The Science
Calcium Phosphate Biomaterials
Calcium phosphates comprise a family of highly biocompatible solid materials that can be used for a variety of grafting and tissue augmentation procedures. Chemically, all calcium phosphates are calcium salts of phosphoric acid. Their physical properties can range from hard and insoluble to soft, friable and soluble. Control of the properties is by a variety of factors that including setting the calcium to phosphorous ratio, the presence of traces of other biocompatible metal ions, the control of density and the inclusion of porosity1.
The only calcium phosphate that is stable in contact with water is hydroxyapatite (HA)2, and this is the form found naturally as bone mineral. In applications where a stable, but biocompatible material is desired, HA is the material of choice. If the goal is remodeling, then more soluble forms might be of interest. Additionally, calcium phosphate dissolution can provide a local source of calcium and phosphate ions in the graft site, which serve as the raw materials from which new bone can be constructed by osteoblast cells.
Regardless of the composition, all calcium phosphates are osteoconductive. This means bone bonds directly to the calcium phosphate surface without an intervening soft tissue layer3. Osteoconduction helps to increase the activity of bone forming cells (osteoclasts). Benefits include the ability to bridge larger gaps4, and a better chance to form bone in the presence of micromotion5, which is found in unstable grafting sites.
As well as the chemical composition, the physical forms of calcium phosphate materials can also be optimized for specific applications. Bone cells preferentially grow inside of porous materials. An ideal scaffold consists of interconnected pores in the size range of 50 to 400 microns6,7,8. This tendency of bone to grow in pores is so strong that bone will even grow inside of materials that normally will not support bone growth at all, such as polyethylene9. Porosity, especially interconnected porosity with open pores, combined with an osteoconductive calcium phosphate composition, provides a powerful bone growth scaffold.
In contrast, for applications where inertness is required such as soft tissue augmentation, dense, nonporous, nearly insoluble HA particles with a minimal surface area are ideal.
To access a wide variety of calcium phosphates with properties tailored for virtually any application, please call us at 262.642.2760.
References
S.Radin and P.Duchane; “Kinetics of the in vitro surface transformation of bioactive ceramics to biologically equivalent apatite” in P.Ducheyne and D. Christiansen; “Bioceramics” Vol 6, Nov. 1993 Butterworth-Heinemann Ltd., Oxford UK; pg 59-65
P.K.Bajpai and W.G.Billotte; “Ceramic Biomaterials”, in J.D.Bronzino “The Biomedical Engineering Handbook”, 1995 CRC Press inc.: page 552-580
D.F.Williams; “Proceedings of a Consensus Conference of the European Society for Biomaterials”, 1987 March 3-5, Elsevier, Chester (1986)
J.A.M.Clemens, et. al.; “Healing of gaps around calcium phosphate-coated implants in trabecular bone of the goat”; J. Biomedical Res., Part A, Vol. 36 Issue 1, pg 55-64 (1997)
K. Soballe, et. al.; “The effects of osteoporosis, bone deficiency, bone grafting, and micromotion on fixation of porous-coated versus hydroxyapatite-coated implants”, in, R.G.T.Geesink and M.T.Manley, “Hydroxyapatite Coatings in Orthopaedic Surgery” 1993, Raven Press, NY; pg 107-136
A.I.Itala, et. al.; “Pore diameter of more than 100 microns is not requisite for bone ingrowth in rabbits”, J Biomed Mater Res. Part B Appl Mater, 2001 Oct;58(6):679-83
J.X.Lu, et. al.; “Role of interconnections in porous bioceramics on bone recolonization in vitro and in vivo”, J Mater Sci: Mater in Med, 1999 Feb. 10(2): 111-20
l. Galois and D. Mainard; “Bone ingrowth into two porous ceramics with different pore sizes: an experimental study”, Acta Ortho Belg. 2004 Dec;70(6):598-603
J.J.Klawitter, et. al.; “An evaluation of bone growth into porous high density polyethylene”; J Biomed Mater Res. 1976 March;10(2):311-23