Eccentricity Error Correction for Automated Estimation of Polyethylene Wear after Total Hip Arthroplasty

Acetabular wear of total hip replacements can be estimated from radiographs based on the apparent displacement of the femoral head relative to the acetabular cup. A wire marker is often attached to the polyethylene cup rim and its projection can be modelled as an ellipse. The centre of this ellipse is not the projection of the centre of the rim so its use as a reference point to measure wear can be problematic. The implications of the resulting eccentricity errors were investigated. The 3D poses of acetabular cups estimated from projected ellipse parameters were used to estimate error bounds and expected error values. The effect of correcting for these errors on wear measurements was investigated using standard clinical anteroposterior radiographs and an automated ellipse fitting method

Eccentric Elliptical Contours in Total Hip Replacements

The active ellipses method for assessing wear in total hip replacements uses robust ellipse fitting to localise the contours of the femoral head and acetabular rim wire marker. In the case of the latter these ellipses can be very eccentric and the standard algebraic distance was shown to be inadequate. The geometric distance from an ellipse to a point is not trivial to compute and thus numerous error of fit functions have been created. In this work several of these error of fit functions are compared, including a geometric error of fit function, on both synthetic data and by using active ellipses on a set of test radiographs containing eccentric rims. Least squares estimation using a geometric error function was most accurate in the presence of Gaussian noise. However, least median of squares estimation using a geometric error function was most accurate in the presence of outliers. Furthermore, its performance was similar to that of a computationally cheaper error function known as the foci bisector distance to the extent that the two were almost interchangeable

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

Measurement of Acetabular Wear Using Intelligent Ellipses

A summary of recent studies reported that of a sample of 100 Total Hip Replacements (THRs) 18 revisions occurred, 16 of which were due to osteolysis and loosening of the acetabular cup. Thus tracking acetabular wear is a key step in the early detection of failure in THRs. The femoral head and rim of the acetabular cup appear elliptical in radiographs regardless of orientation. 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. It involves the use of intelligent ellipses that learn to identify the typical change of the grey level at the femoral head from a set of manually annotated digitized radiographs. When trained these intelligent ellipses are placed within a previously unseen digitised radiograph of a THR. Searching occurs along normals to the intelligent ellipses and comparing the normalized grey level derivative profiles (a measurement of the gradient) to a statistical model of the typical grey level derivative around a femoral head. The location and other parameters of the intelligent ellipses are altered to best match the model. After several searches the ellipse converges around the femoral head in the radiograph. In a similar fashion the rim of the acetabular cup can be located. Once the centres have been found the distance from the centre of the femoral ellipse to the centre of the acetabular ellipse can be calculated