Table of Contents
"Those metal ions are going to poison you."
All artificial implants release wear and corrosion particles into your body. 70 years of joint replacement history tells us that our bodies can tolerate these fairly well. There is probably individual variability in our body’s response to these foreign particles, but this is poorly understood. At this point, there is no test that has been validated to predict any individual response to foreign particles. There are several issues that I will discuss as it relates to foreign material.
- Metal allergy
- Cobalt toxicity
- Cobalt carcinogenicity
Metal allergy appears to be a myth. There is no convincing scientific evidence that our bodies are allergic to implanted metal. It is well documented that skin sensitivity to metal does exist. The most common skin sensitivity is to Nickel. Skin patch tests to nickel show a positive result in 20% of the population. But that 20% do not have a higher failure rate with implants containing small amounts of nickel (stainless steel, cobalt-chrome).
The blood Lymphocyte Transformation Test (LTT) has been touted by many to determine if a person is allergic to metals. Your blood is drawn and sent to a lab where the reactivity of your lymphocytes is tested for various metals. We have found that 58% of people tested as “allergic” to at least one metal on the panel. The most common was nickel at 39% (present in cobalt-chrome alloy). Titanium “allergy” was seen at 19%. This would mean that at least 40% of people are allergic to their knee replacement or hip resurfacing and at least 20% to their hip replacement. In our study, a positive LTT preoperatively did not predict any negative outcome with hip resurfacing. Therefore, we would have to conclude that the LTT has no predictive value…it does not do what the lab says it does. In fact, a $500 lab test is being sold without any data that actually validates it. There is no value in getting metal allergy tests.
Cobalt toxicity is a real but rare problem. There was a health crisis in Canada when cobalt sulfate 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 for patients without implants 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 even though you are not sure whether the symptoms are related to the cobalt level. 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.
Metal ions released from implants do not cause cancer. This concern arose from basic science studies where DNA changes were seen in cells in tissue culture when exposed to cobalt at sufficient concentrations. Several long-term clinical studies have found no difference in cancer rates in patients with metal-metal bearings, metal-plastic bearings, and those in the general population without implants. The studies in Finland by Visuri are the best. Also, more recent studies of the British and Australian registries indicate a lower 10-year all-cause mortality in hip resurfacing compared to THR.
The most significant problem with metal debris is a local inflammatory reaction to either wear or corrosion debris around the hip joint. 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 (metallosis, or Adverse Wear Related Failure [AWRF]). However, if the socket component is placed too steeply (high inclination) or too tilted forward (high anteversion) an abnormal wear pattern termed “edge loading” can rarely occur leading to metallosis (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 who 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 ensure 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.
But THR continues to have a major problem with trunion corrosion accounting for a 0.1-5% 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 (metallosis). 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 know that the cobalt and chromium particles coming off the THR trunion in cases of trunion corrosion is NOT the source of the tissue reaction. Many THR surgeons think it is 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 0.1-5% (5-250/5000) 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.