Total hip joint prostheses made of CoCrMo heads versus ultra high molecular weight
polyethylene (UHMWPE) cups have a limited lifetime, mainly due to the wear of the
UHMWPE cups as a result of high friction between the articulating surfaces leading
to osteolysis and implant loosening with revision surgery becoming inevitable in
more active patients. Tribology plays an important role in developing the design,
minimizing wear and reducing friction of hip joint prostheses in order to improve
their long-term performance, with good lubricating properties. Metal-on-metal hip
resurfacing prostheses have shown significantly lower wear rates compared with
conventional metal-on-polyethylene implants and thus osteolysis is potentially
reduced leading to increased lifetime of the prosthesis. Nevertheless, excessive wear
of metal-on-metal joints leads to metal ion release, causing pseudo-tumours and
osteolysis. An alternative approach to such bearings is the use of newly developed
carbon fiber-reinforced poly-ether-ether-ketone (CFR PEEK) acetabular cups
articulating against ceramic femoral heads due to their better wear resistance
compared to UHMWPE. In this study, therefore, friction and lubrication properties
of large diameter, as cast, Co-Cr-Mo metal-on-metal hip resurfacing implants with
various diameters and clearances have been investigated and compared to those of
the newly developed zirconia toughened alumina (ZTA) ceramic femoral heads
articulating against carbon fiber reinforced poly-ether-ether-ketone (CFR PEEK)
acetabular cups with different diameters and clearances. Friction hip simulator was
used to measure frictional torque and then friction factors were calculated along with
Sommerfeld numbers leading to Stribeck analysis and hence the lubricating mode
was also investigated. This involved using lubricants based on pure bovine serum
(BS) and diluted bovine serum (25 vol. %BS+75 vol. %distilled water) with and
without carboxymethyl cellulose (CMC) (as gelling agent). Standard Rheometer was
used to measure lubricant viscosity ranged from 0.0014 to 0.236 Pas at a shear rate of
3000 . Pure bovine serum, diluted bovine serum without CMC and with CMC
(25BS+75DW+0.5gCMC and +1gCMC) showed pseudoplastic flow behaviour up to
shear rate of ¿139 above which a Newtonian flow with significant increase in
shear stress was observed. The viscosity flow curves for the 25BS+75DW+2gCMC,
+3.5gCMC and +5gCMC showed only shear thinning up to a shear rate of 3000 .
The shear rate application modified the flow behaviour of bovine serum from a
pseudoplastic to a Newtonian flow depending on its purity and CMC content. This
will cause a different frictional behaviour depending on joint diameter and clearance,
as seen in this work. The experimental data were compared with theoretical
iv
predictions of the lubricating regimes by calculating theoretical film thickness and
lambda ratio. The metal-on-metal Biomet ReCaps showed similar trends of Stribeck
curves, i.e. friction factors decreased from ~0.12 to ~0.05 as Sommerfeld numbers
increased in the range of viscosities ~0.001-0.04Pas indicating mixed lubrication
regimes above which the friction factor increased to ~0.13 at a viscosity of 0.236Pas.
The Stribeck analyses suggested mixed lubrication as the dominant mode with the
lowest friction factor in the range ~0.09 - ~0.05 at the physiological viscosities of
~0.01 to ~0.04 Pas and that such joints can be used for more active patients as
compared to the conventional total hip replacement joints with 28mm diameter. The
Stribeck curves for all ZTA ceramic-on-CFR PEEK components illustrated a similar
trend with BS fluids showing higher friction factors (in the range 0.22-0.13) than the
diluted BS+CMC fluids (in the range 0.24-0.05). The friction tests revealed
boundary-mixed lubrication regimes for the ZTA ceramic-on-CFR-PEEK joints. The
results, so far, are promising and suggest clearly that the newly developed ZTA
ceramic femoral heads articulating against CFR PEEK cups have similar friction and
lubrication behaviour at optimum clearances to those of currently used metal-onmetal
hip resurfacing implants at the range of viscosities 0.00612 to 0.155Pas. These
results clearly suggest that the ZTA ceramic-on-CFR-PEEK joints showed low
friction at the physiological viscosities of ~0.01Pas in the range ~0.1-0.05,
suggesting that these novel joints may be used as an alternative material choice for
the reduction of osteolysis. The result of this investigation has suggested that the
optimum clearance for the 52mm diameter MOM Biomet ReCaps could be ~170¿m.
However, 48 and 54mm joints showed lower friction due to clearances to be
>200¿m. For the 52mm ZTA ceramic-on-CFR-PEEK joints the optimum clearance
seems to be ¿ 630¿m radial clearance. These results suggested that increased
clearance bearings have the potential to generate low friction and hence no risk of
micro- or even macro-motion for the ceramic-on-CFR-PEEK joints. This study found
no correlation between theoretical predictions and experimental data for all metal-onmetal
and ZTA ceramic-on-CFR PEEK bearings at the physiological viscosity
(0.0127Pas). However, at lubricant viscosity of 0.00157Pas, the theoretical
prediction of lubrication regime correlated well with the experimental data, both
illustrating boundary lubrication. As expected, a decrease in viscosity resulted
decrease in the film thickness.
| Identifer | oai:union.ndltd.org:BRADFORD/oai:bradscholars.brad.ac.uk:10454/5665 |
| Date | January 2012 |
| Creators | Ehmaida, Mutyaa M. |
| Contributors | Youseffi, Mansour |
| Publisher | University of Bradford, School of Engineering, Design and Technology |
| Source Sets | Bradford Scholars |
| Language | English |
| Detected Language | English |
| Type | Thesis, doctoral, PhD |
| Rights | <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-nc-nd/3.0/88x31.png" /></a><br />The University of Bradford theses are licenced under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/">Creative Commons Licence</a>. |
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