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

Abstract

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, P < .001) and a weak positive correlation with RSA-HARA (R = 0.37, P < .001). HARA ranged from 0° to 43° (mean 12°), and RSA-HARA from 0° to 54° (mean 26°). An RSA-HARA of 30° significantly increased ROM for external rotation (74° vs. 38°, P < .001) and internal rotation (101° vs. 70°, P < .001) compared to 0°. Semiconstrained inserts at 20° showed superior ROM for internal (84° vs. 70°, P < .05) and external rotation (58° vs. 38°, P < .05) compared to standard inserts at 0°.

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.