Wear has been recognized as the key factor affecting the long-term performance of artificial joints such as hip, knee
and spine prostheses. In this paper, the wear of ultra-high molecular weight polyethylene (UHMWPE) and cross-linked
UHMWPE counter with cobalt chromium alloy was investigated using a new BiotriboPOD-Multi wear testing apparatus
for orthopedic implant materials. Via this apparatus, an effective testing method for the wear evaluation of orthopedic
implants can be achieved.Osteolysis induced by ultra-high molecular weight polyethylene
(UHMWPE) wear particles is regarded as an important
factor in the long-term durability of total joint arthroplasty. Up to 100,000 micron sized UHMWPE wear particles are
released per footstep.
Such wear particles can activate the
immune system as well as be deposited into the surrounding
tissue, which results in the lifetime of the most artificial joints
limited to about 10-15 years. Therefore, the investigation
on the wear performance of UHMWPE material is one of
the important research areas in artificial joints. Since material
selection and component design are two essential factors in
the wear performance and durability of orthopedic implants,
both biomaterial and product wear test are needed.
Because the tribological property of orthopedic biomaterials
has significant influence on the wear performance of implants,
the bench wear evaluation of orthopedic biomaterials is regarded
as another important testing methodology. A proper
bench wear test can be used to develop an understanding of
wear mechanisms and the influence of environmental, design,
and material parameters on wear behavior. Moreover,
due to the low productivity, complex structure and high cost
of joint simulator wear testing, a reliable and effective wear
testing methodology for orthopedic biomaterials is expected
and required.
In the tribological evaluation of orthopedic implants, POD
testing methods have also been applied.
However, unfortunately,
the conventioncal POD test devices with unidirectional
relative motions between a pin and a disk could not reproduce
the wear factors obtained in clinic. For example, the wear factors
obtained for the most common combination, ultra high
molecular weight polyethylene (UHMWPE) against polished
CoCrMo alloy in bovine serum are of the order of 10-8 mm3
/
Nm whereas the true clinical wear factors measured from retrieved
acetabular cups of artificial hip joints are of the order
of 10-6 mm3
/Nm.
According to the study on the joint movements in human activities,
researchers have observed a directional dependence
on the wear rate of orthopedic biomaterials. Thus, the
multi-directional motion has been regarded as fundamental importance in laboratory wear tests for orthopedic biomaterials.
Following this rule, studies on the development of new
POD devices for orthopedic biomaterials are being carried
out. Saikko developed a high-capacity hip wear simulator of
the circular translation pin-on-disk type, call super-CTPOD,
which could provide 100 separated test stations and reproduced
the wear of artificial hip joints in vitro. The present authors
designed a single-station POD apparatus, and validation
test results show the wear factor agreed with those obtained
in clinic.
In this study, the wear performance of conventional ultra high
molecular polyethylene (UHMWPE) and cross-linked UHMWPE
were investigated by a new BiotriboPOD-Multi method
in a long-term testing duration. Wear factors were calculated
and compared with clinical results.
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