Abstract
It has been known for a while that the cup angle has an influence on the longevity of artificial hip joints. Retrieval analysis has, however, been difficult, because traditional techniques for retrieval analysis have a number of disadvantages. They are either slow, have a low resolution or a low coverage of the joint under study. A new measuring instrument has been developed and is described to overcome these limitations.
At the example of a simulator device, a McKee-Farrar device and a failed resurfacing device, the different characteristics of wear scars in an ideal case (simulator device at correct angle), a first generation metal-on-metal joint (McKee-Farrar device at correct angle) and a resurfacing device (at wrong angle) are demonstrated.
The simulator device and the McKee-Farrar device show comparatively low wear, while the failed resurfacing device has large edge wear, emphasizing the importance of cup angle on joint performance.
Introduction
Retrieval analysis is an important tool for gaining an understanding of in vivo wear and failure mechanisms in artificial hip joints. However, the 3D measurement of wear patches on large spherical bearings has been difficult.
At present, there are a number of different techniques used to evaluate wear on artificial hip joints. The most common techniques are the gravimetric method, roundness measurement and measurement with a coordinate measuring machine (CMM).
Gravimetric Methods
As the hip joint wears, material is eroded from the surface of the ball and cup. This loss of material can be measured by weighing the components before and after the wear process. The challenge here is to measure weight loss in the order of milligrams on a component that can weigh a few hundred grams.
Because of possible contamination from the fluids surrounding the joint, gravimetric methods generally show an increase in mass before showing a decrease and additional techniques have to be employed to compensate for this error 1. The gravimetric method cannot give information on the shape of the wear patch and can only be used on joints that are being evaluated in wear simulators.
Roundness Measurement
A better way of measuring wear uses a roundness instrument to measure deviations from roundness. This is a two dimensional method that gives the user information on a trace around the surface. Such a machine can measure with a resolution of 10 nm, and because it can evaluate discrete surface traces, the original unworn areas can be compared with the worn ones. Thus the wear of clinical retrievals can be evaluated.
The roundness instrument is well suited to measuring linear wear, provided enough measurements in various planes around the joint are made and the deepest point of the wear scar is found. Kanada [2] calculates that for a ball of up to 38 mm (1.5″) in diameter an average of 9 measurements are required, while for a ball above 50 mm (2″) in diameter, 18 or more measurements can be necessary to characterise its roundness.
Coordinate Measuring Machine
Recent studies have used CMMs to measure wear on artificial hip joints 1, 3, 4. A coordinate measuring machine gives a three dimensional image of the joint by scanning the surface with a stylus. The CMM can scan the whole spherical area of a hip joint, but it has a lower resolution of typically 4
