Composites of different material systems can combine the advantages of both. Therefore a variety of composite materials have been developed. One distinguishes composites which work as bone cements [172, 219, 220] and those that include living cells and were used for tissue engineering [176, 221].

To quantify the bone-implant interface of high-strength HA/poly(L-lactide) (PLA) composite rods an affinity index was calculated, which was the length of bone directly joined to the rods, expressed as a percentage of the total length of the rod's surface [219]. Calcined and uncalcined HA particles which amount to 30 or 40% by weight of the composite were implanted in the distal femora of 50 rabbits and after 2, 4, 8, and 25 weeks they were examined by SEM, TEM, and light microscopy. In all composites new bone formation could be examined after 2 weeks. SEM showed direct bone contact with the composites without intervening fibrous tissue. The affinity indices of all the composite rods were significantly higher than those of unfilled control PLA rods. The maximum affinity index (41%) was attained at 4 weeks in 40% calcinated HA-containing rods.

Another approach to use composites is to increase the compressive strength of calcium phosphate cement (CPC) [220], which limits their use to non-load-bearing applications, by the means of water-soluble polymers. Composites formulated with the polycations poly(ethylenimine) and poly(allylamine hydrochloride) exhibited compressive strengths up to six times greater than that of pure CPC material. SEM results indicate a denser, more interdigitated microstructure. The increased strength was attributed to the polymer's capacity to bridge between multiple crystallites and to absorb energy through plastic flow. Glass-ceramic, as mentioned previously, is also used in composites with bisphenol-a-glycidyl dimethacrylate (Bis-GMA)-based resin. These were compared with composites containing HA or ^-tricalcium phosphate (TCP) as the inorganic filler [172]. After 10 weeks of implantation into tibial metaphyses of rabbits, the ceramic-containing sample was in direct contact with bone and ceramic particles were partially adsorbed at the surface. The HA-containing cement was in contact with partially mineralized extracellular matrix. In 25-week specimens, ceramic particles were completely absorbed and replaced by new bone, and there was no intervening soft tissue. The tissue response of nano-hydroxyapatite/collagen composite was investigated in [176]. At the interface of the implant and marrow tissue, solution-mediated dissolution and giant-cell-mediated resorption led to the degradation of the composite. Interfacial bone formation by osteoblasts was also evident. The composite can be incorporated into bone metabolism instead of being a permanent implant. Nano-HA was also used in a porous collagen composite, to build up a three-dimensional osteogenic cell/nano-HA-collagen construct [221]. SEM and histologi-cal examination have demonstrated the development of the cell/material complex. Other biodegradable composites are based on polyhydroxybutyrate-polyhydroxyvalerate (PHB-PHV) [222]. SEM showed that the intended compositions of composites were achieved and bioceramic particles were well distributed in the polymer.

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