Material selection in artificial joints
R. David Whitby | TLT Worldwide March 2018
Common wear mechanisms have driven the trend toward metal-on-plastic hip prosthetics.
Metal-on-metal prosthetic joints are more likely to release metal ions into the patient’s bloodstream.
© Can Stock Photo / Eraxion
In January I summarized recent research into understanding the coefficient of friction in artificial joints. The other significant factor in limiting the utility of replacement hip, knee and shoulder joints is wear.
Early artificial hip joints were metal on metal, which were hard wearing but caused other problems. They are more likely to release metal ions into the patient’s bloodstream, which can cause inflammation and bone erosion. The shaft of the prosthesis can become loose in the hollow of the thigh bone due to thinning of the bone around the implant, causing pain and a feeling that the joint is unstable. This happened in around 10% of cases.
Newer joints tend to be metal on plastic. However, the ultra high molecular weight polyethylene (UHMWP) used to line the joint’s cup tends to wear faster. Analysis by the Orthopedic Research Center in Los Angeles of wear on UHMWP components that have been retrieved after use in patients has provided invaluable understanding of how wear occurs
in vivo. The analysis indicates that the classical wear mechanisms that apply to prosthetic joints include adhesion, abrasion and fatigue. Although a wide variety of terms have been used, an overview of the literature indicates that about eight terms have been sufficient to describe damage to retrieved UHMWP components. These are burnishing, abrasion, scratches, plastic deformation, cracks, pits, delamination and embedded third bodies.
Laboratory wear studies of metal-on-plastic artificial joints have shown the wear factor used in Archard’s law for the conventional polyethylene to be highly dependent on contact pressure. This has produced variability in experimental data and has constrained the reliability and applicability of previous computational predictions. As I illustrated in my previous column, loads and contact pressures in artificial joints vary greatly from patient to patient. Researchers at the University of Leeds in the UK proposed an alternative law, based on wear volume being dependent on, and proportional to, the product of the sliding distance and contact area. The dimensionless wear coefficient, which was independent of contact pressure, was determined from a multi-directional pin-on-plate study. The wear of the polyethylene cup was independently experimentally determined in physiological full hip joint simulator studies. The predicted wear rate from the new computational model showed an improved agreement with the experimental measurement. Wear in the UHMWP hip joints increased as head size and contact area increased. This resulted in a much larger increase in the wear rate as the head size increased compared with the previous computational model and is consistent with clinical observations.
Almost 10 years ago work at the University of Colombia in Medellín attempted to understand the wear mechanisms acting on the surfaces of metal-on-plastic joints, particularly when factors such as surface finish, chemical composition, stress distribution and medical history of the patient were considered. Wear tests were performed in a modified pin-on-disc machine, which allowed lubrication of the contact surfaces with bovine serum to provide chemical and biological characteristics similar to those of synovial fluids found in natural joints.
The most important wear mechanisms in the metal-polymer pairs under the specific conditions studied were the adhesion of the polymer to the metallic surface and subsequent failure at the sub-surface of the polymer. The transference of a thin polyethylene layer to the metallic surfaces promoted a lubricating effect at the interface for low normal loads and short testing times. The amount of polymer adhering to the surface increased with the normal load and was affected by the viscoelastic response of the UHMWP, especially for normal loads over 30 N. For higher normal loads and longer testing times, the lubricating effect was not predominant and significant plastic deformation of the polyethylene was observed.
It would be very useful to study the wear of artificial joints
in vivo in patients. This has been done in the past for metal-on-metal hip joints using X-rays, but this is much more difficult to do with metal-on-plastic joints because the wear of the UHMWP cannot be seen as easily. Magnetic resonance imaging (MRI) cannot be used, because patients with metallic implants cannot be scanned using this method.
Fortunately for most patients, artificial knee and shoulder joints do not appear to experience the same wear problems as artificial hip joints.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.