Radiographic Analysis of Total Hip Replacements
Measurement of Acetabular Wear Using Active Ellipses
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Total Hip Replacement (THR) is an increasingly common operation within the United Kingdom, with over 40,000 occurring per annum, 5,000 of which are revisions. Patients are typically elderly, but there is an increase in the number of younger patients, requiring a longer life-span of prosthesis.

The majority of failures are due to the displacement of the centre of the femoral head relative to the centre of the acetabular cup (acetabular wear). The femoral head and rim of the acetabular cup appear elliptical in radiographs. This knowledge of shape can be used to measure acetabular wear. Current approaches range from manual methods that estimate the centre of the femoral head using transparent overlays of concentric circles to semi-automated methods using Sobel edge detection.

Our aim is to create an automated, accurate and repeatable method that improves on existing performance, thus a new method is under development. Active ellipses with prior knowledge of the intended contour are used to search for the boundaries of the femoral head and acetabular rim. The method has been validated by comparison with a labour intensive method. Active ellipses have been found to be more repeatable and the two methods agreed to the extent that they were considered clinically interchangeable.

Funded by: Zimmer Corporation