Orthopaedic Crossfire® Tackles Metal on Metal

Dr. Jacobs: “What better way to enter the twilight zone than to quote my favorite philosopher, Donald Rumsfeld…“There are known knowns, there are things we know we know, we also know there are known unknowns, etc.” It’s the unknown unknowns that have been the Achilles heel with some of these reconstructions. It’s not all bad news…there are published studies with promising intermediate term results with both metal on metal (MoM) total hips and surface replacements. However, there are multiple reports of osteolysis aseptic loosening, soft tissue masses in MoM total hip replacement systems and also surface replacements that are possibly associated with hypersensitivity. To understand this we need to understand the fate of the debris that is generated from MoM bearings because I think this determines the ultimate soft tissue and local response. Not only do we have to deal with particles from wear but also free metallic ions that come off as a result of corrosion and colloidal organometallic complexes that form from these ions complexing with local proteins.”

“Metal particles: we don’t know as much as we’d like. One MoM retrieval study showed that most of these particles are less than 50 nanometers. This is an order of magnitude smaller than polyethylene particles…meaning that even though the volumetric wear rate might be less with MoM, the number of particles produced may be up to three orders of magnitude higher depending on what sizes you use for the calculation. It has also been shown that the chrome to cobalt ratio in periprosthetic tissue is reversed. It should be 1 to 2 in the alloy and it’s 9 to 1 or 5 to 1, depending on how you measure it…so what you’re actually seeing in the tissues is a preponderance of corrosion products—either those precipitated from metal ions or those from the nano debris that have been generated.”

“Metal ions: we have found, as have other labs, that there are elevations of 10 20-fold in patients that have MoM bearings. But we don’t know if there is any toxicity. Acute toxicity reported is rare, but there are reports from the UK showing a higher chromosomal aberration rate in patients with MoM bearings…but the clinical significance is unknown and whether this is related to long term carcinogenesis is unclear. Data from our lab and others have shown that patients with MoM bearings do have a higher rate of hypersensitivity; the cytokine profile shows high levels of interferon gamma and IL-2, which are the signature cytokines for a Th1 response. This specific response leads to pro-inflammatory mediators that can actually upregulate the whole innate immune system leading to what I would call a hypersensitivity induced osteolysis. This is different from the typical particle associated osteolysis…this may actually be mediated by a hypersensitivity reaction. Why discuss this? Because we are starting to see reports of these lymphocytic tissue reactions in these modern generation metal on metal bearings.”

“In one study, pseudo-tumors were reported in 17 patients with 20 hips; the histology reveals necrosis and lymphocytic infiltration. This is a case of a pseudo-tumor, not with a MoM bearing but with a highly corroded stem showing necrosis of the tissues, the bone, and the corrosion products of massive necrosis. As was the case with ceramics, with MoM we can’t throw out the baby with the bath water.”

Dr. Schmalzried: “With regard to metal-metal the benefits include high stability and low wear; they’re unbreakable, and they form the foundation for a higher performance arthroplasty. There is a risk…a rare adverse local tissue response. And then there is the ‘compared to what?’ We’ve heard the debate on cross linked polyethylene and ceramic on ceramic…there is no risk free alternative.”

“It’s no mystery why both surgeons and patients have embraced metal-metal bearings. The high stability favors the larger diameter; metal-metal bearings work better and have lower wear as the diameter goes up…that gives you a higher, impingement free ROM and greater jump distance…. And it enables resurfacing. There’s already documentation of very low wear in vivo for more than 30 years and no evidence of gross material failures.”

Showing a video of a rock climber, Dr. Schmalzried stated, “This is a patient in my practice who said, ‘Rock climbing is my passion, but I can’t do it anymore. Do you have a total hip that would allow me to return to rock climbing?’ We used a bilateral, 36mm on a standard new small generation trunion, giving him the ROM, stability, and strength he would need.”

“There are more than 400, 000 second generation implants worldwide with more than 20 years of follow up. There are several reports indicating no discernable difference in pain relief or clinical outcomes; and if you have a less than 5mm thick mono-block cup it conserves bone. You can’t accomplish that with an ingrowth surface using either polyethylene or ceramic.”

“Even within manufacturers there is variability in the process. The formula for low wear involves high carbon, large diameter, and small clearance; if it’s well made and well mated, it will do well. It’s good in young patients; high activity does not influence ion levels. What does influence ion levels is urine volume. Quite simply, chromium excretion is a function of urine volume; if your patients want to decrease their circulating levels of ions, they should drink a lot…. They will pee a lot, and they will pee out their ions.”

“You’ve heard that vertical cups don’t do well with cross linked poly; you’ve heard that malpositioned vertical cups don’t do well with ceramic on ceramic…. The same is true for metal-metal. Higher wear is associated with vertical component positioning. The risk of high ion levels goes up with vertical component positioning; you need to avoid components at 55 degrees or higher regardless of the bearing. Large diameter does not compensate for vertical positioning, and there is some data indicating that excessive combined anteversion has a similar effect.”

“What determines serum ion level? Component orientation (avoid vertical positioning); the loads and motion patterns will be different from patient to patient; the lubrication regime can be different; the wear mechanism, the presence or absence of any third bodies; non-bearing sources such as impingement and taper corrosion; and the ion excretion rate.”

“Pseudo-tumors are not a new phenomenon. We need more information on the role of mechanics, and the role of corrosion. What is the incidence worldwide? There appear to be some centers that have a high incidence and other centers with a low incidence. My personal experience, through April 2008 with 571 hips having at least a 36mm bearing is that I have one patient with bilateral surface replacements who developed ALVAL [aseptic lymphocyte dominated vasculitis associated lesion]. I participated in three metal-metal surface replacement IDEs [investigational device exemption] with greater than 2, 500 hips, minimum two year follow up; none of those have been revised for an adverse immune response.”

“I don’t want anyone to think that I’m not concerned about the rare local immune response. If there is anything weird going on it has to do with the fact that the incidence appears to be quite variable from center to center and region to region. We must identify risk factors from an implant, patient, and surgical technique perspective. But it is my position that if a bearing is well made and well mated it will do well.”

Moderator Thornhill: “Tom, which is more high performance, a conventional femoral stem, large head, metal on metal, or a resurfacing?”

Dr. Schmalzried: “This isn’t a debate about resurfacing. You can get the same level of performance from either type of arthroplasty if the reconstruction is mechanically done well.”

Moderator Thornhill: “Josh, do you do use MoM hips?”

Dr. Jacobs (hesitating): “Uh, no.”

Moderator Thornhill: “Tom, what do you tell your patients about the downside of a MoM hip?”

Dr. Schmalzried: “I make sure they understand that there are possible local tissue reactions…and I tell them that I’ve only had one such case.”

Moderator Thornhill: “Josh, what are you doing now and what can you tell us from your results so far?”

Dr. Jacobs: “We should distinguish pseudo-tumors from metal hypersensitivity. It’s unclear what the pathogenesis of these pseudo-tumors are, and to what extent they may or may not be related to hypersensitivity. We’ve seen pseudo-tumors around metal on polyethylene. But it seems to be more prevalent with metal on metal (anecdotally). In terms of systemic testing, there are various ways that we have tried in a laboratory to determine whether the patient might be hypersensitive to metal, but that may or may not be predictive of whether they get a pseudo-tumor. This is going to require extensive research…and we should try to get an idea if there are some peripheral markers that can identify them even before surgery.”

Moderator Thornhill: “Tom, I know everyone in southern California wears 24 carat gold, but the bling in Boston has a lot of alloys. When you have a patient saying they have metal sensitivity, what do you do?”

Dr. Schmalzried: “I screen for overall allergic history and a cutaneous metal allergy history specifically. If someone has a long list of allergies I tell them, ‘I think that you’d be better off with an implant that doesn’t have cobalt chromium molybdenum nickel alloy. If someone says they have a ring or bracelet that gives them a rash or redness I say, ‘I think you’d be better off with something that doesn’t contain that alloy.’”

Moderator Thornhill: “It’s a problem…I agree.”

Dr. Schmalzried: “But it may be one of the reasons why it’s low in my practice because I’ve avoided people with an allergic propensity.”

Moderator Thornhill: “I solve it by not doing metal on metal and sending the patient to someone who does. Josh, if you’re going to do metal on metal should you avoid modular stems?”

Dr. Jacobs: “The problem with implant debris and metal debris in particular is that the effects can be cumulative and additive—and may be synergistic. The trouble with modular stems is that that (modular junctions) can be an additional source of debris generation. And in addition the debris generated from these can accelerate third body wear at the articulation. So I’m afraid that we may see even more complications in those reconstructions that have more than one or two modular junctions, so it’s something to avoid if at all possible.”

Dr. Schmalzried: But there’s no data at this point to indicate that the risk is increased, right, because all the surface replacements are monoblock implants?”

Dr. Jacobs: “There is no clinical data yet; however the current generation of some of these modular neck body stems has not been around long enough to generate that data.”

Moderator Thornhill: “Tom, concerning systemic toxicity, people say ‘don’t do it in a woman of childbearing age or someone with renal failure.’ What are the things that you avoid in terms of systemic toxicity?”

Dr. Schmalzried: “There’s no reason to do it in a patient with renal failure. If it’s a woman of childbearing age, I have a frank discussion with the patient about whether the benefits outweigh the risks. In the resurfacing trials we’ve had a number of women who became pregnant, went to term, and delivered healthy babies, so again we have not seen a problem in that age group.”

Moderator Thornhill: Thank you both.

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