Biomechanical analysis of stability and fixation strength of total shoulder prostheses

The Neer I (polyethylene glenoid), Neer II (plastic glenoid surface with metal backing), Cofield, and Gristina monospherical total shoulder prostheses were tested for joint subluxation resistance and glenoid component fixation strength. Synthetic glenoid models with consistent structural properties and standard under-cutting geometry were used for glenoid component fixation with bone cement. It was found that joint subluxation resistance varied linearly with axial compressive force applied. Higher subluxation strength was associated with the amount of curvature of the glenoid articulating surface. Consequently, joint stability was consistently higher for the Gristina design, and all had lower resistance in the anteroposterior direction. After complete joint dislocation under high axial compressive force, the polyethylene material at the site of subluxation experienced gross plastic deformation. The fixation strength of the glenoid component was lowest for the Neer I design and highest for the Cofield design. Fatigue loading at the joint did not seem to affect the glenoid component fixation strength. In general, all glenoid components appeared to have sufficient fixation strength against normal shoulder joint forces except the Neer I design. Loosening and fracture of the plastic glenoid component may occur under excessive loads.