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  Fig. 2 Upper: Calcium sulphate + hydroxyapatite. Lower: Calcium sulphate + hydroxyapatite + vitamin E.  Fig. 3 Histology at 6 weeks, showing bone ingrowth around the synthetic bone (HA). |
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The osteoconductive (bone-stimulating) synthetic grafts that are used fall mainly under the calcium sulphate and calcium phosphate groups. They can be used as preset and injectable materials. The use of calcium sulphates was first reported by Dressman from the Trendelenburg clinic in 1892 and then later by Peltier in the United States, who gained extensive clinical experience from the 1950s to the 1970s. There is now renewed interest in treatment of contained bone defects. The drawbacks of calcium sulphates are their weak mechanical strength and rapid resorption within 6–12 weeks.
For clinical use, injectable osteoconductive grafts should ideally be biphasic with a compressive strength >25 Mpa. Their injection time should be between 2 and 6 min, with a setting time of less than 10 min.
There are a number of phosphate substances which, with the addition of water and different accelerators, will set into solid phosphates. Of these, hydroxyapatite is the least soluble. So far, at least 25 phosphate compounds have been reported, but they are at best mouldable and not easily injectable, thus restricting indications. They often have very low strength, especially during the first few days, and no interconnecting porosity and, most importantly, are very expensive.
Recently, polymer phosphate compounds have come into limited clinical use. The combination of polyethylene and apatite for middle ear implants is one example, but also degradable screws for fracture fixation, such as polylactic acid combined with tri-calcium phosphate, are used today.
Development of Phosphate Cement
The development of phosphate cement will be to improve the biological response and injectability. The graft should have a construct that creates interconnection porosity for bone ingrowth. We have added vitamin E, a radical scavenger and anti-oxidant, to improve fracture healing. A small amount of vitamin E also increases injectability and creates a certain porosity within the material (fig. 2). In an animal bone harvest chamber model, the composition of apatite and calcium sulphate has been studied. The sulphate is resorbed within a few weeks and replaced by bone ingrowth, providing very close contact between natural and synthetic bone (hydroxyapatite) (fig. 3).
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