Classification of artificial hip joint

3.2.1 Metal femoral heads articulating against UHMWPE cups

Traditional metal femoral heads articulating against ultra-high molecular weight polyethylene (UHMWPE) acetabular cup are widely employed in prosthetic hip replacements and most of them are successful clinically, as shown in Fig. 2.4 (e) and Fig. 3.3 [24-25].

However, about 10¹⁰-10¹¹ polyethylene wear particles with the size range of 0.1 µm ~ 10 µm are generated, which weaken surrounding bone causing bone resorption, leading to the loosening of the prosthesis and ultimate failure [26-28]. In addition, the average wear rate of the polymer cup is in the range of 20-60 mm³/year for metal-on-UHMWPE hip joint.

Figure 3.3 Metal-on-UHMWPE hip joint [25]

According to the ref. [29], cobalt-base alloys are the most commonly used metals for current metal-on-UHMWPE implants. The oxide surface layer on titanium alloy (Ti-6Al-4V) femoral heads result in high UHMWPE wear due to breakdown of titanium surface.

3.2.2 Ceramic femoral heads articulating against UHMWPE cups

Ceramic materials due to excellent biocompatibility and low friction coefficient have been used instead of a metallic femoral head for artificial joints [30]. Comparing to the traditional metal and polyethylene materials, the average wear rate of the UHMWPE can be decreased by 50 % when a ceramic femoral head articulates against UHMWPE [31]. Although ceramic

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materials generate significantly less polyethylene debris when used in conjunction with polyethylene acetabular components in bearing couples, there is still a large number of particles.

Additionally, the stress shielding due to the high elastic modulus of ceramic materials may be responsible for cancellous bone atrophy and loosening of the acetabular cup in older patients with senile osteoporosis or rheumatoid arthritis [32].

Figure 3.4 Ceramic-on-UHMWPE hip joint [25]

3.2.3 Metal femoral heads articulating against metal cups

As is well known, the wear particles produced during the motion of joint have been identified as the main factor limiting the lifetime of the implants, so the number of wear particles have attracted much researchers attention. As described above the volume of debris is inversely proportional to the materials hardness, thus hard-on-hard bearings have also been investigated [19].

By comparing with UHMWPE, metal-on-metal hip joints have a lower rate in the range of 1-5 mm³/year and the wear debris particles are decreased, smaller than those of UHMWPE in metal-on-UHMWPE and ceramic-on-UHMWPE implants (see Fig. 3.5) [28-29, 33]. The amounts of particles are about 10⁹ (micrometer-size). The ball and cup are made of Co-Cr-Mo which is only in clinical use in Europe [29].

However, it is observed that the blood levels of metal ion releasing from the metal joint are increased, which may negatively influence the haemocompatibility of the surface and cause a delayed-type metal hypersensitivity [5, 34]. Additionally, the number of allergies is increasing at

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3. Hip joint replacement background

39

a rate of about 10 % each year [34] and many people have allergies to implant metals or metallic particles. According to the report in ref. [35], patients with total hip replacements by implants of stainless steel or of Co-Cr alloys who experienced difficulties after two to fifteen years due to a loosening of the prosthesis and / or allergic reactions to Cr, Co or Ni were found to have an increased content of these elements in their urine, plasma and blood. Already fifteen months after removal the contents were excessive in these fluids. Moreover, a few elements (Cr, Co, Ni and V) have toxic effects. Ti and its alloys, Nb and Ta exhibit an inert behavior, nevertheless the steel 316L and the Co-Cr alloy are encapsulated by a tissue membrane and their behavior is not inert [36].

Figure 3.5 Metal-on-Metal hip joint [25]

3.2.4 Ceramic femoral heads articulating against ceramic cups

Ceramic-on-ceramic hip joints, where an alumina femoral head is combined with an alumina acetabular cup, show very low wear in the range of 0.05 mm³/ year [31] and reduce wear rate of just 0.032 mm³ per million cycles. In addition, the use of ceramic-on-ceramic hip joint not only resolves the problems caused by wear debris, but also alleviates any concerns about metal ion release into the body, compared to other hip joint systems discussed above.

Although ceramic-on-ceramic hip joint has these advantages, it is not in clinical use in the United States [29], because they may crack and release millions of hard particles which cannot all be removed surgically [34]. Actually, the best choice for ceramic hip joints is the type of

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ceramic-on-ceramic, because ceramic particles are so hard that they can make metals or UHMWPE wear out quickly if they migrate onto the articulating surfaces.

Figure 3.6 Ceramic-on-Ceramic hip joint [37]

3.2.5 DLC coated artificial hip joint

In ref. [38], it was shown that polyethylene, cobalt-chromium or bone cement particles, introduced into implanted bone harvest chambers in rabbits, caused inflammatory reaction and reduced bone ingrowth.

As described above, in order to overcome the problem of wear particle generation in artificial implants, a new low wear material is required. The importance of the surface finish is reported by Dowson [31] in a comparative study of the performance of metallic and ceramic femoral heads. So coating is applied to the hip joint replacement because the implant surface can be modified by deposition. The advantages of DLC coatings used as material for joint have been discussed above in Chapter 2.

DLC coating, deposited on the metal substrate, can be used as a barrier to prevent leaching of metallic ions into the body. Additionally, the adhesion of DLC coating to substrate can be improved by interface layer, about which the details will be discussed in Chapter 4. Usually there are three different hip joints coated with DLC as reported by R. Hauert in ref. [34]: DLC-coated femoral heads against UHMWPE implanted cups, DLC-coated femoral heads against metal cups and DLC-coated femoral heads against metal DLC-coated cups. As described in biomedical

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3. Hip joint replacement background

41

applications of DLC coatings in Section 2.2.1, these DLC-coated hip joints are studied in laboratory and still in process.

To date, the study of the amount of wear particles has attracted more and more researchers [11, 39-42]. However, reports about wear particle size distribution are rare to find, especially for

DLC coatings in hip joints. In this work, the wear particle size distribution is studied in metal-on-metal joints with both sides coated with DLC.