BONE STRENGTH AND MATERIAL PROPERTIES OF THE GLENOID

The qualify of the glenoid bone is important to a successful total sho ulder replacement. Finite element models have been used to model the r esponse of the glenoid bone to an implanted prosthesis. Because very l ittle is known about the bone strength and the material properties at the glenoid, these models were all based on assumptions that the mater ial properties of the glenoid were similar to those of the tibial plat eau. The osteopenetrometer was used to assess the topographic strength distribution at the glenoid. Strength at the proximal subchondral lev el of the glenoid averaged 66.9 MPa. Higher peak values were measured posteriorly, superiorly, and anteriorly to the area of maximum concavi ty of the glenoid joint surface known as the bare area. One millimeter underneath the subchondral plate, average strength decreased by 25%, and at the 2 mm level strength decreased by 70%. The contribution of t he cortical bone to the total glenoid strength was assessed by compres sion tests of pristine and cancellous-free glenoid specimens. Strength decreased by an average of 31% after the cancellous bone was removed. The material properties of the glenoid cancellous bone were determine d by axial compression tests of bone specimens harvested from the cent ral part of the glenoid subchondral area. The elastic modulus varied f rom approximately 100 MPa at the glenoid bare area to 400 MPa at the s uperior part of the glenoid. With the elastic constants used as a pred ictor of the mechanical anisotropy, the average anisotropy ratio was 5 .2, indicating strong anisotropy. The apparent density was on average 0.35 gr. cm(-3), and the Poisson ratio averaged 0.263. According to ou r findings the anisotropy of the glenoid cancellous bone, details conc erning the strength distribution, and the load-bearing function of the cortical shell should be considered in future finite element models o f the glenoid.