Abstract
The current choice of materials for the knee prosthesis is limited to metals for the femoral component and UHMWPE for the tibial bearing surface. The clinical experience in the hip surgery showed the great advantage of using ceramic heads regarding the reduction of polyethylene wear debris, especially zirconia. The first implantations of an alumina knee prosthesis in the 80ies confirmed the expected wear reduction but remained isolated because of the poor mechanical properties of alumina. The aim of this study was to evaluate the mechanical safety of a zirconia knee femoral component, with the help of Finite Element Analysis (FEA) and experimental burst testing.
A 3D FEA model reproduced the Cemented Foundation Total Knee System (Encore Orthopedics) with a zirconia femoral component. Flexion was reproduced up to 100° under a 7 kN load. When the flexion angle increases, the maximum tensile stress in zirconia moves from the base of the fixation pegs to the posterior end. The stress value remains around 40 MPa, which must be compared with the flexural strength of zirconia (> 1300 MPa). Concerning the UHMWPE tibial insert, the stress levels observed are equivalent to current metal-UHMWPE knees.
Rupture tests were also defined in order to evaluate the burst strength of the ceramic part. In the first testing configuration the maximum load occurs at the inner chamfers and the mean burst strength of the ceramic implants is 34.8 kN (±6.8). In the second rupture test, the parts broke at 13.0 kN (±2.0). Both types and levels of rupture were confirmed by FEA simulation.
In conclusion the FEA showed that regular physiological conditions are secure for the zirconia component. Experimental burst loads show a safety factor of 2 or more in comparison with the maximal load in the body.
