Abstract:
A method and apparatus are provided for processing biological tissue and other materials which involves providing or distributing a substance containing abrasive particles to an area in front of at least a portion of a surface of the material and irradiating both the substance and the portion of the surface with light from a selected source, the light being selected and delivered in a manner such that selective ablation is caused on the substance sufficient to force the abrasive particles under a selected pulse against the portion of the surface. Ablation may be of the particles themselves or the particles may be contained within a shell, with ablation being of the shell. The substance is preferably delivered as a series of distribution pulses with the light being delivered either continuously or as light pulses having a predetermined relationship to the distribution pulses.

Description:
FIELD OF THE INVENTION 
     The invention concerns the processing of materials and medical engineering and can be used for the processing of materials in dentistry, orthopedics, surgery, dermatology and other fields of medicine, for removal and destruction of hard and soft materials and tissues and for modification of properties of hard and soft materials and tissue surfaces. 
     BACKGROUND OF THE INVENTION 
     The processing of hard dental tissues by simultaneous influence of laser radiation and the flow of abrasive particle is shown in U.S. Pat. No. 5,334,016, where a pneumatic system forms an air jet saturated by abrasive material and directs it on an object inside a patient&#39;s mount. A disadvantage of this method is the necessity of delivering the abrasive particles through a pipe at very high speed, this causing loss of essential energy by the particles. Therefore, near the tissues, there are particles with kinetic energies that differ considerably, so that only some of them participate in the removal of tissues, enamel and dentin and the remaining particles are stored in the patient&#39;s mouth, producing no useful effect. It can also be difficult to completely remove these particles, even with the help of an additional aspiration system. 
     The application of laser radiation in this prior art method results in a negligible increase in efficiency, since the air-abrasive flow and laser radiation do not interact, but independently produce additive influence so that the efficiency of laser processing is limited by the finite value of reflection and absorption indices of tissues. The removal, cutting, drilling and other specific methods of laser-tissue interaction are also accompanied in a number of cases with increased necrosis. 
     Another prior art method and apparatus for processing of materials including tissues uses particles of condensed substance (drops of liquid), which acceleration in the direction of a surface of the material being processed as a result of interaction with electromagnetic radiation directed to the processed surface. The main disadvantage of this technique is the insufficient hardness of liquid particles, so that, even at high speed, they cannot cut most materials, particularly hard materials such as metals, ceramics, enamel or dentin of tooth, with high efficiency. 
     A need therefore exists for methods and apparatus for the improved utilizing of both radiation and particles in the efficient and accurate processing of materials, including tissues. 
     SUMMARY OF THE INVENTION 
     In accordance with the above, this invention provides a method for processing a material having an outer surface, the term “material” as used herein including biological tissue, and biological tissue being the material on which the invention is primarily adapted for use. However, the invention is by no means limited to use on biological tissue. The method includes distributing a substance containing abrasive particles in an area in front of at least a portion of the surface and irradiating both the substance and the portion of the surface with light selected to cause selective ablation of the substance sufficient to force the abrasion particles under a selected pulse against the portion of the surface. For some embodiments of the invention, the substance is the abrasive particles, the particles being of a size to be selectively ablated by the irradiation to force the particles against the surface. For this embodiment, the particles should have a size d&gt;&gt;k −1 (λ), d, the characteristic size of particle, being between 1 and 1000 μm and k(λ), the absorption index of the particle material at wavelength λ, being between 10 5  cm −1  and 10 2  cm −1 . These particles may for example be distributed into the area in front of the surface by airflow. 
     Alternatively, the substance may include the particles, each enclosed within a shell, the selective ablation being of the shell. For this embodiment, each shell should have a thickness δ such that δ&gt;&gt;k −1 (λ). For this embodiment, it is also preferable that 1&lt;δ&lt;1000 μm and that 10 5  cm −1 &gt;k&gt;10 2  cm −1 . The substance may be a suspension of the particles in a liquid, the liquid being selectively ablated by the light. The distributing step distributes the suspension to the surface to cool the material and the irradiating step irradiates a small portion of the surface, particles being forced for the most part only against such small portion to cause the processing thereof. The shell may be substantially completely ablated by the irradiation thereof so as not to interfere with the action of the particles on the surface. 
     For still another embodiment of the invention, the substance is a substantially solid body containing the particles, a component of the body being selectively ablated by the light, and the distributing step includes the step performed throughout the processing of the material of maintaining a portion of the body between the light and the portion of the surface being processed. The solid body may be formed as a suspension of the particles in a substantially solid binder, the binder for example being the component selectively ablated by the light, or the solid body may be formed of the particles processed, for example by sintering or under pressure to adhere in the desired shape. 
     The substance may be distributed to the area in front of the surface for a duration τ, the light having an energy density E near the surface such that τ&lt;&lt;d 2 /4α and E&gt;&gt;k −1 (λ)ρQ. E may be, for example, between 10 −1  and 10 4  j/cm 2 . 
     The irradiation may be performed continuously or may be performed as a sequence of time-spaced light pulses. Similarly, the distribution of substance may be by a series of distribution pulses. The distribution pulses may be synchronized with the light pulses or may occur before each light pulse, the duration of the distribution pulses being less than the time between light pulses in the latter case. A distribution pulses may also have a repetition rate which is less than that for the light pulses. 
     The invention also includes apparatus for processing a material, as previously defined, having an outer surface, which apparatus includes a mechanism selectively providing a substance containing abrasive particles in an area in front of at least a portion of the surface to be processed, a light source, and a system for selectively directing light from the source to irradiate both the substance and the portion of the surface, the source and the system being selected to cause selective ablation of the substance in response to irradiation thereof sufficient to force abrasive particles under a selected pulse against the portion of the surface. The light source may for example be a laser, an incandescent lamp or a flash lamp. The system for selectively directing light may include a control which operates the source in a selected pulse mode and an optical system directing light from the source to a tip, the mechanism selectively providing the substance through the tip to the area and the tip being adapted to direct light through the area to the portion of the surface. The substance may also be provided to the area by the mechanism as a series of distribution pulses. The distribution pulses may be provided in synchronism with the light pulses from the source or the distribution pulses may occur before each light pulse, the duration of the distribution pulses being less than the time between light pulses. The distribution pulses may have a repetition rate which is less than the repetition rate for the selected pulse mode in which the source is operated. Alternatively, the control may operate the source continuously. 
     The mechanism may be operated to distribute the substance to the area for a duration τ, and the source may deliver light radiation having an energy density E near the surface of the material such that 10 −1 &lt;E&lt;10 4  j/cm 2 . In this case, E is preferably between 10 −1  and 10 4  j/cm 2 . 
     For some embodiments, the substance is the abrasive particles and the mechanism includes a source of the particles, a carrier for delivering the particles to the area, and a control for operating on at least one of the carrier and the source to provide a controlled delivery of particles to the area, the particles being of a size to be selectively ablated by light irradiation directed thereat from the source. The carrier may be delivered under pressure to an air pipe under control of a valve operated by the control, the particles being delivered from a source thereof to the air pipe to be carried therethrough by air to the area in front of the material surface. Alternatively, the carrier may be a liquid, the substance being a suspension of the particles in the liquid, the liquid being selectively ablated by the light. In this case, particularly where the material is biological tissue, the mechanism may be controlled to deliver the suspension to the surface to cool the material prior to the system applying light to the substance to ablate the liquid, forcing the particles for the most part only against a small portion of the surface irradiated by the light to cause processing of material at the portion. 
     For some embodiments of the invention, the substance is in the form of a substantially solid body containing the particles, an indexing mechanism being provided for both supporting and maintaining the body in an area between the light and the portion of the surface to be processed. The solid body may be a suspension of the particles in a substantially solid binder, the binder for example being selectively ablated by the light, or the particles may be processed to adhere in a desired shape, for example by sintering, pressure or the like. 
     The substance may be sapphire particles suspended in water, with a light source for such substance being, for example, an ER-laser. Where a laser is used as the light source, the light source may for example be CO 2  laser or an excimer laser. For one embodiment, a neodymium laser is used with carbon particles as the substance. Where the material being processed is biological tissue, the particles are preferably biologically safe materials such as hydroxyapatite, carbon, silicon or ice. 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more detailed description of preferred embodiments as illustrated in the accompanying drawings, the same reference numerals being used for common elements in all the drawings. 
    
    
     IN THE DRAWINGS 
     FIGS. 1 a - 1   c  are diagrammatic representations of a particle showing the principle of conversion of light energy into mechanical energy of the particles for three different types of particles; 
     FIGS. 2 a - 2   e  are temporal diagrams of light and abrasive flow for various embodiments of the invention; and 
     FIGS. 3 a - 3   c  are schematic representations of one aspect of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 a  illustrates the principle of conversion of laser beam or other light energy into mechanical energy of abrasive particles. Light flow or radiation  1  at a wavelength λ falls on an abrasive particle  2 . While the form of the particle is arbitrary, for simplicity it will be assumed to be a sphere. The wavelength of light flow  1  and the size and material of abrasive particle  2  are selected so that the conditions d&gt;&gt;k −1 (λ) are satisfied, where d is a characteristic size of a particle  2 , and k(λ) is the absorption index of the particle material at wavelength λ. In this case, the depth of light penetration into particle  2  will be less than its size. That is a necessary condition, together with the limitation of exposure duration to provide heating of only a part of the abrasive particle. The duration τ and the energy density E of an exposure of particles satisfy the following conditions: 
     
       
           τ&lt;&lt;d   2 /4α 
       
     
     
       
           E&gt;&gt;k   −1 (λ) ρQ   
       
     
     where: 
     α=index of temperature conductivity for the material of abrasive particle  2 , 
     ρ=density of the material of abrasive particle  2 , 
     Q=specific energy of transition of the material of abrasive particle  2  from solid to gaseous state. 
     The first condition means that heating of the surface of particle  2  which is turned to the source of light radiation  1  takes place adiabatically and does not result in the uniform heating of the whole particle. The second condition means that the energy absorbed by a part  3  of the particle  2  adjacent the surface turned to light radiation  1  is sufficient for evaporation (ablation) of this part. Thus, when these conditions are realized, a part  4  of particle  2  (part  3  after ablation) is vaporized and saturated steam  5 , produced due to ablation of part  4 , transfers a mechanical pulse P of jet recoil to the non-vaporized part  6  of particle  2 , accelerating part  6  of the abrasive particle in the direction of light propagation. 
     Similarly, referring to FIG. 1 b , for the abrasive particle  2  inside a shell  9 , the mechanism of conversion of light energy into mechanical energy of the abrasive particle involves ablation of a part  7  of the heated area  8  of shell  9 . Shell  9  is formed from a material with absorption index k(λ) which satisfies to the condition δ&gt;&gt;k −1 (λ), where δ=thickness of shell  9 . 
     The material of shell  9  may be a hard inorganic or polymeric; it can also be doped by ions or molecules which strongly absorb the optical radiation. The shell may also be a liquid layer. If the liquid strongly absorbs light radiation at wavelength λ, the conversion of energy is provided due to ablation of the liquid. The shell may be partially ablated, or may be fully ablated so as not to interfere with passage of and/or processing by the particles. 
     If the shell is transparent to the light radiation, the mechanism of conversion of light energy into mechanical energy of a particle is illustrated by FIG. 1 c . The light flow  1  causes the ablation of a part  4  of a particle  2  resulting in saturated steam  5  swelling transparent shell  10  and ultimately tearing it. Since in the beginning, the evaporation takes place in a volume closed by shell  10 , the pressure of saturated steam  5  reaches a value considerably greater than in the case when there is no transparent shell, resulting in an increased jet pulse. Saturated steam  5  pulled out under shell  10 , together with breakdown products  11 , transfers the mechanical pulse P to non-vaporized part  6  of particle  2 . 
     The duration τ and the energy density E of exposure in the case of an absorbing shell  9  as shown in FIG. 1 b  should satisfy to the conditions: 
     
       
           τ&lt;&lt;δ   2 /4α 
       
     
     
       
           E&gt;&gt;k   −1 (λ) ρQ   
       
     
     where, 
     α=index of temperature conductivity of the material of shell  9 ; 
     ρ=density of the material of shell  9 ; 
     Q=specific energy of transition of the material of absorbing shell  9  from solid to gaseous state. 
     Thus, due to jet recoil arising because of asymmetric evaporation of material of an abrasive particle or its shell, the abrasive particle gets a mechanical pulse {right arrow over (P)} 1  which, being added to an initial pulse drive {right arrow over (P)} 0  applied to the abrasive particle, yields a total pulse {right arrow over (P)} applied to the particle 
     
       
         
           {right arrow over (P)}={right arrow over (P)} 
           1 
           +{right arrow over (P)} 
           0 
         
       
     
     For preferred embodiments, the initial pulse of abrasive particles is negligibly low in comparison with {right arrow over (P)} 1 ; therefore in practice {right arrow over (P)}={right arrow over (P)} 1  and the direction of {right arrow over (P)} substantially coincides with the direction of light radiation. 
     It is known that the interaction of abrasive particles with a material surface under low values of pulses and energies results in the hardening or other modification of the material surface, and that, under strong pulses and energies, removal or cutting of the material surface is observed. The efficiency of these processes depends on the ratio of microhardness of the abrasive particles exceeding the microhardness of the processed material. Universal abrasive particles include particles of diamond and/or sapphire which have hardnesses exceeding the hardness of most other materials. Where the material being processed is tissue, the material(s) for the abrasive particles are preferably biologically safe materials, for example hydroxyapatite, carbon, silicon, ice and other materials. 
     The size of the abrasive particles and/or their shells should satisfy the above conditions; but in any case, the size of the abrasive particles and/or the thickness of the absorbing shell should be more than the radiation wavelength in order to provide the asymmetric irradiation. Taking into account the optical range of wavelengths, the size of the abrasive particles or absorbing shell should not be less than one micrometer. However, it is necessary that the size of an abrasive particle be less than the transversal size of the light flow or beam  1  in the field of their intersection (i.e., it should not be more than 1 millimeter). 
     The index of absorption of the material for particles  2  or shell  9  for dimensions according to the above formulas should be 10 2 −10 5  cm −1 . The duration of exposure can be within the range 10 −15 -10 −1  s, and the energy density of exposure can be within the range 10 −1 -10 4  J/cm 2 . 
     The realization of the method of this invention is possible if several temporal operation modes are observed. In the first case (FIG. 2 a ), the light flow  1  is represented by light pulses having a selected repetition rate and the flow  12  of abrasive particle  2  is continuous. This mode is simplest and cheapest. However, in this case it is possible that the accumulation of waste particles may obstruct the effective utilization of light energy in the zone of processing. The same result is possible under application of continuous light and abrasive flow. In the second and third modes (FIGS. 2 b  and  2   c ), light flow  1  and flow  12  of abrasive particles  2  are both pulsed. In these modes, the repetition rates of particle and light pulses are equal. In the second mode (FIG. 2 b ), the light and particle pulses are superimposed. By regulation of the pulse duration of flow  12 , it is possible to precisely set the consumption of abrasive particles, and to also avoid the formation of a layer of abrasive particles on the material surface being processed. Where tissue is the material being processed, it is possible to reduce the invasiveness of the procedure by lowering the energy of light flow  1  directly interacting with tissue. In this case, the light pulses only accelerate the abrasive particles, removal, cutting or modification of material taking place only as a result of the collision of abrasive particles with the tissues because of fragile or viscous cracking, and also due to elastic impact extrusion. 
     In the third mode (FIG. 2 c ), each pulse of particle flow  12  precedes the corresponding pulse of light flow  1 . In this case a lamina of abrasive particles is formed on the surface of the material before the arrival of the light pulse, and removal, cutting or modification of the material is produced by direct transfer of a jet recoil pulse of an abrasive particle resulting from ablation and evaporation of parts of the abrasive particles or their shells. For the fourth operation mode (FIG. 2 d ), the repetition rate of light pulses  1  is greater than the repetition rate of the flow  12  of abrasive particles, but the duration of each light pulse is shorter then that of each particle pulse. In this case, a number of the light pulses affect the material together with abrasive particle pulses and the rest of the light pulses influence the material directly. Such a mode can be useful when the light influence differs essentially from the abrasive one and can itself be useful for material processing. For example, for an illustrative embodiment, the cutting of soft tissue takes place under the combined influence and the coagulation of blood vessels occurs under the influence of light pulses. The same result is achieved under continuous irradiation and pulsed flow of particles (FIG. 2 e ). 
     A device for realization of the proposed method will now be described. It comprises a source of optical light radiation, a control unit, a power supply for the source, an optical system for delivering radiation to a zone of processing of the material being processed, a tank with abrasive particles and a mechanism which delivers abrasive particles to the zone of processing. The mechanism for delivering abrasive particles includes a valve connected to an output of the control unit. Another output of the control unit is connected to an input of the power supply. The radiation delivery system can be made as an optically conjugated lens, an optical fiber and a mirror. The delivery mechanism for abrasive particles is, in the simplest case, represented by an airpipe connected to an air compressor and a tank with abrasive particles. Alternatively, the tank can be connected to a tank with liquid, particles flowing with the liquid from the tank through the pipe leading to the zone of processing. The tanks can be joint, in which case a liquid suspension of abrasive particles from the joint tank is supplied to the zone of processing through the pipe. The valve is preferably an electromechanical switch and is located on an output to the airpipe from the air compressor. The tank is connected to a pump. The outputs of the airpipe and the pipe, together with an output of the optical system delivering radiation to the zone of processing of the material, are joined in a tip containing the outputs from the optical system, the airpipe and the pipe respectively (See FIGS. 3 a - 3   a ). 
     In operation, radiation from the optical source  13  is delivered to zone of processing  17  of a material through radiation delivery system  16 . The flow of abrasive particles  2  is delivered to the same zone from mechanism  19  in the form of a flow of abrasive particles in gas or in liquid stream through airpipe  24  and/or pipe  27 . The light flow  1  and the abrasive flow  12  intersect in the zone of processing. Under this condition, light flow  1  causes the ablation of abrasive particles  2  which transfers the mechanical pulse obtained as indicated above to the surface of the processed material in zone  17 , producing the elastic implantation or cracking. If a liquid-abrasive flow is used, the role of the liquid consists additionally in the cooling of the processed material in zone  17  and preventing its overheating. The light flow can also affect directly on the processed material, producing its ablation or selective heating. Control unit  14  regulates operation of the device by controlling its main parameters; namely, energy of optical radiation and consumption of abrasive material. Control unit  14  may also control the processing mode (FIGS. 2 a - 2   e ). Abrasive flow  12  may for example be represented as a pulse having a duration and synchronization with respect to the light pulse which controls the processing mode. While in the discussion above, it has been assumed that it is the particle being ablated, the light being at a wavelength for which the liquid carrier, for example water, is transparent, the wavelength of light and the liquid carrier can be selected such that the carrier functions as a FIG. 1 b  shell, being ablated by the light to propel the particles. A transparent carrier may result in operation as per FIGS. 1 a  and/or  1   c.    
     Various versions of the device tip  29  are shown in FIGS. 3 a - 3   c.  The tip with light output  30  and air-abrasive output  31  is shown in FIG. 3 a.  The light output  30  is for example an optical fiber. The light radiation is directed on processed material zone  17 , in this case by the optical fiber  30  at an angle φ. The tip  29  is oriented as a whole to the surface of processed material zone  17  so that the angle φ between the axis of the light beam and the perpendicular to the surface of material zone  17  has a predetermined value. The axis of flow  12  of abrasive particles  2  emitted from the air-abrasive output  31  is directed at an angle θ to the axis of light flow  1 . This angle is equal to φ under the perpendicular incidence of the flow  12 . By changing the φ and θ, it is possible to control the efficiency of material removal and the microcontour of the processed surface. 
     FIG. 3 b  illustrates a tip  29  where the liquid-abrasive flow is supplied under low pressure through the nozzle  32  toward one  17 , but is taken before reaching zone  17  in a gas jet directed by nozzle  31  to the area of intersection with light flow  1 . As discussed earlier, the further direction of abrasive particle motion coincides with the direction of light flow. 
     FIG. 3 c  shows a version of tip  29  in which, besides the light output  30 , air abrasive output  31  and output for liquid abrasive  32 , there is an extra output  33  for air. In this case, the air-abrasive jet is supplied through the output  31  under low pressure, and the flow of liquid supplied through the nozzle  32  is taken by high pressure air flow from the nozzle  33  in the direction of the irradiated zone. All parts of each tip are assembled inside a housing  34 . In addition to the tip configuration shown in FIGS. 3 a - 3   c , it is also possible to have only fluid flow through output  32  at, for example, an angle (FIG. 3 a ) without an output  31 . Other nozzle configurations are possible. 
     The source of optical radiation  13  can be either coherent (laser) or incoherent (incandescent or arc lamp). The term “light” as used herein shall mean radiation in a wavelength range of approximately 1 cm to 100 nm. The coherent sources for the proposed method are CO 2  or excimer lasers since most of the hard substances of abrasive particles, including these described above, have strong absorption in the far IR range (9-11 μm) and in UV range (the wavelength is shorter than 0.3 μm). The radiation of a mercury lamp may possibly be used in the UV range. Lasers based on yttrium-aluminum-garnet crystal doped by erbium or neodymium ions may be used as pulsed lasers. The radiation wavelengths for these lasers are 2.94 μm and 1.064 μm respectively. The energy of generated pulses is about 1 J, the duration of pulses may be varied from 50 to 5000 microseconds, the pulse repetition rate may be 25 Hz and the average power may be 15 W for an illustrative embodiment. 
     Pure water or a water solution of food dye can be used as the liquid. Where a neodymium laser is used, carbon particles with dimensions from 10 to 100 microns may be added to the food dye in water, this providing an absorption index value as high as 10 5  cm −1 . If an Er-laser is used, water may be used as a shell, water having an absorption index on 2.94 μm, which is more than 10 6  cm −1 . Saphire particles may be used as the abrasive particles for this embodiment. 
     A SEM photo of the crater formed in hard dental tissue (dentin) by YAG:ER laser radiation without application of abrasive particles and a similar photo of a crater formed with application of sapphire particles with diameter 12 μm in a water shell accelerated by a laser pulse with duration 200 μm and energy density 50 J/cm2 show that the application of the method and device described above results in a nearly twofold increase in the efficiency of processing of human hard tooth tissues. 
     In an alternative embodiment of the invention, abrasive particle delivery mechanism  19  is replaced by a solid body  40  mounted in an indexing mechanism  42  so as to be in the path of light radiation from radiation output  30 . Body  40  may be in the form of a rod, ribbon, fiber, film or other suitable shape, and may for example be formed by presenting or pressing the abrasive particles to a solid body of the desired form. Conversely, the body  40  may be formed of abrasive particles embedded in a suitable binder, which binder may form a shell  9  for the particles  2 , which shell is to be ablated or may form a transparent shell  10  (FIG. 1 c ). As ablation of body  14  under the influence of light radiation occurs, causing abrasive particles to be accelerated to material treatment zone  17 , indexing mechanism  42  is operated under control of control unit  14  to maintain the end of body  40  in the path of light radiation to the processing zone. 
     Thus, while the invention has been particularly shown and described above with reference to preferred embodiments, and variations on the preferred embodiments have also been discussed, such variations and others may be made therein by one skilled in the art while still remaining within the spirit and scope of the invention, which is to be defined only by the appended claims.