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.