Applications in Dental and Orthopedic Medicine - Report Example

Hydroxyapatite: Applications in Dental and Orthopedic Medicine Advancement in science has led to the discovery of various minerals which can be usedas substitutes for body matter. Hydroxyapatite is one such substance and is one of the remarkable discoveries in dental medicine and orthopedics which had led to the creation of dental implants and orthopedic implants. Proust and Klaproth in 1788 were the first investigators to establish hydroxyapatite with bone mineral. Dejong in 1926 was the first to confirm, after the use of x-ray diffraction, that bone is made up of apatite and was made up of a form of hydrate calcium phosphate, known as hydroxyapatite. (D’ Antonio, 1992) It was in 1920 when Albee and Morrison discovered the use of calcium-phosphate materials in humans, through observation of accelerated formation of callus. Since then, calcium phosphate materials began its use as a bulk implant for dental applications and as a bone graft substitute. There were no adverse effects associated with the use of hydroxyapatite in dental medicine and in orthopedics, and it was associated with evidence of osseointegration. Dentists since then have observed the advantage of metal implants coated with hydroxyapatite as compared to uncoated implants of the same design and with the same metallurgical properties. The use of hydroxyapatite in orthopedics was in 1985, when Furlong and Osborn began using hydroxyapatite coated femoral stems in hu8mans; they were later on succeeded by Geesink in 1986. (D’ Antonio, 1992) Physical and Chemical Properties The term apatite was once applied to minerals by Werner in 1788. It now denotes a family of crystals with the formula M10(RO4)6X2, where M is usually calcium, R is usually phosphorus, and X is hydroxide or a halogen such as fluorine. Ever since its creation, its high demand as a biomaterial for surgical procedures and industrial innovation is increasing. Thus this paper aims to discuss hydroxyapatite in general— its physical and chemical properties and its uses on medicine. (Ben-Nissar, 1995) Hydroxyapatite occurs naturally as a mineral form of calcium apatite with the formula Ca5(PO4)3(OH). It is usually written Ca10(PO4)6(OH)2 to show that the crystal unit cell comprises two entities. Hydroxyapatite is the hydroxyl end member of the complex apatite group, in which the OH- ion can be replaced by fluoride, chloride or carbonate. This substance crystallizes in the hexagonal crystal system. (Deer, 1985) It has a specific gravity of 3.08 and is 5 on the Mohs hardness scale. Pure hydroxyapatite powder has a white color, but naturally occurring apatites can however also have brown, yellow or green colorations, comparable to the discolorations of dental fluorosis. (Ben-Nissar, 1995) Hydroxyapatite is a member of the calcium phosphate family whish has been studied extensively for its applicability on the manufacture of dental and orthopedic implants. Hydroxyapatite and its brother compound tricalcium phosphate differ from each other through their calcium-phosphate ratio. The ratio for tricalcium phosphate (Ca3[PO4]2) is 1.5, while that for Hydroxyapatite (Ca10[PO4]6[OH]2) is 1.67. Because solubility at physiological pH is a major factor responsible for bioresorption, hydroxyapatite is better as compared to tricalcium phosphate as a biological implant material. All calcium phosphates, including hydroxyapatite, are highly soluble and unstable in acidic environments. In an acid medium tricalcium phosphate appears to be dissolved 12.3 times much faster than hydroxyapatite, and in a basic medium it is seen to dissolve 22.3 times faster. (Dallemagnes, 1973) The most common precursor to hydroxyapatite is amorphous tricalcium phosphate during both in vivo bone formation and in vitro manufacturing. This substance is highly soluble and unstable at physiological pH. Hydroxyapatite, mostly found in organic bone matrix, exists over a compositional range which can be characterized by its calcium-to-phosphate ratio. It varies from stoichiometric hydroxyapatite (Ca10[PO4]6[OH]2), with a calcium-to-phosphate ratio of 1.67, to calcium-deficient hydroxyapatite (Ca[10-X][HPO4]X[OH][2-X]), with a calcium to- phosphate ratio as low as 1.5. (Friedman, 1995) As to the types of apatites, it was found out that biological apatites are known to be calcium-deficient, bone apatites are found out to contain carbonate, and dental apatites often contain substantial amounts of fluoride. Commercially available calcium phosphate materials can be classified as biphasic calcium phosphate (mixed b-tricalcium phosphate and hydroxyapatite phases), b-tricalcium phosphate, hydroxyapatite, non-sintered calcium-phosphate powders, coralline hydroxyapatite, and bone-derived materials. (De Bruijn, 1992) Biological fixation is defined as the process by which prosthetic components become firmly bonded to host bone by ongrowth or ingrowth without the use of bone cement. Because the goal of the use of hydroxyapatite is to obtain prompt biological fixation to bone, it is of interest to consider the mechanism by which such fixation occurs. It has been suggested that the apatite must first partially dissolve, thereby increasing the concentration of calcium and phosphate in the microenvironment. Carbonate apatite microcrystals then form with the organic matrix of bone, thus causing biological growth of bone tissue. There appears to be a conflict between the ability to manufacture a stable, slowly resorbing material and the ability to achieve rapid union with host bone. (de Groot, 1987) Additional stabilization of a calcium-phosphate apatite by increasing its crystallinity results in a decrease in the release of calcium and phosphate from its surface, as in vivo studies have clearly demonstrated that the higher the percentage of crystallinity of a coating the lower the rate of degradation. The source of free calcium and phosphorus that is present even at the interface of highly crystalline, stable hydroxyapatite coatings appears to be the amorphous calcium-phosphate phase, which is found in all hydroxyapatite coatings. (Geesink, 1988) It is likely that some critical amount of degradation is essential to obtain rapid biological fixation, but premature dissolution of a coating or loss of mechanical bonding to a metal substrate must be avoided. Currently, highly crystalline, pure, stable hydroxyapatite appears to contain adequate amorphous calcium phosphate to allow early biological fixation to be achieved without the use of less crystalline hydroxyapatite or more soluble tricalcium phosphate. (Bauer, 1994) There have been several attempts to prepare hydroxyapatite/ceramic composites through the addition of various ceramic reinforcements. These attempts include the creation of metal fibres, Si3N4 or hydroxyapatite whiskers, Al2O3 platelets and ZrO2 particles. In many cases, the composites could not be successfully prepared and, because of problems related to a poor densification the mechanical properties could not be improved. (Driessens, 1988) Hydroxyapatite/metal and hydroxyapatite/polymer composites are two typical classes of materials, which have been examined for improving the toughness characteristics of synthetic hydroxyapatite. In both cases, a toughness improvement can be found, due to a crack-face bridging mechanism operated upon plastic stretching of metallic or polymeric ligaments. (Zyman, 1993) Zhang et al. proposed a toughened composite consisting of calcium hydroxyapatite dispersed with silver particles. This material was obtained by a conventional sintering method. It was reported that the toughness of these composites increased up to 2.45 MPa m1/2 upon loading the mixture, with (30 vol%) silver. (De Bruijn, 1992) The use of silver is not only for taking advantage of the ductility of silver in terms of fracture toughness, but also because silver is inert and has anti-bacterial properties. Attempts to supersede metal alloys by carbon-fibre reinforced plastics and by various composites to stabilize fractures have met with limited success. Although a new titanium metal core composite hip implant has been clinically assessed in Europe with promising results. (Cook, 1991) References 1. Bauer, T. W.; Taylor, S. K.; Jiang, M.; and Medendorp, S. V.: An indirect comparison of third-body wear in retrieved hydroxyapatitecoated, porous, and cemented femoral components. Clin. Orthop., 298: 11-18, 1994. 2. Ben-Nissar, B.; Chai, C.; and Evans, L.: Crystallographic and spectroscopic characterization of morphology of biogenic and synthetic apatites. Part B. In Encyclopedic Handbook of Biomaterials and Bioengineering, p. 196. Edited by D. L. Wise. New York, Marcel Dekker, 1995. 3. Cook, S. D.; Thomas, K. A.; and Kay, J. F.: Experimental coating defects in hydroxylapatite-coated implants. Clin. Orthop., 265: 280-290, 1991. 4. Dallemagnes, M. J., and Richelle, L. J.: Inorganic chemistry of bone. In Biological Mineralization, p. 23. Edited by I. Zipkin. New York, John Wiley and Sons, 1973. 5. D’Antonio, J. A.; Capello, W. N.; Crothers, O. D.; Jaffe, W. L.; and Manley, M. T.: Early clinical experience with hydroxyapatite-coated femoral implants. J. Bone and Joint Surg., 74-A: 995-1008, Aug. 1992. 6. de Bruijn, J. D.; Klein, C. P.; de Groot, K.; and van Blitterswijk, C. A.: The ultrastructure of the bone-hydroxyapatite interface in vitro. J. Biomed. Mater. Res., 26: 1365-1382, 1992. 7. Deer, W. A.; Howie, R. A.; and Zussman, J.: An Introduction to the Rock Forming Minerals, pp. 504-509. Hong Kong, Longman, 1985.\ 8. de Groot, K.: HA coatings for implants in surgery. In High Tech Ceramics, pp. 381-386. Edited by P. Vincencini. Amsterdam, Elsevier Science, 1987. 9. de Groot, K.; Geesink, R.; Klein, C. P.; and Serekian, P.: Plasma sprayed coatings of hydroxylapatite. J. Biomed. Mater. Res., 21: 1375-1381, 1987. 10. Driessens, F. C.: Physiology of hard tissues in comparison with the solubility of synthetic calcium phosphates. Ann. New York Acad. Sci., 523: 131-136, 1988. 11. Friedman, R. J.: Advances in biomaterials and factors affecting implant fixation. In Instructional Course Lectures, The American Academy of Orthopaedic Surgeons. Vol. 41, pp. 127-136. Park Ridge, Illinois, The American Academy of Orthopaedic Surgeons, 1992. 12. Geesink, R. G. T.; de Groot, K.; and Klein, C. P. A. T.: Bonding of bone to apatite-coated implants. J. Bone and Joint Surg., 70-B(1): 17-22, 1988 13. Zyman, Z.; Weng, J.; Liu, X.; Zhang, X.; and Ma, Z.: Amorphous phase and morphological structure of hydroxyapatite plasma coatings. Biomaterials, 14: 225-228, 1993. Read More