Fig. 1 Magnetic resonance images of a type-2a pseudotumor (arrow) adjacent to a metal-on-metal hip prosthesis that was well functioning (A) and to one that was painful (B). (Reproduced with permission from Hart AJ, Satchithananda K, Liddle AD, et al: Pseudotumors in association with well-functioning metal-on-metal hip prostheses: A case-control study using three-dimensional computed tomography and magnetic resonance imaging. J Bone Joint Surg Am 2012;94:317-325.)

AAOS Now

Published 5/1/2012
|
Terry Stanton

Metal-on-Metal Hip Implants: The Clinical Issues

Surgeons share thoughts on treating patients who have MoM implants

Recent concerns about metal-on-metal (MoM) hip replacement prompted a symposium of surgeons to discuss clinical considerations for patients with MoM devices during the 2012 AAOS Annual Meeting.

If the participating physicians achieved any consensus, it was that much remains to be learned about what causes problems in patients with MoM total hip and hip resurfacing arthroplasties (THAs and HRAs)—especially in regard to metal ions and the extent to which failures are attributable to metal sensitivity and reactions.

The panelists—Keith R. Berend, MD, New Albany, Ohio; C. Anderson Engh Jr, MD, Alexandria, Va.; Thomas P. Schmalzried, MD, Los Angeles; Joshua J. Jacobs, MD, Chicago; William J. Maloney, MD, Redwood City, Calif.; Brad L. Penenberg, MD, Beverly Hills, Calif; David W. Murray, MD, United Kingdom; and Andrew J. Shimmin, MD, Windsor, Victoria, Australia—agreed that these patients need annual follow-up but differed over what to include in the examination.

They generally agreed that the visit should involve clinical exam, history, and plain radiographs, but several were also inclined to order magnetic resonance images (MRI) on a nearly routine basis. They noted that the issue of blood tests for metal ions is complex, and decisions on ordering tests and interpreting results encompass a number of variables, including the type of implant, signs and symptoms, and even the patient’s age and sex.

“Follow-up is a significant issue,” said Dr. Berend. “If a problem with a device occurs, it is difficult to enforce follow-up. We have almost 3,000 MoM hip patients in our practice, going back to the mid-90s. It is impossible to find them all and get them to return a survey or come in for a visit. In private practice, that is the biggest problem.”

The symptomless patient
Dr. Engh provided a review of treatment protocols for the asymptomatic patient. He first noted that the U.S. Food and Drug Administration (FDA) recommends using three-dimensional (3D) imaging, blood ion tests, and aspirations only if the patient is experiencing symptoms; no evidence supports such measures in the asymptomatic patient. This is in contrast to the 2010 recommendation from the United Kingdom’s Medicines and Healthcare products Regulatory Agency (MHRA), which called for consideration of 3D imaging and/or ion testing if radiographs indicated abnormalities, in patients with smaller-diameter resurfacing implants, and in cohorts with higher failure rates.

“I think it’s important to understand what implant we’re looking at,” said Dr. Engh. “Has the implant been recalled or withdrawn from the market? Is it a modular or a nonmodular implant?

“I think orthopaedic surgeons need to be active in seeking symptoms in our patients with THAs,” he continued. “THAs are fantastic at alleviating pain, and patients can have relatively minor symptoms compared to what they had preoperatively. We need to ask these patients about swelling, asymmetry, and, of course, any persistent tendinitis. Those conditions will demand further evaluation.”

Dr. Engh recommended measuring the vertical position of the implant as part of the radiographic examination. “It’s not enough to look at radiographs and say they look OK,” he said. “We should measure them and look for cups that are angulated or inclined greater than 50 degrees.” He uses a shoot-through lateral view to help identify impingement and any anterior edge loading.

He advised obtaining serial radiographs for all patients, especially those with nonmodular shells, which are seen as prone to loosening. Osteolysis, he noted, should always raise concern, even in the asymptomatic patient. “Any osteolysis that is seen around the MoM hip within the first 10 years warrants further evaluation,” he said.

When radiographic findings warrant, Dr. Engh will order a blood test for metal ion levels, but as the other panelists agreed, several issues make ion testing less than conclusive.

“I think we all agree that ion levels are a proxy for metal debris,” he said. “It would be nice if we knew this debris was occurring just at the surface, but I suspect some component of corrosion and some component of unintended wear and impingement.”

In many areas, finding a qualified laboratory may be difficult. “Orthopaedic surgeons who plan to do metal ion level testing on a regular basis need to identify a trained laboratory and educate the patient. I warn you,” he told the audience, “sometimes you’ll get back arsenic and lead levels when you wanted cobalt and chromium. The sensitivity and specificity of these tests is poorly defined. I look at ion levels as identifiers of a potential problem.”

He contrasted this with 3D imaging, particularly MRI, “which identifies a real problem—tissue damage.” However, it requires “training and interest” of the technician and the radiologist. High ion levels can also be a cue to perform aspiration; Dr. Engh looks for “high fluid volumes and pink cloudy appearance.”

Summing up his approach, he said, “I would like to remind everyone that MoM adverse reactions are infrequent, but they are occasionally severe. We—not the media, not the New York Times—need to educate our patients, primarily on the issue of follow-up.”

The meaning of symptoms
Dr. Berend addressed the symptomatic—ie, painful—MoM THA. The history is the first step, he said. “During the patient work-up, we need to rule out increased risk factors for infection. Frequently, when implants fail, they will mimic infections, so things like the wound status, drainage after surgery, and the use of antibiotics are important to rule out, especially in an ambiguous scenario like a painful MoM THA. For the pain, we need to understand the temporal onset, the duration, the severity, the site, and the character.”

If the patient has had pain since the initial surgery, Dr. Berend said, the cause might be infection, failure of initial implant stability, impingement, periprosthetic fracture, or incorrect initial diagnosis. “Research by our group and others has shown that initial failure is more common with the monoblock cobalt-chromium shell with some type of applied biointegrated surface on it.”

Patients may also experience a return of preoperative symptoms after a pain-free interval.

Dr. Berend said groin pain “is the number one symptom in patients who have a problem with the MoM implant.” In the physical examination, “specific findings can point to a failure of the acetabular component or a complication related to metal ions or an adverse reaction to metal.”

For example, pain at the first initiation of straight leg raises may indicate iliopsoas tendinitis caused by inflammation or pseudotumor. Pain with resisted hip flexion or passive extension may be another indicator. “In the MoM scenario, these can be very concerning,” Dr. Berend said.

Researchers continue to study the relationship between metal debris and adverse tissue reactions, including hypersensitivity, metallosis, pseudotumor, and the phenomenon that has been termed ALVAL (aseptic lymphocyte dominated vasculitis-associated lesion). ALVAL is marked by a prominent perivascular and/or diffuse lymphocytic infiltration reminiscent of a delayed type hypersensitivity response.

Dr. Berend said that in the wake of the 2010 MHRA alert, cobalt and chromium blood levels of 7 parts per billion (ppb) have come to be viewed as a threshold above which 3D imaging and other diagnostic measures should be taken. “The problem, though, is that the literature does not support using that test to diagnose a failed MoM replacement,” he said. “The cut-off level of 7 ppb has poor specificity and sensitivity. Even the optimal level of 5 ppb still has only moderate specificity and poor sensitivity. So we don’t know what number will tell us when we need to take action. If the levels are not predictive of soft-tissue damage, they should not be used in isolation. They should be used as screening tools.”

Dr. Berend believes that community laboratories can be used to process ion level tests. “As long as you use their reference values, they are relatively accurate,” he said, adding that either whole blood or serum may be used.

However, he believes that imaging studies obtained in the community setting may be less useful. MRI images “may be uninterpretable, and it can be very difficult to get good radiographic evaluation for them.” Ultrasound might be considered as an alternative to evaluate soft tissues and the psoas tendon, detect fluid collections, identify pseudotumor, and guide aspiration, he said.

Aspiration fluid evaluation should include leukocyte esterase, cell count with differential, and culture/sensitivity testing, both aerobic and anaerobic. Dr. Berend recommends requesting a manual count because “a mechanical device will frequently show hundreds of thousands of white cells because it’s counting the dead fibrous tissue instead of white cells.”

Unfortunately, said Dr. Berend, no consensus has been reached about definitions for terms such as metallosis, hypersensitivity, ALVAL, and pseudotumor. Additionally, no cutoff value for serum metal ions has been established and no standardized radiographic protocol exists.

“The problem is that we are left with a diagnosis of exclusion. We need to take an individualized, patient-specific approach, using all the information we have available to us—from a careful radiographic evaluation of the abduction angle and anteversion, to serial radiographs to evaluate radiolucencies and component migration, to nuclear medicine studies to rule out loosening and sepsis, and ultrasound and/or MRI to rule out pseudotumor,” he concluded.

When to revise
Dr. Schmalzried addressed the risk factors for failure of MoM devices and the timing of intervention. He noted that head size looms as a factor for loosening in MoM monoblock sockets; bearing diameters greater than 36 mm have a greater potential for taper corrosion and soft-tissue irritation.

“Monoblock sockets were developed for resurfacing and accommodate very large heads,” he said. “We all embrace that for the functional benefits of range of motion for dislocation. But they are more difficult to properly insert. There are no holes to see through them. You don’t know if they’re fully seated or not. They are cobalt-chrome, which makes them stiffer than titanium 6-4 and less biocompatible. There may be failure to osteointegrate. This leads to activity-related groin pain and subtle radiolucencies.”

Dr. Schmalzried distinguished the following two pathologies in adverse local tissue response (ALTR):

  • high wear and ions, with a foreign body response but no pseudotumor
  • lower wear with pseudotumor

Potential stimuli for reactions are wear particles, metal ions, and corrosion products, and tissue reaction is dependent on the concentration of various stimuli. “Pseudotumors are not simply from wear,” he noted. “One of the original reports on pseudotumors did not even involve an MoM bearing.”

In radiographic assessment, Dr. Schmalzried generally orders a low, AP pelvis beam, a true lateral Johnson shoot-through view, and a table-down lateral view, seeking to assess for fixation, position—including lateral opening angle and version—and osteolysis.

“Numerous studies showed that vertical sockets did not do well,” he said. “It’s also been shown that version outliers—either too much anteversion or retroversion of the cup—are at risk for ALTRs. Cup position on true lateral radiographs is a surrogate for anteversion.”

Addressing the issue of ion blood levels, Dr. Schmalzried said he views a reading of 2 ppb as indicator of low risk. He does not view the MHRA threshold of 7 ppb as stringent enough, however.

For 3D imaging, he uses metal artifact reduction sequence (MARS) MRI, and assesses for extracapsular fluid, capsular thickening, muscle edema, and solid masses.

Dr. Schmalzried considers revision surgery in patients who have a poor clinical result, pain and mechanical symptoms, pseudotumor or other soft-tissue involvement, such as edema or necrosis, and osteolysis. He advises not delaying revision on patients with pseudotumors.

“It doesn’t have to be tomorrow, but should be within 3 months,” he said. He also views “double-digit” metal ion levels as an indication for revision. “It is not in itself a reason for revision, but I have yet to see a case where there wasn’t some other abnormality necessitating a revision,” he said.

He continues a course of observation for patients with “clean” MRI and ion levels less than 7 ppb.

For revision surgery, he offers the following recommendations:

  • Débride reactive tissues. “Pseudocysts” are most common.
  • Remove the MoM bearing; generally, replace it with polyethylene.
  • Leave in place well-fixed, well-positioned modular sockets and stems.
  • Avoid using a femoral head of mixed metal as the revision implant. (“If the taper shows evidence of corrosion,” he said, “I’d be worried about putting another cobalt-chrome source in that patient.”)

Other issues
In the discussion that followed the presentations, Dr. Jacobs noted that the examination should include questions about issues other than the hip, because signs and symptoms in other bodily systems may reveal systemic effects.

“We should be asking patients if they have any hearing changes, visual changes, or any other systemic problems,” he advised. “I get a lot of referrals and routinely ask those questions, and I get some interesting answers.”

Dr. Maloney agreed: “When we’re taking a history, we ask... how far can you walk? Are you limping? How do you do on stairs? But we don’t ask…have you had to go to a cardiologist? Have you had any problems with your hearing? Have you had any neurologic issues? Including those questions may reveal the systemic toxicity issue as a problem.”

Dr. Jacobs added: “It’s a challenge, but we need to be attuned to this issue. Potential metal toxicity is really a syndrome, not just one symptom. It tends to be a syndrome. I refer patients concerned about a new symptom developing to a toxicologist for evaluation, which might be a good idea in general. ”

Disclosure information: Dr. Engh—DePuy; Dr. Berend—Biomet, American Association of Hip and Knee Surgeons, Clinical Orthopaedics and Related Research, Journal of Arthroplasty; Journal of Bone and Joint Surgery–American, Orthopedics; Dr. Schmalzried—DePuy, Stryker, Orthopaedics Research and Education Foundation, Orthopedics Today; Dr. Jacobs—Implant Protection, Medtronic SofamorDanek, Nuvasive, Zimmer; Dr. Maloney—Abbott, Gillead, ISTO Technologies; Johnson & Johnson, Merck, Moximed, Pfizer, Pipeline Orthopaedics, TJO, AJRR, Western Orthopaedic Association, Journal of Orthopaedic Research, Journal of Orthopaedic Science.

Terry Stanton is senior science writer for AAOS Now. He can be reached at tstanton@aaos.org