Development of a computational model to study instability and scapular notching in reverse shoulder arthroplasty
To my loving wife, caring family, and supportive friends, I could not have accomplished so much without you. iii ACKNOWLEDGEMENTS I would like to thank my advisors Don Anderson and Jess Goetz. They provided me an excellent environment to develop my engineering, technical communication, and research skills. Also, I would like to thank the other members of the lab for helping me conduct research. In addition, I would like to thank my fellow graduate students, past and present, for pushing me to
... hieve more. Finally, I would like to thank my loving wife, my family, and my friends for their care and support throughout this process. iv ABSTRACT Reverse shoulder arthroplasty (RSA) is a common treatment for individuals with arthritis of the glenohumeral joint in the presence of a massive rotator cuff tear. Though this procedure has been effective in restoring function to these individuals, it has also been associated with high early to mid-term complications, such as scapular notching and instability. A finite element (FE) modeling approach has previously been used to study the range of motion an individual with RSA could adduct their arm the polyethylene liner impinged on the inferior scapular bone and the contact stress at the impingement site. This model was then validated in a physical experiment using cadaveric tissue. In this document, I introduce modifications to that FE model to further study instability and scapular notching risk. First, modern RSA implant geometries were introduced into the model, and the effect of polyethylene liner rotation and glenoid version on impingement-free range of motion and instability risk was assessed. Then, a physical material property characterization of rotator cuff tissues present after RSA was performed. Finally, those material properties and continuum elements representative of the rotator cuff tendons were introduced into the FE model. Throughout all of these studies, greater complexity and fidelity was added to improve the ability to model both contact at the impingement site and potential dislocation events through more accurate loadings and boundary conditions. v PUBLIC ABSTRACT Rotator cuff tears are the most prevalent shoulder injury in the US, accounting for over 4.5 million clinical visits a year. Patients suffering from massive rotator cuff tears over a prolonged period can develop painful arthritis, leading to shoulder dysfunction. Fortunately, a new type of implant system was developed for these patients called a reverse shoulder arthroplasty (RSA). In this design, the natural ball-in-socket anatomy of the shoulder is reversed, replacing the humeral head (ball) with a humeral cup (socket) and the glenoid cavity (socket) with a glenosphere (ball). This change in anatomy provides function and relives pain for these patients. Unfortunately, this implant has also been associated with high rates of complications, such as instability or dislocation and scapular notching, a phenomenon where the humeral cup contacts and grinds against the bone of the shoulder blade, causing the bone and polyethylene cup to wear away. In this document, I introduce a computer model studying RSA in which contact stresses can be computed. This allows for testing and experimentation of various implantation techniques to determine which techniques produce greater risks for these complications. Specifically, I analyze what effect glenosphere tilt and humeral polyethylene cup rotation have on dislocation risk. In addition, I conducted physical testing of cadaveric shoulder tendons that prevent dislocation to determine their mechanical behavior. These values were then used to enhance the computer model to better study how weak or strong shoulder tendons affect dislocation and scapular notching.