STRESS DISTRIBUTION WITHIN THE CEMENT MANTLE OF THE KEELED GLENOID COMPONENT: A COMPARITIVE STUDY OF KEEL ANGLE

Abstract

The purpose of the study was to reduce peak cement mantle stresses occurring at the tip of the keel for an all-polyethylene cemented glenoid component using finite element (FE) techniques.

Loosening of the glenoid component remains to be one of the most determinant factors in the outcome of total shoulder arthroplasty. Due to the off-centre loading that occurs, there is bending of the glenoid component with high shearing forces. These forces are transmitted to the underlying cement mantle and bone. It has been reported in previous FE studies that high cement mantle stresses occurs at the tip of the keel and at the edges of the cement flange. These stresses at the bone-cement interface can exceed the fatigue life of the cement, therefore initiating crack formation and damage accumulation. This results in loosening of the component and thus failure.

A three-dimensional (3D) model of the scapula was developed using CT data. Surfaces of the inner and outer contours of the cortical shell were created within commercially available software, using a threshold algorithm. The glenoid bone geometry was then produced. Material properties for the reconstructed glenoid were taken from literature, using four differing material properties. The articulating surface of the keeled glenoid component was modelled with a 3mm radial mismatch. This was positioned in the glenoid bone with a uniform cement mantle thickness of 2mm. The resulting FE mesh consisted of solid parabolic tetrahedral elements.

The effect of varying the angle on the keel of the component in the superior/inferior (S/I) direction was studied with uniform cement mantle thickness. The S/I length of the keel at the lateral end where it meets the back face of the component was maintained (juncture with flange), whilst the S/I length of the keel at the medial end (tip of the keel) was reduced as the change in angle increased. Two load cases were studied, involving a physiological load for 90 degrees of abduction and a central load of same magnitude.

It was found that by increasing the angle of the keel, where the S/I length at the tip of the keel was reduced, resulted in lower cement mantle stresses in this area of interest. This can be attributed to it being further away from the stiffer cortical bone where high tensile stresses exist due to inherent bending of the glenoid construct under loading. Therefore by reducing these high cement mantle stresses at the tip of the keel, fatigue failure of the cement mantle could be reduced.