Abstract
Introduction
Reverse Shoulder Arthroplasty (RSA) improves the mechanics of rotator cuff deficient shoulders. To optimize functional outcomes and minimize failures of the RSA manufacturers have recently made innovative design modifications with lateralized components. However, these innovations have their own set of biomechanical trade-offs, such as increased shear forces along the glenoid bone interface. The objective of this study was to develop an efficient musculoskeletal model to evaluate and compare both the muscle forces and joint reactive force of a normal shoulder to those implanted with varied RSA implant designs. We believe these findings will provide valuable insight into possible advantages or shortcomings of this new RSA design.
Methods
A kinematic model of a normal shoulder joint was adapted from publically available musculoskeletal modeling software. Static optimizations then allowed for calculation of the individual muscle forces, moment arms and joint reactive forces relative to net joint moments. An accurate 3D computer models of humeral lateralized design (HLD) (Equinoxe, Exactech, Gainesville FL, USA), glenoid lateral design (GLD) (Encore, DJO Global, Vista CA, USA), and Grammont design (GD) (Aequalis, Tornier, Amsterdam, NV) reverse shoulder prostheses was also developed and parametric studies were performed based on the numerical simulation platform.
Results
As expected, there were decreases in muscle forces in all RSA models (Table 1). These decreases were greatest in the middle deltoid of the HLD model for abduction and flexion (Figure 1) and in the rotator cuff muscles under both internal and external rotation (Figure 2). In all RSA models the muscle forces of the rotator cuff were diminished to near zero in all range of motions. The joint reactive forces in abduction and flexion decreased similarly for all RSA models compared to the normal shoulder model, with the greatest decrease again seen in the HLD model (Table 1).
Conclusion
These findings demonstrate that the design characteristics implicit in these modified RSA prostheses result in kinematic differences most prominently seen in the deltoid muscle and overall joint reactive forces. These differences could have a profound effect on the ultimate clinical success and long term outcomes for RSA. These results can help guide continued optimization of RSA design and clinical outcomes. The developed innovative shoulder modeling simulation could serve as a prototype for testing of future implant design concepts.
