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Boundary Lubrication in Hip Prostheses

Source – Surface Science & Technology

1

Artificial joints are of increasing importance for the modern human lifestyle. Among the most successful material pairings is the ceramic/polymer Charnley-type joint (named after Sir John Charnley at the Center for Hip Surgery at Wrightington, England), which however has a limited lifetime of 15-20 years due to polymer wear particles. These polymer wear particles trigger a macrophage reaction that is found to loosen the implant shaft in the femur bone. Loosening further increases wear and a replacement is thus necessary. Similar to the natural joint, the artificial joint is lubricated by a body fluid (synovia), that contains biological macromolecules. Such macromolecules are thought to act as boundary lubricants in the natural joints. However, the surface of natural joints, the so-called cartilage, has completely different properties than an implant polymer surface. Namely, the cartilage is hydrophilic, whereas the polymer (polyethylene) surface is very hydrophobic, which has a drastic effect on the way these macromolecules from the synovia can adsorb onto the surface. The main aim of this project is to understand and enhance the boundary lubrication properties of synovial macromolecules on polymer surfaces. Therefore, we have been modifying the polymer surface to increase hydrophilicity.

Different test methods such as contact angle measurement, pin-on-disc tribometry, X-ray photoelectron spectroscopy (XPS) and optical waveguide spectroscopy (OWLS) have been used to measure surface hydrophilicity, dynamic and static friction forces, surface chemical composition and kinetics of protein adsorption on the polymer surfaces.

The first part of the study aims to modify boundary lubrication by means of oxygen-plasma treatment of the ultrahigh-molecular-weight polyethylene (UHMWPE). In the second part of the project, an alternative polymer system has been applied in order to render the polymer bulk chemistry more hydrophilic.

We carry out conventional pin-on-disc tribometer measurements. Pins made of the polymer are rubbed against Al2O3 discs, in an environment of saline-aqueous human serum albumin (HSA) to mimic the synovial fluid.

We note that pin-on-disc tribometer measurements are not equivalent to experiments done with a hip-joint simulation – not to mention the complex biomechanical situation in the natural human joint. The relationship between friction and wear is another important clinical issue, which we are neglecting in this feasibility approach. It is also understood that plasma treatment is not a viable approach to improving lubrication in actual hip joint, due to the inherently short lifetime of the modified surface (~hours).

UHMWPE

The advancing contact angle, q, of a water drop on a surface is a measure of hydrophilicity or surface energy. The effect of oxygen-plasma treatment on the hydrophilicity on the UHMWPE surface can be clearly seen by contact-angle measurements. A rapid decrease of the advancing contact angle, as a function of the plasma treatment time, with a characteristic time constant of around 5-10 s has been observed.
Subsequent friction measurements have been performed using a pin-on-disc tribometer in different lubricating media such as pure water, saline Ringer’s solution and HSA in Ringer’s solution. A comparison between untreated and oxygen-plasma treated (10 s) UHMWPE is shown in Figure 1.

In protein solution, a significant reduction of up to 40% of dynamic friction was measured after plasma treatment, while no significant effect occurs in water or Ringer’s solution alone. The relatively low friction in Ringer’s solution compared to water is thought to relate to electrostatic effects between the two interfaces.
XPS measurements on the sliding tracks of the Al2O3 discs showed that there was no transferred ma-terial of the polymer in the case of protein or Ringer’s solution lubricated systems.
The effect of oxygen-plasma treatment on the adsorption kinetics of HSA on poly-ethylene (PE) was monitored by OWLS LINK and is illustrated in Figure 2. Very thin (20-40nm) spin-coated PE films were used for these experiments.

It was found that after oxygen-plasma treatment a higher amount of HSA proteins adsorb faster on the PE and build a denser boundary layer. The proteins adsorbing on the plasma-treated surface are thus thought to form a more natural, lower-shear-strength layer that can better serve as a boundary lubricant.

The modified adsorption of the HSA proteins after oxygen-plasma treatment onto the PE surface appears to be responsible for the reduced dynamic friction in protein solution observed in the tribometer tests. The smaller surface area per molecule of HSA after oxygen-plasma treatment suggests a less denatured boundary layer and the protein maintains its natural conformational degrees of freedom. The reduction in dynamic friction after plasma treatment is interpreted as enhanced boundary lubrication.

Additionally, XPS measurements on the plasma-treated UHMWPE showed the presence of two main functional groups: C-OH (hydroxyl) and C=O (carbonyl). Consequently, alternative poly-mer chemistries containing such functional groups were the focus of part II in this study. The advantage of a bulk-modified polymer would undoubtedly be a maintained friction-reducing effect due to surface-hydrophilicity.

Polyamides

A series of different polyamides (Nylon 12, 6, and 4.6) of different ratios (C=O, N-H, CH2) was chosen as alternative model system – the numbers indicating the average number of CH2 groups between polar functional groups.
The pin-on-disc measurements presented here have been carried out against zirconia discs.
Contact angle measurements confirmed an increasing hydrophilicity with increasing proportion of CONH groups in the polymer. All polyamides used display a greater hydrophilicity than UHMWPE. An additional plasma-treatment of the polyamides was still applied as a control series (Figure 4).

The corresponding friction measurements are shown in Figure 5. Two main conclusions can be drawn from these measurements:

  • Friction coefficients decrease with increasing hydrophilicity due to plasma treatment.
  • There is no correlation of hydrophilicity with friction coefficient between different polymer systems.

In other words, a friction-reducing tendency due to further increase of surface hydrophilicity can still be observed. However, effects of mechanical properties are found to dominate the measured friction. This finding confirms that all used polyamides have comparable mechanical properties, as compared to the superior properties of UHMWPE.
This illustrates a general dilemma in finding alternative, more hydrophilic polymer systems.

In summary, the significant effects achieved by modifying the polymer hydrophilicity by oxygen-plasma treatment show the direction for further investigations in this area. More hydrophilic polymers with mechanical properties comparable to UHMWPE are sought.

References:

  1. M.R. Widmer, M. Heuberger, J. Vörös, and N.D. Spencer, Tribology Letters 10, No.1-2 (2001) 111.
  2. M.R. Widmer, M. Heuberger, N.D. Spencer, Proceedings of the 28th Leeds-Lyon Symposium on Tribology, 2001, Vienna, submitted.

Collaboration and Support:

  • ETHZ–IFP (Polymertechnology)
  • Sulzer Innotec AG
  • Sulzer Orthopaedics Ltd.
  • Metoxit AG
  • ETH KTI

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