Basically, things run as smooth as clockwork in hip-joints. But modern lifestyles, which include excessive sporting activity, overweight and longer life expectation, are leading to a failure of the largest joint in our skeleton in an increasing number of people. In Germany between 150,000 and 180,000 artificial hip replacements are implanted every year and a study carried out in 1999 estimates that in western societies around a third of the population will undergo such an operation sooner or later.

Unfortunately, an artificial hip-joint, which typically comprises a joint socket made of polyethylene and a ceramics or metal joint head, often fails to eliminate all the problems for the remainder of the patient's life. After 10 or 15 years a new replacement usually becomes necessary. The biggest single problem comes from the mechanical abrasion of the joint socket. On top of this, in some cases the abrasion of the polyethylene can activate the immune system, as it is deposited in surrounding tissue. This can even lead to a resorption of the bone, which results in a loosening of the joint.

Neglected joint lubricant

In order to arrive at a better understanding of the mechanics of an artificial hip-joint it is not enough to study the nature of the joint socket and the joint head. A further factor that must be taken into account is the joint lubricating fluid. Just as in real joints, this fluid also plays a decisive part in assuring a more or less smooth functioning in artificial joints. Surprisingly, not much attention has been paid to this factor until now. Nicholas Spencer, Professor at the Laboratory for Surface Science and Technology (LSST), a part of the Department of Materials at the ETH Zurich (1), and his team have changed that. With an article published in the specialist magazine "Biomaterials“ the team provides new fundamental insights on the lubrication of artificial hip-joints (2). "The combination of an analysis of the folding of proteins in the joint lubricant, the adsorption of the polymer surface and the corresponding friction is unique," says Manfred Heuberger, senior assistant in Spencer's team. He also points out that the underlying research was only possible thanks to co-operation with the ETH Institute of Molecular Biology and Biophysics, headed by Professor Rudolf Glockshuber.

Protein structure affects friction

The team of ETH researchers turned their attention to albumin, the most common protein in joint lubricant–in fact in the entire body. What they discovered is that, in its folded state, this protein prefers to adsorb onto hydrophilic surfaces of a specially modified polyethylene, while in its unfolded, denaturised state, it prefers to hold on to a hydrophobic, unmodified surface.

Friction, such as that which occurs in the hip-joint, can be measured with this piece of equipment: Manfred Heuberger demonstrating the Tribometer. large

In its water repellent state albumin also builds a complex layer on the adsorbent surface. This results in a higher sliding friction. Analysis of this with the tribometer provides data on just how high this is. To measure the level, the tribometer presses with a polymer coated pin on to a revolving ceramic disc. In this apparatus, that resembles a record player, friction is measured as a function of the applied torque. The team of researchers also shows how, when heated to 70 degrees Celsius, the albumin in the joint lubricant practically only appears in its denaturised state. Obviously, this affects adsorption behaviour.

"This finding has clear implications when it comes to the standard tests for artificial hip-joints," says Heuberger. These tests often employ temperatures of up to 90 degrees Celsius, which obviously doesn't correspond to the situation in the body. The ETH researcher says that attention must also be paid to the sort of lubricant fluid that is used in the tests. "It can no longer be considered irrelevant whether one uses water, a saline solution or a protein solution."

More groundwork needed

When asked about the best possible surface for artificial hip-joints, Heuberger isn't ready to commit himself. More research is needed. For example, there was no thorough understanding of the correlation between friction and wear. Contrary to the simple assumption that higher friction leads to stronger wear, it is possible that hydrophobic surfaces with higher friction will prove more suitable. This is because a more compact protective layer is formed, which means that there is less direct contact between the ceramic and polyethylene and, possibly, less wear. Heuberger is not, however, turning his attention to wear for the moment. First of all, because he doesn't have access to a hip-joint simulator, but also because he first wants to investigate to what extent lipids–as distinct from proteins–affect the friction between ceramic and polyethylene.