Bioactive glass 101
Bone graft substitutes are commonly used to replace and regenerate bone lost due to trauma, infection, or disease [1]. Current generation biomaterials are designed to stimulate specific cellular responses at the molecular level. This generation of biomaterials are bioactive degradable and might be osteoconductive or osteoinductive. Bioactivity refers to any interaction or effect that materials have on cells to activate specific cellular recruitment and cellular responses. Bioactive glass is an osteostimulative material that is currently used as bone graft substitute and in the treatment of osteomyelitis.
Osteostimulation refers to osteoblast cell recruitment and/or differentiation and osteoblast activation to produce new bone in a bony environment [2]. Osteostimulation should not be confused with osteoinduction, which is the ability of materials to recruit stem cells to differentiate into bone forming cells and form ectopic bone. Also it should not be confused with osteoconduction, which is the possibility for bone to grow along the material, in other words only providing a scaffold for bone formation [3]. Bioactive glass is an osteostimulative material; thus it is osteoconductive and serves as a scaffold for bone formation in vivo, but it is not fully osteoinductive. The composition diagram for bone bonding based on a Na2O–CaO–SiO2 ternary state diagram is shown in Figure 1.a. in which only region A displays the area where composition leads to bioactive glass with bone bonding properties.
An excellent review paper on bioactive glasses was published by Julian R. Jones in 2015 and it is highly recommended [4].
Figure 1.a. Composition diagram for bone bonding based on a Na2O–CaO–SiO2
Area A: Bioactive glass – bone bonding properties; Area B: Inert glass – no bone bonding properties; Area C: Soluble glass (water glass) – no bonding properties; Area P: No glass formation – no bonding properties.
Figure 1.b. Section of a structural model of bioactive glass with Na and Ca removed for clarity. The structure contains bridging oxygen (BO) and non-bridging oxygen (NBO)
S53P4 bioactive glass (Bonalive®) is a synthetic, bioactive, osteoconductive, osteostimulative and bacterial growth inhibiting bone graft substitute, composed of SiO2 53%, Na2O 23%, CaO 20%, P2O5 4% (by weight). The structuring of the atoms of bioactive glass can be seen in Figure 1.b. This bioactive glass is characterized by its ability to attach firmly to living tissue, facilitate tissue growth, bond chemically with surrounding bone and promote new bone formation in the implanted area. This material is not only osteoconductive, due to its bioactive composition it stimulates and promotes new bone formation. It works by leaching out ions that react with the body fluids, which results in transforming the granule surface chemically into one that resembles the chemical composition and structure of natural bone tissue. Leaching of the ions stimulates continuous bone tissue growth (osteostimulation). The simultaneous increase of osmolarity and pH and is also the reason for the unique bacterial growth inhibiting property of S53P4 bioactive glass. Currently it is the only bioactive glass that is proven to inhibit bacterial growth.