The 2012 Otto Aufranc Award: The interpretation of metal ion levels in unilateral and bilateral hip resurfacing

Abstract

Background: The interpretation of metal ion concentrations and their role in clinical management of patients with metal-on-metal implants is still controversial.

Questions/purposes: We questioned whether patients undergoing hip resurfacing with no clinical problems could be differentiated from those with clinical (pain, loss of function) and/or radiographic (component malpositioning, migration, bone loss), problems based on metal ion levels, and if there was a threshold metal level that predicted the need for clinical intervention. Furthermore, we asked if patient and implant factors differed between these functional groups.

Methods: We retrospectively identified 453 unilateral and 139 bilateral patients with ion measurements at minimum followup of 12 months (mean, 4.3 years; range, 1-12.9 years). Patients were designated as well functioning or poorly functioning based on strict criteria. The acceptable upper levels within the well-functioning group were determined from the 75th percentile plus 1.5× interquartile range. The sensitivity and specificity of these levels to predict clinical problems were calculated.

Results: Well-functioning group ions were lower than the poorly functioning group ion levels. The acceptable upper levels were: chromium (Cr) 4.6 μg/L, cobalt (Co) 4.0 μg/L unilateral and Cr 7.4 μg/L, Co 5.0 μg/L bilateral. The specificity of these levels in predicting poor function was high (95%) and sensitivity was low (25%). There were more males in the well-functioning group and more females and smaller femoral components in the poorly functioning group.

Conclusions: Metal levels higher than these proposed safe upper limits can predict problems with metal-on-metal resurfacings and are important parameters in the management of at-risk patients.

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

Fig. 1A–B

(A) Box plots of serum Cr and Co levels in the well-functioning and poorly functioning groups for unilateral HRA. Well-functioning group patients have lower (p < 0.001) levels compared with poorly functioning group patients. There is a predominance of females among the outliers. (B) Box plots of serum Cr and Co levels in the well-functioning and poorly functioning groups for bilateral HRA. Like in the unilateral group, well-functioning group patients have lower (p < 0.001) levels compared with poorly functioning group patients and there is a predominance of females among the outliers.

Fig. 2

Acceptable upper limits of Cr and Co levels for unilateral and bilateral HRA were established as the highest values, which were not considered as outliers for the well-functioning group patients. The definition used for the upper limit was (75th percentile) + 1.5 × (interquartile range) = top margin of the box and whisker plot [12]. Lower ion levels (p < 0.001) were found with unilateral HRA compared with bilateral HRA.

Fig. 3A–B

(A) ROC curve demonstrating the power (sensitivity and specificity) of serum Cr and Co for diagnosing clinically relevant problems in unilateral HRA. For Cr, area under the curve (AUC) was 0.67 (0.62–0.72), whereas for Co, AUC was 0.65 (0.59–0.70). Sensitivity and specificity of the upper limits in predicting poor function were, respectively, 25% and 95% for Cr and 22% and 96% for Co. (B) ROC curve demonstrating the power (sensitivity and specificity) of serum Cr and Co for diagnosing clinically relevant problems in bilateral HRA. For Cr, AUC was 0.79 (0.71–0.86); for Co, AUC was 0.76 (0.68–0.84). Sensitivity and specificity of the safe upper levels for bilateral HRA were, respectively, 43% and 93% for Cr and 38.6% and 91% for Co.

Fig. 4A–B

(A) Differences in sex and component size between the well-functioning and poorly functioning group in unilateral HRA. The majority of males (57%) were in the well-functioning group compared with the majority of females (64%) who were in the poorly functioning group. Well-functioning group patients had larger sized components (mean femoral head size, 50.4 mm) compared with poorly functioning group patients (mean, 48.6 mm) (p < 0.001). (B) Differences in sex and component size between the well-functioning and poorly functioning groups in bilateral HRA. Like with unilateral HRA, the majority of males (61%) were in the well-functioning group compared with the majority of females (72%) in the poorly functioning group. Optimum group patients had larger sized components (mean femoral head size, 51.5 mm) compared with poorly functioning group patients (mean, 48.0 mm) (p < 0.001).

Fig. 5

Diagnostic and therapeutic algorithm for the followup of a hip resurfacing arthroplasty is shown.

Fig. 6A–B

(A) Coordinate measuring machine (CMM)-derived wear depth map of a retrieved Durom HRA, head size 44 mm, showing low wear (maximum, 8.5 μm). This was implanted in a 26-year-old female patient with congenital hip dysplasia and revised after 19 months for cup loosening. Prerevision cup position measured with EBRA was 50° inclination and 14° anteversion. Metal ions prerevision were Cr: 1.6 μg/L, Co: 0.5 μg/L. (B) Same retrieved Durom HRA: scanning electron microscopy (SEM) picture of the bearing area approximately 5000× displaying only occasional scratches and a smooth background surface.