Implants penetration rates

With the introduction of crosslinked polyethylene in hip replacement implants, a number of recent clinical studies measmed the wear rates in the human body. Ise et al. (2009) examined implants at a minimum follow-up of three years that had articulations of UHMWPE against zirconia, crosslinked polyethylene against zirconia and crosslinked polyethylene against stainless steel. The penetration rates were reduced when crosslinked polyethylene was used (Table 7.3). Similar results were found by Glyn-Jones et al. (2008), who found penetration rates for UHMWPE and highly crosslinked polyethylene of 0.1 mm year and 0.06 mm year", respectively. 

FIGURE 12.14 UHMWPE dome penetration and penetration rate versus implantation time for retrieved CHARITE Artificial Discs. Adapted from [73]. 

Methods for tracking nanotubes in biological materials are needed to measure the rate of nanotube transport, distance of penetration from places of introduction at inhalation, with food and water, and implantation of nanotubes ... 

Nondegradable polymers are also useful as matrices for ocular implants. This application requires the polymer to be hydrophilic, to minimize local tissue irritation. Need for ocular implants stems from the challenges posed to conventional ocular medicines (i.e., eye drops) such as rapid dilution, tear washout, poor patient compliance, and limited bioavailability. Ocular implants from hydrophilic polymer matrices that provide localized sustained release may overcome the above limitations. The first polymeric sustained release product to reach the market was Ocusert , a pilocarpin sustained release ocular implant developed by Alza. Ocusert has the drug reservoir as a thin disc of pilocarpine-alginate complex sandwiched between two transparent discs of microporous membrane fabricated from ethylene-vinyl acetate copolymer. The microporous membranes permit the tear fluid to penetrate into the drug reservoir compartment to dissolve pilocarpine from the complex. Pilocarpine molecules are then released at a constant rate of 20 or 40 pg/hr for a four- to seven-day management of glaucoma. 

The etch rate of thermally grown oxide may be altered by postoxidation treatments. Ion implantation at a certain level, depending on the nature of damage and distribution, has been found to increase the etch rate. Also, an electric field, by applying an anodic potential onto a thermally grown oxide film, is able to inject hydroxyls into the oxide. The etch rate of the hydroxyl penetrated region, depending on the hydroxyl concentration, is much faster than the unaffected thermal oxide... 

Regardless of the properties of the implant, it is not possible to release drug into the tissue at a rate faster than determined by Equation 10-24. If the release rate from the implant is less than this maximum rate, q (the concentration in the tissue immediately outside the implant) is less than the saturation concentration, q. The actual concentration c can be determined by balancing the release rate from the implant (dMt/df, which can be determined from Equation 10-11 provided that diffusion is the mechanism of release from the implant) with the rate of penetration into the tissue obtained by substituting q for Cg in Equation 10-24 ... 

Because of the instability of the anhydride bond in the presence of water, special properties are required for stable polyanhydride devices. A critical element in the development of polyanhydride biomaterials is controlling hydrolysis within a polymeric device. To obtain implants where hydrolysis is confined to the surface of the polymer, hydrophobic monomers can be polymerized via anhydride linkages to produce a polymer that resists water penetration, yet degrades into low molecular weight oligomers at the poly-mer/water interface. By modulating the relative hydrophobicity of the matrix, which can be achieved by appropriate selection of monomers, the rate of degradation can then be adjusted. For example, copolymers of sebacic acid, a hydrophilic monomer, with carboxyphenoxypropane, a hydrophobic monomer, yield ... 

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