Abstract
Reverse total shoulder arthroplasty was developed to address the treatment of patients with Cuff Tear Arthropathy. Despite of the clinical improvements seen with initial reverse shoulder replacements, several mechanical problems remain. Scapular notching has been reported between 24.5% and 96% of cases. Patients have also exhibited limited external rotation, either from impingement or slackening of remaining cuff musculature. Additionally, by medializing and moving the humerus distally, patients note a loss of the normal deltoid contour leading not only to cosmetic concerns, but possibly decreasing deltoid efficiency and creating a prosthesis with less inherent stability. Finally, although mechanical failure on the glenoid side initially was thought to be uncommon, various glenoid sided problems have been reported.
Recognition of these problems led to clinical and basic science studies aimed at improving surgical technique and the design of reverse shoulder implants. During the last 10 years, our institution has been conducting biomechanical research examining the forces across the glenohumeral joint. Several different models have been created to replicate mechanical failures by integrating biomechanical information with our clinical investigations, including altering the position of the implant (tilt), the type of fixation of the implant (screw or peg), and glenoid-sided bone loss. We were able to address glenoid component failure (with initial rates of 10% in our clinical studies) by recommending locking screws to neutralize forces at the fixation site. These discoveries have reduced glenoid-sided fixation failures to less than 0.1%.
In vitro kinematic function and factors that affect impingement free glenohumeral motion of reversed implants is another area of interest. The clinical relevance of impingement includes scapular-notching, pain from impingement, instability and excessive prosthetic wear. Several models that include motion in three different planes (flexion-extension, abduction-adduction and internal-external rotation) have been developed to study multiple prosthetic, technique and anatomic factors which can result from varying degrees of impingement. By integrating the results from these models into our clinical practice (e.g., selecting a more lateralized glenosphere, selecting a varus humeral component and inferiorly translating the glenoid component on the glenoid surface), we have been able to maintain low rates of notching (∼10% at 8 year follow-up). Finally, our current work involves development of a model that attempts to understand which factors might be influential in causing instability and stiffness. Thus, biomechanics research offers an excellent opportunity for interdisciplinary collaboration to solve complex clinical problems.
