Osteoconductive bone grafts

In early experiments, degradable polyurethane scaffolds derived from lysine polyisocyanates and hexamethylene diisocyanate (HDI) were shown to support attachment, proliferation, and differentiation of osteoblasts. To enhance the modest osteoconduc-tivity of polyurethane scaffolds, recent studies have investigated addition of a ceramic component. Both polyurethane scaffolds and polyurethane/ceramic composites have been shown to support new bone formation in a number of preclinical studies. 

Scaffolds synthesized from HDI-derived segmented polyurethane elastomers have been reported to support bone healing in an ovine iliac crest defect model [46,47]. Chain extenders comprising 1,4-butanediol, 2-amino-l-butanol, 2-mercaptoethyl ether, and isosorbide diol [48-50] were investigated, as well as poly(ethylene oxide) (PEO), poly(ethylene-fc-propylene-Z -ethylene oxide) (PEO-PPO-PEO) block copolymers, and poly(caprolactone) macrodiols. Using a salt leaching/phase inversion technique, porous scaffolds with up to 90% porosity were fabricated from the HDI-derived 

Beta-tricalcium phosphate ( 3-TCP) is a highly porous ceramic with chemical and physical structures that mimic bone [66,67]. While it is osteoconductive and porous, the compressive mechanical properties of p-TCP are considerably lower than those of HA [66-69] and it degrades more rapidly than HA [70]. p-TCP has been incorporated in both injectable and implantable polyurethane composites to capitalize on its osteoconductive properties and overcome weaker mechanical strength [39]. Addition of as little as 10wt% p-TCP increased the modulus and strength of the composites, and the composites approach the strength of cancellous bone with incorporation of 70wt% 

P-TCP [23,69]. Furthermore, p-TCP/polyurethane composites showed enhanced osteoblast attachment and proliferation compared to polyurethanes alone [23,71]. 

Allograft bone has been very extensively investigated as a commercial bone graft material. One of the first studies on the development of an injectable lysine-derived polyurethane bone graft investigated an LDI-based carrier for demineralized bone matrix [18]. An LDI-poly(p-dioxanone-co-glycolate) prepolymer was mixed with demineralized bone matrix to form a reactive putty. Implantation of the putty in an intramuscular site did not elicit an adverse inflammatory response. Several more recent studies have 

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Bucholz RW. Nonallograft osteoconductive bone graft substitutes. Clin Orthop. 2002 Feb(395) 44-52. 

Beardmore, A.A., Brooks, D.E., Wenke, J.C., Thomas, D.B. 2005. Effectiveness of local antihiotic delivery with an osteoinductive and osteoconductive bone-graft substitute. J Bone Joint Surg Am 87(1), 107-112. 

Hak, D.J. 2007, The use of osteoconductive bone graft substitutes in orthopaedic trauma. Journal of the American Academy of Orthopaedic Surgeons 15(9), 525. Halloran, D.O., Grad, S., Stoddart, M., Dockery, P., Alini, M., Pandit, A.S. 2008. An injectable cross-linked scaffold for nucleus pulposus regeneration. Biomaterials 29(4), 438-447. 

Tabata, M., Shimoda, T., Sugihara, K., Ogomi, D., Serizawa, T. and Akashi, M. (2003) Osteoconductive and hemostatic properties of apatite formed on/in agarose gel as a bone-grafting material. Journal of Biomedical Materials Research Part B Applied Biomaterials, 67B, 680-688. 

Bioresorbable p-tricalcium phosphate is occasionally used in conjunction with hydroxylapatite to improve solubility (Klein et al. 1984, Yamada et al. 1997) and hence the osteoconductivity. Applications include nose reconstruction (Abe et al. 2001), fusion of the backbone (Ueda et al. 2001), and use as a bone graft (Fujibayashi et al. 2001). 

There are two t5 es of bone tissue in the human oiganism. Cortical bone thanks to the presence of Haversian channels shows good osteoconductive properties. Thanks to its mechanical properties it can be used in cases when recreation of tridimensional cavities within the facial part of the skeleton is required. As opposed to cortical bone, cancellous bone is extremely rich in osteogenic cells. Living osteoblasts of cancellous bone may survive even for a few hours from the time of harvesting of the tissue early revascularization in closed cavities takes place after 48 hr. The disadvantage of the cancellous bone grafts is their small mechanical endurance. It is also connected with the lack of possibility to use them in case of tridimensional reconstructions [3]. 

One of the major classes of synthetic bioresorbable polymers is that of aliphatic polyesters or poly(a-hydroxy acids). Poly(a-hydroxy acids) such as PGA, poly(lactic acid) (PLA) stereoisomers poly(L-lactic acid) (PLLA) and poly(D-lactic acid), and pol-y(lactic-co-glycolic acid) (PLGA) copolymers are the most widely used and most popular bioresorbable polymers since they received Food and Drug Administration (FDA) approval for clinical use in humans in different forms (eg, fibers for sutures, injectable forms) (Nair and Laurencin, 2007). These polymers are commonly used in regenerative medicine applications. An example is the InQu Bone Graft Extender Substitute (ISTO Technologies), an osteoconductive biosynthetic product used as bone graft substitute in the skeletal system to support new bone formation. The resorption rate of... 

Vaccaro A.R. (2002) The role of the osteoconductive scaffold in synthetic bone graft ortopedics, 25,... 

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