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lagunabbSpectatorDr Fried:
I asked Bob Gregg the question below to assess the difference between theory and application for Nd:YAGs. Can you please comment specifically on the way that Nd:YAGs are being used and how potential problems are avoided in practice. Some manufacturers such as Lares suggest that Nd:YAGs can be safely used to selectively remove pigmented caries.
Bob,
I talked to someone last week that held your system in high regard. In reviewing (again for the 4th time) the Fried et al’s paper “Mechanism of Water Augmentation” Laser in Surgery and Medicine 31 pp 189-190 (2002), they wrote:quote:
Extensive damage to the mechanical intergrity of the bovine block and thermal damage to the underlying dentin was observed after irradiation with the free running Ho:YAG laser at 2.1 um (see Figure 5) even with a layer of water applied to the surface. Laser light at 2.1 um is only weakly absorbed by water. Therefore, it is able to penetrate through that water layer without the transfer of much energy and through the outer 1-2 mm layer of enamel where it is absorbed by the dentin resulting in thermal damage. Monte Carlo simulations of light deposition in teeth followed by calculation of subsequent temperature rise have predicted localized thermal damage at the dentin-enamel junction (DEJ) during Nd:YAG (1.064 um) irradiation without ablation of the outer enamel. Localized subsurface damage was visible at the DEJ after Ho:YAG irradiation. At higher incident fluence, thermal stresses generated at the DEJ produced large fractures transverse to the path of laser irradiation resulting in the destruction of the bovine enamel block. This result exemplifies the danger of irradiating teeth with free-running Nd:YAG and Ho:YAG laser pulses — even with entensive water spray.
Their Figure 5 shows SEM’s of subsurface damage to dentin at 70 J/cm2 and extensive thermally induced DEJ fractures at 150 J/cm2.I would appreciate any thoughts or comments about this experiment and their interpretations. Thanks.
dfriedSpectatorI agree that pigmented caries preferentially absorb light at 1064-nm, see Harris et al. Selective Ablation of Surface Enamel Caries with a Pulsed Nd:YAG Laser, Lasers in Surg. Med. 30(5) 342-350(2001) and our acoustic measurements confirmed that. However, that does not mean it is safe or very useful. Simulations show that Nd:YAG laser energy goes right through the enamel and is scattered/absorbed near the Dentin-enamel junction (DEJ) see Seka et al.. Light deposition and thermal response in dental hard tissue. J Dent Res, 74, 1086-1092 (1995). So one might possibly have the same awful effect we observed with the Ho:YAG . The reason we don’t readily see the sub-enamel charring that we observed with the Ho:YAG with the Nd:YAG, is that the water absorption is so low that the energy is deposited over a much larger area beneath the DEJ.
Another problem is that we have no idea what is absorbing the light at 1064-nm. I can see how you can use a low energy setting on your Nd:YAG and just remove the pigmented tissue, i.e. you hear a pop when there is absorption and after the pigmentation is gone, there is no audible pop. I guess that if the energy setting is low enough so that the total energy required is low, the heat accumulation may be at a safe level. However, it is not clear what you are removing, i.e., whether or not you are removing a superficial stain or actual decay. I think you are better off using an Er:YAG or an Er:YSGG where the absorber is well established and the absorption is very strong.
If I was a dentist, I wouldn’t use an Nd:YAG on hard tissue. However, I am very curious about the actual source of pigmentation, i.e., whether it is tanin stains or necrosed bacteria or some other source.
Robert GreggParticipantHi Ray,
Am I Dr. Fried or Bob?? Or are you really me??:confused::biggrin:
Good questions.
One answer to the question about pulsed Nd:YAGs and caries and dentin removal is the pulse duration needs to be short (e.g 100 usec) to minimize lateral thermal burn. (Didn’t I show you 7 different pulse duration on an extracted root when you were here?)
The Lares laser has a pulse duration of 150 usec at the shortest–which is OK for caries removal, just not as good as 100 usec–less lateral thermal damage.
The Fr, Nd:YAG will NOT removal HEALTHY enamel. But it will remove diseased enamel, decalcified enamel and healthy dentin. Which is actually a neat thing when you think about it as a clincian. We can undermine the enamel that’s healthy, and not damage it, we can prep VERY thin area of DEJ (like incisals of lower anteriors), while removing decayed or even healthy dentin. Not many tools in my dental tool box can do that as selectively as a pulse 60 microns deep at a time!!
We NEVER use Fr, Nd:YAGs to prep under water. We use water only after the prep is completed to remove any discoloration from the plasma deposits on the tooth surface (carbon). Don’t see any problems in regards to fracturing or crazing with the FR Nd:YAG on vital tooth structure. The pulse preps are very clean.
Since I have a 2.1 micron Fr, Ho:YAG, (150 usec, 5-10 Hz, up to 500 mj/pulse) I can tell you I have been disappointed with its hard tissue capabilities. It just doesn’t absorb on dentin or pigmented caries much at all. But man does it heat up the vital tooth. Patients do NOT like it even when theya re numb!!
QUOTEAt higher incident fluence, thermal stresses generated at the DEJ produced large fractures transverse to the path of laser irradiation resulting in the destruction of the bovine enamel block. This result exemplifies the danger of irradiating teeth with free-running Nd:YAG and Ho:YAG laser pulses — even with entensive water spray.I question his conclusions. But I would need to know what his parameters of operation.
We typically use 2.5 Watts 10 Hz 250 mj/pulse at 100 usec, no H2O. What were Fried’s parameters?
Bob
Robert GreggParticipantHello Dr. Fried,
Welcome to the forum.
I enjoyed your presentation on P-OCT in Florida.
My Fr, Nd:YAG colleagues and I have been using them for caries removal, dentin prepping, dentin etching, etc for over 12 years without any adverse effects.
Our personal observations are we obtain better bonds to dentin when we etch with Fr, Nd:YAGs.
Fr, Ho;YAGs aren’t good on hard tissue even at 500 mj/pulse.
The reason many of us like the Fr, Nd:YAGs over Erbium in certain cases for hard tissue, is its ability to selectively remove only the organic plug, stain, caries, or dentin. Many times this means we do not have to prep the enamel, thereby conserving maximum tooth structure.
Erbiums are great for enamel preparations, and do so quickly without any thermal injury or fracturing to surrounding tooth, to be sure. It’s just I don’t always want to remove or etch or prep enamel, especially if I only want to determine if the decay remains in enamel.
Or maybe I just want to remove smoker’s stain or calculus from the pits and fissures. Easy to do with Fr, Nd:YAG.
Very useful diagnostic aid until Lantis makes P-OCT available.
Bob
Glenn van AsSpectatorBob: please direct Dr. Fried to the Hydrokinetics article on the Erbium section. I think it would be wonderful to get him and Ray to discuss the merits of various erbium yag and ER YSGG wavelengths in respect to their modality of action.
I as an erbium yag user was quite intrigued by his article.
GRIN
Glenn
(stirring the pot which they wont let me do on DentalTown anymore – I cant even mention that forbidden term (HK).)
signed Glenn the s_it disturber.
lagunabbSpectatorBob,
“We typically use 2.5 Watts 10 Hz 250 mj/pulse at 100 usec, no H2O. What were Fried’s parameters?”
Your settings convert to 156 J/cm2 per pulse compared to Dr. Fried’s experiments at 70 J/cm2 and 150 J/cm2 (see his Figure 5) assuming you are using 400 um fiber.
Your comments suggest that you don’t believe that the results from the Ho:YAG is valid (at least based upon clinical observations) for Nd:YAG. If the damage is subsurface as shown by Dr. Fried, is there a way to tell or does one just wait and see if nothing nasty happened months later? Or maybe the damage (if does happen) heals given time?
(Edited by lagunabb at 8:15 am on Mar. 18, 2003)
Robert GreggParticipantHi Ray,
I need to get you over here again to watch some clinical hard dentin and enamel lasing with a pulsed Nd:YAG, and then on some extracted teeth. We’ll look under light microscopy.
Subsurface? I don’t believe that’s how pulsed Nd:YAGs or pulsed anythings work. They are a surface phenomenon, with some acoustical and mechanical effects.
It’s never been a question of waiting for there to be an adverse clinical situation afterwards. It just doesn’t happen…ever.
Our fiber-optics are 320, not 400. So the power density is almost 50% greater with a 300.
Thanks,
Bob
lagunabbSpectatorBob,
Sorry I have such a thick skull and forgot so much of what you showed me. I am a poor student and will have to drop by once again to do a makeup session. The 320 um fiber translates to 250 J/cm2 per pulse at your settings for pigmented caries removal.I don’t think you are trying to tell me that the enamel-laser interaction is wavelength independent? I am stuck on Dr Fried’s point that 1064 um is not well absorbed in hydroxyapatite and water so laser energy is transmitted until it is absorbed by other tissue underneath. I am hearing that from your clinical observations that it does not seem to matter even at 250 J/cm2. Sounds like there is a need for additional experiments at 1064 um of the type that Dr Fried conducted on Ho:YAG.
Robert GreggParticipantRay,
I may be the “thick head” here! I was trying to understand the experiment……
QUOTEI don’t think you are trying to tell me that the enamel-laser interaction is wavelength independent?No, not in this instance. But I think I see where the confusion may be.
The Fr Nd:YAG, even at the power densities of 250 J/cm2–and thanks for that calculation–will PASS through dentin if not in focal contact with it. So that means if I am on enamel and ablating away a organic plug, the interaction will stop once the pigment is gone, or the fiber comes within the focal spot of the underlying dentin. But simply irradiating dentin through enamel will not result in damage to the underlying dentin–not in vivo in human teeth anyway.
Too bad the dental researchers don’t use J/mm2 since it is more meanigful than J/cm2 and more to our scale.
Bob
lagunabbSpectatorThanks Bob, I think I get it. The beam is scattered and so is the energy when it passes through the enamel. In the experiments, Dr. Fried used polished and flat bovine samples and that could mean less scattering than a vital tooth where the caries is located in a concave shaped depression. Sort of like sending a beam through a concave lense and scattering the beam. Your comments on patients’ reaction to Ho:YAG does indicate some difference in interaction between Ho and Nd:YAGs, so it’s still probably worthwhile to run the equivalent experiments (Fried et al) for Nd:YAGs.
Thanks again for the helpful comments.
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