Biomedical Engineering Reference
In-Depth Information
during brushing. Acrylic resin and porcelain are commonly used materials for artificial teeth. Acrylic
resins are cross-linked and made using molding methods such as injection molding. It is a good ther-
mal insulator but susceptible to abrasive forces and has poor resistance to fatigue fracture and impact
strength. Porcelain is made from an inner opaque core material overlaid with translucent dentine
material and a final layer of porcelain on the outside. It is very rigid, hard, and brittle. One of the
solutions to improving the properties is to use nanosized powders of alumina to the porcelain.
Natural and artificial human teeth need regular treatment from qualified dentists. Due to the com-
plex environment, dental technologist and dentists have developed numerous tools of various sizes
and shapes to manipulate, drill, and remove unwanted materials. The correct use of the dental tool is
dictated by the function required and preference of the dentist. One of the most common dental tools
is the dental bur. These also come in various shapes and sizes.
15.2
DENTAL BURS
Dental burs have revolutionized the dentistry field and are used for cutting hard tissues such as bone
or tooth
[1,4,6-9]
. They are normally made of stainless steel, diamond grit or particles, and tungsten
carbide, and fitted to a dental drill incorporating an air turbine. The dental bur was developed
300
years ago
[8]
.
The dental bur
[4,8,9]
has three parts: the head, the neck, and the shank. The head contains the
blades, which produce cutting action by rotary motion. The blades are positioned at various degree
angles in order to change the property of the bur. Dentists use this to bore through tooth enamel and
to clean and remove plaque from the tooth's surface, and dental technicians use this to prepare dental
materials in laboratory. Decayed tooth material is also removed before applying a filling. Dental burs
come in many shapes and sizes designed for specific applications and can rotate at speeds of up to
500,000 rpm. They can be made of steel and then coated with a hard coating, such as tungsten carbide
coating, or they can be entirely tungsten carbide
[8,10-13]
. Numerous shapes of burs are manufac-
tured for various applications, cutting, and drilling abilities.
The basic design of an eight blades fissure bur
[14]
is shown in
Figure 15.2
.
Generally geometrical features of the bur are manufactured to have a negative rake angle
(
Figure 15.3
). However, those with a positive rank angle are designed mainly for cutting soft materi-
als (e.g., acrylics) to remove material during cutting to prevent the tool from clogging with the chips.
When the rake angle of a bur blade is too steep, it can damage the subsurface of the tooth. These
weak areas become sites for subsequent bacterial infection. When the rake angle is decreased, a gen-
tler action is achieved. The life of the bur is decreased due to the acute angle of the cutting tips, but
less residual subsurface damage occurs to the tooth.
As mentioned earlier, the two most common dental bur materials used are stainless steel and tung-
sten carbide
[7-9,11,16,17]
. Each material has its own specific advantages and disadvantages. Steel wears
rapidly and corrodes easily and thus appears to be a poor choice of material for burs. However, at lower
speeds, the wear rate associated with the cutting process is reduced significantly and steel may be the pre-
ferred material. The main corrosive environment experienced by the bur is during the sterilization process,
although the conditions in the oral cavity will also contribute. Stainless steel provides a less efficient cut-
ting edge than carbon steel but has a greater corrosion resistance. Stainless steel is a cheap material which
makes it suitable for burs provided that they are considered single-use and classed as disposable burs.
