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Selective removal of caries lesions with high precision is best accomplished using lasers operating at high pulse repetition rates utilizing small spot sizes. Conventional flash-lamp pumped Er:YAG lasers are poorly suited for this purpose, but new diode-pumped solid-state (DPSS) Er:YAG lasers have become available operating at high pulse repetition rates. Microradiography was used to determine the mineral content of the demineralized dentin of 200-μm thick sections with natural caries lesions prior to laser ablation. The purpose of this study was to explore the use of a DPSS Er:YAG laser for the selective removal of demineralized dentin and natural occlusal lesions on extracted teeth.
DPSS Er:YAG lasers with high pulse repetition rates are more suitable for the selective removal of dental caries than existing Er:YAG lasers . The flash-lamp pumped erbium solid-state lasers presently being used for dental hard tissue ablation are not suitable for this approach since they utilize high energy pulses and relatively low pulse repetition rates. Diode pumped Er:YAG lasers are now available operating with pulse repetition rates as high as 1–2 kHz and initial studies have been carried out demonstrating their utility for the ablation of dental hard tissues and bone [2–4]. Last year we investigated the use of this laser for the selective removal of caries lesions in the enamel .
For selective removal, low energy pulses and small spot sizes must be used to minimize the amount of tissue removed per laser pulse, therefore the laser has to be operated at high pulse repetition rates for practical removal rates. In addition the laser needs to be integrated with a scanner. Computer control is now feasible due to the recent advances in compact high-speed laser scanning technology such as MEMS (Micro-Electro-Mechanical Systems) mirrors and miniature galvanometer “galvo” based scanners. One approach is to remove the lesion layer by layer, i.e. image the lesion then scan the laser and remove an outer layer of the lesion, then re-image and scan again, repeating that process until the lesion is completely removed.
Therefore, it is advantageous to couple the laser with an imaging system for further selectivity. Image-guided laser ablation of dental caries requires the rapid acquisition of high-contrast images of areas of enamel and dentin demineralization that can be input into the laser-scanning system to selectively remove areas of demineralization. Previous approaches for guiding laser ablation have included fluorescence [5–7] and near-IR transillumination . Near-IR reflectance imaging is ideally suited for acquiring high-contrast images for image guided ablation due to the weak light scattering from sound enamel and the lack of interference from stains [9–11]. Stains which greatly interfere with visible light based methods do not interfere at near-IR wavelengths beyond 1300-nm, this is why visible reflectance measurements and fluorescence are of limited effectiveness in tooth occlusal surfaces [12, 13] and are not well-suited for image guided laser ablation. In previous studies we showed that near-IR reflectance in the wavelength regions coincident with increased water absorption namely 1450-nm and wavelengths higher than 1500-nm were ideally suited for this approach [14–16]. An important concern, essential for image-guided ablation, was that thermal modification of sound and demineralized tooth surfaces by the laser, could increase reflectivity from irradiated areas adversely influencing the lesion contrast and performance. We measured the lesion contrast at 1450-nm and from 1500–1700-nm before and after CO2 laser ablation had been initiated and those changes were acceptable . The enamel surfaces appear much rougher after Er:YAG ablation than they do after CO2 laser ablation  and that may prevent the use of NIR reflectance imaging with the Er:YAG laser for image-guided ablation on enamel surfaces.
The purpose of this study was to explore the use of a DPSS Er:YAG laser for the selective removal of dentin and natural occlusal lesions on extracted teeth.
Human tooth samples were divided into two groups (Fig. 1). Group 1 consisted of human tooth samples with dentinal lesions that were used for mineral content analysis. Group 2 consisted of whole teeth with occlusal lesions. Group 1 samples were prepared by cutting whole teeth into 200-μm slices using a IsoMet 5000 Precision Saw from Buehler (Lake Bluff, Il). Transverse Microradiography (TMR) was used on the slices to determine mineral content. A total of 9 samples were used for mineral content analysis. Group 2 samples were prepared by removing the roots of extracted teeth and mounting the crowns onto black orthodontic acrylic blocks. A total of 16 samples were used for laser ablation. The teeth were collected from dental offices in San Francisco Bay Area and sterilized with Gamma radiation.
Samples were irradiated using a DPSS Er:YAG laser, Model DPM-30 from Pantec Engineering (Liechtenstein) operating with a pulse duration of 50-μs. The laser energy output was monitored using a power meter EPM 1000, Coherent-Molectron (Santa Clara, CA), and the Joulemeter ED-200 from Gentec (Quebec, Canada). A high-speed XY-scanning system, Model ESP 301 controller with ILS100PP and VP-25AA stages from Newport (Irvine, CA) was used to scan the samples across the laser beam. The laser was focused to a spot size of ~150-μm using an aspheric ZnSe lens of 25 mm focal length. A pressure air-actuated fluid spray delivery system consisting of a 780S spray valve, a Valvemate 7040 controller, and a fluid reservoir from EFD, Inc. (East Providence, RI) was used to provide a uniform spray of fine water mist onto the tooth surfaces at 2 mL/min.
For the Group 1 samples (dentin) the laser was operated with a pulse repetition rate of 100-Hz and the samples were scanned at a rate of 5 mm/sec. Fiducial squares 2 mm by 2 mm were created on each sample. Incisions were produced at a fluence of 10 J/cm2 by one pass in one direction.
For the Group 2 samples (occlusal lesions), a rectangular box was cut across the lesion on the enamel samples with the box size encompassing the entire lesion. Incisions were produced by scanning the laser at a rate of 20 mm/sec with a pulse repetition rate of 200 Hz. The laser was scanned in one direction and each scan was separated by 25-μm for each iteration (laser spot size 150-μm). A fluence of 50 J/cm2 was used. The iterations were repeated until the lesion was completely removed inside the box.
Tooth surfaces were examined before and after laser irradiation using an optical microscopy/3D surface profilometry system, the VHX-1000 from Keyence (Elmwood, NJ). The VH-Z100R lens with a magnification of 100x-1000x. Depth composition digital microscopy images (DCDM) and 3D images were acquired by scanning the image plane of the microscope and reconstructing a depth composition image with all points at optimum focus displayed in a 2D image. The Keyence 3D shaped measurement software, VHX-H3M, was used to correct the tilt of the sample and measure the variation in depth over the enamel in the ablated areas.
A custom-built digital TMR system was used to measure mineral loss in the different partitions of the Group 1 samples. A high-speed motion control system with Newport (Irvine, CA) UTM150 and 850G stages and an ESP300 controller coupled to a video microscopy and laser targeting system was used for precise positioning of the tooth samples in the field of view of the imaging system. The volume percent mineral for each thin section was determined by comparison with a calibration curve of X-ray intensity vs. sample thickness created using sound enamel sections of 86.3±1.9 vol.% mineral varying from 50 to 300 μm in thickness using image analysis software. The calibration curve was validated via comparison with cross-sectional microhardness measurements. The volume percent mineral was determined using microradiography for section thickness ranging from 50 to 300-μm highly correlated with the volume percent mineral determined using microhardness, r2 = 0.99 . Microradiography images were loaded onto Igor Pro. The images contain the percentage of mineral content at each pixel. The VHX-1000 digital microscope was used to measure the depths of the incisions. By matching the positions in the TMR images and the digital microscope the removal depth was measured for varying mineral content.
A cross-polarization OCT system purchased from Santec (Komaki, Aichi, Japan) was used to acquire 3D tomographic images of the Group 2 occlusal lesions before and after removal. This system acquires only the cross-polarization image (CP-OCT), not both the cross and co-polarization images (PS-OCT). The device, Model IVS-300-CP, utilizes a swept laser source; Santec Model HSL-200-30 operating with a 33 kHz a-scan sweep rate. The interferometer is integrated into the handpiece which also contains the microelectromechancial (MEMS) scanning mirror and the imaging optics. This CP-OCT system can acquire complete tomographic images of a volume 6×6×7 mm in size in ~3 seconds. This system operates at a wavelength of 1,321 nm with a bandwidth of 111 nm with a measured axial resolution in air of 11.4 mm (3 dB). The lateral resolution is 80 μm (1/e2) with a transverse imaging window of 6×6 mm and a measured imaging depth of 7 mm in air. The polarization extinction ratio was measured to be 32 dB. Image registration and volumetric measurements were carried out using Avizo software from FEI (Hillsboro, OR).
After sample imaging was completed, approximately 200 μm thick serial sections were cut using an Isomet 5000 saw (Buehler, IL), for polarized light microscopy (PLM). PLM was carried out using a Meiji Techno RZT microscope (Meiji Techno Co., LTD, Saitama, Japan) with an integrated digital camera, Canon EOS Digital Rebel XT (Canon Inc., Tokyo, Japan). The sample sections were imbibed in water and examined in the brightfield mode with crossed polarizers and a red I plate with 500-nm retardation.
Initial Er:YAG dentin ablation rates were assessed using Group 1 samples. PLM and TMR were used to confirm lesion presence. Incisions were produced on the Group 1 sections by repeated Er:YAG laser scans as shown in Fig. 2. The ablation depths were analyzed using DCDM. The results indicate that the ablation rate correlates with the mineral content and that the incisions are deeper in areas with lower mineral content. DCDM images show that the dentin and demineralized dentin were ablated without any apparent thermal or mechanical damage.
Lesions were removed from tooth occlusal surfaces by scanning the Er:YAG laser over selected regions of interest. DCDM images were taken after every 1–2 iterations (laser scans). Figure 3 shows the DCDM images during treatment of one tooth at three different time points. Ablation was stopped after the lesion was removed. Figure 4 shows CP-OCT images of the same tooth before and after ablation. The PLM post-ablation image (not shown) of the sectioned sample shows that most of the lesion was removed.
Ablation of dental hard tissues was achieved using the Er:YAG laser operating at high pulse repetition rates with minimal peripheral thermal damage. In addition, since water is the primary absorber of Er:YAG radiation and demineralized areas are more porous and have a higher water content, the ablation rate is significantly higher for demineralized enamel  and dentin vs sound tissues. In the future we plan on publishing an expanded manuscript of these results including statistical analyses.
The authors would like to thank William Fried and Andrew Jang for their contributions. This work was supported by NIDCR Grant R01-DE19631.