Summary

In-vivo data suggests that surgical technique and implant characteristics appear to affect contact patterns during abduction, but not ROM or strength after RSA.

Abstract

Introduction

Reverse shoulder arthroplasty (RSA) reduces pain and dysfunction in patients with rotator cuff arthropathy. Controversy remains regarding ideal implant configuration to maximize shoulder function. Our aim was to determine effects of surgical technique and implant geometry on in-vivo kinematics and strength during abduction. We hypothesized greater humeral neck shaft angle would be associated with more GH abduction, more humeral retroversion would be associated with a more posterior contact path on the glenosphere, and that more lateralization would be associated with greater strength and more normalized scapulohumeral rhythm (SHR).

Methods

35 patients received RSA (17M,18F,72.8±7.3years) using a standard 145° onlay humeral implant (145°OHI)(Wright-Med/Tornier) or a 135° inlay humeral implant (135°IHI)(Arthrex) and consented to participate in this IRB approved study. Synchronized biplane radiographs were collected at 50 images/second during 3 abduction trials 1-5 years after RSA. Scapular and glenohumeral kinematics were determined using a validated technique that matched subject-specific digitally reconstructed radiographs to biplane radiographs with sub-millimeter accuracy. A 3D CAD model of the polyethylene cup was fit into the humeral tray using Geomagic. Location of the center of contact between the polyethylene and glenosphere was determined throughout the entire abduction motion. The superior/inferior (S/I) and anterior/posterior (A/P) center of contact locations were averaged across trials at corresponding GH abduction angles. GH abduction and scapular upward rotation were averaged across trials at corresponding humerothoracic rotations and used to calculate the average SHR. Glenoid tilt and humeral retroversion were calculated from 3D models from CT-scans. Lateralization, neck-shaft angle, glenosphere size, and eccentricity were recorded from surgical notes. A Biodex machine set to 30°/second was used to determine strength by measuring isokinetic torque throughout abduction and adduction ROM. Peak torque and total work were found from torque/angle curves and normalized to bodyweight. Implant characteristics and surgical techniques that predicted kinematics, strength, or contact path locations were identified using multiple linear regression using forward selection, significance set at p<0.05.

Results

The most posterior contact points for 135°IHI were 3.4mm more posterior than 145°OHI (p<0.007). The most anterior contact points were 5.8mm more posterior for 135°IHI than for145°OHI (p<0.001). Increased retroversion was associated with a more anterior contact point for the most anterior (p=0.036) and posterior points (p=0.027). Average SHR was 0.9±0.3°. Abduction and adduction torque were positively correlated with glenosphere size (p<0.001, R=0.628 and p=0.01, R=0.448, respectively). No other associations between implant characteristics or surgical techniques and kinematics were found (all p>0.05).

Discussion

Surgical technique and implant characteristics appear to affect contact patterns during abduction, but not ROM or strength after RSA. This highlights the importance of in-vivo data to confirm cadaver-based research and computer simulations that do not account for healing and changes in neuromuscular control after surgery. The more anterior contact path associated with either 145°OHI or increased retroversion may impact polyethylene wear patterns. Contrary to our hypothesis, increased glenosphere size, not lateralization generated more torque. This may further support prior studies that found increased deltoid load with larger glenosphere size though this may not impact PROs.