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Finsbury Orthopaedics Ltd

This application was developed in collaboration with Finsbury Orthopaedics Ltd For this hip socket wear volume application the Zygo GPI Inteferometer was used to cover as large an area as possible. The GPI is controlled by MetroPro, a powerful software package that allows sophisticated processing of interference images to generate quantitative data from measured surfaces. The GPI allows form, flatness and sphericity to be measured with unprecedented accuracy and can measure the radius of curvature for both concave and convex spherical surfaces.Below is shown the output interference image from a Zygo GPI interferometer measuring a hip joint socket that is 50mm diameter. The almost circular wear area can be clearly seen offset from the centre. The diameter that can be observed with this GPI particular configuration (using a Reference Sphere =0.58), allows observation of a diameter of 43 mm. The interference pattern indicates the steepness of the surface slope, the closer the interference fringes the steeper the surface.

Interferometer measurement of wear on hip joint
Interferometer measurement on hip joint, showing worn area and unworn area.


In this image the interference data has been processed by the Zygo MetroPro software to provide height information and colour coded for clarity (red indicates high areas and blue low) showing the spherical form of the hip joint socket and the depression caused by wear of the socket in contact with the head.

Colour-coded image of wear on hip joint
Colour-coded interferometer image of hip joint, showing worn area and unworn area.


To measure wear volume the unworn spherical surface around the periphery of the wear area is used as a reference surface through an image masking technique in order for the MetroPro software to calculate the wear volume

Wear volume on hip joint
Wear volume is 1.104 cubic mm, referenced to unworn spherical surface


Wear depth 7 microns
Snap line taken across the colour coded height image provides a 2D profile plot of the worn area. In this sample peak depth is approximately 7 microns.


2D profile plot
Snap line taken across the colour coded height image provides a 2D profile plot of the worn area. In this sample peak depth is approximately 7 microns.