AU3130199A - Laser ablation of tooth material - Google Patents

Description

WO 99/51161 1 PCT/AU99/00245 LASER ABLATION OF TOOTH MATERIAL Field of the Invention The present invention relates to a method and apparatus for the laser ablation of dental tissue, for example in the removal of hard and soft dental tissues. 5 Background Art Lasers emitting a wavelength of around 3 microns are considered to be desirable in medical procedures, especially refractive corneal surgery, owing to their ability to interact with materials containing water. The small absorption depth of the 3 micron wavelength electromagnetic radiation in hydrated material results in the 10 concentration of the energy deposited by the laser; and hence the ablation of the irradiated tissue which itself removes the deposited energy. The result is a larger penetration into the tissue with little thermal damage to surrounding structures. Reflecting this trend, the Erbium:YAG laser, operating at close to 3 microns has begun to find application in dentistry involving the ablation of hard dental tissue 15 (teeth). However, even at 3 micron wavelength, the Er:YAG ablation process for teeth is very thermal, forcing the requirement of sophisticated cooling mechanisms so that the temperature of the tooth does not rise so much that the pulp tissue inside the tooth is damaged. It is therefore an object of the invention to at least in part resolve this difficulty for 20 utilising in dental procedures lasers emitting a wavelength around 3 micron. Optical parametric amplification, or oscillation, is known in the general prior art of laser devices to produce short pulse, high peak power laser emissions of varying wavelengths. It is a process whereby a beam of sufficient intensity at one wavelength, is used to generate optical energy at a different wavelength, through 25 an interaction with a frequency converting compound such as a non-linear optical (NLO) crystal. The first wavelength is supplied by an external, or pumping, source and is directed through the birefringent NLO material. The interaction of the pump WO 99/51161 2 PCT/AU99/00245 output with the NLO crystal produces optical gain at two longer wavelengths, commonly known as the signal and idler frequencies. Reflective end mirrors are used within the cavity to reflect the beams back and forth, causing optical parametric oscillation or resonant energy amplification. Useful energy build up 5 then occurs in the laser cavity if the beams are in phase-matched conditions. A partially reflective output mirror allows the beam of the desired frequency to be extracted from the cavity. U.S. Patents 5,579,152 and 5,606,453 describe recent embodiments of optical parametric oscillation technology. Optical parametric oscillation (OPO) allows tuning of the signal and idler 10 frequencies, enabling the production of a broad range of wavelengths in the infrared, UV or visible electromagnetic spectrum. The orientation of the NLO crystal with respect to the polarization of the pump beam helps to determine the signal and idler wavelengths. Tuning of the signal and idler beams can be achieved by changing the orientation of the NLO material, or by varying the 15 temperature of the crystal. The wavelengths of signal and idler beams also depend on the initial pump wavelength and the non linear optical material used. Tunable pump sources might include dye lasers or solid state transition metal or rare earth mediums. Suitable non-linear optical crystals may include those that can phase-match the OPO process and transmit light of particular wavelengths, 20 including three microns; such crystals include potassium titanyl phosphate, potassium titanyl arsenate or potassium niobate. Optical parametric oscillation, such as that described above, has been used to produce laser energy at variable wavelengths. Currently, this technology is utilised in a number of commercial and research ventures, including optical 25 communications, chemical reactivity, and laser fusion. However, OPO has not been commonly applied to procedures involving the ablation of materials. Further, lasers employing optical parametric oscillation have rarely been applied to medical procedures. U.S. Patent 5,144,630 describes a medical laser apparatus that may be used for ablative medical processes, particularly for refractive surgery of the 30 eye. This patent describes a laser system that uses a number of frequency converting techniques to provide a range of different laser wavelengths, from the WO 99/51161 3 PCT/AU99/00245 ultraviolet to the mid-infrared. It teaches the use of harmonic generation to produce UV energy in the range of 200 nm, and an OPO process to generate infrared energy tunable from 1.5 to 4.5 microns. KTP or KNbO3 crystals are used for optical parametric oscillation in three embodiments of optical cavity 5 arrangements. The first arrangement utilises the usual OPO configuration, as outlined above, and the second comprises two NLO crystals orientated in the extraordinary direction. The third suggests that the output beam is further amplified by passing through a NLO crystal external to the OPO cavity. Wavelength selection can then be controlled by a computer system, with desired 10 wavelengths being deflected by beam splitters. Optical parametric oscillation involving the production of infrared radiation through the use of a KTP crystal pumped by a solid state laser has been further examined in Kato (IEEE Journal of Quantum Electronics 27 (5): 1137-39). This paper describes the generation of stable power, short pulses around 3.2 jm. Rapid 15 transmission fall-off occurred around 3.3 gm, indicating that the described design of optical parametric oscillator is best used to produce tunable infrared wavelengths near the peak of water absorption in water. International patent publication WO 98/41177 discloses an arrangement for performing corneal refractive surgery, especially photorefractive keratectomy 20 (PRK), in which an OPO converts the output of an Nd-doped laser source to produce a treatment beam of wavelength in the mid-infrared range 2.75 to just over 3 micron, preferably near the 2.94 micron water absorption peak. Summary of the Invention The invention essentially entails an appreciation that an OPO-generated laser 25 beam is especially suitable to resolve the aforementioned difficulties encountered with the application of mid-infrared laser radiation in dental procedures. An OPO operating in the region of 3 microns can provide pulses with a much shorter pulse width (<20 ns) than previously used in dental procedures, which results in more efficient ablation and lower ablation thresholds. This means the WO 99/51161 4 PCT/AU99/00245 teeth can be ablated with lower temperature rises and with less risk of damage to the pulp of the tooth. An OPO also has the advantage that its wavelength can be tuned to other absorption peaks of tooth material, in the range 2.6 to 3.2 microns, allowing further improvements in the efficiency of ablation. 5 The invention therefore provides a method of ablating tooth material, which includes providing an initial laser beam of wavelength unsuitable for the ablation, and deriving, from this initial laser beam, a pulsed target laser beam of wavelength in the range 2.6 to 3.2 microns. The target beam is directed onto the tooth material, and the tooth material is ablated with the target beam. 10 Preferably, the target beam comprises pulses of a width no greater than 20 ns. Advantageously, the wavelength of the target beam is tuned to one or more absorption peaks of tooth material within said range. Preferably, the pulsed target laser beam is derived from the initial laser beam by a process which includes optical parametric oscillation. 15 In an embodiment, the aforesaid deriving of the pulsed target laser beam includes directing the initial laser beam through a first reflective end means into a laser cavity defined by said first and a second reflective end means, and directing the initial laser beam through a frequency converting compound to produce second and third beams of second and third wavelength respectively. Said second and 20 third beams are reflected back and forth within the cavity, and the target beam of a wavelength in said range is transmitted out of the cavity. The invention further provides dental procedure ablation apparatus for ablating tooth material that includes a source of an initial laser beam of wavelength unsuitable for the ablation, and frequency conversion means for deriving from the 25 initial laser beam a pulsed target laser beam of a wavelength in the range 2.6 to 3.2 microns. Means is further provided for directing the target beam onto the tooth material, for ablating the tooth material, which beam directing means includes means to deliver the laser energy to the tooth material in a focused or nearly focused form.

WO 99/51161 5 PCT/AU99/00245 The source of the initial beam is preferably a solid state laser source, and the initial laser beam is preferably a pulsed beam. Preferably, the frequency conversion means includes an optical parametric oscillator, which is advantageously singly resonant. 5 The frequency conversion means may include a frequency converting non-linear compound, eg. a potassium titanyl phosphate (KTP) crystal. The beam directing means may include a handpiece and one or more of a fibre optic cable, a hollow waveguide and an articulated arm that includes laser reflecting mirror means. 10 In an embodiment, the frequency conversion beams includes an optical cavity defined by two parallel reflecting means, and a frequency converting compound in the cavity between the reflecting means for producing second and third beams of second and third wavelengths respectively. The reflecting means are partially reflective, and transmissive of electromagnetic radiation of substantially only a 15 desired wavelength, whereby the second and third beams are reflected back and forth by the reflecting means within the cavity, and the target beam of wavelength in said range is transmitted out of the cavity. The invention still further encompasses use of the aforedescribed apparatus in the ablation of tooth material. 20 Brief Description of the Drawing In order that the invention may be more clearly ascertained, a preferred embodiment will now be described by way of example with reference to Figure 1, which is a schematic view of a dental procedure laser ablation apparatus according to a preferred embodiment of the present invention.

WO 99/51161 6 PCT/AU99/00245 Preferred Embodiment The illustrated dental procedure ablation apparatus 10, includes a solid state laser source 12, an OPO frequency conversion stage 11, beam delivery optics 30, and a handpiece 40 by which the dentist aligns the laser beam with the part of the 5 tooth 50 to be treated. The target laser beam 26 is employed to ablate a portion 52 of material of the tooth 50 for removal of diseased material prior to filling a dental cavity.. Source 12, conveniently an optical pump in the form of an Nd:YAG laser, emits an initial pulsed laser beam 14 of wavelength 1064 nm (unsuitable for hard tissue 10 dental ablation), which passes through a first mirror 16 of stage 11, coated for high transmission at this wavelength. After transmission through first mirror 16, the beam 14 passes through a frequency converting compound in the form of potassium titanyl phosphate (KTP) crystal 18. KTP crystal 18 is aligned for proper phase-matching conditions. The interaction of the beam 14 with the non-linear 15 KTP crystal 18 produces parametric output beams 20 and 22 at signal and idler frequencies respectively. Signal beam 20 oscillates between first mirror 16 and a second mirror 24 in a laser cavity 13 defined by mirrors 16, 24, while beams 14 and 22 double pass KTP crystal 18. Each mirror 16 and 24 is coated for reflection and transmission of energy with specific wavelengths: first mirror 16 is 95% 20 transmissive of the pump 12 wavelength (1064 nm), 99% reflective of the signal 20 wavelength (1650 nm), and 90% reflective of the idler 22 wavelength (3000 nm). Second mirror 24 is 80% reflective of the pump wavelength. Approximately 10% of light in the signal (1650 nm) range is transmitted by second mirror 24, which is transparent to 90% of wavelengths above 2.9 microns. 25 Thus, the cavity defined by mirrors 16 and 24 and containing KTP crystal 18 acts as an optical parametric oscillator singly resonant at the signal wavelength, while double passing the pump and idler wavelengths (ie. 1 pass in each direction). Second mirror 24, or the output coupler, therefore transmits most of the infrared light of idler beam 22 at around 3 microns to the exterior as target beam 26. The 30 reflectivity of second mirror 24 at the signal wavelength is optimised depending on WO 99/51161 7 PCT/AU99/00245 the pump power. Pulses preferably no greater than 20 nanoseconds in width eg. 5 nanoseconds, and at wavelength 3000 nm are extracted from the cavity and directed through optional focusing optics. A suitable beam delivery system 30 (not detailed), typically including a fibre optic 5 cable or hollow waveguide or mirrors in an articulated arm, in co-operation with handpiece 40, is utilised to direct the beam onto the tooth surface to be ablated. By fine tuning of OPO frequency conversion stage 11, or by utilising other source/OPO compound combinations, the target beam may have a wavelength set at an absorption peak of tooth material in the aforementioned range, or in the 10 narrower preferred range 2.85 and 3.05 microns. It is found that a laser beam derived in this way, by OPO frequency conversion from a solid state neodynium-doped infrared laser, and pulsed to obtain pulse widths no greater than 20 nanoseconds, is especially effective for ablation of teeth with lower temperature rises, and consequently reduced risk of damage to the 15 pulp of the teeth. A useful additional advantage of the described arrangement using an Nd:YAG pump laser is that this laser and its harmonics - including the 5th harmonic at 213 nm - have other potential applications in dentistry. The 213nm 5 th harmonic can assist in the cold ablation of tooth material, e.g. caries removal, and the 1064nm 20 fundamental and 532nm 2 nd harmonic can be used to thermally coagulate gum tissue e.g. gingivectomy.

Claims (1)

1. Claims

   1 A method of ablating tooth material, including:

   providing an initial laser beam of wavelength unsuitable for said ablation;

   deriving from said initial laser beam, a pulsed target laser beam of wavelength in the range 2.6 to 3.2 microns;

   directing said target beam onto the tooth material; and

   ablating the tooth material with the target beam.

   2 A method according to claim 1 wherein said target beam comprises pulses of a width no greater than 20 ns.

   3 A method according to claim 1 or 2 further including tuning the wavelength of the target beam to one or more absorption peaks of tooth material within said range.

   4 A method according to claim 3 wherein said absorption peak includes the peak for water contained in the tooth material, ie. 2.94 microns.

   5 A method according to any preceding claim further including providing said initial laser beam by means of a solid state laser.

   6 A method according to any preceding claim wherein said initial laser beam is a pulsed laser beam.

   7 A method according to any preceding claim wherein said pulsed target laser beam is derived from the initial laser beam by a process which includes optical parametric oscillation.

   8 A method according to any one of claims 1 to 7 wherein said deriving of the pulsed target laser beam includes: directing the initial laser beam through a first reflective end means into a laser cavity defined by said first and a second reflective end means;

   directing said initial laser beam through a frequency converting compound to produce second and third beams of second and third wavelength respectively;

   reflecting said second and third beams back and forth within said cavity; and

   transmitting said target beam of a wavelength in said range out of said cavity.

   A method according to 8 wherein said wavelength of the initial laser beam is 1064 nm, said second wavelength is about 1650 nm, and both said third wavelength and said wavelength of the target beam are about 3000 nm.

   Dental procedure ablation apparatus for ablating tooth material including:

   a source of an initial laser beam of wavelength unsuitable for said ablation;

   frequency conversion means for deriving from the initial laser beam a pulsed target laser beam of a wavelength in the range 2.6 to 3.2 microns; and

   means for directing said target beam onto the tooth material, for ablating the tooth material, which beam directing means includes means to deliver the laser energy to the tooth material in a focused or nearly focused form.

   Apparatus according to claim 10 wherein said target beam comprises pulses of a width no greater than 20 ns.

   Apparatus according to claim 10 or 11 further including means to tune the wavelength of the target beam to one or more absorption peaks of tooth material within said range. 13 Apparatus according to claim 10, 11 or 12 wherein said source is a solid state laser source.

   14 Apparatus according to any one of claims 10 to 13 wherein said source generates said initial laser beam as a pulsed laser beam.

   15 Apparatus according to anyone of claims 10 to 14 wherein said frequency conversion means includes an optical parametric oscillator.

   16 Apparatus according to claim 15 wherein said oscillator is singly resonant.

   17 Apparatus according to any one of claims 10 to 16, wherein said frequency conversion means includes a frequency converting non-linear compound.

   18 Apparatus according to claim 17 wherein said compound is a potassium titanyl phosphate (KTP) crystal.

   19 Apparatus according to any one of claims 10 to 18, wherein said frequency conversion beams includes an optical cavity defined by two parallel reflecting means, and a frequency converting compound in said cavity between said reflecting means for producing second and third beams of second and third wavelengths respectively, and wherein said reflecting means are partially reflective, and transmissive of electromagnetic radiation of substantially only a desired wavelength, whereby said second and third beams are reflected back and forth by said reflecting means within said cavity, and said target beam of wavelength in said range is transmitted out of said cavity.

   20 Apparatus according to claim 19 wherein one of said reflecting means is >95% transmissive of said initial laser beam, >99% reflective of said second beam and >90% reflective of said third beam, and the other of said reflecting means is >80% reflective of the initial laser beam, around 10% transmissive of the second beam, and >90% transmissive of the third beam, which on transmission therethrough forms said target beam.

   Apparatus according to any one of claims 10 to 20, wherein said beam directing means includes a handpiece and one or more of a fibre optic cable, a hollow waveguide, and an articulated arm that includes laser reflecting mirror means.

   Use of apparatus according to any one of claims 10 to 21 in the ablation of tooth material.