Table of Contents
Is failure due to wear debris/metal ions a problem?
Metallosis from hip resurfacing is rare, trunion corrosion in THR is much more common. Internal allergy to metal is a myth. Cobalt toxicity is very rare.
Metallosis from excess bearing wear (Hip Resurfacing)
Excess wear debris from metal-on-metal bearings can rarely accumulate to a point where serious tissue inflammation leads to pain requiring revision surgery. I call these Adverse Wear Related Failures (AWRF). Other terms with similar meanings are Altered Local Tissue Response (ALTR), Adverse Response to Metal Debris (ARMD), Acute Lymphocytic Vasculitis Associated Lesion (ALVAL), metallosis, or Pseudotumor (false tumor). All of these imply some degree of inflammatory tissue response to either wear or corrosion debris from implants.
Some experts believe allergy to metal particles may be involved. There is, however, no convincing evidence that an allergic response is occurring. 20% of people have skin sensitivity (allergic rash) to nickel. This skin reaction is much less common with other metals. Cobalt chrome alloy contains trace amounts of nickel. People with nickel skin sensitivity have no difference in THR outcomes than people who are skin test negative. Skin patch tests do not predict implant failures. No evidence supports that internal allergies to metals exist. The Lymphocyte Transformation Test (LTT) has been promoted to be an indicator for internal allergies but has never been successfully validated for this purpose. I tried to test this hypothesis but failed in a study comparing LTT tests done preoperatively to outcomes at 2 years after surgery. All artificial joint implants contain metal, there is no reliable test to determine if your body may tolerate one type better than another. In my LTT study 40% of patients tested “positive” for allergy to some metal in the implant. The most common “positive” allergy test was for titanium, which is present in virtually all THR implants. None of the patients that tested posive t any metal in their implant had any problem at 2 years. 3/135 patients had residual unexplained pain, all of these had a negative LTT. Metal allergy to implants is a myth.
Systemic cobalt toxicity (mild if cobalt >20ug/L vs severe if cobalt level is >>100ug/L)
Cobalt toxicity is a real but rare problem. There was a health crisis in Canada when cobalt was added to beer years ago. Cardiac failure occurred in some patients, but unfortunately, blood levels of cobalt were not reported. From other rare case reports of failed THR, it appears that a cobalt level well above 100 ug/l (perhaps over 500ug/L) is required to cause cardiac toxicity. Cobalt and chromium are both naturally occurring in our bodies, but very high doses can be problematic.
A normal blood level in most labs is less than 1.5ug/L. 80% of my patients fall in this range even with a metal-metal bearing. The mean level for my hip resurfacing patients is 1.4ug/L. Metal-plastic THR mean levels are 0.5ug/L, while Total Knee Replacement (TKR) mean levels are 3.3ug/L. If you are one of the nearly 1 million patients having a TKR in the US annually, you are more likely to have an elevated cobalt level than if you have a hip resurfacing!
There is some evidence that levels above 20ug/L may cause mild systemic toxicity in some patients. This includes neuropathy, tinnitus, and hearing loss, NOT cardiac failure. Unfortunately, these “mild” toxicity symptoms are very common for other reasons in aging people, therefore it is never clear if they are related to the cobalt level itself in any individual case. The best evidence indicates that these types of symptoms are most likely not caused by the blood ion level unless the level is over 20ug/L. I have used chelation with N-acetyl cysteine (NAC) rarely in patients with levels above 20ug/L without symptoms if the hip itself is functioning well. With a level above 20ug/L and systemic symptoms, I believe it is best to remove the implant. Although I have revised patients with metallosis (who have a mean Cobalt level of 70ug/L), I have not yet revised anyone for isolated elevated ions.
Adverse Wear Related Failure (AWRF) in metal-on-metal Hip Resurfacing.
The most significant problem with metal debris is a local inflammatory reaction to either wear or corrosion debris around the hip joint itself. Think of having dust blown into your eye. This causes severe inflammation, which is driven by the immune system, but it is not an allergic response to dust. All people’s eyes will become irritated to some degree. In the same fashion excess wear or corrosion debris can cause irritation of your hip.
In controlled laboratory wear testing, the metal-on-metal bearing of hip resurfacing releases the least amount of wear debris of any implant except for ceramic-on-ceramic THR bearings. A well-positioned implant will never fail due to the accumulation of excess wear debris (AWRF). However, if the socket component is placed too steeply or too tilted forward an abnormal wear pattern termed “edge loading” can rarely occur leading to AWRF (5% of sockets outside the established safe zone).
Irritation from excess metal wear debris results in a large fluid collection with thick white fluid resembling pus (but no bacteria are present). The wall of the fluid collection is very thick and permeated with grey metallic debris. There is almost never any damage to muscles or other vital structures.
The correct treatment is the removal of the fluid and careful excision of most of the cyst wall with the metal debris. And of course, correction of the faulty cup position or change to a different bearing type. Reports of damage to muscles or vital structures from AWRF in failed hip resurfacing cases are usually due to overly aggressive surgery. Those that believe in the allergy myth want to remove every bit of tissue and then the overly aggressive revision operation causes more harm.
A well-positioned hip resurfacing cup puts off less wear debris than most THR components in use today. However, if the cup is malpositioned, a small percentage (5%) will begin to edge-load and produce large amounts of wear debris causing a failure due to wear debris overload (AWRF). I began to understand this problem in 2007; by 2009 we developed a safe zone (RAIL) for placing the components as well as intra-operative x-ray techniques to assure the correct placement of the cup within the RAIL guidelines (Relative Acetabular Inclination Limit). Since 2009 (>5000 cases), not a single cup has failed to meet the RAIL guidelines, and no wear failures have occurred. Problem solved.
Trunion Corrosion in THR
But THR continues to have a major problem with trunion corrosion accounting for a 1-5% rate of failure by 10 years. Furthermore, in my experience, trunion corrosion results in much more severe soft tissue inflammation than occurs in a hip resurfacing bearing wear failure (AWRF). The THR problem with wear was solved with the introduction of crosslinked polyethylene 20 years ago. But the problem of trunion corrosion has now become a major source of concern. We do not fully understand this problem, nor do we yet have a solution for this problem.
The trunion is the connector between the head and the stem (hip resurfacing does not have this junction). Corrosion at this connector releases metal ions at a low rate but also something else that is very irritating to the tissues. The metal blood levels are typically mildly elevated. Severe inflammation and even extensive muscle damage can be seen. But similar metal levels are seen in many well-functioning hip resurfacing patients without any tissue reaction. Therefore, I suspect that the cobalt and chromium coming off the THR trunion in cases of trunion corrosion is NOT the source of the tissue reaction. Many THR surgeons think low elevated cobalt levels are the cause because they are unaware that well-functioning hip resurfacing patients often (20%) have mildly elevated levels.
With hip resurfacing I have had a rate of wear failure of 0/5000 since 2009, with THR the rate of trunion corrosion failure is 1-5% (50/5000) by 10 years. And THR surgeons claim that metal wear failure is the reason NOT to have a hip resurfacing? Instead, the risk of trunion corrosion is another reason not to have a THR.