Though the literature contains many computational models studying RSA, very few utilize finite element analysis to study stresses in the implant and the surrounding bone. The introductions section shows that many parameters (center of rotation lateralization, center of rotation superior or inferior position, tilt of the cut glenoid surface, glenosphere shape design, glenosphere size, humeral design, notch severity, etc.) have been studied independently utilizing many different methods (finite element modeling and non-FE computational modeling). However, the introduction section also detailed the current limitations in modern modeling as well as many examples of the heights to which finite element modeling can be taken to study RSA. Using these limitations as guidelines, the goal of this project is to create a robust FE model of RSA to study the effect of lateralization on scapular notching and shoulder function. In the following chapters, the development of the model is detailed. In addition, results produced by the incrementally advanced models are shown. In Chapter 2, the initial finite element model encompassing scapular and RSA hardware geometry is described. Chapter 3 contains description of incremental changes to the model including humeral geometry and muscle element incorporation. An anatomically realistic configuration of the finite element model with increased functionality is detailed in Chapter 4. Finally, Chapter 5 discusses the assets and limitations of the current model as a platform for future research. In addition, a proposed validation protocol is presented.

Reverse shoulder arthroplasty (RSA) is performed to decrease pain and improve function and range of motion (ROM) primarily for patients with rotator cuff arthropathy, an arthritis of the shoulder secondary to rotator cuff insufficiency. However, RSA has suffered from high early to mid-term rates of complication, with instability being one of the most common. The shoulder biomechanics post-RSA depend on multiple factors such as implant geometry, positioning, and cuff integrity. This study built upon prior finite element (FE) analysis of RSA to investigate the effects of glenoid lateralization and retentive liner design on shoulder stability. A previously validated FE model was extended to model shoulder external rotation (ER) after implantation of the Zimmer Trabecular Metal RSA system. The FE model included the scapula bone with an implanted glenosphere implant, the humerus bone with implanted humeral sections of the RSA implant, and muscle tendons representing the subscapularis, infraspinatus, and deltoid. Six different models matched glenospheres in three cases of lateralization (2mm, 4mm, and 10mm) with two humeral poly liner designs (normal: 150° neck shaft angle or retentive: 155° neck shaft angle). Using Abaqus/Explicit FE software, the proximal ends of the soft tissues were pulled to their anatomical positions, and then fixed in space while the humerus was externally rotated 80° about the humeral long axis from a neutral position with the shoulder abducted 25°. The displacements, deltoid and subscapularis forces, impingement-free ROMs, and subluxation gap distances were recorded. Although greater glenosphere lateralization was associated with higher impingement-free ROM, larger deltoid and subscapularis forces developed. Deltoid tension contributes to shoulder stability and control, but elevated amounts of deltoid tension may contribute to scapular fractures and greater stress at impingement sites post-RSA. Further analysis such as inclusion of more anatomical features and additional motions may offer greater insight to orthopedic surgeons when planning for RSA insertion.

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.

Shoulder arthroplasty surgery has shown remarkable progress during the last few decades. A number of factors affect postoperative range of motion, pain and prosthetic durability. Among these factors, the length of the lever arm and joint stability is the ones that can be altered by the selected prosthetic component. It is uncertain how much of the normal anatomy needs to be re-established. Stemless prostheses with total reliance on metaphyseal fixation were introduced in France in 2004 (TESS, Zimmer Biomet). The goals were to avoid stem-related complications. Stemless implants have other potential benefits, including the ability to restore shoulder anatomy. Study I: This is a prospective cohort study of 49 patients with one of two versions of the TESS prosthesis (anatomical or reverse) with clinical and radiological follow-up ranging from 9–24 months. The TESS prosthesis showed short-term results that were comparable with other shoulder prosthetic systems. Study II: This is a prospective comparative non-randomised study of 37 patients (40 shoulders) who underwent TESS reverse shoulder arthroplasty (RSA) with a follow-up ranging from 15–66 months. We found a significant improvement in functional outcome and reduction of pain in both stemmed and stemless groups. Glenoid overhang influenced the occurrence of scapular notching (SN). Study III: This is a radiological study showing that CT had a good reliability and reproducibility in estimating LHO. Study IV: This is a prospective radiological study of 69 patients (70 shoulders) with primary osteoarthritis (OA) who had undergone stemless total anatomical shoulder arthroplasty (TSA). This study showed that stemless anatomical TSA could be useful in restoring shoulder anatomy. Study V: This is a prospective study of 44 patients with OA who had undergone stemless anatomical TSA with a clinical and radiological follow up ranging from 12 – 50 months. Our study showed that LHO reconstruction close to the anatomy of a healthy contralateral shoulder improved shoulder function. Stemless anatomical TSA help to restore LHO. Increasing LHO may have a negative effect on shoulder function at three months but had no effect at 12 months. The main conclusions of this thesis are: 1. TSA (anatomic and reverse) using stemless humeral components is reliable if bone quality is adequate. The complication rate is comparable with other shoulder prosthetic systems. 2. Glenoid overhang decreased complications in RSA. 3. LHO measurement on AP radiographs is less reliable and underestimates the distance when compared with CT. 4. Stemless TSA could be of help in reconstructing shoulder anatomy. 5. Shoulder reconstruction close to the anatomy of a healthy contralateral shoulder improves shoulder function. / Axelprotes kirurgi har visat avsevärd utveckling under de senaste decennierna. Ett antal faktorer påverkar postoperativt rörelseomfång, smärta och proteshållbarhet. Bland dessa faktorer utgör längden av hävarmen och ledstabilitet de faktorer som kan ändras genom val av proteskomponent. Det är osäkert om den normala anatomin måste återupprättas. Oskaftad protes med eliminering av humerusstamm och tillit till metafysär fixering introducerades i Frankrike år 2004 (TESS, Zimmer Biomet). Målen var att undvika stam relaterade komplikationer. Oskaftat implantat har andra potentiella fördelar, inklusive möjligheten att återställa axelnsanatomi. Syftet med denna avhandling var: (1) Att undersök radiologisk stabilitet av oskaftade axelproteser. (2) Att studera effekten av protes placering vid omvänd axelartroplastik både radiologiska och kliniskt utfall. (3) Att studera tillförlitlighet av mätningen av den laterala humeral offset (LHO), avståndet mellan processus coracoideus till laterala kanten av tuberkulum majus, med användning av CT eller röntgen. (4) Att studera oskaftad axelprotes förmåga att återställa axelnsanatomi. (5) Att studera den kliniska betydelsen av LHO återställning i för det funktionella resultatet efter axelartroplastik. Studie I: Detta är en prospektiv kohortstudie av 49 patienter med en av de två versionerna av TESS (anatomisk eller omvänd) med klinisk och radiologisk uppföljning från 9-24 månader. TESS protes visade lovande resultat på kort sikt med komplikationer som var jämförbar med andra axelprotessystem. Studie II: Detta är en prospektiv jämförande icke-randomiserad studie av 37 patienter (40 skuldror) som opererades med TESS omvänd axelartroplastik med en uppföljning från 15-66 månader. Vi fann en signifikant förbättring av funktion och minskning av smärta i både skaftad och oskaftad grupper. Glenoid overhang bedöms påverka risken för scapular notching (SN). Studie III: Detta är en radiologisk studie som visade att CT hade god tillförlitlighet och reproducerbarhet att mäta LHO. Studie IV: Detta är en prospektiv radiologisk studie av 69 patienter (70 skuldror) med primär artros som hade genomgått oskaftad total anatomisk axelprotes. Denna studie visade att oskaftad axelprotes kan vara till hjälp att återställa axelnsanatomi. Studie V: Detta är en prospektiv studie av 44 patienter med unilateral primär artros som hade genomgått oskaftad total axelprotes med en klinisk och radiologisk uppföljning från 12 - 50 månader. Vår studie visade att LHO rekonstruktion till den friska axeln förbättrar axelfunktion. Oskaftat implantat kan vara av hjälp till att återställa LHO. Ökad LHO kan ha en negativ effekt på axelnsfunktion vid tre månader, men denna effekt påvisade ej vid 12 månader. De viktigaste slutsatserna i denna avhandling är: 1. Oskaftad total axel artroplastik (anatomisk och omvänd) är tillförlitlig om benkvalitén är god med komplikationer som var jämförbar med andra axelprotessystem. 2. Glenoid overhang minskar komplikationer vid omvänd axelartroplastik. 3. LHO mätningen på röntgen är mindre tillförlitlig och underskattar avståndet jämfört med CT. 4. Oskaftad axelprotes skulle kunna vara till hjälp för att rekonstruera axelnsanatomi. 5. Axel rekonstruktion inom anatomi till att efterlikna anatomi på den friska kontralaterala axeln förbättrar axelfunktion.