Biomedical Engineering Reference
In-Depth Information
interface with the patient's bone are ultimately resorbed and are replaced by ingrown new bone tissue.
This approach is in the experimental stages.
Rubber used in catheters, rubber gloves, and so on is usually reinforced with very fine particles of
silica (SiO
2
) to make the rubber stronger and tougher.
Teeth with decayed regions have traditionally been restored with metals such as silver amalgam.
Metallic restorations are not considered desirable for anterior teeth for cosmetic reasons. Acrylic resins
and silicate cements had been used for anterior teeth, but their poor material properties led to short
service life and clinical failures. Dental composite resins have virtually replaced these materials and are
very commonly used to restore posterior teeth as well as anterior teeth (Cannon, 1988).
The dental composite resins consist of a polymer matrix and stiff inorganic inclusions (Craig, 1981).
A representative structure is shown in Figure 4.4. The particles are very angular in shape. The inorganic
inclusions confer a relatively high stiffness and high wear resistance on the material. Moreover, since
they are translucent and their index of refraction is similar to that of dental enamel, they are cosmeti-
cally acceptable. Available dental composite resins use quartz, barium glass, and colloidal silica as fillers.
Fillers have particle size from 0.04 to 13 μm, and concentrations from 33% to 78% by weight. In view
of the greater density of the inorganic filler phase, a 77 wt% of filler corresponds to about 55 vol%. The
matrix consists of a polymer, typically bisphenol A glycidyl methacrylate (BIS-GMA). In restoring a
cavity, the dentist mixes several constituents, and then places them in the prepared cavity to polymerize.
In order for this procedure to be successful, the viscosity of the mixed paste must be sufficiently low and
the polymerization must be controllable. Low-viscosity liquids such as triethylene glycol dimethacrylate
are used to lower the viscosity and inhibitors such as butylated trioxytoluene are used to prevent pre-
mature polymerization. Polymerization can be initiated by a thermochemical initiator, such as benzoyl
peroxide, or by a photochemical initiator (benzoin alkyl ether) which generates free radicals when sub-
jected to ultraviolet light from a lamp used by the dentist.
Dental composites have a Young's modulus in the range 10-16 GPa, and the compressive strength
from 170 to 260 MPa (Cannon, 1988). As shown in Table 4.1, these composites are still considerably
less stiff than dental enamel, which contains about 99% mineral. Similar high concentrations of
mineral particles in synthetic composites cannot easily be achieved, in part because the particles
do not pack densely. Moreover, an excessive concentration of particles raises the viscosity of the
unpolymerized paste. An excessively high viscosity is problematic since it prevents the dentist from
adequately packing the paste into the prepared cavity; the material will then fill in crevices less
effectively.
The thermal expansion of dental composites, as with other dental materials, exceeds that of tooth
structure. Moreover, there is a contraction during polymerization of 1.2-1.6%. These effects are thought
FIGURE 4.4
Microstructure of a dental composite. Miradapt
•
(Johnson & Johnson) 50% by volume filler: barium
glass and colloidal silica. (Adapted from Park, J.B. and Lakes, R.S. 1992.
Biomaterials
, 2nd ed., Plenum Press, New York.)
