Metal on Metal
The bearing surface is all important.
The hard metal on metal 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 chrome bearing surfaces have a 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. 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 would result 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 dramatically increased in the United Kingdom and Europe in the late 1990s, as have the number of designs. In the United Kingdom about 10% of all total hip replacements performed are RHRs and are the prosthesis of choice in 34% of patients under 55 years of age.
The new millennium has seen an increase in the use of big head technology. Manufacturers have 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 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 from impingement, increased cup inclination, as well as an excessively large or small diametrial clearance.
The presence of a peri-vascular lymphocytic infiltrate around metal on metal total hip replacements and RHRs is thought to be due to a delayed hypersensitivity reaction (Type IV) to the metal.
There has been sporadic documentation of soft tissue reactions and an ALVAL response (acute lymphocytic vasculitis and associated lesions) to both types of metal on metal implants. Further research and clinical studies are required regarding these hypersensitivity reactions.
The metal ions have been found in the lymphatic system and numerous organs, and are also known to cross the placenta.
High levels of metals such as cobalt chrome and nickel in the human body are thought to be potentially toxic. To date, no causal link with cancer has been demonstrated in patients with metal on metal implants, although the issue of potential carcinogenesis still remains a concern.
Modern large head metal on metal total hip replacements can be used in patients of all ages, in those with varying bone quality as well as in patients with an active lifestyle. The criteria for performing RHRs, however, are more specific.
Appointments
To make an appointment e–mail Sue Misir, secretary to Mr Evert Smith, or telephone:
- 0117 907 4228 (private)
- 0117 323 5194 (NHS)
For an NHS appointment your GP will need to refer you. How?
Evert Smith is an Orthopaedic Surgeon in whom I have absolute faith and confidence.
Bob Gibbons, 2007.