The biomechanics of reverse shoulder arthroplasty.

摘要

Rotator cuff deficiency with glenohumeral arthritis presents a unique challenge to the orthopaedic surgeon. Under these conditions, total shoulder replacement has yielded poor results as a result of eccentric loading of the glenoid leading to loosening and early failure. Multiple procedures have been recommended to resolve this problem including total shoulder arthroplasty, shoulder arthrodesis, and hemiarthroplasty. Hemiarthroplasty, the current standard of care for this condition, offers only limited goals for functional improvement and only a modest improvement in pain.;Recently, there has been renewed interest in reverse shoulder arthroplasty. The main concept behind the reverse shoulder implant is the stabilization of the joint by replacing the head of the arm with a socket and placing a ball on the shoulder side. This "reverse" configuration creates a fixed fulcrum through which the deltoid can act more efficiently at raising the arm and thus increasing range of motion and returning the patient to a more normal level of function. This dissertation attempts to fill in some of the gaps in reverse basic science with six published studies. The important results found in these studies were: (1) Implantation of the glenosphere with an inferior tilt reduces the incidence of mechanical failure of the baseplate. (2) A positive linear correlation is present between abduction range of motion (ROM) and center of rotation offset (CORO). (3) When comparing several factors affecting ROM and scapular impingement, CORO had the largest effect on ROM, followed by glenosphere position. Neck-shaft angle had the largest effect on inferior scapular impingement, followed by glenosphere position. (4) Stability is determined primarily by increasing joint compressive forces and, to a lesser extent, by increasing humerosocket depth. (5) There are three distinct classes of arc of motion relative to the articular constraint: I -- arc of motion decreased with increased constraint, II -- arc of motion with a complex relationship to constraint, and III -- arc of motion increased with increased constraint.;The information presented in this dissertation may be useful to the orthopaedic surgeon when deciding on an appropriate reverse implant and improving surgical technique, as well as aiding engineers in improving reverse implant design.