Ceramic-on-metal THR shows lower whole blood metal ion levels than metal-on-metal construct

Good component position provides optimum clinical results, according to researchers.

Investigators found lower whole blood metal ion levels after total hip replacement using a ceramic head and metal liner compared to replacement using a metal-on-metal construct.

“Early, 1-year clinical data support the laboratory finding that ceramic-on-metal (CoM) wears less than metal-on-metal (MoM),” Graham Isaac, PhD, told Orthopaedics Today Europe. Isaac presented the study at the 55th Annual Meeting of the Orthopaedic Research Society.

Same manufacturer

In a prospective clinical trial, Isaac and his colleagues compared the whole blood metal ion levels of 24 patients who underwent total hip replacement (THR) using a ceramic head and metal liner with a group of 22 patients who had the procedure performed with a metal head and metal liner. All of the patients received 28-mm diameter heads, and all of the components were manufactured by DePuy International.

The investigators collected whole blood samples preoperatively and at 3 months, 12 months and more than 24 months postoperatively. The samples were analyzed using high-resolution Inductively Coupled Plasma-Mass Spectrometry. The detection limits for cobalt and chromium were set at 0.015 ug/l and 0.1 ug/l, respectively.

The investigators found that the groups showed similar metal ion levels preoperatively, with higher levels of cobalt compared to chromium. Interestingly, the background ion levels measured preoperatively comprised 30% to 50% of the overall value 12 months after implantation, he said.

Postoperatively, they discovered that the CoM group had lower median cobalt and chromium levels than the MoM group at all measured time points.

Increased levels

The CoM group showed lower increases in cobalt, chromium and combined ion levels than the MoM group at both 3 and 12 months. At 12 months, the CoM group showed a median increase of 0.35 µg/l in combined cobalt and chromium metal ion levels, while the MoM group showed a median increase of 0.96 µg/l.

This difference was significant in the chromium and combined ion levels compared to the MoM group at 12 months. The differences for cobalt alone were not significant at any time point, he said.

The investigators identified four outliers (ie, either cobalt or chromium levels greater than 10 µg/l) in each group.

“In common with previous studies, these were generally related to component position,” Isaac said. “With the exception of two marginal outliers, one in each group, the other six outliers had either a cup abduction angle of greater than 55°, an anteversion angle of greater than 30°, or both.”

Isaac also cited the short-term follow-up as a study limitation and offered a word of caution.

“It is important to emphasize that the combination being referred to is a ceramic head articulating against a metal cup liner. The reverse combination has been evaluated in the laboratory and does not perform as well,” he said.

Isaac also noted that the laboratory and clinical data presented in the study refer to a particular combination of head and cup supplied by DePuy, which is CE marked. “Other components may perform in a different manner,” he said.

For more information:

• Graham Isaac, PhD, can be reached at DePuy International Ltd., Number 1 White Rose Office Park, Leeds LS 11 OEA, England; +44-(0)113-387-7800; e-mail: gisaac@its.jnj.com. The work was supported by DePuy International Ltd.

Reference:

• Isaac G, Brekon A, van der Jagt D, et al. Whole blood metal ion levels after total hip replacement: A comparison of ceramic-on-metal and metal-on-metal bearings. Paper #45. Presented at the 55th Annual Meeting of the Orthopaedic Research Society. Feb. 22-25, 2009. Las Vegas.