Summary

Retroversion, neckshaft angles and eccentricity are associated with kinematics, contact path, and clinical outcomes after reverse shoulder arthroplasty during hand to head motion.

Abstract

Introduction

External rotation (ER) is a post-surgical assessment after reverse shoulder arthroplasty (RSA) because it is essential to activities of daily living, such as the hand-to-head motion. The aim of this study was to determine effects of surgical technique and prosthesis geometry on in vivo kinematics, contact path, and patient-reported outcomes (PROs) during the hand-to-head motion after RSA. We hypothesized that greater lateralization, greater retroversion, and lower humeral neck-shaft angles would associate with kinematics and contact paths during hand-to-head that are associated with improved PROs.

Methods

Patients who received RSA within the previous 1-5 years consented to participate in this IRB-approved study. RSA was performed using standard 135-degree or 145-degree humeral implants. Lateralization, glenosphere size, and eccentricity were recorded from surgical notes. Humeral retroversion and glenoid tilt were measured on post-operative CT. Participants performed 3 trials of hand-to-head motions while synchronized biplane radiographs were collected at 50 images/s for 2 seconds. Digitally reconstructed radiographs from subject-specific segmented bone tissue of humerus and scapula with respective implants were matched to biplane radiographs with sub-millimeter accuracy to determine six degree-of-freedom kinematics. The contribution of each glenohumeral (GH) and scapular rotation (GH abduction, plane of elevation and internal/external (I/E) rotation, as well as scapular upward rotation, protraction, and tilt) to overall shoulder motion was calculated. Average end position of all 6 rotations was found. The path of the center of contact between a 3D CAD model of the polyethylene and the glenosphere was calculated. ASES, DASH, and CMS scores were collected at testing. Implant characteristics and surgical techniques that predicted kinematics (peak angles, ROM, contribution, end position) and contact path location were identified using multiple linear regression using forward selection with SPSS 29.0 software. Associations between kinematics and PROs were evaluated with Pearson correlations. Significance was set at p<0.05.

Results

This study includes 35 RSA patients (17M,18F,72.8±7.3 years) with average follow-up of 2.2±1.1 years. Greater retroversion angles correlated with less peak abduction and scapular protraction contribution (p=0.038 & p=0.005, respectively). Greater eccentricity was associated with more scapular upward rotation contribution (p=0.005). 145° neckshaft angles displayed more anterior contact path (p=0.006). CMS scores improved with more abduction (all p<0.05). ASES and DASH scores improved with more scapular upward rotation (all p<0.05). DASH and CMS scores improved with more superior contact paths (all p<0.05).

Discussion

During hand-to-head, favorable PROs are associated with more glenohumeral abduction and higher upward scapular rotation. Those kinematic outcomes are associated with the surgical techniques of less retroversion, greater eccentricity, and more superior contact path. Contrary to our hypothesis, greater retroversion was not associated with kinematics that were associated with better PROs, and lateralization and neck-shaft angle did not have significant effect on this motion. Additional study is needed to identify the point at which less retroversion and more eccentricity begin to worsen outcomes. Control subject kinematics data are needed to evaluate RSA’s restoration of healthy kinematics. Satisfaction and function may improve by identifying techniques that affect kinematics associated with better outcomes after RSA. Study was funded by NIH Grant: R03AG064417