Unique inertness, including electrolytic passiveness, biocompatibility, corrosion resistance, andhardness characteristic of sapphire, define its main fields of application in medicine: implants,medical instrument and devices making.
Medical-biological investigations have shown that sapphire is not toxic for humans and does notcause changes in the functions of the central nervous system, liver, kidneys, protein and fatmetabolism, and general reactivity. It does not possess carcinogenic, mutagenic, embryotrophic,or other types of remote effects. Collagen fibrillar capsule growing on the implant passes intobone and muscular tissue, which preserves the normal structure. In contrast to titanium,electrically neutral sapphire is not carried by electrochemical reactions into lymph nodes andother parts of the body, does not cause immunodepressions and other changes in the immunesystem, and does not lead to demineralization of adjacent bone tissue.
Crystallographic aspects. Beyond other conditions, the rate of implant adaptation in the patientdepends on the crystallographic conformity between the structures of the implant and the mineralcomponent of the bone tissue (crystalline fibers contained in microfibrils). The mineralcomponent is essential and it is necessary to take into account the crystallographic interactionbetween the crystals of the bone tissue and the lattice of the implant. Such considerations aresimilar to heteroepitaxy and intergrowth of crystals. Biochemical and biomechanical testing ofimplants made from crystals with different syngony and lattice parameters showed theadvantages of sapphire.
Functional merits of the sapphire are clearly seen in the comparison of their basic, functionalproperties with those of the widely used titanium analogs.
It is known that immunologic disturbances are defined by the implant material. The resistance ofsapphire to any acid and alkali is immensely higher than that of metals and even ofpolycrystalline aluminum oxide. This likely explains the fact that sapphire does not change theimmunological status of patients. At the same time, 73% of the cases involving metallic implantinsertion are followed by immunodepression, changes of immunoreactivity, and otherimmunologic disturbances (especially for male patients). All these manifestations raise theprobability of postoperative complications.
It is well-known that the nickel and chromium contained in some implants are carcinogenic,possess cytotoxic effects, and may affect allergies. Approximately 15% of people are especiallysensitive to nickel, and 8% to chromium. Survival tests for L-132 cells (RPE, relative platingefficiency) placed in media of different implant materials for a long period of time show thataluminum oxide, platinum, and TiAl6V4 alloy possess a level of survival close to 100%. For Ni—Cr—Co alloy this characteristic is on the order of 23%, while for Ni—Cr—Mo alloy it is evenlower. The sapphire is especially suitable for those patients who suffer from intolerance tometals, or already have metallic implants inserted.
The use of sapphire in friction pairs
is of particular interest. As a rule, orthopedic implants areof large size and, therefore, an essential amount of metal is introduced into the patient duringtraditional implantation. For instance, a hip joint endoprosthes contains 300–350 g of titaniumand alloyed steel (the latter being even less physiological). The metal corrosion productspenetrate into the bone and may lead to traumatic osteomyelitis. The service life of the widelyused metal-plastic hinges on hip joint prostheses is insufficient (5–7 years) due to the highfriction coefficient of the metal-polyethylene pair, which increases over time with use. Theservice life of joint prostheses depends on the quality of spherical surface polishing. Metallicand ceramic surfaces cannot maintain a high finish class because of the presence of disorientedgrains and intergranular boundaries with different physical and mechanical characteristics thanthose of the grains. Different rates of wear of this microstructure raise the friction coefficient ofthe pair and lead to elevated wear of the mating component. The rough surface which is goodfor conventional intraosteal implants is undesirable for friction pairs. The probability ofadherence of organic molecules to such a surface increases, thus deteriorating the performance ofthe friction pairs. Sapphire not containing block (grain) boundaries allows surfaces with a highpolish quality to be obtained (possessing a slight roughness and containing practically noscratches and pits). Moreover, sapphire is one of the most wear-resistant materials (this fact ledto its wide employment in the friction pairs of watches and other devices). The frictioncoefficient of polished sapphire pairs actually decreases over the life of their operation; the wear index approaches the corresponding value of natural jointsThe wear of the polyethylene component of polyethylene – alumina ceramic pairs (even if thelatter is of low quality) is less than with polyethylene – zirconium dioxide pairs . It is natural thatfor polyethylene coupled with sapphire this characteristic is much better compared to the ceramicmaterials.