2015 ISAKOS Biennial Congress ePoster #2118

A Finite Element Model Measuring the Mechanical Trade-Offs of Changing Centers of Rotation in Reverse Shoulder Arthroplasty

Carolyn M. Hettrich, MD, MPH, Iowa City, IA UNITED STATES
Vijay N. Permeswaran, MS, Iowa City, IA UNITED STATES
Donald Anderson, PhD, Iowa City, IA UNITED STATES
Jessica E. Goetz, PhD, Iowa City, IA UNITED STATES

University of Iowa Hospitals and Clinics, Iowa City, Iowa, USA

FDA Status Not Applicable

Summary: A finite element model was created to study the effect changing centers of rotation have on range of motion and effective deltoid moment arm.

ePoster Not Provided
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Abstract:

Introduction

Reverse Shoulder Arthroplasty (RSA) has become a common surgical treatment for relieving the symptoms associated with cuff tear arthropathy. The RSA design moves the shoulder center of rotation medially, increasing the effective power of the deltoid muscle. However, this change in anatomy can lead to complications, such as scapular notching, caused by contact between the humeral cup and the scapula. A common technique to relieve scapular notching is to lateralize the center of rotation, but there are associated mechanical trade-offs. In this study, a finite element model was used to study the effect of lateralization on range of motion and deltoid moment arm.

Methods

A series of five finite element models were created for analysis, including a 5 mm medialized model (to represent a deteriorated glenoid), as well as four lateralized models (2.5, 5, 7.5, and 10 mm). All models were created using the same anatomical template, acquired from segmentations of the Visible Human Female. The models contained a scapula, glenosphere, humerus with implant, and a deltoid cable element. The models were subjected to range of motion evaluation and a deltoid muscle force test. The impingement-free range of motion was tested in the coronal and scapular planes with a rotating scapula model simulating scapulo-thoracic motion. In addition, all models underwent a muscle force test where an external moment was applied to the humerus, and the force required by the deltoid element for static equilibrium was computed.

Results

Lateralization was found to be an effective technique for decreasing scapular notching by reducing impingement. In the scapular plane, only the 5 mm medialization resulted in impingement before the humerus reached neutral position. In the coronal plane, increases in lateralization were matched with increases in range of motion; the 5 mm medialization impinged at 40° of abduction, while the 10 mm lateralization did not impinge at all. However, increases in lateralization also increased the deltoid force required to resist the applied moment; the 5 mm medialized model required 546 N, while the 10 mm lateralization required 660 N, a 21% increase. Furthermore, for every mm of increased lateralization past 2.5 mm, an additional 14.27 N of force was required of the deltoid.

Discussion

The findings of this study underscore the fact that lateralization must be chosen carefully. While too little lateralization reduces the impingement-free range of motion, too much lateralization can overstress the deltoid. These data suggest that an ideal amount of lateralization could be calculated for each patient, maximizing the impingement-free range of motion while preserving adequate deltoid power.

Conclusion

Lateralization has been shown to effectively increase the range of motion in RSA. In addition, increases in lateralization have been shown to increase the deltoid force required to resist external loads. This indicates that, while lateralization can help reduce scapular notching due to contact, excessive lateralization can overload the deltoid muscle.