 141 shoulders (121 patients) were analyzed to measure HARA (humeral center line to vertical plumb line) and RSA-HARA, defined as the angle between the humeral center line and supraspinatus fossa line influenced by RSA implantation and scapular posture. A subsequent computer modeling study of 22 computed tomography scans simulated RSA-HARA positions of 0°, 15°, 30° (Altivate; Enovis, Austin, TX, USA; 36 mm and 36-4 mm glenoid heads, standard insert, jump height = 10 mm, liner stability ratio = 202%), and 20° (semiconstrained insert, jump height = 11.3 mm, liner stability ratio = 202%). The ROM parameters-extension, external rotation, internal rotation, and adduction-were compared using a 135° humeral stem and gender-specific glenoid head configurations. Flexion, abduction and extension were not recorded. Results: Body mass index showed a moderate positive correlation with HARA (R = 0.48, Conclusions: Body mass index was moderately correlated with HARA (R = 0.48) and weakly correlated with RSA-HARA (R = 0.37). A greater RSA-HARA was associated with improved impingement-free ROM. Patients with increased HARA may benefit from more retentive liners, offering stability without compromising ROM. Customizable preoperative planning software incorporating HARA, RSA-HARA, and scapulothoracic posture are important to optimize RSA planning.