Analysis of Total Hip Replacements Using Active Ellipses

In this paper we propose a new method for the measurement of wear of a total hip replacement. Our
method exhibits a greater degree of automation and is to be both accurate and repeatable. Measurement of wear
can be quantified as the displacement of the centre of the femoral head relative to the centre of the acetabular
cup or acetabular rim. Our method uses active ellipses - ellipses that, with prior knowledge of the intended
contour, search for and alter shape to segment the boundary of the head and rim.
A set of radiographs are manually annotated and the characteristics of the boundary of the femoral head and
acetabular rim are learned. Two ellipses are sequentially placed on the radiograph, the first deforming around
the boundary of the femoral head, the second placed using the previously learned average shape of the acetabular
rim and converges around the wire marker. Once both ellipses have converged the distance between their two
centres can be calculated and converted to mm as a measure of wear.
Our method is validated by comparison with manual fitting of ellipses. A discussion of the results, the clinical
relevance and further investigations concludes this paper.