Toxic hip

Recent media attention has brought the focus to toxic hip replacements (Figure 1). This article presented here is meant to dispel the rumors and give a sense of perspective to the area of controversy.


Figure 1. This article highlights the problem of toxicity in hip resurfacing implants. In essence the title is accurate if misleading. The hip itself is not toxic really and uses implant materials well accepted for hip replacement surgery. The hip is not 'faulty'. It was designed as is and appears to have been put in well. Really the 'fault' (if there is a fault) is the fact that when these hips were being put in no one anticipated the toxic effects we are now aware of. This toxic effect is due to the high amounts of metal debris formed by metal-on-metal articulations. Most orthopaedic surgeons do not now advocate their use.
 

Hype or real?

Unfortunately, there is merit to the writing in these articles. In other parts of this site I have written about metal allergies in general. There we mention that one of the effects of metal toxicity is necrosis. The effects of metal ions on the body are delayed type IV T-cell hypersensitivity, dose-dependent cell necrosis, and mutagenic changes amounting to carcinogenesis. The effect that is most relevant in the present article in dose-dependent cell necrosis.

 

Who is at risk?

From the foregone discussion the term “dose-dependent” means that the more the metal the higher the chance of necrosis. Therefore if a metal on metal joint replacement of the type being described is used then the amount of metal being formed due to wear and tear in the hip is most prevalent. Hence, these sorts of designs are most at risk of developing clinically relevant necrosis and pain in the hip requiring a revision.

 

Nevertheless it has been shown that such necrosis can happen in any  joint replacement involving a metal surface – whether metal on metal or metal on polyethylene. It is just that in metal on metal replacements the number of metal particles is a lot higher. For this reason surgeons at our clinic have long since advocated the use of non-metal articulations and have one of the longest experiences in Singapore using non-metal articulations like ceramic. These are used both in hip and knee replacement surgery.

 

Why were these joints used in the first place?

Well, it all boils down to size.

 

The evolution of hip replacements, on which all other joint replacements are based (whether in the hip, knee, shoulder, elbow, wrist, ankle or even spine), originally culminated in a 28mm diameter metal ball made of cobalt chrome articulating on a polyethylene socket. This combination remains the gold standard in hip replacements and is responsible for the longest lasting hips in the world based on the Exeter hip system – a system introduced in Singapore by A/Prof Suresh Nathan himself some 10 years ago although they have been around for 40 years already.

 

The advantage of making the head bigger however is the hip becomes more stable and the patient has a higher range of motion (Figure 2). The disadvantage is with bigger heads the wear and tear on the polyethylene cup is worsened and there is less material on the cup to begin with so they will fail earlier.


Figure 2. In younger patients like this, large heads allow the patient extreme stability and range of motion (a) with implants made with hard materials articulating on hard sockets as in the ceramic on ceramic hip replacement inserted through the anterior minimally invasive surgery approach here (b). Whenever the head becomes bigger the socket becomes thinner (arrowed) and more prone to early failure (c).
 

The solution is to make a harder cup. This has been achieved in three ways – making the polyethylene harder and more resistant to wear by increasing cross-linkages in the material (ie. highly cross-linked polyethylene), using a durable material like ceramics or using metal. And that was how metal on metal implants were born.

 

All three new-age materials are brittle and prone to cracking. Ceramics are completely biologically inert, polyethylene causes osteolysis and loosening and metals are toxic.

 

For this reason surgeons at our clinic advocate the use of ceramics if big heads are desired. We may occasionally offer polyethelene-large head couples but we never offer metal on metal articulations.

 

What is the effect of metal ions in the body?

There are 2 effects. In the blood stream they become lodged in the spleen and lymph nodes and these structures enlarge and can predispose to cancers there.

 

In the soft tissues around the joints they can cause a vasculitis around vessels or alval (“aseptic lymphocyte-dominated vascular associated lesion”) which causes blood flow to stop and tissue to die. In addition, the metal ions can be directly toxic to the tissues causing pus and swelling with necrosis. This is aseptic but behaves like an infection (Figure 3).


Figure 3.  Metal debris in the tissue around joints incites an inflammatory reaction with macrophages ingesting metal particles (a). This leads to extensive necrosis (b) which while associated with a lot of pus is not infected (ie. aseptic). Granulomas can form which result in lumpy tissue and pain (c). Vasculitis is represented by lymphocytes aggregated around blood vessels and result in ischemic necrosis (d). All pictures are derived from Acta Orthopaedica 2009; 80 (6): 653–659.
 

What to do?

If one has a metal on metal implant the best advice appears to be to monitor the serum metal ion levels and revise the implants if the levels are high. Some papers suggest that failures are more common in only some designs more than others but this appears to be at least contentious. A recent review (Acta Orthopaedica 2009; 80 (6): 653–659) suggests that all such implants are susceptible to failure - a finding supported by most joint replacement registry data. Therefore if such an implant is associated with pain for a patient it probably should be revised before the necrosis becomes so extensive that it is difficult to revise without risking dislocations after the revision. These revisions are covered elsewhere on the site.