Abstract
Introduction
In knee arthroplasty a ceramic component has several advantages: first, there is no ion release implying a risk for potential allergies. Second, the hardness of the material leads to a scratch resistance which ultimately reduces PE wear over time. In the past, ceramic components in knee applications were limited in the variety of design possibilities due to necessary thickness of the component resulting from the associated fracture risk of ceramics.
By the development of an alumina matrix composite material with increased mechanical properties it is possible to develop ceramic knee components which have nearly the same design as a metal component and use the same implantation technique as well as the same instruments. This offers the surgeon the opportunity to choose intraoperatively between metal or ceramic knee components. Extensive in-vitro testing shows that ceramic knee components achieve superior mechanical test results. The reliability of the components is proven by two different burst tests and a fatigue test for both a femoral and a tibial ceramic knee component.
Material and method
The mechanical proof-test was developed by subsequent steps of numerical load/stress analysis and design of an adequate mechanical test equipment. The procedure was organized as follows:
Oncologic: Analysis of relevant maximum in-vivo loading conditions
Analysis of the “boundary conditions”
Finite Element analysis: Identifying regions of highest stress concentration
Design analysis and accommodation if necessary
Development of an adequate mechanical test equipment which produces stresses comparable to the in-vivo conditions
Performing mechanical tests with ceramic femoral components
Validation of the test concept: comparison of test results and stress analysis
Assign “safety margin”,
Establish “proof test”
Results
Two independent load scenarios have been determined for each type of components as being in-vivo relevant. Hence, the developed proof-test consists of two subsequent load tests, the so-called regular test and the tension test for the femoral components, and the upper side test and the lower side test for the tibial 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.
Discussion
The procedure to determine the proof loads using the maximum in-vivo loads together with a safety factor ensures the mechanical safety of the ceramic knee component. Together with the well-known excellent wear and biological behaviour of ceramics, this application provides an alternative to common metallic knee components. Clinical observations in the framework of a multi-centre study in different European countries have been started and show very promising results.
