Knee-resistance mechanisms

Abrasive wear of polymers has two components material can be removed by the rasping action of a countersurface or it can be sheared off viscoelastically by a countersurface to which it adheres. The precise balance of mechanisms depends on the characteristics of the counterface and the conditions under which the abrasion takes place. Many polymers exhibit excellent wear resistance, which in combination with their low coefficients of friction suit them for applications where lubrication is either impossible or undesirable. We use wear resistant polymers in such diverse applications as bushings in business machines, pump seals, and replacement hip and knee joints. 

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]. 

Biomedical-grade zirconia was introduced 20 years ago to solve the problem of alumina brittleness, and the consequent potential failure of implants. The reason for this is that biomedical-grade zirconia exhibits the best mechanical properties of oxide ceramics as a consequence of transformation toughening, which increases its resistance to crack propagation. Likewise, partially stabilized zirconia shows excellent biocompatibility, and it has therefore been applied to orthopedic uses such as hip and knee joints [255]. 

Bioceramic Applications The performance requirements of yttria-stabilized tetragonal zirconia polycrystal (TZP) to form biocompatible, strong components for use as hip, knee, and dental prostheses, and which demonstrate long-term resistance against aggressive body fluids and mechanical wear and tear, during a predicted lifetime of 15-20 years in the human body, include ... 

Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints, for fracture healing aids as bone plates and screws, spinal fixation devices, and dental implants because of their excellent mechanical properties and corrosion resistance. Some metallic alloys are used for more active roles in devices such as vascular stents, catheter guide wires, orthodontic archwires, and cochlea implants. 

More recently, knee simulator experiments have been performed with GUR 1050 blended with 3000ppm vitamin E and likewise observed a reduction in wear as compared with unstabihzed controls [21]. These recent experiments were performed without sterilization because the material is ethylene oxide sterilized when used clinically in Japan by Nakashima Medical. Several mechanisms, including improved mechanical properties, reduced oxidation during consolidation, and slight differences in protein adhesion, have been postulated by the authors to explain why, in the absence of ganuna sterilization, the addition of vitamin E results in improved wear resistance in a knee simulator. 

The UHMWPE/HA biomaterials were designed to take advantage of natural synovial lubrication mechanisms. UHMWPE/HA is UHMWPE with a small amount of HA in the bulk that extends HA roots at the surface of the UHMWPE to which an optional HA surface coating can be applied. The HA-rich material is hydrophilic, lubricious, and well hydrated. The UHMWPE/HA family of materials includes UHMWPE/HA with nonintentionally crosslinked UHMWPE, crosslinked UHMWPE/HA, and crosslinked compatibilized UHMWPE/HA. UHMWPE/HA wears considerably less than plain UHMWPE, and crosslinked compatibilized UHMWPE/HA has a wear resistance on par with UHMWPE having a similar level of crosslinking. Furthermore, after 2 million cycles of wear, crosslinked compatibilized UHMWPE/HA surfaces look similar to unworn surfaces, indicating little to no UHMWPE wear. Preliminary studies indicate that UHMWPE/HA is noncy-totoxic and well tolerated in the knee. 

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