Metal on Metal
The bearing surface is all important.
The hard metal on metal (MoM) bearing surface is an excellent tough wearing surface for total hip and resurfacing replacements. It is self-repairing because it is polished. The metallurgy is important. Most Cobalt-Chromium bearing surfaces have high carbon content. Lubrication reduces friction, which reduces wear. Optimum joint motion is achieved with fluid-film lubrication. The friction is influenced by the fluid film thickness, the roughness of the bearing surfaces and the chemical composition of the material and is dependent on the load across the joint, speed of joint movement and the viscosity of the synovial fluid. Given an optimum clearance between the liner and the ball, the steady-state wear rate of metal surfaces is about 5 microns a year, following the initial run in period. By comparison, conventional polyethylene liner and metal head couples wear at a rate of about 200 microns a year.
In 1951, George McKee and John Watson-Farrar designed a metal on metal total hip arthroplasty. The first generation metal on metal bearing articulations such as the McKee-Farrar, Huggler, Sivash and Ring prostheses had high failure rates due to poor manufacturing and loosening. However, it was known that metal on metal hip replacements caused little bone reaction.
1989 saw a new era in resurfacing hip replacements (RHRs). Modern hip resurfacing designs utilise the concept of metal on metal bearing couples. This design development occurred as it had become clear from the failures of the first generation RHRs, that conventional polyethylene could not be used as the bearing surface, because the large femoral head resulted in excessive volumetric wear, increased polyethylene debris and osteolysis. The first new procedure was performed in February 1991, and further design adjustments and improvements were made over the next three years.
The use of metal on metal RHR has recently decreased in the United Kingdom. About 6% of all total hip replacements (THRs) performed are RHRs. Large head metal on metal THRs have decreased from 7% to 4% of the total number of THRs (65229) performed in 2009 (as documented in the National Joint Registry for England and Wales).
The new millennium saw an increase in the use of big head technology. Manufacturers utilised theoretical as well as in vitro evidence to predict that large head metal on metal articulations have good fluid film lubrication and therefore benefit from even lower wear rates. The use of metal on metal bearings account for a third of all THRs performed in the USA.
The metal particles produced through wear are smaller (22-90 nanometres) and are more numerous than polyethylene particles. The shape of these metal particles typically tends to be oval or round with very few needle forms.
Increased wear can occur with impingement, increased cup inclination and excessively large or small diametrial clearance. The presence of a peri-vascular lymphocytic infiltrate around metal on metal THRs and RHRs, is thought to be due to a delayed hypersensitivity reaction (Type IV) to the metal. The tissue/histological features are known as ALVAL (acute lymphocytic vasculitis and associated lesions). The metal ions have been found in the lymphatic system and in numerous organs, and are also known to cross the placenta. High levels of metals such as Cobalt, Chromium and Nickel in the human body are thought to be potentially toxic. To date, no direct causal link with cancer has been demonstrated in patients with metal on metal implants, although the issue of potential carcinogenesis still remains a concern.
On 22 April 2010 Medical Device Alert [MDA/2010/33] was generated, due to reports received by the Medicines and Healthcare Products Regulatory Agency (MHRA) UK. In the revision of these implants, soft tissue reactions were noted, which may be associated with unexplained hip pain.
On 7 September 2010 a further Medical Device Alert [MDA/2010/069] was issued for the DePuy ASR system, which includes the ASR surface replacement heads for hip resurfacing arthroplasty, the ASR XL femoral heads for total hip replacement and ASR acetabular cups for hip resurfacing arthroplasty or total hip replacement.
The MHRA established a Joint Working Group to study the association between metal on metal hip replacements and soft tissue injury and to produce advice and guidance for the Health Services based on the results.
At the Bristol Hip Arthroplasty Course (BHAC) in December 2010 it was noted that big ball metal on metal THRs were being 'abandoned' by the surgeons in the UK. On a 'show of hands' not one meeting attendee was using this form of hip replacement. The meeting clearly identified surgeons' concerns with metal on metal bearings, originally thought to be affecting only the resurfacing type of arthroplasty. However, the faculty interactions clearly demonstrated that there was a significant problem with all metal on metal replacements.
At the British Hip Society (BHS) Annual Conference, in Torquay in March 2011, MoM bearing surfaces were the focal topic. Several units presented audited results of these devices at short to mid-term. There was a predominance of the ASR XL device, which has been withdrawn, but large diameter MoM devices from other manufacturers may also be showing similar results.
Large head metal on metal THRs were discussed in a session where research papers were delivered showing higher than anticipated early failure rates. These ranged from a 21% revision rate at 4 years, to a 49% revision rate at 6 years for the ASR XL device. Other devices were shown to have high revision rates of around 12-15% at 5 years.
As in the resurfacing arthroplasties, increased wear of the metal on metal bearing surfaces have been noted. However, wear and/or corrosion has now also been identified at the trunnion-taper junction between the metal head and the stem.
Patients present with pain and this should be investigated appropriately. Radiographs may show loosening and osteolysis. Ultrasound and MRI scans may show fluid collections, as well as cystic and/or solid masses. Cobalt and Chromium ions may be elevated. The use of large diameter metal on metal bearings in primary total hip replacement should be carefully considered and possibly avoided.
Revision procedures can be complicated if there is significant soft tissue damage and may require specialist reconstruction techniques.
The BHS, National Joint Registry UK (NJR), British Orthopaedic Association (BOA) and the MHRA will continue to monitor the outcome of these large diameter metal on metal bearing THRs and further updates will be published as more information on ARMD and implant failure becomes available.
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Bob Gibbons, 2007.