Structural analysis of an offset-keel design glenoid component compared with a center-keel design

Many different designs of glenoid prostheses have been developed in an attempt to reduce the loosening rates and improve the prognosis of total shoulder arthroplasty. This study investigated a design in which the keel is positioned anterior to the central plane of the component, an offset-keel design. The primary purpose of anterior location of the keel is to avoid contact between the keel and the cortical bone surface. However, anterior placement of the keel also situates it more directly under the line of action of the contact force in abduction; this has the possible advantage of reducing the bending stress on the cement mantle. Our purpose was to establish whether an offset-keel design reduces the cement stresses below those obtained with conventional central-keel designs. A computed tomography-based finite element model of the glenoid region is used and dynamic loading for 0 degrees to 180 degrees in both flexion and abduction is simulated with the use of data from van der Helm (J Biomech 1994;27:527-50). Finite element analyses are carried out for both the normal and the rheumatoid arthritic case. For the rheumatoid arthritic joint, a Larsen grade IV type destruction is reproduced and proximal subluxed loads are applied, associated with a deficient rotator cuff for 0 degrees to 180 degrees in flexion and abduction. Results predict that the cement mantle in the offset-keel design is much less stressed compared with that in the center-keel design for the maximum glenohumeral joint load in abduction for both the normal and the rheumatoid arthritis case. In flexion the offset-keel design still has lower cement stresses even though the load is acting on the opposite side of the glenoid cup from the keel; one explanation for this is that insertion of the offset keel involves removal of the lower stiffness cancellous bone, leaving the glenoid component flanges to be supported by the stronger bone remaining in the glenoid cavity. From a biomechanical point of view, the advantages of an offset-keel design would appear to be considerable.