Insufficient acetabular version increases blood metal ion levels after metal-on-metal hip resurfacing

Abstract

Background: Many factors affect the blood metal ion levels after metal-on-metal (MOM) hip arthroplasty. The main surgically adjustable variable is the amount of coverage of the head provided by the cup which is a function of the inclination and version angles. However, most studies have used plain radiographs which have questionable precision and accuracy, particularly for version and large diameter metal heads; further, these studies do not simultaneously assess version and inclination. Thus the relationship between version and blood metal ions levels has not been resolved.

Questions/purposes: We determined whether cup inclination and version influence blood metal ion levels while adjusting for age at assessment, gender, body mass index, horizontal femoral offset, head size, manufacturer hip type, and Oxford hip score.

Patients and methods: We prospectively followed 100 individuals (51 females, 49 males) with unilateral MOM hip resurfacing who underwent clinical assessment, CT scanning, and blood metal ion measurement. Multiple regression analysis was used to determine which variables were predictors of blood metal ion levels and to model the effect of these variables.

Results: Only cup inclination, version angles, and gender influenced blood cobalt or chromium levels. Cobalt and chromium levels positively correlated with inclination angle and negatively correlated with version angle. The effect of changes in version angle was less than for inclination angle. Based on our observations, we developed a formula to predict the effect of these parameters on metal ion levels.

Conclusions: Our data suggest insufficient cup version can cause high blood metal ions after MOM hip arthroplasty. We were unable to show that excessive version caused high levels.

Level of evidence: Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.

Fig. 1

An example of the 3-D CT image shows the cup face plane differentiated from the femoral head and the measurement points located around the rim of the cup.

Fig. 2

A graph shows CT-measured 3-D cup positions for 100 unilateral MOM hip arthroplasties. The lowest and highest values of each inclination and version angle are shown. The box delineates the safe zone suggested by Lewinnek et al. [30] that most surgeons aim for.

Fig. 3A–B

The relationship between blood Co levels and cup angles is shown by two line plots of the final statistical model. The estimated blood Co levels (ppb) for a given (A) inclination angle with all other explanatory variables kept constant and version set at the median for the data of 24.1°, and (B) for a given version angle with all other explanatory variables kept constant and version set at the median for the data of 48.2° are shown.

Fig. 4A–B

The relationship between blood Cr levels and cup angles is shown in line plots of the final statistical model used to estimate blood Cr levels (ppb) for a given inclination angle with all other explanatory variables kept constant and the version set at the median for the data of (A) 24.1°, and (B) 48.2°.