Volume 87 - Number 1

January - February 2008
Disaster Training Enters the 21st Century

Atraumatic Tooth Preparation

Atypical Odontalgia: A Review

The Dean

Classified Ads
MDA News
Resumes

Clinical Feature

Atraumatic Tooth Preparation



Thomas D. Larson, D.D.S., M.S.D.*




A Note from the Publications Committee

Dr. Thomas Larson continues to supply us with articles of clinical relevance. The Publications Committee feels his topics address clinical situations and choices we deal with on a daily basis. Does the tooth need a post? How long should the post be? What type of post is best, and should it be bonded or cemented? What are the products that have undergone scientific studies? All these questions were answered in a single review of literature article that we published.

Most of us and our readers are busy private practitioners with limited time to read scientific studies. To have someone of Dr. Larson’s academic experience and credentials providing our journal his extensive yet concise reviews of the literature is of great value. To that end, Northwest Dentistry will be presenting Dr. Larson’s articles in a category of their own within our Clinical Department, and on a more frequent basis. “I chose Northwest Dentistry,” Dr. Larson told us, “because if I were to publish in national journals, many of my former students would never see the articles. They are the audience to which my academic career is devoted. They are the audience that I am trying to affect by publishing best practices.” We can only add our thanks for this opportunity.

The Editors



Abstract

One of 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.

Introduction
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.

Controlling Temperature
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 F).vi

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.

In 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 of 1.2N.xx
Cutting with carbide burs is most efficient at ultra high speed in dentin, with intermittent cutting compared to continuous cuttingxxi (Figures 4
and 5).

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).

   Dry 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 damage.xxxi
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

Pulpal Response
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

Summary
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.
n

References

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*Dr. Larson is Associate Professor, Department of Restorative Sciences,Division of Operative Dentistry, University of Minnesota School of Dentistry, Minneapolis, Minnesota. E-mail is larso004@umn.edu. 





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