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Metal on Metal ion levels: safe upper limits for MoM

Josephine

NURSE DIRECTOR EMERITUS
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Researchers set safe upper limit metal ion levels for metal-on-metal hip resurfacings
Orthopaedics Today - Europe [Archives], Issue 4
The limits here are lower than those of the UK’s Medicines and Healthcare Products Regulatory Agency.

An award-winning study by researchers from Belgium identified safe upper limits of 4.6 µg/L for chromium and 4.0 µg/L for cobalt in unilateral metal-on-metal hip resurfacing arthroplasties and 7.4 µg/L for chromium and 5.0 µg/L for cobalt in bilateral procedures.

“Levels higher than [these] established limits were significantly correlated with clinical symptoms, smaller component size, a smaller coverage arc, smaller contact patch to rim distance and a higher cup inclination,” Catherine Van Der Straeten, MD, said. “Females and small head sizes are more at risk of having problematic hips.” Van Der Straeten presented results of the Otto Aufranc Award-winning study at the Hip Society Specialty Day 2012 in San Francisco. A receiver operating characteristic (ROC) analysis showed that the limits had high specificity and low sensitivity for predicting poor function.

“These upper acceptable limits are lower than the recommended threshold by the Medicines and Healthcare Products Regulatory Agency [in the United Kingdom],” Van Der Straeten said. “But, we had low tolerance for what we call the clinically problematic hip.”

Database study
Van Der Straeten and colleagues performed a retrospective study of metal ion levels in unilateral and bilateral metal-on-metal hip resurfacings at minimum 12-month follow-up using from a database in Ghent. They excluded any other sources of metal ions and any renal insufficiency, and identified 453 patients with unilateral and 139 patients with bilateral hip resurfacings. The patients had a mean 4.3 years follow-up. There were eight resurfacing designs in the unilateral group and seven designs in the bilateral group. The researchers then categorized the patients into either an optimum (well-functioning) or non-optimum group.

“The criteria to be fulfilled for allocation into the optimum group are strict: no patient-reported hip complaints, no surgeon-detected clinical findings, a Harris Hip score higher than 95, the contact patch to rim distance greater than 10 mm, no abnormal radiological findings and no further operations scheduled,” Van Der Straeten said.

The optimum group consisted of 55% of unilateral and 42% of bilateral hip resurfacings. Most of the male patients were in the optimum group, while most female patients were in the nonoptimum cohort. The researchers found larger sized components in the optimum group. They also discovered significantly lower metal ion levels for the unilateral and bilateral resurfacings in the optimum group compared those in the non-optimum cohort.

The researchers defined safe upper metal ion limits as “the highest values not considered as outliers in the optimum group,” Van Der Straeten said. Using the safe upper limits established by the study showed 95% specificity for unilaterals and 91% for bilaterals. Overall, the limits had 25% sensitivity. When cobalt or chromium levels reached higher than 10 µg/L, specificity was 100%, Van Der Straeten noted.

Therapeutic algorithm
Symptoms other than metal wear may cause painful hips, Van Der Straeten said. Therefore, the researchers developed a diagnostic and therapeutic algorithm for these patients.

“We always start from the clinical symptoms,” she said. “Second, we look at the X-rays and thirdly, at metal ions in order to decide whether additional investigations are necessary or therapeutic action needs to be undertaken.”

If the researchers find metal ions levels lower than 4 µg/L in the absence of clinical and radiological symptoms, patients undergo routine follow-up. If levels are between 4 µg/L and 10 µg/L, metal ions are moderately elevated and “thorough diagnostic investigations have to be repeated until you find the cause,” Van Der Straeten said. If metal ion levels are higher than 20 µg/L, there may be systemic toxicity and diagnostic investigations are required. Revision should be “considered even without symptoms,” she said.

“Metal ions are to be used as an adjunct to standard clinical and radiographic follow-up,” Van Der Straeten added.
 

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