Abstract
Today, hip prostheses are validated with Standards for fatigue testing: The Standard ISO 7206-4 requires to test 6 components at 230daN during 5 × 106 cycles without crack. For the neck region of stemmed femoral components, the Standard ISO 7206-6 requires 6 tests at 534daN during 10 × 106 cycles without crack. But these tests don't provide provide any indication on reliability level for an implantation in population.
At the same time, the number of hip prosthesis implantation is growing, patients are implanted younger and younger and they want to be able to maintain a “normal” life. This way the average “loading spectrum” is getting tougher and tougher, due to this modification of the use of prosthesis in comparison with some years ago. On the other hand, there is new materials, new processes (additive manufacturing), new methods (customized stems…) with no feedback on reliability. A method is then necessary to manage the reliability in fatigue for actual and new products.
The objective of this study is to establish a statistical distribution of loading of hip prosthesis in order to define at best a minimum value of strength required for a good fatigue design.
To define this strength, the Stress-Strength (well known in automotive sector) approach is applied (fig 1). This approach will allow better assess the reliability in a population, depending on the mean strength and the scattering in fatigue.
The first step is to establish the distribution of the loads for a hip prosthesis. Then, for a given risk level, the required strength can be defined, knowing the scattering of this strength.
The strategy to have the distribution is based on:
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In vivo load recordings on hip prosthesis (find on Orthoload.com),
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Analysis of frequency of everyday activities,
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Activity level of different category of the population,
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Statistical distribution of key parameters, like weight, age…
All these data are collected in the literature, and combined, then processed with the software DEFFI®. The goal is first to assess the reliability reached by a “nominal” stem and compare it to the reliability described in implant registers. Another goal is to analyse the stress distribution and compare it to standard requests (ISO 7206-6), in order to assess the reliability of an implant that succeeded this standard.
A last, this method is a way to define the minimum strength for implants dedicated to particular populations: young and active patients, patients with high Body Weight, etc…
For any figures or tables, please contact authors directly (see Info & Metrics tab above).
