Blood metal ion levels remained stable 5 years after MoM THA

An analysis showed patient gender, never-smoking status were associated with increased cobalt levels in small-head hip arthroplasty.

Issue: December 2017

At 5 years postoperatively, most patients who underwent metal-on-metal total hip arthroplasty had stable levels of blood cobalt ions according to recently presented data.

“We know there are some predictors for elevated blood metal ion levels in large metal-on-metal (MoM) THA. Our study shows there are also risk factors in small-head MoM THA, which are independent of the implant and we have to be aware about that,” Amanda Gonzalez, MD, of the Hôpitaux Universitaires de Geneve, told Orthopedics Today.

Although only a few patients with THA in this study had a second measurement of their cobalt ion levels, “the cobalt level remained stable over time in the majority of patients,” she said.

Gonzalez and her colleagues stratified 447 MoM THA cases in which patients presented with postoperative blood metal ion concentration into a small-head group (n=355) and a large-head group (n=92). They evaluated patients according to age, sex, BMI, smoking status, American Society of Anesthesiologists (ASA) score, Charnley score, aspirin use, diagnosis, bilateral MoM THA, stem type, cup diameter, cup inclination and side operated. Blood cobalt levels of 2 µg/L or more were considered elevated.

“In the multivariate analysis, female [gender], ASA score 3 to 4, bilateral MoM THA, aspirin use and never-smoking remained significant predictors of cobalt concentration [greater than] 2 µg/L in small-head THA,” Gonzalez said at a meeting.

To assess the evolution of cobalt levels over time, only unilateral THAs were included: 239 patients in the small-head group; and 56 patients in the large-head group. Researchers noted cobalt levels were 2 µg/L or more in 17% of patients in the small-head group and in 45% of patients in the large-head group. Among the 48 patients who had second cobalt measures, six patients had their cobalt levels change in concentration between the first and second measurement, Gonzalez said. However, about 86% of patients had stable cobalt levels.

Gonzalez said she was surprised aspirin was a predictor of elevated blood cobalt levels. “Patients with cardiac implants, such as bare metal stents, are usually taking aspirin,” she told Orthopedics Today . “We hypothesize that their presence, but not the aspirin itself, might be the reason for higher cobalt levels in these patients. However, further investigation is needed.”

Regarding the cobalt level evolution over time, these are primary results and more data are needed to confirm them, she noted.

“We are going to update our data and redo the analysis,” Gonzalez told Orthopedics Today . “I do not think the results are going to change but, by including more patients, the study will have more power. The research question is an important one, because if cobalt levels remain constant over time (beyond 5 years after surgery), repeat testing, as recommended currently, may become unnecessary.” – by Casey Tingle

Reference:
Gonzalez A, et al. Paper #2171. Presented at: 18th EFORT Annual Congress; May 31-June 2, 2017; Vienna.

For more information:
Amanda Gonzalez, MD, can be reached at Rue Gabrielle-Perret-Gentil 4, 1205 Geneva, Switzerland; email: amanda.gonzalez@hcuge.ch.

Disclosure: Gonzalez reports no relevant financial disclosures.