Bioactivity is defined as the ability to chemically bond to living tissue [12], but what differentiates a standard soda-lime silicate glass and these bioactive compositions? There are three major differences; the low S i02 content
( < 60 mol% ), and correspondingly, the higher than average alkali/ alkaline earth content and the high CaO:P2Oj ratio.
The base Bioglass composition is 45S5, a nomenclature explained as [1] 45 weight% S i02 with a calcium to phosphorus ratio of 5. Table 5.1, from reference [26] details a variety of materials developed from this base composition. These alterations from 45S5 do affect the bioactive properties, decreasing the CaO:P2Oj reduces the level of bioactivity, but does provide for other desirable features such as machinability, mechanical strength, lower melting temperatures and
C om position o f B loactlva plasm aa and O lm C w m lc i (wt%)
46SS 4SSS4F 4681SSS S2S4 6 &&S4 3 KOC KGS KGy?U a/w MB
Component B o g m i* e o 0 » i k C i t a i t * Cm me«** gl— w m n t J - l M tO Tt S45P? so, 4 5 4 5 3 0 5 2 55 46 2 4 6 3 8 3 4 2 1 9 -5 2 4 5
SÔ
6 6 6 • 6 1 6 3 4 - 2 4 7 2 4 5 14 7 2 4 5 21 19 5 2 0 2 3 3 31 44 9 9 - 3 2 2S P *
Kg
9 8 2 5 5 16 1 3 5 0 5 2 9 4 6 5 -1 5 2 4 5 2 4 5 2 4 5 21 19 5 4 8 S 4 3 - 5 2 4'S’ '
T a , d , / T i O , 0 4 7 3 - 5 1 2 -3 3 15 6 5 2Slructure Glass and Glass Glass Glass ( M M Glass- Glass Glass Glass glass ceramic ceramic ceramic ceramic ceramc
ce ra m ic
Table 5.1. The composition o f various bioactive glasses and glass-ceramics as given in reference / 26/. Note that the concentration o f each component is given in wt %.
open gel structure then permits rapid migration of Ca2+ and P 0 43+ to the interface region which subsequently produces a CaO-P2Oj rich film on the gel layer which develops into a thicker and denser amorphous layer via the arrival of more ions from the bulk glass absorption of calcium and phosphate ions from the physiological solutions. This stage has been detected after only ten minutes of immersion in a controlled in vitro examination [26], The final stage involves the crystallization of the amorphous calcium phosphate into a mixed hydroxyl, carbonate and fluoro- apatite layer, the necessary OH-, C 0 32" and F" anions again being derived from the host tissue. Laboratory in vitro simulations with the implant surface examined by FTIR indicate that by \Vi hours the P-O vibration is more consistent with crystalline apatite. Concurrently C-O modes are also observed [52], Around 10 hours
subsequent to implantation, the mixed apatite layer is approximately 4 microns thick
Figure 5.1. An illustration o f the 3 stage interface between a bioactive glass and a rabbit tibia 6 weeks after insertion. Concentration profiles o f Si and Ca drawn from an SEM micrograph /49/
[S3]. Figure 5.1, taken from Karlsson et al [34], illustrates the three stage profile of the tissue-glass interface.
The bioactivity • compositional dependence of glasses within the soda-lime silicate system is illustrated in figure 5.2. Compositions within region A form strong bonds with living tissue, hence the region is known as the bioactive
open gel structure then permits rapid migration of Ca2+ and P 0 43+ to the interface region which subsequently produces a Ca0-P205 rich film on the gel layer which develops into a thicker and denser amorphous layer via the arrival of more ions from the bulk glass absorption of calcium and phosphate ions from the physiological solutions. This stage has been detected after only ten minutes of immersion in a controlled in vitro examination [26]. The final stage involves the crystallization of the amorphous calcium phosphate into a mixed hydroxyl, carbonate and fluoro- apatite layer, the necessary OH-, CO32" and F- anions again being derived from the host tissue. Laboratory in vitro simulations with the implant surface examined by FTIR indicate that by 1 Vi hours the P-O vibration is more consistent with crystalline apatite. Concurrently C-O modes are also observed [52]. Around 10 hours
subsequent to implantation, the mixed apatite layer is approximately 4 microns thick
Figure 5.1. An illustration o f the 3 stage interface between a bioactive glass and a rabbit tibia 6 weeks after insertion. Concentration profiles o f Si and Ca drawn from an SEM micrograph 149/
[53]. Figure 5.1, taken from Karlsson et al [54], illustrates the three stage profile of the tissue-glass interface.
The bioactivity • compositional dependence of glasses within the soda-lime silicate system is illustrated in figure 5.2. Compositions within region A
increased adhesion to substrates. Perhaps, most importantly, controlled
crystallization of the bioactive glasses into glass-ceramics produces no detectable reduction in the tissue bonding ability. Certain halides such as CaF2, substituting for up to 12.5 weight% of CaO, have been introduced as nucleating agents to ensure the crystallization of required phases such as fluroapatite. One important group of oxides does have a distinctly destructive affect upon bioactivity [47-49], transition metal oxides. As little as 3 weight% of oxides such as Ta20 3, T i02 and Z r02
destroys all bone bonding ability. A similar concentration of Sb203 or A1203 also has the same adverse effect. However the bioactive glass-ceramic developed by Vogel, which does contain alumina, remains bioactive. This is achieved by the A1203 being incorporated into the mica crystalline phase and as such it does not interfere with the initial surface reaction kinetics [36). Andersson et al [50] have more recently shown that up to approximately 1.6 mol% of alumina can be tolerated without destroying the ability to bond to living tissue.
These surface reaction kinetics have been studied most extensively in the 45S5 Bioglass [2] and apparently proceed via 5 stages after implantation [26]. Initially there is an ion exchange between the implant surface and the surrounding physiological environment with Na* being replaced by H* or H30 +. This process is diffusion controlled and is therefore time dependent, proportional to t '1/2, with a depletion depth in excess of 0.5 microns within minutes of implantation [26]. The second stage involves the break up of the silicate network to form silanols viz;
S i - O - S i + H 20 ---> 2 Si - OH ^
This is an interfacial reaction and hence is directly proportional to time. The third stage involves the equilibrium condensation and partial repolymerisation of these silanols to form an S i02 rich gel layer at the interface. The formation of this gel layer has been observed after as little as 20 minutes after implantation [51]. The
\S iO ,
Oi» O n O f
ft ft a a •- ?,
Figure 5.2. B io g la ss bonding behaviour represented on a triaxial com position diagram. A ll com positions contain a constant 6 wt% P f l y Illustration fr o m ¡9 /
bonding boundary. Increasing the S i02 content of the glasses, region B, to compositions typical of window or bottle glass destroys the bioactivity, these compositions behave in a similar manner to 'inert' implants in that they elicit encapsulation by fibrous membranes from the host. Compositions within region C are found to be completely resorbable, disappearing completely after 10 to 30 days of implantation, region D is a non-glass forming region due to the very low S i02
content. Interestingly, within the region A. the degree of bioactivity decreases as the composition nears the region boundary [26].