Biomechanical assessment of RTSA functional outcomes towards optimising the prosthesis configuration

• Main Author: Glenday, Jonathan Daniel
• Other Authors: Sivarasu, Sudesh
• Format: Doctoral Thesis
• Language: English
• Published: Faculty of Health Sciences 2019
• Online Access: http://hdl.handle.net/11427/30031

Reverse total shoulder arthroplasty (RTSA) alleviates pain and restores function to patients with cuff tear arthropathies or massive rotator cuff tears. The procedure uses a semi-constrained prosthesis to reverse the orientation of the glenohumeral joint, thereby altering the biomechanics of the deltoid and allowing it to restore shoulder function in the presence of an irreparably damaged rotator cuff. However, there are complications that can impact long-term success of RTSA. Adaptations to the design and placement of the prosthesis have been investigated to address these complications and this has led medical device manufacturers to develop divergent implants. This divergence, as well as previous literature regarding RTSA biomechanics, suggest that a configuration that optimises reverse shoulder functional outcomes has yet to be determined and that it can be obtained by combining multiple modifications. A biomechanical assessment framework was established to characterise reverse shoulder function and the effect of modifying prosthesis configuration. It utilised the Newcastle Shoulder Model (NSM) and a custom-made impingement detection algorithm to simulate seven standardised motions that either elevated or rotated the humerus. Four outcome measures (deltoid elongation, deltoid moment arm, joint stability and impingement-free range of motion) were evaluated for each motion. The framework took anatomical variability into consideration by performing the simulations using a subject-specific reverse shoulder cohort. Further, 36 modified configurations of the prosthesis (based on offsets to the placement of glenosphere, humeral tray and greater tuberosity) were evaluated. The effect of each of these modifiable parameters on the outcome measures was characterised as beneficial, detrimental, or negligible, in comparison to a default prosthesis configuration. Seven of the most beneficial parameters were then selected for combination and evaluated using the assessment framework. Due to an antagonistic relationship between the outcome measures, and differing functional requirements of the motions, none of the configurations were able to simultaneously maximise all outcome measures. Rather, the optimised configuration (which inferiorly translated the glenosphere and posteromedially translated the humeral tray) provided balanced, moderate improvements to majority of the outcome measures. Overall, the deltoid did not excessively elongate, and deltoid moment arms, joint stability, and impingement-free range of motion improved by 17.9%, 57.1%, and 32.1%, respectively compared to the default configuration. Subsequently, comparisons between the effect of the default and optimised configurations on muscle fatigue and micromotion at the bone-implant interface were made. Muscle fatigue was assessed by adapting the NSM, and micromotion was assessed through a finite element analysis of a subset of the reverse shoulder cohort. The optimised configuration had a beneficial impact on the time to initiate muscle fatigue by decreasing the force required by the middle deltoid to initially elevate the humerus, and it had no appreciable effect on micromotion. In summary, an optimised RTSA configuration has been presented in this thesis. For a rotator cuff deficient reverse shoulder, the proposed configuration provided balanced, moderate improvements to majority of the functional outcomes. Additionally, the configuration was able to mitigate the effect of muscle fatigue and did not affect micromotion. Future studies should look to experimentally validate these findings, determine their clinical significance, and enhance both the assessment techniques and framework.