The wear scar and the surrounding area have been measured. In this particular case, the hip head has been measured on an OrthoLux 4 AB with 901,800 data points.
This is usually achieved by fitting the nominal shape to the surface. Initially this includes all data points:
From this initial fit, it is possible to separate the original surface, which is later used as the datum for wear measurements from the wear scar.
The next step is to exclude measurement points on the wear scar. This important step allows establishing a clear datum surface for further calculations.
Now that the original, unworn surface has been established as a datum, the sphere fit can be repeated.
Steps 2 and 3 might have to be iterated a few times until all of the wear scar has been isolated and removed.
In this case, the wear scar is 53.7 µm deep and the wear volume is 18.6 mm ³.
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