News | New MHRA guideline: annual checks on metal hip recipients | Which Medical Device

Patients with a common type of metal hip implant should have annual health checks for as long as they have the implant, according to the MHRA. The all-metal devices have been found to wear down at an accelerated rate in some patients, potentially causing damage and deterioration in the bone and tissue around the hip. There are also concerns that they could leak traces of metal into the bloodstream, which the annual medical checks will monitor.

Hours before critical coverage from the BMJ and the BBC, the MHRA issued new guidelines on larger forms of ‘metal-on-metal’ hip implants. Advice on smaller metal devices or those featuring a ceramic head has not changed. Previously, guidelines suggested larger implants should only be checked annually for 5 years after surgery. The agency now says the annual check-ups should be continued for the life of the implant as a precautionary measure to reduce the “small risk” of complications and the need for further surgery.

There are numerous designs and materials used to make hip implants. In recent days the MHRA has issued major updates to its advice on a type of metal-on-metal (MoM) hip replacement. As the name implies, MoM implants feature a joint made of two metal surfaces – a metal ‘ball’ that replaces the ball found at the top of the thigh bone (femur) and a metal ‘cup’ that acts like the socket found in the pelvis.

The MHRA’s updated advice concerns the type of MoM implant in which the head of the femur is 36mm or greater. The agency now says that patients fitted with this type of implant should be monitored annually for the life of the implant and that they should also have tests to measure levels of metal particles (ions) in their blood. Patients with these implants who have symptoms should also have MRI or ultrasound scans and patients without symptoms should have a scan if their blood levels of metal ions are rising.

It is estimated that 49,000 people in the UK have been given metal-on-metal implants with at minumum width of 36mm. This represents a minority of the patients given hip replacements, who mostly have devices featuring plastic, ceramics or smaller metal heads. In 2010 there were 68,907 new hip replacements fitted, and approximately 1,300 of these surgeries used an MoM implant sized 36mm or above – a rate of around 2%.

All hip implants will wear down over time as the ball and cup slide against each other during walking and running. Although many people live the rest of their lives without needing their implant to be replaced, any implant may eventually need surgery to remove or replace its components. Surgery to remove or replace part of the implant is known as ‘revision’ and, of the 76,759 procedures performed in 2010, some 7,852 were revision surgeries.

However, data now suggest that large head MoM hip implants (those with a width of 36mm or greater) wear down at a faster rate than other types of implants. As friction acts upon their surfaces it can cause tiny metal particles (medically referred to as ‘debris’) to break off and enter the space around the implant. Individuals are thought to react differently to the presence of these metal particles, but, in some people, they can trigger inflammation and discomfort in the area around the implant. Over time this can cause damage and deterioration in the bone and tissue surrounding the implant and joint. This, in turn, may cause the implant to become loose and cause painful symptoms, meaning that further surgery is required.

News coverage has also focused on the MHRA’s recommendation to check for the presence of metal ions in the bloodstream, potentially released either from debris or the implant itself. Ions are electrically charged molecules. Levels of ions in the bloodstream, particularly of the cobalt and chromium used in the surface of the implants, may, therefore, indicate how much wear there is to the artificial hip.

There has been no definitive link between ions from MoM implants and illness, although there has been a small number of cases in which high levels of metal ions in the bloodstream have been associated with symptoms or illnesses elsewhere in the body, including effects on the heart, nervous system and thyroid gland.

The MHRA advises that most patients with MoM implants have well functioning hips and are thought to be at low risk of developing serious problems. However, a small number of patients with these hip implants develop soft tissue reactions to the debris associated with some MoM implants.

Source: Dinnington Guardian, 20 August 2012 (UK)

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