Measurement of femoral head penetration in polyethylene using a 3-dimensional CT-scan technique

Abstract

Background Current techniques for measuring in vivo polyethylene wear suffer from a range of problems, resulting in an unacceptable lack of repeatability and/or insufficient accuracy when they are used to measure the low wear rates associated with new, highly crosslinked polyethylene. We describe an improved CT method for measurement of 3D femoral head penetration in PE acetabular cups that has sufficient accuracy and repeatability to allow assessment of the wear potential of modern implants.

Method The accuracy and repeatability of the CT-scan method was determined by blindly repeating measurements on a precisely calibrated 28-mm prosthetic head and by comparing them with direct metrological measurements on 10 acetabular specimens with in vitro wear from machining, and on 8 explanted acetabular specimens with in vivo wear.

Results The intra- and interobserver errors in femoral head diameter were 0.036 mm (SD 0.044) and 0.050 mm (SD 0.022), respectively. CT estimated femoral head penetration in both all-poly and metal-backed acetabular components with accuracy ranging from 0.009 to 0.245 mm (mean 0.080; SD 0.067).

Interpretation We found that the CT method is rapid, is accurate, and has repeatability and ease of availability. Using a slice thickness of 0.0625 mm, this method can detect wear—and also the threshold for the wear rate that causes osteolysis—much earlier than previous methods.

TG was the principal investigator of the study and was supervised by BA and UH. MS established the methodology of the study and carried out the statistical analysis, while EV determined the accuracy and repeatability of the technique. CM and FT carried out the CT scanning and supervised analysis of the images. All authors contributed to interpretation of the results and to writing of the article.

No competing interests declared.