the tenets of operative dentistry is to maintain the health and vitality of
teeth whenever possible. Methods of tooth preparation can adversely affect the
tooth structure and the pulp. This paper will review the research on methods
and devices used to prepare teeth, the possible sequelae of tooth preparation,
and the best methods that can be recommended from the research to maintain
tooth vitality and marginal integrity.
The air turbine high-speed handpiece, as an example of operative instrumentation, has been used in dentistry
since the 1950’s, with some changes to the design. Most notably water spray
patterns have been improved, fiber optic light sources have been added, and
improved design and materials have made the turbine more concentric and longer
wearing. Modern dental handpieces are durable, and coupled with new burs, can
be used to prepare teeth quickly and accurately. There are methods of
instrumentation that can improve dental outcomes and limit iatrogenic effects.
This paper will review the research relating to those methods.
One of the initial concerns about cutting
with ultra-high-speed handpieces (200,000-400,000 rpm) is the temperature rise
caused by the frictional heat created when cutting at such high speeds. Dry
cutting with air-cooling only is reported to contribute to increasing
temperatures that negatively affect the pulp and cause cracking of the dentin and
enamel.i The recommendation is that water-cooling is superior to
air-cooling in moderating temperature rises in tooth structure.ii,iii,iv,v
Even polishing intra-orally can increase the temperature of the pulp. In an in vitro study, the temperature in the
pulp chamber of extracted teeth while polishing an amalgam increased more than
20 degrees Celsius (11 degrees F) within 30 seconds while polishing
continuously and dry. Using a water coolant while polishing with intermittent
pressure causes a decrease in temperature of four degrees Celsius (-2.2 degrees
Temperature increases in teeth have been
studied as a function of tooth preparation and also separately to determine the
effect such temperature rise has on the health of the pulp. An early study from
1965 looked at how a temperature rise without tooth preparation changed the
histologic appearance of the pulp. After applying heat to the teeth of dogs, a
four degree F increase showed no effect on the pulp tissue, whereas a ten
degree F increase showed immediate and prolonged changes, with 15% necrosis of
the pulp 56 days after exposure. A 20 degree F increase showed a severe inflammatory response with 65% pulpal
necrosis after 56 days.
A 30 degree F rise showed complete pulpal necrosis.vii
Early studies have looked at the pulpal
temperature rise during tooth preparation in extracted teeth with different
rotating speeds, with and without coolant. Using eight ounces of force and
comparing a steel bur to a carbide bur to a diamond bur, at 20,000 RPM. With no
coolant the Fahrenheit temperature rise is 125°, 90°, and 85°, respectively.
All are sufficient to cause pulpal necrosis. These same conditions with water
coolant change the temperature rise to 10° for the steel and carbide burs and
seven degrees for the diamond bur.viii A recent study in vitro showed that increasing the
amount of water cooling and decreasing the load during cavity preparation
causes minimal temperature rises in teeth.ix
Recent studies of temperature rise in vitro and in vivo demonstrate that measured increases in temperature are
greater in extracted teeth than in vivo
measurements. Interpreting data from in
vitro studies is valuable in showing the relative order of temperature rise
rather than absolute values to temperature increases. For example, diamond burs
increase temperature more than carbide burs.
vivo measurements of tooth temperature rise show water-cooled
high-speed tooth preparation resulted in a three to four degree C rise
(1.67-2.2°F), whereas dry, air-cooled high-speed bur preparations caused a 14
degree C rise (7.78°F) in temperature.x This experiment was
performed on beagle dogs, with a temperature probe placed in the pulps of the
teeth to record the temperature rise.
The temperature rise created by cutting with
a high-speed in vitro has also been
shown to cause thermal expansion and contraction of the dentinal fluid as well
as of the enamel.xi Temperature rise causing an expansion of the
dentinal fluid causes an outward fluid flow. Cooling during high-speed cutting
diminishes outward fluid flow, whereas cutting dry causes inward fluid flow.xii
Inward fluid flow stimulates the A-delta neurons of the pulp, eliciting a sharp
pain response dulled with the use of anesthetics.xiii
Temperature of the water has been studied,
and it is reported that a cooling temperature of 30-34 degrees C (86-93° F)
prevents adverse temperature rise.xiv,xv Generally increasing the
water flow (mL/min) from minimal to 45mL/min with a low load causes either no
change or a slight decrease in pulpal temperature in vitro.xvi,xvii
Cutting Force and Bur Design
In a survey of North American dental schools
in 1999, it was reported that there is consensus among educators that medium
grit diamonds are used for gross tooth reduction and axial wall reduction.
Postgraduate prosthodontists prefer a coarse grit diamond for gross reduction.
Diamonds and carbide burs are used for marginal finishing. Carbide burs are
used for internal cavity outline form in predoctoral operative dentistry,
whereas a combination of fine diamond and carbide burs are used for finishing
composite restorationsxviii (Figures 1, 2, and 3).
Improved concentricity of one-piece carbide
burs, where the cutting edges are aligned with the steel shaft more accurately
than with two-piece construction of the bur, have been shown to produce
superior quality of cut surfaces at entry, exit, and advancing fronts, as well
as less sub-surface cracking.xix Plain and cross-cut carbide burs
were compared while assessing the magnitude of cutting forces. In this in vitro study there was no difference
between cross-cut and plain burs on cutting force wet or dry, but the handpiece
torque produced significantly different forces. The higher torque required a
mean cutting force of 1.44N (Newton) compared to the lower torque with a force
Cutting with carbide burs is most efficient at ultra high speed in dentin, with
intermittent cutting compared to continuous cuttingxxi (Figures 4
Cutting force has been mentioned previously
as affecting the temperature, with increasing forces causing a greater
temperature rise. Cutting force is optimal at about 100g of force at the
diamond tip for medium-grit diamond burs, which also is the average force most
dentists use.xxii The design of diamond burs can specifically affect
cutting efficiency and temperature increases. The cutting rate of super coarse
grit diamonds is not significantly greater than the coarse grit diamond, but is
significantly greater than the medium grit diamond.xxiii The cutting
efficiency of the coarser diamond is less than that of the less coarse diamond,
and the conventional diamond design is more efficient than channeled diamond
design.xxv Cutting efficiency is improved with higher coolant flow
rates,xxv and with multiple spray ports on the handpiece which clear
cutting debris more efficiently.xxvi
Effect of Cutting on Tooth Structure
Cutting at 400,000 rpm produces cleaner
cavity surfaces than 200,000 rpm, and plain carbide burs leave cleaner surfaces
than diamond burs.xxvii,xxviii Preparing a dentinal surface with a
diamond bur decreases the dentin permeability compared to a carbide bur,
presumably because of the collection of smear layer the diamond bur createsxxix
(Figure 6). When heavy forces are used with an air turbine handpiece, the bur
can stall, causing a rippling effect on the cut surface. This rippling occurs
when the blades of the bur become clogged with debris created with heavier
forces. Lighter loads leave a smoother surface, without the ripplingxxx
(Figures 7 and 8).
cutting can induce thermal stresses high enough to fracture enamel (Figure 9).
Dry cutting, when close enough to the pulp (~1-2mm) causes biologic pulp
Diamond burs create enamel subsurface damage by initiating inter- and intra-rod
cracks, with coarser diamonds creating the longest cracks of 84+/- 30 microns.
Finishing the surface of enamel with a fine diamond eliminates this cracking.xxxii
Cracks initiated in enamel can propagate to the dentino-enamel junction (DEJ),
but are generally arrested when they reach the mantle dentin. The DEJ has a
fracture toughness approximately five to ten times greater than the enamel, but
about 75% lower than dentin.xxxiii,xxxiv Cutting on dentin can also
create cracking (Figure 8). Using both in
vitro and in vivo models,
researchers have found that there is a positive correlation between creep rate
and stress in dentin, and that loading with a constant force over time showed
stress relaxation. Such stress, cyclically applied, can cause flaws in dentin,
caused by tooth preparation, which contribute to crack propagation. Sub-surface
cracks in dentin of 25 microns have been tested in finite element analysis to
propagate and cause fractures an estimated 25 years later. Cracks of 100
microns were shown to propagate and reduce the fatigue life fracture to an
estimated five years.xxxv
Preparation of a margin for bonding can
affect the quality of the bond, with use of concentric straight burs and
finishing burs causing the least damage to enamel marginal structure.xix,xxvii,xxviii
Intermittent cutting with light pressure
diminishes any histologic pulpal response to tooth preparation. However,
high-speed cutting causes movement of dentinal fluid outward, resulting in
movement of odontoblastic nuclei. There also is an exudation of dentinal fluid,
normal due to a hydrostatic pressure of 5-20mm/Hg. This redistribution of
cytoplasmic constituents is thought to cause degeneration of odontoblastic
processes, possibly resulting in formation of dead tracts in the dentin.xxxvi
High-speed tooth preparation dry, close to
the pulp (0-1mm of remaining dentin thickness) with diamond and carbide burs
increases pulpal pressure by 12 kPa and 6kPa, respectively, whereas cutting wet
causes an increase in pulpal pressure of 0.6 kPa and 0.15 kPa, respectively.xxxvii
Cutting wet decreases the pressure on the pulp by allowing dentinal fluid flow.xxxviii
Cutting dry can cause excessive evaporative water loss through the dentinal
tubules, even through smear layers produced by cutting instruments, possibly
causing disruption of odontoblasts.
Dry preparation decreases blood flow, and
within one mm of the pulp causes a decreased blood flow due to inhibition of
sympathetic nerve stimulation. When cutting this close to the pulp, plasma proteins
will enter the dentinal tubules and clot, decreasing the diameter of the
dentinal tubules, and limiting permeability. Cutting wet, with light
intermittent pressure more than one mm from the pulp, will not cause
displacement of the odontoblastic nuclei. It is not recommended to cut close to
the pulp with a high-speed bur.xxxvi
Ultra-high-speed, high-torque cutting causes
greater pulpal response than low torque cutting. All pulpal changes resolved
within 30 days for normal depth cavity preparations in this in vivo study.xi In an in
vivo study, 30 premolars scheduled for orthodontic extraction were prepared for
full crowns and cooled with water spray and without. After extraction, the
teeth were prepared histologically. The most severe reaction of the pulp was
from dry cutting. The odontoblastic layer was significantly altered, with
inflammatory infiltrates present in the absence of bacteria.xli
Using laser Doppler flowmetry in vivo, blood flow was measured in
human teeth. As a tooth was cooled, pulpal blood flow decreased but did not
completely stop. This altered the sensory threshold, causing little to no nerve
response.xlii Pulpal nociceptors are regulated by temperature and pH
changes, with cooling to 26 degrees C (46.3°F) causing a significant diminution
of nerve response.xliii Cooling while cutting a tooth can have the
beneficial effect of decreasing the inflammation caused by increased blood
flow. The temperature-related increase of blood flow could result in the
extravasations of plasma.xliv Blood flow is not affected by the use
of mandibular nerve blocks with three percent mepivacain.xlv
With the insults inherent in the surgical
preparation of a tooth, it is necessary to realize the pulp-dentin complex will
react in many ways to protect the pulp. Normal aging will result in a decrease
in the number of fast-conducting afferent nerves. Mineral apposition causing
secondary and tertiary dentin formation will insulate the nerves from being
activated, especially by the hydrodynamic mechanisms of dentinal fluid flow.xlvi
Pulpal tissues, when faced with increased blood flow, increases in pulpal
pressure, and outward movement of dentinal fluid, will react physiologically by
lining the tubules with proteins, mineral deposits, or tertiary dentin,
limiting damage to the pulp.xlvii
Minimizing pulpal trauma from surgical tooth
preparation requires that ultra-high-speed tooth preparation use water spray
from multiple ports, with a water temperature of 30-34 degrees C (86-93.2
degrees F) using light, intermittent pressure with lower torque handpieces
(Figure 10). Controlling temperatures requires use of carbide and medium grit
diamonds with high-speed handpieces. Coarse diamond burs can create too much
heat, resulting in damage to underlying enamel and dentin. To minimize pulpal
damage, as a tooth preparation comes within one mm of the pulp, the preparation
should be completed using a slow-speed handpiece with light intermittent
pressure. One-piece carbide burs offer greater concentricity and smoother cuts
to the tooth structure.
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is Associate Professor,
Department of Restorative Sciences,Division of Operative Dentistry, University of Minnesota School of Dentistry, Minneapolis, Minnesota.
E-mail is email@example.com.