Thomas P. Ertl

Flexible waveguides for Er-YAG laser radiation delivery

Flexible plastic waveguides (FPW) were devised for the delivery of Er-YAG laser radiation. The FPW characteristics were studied under various conditions. In vitro studies were carried out to explore the drilling procedure on extracted teeth and the FPW-tissue mutual effects. The results which were obtained proved that the FPW as a delivery device might be a substitute hand applicator for the pneumatic turbine for drilling in teeth.<<ETX>>

Hard-tissue ablation with pulsed CO 2 lasers

CO2 Lasers at (lambda) equals 9.6 and 10.6 micrometers and pulse lengths in the range of 100 ns to 150 micrometers operating in multimode were used as a drilling instrument with energy densities up to 200 J/cm2. The laser light was focused perpendicularly onto the surface of 1 mm slices of dentin and enamel. Different methods of cooling were applied. Temperatures were measured during ablation with thermocouples. The quality of ablation was checked with a stereo microscope and the diameters of the ablated areas were measured. Ablation rates (mm3/s) and ablation efficiencies (mm3/J) were calculated. Best results were obtained with pulse lengths of 100 to 130 micrometers . Ablation without carbonization of the organic matrix of dentin was only possible with water or water/air spray cooling. Ablation rates of up to 0.2 mm3/s and an ablation efficiency of up to 50 X 10-3 mm3/J were reached.

Thermal and acoustic problems on root canal treatment with different lasers

Side effects of root canal preparation with lasers such as the generation of acoustic shockwaves and heat transfer were investigated. Shockwaves may cause disintegration of root hard substance and too high temperatures may damage the periodontium. Three types of pulsed lasers with different ablation characteristics were chosen for the study. (1) Excimer laser 308 nm/120 ns. (2) Er:YSGG laser 2.78 micrometers /500 microsecond(s) . (3) Nd:YAG laser 1.06 micrometers /180 microsecond(s) . Delivery systems for all lasers were quartz fibers with 400 micrometers core diameter. Canals were pretreated up to size 40 to obtain a comparable root canal shape. The teeth were positioned with the root in chicken egg protein as a heatsink during the laser operation. Shockwaves were measured with a needle hydrophone and visualization of the ablation process was made with high speed flashlamp photography. Temperatures were measured with a fiberoptic device. Results show that lasers with medium pulse length, operating at wavelength highly absorbed by hard and soft tissue, caused minimum side effects. The ablation process with lasers emitting at a low absorbed wavelength rapidly shifts from an initial heat transfer at the beginning of preparation to a noncontrollable ablation and temperature rise when carbonization occurs in the canal. Very short pulsed lasers such as excimer lasers cause stronger shock waves than lasers with a pulse length in the microsecond(s) region. One can conclude that Er:YSGG lasers offer the best ratio between efficiency and side effects.

Biocompatibility of Er:YSGG laser radiated root surfaces

Pulsed Er:YAG and Er:YSGG lasers are well known to be effective instruments for the ablation of dental hard tissues. Developments in the last years made it possible to transmit the laser radiation at these wavelengths with flexible fibers. Therefore the application in the periodontal pocket may be possible. The aim of this study was to evaluate the in-vitro conditions to generate a bioacceptable root surface. Twenty extracted human teeth, stored in an antibiotic solution, were conventionally scaled, root planed and axially separated into two halves. Two main groups were determined. With the first group laser radiation was carried out without and in the second group with spray cooling. The laser beam was scanned about root surface areas. Laser parameters were varied in a selected range. The biocompatibility was measured with the attachment of human gingival fibroblasts and directly compared to conventionally treated areas of the root surfaces. The fibroblasts were qualified and counted in SEM investigations. On conventionally treated areas gingival fibroblasts show the typical uniform cover. In dependance on the root roughness after laser treatment the fibroblasts loose the typical parallel alignment to the root surface. With spray cooling a better in-vitro attachment could be obtained. Without spray cooling the higher increase in temperature conducted to less bioacceptance by the human gingival fibroblasts to the root surface. These results show the possibility of producing bioacceptable root surfaces with pulsed laser radiation in the range of very high water absorption near 3 micrometer.

Application of lasers in endodontics

Root canal treatment is still a problem in dentistry. Very often the conventional treatment fails and several treatment sessions are necessary to save the tooth from root resection or extraction. Application of lasers may help in this situation. Bacteria reduction has been demonstrated both in vitro and clinically and is either based on laser induced thermal effects or by using an ultraviolet light source. Root canal cleansing is possible by Er:YAG/YSGG-Lasers, using the hydrodynamic motion of a fluid filled in the canals. However root canal shaping using lasers is still a problem. Via falsas and fiber breakage are points of research.

Flexible waveguides for the delivery of high-power Er-YAG laser radiation

In a previous research project a flexible plastic hollow waveguide has been developed in our laboratory. The waveguide was prepared by depositing silver (Ag) layer and silver iodine (AgI) overlayer on the inner surface of a plastic tube. The said waveguide proved to be a very suitable means for delivering high power CO2 laser energy ((lambda) equals 10.6 micrometers ) in any desired tortuous path, having small attenuation. Through the same waveguide it is possible to transmit non-coherent infrared energy from a thermal energy source to a detector. In this paper we present a new type of waveguide which is suitable for the transmission of Er- YAG laser radiation ((lambda) equals 2.94 micrometers ). Such energy can be employed for drilling and operating in hard tissues (bone, tooth). The essential factor which made this device possible for use as Er-YAG laser energy delivery system is based on the known data from the CO2 waveguide research was the control over the thickness and the index of refraction of the dielectric layer (AgI). Another important factor was to avoid the roughness of the plastic substrate and of the Ag/AgI layers. Reducing the roughness enabled us to reduce the scattering of the transmitted radiation to a low value, which is essential for the good functioning of the waveguide. The performed experiments have shown that an energy of up to 900 mJ could be coupled into the flexible waveguide and delivered to a target in straight or bent trajectory. The transmission was 55% in straight and 40% in bent trajectory (waveguide length 1 m, internal diameter 1.9 mm minimum radius of bending 20 cm).

Root canal preparation with Er:YSGG laser

The high level of efficiency of hard tissue ablation with Er:YAG and Er:YSGG lasers is well known. Of these lasers it is possible only to transmit Er:YSGG laser radiation with OH reduced quartz fibers. Most of the fibers we use in this study were prepared as hemispherical fiber tips. Fifty single rooted teeth were divided into ten groups (n equals 5). After conventional opening of the pulp chamber, root canal preparation was performed in five groups under water only using the laser. In the other five groups preparation with K-files to size 35 was performed before treatment with laser radiation. All teeth were axially separated with direct access to the root canal and examined in SEM investigations. The groups were compared by measuring the areas with patent dentin tubules. Representative areas were examined by TEM. The temperature at the root surface was measured during laser irradiation with thermocouples positioned at several points. The in-vitro study of the effect of the high delivered energy (50 - 100 mJ per pulse) in the root canal showed a good ablation effect. Most of the dentin tubules were opened. The increase in temperature at the root surface was tolerable.

Er:YAG laser flexible waveguides

Flexible plastic waveguides (WG) were devised for the delivery of Er-YAG laser energy in curved trajectories. The WG were optically characterized. The WG transmitted energy could drill a hole in teeth enamel hence it can be used as a substitute for a pneumatic drill.

Effects of pulsed Er:YSGG and Ho:YAG laser on the organ of Corti of the guinea pig cochlea: a scanning electron microscopic study

Laser stapedotomy has now become an established method in the surgical treatment of otosclerosis. Recent experimental studies have demonstrated that, apart from the continuous wave lasers, several pulsed laser systems are also suitable for stapes management. Experiments were performed in guinea pigs to clarify which, if any, of the pulsed lasers used can damage the inner ear on application of the laser parameters required for the stapedotomy. The basal convolution of the guinea-pig cochlea was chosen as the application site. Acoustic evoked potentials (compound action potentials) yielded information on inner-ear function. With the Er:YSGG laser (energy: 85 J/pulse, energy density: 36 J/cm2, total energy: 0.425 J), five applications to the cochlea are necessary for a foot plate perforation of 500 - 600 micrometers . With the Ho:YAG laser, an adequately large perforation can be achieved with at least 10 applications of an energy of 210 mJ per pulse (energy density: 90 J/cm2, total energy: 2.1 J). The aim of this study was: (1) to clarify whether the Er:YSGG and Ho:YAG laser could cause morphological changes in the organ of Corti of the guinea pig on application of the laser parameters required for stapedotomy, and (2) to verify our experimental electrophysiological results and correlate them with the morphological changes detected in the organ of Corti in the guinea pig cochlea by scanning-electron-microscopic examination. It shows that the effective laser parameters (5 X 85 mJ) of the Er:YSGG-laser cause no changes of the guinea pig cochlea. Even with the application of 25 pulses with the same energy the guinea pig cochlea shows normal appearance. The effective laser parameters of the Ho:YAG laser (10 X 210 mJ) show changes in the outer hair cells in the form of stereocilie fusion and giant hair cell formation while the inner hair cells and supporting cells are showing normal appearance. Our results clearly demonstrate a high application safety for the Er:YSGG laser, while the Ho:YAG-laser is not well tolerated in the animal experiment and has only a low application safety. Its application in stapedotomy would be unreliable and dangerous for the inner ear. Our results also clearly show that the electrophysiological data correlate well with the scanning- electron-microscopic data.

Thermal measurements of flexible hollow waveguides

The effect of transmitted CO2 laser energy ((lambda) equals 10.61 micrometers ) upon heating of a plastic hollow waveguide (PHWG) was investigated. The temperature rise of the external tubes wall was recorded by thermocouples or thermal imaging system. The most affected regions of the waveguide were the curved segments and the distal end. Gas cooling and external metal layer reduce this undesired effect.