Elevated serum ion levels reported after metal-on-metal hip replacement

Concentrations were twice as high in bilateral THR patients than in a matched unilateral group.

Issue: Issue 2 2004

British flag Serum metal ion concentrations in patients who underwent various types of total hip arthroplasty with metal-on-metal articulations were significantly elevated in those who had bilateral or hip resurfacing arthroplasty, according to a British researcher.

“As the threshold for chronic toxic levels of serum cobalt and chromium are still unknown, long-term follow-up studies on patients with such metal-on-metal bearings are important,” said Paul T.H. Lee, clinical research fellow in the Orthopaedic Research Unit at Cambridge University, England.

During the British Orthopaedic Association (BOA) Annual Congress, Lee presented results of two studies conducted at his center that quantified serum cobalt and chromium ion concentrations following metal-on-metal hip arthroplasty surgery.

In the first study, researchers compared serum ion levels in bilateral and unilateral metal-on-metal total hip replacement (THR) patients. Results showed significantly higher serum cobalt and chromium concentrations in the bilaterally operated group.

Bilateral vs. unilateral cases

BOA 2003 [logo] Participants underwent unilateral or bilateral metal-on-metal THR done by the same senior surgeon at Cambridge Lea Hospital from 1995 to 2001. All patients were implanted with a DePuy Ultima prosthesis with a 28-mm-diameter bearing made of cobalt chrome molybdenum (CoCrMo) alloy through a posterior approach. Minimum follow-up was 12 months.

The pressfit acetabular shells were made of titanium alloy. Cemented femoral stems were CoCrMo alloy in all cases, except for three in the bilateral group, whose stems were made of titanium alloy.

Bilateral and unilateral patients were matched according to time since surgery to within six months, body mass to within a <10 kg difference, and activity level intensity. Lee assessed the average daily activity level during the week prior to blood sampling and categorized patients into four intensity groups.

Matched pairs studied

Researchers used inductively coupled plasma mass spectrometry (ICPMS) to analyze serum samples for cobalt and chromium blood levels. Results were analyzed with Mann-Whitney and Student’s t-tests.

Twenty-two of 108 patients meeting the inclusion criteria were matched to create 11 matched pairs, with no significant differences between them in age, gender, body mass, time since surgery or activity level.

Researchers found median serum cobalt levels were 42 nmol/L in bilateral patients compared to 22 nmol/L in unilateral patients. Median serum chromium levels were 52 nmol/L in bilateral and 19 nmol/L in unilateral patients. These differences were statistically significant.

“We have shown that the median serum cobalt and chromium ion levels in patients after bilateral metal-on-metal THR were about two times more than the median level in patients after unilateral metal-on-metal THR,” he said.

Does bearing size matter?

The second study explored bearing size as a factor affecting serum metal ion levels in unilateral hip arthroplasty patients.

“Hypothetically, larger bearing metal-on-metal resurfacing hip arthroplasty may produce less particulate wear debris with lower subsequent metal ion release compared to small bearing metal-on-metal THR,” Lee said. Metal implant corrosion has been shown to be in part a product of surface area exposed to the body and surface area of any particles produced.

“Most of the metal-on-metal bearings in this study produced elevated serum cobalt and chromium levels,” Lee said. The large bearings used for metal-on-metal hip resurfacing arthroplasty, however, produced higher serum metal ion levels.

THR procedures were done using the DePuy Ultima prosthesis. Hip resurfacing arthroplasty was done with the Birmingham prosthesis manufactured by Midland Medical Technologies or Corin Surgical’s Cormet 2000 prosthesis. Using inclusion criteria from the first study, 22 THR patients were matched with 22 resurfacing arthroplasty patients, with no significant differences between the groups. “The bearing size used for the resurfacing was almost twice the size used for the THR,” he said.

Concentrations were determined by analyzing serum samples using ICPMS. Lee used the same tests for the statistical analysis.

In the resurfacing patients implanted with larger bearings, median cobalt ion levels were 38 nmol/L (range 14-144) compared to 22 nmol/L (range 15 to 87) in those with 28-mm bearing THR (P=.0021). Median chromium ion levels were 53 nmol/L (range 25 to 165) and 19 nmol/L (range 2 to 58) in patients with the large and small bearings, respectively (P<.0001). Differences between the groups were highly statistically significant for both metal levels.

“We have shown that the small bearing metal-on-metal THR produced significantly lower serum metal ion levels than large bearing metal-on-metal resurfacing hip arthroplasty. This finding may be relevant to the potential for long-term side effects for patients with chronically elevated serum metal ions.”

Commenting on the second study, which was published in the British Edition of the Journal of Bone and Joint Surgery, Joshua J. Jacobs, MD, said the surface replacement and total hip components studied were fabricated from two different alloys. The surface replacement components were fabricated from cast CoCrMo alloy, while the total hip components were fabricated from wrought CoCrMo alloy (ASTM F1537: acetabular shell high carbon content; femoral head low carbon content).

“This difference in the composition of the bearing couples really complicates the analysis, so that it’s unclear whether the effect they’re observing is secondary to the diameter of the bearing surfaces or secondary to the wear resistance of the different CoCrMo alloys,” Jacobs said.

In a study Jacobs et al conducted using cast CoCrMo alloys of identical composition (ASTM F-75) for metal-on-metal total hip and resurfacing arthroplasty with 36-month follow-up (minimum), investigators found no difference in serum or urine Co and Cr levels between patients with surface replacements and patients with total hip replacements.

For more information:

Lee PTH. The effect of bilateral metal-on-metal hip arthroplasty on serum metal ion concentrations. Serum metal levels after metal-on-metal hip arthroplasty: does bearing size matter? Both talks presented at the British Orthopaedic Association Annual Congress. Sept. 17-19, 2003. Birmingham, England.

Clarke MT, Lee PTH, Arora A, Villar RN. Levels of metal ions after small- and large-diameter metal-on-metal hip arthroplasty. J Bone Joint Surg [Br] 2003;85-B:913-17.