Serum metal ion levels deemed safe after TDR in two short-term follow-up studies

Cobalt, chromium and titanium levels studied were comparable to those of other orthopedic devices.

Issue: April 2009

TORONTO — In two separate prospective longitudinal studies of patients with different total disc replacement devices, serum metal ion levels were similar at short-term follow-up to those typically seen in individuals with posterior spinal instrumentation or hip arthroplasty prostheses.

The studies, presented at the North American Spine Society 23rd Annual Meeting, here, looked at newer devices that are not FDA-approved; the Maverick Total Disc Replacement used at single lumbar levels and the Prestige LP Cervical Disc Replacement [both Medtronic] used for single-level cervical arthroplasty.

“Now we have quality data that the serum ion levels seen with the Prestige LP are no different than what … we have seen for different types of spine and orthopedic hardware used in patients for decades,” Matthew F. Gornet, MD, told Orthopedics Today.

Serum assays

Medtronic and the investigators felt it was important to patients to have postoperative serum ion levels quantified and possible risks of increased ion exposure identified ahead of more widespread use of these the implants, Gornet, lead author of both studies, said.

In the first study, investigators analyzed serum samples for chromium and cobalt levels from 24 patients with the metal-on-metal, cobalt chrome Maverick prosthesis at L4-S1, but without other internal or external metal exposure. Samples were taken preoperatively, at 3 and 6 months postoperative and at 12 months postoperative in 23 of the patients.

Results of serum assays performed by Joshua J. Jacobs, MD, at Rush University in Chicago, showed preoperative cobalt levels of 0.10 ng/mL increased to 1.03 ng/mL at 3 months. Such early increases are typical right after using metal implants, Gornet explained.

Representative sample

Cobalt levels were 0.98 ng/mL at 12 months, which investigators noted was equivalent to those seen with metal-on-metal hip resurfacing or total hip replacement surgery.

Median 3-month postoperative chromium levels in the Maverick patients were 0.49 ng/mL. Their 12-month levels of 0.43 ng/mL were lower than with metal hip arthroplasties, investigators noted in their abstract.

These patients comprised a representative sample and were well-controlled for study inclusion and exclusion criteria, Gornet said.

Titanium alloy composite

The second investigation involved 30 patients who received the Prestige LP implant made of a titanium alloy and ceramic composite material. Preoperative and 6-month postop serum ion assay results were available for 29 patients, however all 30 patients had 3-month serum ion level test results.

Titanium levels were 0.100 ng/mL preoperatively and 1.216, and 1.159 ng/mL at 3- and 6-months postop, respectively. Other studies that looked at the same ions in blood serum but used different methodologies showed higher amounts in the short-term, investigators noted.

Gornet said he and his colleagues have applied for approval to follow-up these patients through about 7 years postop. “I think this gives us a representation of what metallic implant exposure will do to ion levels,” he said.

For more information:

Matthew F. Gornet, MD, can be reached at the Orthopedic Center of St. Louis-Spine Research Center, 14825 N. Outer Forty Road, Suite 200, St. Louis, MO 63017; 314-392-5089; e-mail: mfgspinal@gmail.com. He receives royalties and travel and research support from Medtronic, royalties from Stryker, is on the scientific advisory boards and has stock ownership in Pioneer and K2m and is on the scientific advisory board of Applied Spine Technologies.

References:

Gornet MF, Burkus JK, Skipor AK, Jacobs JJ. Prospective study of serum metal ion levels in patients with cobalt-alloy metal-on-metal lumbar disc replacements. Paper #117.

Gornet MF, Ceola W, Burkus JK, et al. Serum metal ion levels in patients with a titanium ceramic composite metal-on-metal cervical disc replacement. Poster #128. Both presented at the North American Spine Society 23rd Annual Meeting. Oct. 14-18, 2008. Toronto.