Contact pressure at the implant-tendon interface after total shoulder joint replacement surgery.

Abstract

INTRODUCTION: One of the most common complications of shoulder arthroplasty is anterocranial migration and subachromial impingement due to rotator cuff failure. One hypothesis for this phenomenon is impingement between the prosthesis and rotator cuff tendons. The aim of this study was to evaluate the contact pressure between the rotator cuff tendons and the prosthetic joint replacement component after implantation of the Limra SMR shoulder replacement. METHODS: Eight fresh-frozen, male entire extremity specimens were harvested from human cadavers (mean age: 68 years). Loops of 5-Ethibond Suture were attached to the insertion of the individual tendon of each muscle sub-region and secured. Using a weight-pulley system, physiological muscle force calculated using a musculoskeletal model was applied to each muscle-tendon unit at a variety of joint positions throughout abduction, flexion, and axial rotation. Pressure sensitive Fuji film placed underneath each rotator cuff tendon during testing was scanned and assessed to evaluate contact pressure. Experiments were performed for the natural shoulder, and were repeated after total joint replacement surgery using the Lima SMR. RESULTS: Overall, both joint angle and shoulder joint replacement surgery had significant effects on the maximum contact pressure measured between the humeral head and all rotator cuff muscle tendons except teres minor (p<0.05), with maximum pressure increasing after surgery for all tendons. The supraspinatus, one of the initiators of abduction, was found to have a larger contact pressure after surgery in early abduction (p = 0.008), and demonstrated a significant increase in contact pressure with the humerus externally rotated (mean difference: 0.36 MPa, 95% CI [0.19, 0.54], p = 0.001). CONCLUSIONS: Greater contact pressure between the rotator cuff tendons and the prosthetic humeral head may present increased risk of post-operative tendon damage, particularly in the supraspinatus. Decreasing the size of the humeral head component may reduce the magnitude of the tendon-implant contact pressure.