More studies needed about effects of using metal spine devices

The focus is on what may occur if metal particles are released from scoliosis, fusion or disc replacement implants.

Issue: January 2004

Corrosion or wear of metal spinal implants and the consequences of any subsequent metal particle release are popular topics at orthopedic meetings as more research is being done in this area. Those involved in this work agree that progress has been slow.

Highlights of recent research included a study into the use of a porous nickel-titanium alloy for interbody fusion devices (IFD) implanted near the spinal cord, while other researchers investigated, among other things, metals and polymers used in total disc arthroplasty (TDA) prostheses. To ensure that patients are being treated with implants that will be safe in the long term, more information is needed about the effects of metal implant debris in the spine, particularly with respect to their possibly causing osteolysis, allergic reactions or infections.

Understanding this phenomenon may shed light on what is happening when metal screws, rods and other instrumentation are used in the spine. The phenomenon is seen when patients who have pain associated with their spinal implants report that it is resolved after implant removal.

“It’s not completely understood what that pain is, whether it’s related to inflammation due to the corrosion problems, infection, or whether it’s induced by the mechanical looseness of the implant,” said Robert Urban, director of the implant pathology laboratory in the department of orthopedic surgery at Rush University Medical Center in Chicago.

“We don’t have good studies to have all the answers on that, but in general the limited amount of corrosion and corrosion products that are produced from devices like plates and screws [used elsewhere in the body] is not clinically significant,” he told Orthopedics Today.

Spinal cord studies

[photo]
Nitinol particles are seen on a rabbit’s spinal cord at 12 weeks postimplantation in this macroscopic view. They clung to the soft tissue and remained at the implantation site.

COURTESY OF SOUAD RHALMI

Canadian researchers who conducted a rabbit study reported in a presentation at the 18th Annual Meeting of the North American Spine Society that the animals’ spinal cord response showed it was equally compatible with nitinol, a nickel-titanium alloy, and titanium alloy particles directly implanted in the spinal canal.

The biocompatibility of titanium alloy devices used in orthopedic surgery is fairly well known.

Researchers focused on the spinal cord because in humans this tissue ends up being closest to IFDs. They divided 45 white female rabbits into three groups. Twenty each were implanted with nitinol and titanium particles (<300µm). Five rabbits formed a control group and received a sham operation. In both implantation groups, four animals each were sacrificed at one, four, 12, 26 and 52 weeks post-implantation.

The researchers studied the tissue reaction macroscopically and histologically to identify any inflammatory reaction in the spinal cord, dura mater or soft tissue. They found an inflammatory response restricted to the spinal cord’s loose connective tissue lining, which they described in their abstract as normal and similar between the two metals.

In all instances the spinal cord, dura mater and nerve root tissues did not demonstrate any adverse reaction except for slightly fibrotic tissue limited to the soft tissue and few macrophages and lymphocytes seen surrounding the nitinol and titanium particles. “In all cases, the nitinol and titanium particles remained in the epidural space where the connective tissue behaved as a shield toward the particles,” said Souad Rhalmi, MSC, who presented the findings. “This investigation demonstrates that in the case of debris release, if any, from a porous nitinol device, the particle would be harmless to the spinal cord and nerve roots,” she told Orthopedics Today.

Corroding crevices


At six months postimplantation this spinal cord section (left) showed that the nitinol particles were surrounded by connective tissue. Mild inflammation was limited to the particle/fibrosis interface and it did not go through the dura mater (25x). Researchers saw a similar situation (right) at six months postimplantation in a section of a spinal cord where the titanium particles were implanted (25x).

Based on Urban’s research, modular spinal implants, not IFDs, run the greatest risk of corroding in vivo because wear in tiny crevices can disrupt reformation of the passivation layer and allow corrosion of the underlying metal, which can be clinically significant. “It depends on how much it corrodes and how much of these corrosion products it produces,” Urban said.

Each metal, including stainless steel, titanium alloy and cobalt chromium, produces unique corrosion products, some of which form complexes with the serum. Chromium, for example, can be measured in the serum and urine. “For implants that are corroding, you should be able to measure increased levels,” he said, adding that checking these levels in patients might be warranted in the presence of clinical signs of corrosion, such as pain.

Nadim J. Hallab, PhD, who is also at Rush University Medical Center, was the first author of two recently published studies on particulate response in the spine from total disc arthroplasty (TDA) devices and around implants used for fusion. In both studies cytokine responses were measured. “I think with the introduction and popularization of total disc replacement surgery, clinical concern will likely increase about the effects from implant debris and corrosion,” Hallab told Orthopedics Today.

Total disc arthroplasty: wear debris

The situation could be improved somewhat by having fewer moving parts for spinal devices, he noted. But, due to articulating designs now being used, “The possibility of wear debris increases with TDA. I think that’s the concern that’s being raised. Will the risks associated with that debris still be outweighed by the benefits of mobility, motion, longevity and the improved quality of life of the patient?”

The problems from metal products used near the spine are not new, Hallab added. Effects from gross corrosion problems were observed approximately 20 years ago with some stainless steel rods. “The new corrosion debris and implant wear debris may really be the same sort of phenomenon and induce the same effects,” he said.

Using biologic tissue-engineered solutions to manage disc degeneration is a possible solution. “But, due to the problems associated with that and the lack of fundamental understanding of cellular biology, I don’t expect those to supersede TDA in the near future.”

For more information:

Rhalmi S, Charette S, Assad M, et al. Spinal cord reaction to nitinol and titanium alloy particles: a 1-year study in rabbits. #93. Presented at the North American Spine Society 18th Annual Meeting. Oct. 21-25, 2003. San Diego.

Hallab N, Link HD, McAfee PC. Biomaterial optimization in total disc arthroplasty. Spine. 2003;28(20):S139-52.

Hallab NJ, Cunningham BW, Jacobs JJ. Spinal implant debris-induced osteolysis. Spine. 2003; 28(20):S125-38.

Urban RM, Jacobs JJ, Gilbert JL, et al. Corrosion products generated from mechanically assisted crevice corrosion of stainless steel orthopaedic implants. Standard Technical Publication 1438. West Conshohocken, Pa: ASTM INTERNATIONAL; 2003.