Implants friction coefficients

The implantation of a considerable dose of metals or non-metals in materials led to large changes of the friction coefficient. The effect of Ar" is certainly mainly due to the creation of a new surface topography or to variations of the natural oxide layer of the material. Non-inert ions as Pb, Mo and Se showed some chemical effect dependent on the type of element introduced. The greatest effect came from lead implantation and was interpreted as a change in the plastic properties of the junction events. Eventually, also the formation of PbO at the surface should be considered. 

Polyolefins are the main components used in prosthetic devices, hip joints and knee implants. They exhibit a high degree of biocompatibility (almost neutral), excellent chemical resistance and superior mechanical properties. The main advantage of polyolefins compared to metallic implants is the low friction coefficient and wear resistance due to their self-lubricating characteristics [13-17]. 

Sas] Surface hardness, friction coefficient Surface mechanical properties of TY-implanted Cr-Fe alloys... 

Table 7.6 Friction coefficients of various implant materials from in vitro studies... 

Friction studies were carried out with a hip-joint simulator by Scholes et al. They compared the friction behavior between a number of implant materials using both carboxy methyl cellulose (CMC) solutions and biological fluids as lubricants. They found that friction coefficient for alumina/alumina was higher for biological fluids than for CMC, and attributed this to the inhibition of fluid-film formation by a protein film on the surfaces. For metal/ metal joints, the friction coefficients were lower in biological fluids, leading to the conclusion that the formation of a protein film assisted boundary lubrication. 

The adsorption of albumin, the most abundant protein in synovial fluid, on implant materials was studied in detail by means of XPS and I -labeUed albumin. These techniques showed the formation of a monolayer of albumin on alumina but multilayered islands of protein on the CoCrMo surface. No PE transfer was observed in AFM images of the surface of CoCrMo after tribological tests when an albumin solution was used as the lubricant. However, contrary to the studies described earlier, the addition of albumin to a solution of Hank s balanced salt solution caused a dramatic drop in the friction coefficient for UHMWPE sliding against CoCrMo. ... 

It has been postulated that the formation of thick, multilayer protein films plays an important role in determining the friction coefficient of artificial hip implants. Albumin is sensitive to many factors including heat, protein concentration, salt concentration, and pH, and aggregates through hydro-phobic interactions in solution over time to form solid, insoluble particles. These particles are then deposited on the surface. Under the conditions used in this study, no deposition of protein to form a thick, aggregated film could be observed, either with AFM or with fluorescence imaging. It has been shown elsewhere that protein films with an average... 

For many years, CVD TiN has been used for wear-and erosion-resistant applications. TiN has a low coefficient of friction and is relatively chemically inert, which makes it attractive for this purpose. In addition, the coating of stainless steel with TiN is of interest for increased biocompatibility of surgical tools and human implants. The reactions used to deposit TiN are very similar to those used for the deposition of Ti02. TiCU is the most common titanium precursor. Nitrogen or ammonia can be used as the nitrogen source. 

Ceramics, such as the coinmonly used Alumina (AI2O3) and Zirconia (Zr02), tend to have a very high elastic modulus (around 400 GPa), have a low coefficient of friction, and are resistant to wear. These properties make them useful as the load-bearing surfaces in orthopedic implants. 

Polyurethane is a stiffer and firmer material than silicone introduced in 1977 for its better tear strength and low coefficient of friction. Due to documented cases of stress cracking discovered a few years after implantation, some processing changes were made, increasing performance satisfaction. Compared with silicone, it allows for a thinner lead body and when moist is slippery. This represents an advantage in cases in which two... 

Poly (hexamethylene adipimide) is also known as Nylon 6,6 since its repeat unit has two six-caibon sequences. Nylon is tough, abrasion resistant, and has a low coefficient of friction, making it a popular suture material. Nylon 6,6 is hydrophilic and absorbs water when placed in tissues or in humid environments (9 to 11 percent water when fully saturated ). Absorbed water acts as a plasticiser, increasing the ductility and reducing the modulus of Nylon 6,6. Nylon bioerodes at a very slow rate. Nylon 6,6 implant in dogs lost 25 percent of its tensile strength after 89 days and 83 percent after 725 days. ... 

Future research efforts in IVD arthroplasty should focus on either partial or full disc functional restoration. This may include NP implants and/or reinforcement or total disc replacement. PVA-C, as a hydrogel, has many interesting properties, such as its long-term biocompatibility and nontoxicity. It is also strongly hydrophilic and viscoelastic with nonlinear stress-strain characteristics similar to the IVD. It has a very low coefficient of friction and has good wear resistance [23]. However, its strength is still too low to serve as a practical functional replacement of the annulus fibrosus. PVA-BC may further increase the strength of the PVA-C to make it a viable candidate material for IVD fabrication. 

Table lb.22 Influence of nitrogen ion implantation on the mean wear rate and the coefficient of friction of the cast alloy Co29Cr5Mo (Ref. 25, 26)... 

This TPE has been described as an aromatic elastoplastic polyurethane alloy. It possesses a low coefficient of friction, low extractables, and dimensional stability. Hardness ranges from 65 to 75 Shore D. The material is classified for short-term (29 days or less) implantation. Clear and radiopaque formulations are available. Tubing should be annealed at 80°C for four hours to reduce crystallinity (Tables 4.3, 4.4, 4.12, and 4.14). 

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