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.

Figure 1.

3-dimensional image reconstructions of a CT-scanned all-poly acetabular component assembled with its matching femoral stem and 28-mm diameter head. The images were produced using the surface rendering function in OsiriX and illustrate the ‘partial voluming effect’ that increases with thickness of CT section slice: A. 5 mm; B. 2.5 mm; C. 1.25 mm; D. 0.625 mm.

Figure 2.

Digitization of points on 3-dimensional image reconstructions of CT- scanned implants. A and B. An all-poly acetabular component assembled with its matching femoral stem and 28-mm diameter head. C and D. A metal-backed PE acetabular component assembled with its matching femoral stem and 32-mm diameter head.

Figure 3.

Calculation using 3DReshaper, by the method of least squares, of the center and diameter of a spherical femoral head (panel A) and the center and diameter of a circular rim of a metal-backed acetabular component (panel B).