David, a forward from a friend-
In Laser-Tissue Interactions: Fundamentals and Applications by Dr. Markolf Niemz Ph.D, he states on page 198, ”For the removal of metallic fillings, infrared lasers cannot be used, since the reflectivity of these materials is too high in that spectral range.” Niemz continues, “Amalgam should never be ablated with lasers at all …..as during irradiation, the amalgam melts and a significant amount of mercury is released that is extremely toxic for both patient and dentist.” Niemz then presents a photomicrograph of a melted amalgam crater, in an alloy irradiated with an Er:YAG laser at a 90 micosecond pulse duration of 100 millijules of energy.
The “sparking” effect that is seen, is most definately plasma sparking, (remember, we are using infrared energy and cannot see the beam, so it is not “reflected” laser energy) as the laser induced free mercury vapor from the melted amalgam absorbs more erbium photons in the next series of laser pulses. This immediate interaction rapidly ionizes the free mercury, producing an intense electric field seen as a spark.
As to why the tip fractures, let’s do a thought experiment:
If we are lasing a dark (non polished or corroded) amalgam, and hence ionizing free mercury, because the amalgam restoration is partially absorbing the energy instead of reflecting it (like is seen with a highly polished matrix band) this vaporized mercury is a very fast moving “plume” that then paints the distal end of the sapphire delivering the Erbium laser energy with a thin coat of mercury. As the laser fires again, the energy is now absorbed (remember the very high Watts/pulse with these FRP lasers we are dealing with) by the black “pigment or chromophore” now coated on the end of the sapphire tip. This intense instantaneous absorption then causes the tip to “blow out” as the laser pulse is “absorbed” by the tip, and not “transmitted” by it.
Why some tips blow out, and others do not, has to do with the “shape” and “hardness” of the synthetic sapphires, along with the amout of energy the laser is producing at a given moment in time. It has nothing to do with “recoil shock”
