BIOMECHANICAL ANALYSIS OF A TOTAL JOINT REPLACEMENT

Abstract

The normal shoulder requires the basic mechanical characteristics of range of motion, stability and strength. However, each of these characteristics can be compromised by arthritis or rotator cuff tear and are often associated with strong pain. Shoulder arthoplasty is one of the most common solutions for pain relief and to restore shoulder functionality. There are many available designs of prosthesis trying to address different shoulder pathologies. Despite this, there are relatively few studies investigating the biomechanics of a total joint replacement and suggest advantages, disadvantages and possible solutions.

The Newcastle shoulder model has been used to investigate the biomechanical properties of a total shoulder replacement having a reverse anatomy design. This model allows the simulation of implantation of the prosthesis and the prediction of muscle and joint forces. To address the requirement of accurate insertion of the prosthesis, the standard surgical procedure has been simulated. The current model was modified to represent the bones, muscles and implant alignment after surgery.

Load sharing results for standardised tasks (Abduction, Forward Flexion) showed great differences between anatomical and prosthetic models. In the latter the shear forces on the glenoid site were reversed, the compression stresses were reduced and the joint contact vectors were always within the humeral cup providing joint stability. This is an important effect of the reverse design, which reverses the envelope of the joint forces increasing also the muscle moment arms crossing the GH joint. The most affected group is the m.deltoid that becomes able to compensate for the dysfunctional rotator cuff muscles. The biomechanical model suggests that a reverse anatomy design can restore GH joint stability for patients with severe RC damage. Increased muscle moment arms also compensate for the lost contribution of the RC muscles to elevation.