Caries Removal: Background and Debate

The U.S. National Health and Nutrition Examination Survey (NHANES), in an extrapolated survey (1999-2002) of the noninstitutionalized civilian population, found that 41% of children age 2 to 11 years had dental caries in their primary teeth, and that 42% of children and adolescents age 6 to 19 years and about 90% of adults had dental caries in their permanent teeth.¹ Other studies reflect even greater prevalence of caries, as high as 96% of children 6 to 7 years old in Saudi Arabia.² Such findings indicate a continued demand for interceptive restorative treatment.

In addition to such prevalence, there is growing skepticism toward the two basic mainstays of restorative dentistry: the cavity classification of G.V. Black (Classes I-V) and the continued use of amalgam as a restorative material.³ Black’s ideology, based on the dogma “extension for prevention,” has often resulted in the removal of large areas of otherwise healthy enamel and dentin. This has been exacerbated by the need to address the mechanical success of amalgam: outline form and retention form of cavity design. Studies assessing the iatrogenic damage to natural tooth tissue during restorative procedures report microfractures adjacent to cavity margins, damage to teeth adjacent to interproximal cavity design,⁴ and thermal injury to the pulp caused by rotary instrumentation.⁵

Despite the gradual reduction in amalgam restorations being placed (56 to 52 million in United States ⁶; 8 to 6 million in United Kingdom⁷), there is still a considerable need to service existing metal restorations. However, the evolution of non–metal-based composite restorations as well as a desire for cosmetic treatment from a patient base with greater awareness and expectations has resulted in a shift toward smaller restorations, a change in cavity design (e.g., “tunnel” restorations⁸), and a greater reliance on acid-etch microretention techniques.⁹^,¹⁰

Instrumentation and Lasers

The most common procedures for removing diseased dental hard tissue during restorative procedures involve the use of excavators and burs. The accuracy of such instrumentation is questioned,¹¹ with minimal objective analysis of total removal of demineralized, infected enamel and dentin, as well as the possible removal of excessive amounts of sound tissue.¹² On the other hand, the use of a suitable laser wavelength may allow a much more conservative approach to the preservation of sound, mineralized tooth tissue, with preferential removal of higher-water-content caries, greater precision, and reduced bacterial contamination of the laser-prepared cavity.¹³^,¹⁴

A key factor in pulpal injury is thermal conduction from the range of instrumentation chosen.¹⁵ Studies have established that rotary instrumentation can cause conductive thermal elevation in excess of 20° C above 37.4° C.¹⁶^,¹⁷ With regard to laser irradiation of dental tissue, the explosive defragmentation resulting from water-assisted mid-infrared (IR) wavelengths allows much of the heat to escape from the cavity (carried in ablated particles), resulting in pulpal thermal increase of less than 5° C.¹⁸^–²⁰ The affinity of mid-IR laser wavelengths for water allows the main absorption to take place in demineralized tissue richer in organic material and with a higher percentage of water, thus protecting any sound tissue overlying the pulp, with a reduced penetration of the beam (Figure 11-1). With adoption of operating parameters to standardize the normal clinical use of each instrument, evidence-based protocols now serve to provide what is best for the patient. As treatment shifts toward early diagnosis and interceptive action that is more selective in preserving healthy tissue, the choice of a laser may become more common, placing the dental profession in an increasingly responsible position to deliver patient-centered restorative care.

FIGURE 11-1 • A, Dental caries presents a risk to natural tooth structure and vitality of dental pulp, as well as a challenge to the clinician. In treating dental caries, it is essential to avoid precipitating further damage to the tooth. B, Using Er:YAG laser, selective removal of dental caries can be achieved with maximal preservation of healthy, mineralized tooth tissue. C, Completed restoration achieves functional and cosmetic form.

The claimed precision of laser use in many areas of surgery would appear to complement the demands of the restorative dentist. Also, although a tooth was the first tissue to be exposed to laser light in Maiman’s 1960 investigations,²¹ the first “dental” laser did not become commercially available until 1989. The chosen wavelength of this laser, neodymium-doped yttrium-aluminum-garnet (Nd:YAG), emitting at a wavelength of 1064 nm, has limited therapeutic action on hard dental tissue. More recent developments led to the erbium, chromium–doped yttrium-scandium-gallium-garnet (Er,Cr:YSGG, 2780 nm) and Er:YAG (2940 nm) laser wavelengths, for use in cavity preparation.

In addition to laser use in dental surgery, other, nonsurgical laser wavelengths have been developed for diagnostic procedures, to assist the clinician in caries detection and assessment of the effective removal of diseased tissue.

Table 11-1 summarizes the advantages of laser use in dental restorative procedures.

Table 11-1 Summary of Comparable Benefits of Laser Use over Rotary Instrumentation (“Airotor”) in Tooth Cavity Preparation

Restorative Procedure	Rotary	Laser*
Cutting enamel/dentin	Yes	Yes
Selective removal of caries	No	Yes
Precision	Precise >1000-2000 µ	Precise <300 µ
Smear layer	Smear layer produced	No smear layer
Thermal rise	Thermal rise >15° C	Thermal rise <5° C
Risk of iatrogenic damage	Greater	Less
Noise/vibration	>120 dB/vibration	<120 dB/no vibration
Bactericidal action	No	Surface decontamination
Speed of cutting enamel	Fast	<30% rotary speed
Speed of cutting dentin	Fast	Comparable
Contact with tooth tissue	Contact required	Noncontact possible
Pain response	High	Less pain/no pain

* Erbium family (Er:YAG, Er,Cr:YSGG).

Laser Photonic Energy–Hard Tissue Interaction

Healthy coronal hard tissue comprises enamel and primary and secondary dentin. By volume, enamel is 85% mineral (predominantly carbonated hydroxyapatite), 12% water, and 3% organic proteins; most free water exists in the periprismatic protein matrix. Dentin has higher water content, being 47% mineral (carbonated hydroxyapatite), 33% protein (mostly collagen), and 20% water. Water content in carious dentin may be as high as 54%.²²

Each of these compounds represents a target chromophore, a tissue element or molecule capable of selective absorption of photonic laser energy. Current understanding of chromophores that offer clinically acceptable laser-tissue interaction identifies high absorption of photonic energy by water (free molecules and OH⁻ radical) at about 3.0-micron (µm) wavelength; by the phosphate (PO₄) radical of carbonated hydroxyapatite (about 7.0 µm); and by the carbonate (CO₃) radical of the tooth mineral, about 9.6 µm. Assuming the incident photonic energy is above the nominal ablation threshold of the target, the resulting conversion of photonic energy into heat can lead to structural or phase change in the target material (photothermal change). With the application of laser energy in restorative dental procedures, demineralized and carious hard tissue must also be considered, where the greater water and protein (pigmented, ~1.0-µm wavelength) content are chromophores.

With such a breadth of target elements, several laser wavelengths have been investigated as possible adjuncts in cutting tooth cavities and removing carious tissue. The absorption coefficients of water and mineralized dental tissue are shown in Figure 11-2.

FIGURE 11-2 • Graphic representation of absorption by water and carbonated hydroxyapatite (CHA) of varying mid-infrared and far-infrared laser photonic radiation energy. Coincident peaks of water and CHA between 2780 and 2940 nm represent the great affinity of these wavelengths for water. In CHA, high absorption of laser energy of approximately 10,600 nm is primarily caused by interaction with phosphate radical of mineral molecule.

Early investigation into the Nd:YAG laser, first marketed for soft tissue laser dentistry in the United States, was done to establish its utility for hard tissue.²³^–²⁷ This included the ablation of pigmented and diseased tissue, the antibacterial effect of this laser wavelength, and the possible effects on the vital dental pulp. Although studies established safe and effective parameters, the Nd:YAG’s clinical significance was of only marginal benefit to the restorative dentist because of its very low absorption in sound enamel or dentin. Further, several studies concluded that the Nd:YAG wavelength could cause unwanted heating effects, such as cracking and melting of mineral structures²⁸^–³⁰ (Figure 11-3).

FIGURE 11-3 • A, Environmental scanning electron micrograph (SEM) showing effect of incident Nd:YAG laser photonic energy on human enamel. The cracks represent thermal overload of the mineral matrix. B, Higher-power (×300) SEM of tooth tissue. Thermal damage has melted the crystalline mineral, resulting in globules of amorphous hydroxyapatite. Ironically, such re-formed mineral displays greater resistance to acid dissolution, leading to claims that such laser treatment may help prevent dental caries.

Early studies on enamel ablation also used the other available laser wavelength, carbon dioxide (CO₂, 10,600 nm), but this laser gave poor interactions, with reports of charring, cracking, and damaging heat buildup in tooth and bone structure.³¹^,³² Although examination of absorption data of this wavelength with hydroxyapatite shows an effective interaction, current CO₂ lasers employ continuous-wave (CW) emission with no cooling water, which results in very high energy deposition in the hard tissue (Figure 11-4).

FIGURE 11-4 • A, Buccal cavity in lower-right canine tooth. Patient requested laser-assisted treatment, and the author erroneously assumed the high absorption of the CO₂ laser in hydroxyapatite would allow efficient ablation. B, Lack of water spray and continuous-wave laser emission caused damaging carbonization to surface caries. Consequently, cavity was prepared using hand instruments. C, Completed restoration. Tooth later tested positive for pulp vitality. Use of CO₂ laser in this case predated commercial availability of erbium laser wavelengths.

In the mid-1990s, use of a more suitable laser wavelength by Keller and Hibst ³³^,³⁴ led to the development of the Er:YAG (2940 nm) wavelength, a free-running (inherently microsecond-pulsed) mid-IR laser for effective ablation of dental hard tissues. This was followed by similar investigations into the use of another mid-IR free-running wavelength, the Er,Cr:YSGG (2780 nm), which could effect photothermic vaporization of interstitial water and ablative disruption of target tooth mineral safely and precisely.³⁵^–³⁸

Relatively, enamel, dentin, bone, cementum, and carious tissue have descending mineral density and ascending water composition.³⁹^,⁴⁰ Both Er,Cr:YSGG and Er:YAG laser wavelengths are absorbed well in water, with the Er:YAG slightly more strongly absorbed in water than the Er,Cr:YSGG (see Figure 11-2). This absorption is several orders of magnitude greater than seen with the Nd:YAG wavelength (Figure 11-5).

FIGURE 11-5 • Comparative interaction (absorption) of four laser wavelengths (λ) with carbonated hydroxyapatite: Er:YAG (2940 nm); Er,Cr:YSGG (2780 nm); CO₂ (9600 nm); and CO₂ (10,600 nm). Other symbols used: µ_a = absorption coefficient, σ_a = surface penetration of the beam (microns), ζ_rel =thermal relaxation (microseconds.).

(Data from Fried D, Ragadio J, Akrivou M, et al: J Biomed Opt 6(2):231-238, 2001.)

The absorption in water with the erbium wavelengths results from a relatively broad water band of about 3000 nm, together with small absorption at about 2800 nm by the hydroxyl group of the (carbonated) hydroxyapatite mineral of the tissues.⁴¹^–⁴⁴

When incident laser energy directed onto hard dental tissue is absorbed by the prime chromophores (either water or carbonated hydroxyapatite), one of two effects occurs. For both Er:YAG and Er,Cr:YSGG wavelengths, this energy is absorbed primarily by the water and is rapidly converted to heat, which causes superheating and a phase transfer in the subsurface water, resulting in a disruptive expansion in the tissue. Through this mechanism, whole tissue fragments are ejected and a hole is cut in the tooth, with little or no alteration to the mineral itself. A common term for this effect is spallation (Figures 11-6 to 11-8, pp. 186-187).

FIGURE 11-6 • Graphic representation of interaction of mid-infrared laser wavelength with tooth (mineralized) tissue. Incident-pulsed laser photonic energy of mid IR wavelength is preferentially absorbed by water contained within the target dental tissue. Rapid heating beyond the vaporization temperature of water causes volumetric expansion and structural fragmentation of associated mineral. Such action may be accompanied by an audible “pop” and visual evidence of a microcrater formation in the tooth surface.

FIGURE 11-7 • Scanning electron micrographs showing effects of pulsed Er:YAG (2940 nm) laser energy with water spray on human enamel and dentin. A, Laser-enamel interaction. Note effects of spallation and absence of signs of thermal change in mineral structure or cracking. B, Higher-power (×300) SEM of same specimen (A). C, Laser-dentin interaction. Note absence of thermal damage, no smear layer, and open dentinal tubules.

FIGURE 11-8 • Hand-held (sectioned) specimen (human molar) exposed to pulsed Er:YAG (2940 nm) laser energy. Use of laser energy without water spray results in carbonization of target tissue.

Relatively high fluences (energy density–laser photonic energy per unit area) are needed at these wavelengths for spallation to occur. The emission mode of current mid-IR lasers is defined as free-running pulsed. Currently available lasers emit a pulse train of 50 to 250 microsecond pulses on average, which, when delivered in rates of 3 to 50 Hz (pulses per second), represent high peak-power values sufficient to ablate tooth mineral tissue. Although pulse durations are close to the thermal relaxation times of enamel and dentin, further examination of ultrashort pulsewidths (and associated high peak-power values) is needed to create sufficient ablative force without inducing collateral thermal damage.⁴⁵^,⁴⁶

The rate of ablation of dental hard tissue depends on the amount of incident laser energy delivered to the tissue, as well as the effects of wavelength, pulse duration, pulse shape, repetition rate, power density, thermal relaxation time of the tissue, and delivery mode.⁴⁷^,⁴⁸ In addition, heat buildup in the tissue (and unwanted heat conduction to the pulp) must be avoided and accumulation of products of ablation (char), prevented.

Mid-IR ablation of dental hard tissue has led to the concept of two wave fronts of interaction: an ablation front and a thermal front. It is important that the ablation front always precedes the thermal front, to avoid the risk of a damaging heat increase through the accumulation of ablation debris within a deep cavity.⁴⁹ Studies therefore have examined the effects of excessive incident power and the buildup of ablation products, or their removal by means of a coaxial water spray.⁵⁰ The vital dental pulp is acutely sensitive to thermal change. The explosive defragmentation of mineralized dental tissue, resulting from water-assisted erbium wavelengths, allows much of the heat to escape from the cavity, carried in the ablated particles and resulting in pulpal thermal increases of less than 5° C.⁵¹^–⁵³

Laser Use vs. Conventional Instrumentation

To any clinician who chooses to use a laser in restorative dentistry, the high-speed rotary drill is seen as the “gold standard.” Ease of use and speed are often accepted as plausible, despite several studies showing that high-speed drilling leads to surface and pulpal temperature rise, tissue cracking, and unnecessary removal of healthy surrounding tissue during cavity preparation ⁵⁴^–⁵⁶ (Figure 11-9).

FIGURE 11-9 • Scanning electron micrograph of human enamel showing cavity prepared with high-speed rotary instrumentation. Note cracking caused by vibration.

The precise and selective ablation of tooth tissue with the Er,Cr:YSGG and Er:YAG laser wavelengths is well documented. Generally, the only drawback appears to be the lower “speed” of cutting compared to a slow-speed drill,⁵⁷^–⁵⁹ with claims of 80% slower in enamel and comparable speed in dentin. Also, the desire to match laser cutting speeds with those of rotary instruments has led to power delivery greatly in excess of that postulated by Keller and Hibst, relative to the ablation threshold of enamel. Coexistent with such power levels and heat conversion, studies have shown that by reducing the pulse duration of the laser energy (pulsewidth), peak-power values rise, ablation is more efficient, and heat transfer is minimized,^33,⁶⁰^–⁶² to establish a clinically acceptable rate of interaction commensurate with treatment time.

Different handpiece tips have been designed to address the needs of energy delivery, efficiency of cutting, and access. Round cross-sectional tips may vary in diameter from 200 to 1300 µm. Care should be exercised in assessing the power density of the emitted beam; as the spot size (diameter) is reduced, the energy per exposed target area increases dramatically, assuming all other variables remain the same, and places a high risk of tissue overheating. Most delivery tips are made of quartz and provide efficient transmission of laser energy to the target. With the spallation effect of hard tissue ablation, impact damage from ejected ablation products may cause irregularity in both the beam configuration and the cutting dynamics. Thus, the tips should be regularly inspected for damage, and the ends can be repolished using fine discs and diamond paste. Tips made of sapphire offer marginal improvements in energy delivery, as do hollow-bore tips.⁶³^,⁶⁴ However, sapphire tips are more expensive and less able to be reconditioned, and the rigidity places these tips at greater risk of fracture during use.

Clinical TIP

Clinical Tip: Other factors, such as fluoridation of the tissue, incident angle of the delivery tip relative to the tooth, and presence of ablation products, also affect the speed of ablation.

Several anecdotal reports have shown the effectiveness of directing the delivery tip parallel to the axis of the enamel prisms, to access the interprismatic, higher water-content structure.