Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Master of Engineering Science


Mechanical and Materials Engineering


Johnson, James A

2nd Supervisor

Langohr, G Daniel G



Acromial fractures are a debilitating complication following reverse total shoulder arthroplasty (RTSA). The purpose of this work is to (1) improve the current state of FE bone stress estimates after RTSA and to investigate the effects of (2) plane of elevation, (3) hand loads, and (4) baseplate screw position on scapular spine stress. The FE method used was validated against an in-vitro strain gauge based experiment and found to be accurate in the prediction of stress increases and decreases after RTSA. The coronal plane of elevation was found to increase scapular spine stress compared to more central planes of elevation. Hand load increased stress levels above the fatigue threshold of healthy cortical bone. A divergent screw angle decreased bone stress and if the screw penetrated the scapular spine then leaving the screw in place decreased bone stress compared to withdrawal. This work can help in determining the optimal surgical protocol.

Summary for Lay Audience

Reverse total shoulder arthroplasty(RTSA) is a type of shoulder replacement surgery. As common with surgeries there is a chance of complication, one such complication associated with RTSA is acromial fractures which occur in up to 7% of cases. The acromion is the upper structure of the scapula, and a fracture is a break or crack in the bone. The purpose of this work was to investigate causes of these fractures by determining how altering the surgical procedure changes bone stress.

In Chapter 2 different implant placements were investigated over a range of arm positions to determine their effect on bone stress. Implant position was varied by shifting the scapular component away from the scapula (0,5,10mm), and downwards(0,2.5,5mm), along with shifting the humeral component towards and away from the body(-5,0,5mm). This was done using computer models of 10 scapulae with a simulated implant. A custom algorithm estimated muscle forces in the shoulder using anatomic dimensions and weights. The different implant configurations were compared to determine bone stress patterns. It was found that shifting the scapular component away from the body increased bone stress and shifting it downward decreased bone stress.

In Chapter 3 the computer modelling method was used to investigate the position of screws that are used in securing the shoulder implant. Screw position was altered by changing the angle (0°,15°), using 3 screws as opposed to 4, and altering the screw length to see the effect of proximity of screws to the acromion. The results found that screws did increase bone stress and a 15° angle decreased bone stress compared to 0°.

In Chapter 4 the custom computer modelling method was validated against the gold standard method of measuring strain in bone. An experiment was performed on 6 cadaveric scapulae and recreated with computer modelling. The results of both methods were compared to prove the computer modelling results are meaningful. No significant difference was found between the two methods.

This work can be used to design future experiments with the goal of determining the optimal surgical procedure to minimize the likelihood of unwanted surgical complications.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License