Metal-on-Plastic Hip Replacements

Metal-on-plastic hip replacement was developed in the 1960's, mainly by Sir John Charnley in Wrightington near Wigan in Lancashire. His earlier classic designs of the late 1960's were his best designs. In patients of retirement age these can be expected to last upwards of 12 to 15 years. They do not last as well in younger patients.

The weakest point of the metal-on-plastic hip is that the plastic does wear away. Eventually just enough plastic dust accumulates to cause a rejection reaction by the body and this undermines and loosens the hip replacement. This typically takes 12 or 15 years in older people, but in younger, more vigorous and active people, the rate of wear will provoke earlier loosening. The rejection reaction can sometimes seriously compromise the surgeon's ability to re-do the operation.

Metal-on-Metal Hip Replacements

In the 1960s, before Sir John Charnley's work, there were several metal-on-metal hip replacements in use. They fell out of use because of a failure rate of approximately 50% within five years.

It has taken 30 years to realise that the majority of these failures were due to poor manufacturing techniques and rushed surgery. The 50% of patients whose prostheses did not come loose in those five years have frequently enjoyed 25 or 30 years pain and trouble free function in the hip.

As there is no plastic to wear away, the risk of rejection reaction is much reduced. The principal downside is that the metal does wear, albeit very slowly, and patients do, particularly over the first year, develop measurable levels of the metals in their blood stream. At present it is thought that such levels are harmless - certainly it has been shown there is no increased risk of cancer. One of the uncertainties in younger people who have metal-on-metal bearings is that we do not know whether there will be any ill effects to a baby during pregnancy, although it is believed that none of the metals are transferred across to the baby.

The operation is done through a wound, at most 15cm (6") long on the outer aspect of the hip. Both components are inserted without cement. As the bearing is large in diameter the risk of dislocation is small and patients recover very quickly afterwards.

Frequently Asked Questions

Q Why have a hip replacement, rather than a resurfacing?

A In women above the age of 55 years, where resurfacings have been performed, there have been one or two failures, too many because of menopausal osteoporosis (thinning of the bones) causing a fracture next to the prosthesis. It is therefore not generally recommended for women older than this to have hip resurfacings - hip replacement is more advisable.

There are certain configurations of anatomy, which are not amenable to resurfacing. In many people, as they get older, too much damage has been done to the bone structure for a resurfacing to find a good foundation.

Q What are the benefits of a large metal head hip replacement.

A A metal headed metal-on-metal hip replacement has a large hard bearing, nearly the same size as the original head of the femur, which moves freely within the socket.

It is a great deal more stable than the more ordinary hip replacement head gliding in a plastic socket - this is usually half the size of the metal head and much more prone to dislocate.

The plastic wear particles that accumulate around a metal-on-plastic hip replacement, eventually cause the body to set up a rejection reaction which undermines and loosens what was a previously well-fixed hip replacement. This can happen at between 12 or 15 years onwards in people of retirement age, and earlier than this in younger people - their increased activity creates more wear, dust and thus the rejection reaction occurs earlier.

Q Are there any other advantages of a metal-on-metal hip replacement?

A As they don't dislocate easily, the patients appreciate their stability and they get better quicker because they are more confident that there is little risk of dislocation occurring.

Q What is the general complication rate?

A There is a less than 1% risk of developing a deep infection in the first year after a hip replacement - generally in a large orthopaedic hospital this is 0.3%. In people of retirement age, one would expect 90% of all replaced hips to last more than 10 years but for failures to begin from 12-15 years onwards. In younger patients, below retirement age, the figures are not so good and are being addressed by newer material and harder bearings.

Q What are the consequences of having a metal-on-metal bearing in the body?

A Metal-on-metal bearings containing chromium, cobalt, molybdenum and nickel do produce a very modest amount of wear. The wear particles are absorbed, to an extent, within the body; skin and other tissue samples taken remotely from the operation site are found in all patients to show very minor traces of these elements.

It is worth remembering, in the same context, that dietary supplements containing all of these elements are on sale in health food shops and their effects are not thought to be harmful enough to warrant any form of restriction upon their sale.

Women of childbearing age may be concerned that these metal particles could harm the unborn baby. So far there is no definite evidence that any such harm has ever occurred. Some investigations into this problem have been carried out and whilst some mothers with these traces of metal in their tissues have borne babies, there have not been any problems yet detected and, more importantly, the metal elements do not appear to cross from the mother's circulation to the baby's circulation, as far as we know.