EP1945402A2

EP1945402A2 - Laser femtosecond microtome for cutting out a material slice by a laser beam, in particular in a cornea

Info

Publication number: EP1945402A2
Application number: EP06841997A
Authority: EP
Inventors: François Salin
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2005-11-04
Filing date: 2006-11-03
Publication date: 2008-07-23
Also published as: FR2892962B1; CA2628597A1; WO2007051951A3; US20090018532A1; FR2892962A1; WO2007051951A2; WO2007051951B1

Abstract

The invention relates to a laser femtosecond microtome for cutting out at least one slice of material from a material block by means of a focused laser beam, wherein said material block comprises a front surface and a slice carrying said front surface, the slice extends at least partially on a X, Z plane perpendicular to an axis Z of the block thickness, is separated from the remaining part of the block by a cleavage surface formed by an assembly of bubbles brought together and each bubble is formed in the focus area of at least one convergent laser beam pulse of an optical axis L. According to said invention, the optical axis L of the laser beam convergent part (3) forms an angle ranging from 45° to + 45° with respect to the X, Z plane. An ellipsoid-shaped focus area has the smallest axis thereof in the direction of the axis Y.

Description

Femtosecond laser microtome for laser cutting a slice of material, especially in a cornea

The invention relates to a femtosecond laser microtome for cutting a slice of material in a block of material by means of a focused laser beam. The material may be a cornea of an eye or any other material in which it is possible to obtain cleavage by producing bubbles in focusing areas of the laser beam such as in certain plastics. It has applications in particular in the field of micro-machining parts including optical or in the field of the treatment of visual defects of the eye. In this latter application, it allows in particular the creation of a cavity in the eye compatible with intra-stromal surgery, corneal surgery, correction of myopia, hyperopia or astigmatism.

A large part of the world's population suffers from visual defects. Most of these defects come from a distortion of the eye that is no longer perfectly spherical. Glasses or contact lenses are currently used to correct these defects. In recent years it is possible to correct some of these vision defects by directly sculpting the cornea with a laser. This technique first requires opening a hood on the surface of the cornea to then allow the action of an ultraviolet laser that corrects the shape of the cornea. Finally, it is even possible to eventually correct myopia directly without opening a hood.

Several methods exist for opening this cover. The first uses a mechanical microtome with a cutting blade of a thin slice of cornea. In this method the microtome is called a metal microkeratome. It allows to have a regular surface state that the ultraviolet laser can work. The microkeratome However, the metal has the disadvantage of requiring a material contact with the cornea and therefore a possibility of infection. In addition, there remains a significant proportion of missed cuts due to variations in the corneal dimensional parameters from one patient to another.

The second, called femto-LASER microkeratome, uses a femtosecond laser in the axis of the eye to cut a slice of cornea but the surface state obtained is not satisfactory because the cleavage zone obtained by Bubbles within the cornea are relatively thick and irregular and a postage patch tear is obtained at the interface between the hood and the cornea. However, the femtosecond laser has the advantage of not introducing any risk of infection because there is normally no material contact with the cornea during laser cutting. In practice, so far, it is preferred to use the metal microkeratome to obtain satisfactory results.

Thus the method called LASIK (LAser In Situ Keratomyleusis) for the correction of myopia, which consists in modifying the curvature of the cornea by laser ablation, is the most widely performed refractive surgery procedure in 2002. The first step of a LASIK is the cutting a corneal slice in the form of a superficial cover using a razor blade of a metal microkeratome. This cover must have a thickness of about 150 microns and a diameter of 7 to 9 mm, remains attached to the surface by a tissue hinge respected during cutting. In a second step, the hood is reclined time to perform ablation of surface using an excimer ultraviolet laser. This laser emits ultraviolet radiation at 193 nm strongly absorbed at the surface of the cornea which is then volatilized. The corneal curvature is thus remodeled by selective and internal thinning of the corneal stroma. At the end of the intervention, the hood is simply repositioned. The femto-LASER microkeratome uses a femtosecond laser beam that is focused with a focus at about 150 μm below the surface of the cornea and has an optical axis substantially parallel to that of the eye and therefore corresponds to a frontal application to the cornea of the cornea. the eye of the laser beam. The high intensity produced in the home produces a bubble of vaporized material that causes a local disruption of the cornea. By moving the focus laterally one can thus create a contiguous carpet of bubbles forming a zone of cleavage within the cornea. The cutting of the hood by laser is obtained by performing a sagittal cut from the plane of bubble to the surface. Although this method works, it has the disadvantage of causing a cleavage zone significantly less well defined than with a metal microkeratome and leaves a surface roughness that is detrimental to healing. The cause is due to a location and imperfect shape of the bubbles that comes from the anisotropic spatial distribution of energy deposition of a laser beam around its focus focus.

Indeed, by nature a laser beam is focused with a law of deposition of energy which is largely anisotropic. For example, the shape of the volume formed of points whose illumination is greater than half of the maximum illumination in the case of a Gaussian circular incident beam and of a focusing means with isotropic transfer function can be estimated at home. . The transverse dimension of this volume is related to the "waist" w ₀ (radius 1 / e ² ) and is of the order of 1, 18 W ₀ for the points at half height of the maximum illumination. The longitudinal dimension of this volume, ie in the direction of the focused beam, is given by the Rayleigh length: z _r = πw ² ₀ / λ with λ = λ _o / n. It can be seen that this volume is an ellipsoid which is of revolution in the case of a symmetrical incident beam and an isotropic focusing means. The ratio between the small and the big axis is: z _r / do = πw _o / 1, 18λ. We see so that if w ₀ is much greater than λ, the ellipsoid will be very long longitudinally. For example, for a medium of index n = 1, 3 and w _o = 5 μm, a ratio of the order of 18 is obtained. It is therefore very difficult to obtain a good longitudinal resolution, that is to say in the direction of the optical axis of the incident beam and the cleavage zone is very high and very irregular.

The solution for obtaining small longitudinal dimension bubbles is to use very open optics which complicate and increase the cost of the system and do not allow fields compatible with the LASIK application.

The present invention instead of trying to correct this defect related to the existence of an energy deposit in an iso-energetic distribution (= iso-luminous) anisotropic ellipsoid on the contrary uses it to facilitate and improve the quality of the cut. Indeed, if one illuminates with the laser the eye by the side, that is to say laterally and no longer according to the optical axis of the eye as before, the height of the cleavage zone is given by the small axis of the ellipsoid is a few microns while the depth of field, in the longitudinal direction corresponding to the general plane of cutting, that is to say the long axis of the ellipsoid, facilitates the realization of the cleavage zone . Thus for optical openings of the order of 0.3 to 0.8, the height of the cleavage zone which corresponds to the minor axis, of transverse direction, may be less than one micron.

The invention therefore takes advantage of the ellipsoid shape of the focusing zone, which corresponds substantially to the shape of the bubble created by a laser pulse, on the one hand having the smallest axis of the ellipsoid which determines the height of the zone. cleavage (hence a good accuracy) and the largest axis of the ellipsoid which is in the general plane of the cleavage zone (hence a greater rapidity of cleavage, the bubbles extending widely in said plane of the cleavage zone). Thus, in the microtome of the invention, the optical axis of the laser beam converging towards the focusing zone is arranged substantially laterally with respect to the slice of material to be produced, unlike conventional devices whose optical axis of the laser beam arrives perpendicularly. to the slice of matter. The term "slice" designates an extended flat element or not and of relatively low thickness uniform or not as appropriate.

Thus, the invention relates to a femtosecond laser microtome for cutting by a focused laser beam of at least one slice of material in a block of material, the block having a front surface and the slice carrying said front surface, the slice extending to at least substantially in an X, Z plane perpendicular to a Y-axis of block thickness, the wafer being separated from the remainder of the block by a cleavage surface formed by the joining of a set of bubbles, each bubble being formed in a focusing zone of at least one pulse of the convergent laser beam of optical axis L.

According to the invention, the optical axis L of the convergent portion of the laser beam forms an angle of between -45 ° and + 45 ° relative to the X, Z plane.

In various embodiments of the invention, the following means can be combined according to all technically possible possibilities, are employed:

the optical axis L of the beam forms an angle of between -10 ° and + 10 ° with respect to the X, Z plane and, preferably, the optical axis L of the beam is substantially in the X, Z plane,

the focused laser beam is obtained by focussing an incident laser beam of illumination cross-section defined by a focusing means,

the illumination cross section is chosen from circular or elliptical shapes,

the illumination cross section is circular,

the illumination cross section is non-circular, the focusing means comprises at least one lens,

the focusing means is a lens,

the focusing means is a set of lenses,

the optical transfer function of the focusing means is isotropic or anisotropic,

the focusing means comprises a dynamically addressable wavefront correction system,

the correction system comprises a means chosen from a deformable mirror, a mosaic of micro-mirrors or an optical liquid crystal valve,

the focusing zone has an isoenergetic distribution of bubble formation according to an ellipsoid, the smallest dimension of said ellipsoid being in a direction substantially parallel to the axis Y,

the ratio between the largest axis and the smallest axis of the ellipsoid is greater than 2 and, preferably greater than 10,

the block of material is a cornea of an eye, the Y axis substantially corresponding to the optical axis of the eye,

an adaptation piece in a material of optical index substantially equal to that of the cornea is disposed on and matches at least the frontal surface of the cornea, said piece having an entrance face for the convergent beam so that said beam convergent crosses elements having substantially the same optical index,

the input face of the adaptation piece is plane and is such that the axis L of the convergent beam is substantially perpendicular to said input face,

the adapter piece compresses and deforms at least the cornea,

the space between the input face of the adaptation piece and the focusing means is totally or partially filled with a fluid of index substantially equal to that of the adapter piece or the focusing means; , the focusing zone can be moved along at least the two axes X, Z by actuators under computer control,

the focusing zone is movable along the three axes X, Y, Z by actuators under computer control,

the microtome further comprises means for location, in principle, in at least one axis, of the possible position of the bubble by detecting a point of focus of a light beam that does not produce a bubble,

- The microtome further comprises posterior location means in at least one axis, the position of the bubble by detecting the bubble plasma light.

The invention therefore makes it possible, in the application to corneal surgery, to produce micro-cavities of very low thickness (height) in the eye and thus the production of a very small and therefore precise thickness of the cleavage zone. using a focused laser beam whose optical axis is very far from the optical axis of the eye, the angle between the two axes being greater than 45 °. It is thus possible to make caps for LASIK treatment by cutting with a laser beam focused laterally to the cornea.

The quality and accuracy of the cutting approach the anterior surface of the cornea and produce covers whose thickness can be less than 100 .mu.m.

The invention also provides the possibility of making corrections of myopia without incision in the eye by producing macro-cavities by addition of bubbles in the cornea. This type of treatment is still called intra-stromal correction of myopia. Indeed, the femtosecond laser has been proposed for an intra-stromal LASIK in order to cut inside the cornea a cavity whose collapse causes the variation of the curvature of the eye but the lack of precision of the traditional means frontal with optical axis parallel to the optical axis of the eye does not allow to obtain an accurate correction.

Corneal cuts can also be made for the insertion of implants into the cornea. We can also be satisfied with localized cuts in order to correct residual optical aberrations.

The invention also allows the micromachining of transparent materials, in particular for producing optical components or applied to micro-fluidics or micro-mechanics.

The invention finally relates to an adaptation piece for the microtome according to one or more of the preceding characteristics and which is made of a plastic material of optical index substantially equal to that of the cornea and disposable. The adaptation piece may further comprise one or more of the previously listed characteristics relating to it.

The present invention will now be exemplified by the following description, without being limited thereto, and in relation to: FIG. 1 which schematically represents the convergence of a laser beam in a reference frame X, Y, Z, FIG. 2 which schematically represents in section a side view of the process of cutting a cover of material on the cornea of an eye, Figure 3 which shows schematically in front view the process of cutting a cover of material on the cornea of an eye, Figure 4 which schematically shows in section a side view of a variant of the process of cutting a cover of material on the cornea of an eye, Figure 5 which shows schematically the implementation of the invention with means for retrocontrol a posteriori of the position of the created bubble. Most of the examples of implementation of the invention given below relate to the application to the treatment of visual defects of an eye with the production of a hood resulting from a cleavage zone in the cornea of an eye . More generally, this cleavage zone may be higher or lower depending on the application, in particular of high height by stacking of bubbles in the case of producing a macro-cavity and in particular of low height by producing a single layer of bubbles in the case of a cut of a hood as in the following examples. As an alternative to producing a macro-cavity, it is possible to cut a core (two layers of bubbles separated by the nucleus of corneal material) which will then be expelled from the eye by an incision. The shape of the nucleus can be lenticular (biconvex lens) in the case of the correction of the myopia or biconcave lens in the case of the correction of the hyperopia of revolution or not (if astigmatism to be corrected). Similarly, because the examples relate to a substantially hemispherical eyeball, the general shape of the cleavage zone is related to the general shape of the cornea, in particular because, in the case of cutting a hood that is circular, the latter has a substantially constant thickness. However, and more generally, for example in the case of a micromachining of another type of object, the shape of the cleavage zone will not necessarily be hemispherical but may be flat or have other types of shapes and shapes. in the case of making a wafer (detachable or not the object) its thickness may be constant or not.

We have seen in introduction that already with an incident Gaussian beam, the focusing zone is ellipsoid. It is also possible to accentuate the eccentricity of the ellipsoid or to create other forms of focusing zones and therefore of bubbles using other forms of incident laser beam illumination. Thus, it is possible to use a laser beam whose transverse geometry is not circular. For example, by using an elliptical incident laser beam on the focusing means, a focal spot is obtained whose dimension is still very small in the direction of the optical axis of the eye (Y axis) and large in the other directions. We can thus create in this way bubbles whose dimensions are small, a few microns, in a direction parallel to the optical axis of the eye (according to Y) and large in the other two directions (according to X and Z). This significantly reduces the time required to cut a disk-shaped cover. It is understood that in addition to the form of illumination of the incident beam arriving on the focusing means, a particular spatial transfer function of said focusing means, alone or in combination with the illumination shape of the incident beam, also makes it possible to to obtain a spreading of the focusing zone in a plane corresponding to the general plane of the cleavage zone and a constriction in a plane perpendicular to the cleavage plane.

Arriving through the left-hand part of FIG. 1, an incident laser beam 1 schematized as substantially elliptical passes through a focusing means 2, for example a diopter or lens with an isotropic spatial transfer function, enabling it to be focused towards a focusing zone corresponding to the focus 4 of the optical element. Between the optical element 2 and the focal point 4, the laser beam is convergent 3 and of optical axis L. In the focusing zone, the distribution curve of iso-illumination (or iso-energy) for a level of illumination given, corresponding for example to the level of threshold illumination for the creation of a bubble (for example threshold of breakdown of the material), has a substantially ellipsoid shape whose largest axis is substantially in a plane Z, X and the smallest axis substantially parallel to the Y axis of a three-dimensional repository X, Y, Z. It is also noted that the optical axis L of the convergent laser beam 3 is also substantially in the plane Z, X in FIG.

It is understood that a laser pulse in the material will form a bubble whose shape will be close to an ellipsoid whose major axis will be in the plane Z, X. It is also understood that in the case of the realization of a hood on a cornea the slice of material forming the cover is substantially in a plane parallel to the Z, X plane and that the cleavage zone has a height along the Y axis as small as possible. Thus, not only the precision of the cut is obtained by the low height of the bubble corresponding to the small axis of the ellipsoid but the cutting efficiency is increased by the long length of the bubble corresponding to the major axis of the ellipsoid in the cleavage plane.

Thus, and as applied to the cornea 6 of an eye 5 and shown in Figure 2, the Y axis is substantially parallel to the optical axis of the eye and the Z plane, X is substantially parallel to at least a portion the cleavage zone and hood so that the cleavage zone is as low as possible (corresponding to the small axis of the ellipsoid).

In FIG. 2, for simplicity, the refractive effects have not been taken into account because part of the convergent laser beam 3 passes through part of the cornea 6 before reaching the focus zone 4. However, if we want to limit or avoid these effects, we can implement two solutions, the first consisting in inclining the optical axis L with respect to the plane Z, X and the second by implementing an optical adaptation piece 8 as will be explained in connection with Figure 4.

In order to obtain an extended cleavage zone, the focusing zone is progressively shifted to produce a two-dimensional array of rows x columns of bubbles (in the case where one would like to obtain a cover for the LASIK application) or three-dimensional if we want to achieve a macro-cavity (application to the in-situ treatment of myopia for example).

The displacement / trajectory of the focusing zone for producing this bubble matrix assembly is preferably starting with the production of bubbles in the portion farthest from the laser source and progressively approaching it so that the beam preferably converge passes through a portion of cornea not yet cleaved. Thus one can realize the bubbles starting with a scan along the X axis for a position on Z given but distant distance from the source, then reduce the distance on Z of a step and make a new scan according to X the along the X axis, and repeat iteratively by gradually reducing the distance on Z as shown schematically in Figure 3. In the case where a macro-cavity is made, we will make several scans at different positions along Y before decrementing the distance Z. Other scanning possibilities are possible but they lead to that part of the converging beam passes through a portion of the cornea already having bubbles such as scanning Z in each case starting at a distance far from the source and incremental displacement on X.

It will be understood that in the case of a cornea 6 which is a curved body, the points will be made on a surface depending on the needs and for example for producing a substantially constant thickness cover of approximately 150 μm the cleavage zone 7 must substantially follow the shape of the outer surface of the cornea at least in its central part. The focusing zone is then located at about 150 microns below the surface of the cornea. The Z position is fixed by the position of the lens. The hood which is circular can have a diameter of 9 mm for example. In the case of a hood that has to be folded down, we finish the cut by a circular movement accompanied by a displacement along the Y axis to cut the edges of the hood.

More generally, in order to cut a hood, it is possible to give the focusing zone a trajectory that follows the curvature of the cornea which may, in a variant, possibly be flattened by means of an adapter piece 8 whose index optic is close to or equal to that of the cornea as will be seen in relation with Figure 4.

For the implementation of the invention, the position of the focus is modified using a device allowing electronic control and computer to move the lens, more generally the focusing means or any other optical means placed on the path beam and can act on the position of the focus, at least along the Z axis and, preferably on all axes in order to move the focus area throughout the X, Y, Z. An automatic feedback system a priori of the position of the focusing zone can also be implemented, either additional lighting is implemented in the path of the laser beam is that the power of the laser can be reduced to a level below the creation of bubbles and that an optical measuring device on the eye detects the position of the focussing zone (focus) thus illuminated by the additional illumination or the laser under low power and allows the comparing with an expected position and generating a possible correction signal to the device moving the position of the focus. The additional lighting may be an LED or another laser but power not allowing the creation of bubbles. Once the position of the focus area is correct, the femtosecond laser is activated for one or more light pulses creating the bubble. Regardless of an automatic feedback system, the use of additional lighting may allow the operator to see where the focus area is. We must take into account the possible difference between wavelengths of the laser and additional lighting and ensure that the optics used allow the same coincidence of focus for both wavelengths or that computer means take into account.

The feedback can also be done a posteriori by detecting the position of the plasma created during the creation of a bubble along at least the Z axis and, preferably along the three axes as shown in Figure 5 which will be explained later.

FIG. 4 shows a variant of the invention implementing an optical adaptation piece 8 which is in a material of optical index substantially equal to that of the cornea, that is to say an index of about 1, 33. This piece 8 is for example plastic optionally disposable because in contact with the eye 5. This piece 8 is disposed on the front part of the eye and marries at least the front surface of the cornea. The part has a lateral plane entry face for the converging beam 3 such that the L axis of said converging beam is substantially perpendicular thereto. Thus, the convergent beam passes through elements, piece and cornea, having substantially the same optical index which avoids or limits the effects of refraction. The part 8 is preferably integral with the equipment comprising the focusing means 2 which has an optical index of approximately 1.55 in order to be able to maintain stable dimensional and structural relationships between all the optical elements. The focusing means is here represented at a distance from the inlet face of the adaptation piece 8, between the two, an air gap is left, or, alternatively, a space filled with a gel or a optical adaptation liquid. In a variant not shown, the adaptation piece 8 comprises the focusing means, the input face of said piece 8 being shaped to focus the laser beam. In some cases it is possible to use a part 8 which, in addition, compresses and deforms at least the cornea of the eye. In a variant, the input face of the adapter part which is plane can be inclined with respect to the optical axis L of the convergent beam 3. In a variant, the input face of the adaptation piece is not plane but has a curvature in order to modify the convergence of the convergent beam 3.

It may be noted that the focusing means 2 may comprise more than one lens, in particular to improve the characteristics of the focal point and to guarantee a small extension along the Y axis in the whole of the XZ plane.

External means for modifying the wavefront can be introduced on the path of the beam 1 1 in order to correct the geometrical aberrations of the focusing means 2. These means for modifying the wavefront can be in particular a deformable mirror, a mosaic of micro-mirrors or an optical liquid crystal valve. These means for modifying the wavefront are then dynamically activated in relation to the position of the focal point in the field of the lens by a computer means as a function of information previously stored on the geometric aberrations of said focusing means 2.

The additional means shown in Figure 5 allow a retrospective identification of the position of the bubble that has just been created by detecting the light waves of the corresponding plasma. An adaptation piece 8 is disposed on the cornea and the laser beam 1 1 arrives laterally passing through a blade 10, the focusing means 2, a space 9 optionally filled with a fluid (including gel) optical adaptation and the lateral entry face of the part 8. The focusing means 2 is maintained in a stable relative position relative to the fitting part 8 by spacers and / or by a encapsulation also for confining the fluid in the space 9. The plasma light waves are firstly detected forward by a beam 17 focused in 15 to a first detector 16 preferably matrix on two dimensions and at least 2x2 . The light waves of the plasma are also detected by a second detector 14 preferably also matrix on two dimensions and at least 2x2, the corresponding beam 12 having been returned by the blade 10 and focused 13 on the second detector 14. One or both detectors can be implemented. The first detector 16 alone may be sufficient for three-dimensional bubble position detection, the two-dimensional array sensor and focusing adjustment means of the focused image on the depth detector. For the second detector 14, according to the same principle it is possible to obtain the position of the bubble in the three dimensions, but the axes given by the matrix sensor will be different from the first case because the observation is lateral rather than frontal.

It is understood that these examples are purely indicative and that the invention can be declined in various other obvious ways, without that the technician has to do work of inventiveness and without departing from the general scope of the invention as delimited by the claims.

Claims

1. A femtosecond laser microtome for cutting by a focused laser beam of at least one slice of material in a block of material, the block having a front surface and the slice carrying said front surface, the slice extending at least in part substantially in a plane X, Z perpendicular to a Y axis of thickness of the block, the wafer being separated from the remainder of the block by a cleavage surface formed by the meeting of a set of bubbles, each bubble being formed in a focusing zone of at least one pulse of the convergent laser beam of optical axis L, characterized in that the optical axis L of the convergent portion (3) of the laser beam arrives substantially laterally with respect to the block at an angle of between -45 ° and + 45 ° with respect to the X, Z plane

2. Microtome according to claim 1, characterized in that the optical axis L of the beam is at an angle of between -10 ° and + 10 ° with respect to the X, Z plane and, preferably, the optical axis L of the beam is substantially in the plane X, Z.

3. Microtome according to claim 1 or 2, characterized in that it comprises a focusing means (2) and that the focused laser beam (3) is obtained by focusing by said focusing means (2) of a laser beam incident (1) having a defined illumination cross-section.

4. Microtome according to claim 3, characterized in that the focusing means (2) comprises at least one lens.

5. Microtome according to claim 3 or 4, characterized in that it comprises means for the focusing zone to have an isoenergetic distribution of bubble formation according to an ellipsoid, the smallest dimension of said ellipsoid being in a direction substantially parallel to the Y axis.

6. Microtome according to claim 5, characterized in that it comprises means for the ratio between the largest axis and the smallest axis of the ellipsoid is greater than 2 and, preferably greater than 10.

7. Microtome according to one of claims 3 to 6, characterized in that the focusing means comprises a dynamically addressable wavefront correction system.

8. Microtome according to any one of the preceding claims, characterized in that it further comprises means for location a posteriori along at least one axis of the position of the bubble by detecting the bubble plasma light.

9. Microtome according to any one of the preceding claims, characterized in that the block of material is a cornea (6) of an eye (5), the Y axis substantially corresponding to the optical axis of the eye.

10. Microtome according to claim 9, characterized in that an adaptation piece (8) in a material of optical index substantially equal to that of the cornea is disposed on and matches at least the frontal surface of the cornea, said a part having an input face for the convergent beam so that said convergent beam passes through elements having substantially the same optical index, the input face being lateral.

1 1. Microtome according to claim 10, characterized in that the input face of the adapter piece (8) is flat and is such that the L axis of the converging beam is substantially perpendicular to said input face.

12. Microtome according to claim 10 or 1 1, characterized in that the adapter piece (8) compresses and deforms at least the cornea.

13. Microtome according to any one of claims 10 to 12 characterized in that the space between the inlet face of the adaptation piece (8) and the means of focusing (2) is completely or partially filled with a fluid of index substantially equal to that of the adapter piece (8) or the focusing means (2).

14. Adaptation piece (8) for microtome, characterized in that it is specially adapted for implementation in the microtome of any one of claims 9 to 13 and is made of a plastics material. optical index substantially equal to that of the cornea, that it is disposable, and that it is intended to be arranged on and marry at least the frontal surface of the cornea, that it has an entrance face for a convergent portion of a laser beam, said input face being lateral.