Ion levels do not tend to increase with time or activity, but more data are still needed.Orthopedic surgeons seeking durable hip prostheses for their patients can safely use metal-on-metal articulations in many cases, according to Thomas P. Schmalzried, MD.
Schmalzried, a member of Orthopedics Today's editorial board, said he likes "metal-on-metal because of the high performance characteristics, the range of motion, the stability and the strength to allow patients to perform vigorous activities with confidence."In presentations at the American Academy of Orthopaedic Surgeons 74th Annual Meeting, Schmalzried argued in favor of all-metal hip articulations as a favorable hip coupling to accommodate today's patients' active lifestyles.
He has treated serious rock climbers and other extreme athletes with metal-on-metal hip prostheses.
Schmalzried does not usually restrict his patients' activities with these prostheses, but he does like to track what they are doing.
"There's no question that today's technology is capable of this level of function. I won't be able to tell anybody it's advisable until time passes and I can make some correlations between what I know my patients are doing and what happens," he added.
Several manufacturers offer second-generation metal-on-metal total hip prostheses in nonresurfacing designs. Among the first: Zimmer's Metasul implant, which the FDA approved in 1988.
Modularity optionsMetal-on-metal hip prostheses accommodate modularity and big femoral heads quite well, according to Schmalzried.
"Modularity is a big benefit," he said. But with it comes the risk of added taper corrosion that could result from introducing another interface.
Modularity's upside: Should a well-fixed cup require a bearing change or if a patient becomes concerned over ion problems or develops a metal sensitivity, the bearing can easily be switched out without disturbing the fixation.
Big diameter heads [ by connie...These are the biggest problems today of course...5 years later....]Intraoperatively, surgeons can attain greater head/neck ratios since metal bearings work better in larger sizes. Available tribology information affirms that. "The thing we really want to maximize is the head/neck ratio," Schmalzried said. "Metal-on-metal actually works better as the bearing diameter goes up."
Large diameter bearings provide greater range of motion (ROM) and function with reduced risk of impingement and dislocation vs. what may occur when patients with other (smaller or more fragile) hip articulations attempt activity extremes, he said.
Schmalzried encouraged surgeons to consider metal-on-metal hip articulations because they wear well in the long-term and during the steady state of use, that phase after the initial usage or run-in is complete.
"There are more than 500,000 implanted with greater than 18-year follow-up on the second generation. Very low wear has again been documented clinically with no discernable difference in pain relief or other clinical outcome," he said.
Furthermore, only rare instances of osteolysis are reported with metal-on-metal designs, a phenomenon Schmalzried and colleagues reported on in 1997.
Another plus: The properties of the metal used - typically a high carbon alloy for optimal wear - which lend durability to these implants and allow the bearings to be manufactured with a rounded, smooth finish.
"I'm unaware of any gross material failure such as fracture or any case of runaway wear," Schmalzried said.
Researchers found another benefit of the material: Should the hip dislocate and "scar" the bearing, the metal virtually polishes itself smooth to remove the scratches.
Rarely patients will develop delayed metal hypersensitivity to the implant, a complication unique to metal-on-metal bearings. This occurs a few times in 10,000 cases. But physicians should be aware of clinical symptoms (activity-related pain) of that type of immune reaction because it may lead to a revision, according to Schmalzried.
Metal ion production continues to be of concern and requires continued monitoring and evaluation.
Schmalzried and colleagues studied and published their findings on the relationship between ion production and increased activity. They saw patient-to-patient differences in ion levels which were not linked to the patients' activity level, but to factors like component orientation, load and motion patterns.
In terms of physiology, patients differ in their ion exclusion rate. Those with normal renal function can effectively clear the ions, Schmalzried said.