WO2008072033A1

WO2008072033A1 - A surgical apparatus and a method for treating biological hard tissues, particularly for dental surgery, based on a fibre laser

Info

Publication number: WO2008072033A1
Application number: PCT/IB2006/054738
Authority: WO
Inventors: Stefano Bonora; Paolo Villoresi
Original assignee: Consiglio Nazionale Delle Ricerche- Infm Istituto Nazionale Per La Fisica Della Materia; Universita' Degli Studi Di Padova
Priority date: 2006-12-11
Filing date: 2006-12-11
Publication date: 2008-06-19

Abstract

A surgical apparatus (10) and a method for treating biological hard tissues are disclosed, based on an optical fibre waveguide structure (12) including: a source of a coherent light radiation comprising a fibre section doped with a rare-earth metal element, particularly Thulium, so as to form an active region (14) arranged to emit photons at optical wavelengths comprised in the range between 1400nm and 2400nm, and preferably at the wavelength of 1940nm; and an optical fibre waveguide section (20) extended beyond the active region for guiding the light radiation on the surface of the tissue (T) to be treated.

Description

A surgical apparatus and a method for treating biological hard tissues, particularly for dental surgery, based on a fibre laser

The present invention regards the treatment of biological hard tissues, and in particular of sound dentin and enamel for restorative dentistry.

Specifically, the invention relates to a surgical laser apparatus and a method for ablating biological hard tissues.

In dentistry it is often necessary to act on the "hard" tissues of the tooth, such as enamel and dentin. For instance, in the treatment of carious disease, is of particular importance the ablation of parts of the "hard" tissues of the tooth in order to shape the cavities to better host the restorative materials. As is well known, the enamel is the strongest tissue in the human body and so mechanic diamond burr is needed for its removal.

The use of laser radiation has been proposed as an alternative to conventional mechanical methods, and applied widely during the last decade in order to reduce the use of anaesthetics, which have several contraindications, and the pain that mechanical tools can cause to patients.

Moreover, optical systems make it possible to carry the laser light to the treatment spot, by guiding its radiation in an optical fibre, and to focus the laser beam on a very small area of the target tissue, compatible with the dimensional ranges involved in the procedure, and therefore act with greater precision on the surface to be treated. The techniques so far developed to apply laser radiation to hard tissues, and hard tissues of teeth in particular, depend on the optical characteristics of the tissues (coefficient of absorption and diffusion as a function of wavelength) and their physical characteristics (heat conductivity, distribution of the nervous system) , as well as on the type of operation to be performed (cutting, removal of carious tissue, modelling of the tooth to apply implants or prostheses, et cetera) .

Several kinds of laser have been used for this type of procedure. US 5,020,995 discloses a CO₂ laser operating at a wavelength of 10.6μm, which was applied to the treatment of both soft and hard tissues of teeth. Its main drawback is due - on the one hand - to the increase in the local temperature of the tissue in case of irradiation with high-energy, short- duration pulses, and - on the other hand - to the heat propagation that occurs if the energy is reduced and the application time is increased.

In order to overcome these drawbacks, it is necessary to resort to several technical refinements concerning the energy level used and the duration and frequency of the irradiation, therefore resulting in a method which depends on clearly defined and extremely limited operating conditions.

Hard tissues require actions mostly of the ablative type, both to eliminate carious tissues and to remodel the shape of the tooth with the prospect of applying prostheses. The study and understanding of the thermal and optical properties of the components of the tooth, specifically enamel and dentin, has reached a less advanced stage than that of soft tissues. Some aspects of the propagation of light and heat inside the tooth are in fact very complex. This is linked to the structural anisotropy of the tooth, which is formed to a large extent by radially orientated hydroxyapatite crystals. The presence of nerve endings, blood vessels, fibroblasts and odontoblasts in the pulp chamber makes the tooth sensitive to the overheating produced during the procedure.

Accordingly, irradiation with high-power pulses, required in order to induce tissue ablation, must be limited in time, so as to allow the action of cooling systems that keep the pulp chamber at a tolerable temperature.

Excimer and neodymium in YAG (Nd: YAG) lasers were used in this context together with CO₂ lasers, initially. Considerable progress was achieved later by introducing lasers of the erbium in YAG or YSGG type (Er: YAG, Er: YSGG) operating at 2.94 μm and 2.79 μm, respectively.

The use of Er: YAG lasers to eliminate dental caries has a widespread diffusion, as they are painless and accurate, and exploits the strong absorption peak of both hydroxyapatite and water, which are the main components of the tooth tissue.

Moreover, their size is considerable due to the large cavities and discrete mirrors needed, which must be supported by an adequate stationary frame, and since they are based on the principle of optical pumping of the active medium their efficiency is poor.

US2006127861 discloses a semiconductor diode laser for hard tissues treatment. This patent exploits the advent of advanced optical systems able to couple high power diode laser bars radiation in optical fibres and the use of dyes to increase the local absorption of the hard tissues. The advantages of semiconductor diode laser based systems is the simplified structure, easier handling, affordable costs, low maintenance with respect to a CO₂ laser or to a laser of the Nd: YAG or Er: YAG type.

It should also be noted that problems similar to the ones noted above for the dental sector can also occur in other fields of surgery, when it is necessary to treat other hard tissues, such as for example bones.

The aim of the present invention is to provide a surgical laser apparatus for the treatment of hard tissues, such as for example the surfaces of teeth or bones, which is both reliable and highly efficient, and at the same time easy to handle and compact with respect to the known solutions.

Another object of the invention is to limit the high costs entailed by the prior art technologies.

Still another object is to provide a reliable and highly efficient method for the treatment of hard tissues, such as for example the surfaces of teeth or bones.

The above objects are achieved by a surgical apparatus according to the characterising part of Claim 1, and a method according to Claim 13.

Particular embodiments of the invention are defined in the dependent claims.

Other features and advantages will become clearer from the following detailed description, given as a non limitative example, with reference to the attached drawings, in which:

Figure 1 is an overall schematic view of a first embodiment of the surgical apparatus according to the invention;

Figure 2 is an overall schematic view of a second embodiment of the surgical apparatus according to the invention; and

Figure 3 is a diagram representing the absorption spectrum of water in a wavelength range of interest in surgical laser applications.

Reference is made to figures 1 and 2, where a surgical apparatus is generally identified with reference numeral 10.

The apparatus comprises an optical fibre waveguide structure 12 including an active region 14 in a fibre section acting as a source of a coherent light radiation at a predetermined wavelength. The active region 14 is a fibre section doped with a rare-earth metal element and delimited by semi- reflective mirrors 16 defining a fibre oscillating structure so as to bring about stimulated emission of coherent light at optical wavelengths comprised in the range between 1400nm and 2400nm as a result of radiative recombination of the charge carriers confined therein.

An excitation semiconductor pumping laser 18 is associated with the active region 14 and arranged to alter the thermodynamic equilibrium of the carrier populations therein in order to bring about a condition of population inversion. A length of fibre 20 extends beyond the active region 14 for guiding the light radiation to the surface of the tissue to be treated, and delivering it through a contact tip 22, e.g. conical in shape. At least a portion of said optical fibre is flexible, and the overall dimension of the apparatus resembles that of a common ball pen.

In a first embodiment, the light radiation is directly emitted from the fibre on the surface of the tissue to be treated, here identifies as a tooth T, while in a second embodiment the light radiation is focussed on a limited area of the said surface by means of a focussing assembly, generally indicated 30, which is arranged to focus light radiation on the end of the fibre section proximate to the contact tip 22.

Conveniently, the apparatus comprises a high power Thulium doped fibre laser with emission tunable at a wavelength in the range between 1400nm and 2400nm, preferably in the range between 1800nm and 2200nm and most preferably operating at the specific wavelength of 1940nm. The high absorption coefficient of the water in this range, as depicted in the diagram of figure 3, gives the possibility of the ablation of hard tissues owing to the small water content of tissues. Furthermore, an external water flux can be supplied to enhance the superficial absorption and cool the tissue, whenever necessary.

In the latter case, the laser action is assisted with water, which may be supplied in form of spray, of small drops or of water jet. The laser radiation is preferably generated with an overall power level enough to overtake the ablation threshold, which also depends on the optical coupling of the radiation on the sample, and generally is exceeding 2OW in pulsed operating mode. The duration of the pulses can vary between several picoseconds to several milliseconds and preferably between lμs and 50,000μs. Its repetition rate, if the cutting of the surface to be treated must be continuous, may range between 1 and 10,000 pulses per second, and is preferably higher than 10Hz. As an alternative, the system can operate by single burst or with a low repetition rate.

The laser system has to focus the beam on the dental tissue, in order to limit the extension of the treated zone and offer a good precision to the operator during the cutting, while at the same time exceed the ablation threshold. For example, a beam diameter at the target site in the range can vary in the range of 10-5000 μm, the laser spot preferably varying between lOOμm and 500μm.

The energy of a laser pulse in focused conditions is defined by the relation:

E_L=P_L -t_L where P_L is the power of the laser and t_L is the duration of the pulse. The pulse energy is preferably comprised between 0. ImJ to 5J. The resulting energy density, also termed flu- ence, is given by: F_L=E_L/S, where S is the surface struck by the pulse in focused conditions.

In tests carried out by the applicant, the surface of a tooth was struck with lms pulses at the frequency of 20Hz and at 85% of maximum power, which corresponded to a fluence of 80J/cm². With this specifics it was possible to cut into the tissues of dentin and of tooth enamel.

The described apparatus according to the invention can also be equipped with a system (not shown) for cooling the surface to be treated.

The advantages of an high power Thulium doped fibre laser with respect to previous technology include the possibility of exploiting a new wavelength. A compact arrangement may also be obtained, for instance by using very thin and flexible optical fibres compared with a prior art Er: YAG laser. As an example, the apparatus may occupy a volume that is approximately 10 times smaller, and may be approximately 5 times lighter than a laser having a conventional architecture (Erbium in YAG) .

The extremely limited dimensions of the apparatus allow it to be accommodated source within a hand-piece to be held in the surgeon's hand.

An all-fibre optical system, both for generating and modulating the useful signal and for guiding it to the tissue area to be treated, also guarantees operation of the apparatus without a cooling system as well as without maintenance, which are on the contrary necessary with bulky prior art systems. Suppliers guarantee fibre lasers for approximately 10 billion pulses, equivalent to an operating life of approximately 8 years. On the contrary, crystal lasers require maintenance over a period ranging from 1 to 3 years, e.g. to replace the lamp and the crystal, for realignment, et cetera. Advantageously, also the laser radiation is produced inside the fibre allowing an easier access to a dental hand-piece. Small core fibres can be used without the addition of external optical elements, still a high intensity radiation can be easily achieved.

The characteristics of compactness and low weight make it easy to move and carry the surgical apparatus or to integrate on the dentist chair, and therefore a single device can be used in all sanitary or home environments in which a physician can work.

Moreover, a higher electro-optical efficiency (equal to approximately 10%- 20%) significantly reduces the consumption of electric power and the need for cooling. The typical efficiency of semiconductor pumping lasers is higher than that of optical-pumping lasers, by a factor that varies from 5 to It will be evident to the person skilled in the art that the apparatus and method described hereinabove can be referred to several fields of medicine, with the appropriate technical refinements entailed by the tissue to be treated, which arise from the knowledge and practice in the field. The apparatus according to the invention can be used not only in dentistry, as described extensively, but also more generally in the surgery of hard tissues (such as for example bones) when it is necessary to treat these tissues precisely and without damaging other more sensitive tissues, or without causing pain.

Claims

1. A surgical apparatus (10) for treating biological hard tissues, comprising a source of a coherent light radiation at a predetermined wavelength and an optical system for guiding the light radiation on the surface of the tissue to be treated, characterised in that the apparatus (10) includes a fibre laser arranged to operate at optical wavelengths comprised in the range between 1400nm and 2400nm.

2. A surgical apparatus (10) according to claim 1, wherein the fibre laser medium (14) is doped with Thulium.

3. A surgical apparatus (10) according to claim 1 or 2, wherein the said fibre laser is arranged to emit radiation at optical wavelengths comprised in the range between 1800nm and 2200nm.

4. A surgical apparatus (10) according to claim 3, wherein the said fibre laser is arranged to emit radiation at the wavelength of 1940nm.

5. A surgical apparatus (10) according to any of claims 1 to 4, comprising an optical fibre waveguide structure (12) including: a first section forming an active region (14); an excitation semiconductor pumping laser (18) associated with said active region (14) ; semi-reflective means (16) for defining a fibre oscillating structure including the active region (14) ; and a second section (20) extending beyond the active region (14) for guiding the light radiation to the surface of the tissue (T) to be treated.

6. A surgical apparatus (10) according to any of the preceding claims, wherein the light radiation is coupled with collimation optical means (30) for focusing the radiation on a target site of the tissue (T) to be treated.

7. A surgical apparatus (10) according to claim 6, wherein said collimation optical means (30) are arranged for focussing the light radiation to a spot on the target site (T) having a diameter in the range of 10-5000μm.

8. A surgical apparatus (10) according to any of the preceding claims, wherein said light radiation is emitted as a pulsed output, with a pulse duration between lμs and 50,000μs and an energy between 0. ImJ and 5J.

9. A surgical apparatus (10) according to any of the preceding claims, wherein said light radiation has a pulse repetition rate between 1 and 10,000 pulses per second.

10. A surgical apparatus (10) according to any of the preceding claims, wherein said apparatus (10) is adapted to be coupled to water delivering means for delivering water to the tissue (T) under treatment.

11. A surgical apparatus (10) according to claim 10, wherein said water delivering means include means for supplying water in form of spray.

12. A surgical apparatus (10) according to claim 10, wherein said water delivering means include means for supplying water in form of drops.

13. A surgical apparatus according to claim 10, wherein said water delivering means include means for supplying water in form of water jet.

14. A method for treating biological hard tissues, comprising the steps of: generating a coherent light radiation at a predetermined wavelength, and guiding the said radiation on the surface of the tissue to be treated for absorption by the water content of said tissue, wherein said light radiation is generated by means of a fibre laser operating at optical wavelengths comprised in the range between 1400nm and 2400nm.

15. A method according to claim 14, wherein the fibre laser medium (14) is doped with Thulium.

16. A method according to claim 14 or 15, comprising operating the said fibre laser at optical wavelengths comprised in the range between 1800nm and 2200nm.

17. A method according to claim 16, comprising operating the said fibre laser at the wavelength of 1940nm.

18. A method according to any of claims 14 to 17, further comprising focussing the light radiation on a target site of the tissue (T) to be treated, with an overall power level above a threshold dependent on the said tissue.

19. A method according to claim 18, wherein said light radiation is focussed to a spot on the target site (T) having a diameter in the range of 10-5000μm.

20. A method according to any of the preceding claims, wherein said light radiation is emitted as a pulsed output, with a pulse duration between lμs and 50,000μs and an energy between 0. ImJ and 5J.

21. A method according to any of the preceding claims, wherein said light radiation has a pulse repetition rate between 1 and 10,000 pulses per second.

22. A method according to any of the preceding claims, further including delivering water to the tissue (T) under treatment .

23. A method according to claim 22, wherein water delivering includes supplying water in form of spray.

24. A method according to claim 22, wherein water delivering includes supplying water in form of drops.

25. A method according to claim 22, wherein water delivering includes supplying water in form of water jet.