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My View:

  • Seems to suggest that the water-loving (hydrophilic) plastics can actually absorb onto the cobalt chrome and improve the lubrication by the body’s natural synovial fluids.
  • Exciting if that is the case.

Lubrication with Synovial Fluid Components

Source:  Surface Science & Technology

Under physiological conditions, natural hip joints are lubricated by synovial fluid, an aqueous electrolyte solution produced by the synovial membrane and cartilage and containing proteins, lipids and hyaluronic acid. The lubrication performance also depends on the type of the biomaterials used, for instance: alumina, CoCrMo or UHMWPE (Ultra High Molecular Weight Polyethylene). Hence, the adsorption of the SF components onto the biomaterials has an important role in the lubrication of hip implants.

Figure 1

Figure 1. Protocol for the friction tests combined with fluorescent lubricants. As the previous adsorbed albumin is removed due to the sliding between the pin and disc, labeled albumin adsorbs onto the disc and pin surfaces.

A technique using fluorescent labeling has been used in order to see the adsorption of proteins and glycoproteins from SF onto the biomaterial surfaces. In addition, this technique allows us to see the exchange of proteins (in vitro) during friction tests carried out in pin-on-disc apparatus (Fig. 1). The first results revealed that this technique could be also useful for the detection of UHMWPE transfer onto the metallic surfaces. Fig. 2 shows, respectively, fluorescent images of the discs of the tribopairs UHMWPE-Alumina, CoCrMo-CoCrMo and UHMWPE-CoCrMo (the first one is the pin and the second one is the disc). The lubricant used was labeled bovine serum albumin (BSA) with ATTO dye 488 (green).

Figure 2

Figure 2. Fluorescent images of discs of a) alumina against UHMWPE pin, and CoCrMo discs against b) CoCrMo pin and c) UHMWPE pin.

Spots and a clear trace of fluorescence can be observed in the wear tracks of the Alumina disc, which means that the protein is exchanging during the friction tests. Traces of fluorescence cannot be observed for CoCrMo-CoCrMo because the energy from emission waves was transferred to the metallic surfaces (quenching) and no energy was available for the excitation of fluorescent labeled proteins (Fig. 3a). However, when the pin is replaced by UHMWPE, spots of fluorescence can be observed. Since the fluorescence cannot be detected onto CoCrMo surface due to the quenching, a transfer of polyethylene has taken place onto the CoCrMo, which enables the fluorescence excitation (Fig. 3b).

Figure 3

Figure 3. a) No emission waves due to the transfer of the energy to CoCrMo (quenching), b) Detection of emission waves due to the transfer of UHMWPE, which avoids the transfer of energy to CoCrMo.

The transfer of PE was also detected using AFM by means of the phase imaging and the measurement of surface-distance curves.
These results support that the most abundant protein in the SF, albumin, cannot prevent the transfer of materials and cannot form a protective layer on the CoCrMo and Alumina surfaces. Friction test with BSF mixed with labeled BSA also shows that further components of SF are exchanged during the sliding between the pin and disc, which means that they are not working as a protective layer.
Friction measurements have shown that the transfer of UHMWPE has affected friction. In the control measurement using phosphate buffered saline (PBS), low friction has been observed in comparison to the BSA and BSF, which could mean that not only the amount of transfer affects the friction, but also how homogeneous this transfer is taking place.
Tests using other components of SF, such as alpha-acid glycoprotein (AGP), have been accomplished and results shown that for CoCrMo-UHMWPE pairs, a decrease in the friction can be seen. This decrease is related to the high contents of polysaccharides, which gives a high hydrophilicity, thus capability of retaining water improving the lubrication.

Contacts:

Marcella Roba
Marco Hiroshi Naka
Nicholas D. Spencer


Partners:

Rowena Crockett (Nanoscale Materials Science – EMPA Dubendorf)
FMH Chirurgie Orthopédique – Hôpital Cantonal de Fribourg
Mathys AG

References:

– Roba M, Naka MH, Gasser B, Delfosse D, Gautier E, Spencer ND and Crockett R. Investigation of artificial hip joint lubrication. European Cells and Materials Vol 14, Suppl. 3, 2007, p 13.

Hip Replacement, Plastic on ceramic hips, Hydrophilic polyethylene

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