Abstract
INTRODUCTION
Since over 40 years, ceramics are known for their excellent biocompatibility, extremely low wear rates and excellent wettability. This would make a ceramic-on-Polyethylene bearing also a beneficial combination for a knee implant if potential strength issues could be overcome.
METHODS
A mechanical proof-test for a ceramic femoral knee implant component was developed by subsequent steps of numerical load/stress analysis and design of adequate mechanical test equipment. The procedure was organized as follows:
- 1.
Analysis of maximum in-vivo loading condition and distinguish between alternating regular loading with a high cycle number during life time and irregular worst case loading. The relevant regular loading is represented by rising from a chair and normal walking. The most critical irregular worst cases are stumbling or impact loading. The load transfer, stress distribution and the anticipated cycle number during life-time are distinguished and taken into account for the development of the test concept.
- 2.
Analysis of the “boundary conditions,” i.e. the fixation of the ceramic prosthesis on the bone identifying the worst-case conditions
- 3.
Finite Element analysis: Identifying regions of highest stress concentration at variable external loading
- 4.
Design analysis and accommodation if necessary
From step 3 it is evident that stress concentration is mainly generated by geometric features, e.g. the shape of the corners at the interface to the cement. Significant reduction of stress concentration was achieved by some minor corrections of design details.
- 1.
Development of an adequate mechanical test equipment which produces stresses comparable to the in-vivo conditions and performing of mechanical tests with ceramic femoral components
- 2.
Validation of the test concept: comparison of test results and stress analysis
- 3.
Assign “safety margin,” i.e. required overload tolerance of the ceramic component with respect to worst case load in-vivo
- 4.
Establish “proof test,” i.e. in-production mechanical testing of each individual component in order to provide safety margin
RESULTS
An in-vivo load scenario has been determined for a ceramic femoral component using a regular and a tension test for the femoral components. In the regular test, the mechanical strength of the polished outer condyles is tested using a force which is equivalent to an in-vivo loading of 16 times bodyweight. In the tension test, the interior sides of the condyles are stressed in the sagittal plane ensuring a mechanically reliable implantation. This test is performed with a force equivalent to 10 times bodyweight. The verification of the numerical calculations of the experimental test setup was done using strain gauges. It has been shown that the mechanical tests exactly produce the intended stress distribution for the femoral ceramic knee component. Using these new and validated test methods ceramic femoral components are safe for clinical use.
DISCUSSION
The procedure to determine proof loads using the maximum in-vivo loads together with a safety factor ensures the mechanical safety of a ceramic knee component. Together with its excellent tribological and biological behaviour of ceramics, this application provides an alternative to standard metallic knee components.
