Summary
Inferior glenoid tilt may negatively influence kinematics that lead to poorer outcomes in the hand-to-back motion after reverse shoulder arthroplasty.
Abstract
Introduction
Internal rotation is not reliably improved after reverse shoulder arthroplasty (RSA). Surgical parameters such as glenosphere size, lateralization, and retroversion have been associated with internal rotation after RSA. The aim of this study was to determine effects of surgical technique and prosthesis geometry on in vivo movement patterns and patient-reported outcomes (PROs) after RSA. We hypothesized that kinematics and contact path that are influenced by greater glenosphere size, glenoid tilt, and lateralization during the hand-to-back motion would correlate with better PROs.
Methods
Patients who received RSA within 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 a hand-to-back movement while synchronized biplane radiographs were collected at 50 images/s for 2 seconds. Digitally reconstructed radiographs from subject-specific segmented bone tissue of the humerus and scapula with respective implants were matched to biplane radiographs with sub-millimeter accuracy to determine six degree-of-freedom scapular and humeral kinematics. The contribution of each component of rotation (glenohumeral (GH) abduction, plane of elevation and internal/external (I/E) rotation, as well as scapular upward rotation, protraction, and tilt) to the overall motion was calculated. Average end position, peak angles, and range of motion (ROM) of all rotations were found. ASES, DASH, and CMS scores were collected at testing. Implant characteristics and surgical techniques that predicted kinematics were identified using multiple linear regression using forward selection with SPSS 29.0 software. The center of contact between a 3D CAD model of the polyethylene and glenosphere was calculated and superior/inferior (SI) and anterior/posterior (AP) locations were averaged across corresponding angles to establish contact path. Associations between the most anterior, posterior, inferior, and superior points on the contact path and surgical technique were identified using multiple linear regression using forward selection. Pearson correlation evaluated associations between either kinematics or contact path and PROs. Significance was set at p<0.05.
Results
The study included 35 patients who received RSA (17M,18F,72.8±7.3 years) with average follow-up of 2.2±1.1 years. Inferior glenoid tilt correlated with less GH abduction, more scapular upward rotation, and less scapular protraction (all p<0.05). 145° neckshaft implants and higher retroversion angles both associated with more anterior contact paths, and larger glenosphere had a more inferior contact path (all p<0.05). DASH and ASES scores improved with more scapular protraction (p=0.026 and p=0.014, respectively).
Discussion
During hand-to-back after RSA, increased scapular protraction was the primary kinematic factor associated with better PROs. The only surgical factor associated with scapular protraction was glenoid tilt. This suggests that more inferior glenoid tilt decreases scapular protraction, leading to less favorable PROs. Contrary to previous work, we found lateralization, a surgical parameter that has been associated with improved IR ROM, did not correlate with kinematics or contact path location. The mechanism for lateralization’s influence on IR performance may not be explained by in vivo kinematics.
ACKNOWLEDGEMENTS: Work was funded by NIH Grant: R03AG064417