Acromion-fixation of glenoid components in total shoulder arthroplasty

Successful design of components for total shoulder arthroplasty has proven to be challenging. This is because of the difficulties in maintaining fixation of the component that inserts into the scapula; i.e., the glenoid component. Glenoid components that are fixated to both the glenoid and acromion (a long process extending medially on the dorsal aspect of the scapula) have the possible advantage of greater stability over those that are fixated to the glenoid alone. In this study, a finite element analysis is used to investigate whether or not acromion fixation is advantageous for glenoid components. Full muscle loading and joint reaction forces are included in the finite element model. Reflective photoelasticity of five scapulae is used to obtain experimental data to compare with results from the finite element analysis, and it confirms the structural behaviour of the finite element model. When implanted with an acromion-fixated prosthesis, it is found that high unphysiological stresses occur in the scapula bone, and that stresses in the fixation are not reduced. Very high stresses are predicted in that part of the prosthesis which connects the acromion to the glenoid. It is found that the very high stresses are partly in response to the muscle and joint reaction forces acting at the acromion. It is concluded that, because of the relatively high forces acting at the acromion, fixation to it may not be the way forward in glenoid component design.