Polyethylene medical implants

Research Focus Synthesis of biodegradable and biocompatible polyethylene oxide derivatives containing poly(ester-urethanes) for use as medical implants. 

M.L. Scott and S.C. Jani, Cross-linked ultra-high molecular weight polyethylene for medical implant use, US Patent 6 726 727, assigned to Smith Nephew, Inc. (Memphis, TN), April 27, 2004. 

K.A. Saum, W.M. Sanford, W.G. Dimaio, Jr., and E.G. Howard, Jr., Process for medical implant of cross-linked ultrahigh molecular weight polyethylene having improved balance of wear properties and oxidation resistance, US Patent 6017 975, January 25, 2000. 

Even polyethylene specially treated for medical implants has aluminum as a contaminant ultrahigh molecular mass polyethylene, considered of special quality, may present up to 40 ppm A1 [77]. 

Cross-linked polyethylene may include impurities and voids from which the major causes of premature failures (SCG in pipes, treeing in electric cables, and cracking in medical implants) can originate. Thus, nondestructive evaluation with effective techniques that can discover defects at the incipient stage, before the component is put in operation, is of vital importance. Conventional ultrasound (up to 10 MHz), which is the current technique for detection of flaws in metal piping and vessels, is limited by the attenuated nature of polymers. A scanning acoustic microscope with operating frequencies up to 100-150 MHz has been found to be more effective. 

Functional biomaterial surfaces that absorb proteins minimally are desirable in prolonging the lifetime of medical implants and providing a clean background for introducing specific cell adhesion functionalities. Nonspecific protein adsorption occurs in various degrees to all surfaces, but more readily to hydro-phobic and positively charged surfaces. To date, the most effective way to minimize nonspecific protein and cell adhesion is to use surfaces comprised of chains of polyethylene oxide (PEO also named polyethylene glycol, or PEG). ... 

Dole and co-workers have reported yields of alkyl free radicals in polyethylene irradiated at 77 K ranging from 2.7 to 3.7 (141,145,149). Furthermore, Cracco, Arvia, and Dole (49) reported that on warming, alkyl radicals decay by a first-order process, and they attributed this to reactions between alkyl radicals within isolated spurs. The persistent free radicals on warming to room temperature are the allyl radicals II. The impact of long-term stability of radical species on the stability of polyethylene has been underlined by studies of Jahan and co-workers (150-157) of ultrahigh molecular weight polymer used in medical implants. 

PLA is a transparent plastic whose characteristics resemble common petrochemical-based plastics such as polyethylene and polypropylene. It can be processed on equipment that already exists for the production of conventional plastics. PLA is produced firom the fermentation of starch from crops, most commonly com starch or sugarcane in the US, into lactic acid that is then polymerized. Its blends are used in a wide range of applications including computer and mobile phone casings, foil, biodegradable medical implants, moulds, tins, cups, bottles and other packaging [50]. 

Although the production of polymers has contributed to pollution— many of them are not biodegradable—they have varied uses as synthetic fibers, films, pipes, coatings, and molded articles. Polymers are also being used increasingly as coalings for medical implants. Names such as polyethylene, poly(vinyl chloride) (PVC), Teflon, polystyrene, Orion, and Plexiglas (Table 12-3) have become household words. 

Parth M, Aust N, Lederer K. Studies on the effect of electron beam radiation on the molecular structure of ultra-high molecular weight polyethylene under the influence of alpha-tocopherol with respect to its application in medical implants. J Mater Sci-Mater Med 2002 13(10) 917-21. 

With the fast developments in the plastic industry, some of the lesser known plastics will either find future usage or already be used for devices, general medical instruments and apparatus or as implant aids. Certain plastics now involve alloys, i.e. mixtures of thermoplastics, and thermoplastic and thermoset resins. Improvements in what were the economic five plastics, i.e. polyethylenes, polypropylenes, polyvinylchlorides, polystyrenes and polyesters, are constantly occurring. Use of metallocene catalysts is likely to produce plastics of a controlled chain length. 

Burton et al [25] investigated the sensitivity and specificity of the luminescent bacteria toxicity test (LBT) as compared with the USP mouse safety test, rabbit muscular implantation, mouse systemic injection, and the MEM elution tissue culture test. The samples included industrial plastics/medical devices and low density polyethylene containing different concentrations of toxic organic substances. The results of these comparative tests showed the LBT to be significantly more sensitive than the animal tests and slightly more sensitive than the tissue culture acute toxicity assay for the samples tested. 

Several other common industrial polymers are also used in biomedical applications [51]. Because of its low cost and easy processibility, polyethylene is frequently used in the production of catheters. High-density polyethylene is used to produce hip prostheses, where durability of the polymer is critical. Polypropylene, which has a low density and high chemical resistance, is frequently employed in syringe bodies, external prostheses, and other non-implanted medical applications. Polystyrene is used routinely in the production of tissue culture dishes, where dimensional stability and transparency are important. Styrene-butadiene copolymers or acrylonitrile-butadiene-styrene copolymers are used to produce opaque, molded items for perfusion, dialysis, syringe connections, and catheters. 

A. Metzger, Polyethylene terephthalate and the pillar palatal implant its historical usage and durabiUty in medical appUcations, J. Am. Podiatry Assoc. 65 (1975) 1-12. 

For decades now, hip acetabula, knee plateaus, shoulder joint capsules, and other implant components have been made of, for example, ultrahigh molecular weight polyethylene (UHMW PE). The material used is so-called medical UHMW PE,... 

The final consideration to be addressed in this chapter on the choice of a polymer fortrsein medical devices is cost. Biomedical polymers can range from inexpensive (PVC, polyethylene) to extremely expensive (e.g., polymers with peptide components). A disposable catheter intended for minutes or hotus use in the body will not jrrstily an expensive polymer. On the other hand, a device implanted with the intent of a lifetime of acceptable performance might rrse an expensive polymeric component if long-term performance benefit can be demonstrated. Also, a higher priced polymer might be justified based on reduced complications. For example, as catheter-related bloodstream infections can add over 56000 to a hospital stay, a more expensive antibacterial catheter should be justified. ... 

Reverse shoulder prosthesis system components. The metal screw-fixed ball is implanted in the scapula to replace the glenoid, and the concave polyethylene component mounted on the stem is implanted into the proximal humerus to replace the humeral head (image courtesy of Encore Medical, Austin, TX). 

Polyethylene and polypropylene are ubiquitous commodity plastics found in applications varying from household items such as grocery bags, containers, toys and appliance housings, to high-tech products such as engineering plastics, automotive parts, medical appliances and even prosthetic implants. They can be either amorphous or highly crystalline, and behave as thermoplastics, thermoplastic elastomers or thermosets. 

Materials like polyethylene terephthatlate (PET) and polytetra-fluoroethylene (PTFE) are being used for implantable medical devices due to their hydrophobic nature and neutral surface charge, which is believed to prevent platelet aggregation and clot formation. This observation points to some general relationships, for example, hydrophobic neutral materials tend... 

We have also seen many medical advancements a workable hip joint with its metal shaft set in a PMMA grout and the acetabular cup of ultrahigh molecular weight polyethylene polymeric finger joints that enable the reconstruction of an arthritic hand to reduce pain and allow it to function and, a functioning artificial heart. In 1970 we had still not broken the 100-hour survival and now the group at Utah is ready for the first human implantation. 

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