Revision surgery to replace failed hip implants is a significant health care issue that is expected to escalate as life expectancy increases. A major goal of revision surgery is to reconstruct femoral intramedullary bone-stock loss. To address this problem of bone loss, grafting techniques are widely used. Although fresh autografts remain the optimal material for all forms of surgery seeking to restore structural integrity to the skeleton, it is evident that the supply of such tissue is limited. Even when allografts are made available, donor selection, screening, procurement and storage are time-consuming and expensive. Moreover, the potential transmission of infections such as hepatitis. AIDS, and Creutzfeldt-Jakob disease raises serious doubts about the future of allotransplantation.

In recent years, calcium phosphate ceramics have been studied as alternatives to autografts and allografts. Although several graft systems succeed in the sense that wound healing occurs and some bony consolidation at the recipient site is achieved, the ideal synthetic graft system has not yet been found. The significant limitations associated with the use o f biological and synthetic grafts have led to a growing interest in the in vitro synthesis of bone tissue. The approach is to synthesize bone tissue in vitro with the patient’s own cells, and use this tissue for the repair of bony defects. Various substrates including metals, polymers, calcium phosphate ceramics and bioactive glasses, have been seeded with osteogenic cells. However, none of these studies were directed towards the goal of achieving optimal conditions for in vitro synthesis of bone tissue.

The selection of bioactive glass in this study is based on the fact that this material has shown an intense beneficial biological effect which has not been reproduced by other biomaterials. Even though the literature provides extensive data on the effect of pore size and porosity on in vivo bone tissue ingrowth into porous materials for joint prosthesis fixation, the data from past studies cannot be applied to the use of bioactive glass as a substrate for the in vitro synthesis of bone tissue. First, unlike the in vivo studies in the literature, this research deals with the growth of bone tissue in vitro. Second, unlike the implants used in past studies, bioactive glass is a degradable and resorbable material. Thus, in order to establish optimal substrate characteristics (porosity and pore size) for bioactive glass, it was important to study these parameters in an in vitro model. We hypothesized that the biological effect (the expression of the osteoblastic phenotype, including the elaboration of bone-like tissue) can be greatly enhanced by careful selection of a suitable combination of porosity and pore size.

We synthesized porous bioactive glass substrates (BG) with varying pore sizes and porosity and determined the effect of substrate properties on the expression and maintenance of the osteoblastic phenotype, using an in vitro culture of osteoblast-like cells. We also studied the effect of (β-glycerophosphate (β-GP) treatment on the reaction kinetics of BG.

Our data showed that porous bioactive glass substrates support the proliferation and maturation of osteoblast-like cells. Within the conditions of the experiment, we also found that at a given porosity of 44% the pore size of bioactive glass neither directs nor modulates the in vitro expression of the osteoblastic phenotype. On the other hand, at an average pore size of 92 μm. when cultures are maintained for 14 days, cell activity is greatly affected by the substrate porosity. As the porosity increases from 35% to 59%. osteoblast activity is adversely affected. We observed that substrates with the largest porosity (59%). may have presented a larger surface area for the dissolution reactions to occur. The data also showed that media supplementation with β-GP in concentrations even as low as 3 mM lowers alkaline phosphatase activity levels and increases the calcium content of the mineral deposited in the cell cultures. Furthermore, the presence of β-GP accelerated the glass transformation reactions, and led to the formation and deposition of calcium phosphate phases with unusual morphology and unusually high Ca/P molar ratios.