Classes of materials used in medicine - Biomedical Engineering Desk Reference - page 256

interfacial thickness of the most reactive bioactive glasses
shown in Fig. 3.2.10-2 is largely due to this reaction.
The fourth term in Eq. 3.2.10-1 , þk 4 t y (stage 4), de-
scribes the precipitation reactions that result in the mul-
tiple films characteristic of type III glasses. When only one
secondary film forms, the surface is type IIIA. When
several additional films form, the surface is type IIIB.
In stage 4, an amorphous calcium phosphate film
precipitates on the silica-rich layer and is followed by
crystallization to form carbonated HA crystals. The cal-
cium and phosphate ions in the glass or glass-ceramic
provide the nucleation sites for crystallization. Carbonate
anions (Co 3 ) substitute for OH in the apatite crystal
structure to form a carbonate HA similar to that found in
living bone. Incorporation of CaF 2 in the glass results
in incorporation of fluoride ions in the apatite crystal
lattice. Crystallization of carbonate HA occurs around
collagen fibrils present at the implant interface and
results in interfacial bonding.
In order for the material to be bioactive and form an
interfacial bond, the kinetics of reaction in Eq. 3.2.10-1 ,
and especially the rate of stage 4, must match the rate of
biomineralization that normally occurs in vivo. If the rates
in Eq. 3.2.10-1 are too rapid, the implant is resorbable,
and if the rates are too slow, the implant is not bioactive.
By changing the compositionally controlled reaction
kinetics ( Eq. 3.2.10-1 ), the rates of formation of hard
tissue at an implant interface can be altered, as shown in
Fig. 3.2.10-1 . Thus, the level of bioactivity of a material
can be related to the time for more than 50% of the inter-
face to be bonded ( t 0.5bb ) [e.g., I B index of bioactivity: ¼
(100/ t 0 . 5bb )] (Hench, 1988). It is necessary to impose
a 50% bonding criterion for an I B since the interface
between an implant and bone is irregular (Gross et al. ,
1988). The initial concentration of macrophages, osteo-
blasts, chondroblasts, and fibroblasts varies as a function
of the fit of the implant and the condition of the bony
defect. Consequently, all bioactive implants require an
incubation period before bone proliferates and bonds.
The

and eliminate bioactivity. Also, the sensitivity of fit of
a bioactive implant and length of time of immobilization
postoperatively depends on the I B value and closeness to
the I B ¼ 0 boundary. Implants near the I B boundary re-
quire more precise surgical fit and longer fixation times
before they can bear loads. In contrast, increasing the
surface area of a bioactive implant by using them in
particulate form for bone augmentation expands the
bioactive boundary. Small ( < 200 m m) bioactive glass
granules behave as a partially resorbable implant and
stimulate new bone formation (Hench, 1994).
Bioactive implants with intermediate I B values do not
develop a stable soft tissue bond; instead, the fibrous
interface progressively mineralizes to form bone. Con-
sequently, there appears to be a critical iso I B boundary
beyond which bioactivity is restricted to stable bone
bonding. Inside the critical iso I B boundary, the bioactivity
includes both stable bone and soft-tissue bonding,
depending on the progenitor stem cells in contact with
the implant. This soft tissue-critical iso I B limit is shown
by the dashed contour in Fig. 3.2.10-5 .
The thickness of the bonding zone between a bioactive
implant and bone is proportional to its I B (compare
Fig. 3.2.10-1 with Fig. 3.2.10-2 ). The failure strength of
a bioactively fixed bond appears to be inversely de-
pendent on the thickness of the bonding zone. For ex-
ample, 45S5 Bioglass with a very high I B develops a gel
bonding layer of 200 m m, which has a relatively low shear
strength. In contrast, A-W glass-ceramic, with an in-
termediate I B value, has a bonding interface in the range
of 10-20 m m and a very high resistance to shear. Thus,
the interfacial bonding strength appears to be optimum
for I B values of w4. However, it is important to recognize
that the interfacial area for bonding is time dependent,
as shown in Fig. 3.2.10-1 . Therefore, interfacial strength
is time dependent and is a function of such morpho-
logical factors as the change in interfacial area with
time, progressive mineralization of the interfacial tissues,
and resulting increase of elastic modulus of the inter-
facial bond, as well as shear strength per unit of bonded
area. A comparison of the increase in interfacial bond
strength of bioactive fixation of implants bonded to bone
with other types of fixation is given in Fig. 3.2.10-7
(Hench, 1987).
Clinical applications of bioactive glasses and glass-
ceramics are reviewed by Gross et al. (1988), Yamamuro
et al. (1990), and Hench and Wilson (1993) ( Table
3.2.10-6 ). The 8-year history of successful use of Cera-
vital glass-ceramics in middle ear surgery (Reck et al. ,
1988) is especially encouraging, as is the 10-year use of A-
W glass-ceramic in vertebral surgery (Yamamuro et al. ,
1990), the 10-year use of 45S5 Bioglass in endosseous
ridge maintenance (Stanley et al., 1996) and middle-ear
replacement, and the 6-year success in repair of peri-
odontal defects (Hench and Wilson, 1996; Wilson, 1994).

length of this incubation period varies widely,

depending on the composition of the implant.
The compositional dependence of I B indicates that
there are iso I B contours within the bioactivity boundary,
as shown in Fig. 3.2.10-5 (Hench, 1988). The change of
I B with the SiO 2 /(Na 2 O þ CaO) ratio is very large as the
bioactivity boundary is approached. The addition of
multivalent ions to a bioactive glass or glass-ceramic
shrinks the iso I B contours, which will contract to zero as
the percentage of Al 2 O 3 ,Ta 2 O 5 , ZrO 2 , or other multi-
valent cations increases in the material. Consequently,
the iso I B boundary shown in Fig. 3.2.10-5 indicates the
contamination limit for bioactive glasses and glass-ce-
ramics. If the composition of a starting implant is near
the I B boundary, it may take only a few parts per million
of multivalent cations to shrink the I B boundary to zero