Abstract:
A method and device for photoablation is disclosed wherein photoablation occurs along the interface between a material having a lower energy ablation threshold and a material having a higher energy ablation threshold. The method and device utilize a laser beam having a beam energy density which is less than the higher energy ablation threshold and greater than or equal to the lower energy ablation threshold. By directing such a laser beam to the interface, the material having the lower energy threshold is photoablated while the material having the higher energy threshold is largely unaffected.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to laser surgery procedures. More particularly, the present invention relates to methods and devices for performing intra-material photoablation wherein the energy density of the focused laser beam is selected in accordance with characteristics of the material to be ablated. The present invention is particularly, but not exclusively, useful as a method and device for precise photoablation at the interface between materials that have different laser induced optical breakdown thresholds. 
     BACKGROUND OF THE INVENTION 
     In the last few years, ultra short pulsed laser systems have become widely available for commercial applications. One application of such ultra short pulsed lasers involves intra-material ablation. For intra-material ablation, the local electrical field strength at the focal point of the laser (usually measured in J/cm 2  or equivalent units) must be equal to or greater than the binding energy of the material&#39;s valence electrons to their atoms. When the beam&#39;s energy density is equal to or greater than the material&#39;s energy density threshold, a laser induced optical breakdown (LIOB) of the material occurs at or near the focal point. During LIOB, a microplasma, gas bubbles and shockwaves are generated. 
     With the above in mind, consideration is given here for the use of an ultra short pulsed laser for photoablating a selected material without affecting the adjacent non-selected material. Specifically, consideration is given for the use of such photoablation where adjacent materials have different ablation thresholds. 
     In considering details of a photoablation procedure, the geometry and intensity distribution of a laser beam should be understood. In particular, the shape of the laser focus can be assumed to be hyperbolic, and the intensity distribution can be assumed to be Gaussian. With these assumptions, the laser focus has a minimum radius (the beam waist) and has a length (or focal depth). The boundary of the laser beam is usually defined by the position where the intensity of the beam has decreased to 1/e 2  of the intensity in the center of the beam. 
     With a known ablation threshold for LIOB, it might be expected that the location where LIOB occurs within the laser focus can be simply determined by the position where the necessary energy density is reached. Therefore, the position of LIOB within a material could be changed not only by moving the position of the laser focus, but also by altering the beam energy. In other words, the intensity needed for LIOB scales with the size of the laser focus. However, this relationship is based on the assumption that the whole energy of the laser pulse is instantaneously deposited within the material. In reality, the intensity (and, thus, the energy) of the laser beam also has a distribution along the z-axis that is determined by the pulse length and shape of the laser pulse and can be described by a sech 2 -curve. This leads to a spatial distribution of the intensity along the z-axis. As a consequence, LIOB occurs only when the intensity of the laser pulse reaches the threshold intensity needed for LIOB. 
     If the threshold for LIOB changes along the z-axis (e.g., at the interface between two materials), then the position of LIOB changes significantly. For instance, a laser may induce LIOB at its focal point in one material but have no effect when another material is used. Therefore, appropriate settings for beam geometry and beam energy can be used to control the position of LIOB at the interface of two materials having different ablation thresholds. If the ablation thresholds for the materials are known, or can be identified, the energy level can be calculated to provide LIOB in only one of the materials. Furthermore, by identifying the position of the interface before activating the laser, the focal point may be initially positioned in the targeted material with the appropriate energy level to avoid any unintentional ablation. After the laser beam has been activated, detecting means may be used to detect whether LIOB has occurred and, if so, in which material. Such detection may be through analysis of the size of the bubble resulting from LIOB or through spectral analysis of the plasma resulting from LIOB. 
     While intra-material photoablation may be performed on various materials, its use on corneal tissue or biological tissue is of particular interest. With regard to corneal tissue, it is noted that several surgical procedures exist for modifying its structure. To understand these procedures, the operation and anatomy of the cornea should be understood. 
     Along with the lens, the cornea refracts incoming light and focuses the light on or near the retina. The curvature of the cornea determines where the incoming light will be focused. If the curvature of the cornea is too steep or too flat, it may be modified by photoablating certain corneal tissue. Anatomically, the structurally distinct corneal tissues include, in order from the anterior to the posterior of the eye, the epithelium, Bowman&#39;s membrane, the stroma, Descemet&#39;s membrane, and the endothelium. Of these, the stroma is the most extensive, being generally around four hundred microns thick. Consequently, the stroma provides the most opportunity for correction via photoablation. Additionally, the healing response of the stroma is typically quicker than the other corneal layers. 
     In the past, techniques such as laser-assisted in situ keratomileusis (LASIK) and laser epithelial keratomileusis (LASEK) have been used to reshape stromal tissue. In these procedures, stromal tissue is ablated after being exposed by temporarily removing the overlying tissues. Other ophthalmic procedures rely on subsurface photoablation, i.e., the photoablation of stromal tissue without first exposing the tissue through the removal of overlying tissue. Because the subsurface photoablative procedures provide clear benefits over the prior art&#39;s reliance on the removal of overlying tissues, further efforts have been made to utilize intracorneal photoablative techniques. Specifically, these efforts have involved the use of ultra short pulsed laser systems in intracorneal photoablation procedures. As a result of these efforts, an intracorneal technique has been discovered that allows precise separation of corneal tissues along their interface using an ultra short (femto-second) pulsed laser. 
     In light of the above, it is an object of the present invention to provide an efficient surgical method for creating a discontinuity at the interface between two distinct materials. Another object of the present invention is to provide a method and device for separating two distinct materials having different ablation energy thresholds. It is yet another object of the present invention to provide a surgical method and device for creating a corneal flap that allows for the accurate positioning of the corneal flap at a predetermined location on the cornea. Still another object of the present invention is to provide a method for intra-material photoablation along an interface that is easy to perform and is comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     A device for performing photoablation along an interface between materials includes a source for creating a laser beam having a energy density less than the non-targeted material&#39;s energy ablation threshold and greater than or equal to the targeted material&#39;s energy ablation threshold. The device further includes a means for directing the laser beam to the interface to photoablate a portion of the targeted material adjacent the interface. A means for scanning the laser beam along the interface provides for photoablation of further portions of targeted material adjacent the interface. 
     In use, the energy ablation thresholds of the two materials are identified. Specifically, the materials, which may be corneal or biological tissues, must be identified as having different energy thresholds for ablation. After the ablation thresholds are identified, an ultra short pulsed laser beam is created by selecting a beam geometry, beam energy and pulse duration. These selections are made to create a laser beam having a desired maximum beam energy density. This desired maximum energy density is less than the ablation threshold of the non-targeted material but greater than or equal to the ablation threshold of the targeted material. After selection of the beam parameters, the beam is employed to photoablate the targeted material adjacent the interface. Despite the use of the laser beam at the interface, the non-targeted material is unharmed. 
     In accordance with the present invention, the interface can be found by employing means such as a confocal microscope or an optical coherence tomograph. After finding the interface, the laser beam may be directed thereto to photoablate the targeted material. Specifically, the laser beam is focused to a focal point where the beam reaches its maximum beam energy density. Additionally, the focused beam has a minimum beam energy density that is defined herein to be equal to the threshold of the targeted material. The minimum beam energy is created either at, or spaced from, the focal point. When directing the beam to the interface, the focal point is preferably located in the non-targeted material. In this way, the method avoids or minimizes inadvertent photoablation of targeted material when the laser beam is generated. 
     Preferably, the focal point is located in the non-targeted portion by setting the appropriate distance between the focal point and the laser source. However, the focal point may be located in the non-targeted portion by more generally situating the focal point in the cornea and then generating the laser beam. After the beam is generated, a sensing means is used to sense whether any targeted material is photoablated. If no targeted material is photoablated, then the focal point can be identified as being in the non-targeted material. If targeted material is photoablated, then the focal point is resituated away from the non-interface boundary of the targeted material. Then the beam is generated and the sensing means is used to determine if any targeted material is photoablated. These steps may be repeated until the focal point is identified as being in the non-targeted material. 
     After the focal point is located in the non-targeted material and the laser beam is generated, the response of the targeted material is detected. For instance, a photoablative result can be confirmed by the size of the bubble, or the spectral analysis of the plasma, created by photoablation. If no targeted material is photoablated, then either the focal point is moved toward the targeted material, or the beam energy is increased (though not to the threshold of the non-targeted material). It is noted that either of these steps will effectively advance the minimum beam energy density toward the interface. After one of, or a combination of, the steps is taken, the laser beam is again generated and the detecting means detects whether any targeted material has been photoablated. If not, the minimum beam energy density is again advanced toward the interface. Preferably, these steps are repeated until the portion of targeted material adjacent the interface and nearest the focal point is photoablated. 
     Once photoablation of the targeted material adjacent the interface is detected, the laser beam is scanned to another location. Then the previous steps are repeated to photoablate a further portion of the targeted material. Preferably, the laser beam is utilized in this manner to form a periphery for a flap. Such a flap is formed by incising the cornea between the anterior surface of the cornea and the periphery. The targeted and non-targeted materials can then be separated along the periphery by mechanically peeling the non-targeted material and the targeted material from one another. 
     While certain embodiments are described above, other alternate embodiments are contemplated by the present invention. For instance, the method may be used to photoablate non-corneal tissue and non-biological tissue. In addition, a wavefront detector or other means may be utilized rather than, or in addition to, a confocal microscope or an optical coherence tomography. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a plan view of an embodiment of the ultra short pulsed laser system of the present invention; 
         FIG. 2  is perspective view of an eye; 
         FIG. 3  is a cross sectional view, not to scale, of a portion of the cornea of the eye as seen along line  3 - 3  in  FIG. 2  showing the anatomical layers of the cornea; 
         FIG. 4  is a schematic view, not to scale, of a section of the cornea seen in  FIG. 3 , showing the shape of the laser beam used to ablate a portion of the targeted material with the targeted material positioned downstream of the interface; 
         FIG. 5  is a schematic view of a section through the cornea, as in  FIG. 4 , showing the shape of the laser beam used to ablate a portion of the targeted material with the targeted material positioned upstream of the interface; 
         FIG. 6  is a schematic view of the cornea, as in  FIG. 5 , with the focal point of the laser beam moved to the interface; 
         FIG. 7  is a schematic view of the cornea, as in  FIG. 5 , with the energy level of the laser beam increased to move the position of the minimum beam energy density to the interface; 
         FIG. 8  is a perspective view of a corneal flap created in part using the method of the present invention; 
         FIG. 9  is a logic flow chart of the sequential steps to be accomplished in accordance with the methods of the present invention; and 
         FIG. 10  is a logic flow chart of additional steps that may be accomplished in accordance with the methods of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , a laser system  20  is shown for conducting a intracorneal laser procedure on an eye  22 . As shown, the eye  22  is aligned to receive an ultra short pulsed laser beam  24  from the laser system  20 . As detailed further below, the pulsed laser beam  24  having selected parameters is generated by a laser source  26 . The laser system  20  includes a focusing unit  28  for focusing the beam  24  to its focal point  30 . Also included in the laser system  20  is a scanning unit  32  for directing the beam  24 . In addition to the laser source  26  and focusing unit  28 , the laser system  20  includes a scanning unit  32  for directing the laser beam  24 . Also provided in the system  20  is a telescope  34  for collimating the beam  24  after it is directed by the scanner  32 . Downstream of the telescope  34  is a reflector  36  that redirects the laser beam  24  toward the eye  22  through the focusing unit  28 . 
     While not directly involved with the generation and control of the laser beam  24 , several sensors are also provided in the laser system  20 . Specifically, the system  20  includes a sensor  38 , preferably a confocal microscope or an optical coherence tomograph, for finding the interface  40  at which photoablation is desired. The system  20  further includes a sensor  42  for detecting whether and where photoablation has occurred. The system  20  may include an additional sensor or sensors  44  for identifying photoablation thresholds as discussed below. 
     In accordance with the present invention, photoablation can be performed to provide intracorneal tissue modification to effect a refractive change in the cornea, to create a flap suitable for a LASIK or LASEK type procedure, to create a passageway or drainage channel in the eye  22 , or to effect any other type of surgical procedure, in whole or in part, known in the pertinent art that requires the removal of ocular tissue. 
       FIG. 2  shows the anatomical structure of the human eye  22  including the cornea  46 , the pupil  48 , the iris  50 , and the sclera  52 . In  FIG. 3  it can be seen that the cornea  46  includes five anatomically definable layers of tissue. Going in a direction from anterior  54  to posterior  56  in  FIG. 3 , the tissue layers of the cornea  46  are: the epithelium  58 , Bowman&#39;s membrane  60 , the stroma  62 , Descemet&#39;s membrane  64  and the endothelium  66 . These corneal layers have distinct photoablation thresholds. For instance, depending on the laser pulse length, the stroma  62  may have a base LIOB threshold of 1, Bowman&#39;s membrane  60  may have a relative LIOB threshold of 2 and the epithelium  58  may have a threshold of 0.5. Between the layers of corneal tissue are the interfaces  40  that are of general importance for the present invention. Specifically, the removal or destruction of the portion of one tissue adjacent an interface  40  can be achieved without damage to the other layer of tissue adjacent the interface  40 . In addition, due to the natural delineation between layers of tissue at the interface  40 , very precise photoablation can be achieved. 
     Referring now to  FIGS. 4 and 5 , a laser beam  24  is shown being generally directed to an interface  40  between a targeted material  68  and a non-targeted material  70 . Specifically, the laser beam  24  is focused such that its focal point  30  is positioned in the non-targeted material  70 . The maximum beam energy density reached at the focal point  30  is insufficient to photoablate the non-targeted material  70  due to the proper selection of beam parameters such as intensity, geometry and pulse duration. Equidistantly spaced from the focal point  30  are beam positions  72  at which a minimum beam energy density is reached. As indicated by the arrow indicating the path of the laser beam  24  in  FIG. 4 , the non-targeted material  70  is positioned upstream of the interface  40 . Therefore, the laser beam  24  does not pass through the targeted material  68  before reaching its focal point  30 . Conversely, in  FIG. 5 , the targeted material  68  is positioned upstream of the interface  40  such that the laser beam  24  passes therethrough before reaching its focal point  30 . Both of  FIGS. 4 and 5  depict a preferred starting point for the photoablation procedure in which the focal point  30  is positioned in the non-targeted material  70  such that photoablation does not occur in the targeted material  68 . 
     Referring now to  FIG. 6 , the laser beam  24  of  FIG. 5  is shown after advancement of the focal point  30  (and the position  72  of minimum beam energy density) toward the interface  40 . Specifically,  FIG. 6  shows that the minimum beam energy density contacts the targeted material  68  and causes a portion  74  of the targeted material  68  to be photoablated. Similarly,  FIG. 7  shows the laser beam  24  of  FIG. 5  after advancement of the position  72  of minimum beam energy density due to an increase in intensity of the beam  24 . Again, the minimum beam energy density contacts the targeted material  68  and causes a portion  74  of the targeted material  68  to be photoablated. 
     Referring now to  FIG. 9 , it will be seen that in the operation of laser system  20 , the performance of the methods of the present invention begins by estimating the depth “d” at which the focal point  30  will be positioned in the eye  22  (action block  76 ). Specifically, the interface  40  between the targeted material  68  and non-targeted material  70  is found. Then the depth necessary to place the focal point  30  in the non-targeted material  70  near the interface  40  is estimated. In  FIG. 6 , the estimated distance  78  is shown establishing the focal point  30  in the non-targeted material  70 . In order to provide the appropriate beam energy densities in the non-targeted material  70  and targeted material  68 , the beam parameters such as the intensity or energy level “E” and the treatment duration “τ” are set (action blocks  80  and  82 ). It is assumed that the ablation energy thresholds of the targeted material  68  and non-targeted material  70  are known before these steps are taken. Of course, the thresholds may be estimated if they are not previously identified. 
     Once the desired parameters are set, the system  20  is activated to generate the pulsed laser beam  24  (action block  84 ). Based on the answer to inquiry block  86 , i.e. whether the non-targeted material  70  (i.e., “M 1 ”) is upstream from the interface  40 , specific steps are taken depending on whether the sensor  42  identifies any photoablated targeted material  68  in response to the laser beam  24 . As is known, photoablation of material such as corneal or biological tissue causes formation of a bubble or plasma that can be sensed by the sensor  42 . 
     Assuming that the non-targeted material  70  is upstream from the interface  40 , then inquiry block  88  is reached. If no targeted material  68  is photoablated, then the position  72  of the minimum beam energy density is advanced (action block  90 ). As shown in  FIGS. 6 and 7 , such advance may be performed by moving the focal point  30  toward the interface  40  or by increasing the energy level of the laser beam  24 . After the advance of the position  72  of the minimum beam energy density, the system  20  is again activated at action block  84  to generate the pulsed laser beam  24 . In this way, the position  72  of the minimum beam energy density is advanced toward the targeted material  68  until photoablation occurs. 
     When photoablation occurs and there is a positive response to inquiry block  88 , then the laser system  20  is executed to scan the laser beam  24  to a new location in the non-targeted material  70  (action block  92 ). Here, a loop such as that discussed above is again encountered to ensure that the position  72  of the minimum beam energy density be advanced toward the targeted material  68  until photoablation occurs at the new location. Specifically, inquiry block  94  requires, if photoablation does not occur, that the position  72  of the minimum beam energy density be advanced toward the targeted material  68  (action block  96 ) before the beam  24  is again activated (action block  84 ). If photoablation does occur, then it is determined whether the treatment duration has expired (inquiry block  98 ). If It has not expired, the method is restarted at the scanning step of action block  92 . If it has expired, then it is determined whether the photoablation pattern of the targeted material  68  is complete (inquiry block  100 ). If the entire pattern is completed, then the procedure is completed and the actions are stopped. If not, the procedure is begun again at action block  82 . 
     Turning back to inquiry block  86 , the situation where the non-targeted material  70  is not upstream from the interface  40  must be addressed. In this case, inquiry block  87  asks whether any targeted material  68  is photoablated in response to the activation of the laser beam  24 . If targeted material  68  is photoablated, then the position  72  of the minimum beam energy density is advanced (action block  89 ). Such an advance may be performed by moving the focal point  30  toward or into the non-targeted material  70  or by decreasing the energy level of the laser beam  24 . After advancing the position  72  of the minimum beam energy density, the laser beam  24  is again activated at action block  84  to generate the pulsed laser beam  24 . In this way, the position  72  of the minimum beam energy density is advanced toward the non-targeted material  70  until photoablation does not occur in response to the activation of the laser beam  24 . This loop ensures that photoablation will occur only in the portion  74  of the targeted material  68  that is adjacent the interface  40 . 
     Once photoablation does not occur in response to the activation of the laser beam  24 , it is known that the focal point  30  is properly positioned in the non-targeted material  70 . The beam  24  can then be scanned to a new location in the non-targeted material  70  (action block  91 ). Here, a loop such as that discussed above is again encountered to ensure that the position  72  of the minimum beam energy density be withdrawn toward the targeted material  68  until photoablation occurs at the new location. Specifically, inquiry block  93  requires, if photoablation does not occur, that the position  72  of the minimum beam energy density be withdrawn toward the targeted material  68  (action block  95 ) before the beam  24  is again activated (action block  84 ). If photoablation does occur, then it is determined whether the treatment duration has expired (inquiry block  97 ). If it has not expired, the method is restarted at the scanning step of action block  91 . If it has expired, then it is determined whether the photoablation pattern of the targeted material  68  is complete (inquiry block  100 ). If the entire pattern is completed, then the procedure is completed and the actions are stopped. If not, the procedure is begun again at action block  82 . 
     As shown in  FIG. 10 , additional steps may be included when the non-targeted material  70  is upstream of the interface  40 . Specifically, the method may include a loop to ensure that the focal point  30  is not positioned too deep within the cornea  46  such that the targeted material  68  is photoablated at a location deeper than the interface  40 . For instance, if photoablation occurs as a result of the activation of the laser beam  24 , then the position  72  of the minimum beam energy density is withdrawn toward the interface  40  (action block  102 ). If, after such withdrawal, photoablation still occurs (as noted at inquiry block  104 ), then the position  72  is withdrawn again. Once photoablation does not occur, the minimum beam energy density is known to be at the interface or slightly within the non-targeted material  70 . Therefore, the position  72  of the minimum beam energy density is advanced toward the targeted material  68  (action block  106 ). If photoablation does not occur in response to this advance (as noted at inquiry block  108 ), the position  72  is advanced again. Upon sensing that photoablation occurs, execution of the scanning process begins at action block  92  and the method returns to the process set forth in  FIG. 9 . 
     Referring now to  FIG. 8 , a corneal flap  110  prepared in accordance with the present invention is shown. The flap  110  is prepared by first photoablating a periphery  112  for the flap  110 . Because the periphery  112  is formed along the interface  40  between two corneal tissues as discussed above, it is much more precise than a periphery cut through a tissue. With the periphery  112  established, an incision can be made extending from the anterior surface  114  of the cornea  46  to the periphery  112  to establish an edge  116  for the flap  110 . Once the edge  116  is created, the flap  110  can be peeled from the remainder of the cornea  46  to expose the surface of the underlying tissue  118 . After exposure, the underlying tissue  118  can be photoablated using an excimer laser (not shown). After photoablation with the excimer laser, the flap  110  can be repositioned over the underlying tissue  118  and allowed to heal. The result is a reshaped cornea  46 . 
     While the particular method and device for performing subsurface photoablation as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of the construction or design herein shown other than as described in the appended claims.