2.5.4 Synthetic Bone Grafting
Recently, the use of synthetic substances as bone graft materials has increased in
importance as an alternative to all other bone grafting methods. Synthetic bone
grafting is a surgical process that uses synthetic substances (often called synthetic
bone grafts) to repair or redevelop defective bone tissue. It is also known as
alloplastic bone grafting. To date, a number of synthetic bone grafts have been
developed to eliminate or minimize the complications associated with autografts,
allografts, and xenografts [43–46]. Each synthetic material has different characteristic functions either in vitro or in vivo, which makes difficult to judge the best
system for bone grafting. There are many advantages and disadvantages associated
with each of these materials. Synthetic grafts are considered a good choice for
bone grafting because synthetic grafts are abundantly available, sterilizable, reproducible, shapeable, and cost-effective. Also, some of the limitation of autografts or
allografts such as donor shortage and the chance of rejection or transmission of
infectious disease can be eliminated by synthetic bone grafts. Currently, none of the
synthetic grafts resembles natural bone tissue in many aspects because of the lack of
proper designing techniques. The biomimetic approach is considered to be
promising for the design of such a bone-resembling graft. Research is now
progressing in this direction and the next decade may witness many breakthroughs
in the fascinating field of biomimetics.
3 Hydroxyapatite
Hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 , is the main inorganic component found in hard
human tissues such as bone and teeth and is the most extensively used bioceramic in
bone tissue engineering. The other materials used for this purpose include alumina,
zirconia, titania phosphates, and calcium phosphates [such as calcium
tetraphosphate (Ca 4 P 2 O 9 ) and tricalcium phosphate Ca 3 (PO 4 ) 2 ] and derivatives
[47, 48]. The chemical structure of HAp is presented in Fig. 5 [49].
Most of the biological HAps are weakly crystalline and non-stoichiometric. HAp
contain various foreign ions, primarily carbonate ion (CO 3
2À
) and small amounts of
sodium (Na
+
), magnesium (Mg
2+
), ferrous (Fe
2+ ), chloride (Cl
À ), fluoride (F
À ), and
hydrogen phosphate (HPO 4
2À ) ions. CO 3
2À consist of 3–8 wt% of the calcified
tissue, which varies with bone age [50–52]. Thus, this ion is a major ion in bone
metabolism. Bone mineral is a calcium-deficient HAp with a Ca:P ratio of about
1.5. Bone mineral and β-tricalcium phosphate (β-TCP) are chemically and compositionally similar; however, it is structurally similar to HAp (Ca:P ¼ 1.67).
Calcium-deficient HAps [Ca 10Àx (HPO 4 ) x (PO 4 ) 6Àx (OH) 2Àx ] with Ca:P ratios varying from 1.67 to 1.33 are formed by the loss of Ca
2+ ions from the unit cell [8, 53].
Various methods have been developed for the synthesis of HAp. For example,
Hao et al. have synthesized HAp nanocrystals by the hydrothermal method [54, 55].
146
A. Bhowmick et al.
Recently, the use of synthetic substances as bone graft materials has increased in
importance as an alternative to all other bone grafting methods. Synthetic bone
grafting is a surgical process that uses synthetic substances (often called synthetic
bone grafts) to repair or redevelop defective bone tissue. It is also known as
alloplastic bone grafting. To date, a number of synthetic bone grafts have been
developed to eliminate or minimize the complications associated with autografts,
allografts, and xenografts [43–46]. Each synthetic material has different characteristic functions either in vitro or in vivo, which makes difficult to judge the best
system for bone grafting. There are many advantages and disadvantages associated
with each of these materials. Synthetic grafts are considered a good choice for
bone grafting because synthetic grafts are abundantly available, sterilizable, reproducible, shapeable, and cost-effective. Also, some of the limitation of autografts or
allografts such as donor shortage and the chance of rejection or transmission of
infectious disease can be eliminated by synthetic bone grafts. Currently, none of the
synthetic grafts resembles natural bone tissue in many aspects because of the lack of
proper designing techniques. The biomimetic approach is considered to be
promising for the design of such a bone-resembling graft. Research is now
progressing in this direction and the next decade may witness many breakthroughs
in the fascinating field of biomimetics.
3 Hydroxyapatite
Hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 , is the main inorganic component found in hard
human tissues such as bone and teeth and is the most extensively used bioceramic in
bone tissue engineering. The other materials used for this purpose include alumina,
zirconia, titania phosphates, and calcium phosphates [such as calcium
tetraphosphate (Ca 4 P 2 O 9 ) and tricalcium phosphate Ca 3 (PO 4 ) 2 ] and derivatives
[47, 48]. The chemical structure of HAp is presented in Fig. 5 [49].
Most of the biological HAps are weakly crystalline and non-stoichiometric. HAp
contain various foreign ions, primarily carbonate ion (CO 3
2À
) and small amounts of
sodium (Na
+
), magnesium (Mg
2+
), ferrous (Fe
2+ ), chloride (Cl
À ), fluoride (F
À ), and
hydrogen phosphate (HPO 4
2À ) ions. CO 3
2À consist of 3–8 wt% of the calcified
tissue, which varies with bone age [50–52]. Thus, this ion is a major ion in bone
metabolism. Bone mineral is a calcium-deficient HAp with a Ca:P ratio of about
1.5. Bone mineral and β-tricalcium phosphate (β-TCP) are chemically and compositionally similar; however, it is structurally similar to HAp (Ca:P ¼ 1.67).
Calcium-deficient HAps [Ca 10Àx (HPO 4 ) x (PO 4 ) 6Àx (OH) 2Àx ] with Ca:P ratios varying from 1.67 to 1.33 are formed by the loss of Ca
2+ ions from the unit cell [8, 53].
Various methods have been developed for the synthesis of HAp. For example,
Hao et al. have synthesized HAp nanocrystals by the hydrothermal method [54, 55].
146
A. Bhowmick et al.