Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Surgery

Supervisor

Johnson, Jim

2nd Supervisor

Athwal, George

3rd Supervisor

Langohr, Daniel

Abstract

The utilization of reverse total shoulder arthroplasty (RTSA) has continued to increase as its clinical indications expand. The optimization of the rotator cuff function in the setting of RTSA is poorly understood and poor outcomes are associated with lack of external and internal rotation function. The purpose of this study is to evaluate the role of implant parameters on rotator cuff tendon excursion and moment arms in the setting of RTSA.

Using a cadaveric based model, a custom designed modular RTSA system was implanted that allowed for incremental changes to glenoid and humeral lateralization. Using a shoulder simulator and optical tracking, rotator cuff tendon excursion and moment arms were calculated at various arm positions and implant configurations.

Increased glenoid and humeral lateralization yielded overall increased tendon excursion. Despite lack of statistical significance, there was a trend towards increased rotator cuff moment arms as glenoid and humeral lateralization increased.

Summary for Lay Audience

The use of reverse total shoulder arthroplasty (RTSA) has continued to increase as it has been found to be beneficial for an increasing number of glenohumeral problems. Despite great clinical outcomes, the consistency of internal and external rotation outcomes is poor. There have been a variety of implant designs studied and used in clinical practice that attempt to combat a variety of issues associated with RTSA. Some designs utilize a lateralized glenoid component while others prefer to lateralize the humeral component. There is a lack of literature that assesses the effect of lateralization of RTSA components on the rotator cuff tendons. The purpose of this study was to evaluate the effect of RTSA humeral and glenoid lateralization on the excursion and moment arms of the rotator cuff.

Using six cadaveric shoulder specimens, a modular RTSA implant was utilized that allowed incremental change to glenoid and humeral lateralization. These specimens were mounted onto a custom shoulder simulator that allowed for controlled abduction, internal rotation, and external rotation. Various implant configurations were tested at various arm positions. The excursion of supraspinatus, infraspinatus, teres minor, subscapularis superior and inferior were recorded at each trial. Excursion data was utilized to calculate respective moment arm data for each trial. The moment arms represent the effectiveness of a muscle on applying motion about a particular joint.

Results demonstrated statistically significant increase in tendon excursion as glenoid and humeral lateralization were increased, for all tendons tested. Despite lack of statistical significance, there was an overall trend towards increased rotator cuff moment arms as glenoid and humeral lateralization increased. Further research comparing these relationships to native rotator cuff excursion may further illustrate possible optimal implant positions that may restore native function.

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