Authored by Abdelbary A*
Abstract
Tribology has a significant role in the functioning of artificial joints. The tribological aspects of total artificial replacement joints is reviewed. The basics of tribology are applied to understand the friction, wear and lubrication of natural and artificial joints. The basic wear mechanisms in artificial joint are introduced. Bearing lubrication concepts can be applied to determine the lubrication performance of artificial joints. Major failure criteria of artificial joints, retrieved from patients, are overviewed and discussed. The role of surface and subsurface cracks in polyethylene along with the effect of counterface imperfections in the wear of artificial joints are pointed out. Wear debris are the key factor related with the contacting surfaces of artificial joints which can cause adverse tissue reactions and the loosening of the prosthetic components. In this paper, generation and classification of wear debris are deliberated.
Keywords: Tribology; Artificial joint; Failure criteria; Fatigue; Wear debrisr
Introduction
Artificial joint replacement is one of the most essential arthroplasty for sever diseases of human joints. This surgical treatment involves the replacement of native articulating joints, such as the knee, hip, ankle, and shoulder with artificial components. Currently, it is assumed that there are well over one million artificial joints implanted yearly into patients world-wide [1,2]. By 2030, there may be 500, 000 total hip arthroplasty (THA) and 3 million total knee arthroplasty (TKA) cases per year [3,4]. Various biomaterial combinations include metals, ceramics, carbon and synthetic polymers are now used in biomedical applications.
In particular, polymeric materials have shown a greatest success in this field due to its excellent tribological properties. Low friction coefficient, high wear resistance, compatibility of wear debris with the body, compatibility of being sterilized by γ-irradiation, and the facility to operate efficiently for long time under no maintenance conditions are the factors that have made it a highly successful bearing material in these applications [5-8].
For several decades’ polyethylene has remained the most suitable material choice for the articulating surface in total joint replacements [9-11]. UHMWPE used as an acetabular cup where the femoral head was stainless steel, ceramic or alumina. UHMWPE was first introduced by Charnley J [12,13] as an alternative for the failed previous polytetrafluoroethylene (PTFE), because the superior wear resistance of the former. A new type of polyethylene material has been developed recently which shown excellent wear properties compared to UHMWPE. This material has been named as ultra-low wear polyethylene (ULWPE). In addition, ULWPE has excellent wear resistance, which is also easy to process [14].
Friction, wear and lubrication of artificial joints play important roles in its successful function. However, it is unpredictable due to complex tribological and biological behaviours and long-term wear [15]. Research efforts are currently addressing the evaluation of the determinants affecting the overall wear rate of the artificial joint articulating surfaces, with the aim of reducing wear rate. However, one of the main challenges is the translation of research data from in vitro to in vivo environments [16]. For example, the wear debris produced by the hip joint simulator has a larger average size and wider distribution range compared to those from the implanted artificial joint [17]. The average diameter of the wear debris from joint simulator is 7.54 μm. The average diameter of the wear debris from artificial joint is 1.33 μm (about 18% of the wear debris from joint simulator).
In order to study the failure criteria of artificial joints, a pioneer work was done by Atkinson [13] based on the investigation of a number of worn hip joints ranging in age from 1- 12 years. They observed that the wear and deformation processes of the acetabular cups seemed to be as follows:
1. Initially, a running-in period: the entire cup surface wears abrasively.
2. With increasing mobility, the upper half of the cup wears adhesively.
3. After several years’ wear, fine fatigue cracks formed on the wear area.
Many questions which need answering were recognized:
Are any of the observed wear feature likely to enhance early failure of the acetabular cup or are there other possible mechanisms?
Are the surface fatigue cracks going to be deletrious? Will they shorten the useable lifetime of the cup?
What is the effect of the cyclic loading on the failure resistance and surface cracks of the polymeric material specially in fluid lubricated media?
What is the effect of the generated wear particle size on the clinical infection?
Finally, it was reported that no conclusions could be drawn, and further researches are necessary. For more than four decades, to the best of our knowledge, hundreds of researches were established in order to find answers for those questions. The present paper is aimed to review and argue the failure criteria of artificial joints.
Tribology of Artificial Joints
Tribology has a significant role in the functioning of artificial joints. General fundamentals of tribology can be used to understand the friction, lubrication and wear of natural and artificial joints in the body [18]. Friction played a dominant role in the design of original Charnley low-friction arthroplasty [19]. Also, wear has an important role, not only from the integrity of the prosthetic component point of view, but also from that of wear debris which can cause adversely biological reactions [15]. Finally, both of friction and wear can be reduced effectively by lubrication.
Friction
In his low-friction arthroplasty, Charnley, J. introduced the significance of friction in the design of artificial joints [12]. At first, McKee-Farrar introduced metal-on-metal hips. Then, polytetrafluoroethylene (PTFE) was selected for its lowest frictional coefficient, although massive wear was subsequently found with the cups made of this material. Charnley preserved with polymer on metal combinations to maintain low frictional torques and introduced UHMWPE cups sliding on stainless steel femoral heads. This combination of materials has proved to be extremely successful giving low friction, low wear rates of the UHMWPE and smaller amounts of wear debris, which could be more readily tolerated by the body [10,11,20,21].
The following three laws of dry friction are often defined:
1. The force of friction (F) is directly proportional to the applied load (W).
2. The force of friction (F) is independent of the apparent area of contact.
3. The kinetic force of friction (F) is independent of the sliding speed (V).
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