Development of a Fused Metrology Method for Detailed Analysis of Worn Hip Implant Bearing Components

Due to the prevalence of aseptic loosening induced by wear particles, a common focus of hip implant experimental studies is the wear performance of the implant. Coordinate measurement machines (CMM) are commonly used for volumetric wear measurements. Allowing a wear map of the bearing surface to be created, and the size and location of wear areas to be analysed. To understand wear mechanisms and tribological behaviour on a surface scale, areal surface measurements are used.

Most manufacturing companies engaged in orthopaedic manufacture have access to high quality CMMs and surface metrology systems. In fact, these are required for compliance with company and international quality standards. However, they do not in general have the ability to combine outputs of these systems in a fused data context to allow them to investigate tribological/wear behaviour.

There is therefore the opportunity to develop an advanced fixture system specifically designed to combine CMM and surface measurement data together for detailed hip implant bearing analysis. Allowing for high accuracy positional targeting of surface measurements anywhere on a worn bearing surface. Such a fixture system with data fusion capability will improve future hip implant surface measurement studies, ensuring the most critical surface topography data is captured easily and quickly. This fixture system also aims to stitch surface measurements through wear areas, this currently has never been achieved and would be a novel step forward in analysis of worn hip implant components.

A motorised fixture was developed, utilising custom designed brackets and two precision rotary stages, allowing for measurement access to the entire bearing surface. The size of the fixture is minimal, 271 x 155 x 93 mm (LWH), ensuring it is compatible with the majority of surface metrology instruments. The software for the fixture system was developed on MATLAB, this software allows for: wear map creation from CMM data; selection of surface measurement locations in relation to the wear map; and control of the fixture. Once developed, the fixture system underwent thorough testing using CMM measurement, allowing the positional error of the fixture to be calculated.

The fixture system was utilised to measure a range of retrieved large MoM femoral heads. Of the ten femoral heads, two showed clear wear areas, with wear volumes of 3.5 mm3 and 3.0 mm3. A series of surface measurements were taken on the worn femoral heads, with the corresponding CMM wear maps used as positional reference. The surface within the wear area roughened for both femoral heads, increasing from 32 to 138 nm (Sa), and 11 to 119 nm (Sa).

Surface topography mapping showed that the area of roughening did not closely match the shape of the CMM defined wear area. The area of roughening could extend out of the wear area or be contained to a very small portion of the wear area. Stitches of surface measurements through the wear area were completed, highlighting crucial stages, including the boundary regions. Allowing for a deeper understanding of the tribological mechanisms occurring.

Through the successful development of the fixture system, a novel surface measurement method for worn hip implant bearings has been created, facilitating accurate data fusion of CMM and surface measurement datasets. Newly developed surface analysis tools such as surface topography mapping and stitching through the wear area allows the worn bearing surface to be studied at a level of detail previously unattainable.

This method will allow for better judgment of hip implant performance and can be utilised for both in-vitro and in-vivo testing for all material types. Alternatively, the fixture system can also be useful for quality inspection of newly manufactured components. As the development cost of the fixture was kept minimal, redevelopment across industry or research environments is achievable.