Patent Application
20230201036
Example embodiments of the invention relate to a focusing optical unit for a UV laser-based system for vision correction (UVL-LVC system) having a UV laser source for providing laser radiation and a scanning system for lateral scanning of the laser radiation in the x- and y-directions. An example embodiment of the invention furthermore relates to a planning unit for generating planning data for a UV laser-based system for vision correction having a UV laser source for providing laser radiation, a scanning system for lateral scanning of the laser radiation in the x- and y-directions, a focusing optical unit for directing the laser radiation to a work surface and a control unit designed to control the UVL-LVC system taking the planning data into consideration. Finally, an example embodiment of the present invention relates to a UV laser-based system for vision correction, comprising a UV laser source for providing laser radiation, a scanning system for lateral scanning of the laser radiation in the x- and y-directions, a focusing optical unit, a planning unit for generating planning data and a control unit designed to control the UVL-LVC system taking the planning data into consideration.
UVL-LVC systems currently in use, for example the MEL systems by Carl Zeiss Meditec AG, the Amaris systems by Schwind eye-tech solutions GmbH or the Micron systems by Excelsius Medical GmbH, are systems for vision correction that have been successfully employed for a long time. Nevertheless, they have a number of deficiencies or disadvantages, for which solutions are intended to be revealed here.
A rigid laser beam guidance system is provided in current UVL-LVC systems, virtually without exceptions. Although this facilitates safe laser beam guidance, it requires that the patient on a patient couch is moved under a fixed system aperture in x, y, z coordinates by operation of said patient couch until the patient's eye intended to be treated is correctly positioned in relation to the optical axis of the system. An exception is formed by the system described in US 2013/0226157 A1, in which the laser arm, rigid per se, is positioned as a whole over the patient, albeit in a manner which still requires the positioning of the patient by way of the patient couch. However, for safety reasons, the latter frequently requires the patient couches to be electrically and/or mechanically connected to the laser base unit, which in turn requires a system certification and large amounts of space.
Additionally, there is a conventional manual static alignment of the eye in relation to cyclorotation in such UVL-LVC systems, i.e., without an automatic correction with the aid of registration data, by rotating the patient's head on the couch under visual monitoring. As a rule, this is not possible with great accuracy and a rotation of the head of the patient during the treatment is not precluded either, which is critical for the treatment results if there is no automatic cyclorotation correction.
Moreover, contact interfaces for affixing the eye are known in some current UVL-LVC systems. However, in such UVL-LVC systems, for example as described in US 2013/0226157 A1 and U.S. Pat. No. 9,592,156 B2, these are only implemented for eye stabilization purposes and do not adopt a truly active role. Work is carried out entirely without contact interfaces in many systems.
Large working distances between laser exit aperture and eye were realized with the introduction of spot-scanning UVL-LVC systems. This was also implemented against the background of using microkeratomes, which were used with the patient on the patient bed of the system, in order to cut the LASIK flap, that is to say an opening in the cornea that can be folded to one side. Among other things, this also required great working distances. Eye trackers for registering and compensating the eye movements were introduced at a later stage, and hence the eye movements during the ablation, which arose due to an unaffixed eye, were compensated. The overall optical system concept accompanying this—which in principle is very similar in the various systems—may also be considered to be disadvantageous.
Various problems arise in relation to eye tracking during the application thereof. Overall, the registration speed for the eye movement is limited and the adjustment of the scanner mirrors for correcting the pulse coordinate requires a finite amount of time. In the context of the system performance, the response to the eye movement (“response time”) is always delayed and hence never exact. In the case of current, fast eye tracker systems (with the repetition rate of approximately 1000 Hz), this is no problem for the lateral correction (x, y shift). However, the currently fastest system already responds to the limitations since the eye tracker speed is even below the limit for the repetition rate. In a manner of speaking, a prediction about the future movement of the eye is made on the basis of the preceding movement trajectories. Naturally, this is an oversimplification since eye saccades/nystagmus correspond to statistical movements of the eye in the broadest sense. This also highlights the fact that an increase in the repetition rate is hardly advantageous with current technology, even though this would be of interest for certain applications and in certain ablation timeframes (thermal controlled/mild ablation).
Moreover, purely lateral tracking is a limitation since the rotation of the eye about the z-axis (“dynamic cyclorotation”) and the roll movements about the horizontal and vertical eye axes have to be taken into account. Finally, the distance (z-distance) to the laser exit aperture may also vary, which can likewise be compensated for by appropriate tracking. Despite all technical intricacies and correction options, these systems however are not able to respond exactly to the eye position: The limited quality of the registration and the speed of the registration and correction do not allow this. Normally, the influence on the refractive results is very small and hardly detectable.
However, there are further problems with the eye tracking in the case of uncooperative patients, in the case of whom there are fixation instability, nervousness, cognitive deficits or problems with perceiving the fixation target. Then, the eye surgeons must affix the eyes during the ablation manually by application of a clamp or foam spatula in order to even make ablation possible. This occurs relatively frequently and is due to the fact that the eye movements are guided out of what is known as the (limited) “eye tracker hot zone” and the system must then stop.
In this context, the production of prismatic errors by the eye tracker must also be mentioned. The ablation profiles are not applied in the correct plane, that is to say not to the surface normal, i.e., perpendicular to the visual axis. This may happen if the patient by preference fixates in a largely fixed but “incorrect” direction, that is to say, e.g., permanently looks in a fixed direction that does not correspond to the center of the “fixation cloud” (depending on refractive deficit and treatment duration, the patient can no longer see the fixation target in focus during the operation).
Additionally, eye trackers do not work for all eyes on account of problematic eye color and/or lack of contrast. Percentage range outage rates of the eye tracker occur in practice. All that eye surgeons can then do is decide between terminating the operation and deactivating the eye tracker, which is then accompanied by the risk of an inaccurate correction.
Constant ambient conditions above the operation site cannot be set, or can only be set to a very qualified extent, in current systems.
It is known that the influence of varying ambient conditions, for instance air humidity and changes in the hydration state of the cornea accompanying this, the composition of the air (in the case of evaporation of solvents for example) or the temperature, on the refractive results may be significant. It is also known to be advantageous to ensure that the hydration state of the cornea is maintained during the ablation, or to ensure that drying out of said cornea is avoided. A distinction has to be made between two effects for the hydration: a) the physiological differences in the hydration of the cornea as a distribution over various patients, and b) the maintenance of the hydration during the ablation itself. Both influences lead to an increased variation in the refractive result (for example via an increased variation in the “attempted vs. achieved” prediction). The literature contains many investigations in this respect. Accordingly, the influence of the hydration of the cornea is significant in particular.
A further, very important factor that influences the refractive results is related to the amount and accumulation of ablation products (“debris”) over the ablated cornea, that is to say the operation site. It is sufficiently well known that the radiated-in UV ablation pulses are absorbed and scattered in the debris. This uncontrollably modifies the effective pulse fluence, which decisively controls the ablation process. This may lead to significant fluence deviations in the sequence of ablation pulses. By way of example, there is the risk of central steepening arising in the cornea post surgery in the case of myopia treatments (central islands). Therefore, current systems usually have aspiration structures or a combined air supply and aspiration structures for the debris. However, truly effective removal of the debris is only implementable to a limited extent in practice as a result of the large distance from the supply and removal lines. In principle, this would require a replacement of the entire air volume above the cornea to be treated between successive pulses in the case of pulse repetition rates between 500 Hz and 1000 Hz. As a worst-case scenario, this may lead to “skew” ablations and hence, for example, to induced coma or SIA (surgically induced astigmatism)—especially in the case of non-optimal or non-directed aspiration of the ablation products.
Moreover, there is the risk of corneal dehydration in the case of systems which additionally supply air. This can likewise only be partly prevented. Overall, the operation site cannot be decoupled from the remaining operative surroundings (e.g., airflows in the room) as a result of the open arrangement of current systems either.
The sterile and secure placement of the flap is of extraordinary importance for a LASIK procedure. Typically, a flap is only 100 μm thick and, following the LASIK incision, only fastened to the cornea by way of a very narrow “hinge”. Maintaining the hydration of the flap is very important for pathological reasons and also for maintaining the shape of the flap since dehydrated flaps shrink within seconds. Following the treatment, a shrunken flap no longer “fits” well into the stromal bed (which naturally is also due to the change in shape of the stromal surface as a result of the ablation), which may in turn lead to postsurgical complications (e.g., epithelial ingrowth). Where possible, flaps should not be bent, pulled or otherwise stressed either. Hence, experts these days hardly still use the “calzone technique” that was employed in the past. Moreover, it is imperative to avoid the flap coming to rest in possibly non-sterile regions of the eye. Despite the sterile preparation of the eye, this may nevertheless occur, for example as a result of the tear film or contact with non-sterile parts of the lids.
In want of a solution that is integrated in the current systems, some users cut their own flap repositories from sterile foam spatulas (or similar materials), which are then moistened and serve as safe and sterile rest for the sensitive flap. Thus, a solution is sought after in order to put an end to this situation.
On account of the relatively large working distances A of existing UVL-LVC systems from the eye, there is hardly a difference there in the focusing plane. Therefore, these UVL-LVC systems can be considered to be virtually telecentric on the image side.
On account of the relatively large working distance of known UVL-LVC systems from the patient's eye, it is difficult to impossible to use reflections from the cornea of the patient's eye for analysis purposes, which as a consequence makes a centration difficult as well. The influence of an inaccurate centration is known and discussed multiple times in the literature. The “common” argument that centration errors, that is to say deviations of the ablation center from the target positions on the cornea, as are typically given by the “ophthalmic pole” for centration on the visual axis and as are referred to as decentrations below, have no influence on spherical corrections is physically true only in certain cases, for example spherical corrections on spherical corneas. However, the visual physiology, inter alia, is not considered in this case. As a rule, decentrations will lead to a shift in the physiological visual axis. When processing the visual impression in the brain, the eye is “rotated” by the eye muscles such that the light from the fixated object continues to fall on the point of sharpest vision, which in principle compensates the prismatic offset (“tip/tilt”). This may lead to problems in the case of binocular vision (stereopsis), for example, which problems are known from investigations in relation to poorly centered spectacle lenses, for example.
At the latest in the case of aspherical corrections on ellipsotoric corneas, which corresponds to the real, actual scenario, a decentration also leads purely physically to a non-attainment of the sought-after correction.
There is also no need to explain further that decentrations have a significant influence on the results of “customized ablation”, as this leads to the induction of higher aberrations (“night vision complaints”, etc.) and hence also to an influence on the refractive result. Hence, a centration that is as exact as possible is the absolute basic precondition for a good result in the case of both topography and wavefront corrections.
Aberrations (or optical modes) couple under decentration. As a result of defocus and cylinder coupling to higher-order aberrations (coma, spherical aberration, higher-order astigmatisms) or of aberrations occurring in natural (aspherical) eyes, decentrations in real eyes are critical, even in the case of pure spherocylindrical corrections. By way of example, in the case of a decentration, coma couples to astigmatism and defocus or spherical aberration couples to coma, astigmatism and defocus. Here, a few examples should be provided for an optical diameter of 6 mm, the examples initially only demonstrating the effects for aberrations up to the 3rd order (the sum of the mode indices is 4). The calculations follow from the coordinate transformation of optical modes:
Until now, only optical modes manifesting themselves in ablation profiles in the optical zone were considered. The transition zones have not yet been mentioned. However, in this context decentrations also mean that transition zones may reach into the optically active zone, especially in the case of hyperopia corrections. This then leads to disturbances in the case of mesopic to scotopic light conditions (“night vision complaints post surgery”; this does not refer to night myopia), and hence to patient dissatisfaction.
Pupil centrations (centration in relation to the CSC, “corneal sighting center”) can be brought about well and reliably in refractive surgery by application of eye tracking systems (“eye tracker”) as integrated pupil recognition. However, this type of centration is not the preferred choice since it has in the meantime been settled in the art that a centration in relation to the ophthalmic pole (visual axis, coaxially sighted corneal light reflex, “CSCLR” condition, see below) would be correct. Experience has shown that small and medium myopia corrections are very uncritical in this case. Relatively large astigmatisms and myopia corrections and, in particular, hyperopia corrections are more difficult. This is because hyperopic eyes are typically characterized by a non-negligible angle between the pupil axis (also referred to as pupillary axis) and the visual axis (“angle kappa”). In this case, corneal sighting center and ophthalmic pole are no longer sufficiently close together, leading to a difference between “angle lambda” and “angle kappa”. Moreover, the pupil and the pupil center are no fixed marking. Both vary with the lighting conditions.
A centration on the visual axis is implemented in the case of the current laser systems by searching for the first Purkinje image of the fixation laser (“target”). Under patient fixation, the recommended CSCLR condition is met when the Purkinje image comes into the center of the system optical unit and the optical system axis and the visual axis become coaxial. Finding this reflection is not trivial in current systems. This is made even more difficult in the case of less cooperative patients, for example who exhibit weak fixation, since the reflection then continually disappears. Moreover, the “parallax error” of the surgical microscope, that is to say different directions for the reflection in the right and left observer eye as a result of the binocular arrangement, makes a correct alignment more difficult.
There is no automatic centration in accordance with CSCLR by way of Purkinje images, and also no prospect thereof, in current systems. As it is carried out reliably and automatically by the systems, in contrast to CSCLR centration by use of Purkinje images, this is probably one of the reasons why the pupil centration—despite the problems accompanying this—is currently preferred by many users.
A manual centration on the vertex (CV), which generally represents a reference center for the topography, by entering displacement coordinates is frequently carried out for topography-guided corrections, but also in the case of standard spherocylindrical corrections. The latter is possible since, in the case of normal eyes, the positions of the CV (“corneal vertex”, that is to say the intersection point of the keratometric axis on the cornea under patient fixation) and the ophthalmic pole (OP), that is to say the visual axis, are sufficiently close together. This in turn is due to the fact that the center of curvature of the cornea approximately coincides with the second image-side optical node of the eye (cf. Gullstrand, Liou-Brennan eye models). An automated centration with respect to the vertex is not found currently. The user frequently shifts the treatment center manually purely on the basis of the visual comparison with a topography measurement. Or they enter displacement coordinates into the system, which are generally specified in relation to the pupil center (CSC) and, for example, are taken from a topography measurement. A problem in both cases is that the pupil diameter during the topography measurement does not correspond to the pupil diameter under the laser on account of differences in lighting. The frequently arising shift of the pupil center with the pupil size then leads to a non-optimal centration as the corneal vertex has not been determined correctly.
Current UVL-LVC systems offer no method of tomographic alignment of the anterior chamber and the tomographic centration. By way of example, this may be important in the case of corneal irregularities (e.g., caused by traumas or brief swelling of the corneal surface as a result of bubbles following femtosecond laser flap generation), which lead to the corneal vertex and/or the Purkinje image not being found correctly or, expressed differently, a position being identified as the vertex which does not correspond to the normal physiological vertex position. The pure “surface information” from the cornea is then not suitable, or not sufficiently well suitable, for a determination of the optimal centration.
Example embodiments of the present invention describe apparatuses which address the aforementioned problems of currently used UVL-LVC systems. In particular, embodiments of the present invention facilitate improvement of the predictability of the refractive results following a correction by application of UVL-LVC systems.
A first aspect of the invention relates to a first variant of a focusing optical unit for a UV laser-based system for vision correction (UVL-LVC system) having a UV laser source for providing laser radiation (for example pulsed laser radiation) and a scanning system for lateral scanning of the laser radiation in the x- and y-directions. The scanning system can be additionally designed to scan in the z-direction. The focusing optical unit serves to focus the laser radiation in a focal field. A laser beam provided by the laser source and shaped by the focusing optical unit has what is known as a “spot” in the focal field (which is also referred to as focus plane, focal plane or focusing plane). For example, the spot has a well-defined “spot diameter” or a “spot size” of 0.3 mm to 1.5 mm, for example 0.5 mm to 1.0 mm. Thus, the focal field is described by the requirements in respect of a lateral extent of the laser radiation. The numerical aperture of a laser beam provided by the focusing optical unit can be less than 0.1, for example less than 0.08. This yields a great depth of field for the laser beam. Hence, the focus of a laser beam need not be in the focal field; it may also be situated a few depths of field in front of or behind the focal field. The lateral extent of the spots is also maintained in that case.
The focal field can be partly or completely identical to a work surface suitable for a treatment for vision correction. For therapy, the eye to be treated or the cornea thereof is typically positioned in or close to this work surface.
According to the invention, the focusing optical unit has a first lens arrangement, which is designed to provide a convergent focal field. The convergence of the focal field is characterized in that a totality of laser rays (for different locations in the focal field, provided by way of the scanning system of the UVL-LVC system) leaving the focusing optical unit run convergently to one another. This means that the totality of the laser rays (and their beams) leaving the focusing optical unit have a common point of intersection. There is no such point of intersection for a telecentric or divergent focal field.
The first lens arrangement of the focusing optical unit may have one or more lenses. The first lens arrangement is for example designed to guide UV light without noticeable losses. To this end, the transmission of the first lens arrangement is particularly high, especially in a wavelength range between approximately 193 nm and 213 nm. By way of example, the first lens arrangement may contain glasses with CaF or fused silica and may be provided with suitable optical coatings.
The focusing optical unit according to the invention follows an entirely novel approach for the ablation geometry. An optical concept differing from all other UVL-LVC systems is implemented in this case. This novel concept allows fundamental improvements to be made in many areas of UVL LVC; in particular, an improvement in the predictability of the refractive results is made possible.
For elucidation purposes, the characteristics of the ablation geometry allowed by the focusing optical unit according to the invention are described in comparison with optical systems for UVL-LVC systems according to the prior art. In the case of conventional UVL-LVC systems, the rays are incident on the cornea of the eye at a significant angle (in relation to a perpendicular incidence) since the typical radius of curvature RC of the human eye is approximately 7.86 mm. The working distance Δ of typically 250 mm is also relatively large for these systems.
In the prior art, the focusing optical unit is typically configured such that there is telecentric focusing of the ablation pulses. Expressed differently: if an appropriate beam (also referred to as laser beam) is assigned to each xy-position on the eye provided by the scanner, these beams are only displaced and not tilted with respect to one another. Telecentric focusing occurs. The centroid rays (or chief rays) of the beams have angles with respect to the surface normal of the cornea of the eye which become ever larger with increasing distance from the apex of the cornea and which deviate ever more strongly from a perpendicular incidence on the cornea.
Alternatively, the focusing optical unit in the prior art is designed such that there is divergent focusing of the ablation pulses. Expressed differently, if the totality of the beams are considered, these leave the focusing optical unit in divergent fashion. Hence, the angles between the centroid rays of the beams and the surface normals of the cornea of the eye become even larger with respect to a perpendicular incidence with increasing distance from the apex of the cornea than in the case of the above-described focusing optical unit (with telecentric focusing) according to the prior art.
This is very different in the case of the focusing optical unit according to the invention. As stated previously, the latter has a convergent focal field. As a result of the convergence of the focal field, the laser radiation is incident on the cornea at an angle that is closer to perpendicular incidence than what can be achieved according to the prior art, for all locations in the focal field. The advantages of this geometry are discussed below.
For example, the first lens arrangement of the first variant of the focusing optical unit according to the invention has more than one lens for the provision of the convergent focal field. For this purpose, the lenses can be configured like in the case of a microscope objective which provides the convergent focal field on the image side (on the side of the eye or its cornea) in the case of an oblique incidence of the laser ray (into the focusing optical unit) on the object side. The object-side oblique incidence into the focusing optical unit can be realized by way of suitable scanners of the UVL-LVC system. Thus, laser beams deflected by the scanners are directed to the convergent focal field through the optical units as “spots”.
The challenge lies in reducing the physical aberrations in the case of oblique incidence and the aperture errors of the focusing optical unit because the imaging quality of the optical unit otherwise quickly reduces significantly outside of the paraxial range, that is to say with increasing distance from an optical axis. By way of example, this would lead to the spots produced by the system in the convergent focal field varying both in terms of size and shape (and hence in terms of the fluence distribution), for example as a result of astigmatic aberrations. Moreover, the spot positions would be displaced by stigmatic distortions (e.g., a barrel distortion, pincushion distortion). In the combination of the effects, the (sought-after) spots with the corresponding unchanging quality would not be achieved depending on the scanning direction in the scan field.
To reduce the aberrations to the necessary degree (requirements in respect of the diameter changes and form changes of the spots in the convergent focal field), that is to say in order to achieve a sufficient quality of the imaging, the focusing optical unit can either be composed of spherical optical units and/or be realized by aspherical optical units or contain aspheres. Using the latter, it is possible to reduce the installation size of the objective (focusing optical unit) in particular. In the case of a desired spot diameter of typically round shape with a full width at half maximum (FWHM) ranging from 0.3 mm to 0.8 mm in the focal field (in the case of a supergaussian fluence distribution in the laser beam), the diameters and shape changes should be located in the range below 20%, for example below 10% (RMS radius deviations <50 μm). The optical units may additionally or alternatively have free-form surfaces. Alternatively, the focusing optical unit may also have (imaging) diffractive elements, for example in the form of radially symmetric diffractive structures applied to a curved lens.
According to an example configuration of the first variant of the focusing optical unit, the convergent focal field has a diameter of at least 6 mm, for example at least 8 mm, in another example at least 10 mm.
In this way, a diameter of approximately 6 mm (or 8 mm, 10 mm) can be obtained in the work surface suitable for a treatment for vision correction.
In this case, the focusing optical unit is for example configured such that the above-described quality of the imaging is observed over the entire diameter of the curved surface.
For example, the optical system of the UVL-LVC system is designed such that the laser radiation is fed to the focusing optical unit according to the invention such that the claimed diameter of the convergent focal field is served. The scanning system can be embodied accordingly to this end.
The optical system of the focusing optical unit with such a diameter of the convergent focal field allows a fixed offset to be applied to the scanner coordinates in order thereby to shift the treatment center with respect to an optical system axis of the UVL-LVC system, which for example runs centrally through the focusing optical unit.
According to an example configuration of the first variant of the focusing optical unit, each location in the convergent focal field has a local center of curvature on the side of the convergent focal field facing away from the focusing optical unit. In other words: If the convergent focal field is locally assigned a curvature—at the location where the laser radiation impinges on the convergent focal field—then this local curvature can be assigned a local center of curvature (for example, by way of an approximation to a spherical curvature, with the center of curvature corresponding to the center of the sphere or ball). As considered from the first variant of the focusing optical unit, this center of curvature is behind the convergent focal field. The curvature of the convergent focal field consequently has the same sign as the curvature of the eye whose vision should be corrected. The convergent focal field has a finite radius of a focal field curvature. For example, the convergent focal field has a focal field curvature with a radius RS ranging from 8 mm to 50 mm, for example from 10 mm to 30 mm, in another example from 12 mm to 20 mm. The focal field curvature may also have a radius RS from 6 mm to 25 mm, from 7 mm to 20 mm, or from 8 mm to 16 mm. The focal field curvature may vary locally over the focal field within the specified limits. For example, the focal field has a focal field curvature that corresponds to a sphere or an asphere; an aspherical form is preferred, in particular, when a large focal field diameter (for example of at least 6 mm) is provided.
In a a particular example configuration, the focal field curvature for the ablation is designed such that the latter corresponds to the typical radius of curvature of the cornea RC (“convergent focal field ablation”). This realizes a virtually perpendicular incidence of the incident laser radiation on the cornea. However, on account of limitations when parameterizing the optical system of the focusing optical unit and the required working distance Δ for clinical practice, at least focal field curvatures RS that are very close to RC are possible. This is desirable in order to obtain a significant reduction in the fluence losses (see below), in order to achieve the advantages described there.
If the focal field has a curvature RS, it is possible to determine a difference radius of curvature RΔ with respect to the cornea with the radius of curvature RC. Said difference curvature corresponds to an “effective” corneal curvature for light incident from the z-direction (i.e., parallel to an optical axis of the focusing optical unit). As a result of the difference calculation, the cornea is “bent” upward—figuratively speaking—by the focal field radius of curvature RS, then allowing the calculation of a fluence loss to be carried out in a simplified manner, that is to say on the basis of an “effective” corneal curvature (with an effective corneal radius of curvature RΔ). This effective corneal curvature is less than the actual corneal curvature. Consequently, with a typical corneal radius of curvature of RC=7.86 mm, values of approximately R6hd Δ≈450 mm to approximately RΔ≈15 mm arise for the effective corneal curvature for focal field radii of curvature with values between approximately 8 mm and approximately 16 mm. The advantage of an improved fluence loss function appears clearly in these ranges, as is yet to be shown below.
The invention relates to a second variant of a focusing optical unit for a UV laser-based system for vision correction (UVL-LVC system) having a UV laser source for providing laser radiation (for example pulsed laser radiation) and a scanning system for lateral scanning of the laser radiation in the x- and y-directions. The scanning system can be additionally designed to scan in the z-direction. The second variant of the focusing optical unit serves to align the laser radiation. According to the invention, the second variant of the focusing optical unit has a second lens arrangement, which is designed to provide perpendicular incidence of laser radiation on a curved surface. This means that a chief ray (or centroid ray) of a beam of laser radiation is directed to the curved surface by the second variant of the focusing optical unit, in such a way that the angle between the chief ray and the surface normal at the location where the laser radiation is incident on the curved surface forms an angle of no more than 10°, for example no more than 5°, in another example no more than 2°. According to the invention, each location on the curved surface has in this case a local center of curvature on the side of the curved surface facing away from the second variant of the focusing optical unit. In other words: If the curved surface is locally assigned a curvature—at the location where the laser radiation impinges on the curved surface—then this local curvature can be assigned a local center of curvature (for example, by way of an approximation to a spherical curvature, with the center of curvature corresponding to the center of the sphere or ball). As considered from the second variant of the focusing optical unit, this center of curvature is behind the curved surface. The curvature of the curved surface consequently has the same sign as the curvature of the eye whose vision should be corrected. The curved surface has a finite radius of a surface curvature.
For example, the curved surface has a surface curvature corresponding to that of a sphere or an asphere. An aspherical form is preferred, in particular, if a large diameter of the curved surface (e.g., at least 6 mm) is provided. The local centers of curvature can also be located along the continuation of the respective chief ray.
Attention is drawn to the fact that the second variant of the focusing optical unit need not necessarily provide a focus of the laser radiation on the curved surface. Rather, all rays of a beam may also be incident on the curved surface in perpendicular fashion. When using a low beam quality UV laser source, for example an excimer laser, the beams may also have a focus on the curved surface. By way of example, a solid-state laser can also be used as a UV laser source. At the location of incidence on the curved surface, the laser radiation advantageously has a diameter of 0.3 mm to 1.5 mm, for example of 0.5 mm to 1.0 mm. The location of impingement is often referred to as “spot” and its diameter is referred to as “spot diameter” or “spot size”. The spot size may serve as a measure for the size with which the curvature on the curved surface is averaged for the purposes of determining the local curvature or the center of curvature.
The curved surface can be partly or completely identical to a work surface suitable for a treatment for vision correction. For therapy, the eye to be treated or the cornea thereof is typically positioned in or close to this work surface.
The second lens arrangement of the second variant of the focusing optical unit may have one or more lenses. The second lens arrangement is for example designed to guide UV light without noticeable losses. To this end, the transmission of the lens arrangement is particularly high, especially in a wavelength range between approximately 193 nm and 213 nm. By way of example, the lens arrangement may contain glasses made of CaF or fused silica and may be provided with suitable optical coatings.
As already discussed above, the chief rays in focusing optical units according to the prior art have angles of incidence deviating ever more from perpendicular incidence with increasing distance from the apex of the cornea because the point of intersection of the chief rays is located in the optical unit. This is very different in the case of the second variant of the focusing optical unit according to the invention. In this case, the curved surface (and the sign of the curvature) leads to the deviation from perpendicular incidence of laser radiation on the cornea of the eye being significantly reduced in relation to the prior art because the point of intersection of the chief rays (for increasing distance from the apex of the cornea) is located behind the corneal apex.
For example, the second lens arrangement of the second variant of the focusing optical unit according to the invention has more than one lens for the provision of the curved surface. To this end, the lenses can be configured in such a way that there is perpendicular impingement of the curved surface on the image side (on the side of the eye or its cornea) in the case of an oblique incidence of the laser ray (into the second variant of the focusing optical unit) on the object side. The object-side oblique incidence into the second variant of the focusing optical unit can be realized by way of suitable scanners of the UVL-LVC system. Thus, laser beams deflected by the scanners are directed to the curved surface through the optical units as “spots”.
Requirements and solutions in respect of the imaging quality for the second variant of the focusing optical unit correspond, in principle, to the requirements and solutions for the first variant of the focusing optical unit (and its configurations).
For example, the second variant of the focusing optical unit also has the features of the first variant of the focusing optical unit (and its configurations). In this case, parts of the first lens arrangement can be identical with parts of the second lens arrangement. The first and second lens arrangement can also be completely identical. The convergent focal field and the curved surface can be partly or completely identical.
According to an example configuration of the second variant of the focusing optical unit, the curved surface has a diameter of at least 6 mm, for example at least 8 mm, in another example at least 10 mm.
In this way, a diameter of approximately 6 mm (or 8 mm, 10 mm) can be obtained in the work surface suitable for a treatment for vision correction.
In this case, the second variant of the focusing optical unit is for example configured such that the above-described quality of the imaging is observed over the entire diameter of the curved surface.
In another example, the optical system of the UVL-LVC system is designed such that the laser radiation is fed to the second variant of the focusing optical unit according to the invention such that the claimed diameter of the curved surface is served. The scanning system can be embodied accordingly to this end.
The optical system of the second variant of the focusing optical unit with such a diameter of the curved surface allows a fixed offset to be applied to the scanner coordinates in order thereby to shift the treatment center with respect to an optical system axis of the UVL-LVC system, which for example runs centrally through the focusing optical unit.
According to an example configuration of the second variant of the focusing optical unit, the curved surface has a surface curvature with a radius RF ranging from 8 mm to 50 mm, for example from 10 mm to 30 mm, in another example from 12 mm to 20 mm. The surface curvature may also have a radius RF from 6 mm to 25 mm, from 7 mm to 20 mm, or from 8 mm to 16 mm. The surface curvature may vary locally over the curved surface within the specified limits.
In a particular example configuration, the surface curvature for the ablation is designed such that the latter corresponds to the typical radius of curvature of the cornea RC (“convergent focal field ablation”). This realizes a perpendicular incidence of the incident laser radiation on the cornea. However, on account of limitations when parameterizing the optical system of the second variant of the focusing optical unit and the required working distance Δ for clinical practice, at least surface curvatures RF that are very close to RC are possible. This is desirable in order to obtain a significant reduction in the fluence losses (see below), in order to achieve the advantages described there.
The observations in relation to a difference radius of curvature RΔ for a first variant of the focusing optical unit with a focal field curvature RS also apply to the second variant of the focusing optical unit, described here, with a surface curvature RF.
In an example configuration, the first or second variant of the focusing optical unit has a working distance Δ ranging from 20 mm to 55 mm, for example from 20 mm to 50 mm. Additionally or as an alternative, the focusing optical unit has an optical aperture of greater than 40 mm, for example greater than 50 mm, in another example greater than or equal to 60 mm.
Focusing optical units according to the prior art have significantly longer working distances and typically have smaller apertures of the focusing optical unit. This also leads to the optical acceptance angle for the return of corneal reflections into the optical system being very small. By contrast, the focusing optical unit according to the invention (in both variants) allows particularly good return of reflections (e.g., the Purkinje image) into the remaining UVL-LVC system through the focusing optical unit. This allows a significantly improved alignment of the UVL-LVC system (or of the focusing optical unit) with respect to the eye of the patient and consequently makes possible a further improvement in the predictability of the refractive results following a correction. In a particular example, the convergent focal field (in a configuration of the first variant of the focusing optical unit) additionally has a focal field curvature with a radius RS ranging from 8 mm to 50 mm, for example from 10 mm to 30 mm, in another example from 12 mm to 20 mm. For the application, these values for the focal field radius of curvature RS yield further improved conditions for the detection of a ray reflected by the cornea.
In a particular example, the curved surface (in a configuration of the second variant of the focusing optical unit) additionally has a surface curvature with a radius RF ranging from 8 mm to 50 mm, for example from 10 mm to 30 mm, in another example from 12 mm to 20 mm. For the application, these values for the focal field radius of curvature RF yield further improved conditions for the detection of a ray reflected by the cornea.
The UV laser of the UVL-LVC system may have a spectral width of approximately 0.5 nm, for example. This may lead to the occurrence of transverse chromatic aberrations. Advantageously, the transverse chromatic aberrations can be compensated by way of a combination of lenses of the focusing optical unit, with the lens materials of the lenses having both different refractive indices and different Abbe numbers in the spectral range of the UV laser. Therefore, in an advantageous configuration of the focusing optical unit, the latter comprises a first lens and a second lens, wherein the first lens has a first lens material with a first refractive index and a first Abbe number and the second lens has a second lens material with a second refractive index and a second Abbe number, with the first refractive index differing from the second refractive index and the first Abbe number differing from the second Abbe number. Here, the different refractive indices must be present in this case in the range of the wavelength of the UV laser. CaF and fused silica materials are mentioned here in exemplary fashion.
In a particular example development of the focusing optical unit, the first lens has a negative optical power, the second lens has a positive optical power and the first refractive index is greater than the second refractive index. In this case, too, the refractive indices are considered in the range of the wavelength of the UV laser. The transverse chromatic aberration of the focusing optical unit can be corrected particularly well using the combination according to the invention of the optical powers and refractive indices.
According to a configuration of the focusing optical unit (in both variants), the first or second lens arrangement is designed to guide visible light without noticeable losses, also while avoiding a degradation of the optical components. To this end, lenses of the focusing optical unit may for example have a coating that provides high transmission (for example greater than 80%, in another example at least 90%) for both the spectral range of the UV laser and for the used visible light.
According to a configuration, the first or second variant of the focusing optical unit has at least two lens groups along a beam path with a non-imaging optical element being arranged therebetween. A lens group can comprise one or more lenses. The non-imaging optical element can be a plane mirror, a beam splitter or an optical filter (for example for polarization or wavelengths) or a retarder (for example a quarter-wave plate or a half-wave plate), which is embodied as a plane parallel plate (or mirror). This allows, for example, the realization of a compact structure of the focusing optical unit or the guidance of further light (for example visible light for fixation light) over parts of the same optical unit or the filtering of the guided light (e.g., of the laser source) or the influencing of the polarization of said guided light.
What follows is a brief explanation as to how fluence losses are reduced or eliminated by the focusing optical unit according to the invention; the explanations apply to both variants of the focusing optical unit. A pulse ablation shape corresponds to an ablation-effective fluence distribution of the radiated-in ablation laser pulse in a plane perpendicular to the direction of incidence. As a result of the geometry of the irradiation of the cornea, the pulse ablation shape deforms into a projected pulse ablation shape (also referred to as “pulse ablation footprint on cornea”). Hence, the fluence distribution on the cornea changes with respect to the “pulse ablation shape” that was radiated in. By way of example, this can be calculated from a given pulse shape with the aid of the blow-off model. For an infinitesimal surface element dA in the pulse ablation shape, the geometry loss is modeled as a cosine function and the following applies: cos(α)=dA/dA′, with the angle of incidence α (angle of incidence with respect to the local surface normal on the cornea) and an infinitesimal surface element dA′ of the projected pulse ablation shape. For a focusing optical unit with a convergent focal field (or perpendicular impingement of a curved surface) according to the invention, only small angles of incidence α arise in comparison with the prior art, and so the “pulse ablation footprint on cornea” only deviates little from the radiated-in pulse ablation shape.
Hence, the focusing optical unit according to the invention (in all variants and configurations) has lower geometry losses than solutions according to the prior art.
Fresnel losses, which can be calculated with the aid of the Fresnel equations with knowledge of the refractive indices of the air and cornea (or stroma) and the angles of incidence, may also occur in addition to geometric losses. The polarization of the light also needs to be taken into account. For polarized light, simulations have shown (as discussed below) that the losses due to the focusing optical unit according to the invention can be reduced and that, in particular, a dependence on the pupil radius is reduced.
In summary, it is possible to show that the focusing optical unit according to the invention (in all variants and configurations) produces fewer geometry losses than solutions according to the prior art. Furthermore, the Fresnel losses are also significantly lower. In particular, there is a significantly lower dependence on the pupil radius for both effects. The focusing optical unit according to the invention therefore advantageously allows an improvement in the predictability of the refractive results following a correction by application of UVL-LVC systems.
A second aspect of the invention relates to a planning unit for generating planning data for a UV laser-based system for vision correction (UVL-LVC system). In this case, the UVL-LVC system has a UV laser source for providing laser radiation and a scanning system for lateral scanning of the laser radiation in the x- and y-directions. The scanning system can be additionally designed to scan in the z-direction. Furthermore, the UVL-LVC system has a focusing optical unit for directing the laser radiation to a work surface. In this case, the work surface is a surface in space suitable for a treatment for vision correction. For therapy, the eye to be treated or the cornea thereof is typically positioned in or close to this work surface. The work surface may be curved. Moreover, the UVL-LVC system has a control unit that is designed to control the UVL-LVC system taking the planning data into consideration. To this end, the control unit may provide control signals for the UV laser source and the scanning system via control lines. Consequently, a laser focus can be displaced in the work surface on the basis of the planning data. Furthermore, a UV laser source power (by way of example, by changing the pump power in the case of a pumped laser source or with the aid of a variable attenuator) or else, optionally, a pulse rate can be controlled on the basis of the planning data. The control unit may be a computer comprising a processor and a memory.
According to the invention, the planning unit is designed to consider geometry losses and/or Fresnel losses when calculating the planning data. In this case, the planning unit can be designed to calculate the geometry and Fresnel losses on the basis of data from the focusing optical unit, the work surface and a curvature, determined preoperatively, of the structure in the eye to be treated, for example the cornea or the stroma.
Additionally or as an alternative, a spatial extent of the laser radiation in the work surface is taken into account when calculating the planning data.
In this case, it is possible to consider a variation of the laser radiation within the laser spot and/or diameter changes and shape changes of the spots in the work surface. By way of example, such changes may occur when the laser power changes and/or depending on the position of the laser spot in the work surface.
According to the invention, the aforementioned effects are taken into account in such a way that their influence on fluence losses is compensated (by way of an energy correction). By way of example, the power of the laser radiation can be altered locally to this end. Additionally or as an alternative, it is possible to adjust a dwell time of the laser radiation at a location in the work surface (by way of control signals to the scanning unit); in this way, it is possible, for example, to adjust the spatial distance between laser pulses or the number of laser pulses at the same location. Consequently, the planning unit can advantageously realize a fluence loss compensation.
Furthermore, the planning unit has an interface, by application of which the planning data can be made available to the control unit.
The planning unit may have a computing unit for calculating the planning data.
In this case, the planning unit according to the invention is not restricted to a UVL-LVC system with a focusing optical unit according to one of the above-described variants and configurations according to the invention. Rather, the planning unit also reveals—as shown below—an advantageous effect for systems having a focusing optical unit according to the prior art.
The planning unit can be part of the control unit of the UVL-LVC system. It may also have a separate computer with processor and memory.
Attention is drawn to the fact that the planning data can be created by the planning unit independently of the actual laser-surgical intervention on a patient's eye.
As will still be discussed in detail below, simulations have shown that it was possible with the aid of the planning unit according to the invention to significantly reduce, in relation to the prior art, the deviation of an effective etch rate from a normalized target etch rate as a function of the (preoperative) radius of curvature of the cornea for a UVL-LVC system with a fluence loss compensation according to the invention. This effect is improved even further if use is made of a focusing optical unit according to the invention.
According to an advantageous configuration of the planning unit, a decentration of an optical axis of the eye with respect to the visual axis or irregularities in the cornea and/or biomechanical effects of the eye are also considered when calculating the planning data. Furthermore, the planning unit can also calculate correction data required in the case of a decentration of the eye with respect to the optical axis of the focusing optical unit. These can then be used, for example, if an offset of the patient's eye occurs.
In a further configuration, the planning unit has an interface, by operation of which information data can be provided to the planning unit from the control unit of the UVL-LVC system. By way of example, the information data can be information about an offset in the case of a non-coaxial alignment of the patient's eye (or fixation of the patient's eye) with respect to the focusing optical unit (corresponding to the CSCLR condition). Additionally or as an alternative, this can relate to image data or information about a reflection; the planning unit can be designed to calculate the offset. According to a configuration of the planning unit, the latter is designed to consider the offset when calculating the planning data.
According to an example configuration of the planning unit, the UVL-LVC system has a focusing optical unit according to any one of the above-described variants and configurations. The ablation geometry with the convergent focal field and/or curved surface together with the optimized fluence loss compensation function according to the invention (realized by way of the planning unit according to the invention) enables a virtually perfect realization of the target ablation rate. It is possible to reduce the variation in the prediction of refractive results (reduced variation in “attempted vs. achieved”). As shown below, a reduction in the variation of the “achieved outcome” versus “attempted” may be of the order of up to ±0.25 diopters in individual cases—purely as a result of an improved energy correction on the basis of the ablation geometry according to the invention!
The almost perfect ablation geometry results in practical elimination of the influence of the optical system on nomograms (“rectification of nomograms”) since a deviation of the refractive results no longer occurs in this respect and consequently is no longer reflected in the nomograms. Ultimately, the “rectification” of the nomograms moreover leads to an improved determinability, acquirability and correctability of other relevant influencing variables of the refractive correction by way of nomograms (reduction in the “bias” in the nomograms) and to a reduction of disadvantageous interactions between various nomogram influencing variables.
A third aspect of the invention relates to a UV laser-based system for vision correction (UVL-LVC system). The UVL-LVC system comprises a UV laser source for providing laser radiation and a scanning system for lateral scanning of the laser radiation in the x- and y-directions. The scanning system can be additionally designed to scan in the z-direction. Furthermore, the UVL-LVC system has a focusing optical unit according to any one of the above-described variants and configurations. The UVL-LVC system moreover comprises a planning unit for generating planning data according to any one of the above-described configurations. Furthermore, the UVL-LVC system has a control unit that is designed to control the UVL-LVC system taking the planning data into consideration.
For example, the optical unit of the UVL-LVC system is designed such that the focusing optical unit, in conjunction with a remaining system optical unit, is able to effectively “collect”, by way of the system, the light reflected by the cornea (e.g., Purkinje images) and when necessary guide said light back to a base unit of the optical system by virtue of the same optical unit as for the laser beam delivery to the eye. Inter alia, this is made possible by virtue of the fact that the scanners, unlike in solutions according to the prior art, are able to be integrated at the start of the beam path (from the view of the UV laser source).
According to a configuration of the focusing optical unit, the latter is configured so that an observation of the operation site in the visible spectral range is possible therethrough. To this end, there is suitable imaging on a camera. Additionally or as an alternative, the UVL-LVC system has a surgical microscope for visual observation. In this case, the focusing optical unit acts as a common “front lens” of the surgical microscope. In this case, it is also possible to make use of auxiliary lenses which improve the imaging of the focusing optical unit for the purposes of observation in the visible spectral range to the extent that is necessary in order to obtain a sufficiently high resolution. However, instead of the visible spectral range, it is also possible to use, for example, IR light and cameras for the observation of the operation site.
In a further configuration of the focusing optical unit and the remaining system optical unit, the optical unit is configured such that both visible light and the UV laser radiation between approximately 193 nm and 213 nm can be guided without noticeable losses, also while avoiding a degradation of the optical components. To this end, the optical units can for example include CaF (calcium fluoride) or fused silica and may be provided with suitable optical coatings.
According to a configuration, the UVL-LVC system may have a base unit which comprises the UV laser source and one or more articulated arms, by operation of which the laser ray is guided to the focusing optical unit. The articulated arms may be interconnected by way of rotary joints. Then, the planning unit is particularly advantageously designed to consider a decentration of the laser ray in a zero position of the scanners, produced by the position of the articulated arms, with respect to an optical axis of the focusing optical unit. By way of example, the articulated arms may have sensors to this end, the latter detecting the position of the articulated arms with respect to one another and with respect to the base unit and making available the corresponding measurement values to the planning unit (optionally via the control unit).
According to a further configuration, the UVL-LVC system has a contact interface. The contact interface permits an alignment and/or a fixation of the eye with respect to the UVL-LVC system. To this end, the contact interface can be configured such that it permits etching of tissue; in the affixed state of the eye, the eye comes into contact with the contact interface only outside of the region in which an etching is intended to take place.
It is understood that the features mentioned above and the features still to be explained below can be used not only in the specified combinations but also in other combinations or on their own without departing from the scope of the present invention.
The invention is explained in greater detail below for example with reference to the accompanying drawings, which also disclose features essential to the invention. In the drawing:
An ablation profile 150 is not applied at the correct angle (i.e., with the center not on the surface normal, that is to say perpendicular to the visual axis 130) as a result of the “wrong” fixation of the patient's eye 110. This may occur if the patient preferably fixates in a largely fixed, but “wrong” direction, that is to say for example permanently gazes in a fixed direction that does not correspond to the center of the “fixation cloud” 120. This may occur should the patient, depending on refraction deficit and treatment duration, no longer be able to see the fixation target in focus during the operation. A prismatic correction error (tip/tilt) arises on account of the wrong fixation.
Furthermore, the focal area 260 has a radius of curvature RS in the example shown. This radius of curvature has the same sign as the corneal curvature with the radius of curvature RC (“scanning radius of curvature”). Additionally, the two radii of curvature RC and RS have almost the same magnitude such that the focal area 260 extends close to the cornea 215.
The focusing optical unit shown in
In
The focusing optical unit shown here is particularly compact and has an installation size (length) of 54 mm in the case of an optical diameter of 56 mm, and provides a working distance of Δ=30 mm in the process.
According to the sphere model, z(R,r)=r2/(R+√{square root over (R2−r2)}) applies. In this case, R describes the radius of a sphere, z describes the heights with respect to a tangent to this sphere, and r describes the distance along the tangent from the point of contact between circle and tangent.
The right-hand side of
Δz(RC, RS, r)=z(RC, r)−z(RS, r)=z(RΔ, r)
In this case, zC:=z(RC, r) and zS:=z(RS, r). This difference height Δz is intended to be determined by a difference radius of curvature RΔ. This corresponds to an “effective” corneal curvature for light incident from the z-direction, for example like for solutions according to the prior art (see also
Using the aforementioned formula for a sphere model, the following arises from the equation for the difference height Δz:
This relationship for determining RΔ applies to all r (in particular to r=0). This results in:
Consequently, the following arises for the “effective” corneal radius of curvature:
Consequently, with a typical corneal radius of curvature of RC=7.86 mm, values of approximately RΔ≈450 mm to approximately RΔ≈15 mm arise for the effective corneal curvature for focal field radii of curvature with values between approximately 8 mm and approximately 16 mm. In these regions the advantage of an improved fluence loss function is particularly clear since these effective radii are significantly larger than the typical radius of curvature of the cornea, and so an impingement of the cornea with laser light is significantly closer to perpendicular incidence than in the prior art.
Even though the observations shown here were made for a first variant of the focusing optical unit, they also apply to a focusing optical unit according to the second variant. In this case, the curved surface is identical to the focal field 460 (with RF=RS).
In this case, γ is the angle between an incident laser ray for the focusing optical unit according to the invention with a convergent and curved focal field with respect to a focusing optical unit according to the prior art with telecentric focusing on a plane focal field. The following arises for the angle of incidence αSdT according to the prior art:
By contrast, the following applies to the focusing optical unit according to the invention:
αMB≈αSdT−γ
Then, the angle of reflection 2αMB+γ arises for the reflected laser ray 590 in the case of a lateral displacement ΔLS. In this case, ΔLS is used in the calculation for the pupil coordinate rSdT(≈rMB). In this case, the ophthalmic pole (OP) and the corneal vertex (CV) are equated to one another WLOG. In this example, a reflection angle of approximately 2αMB+γ=20° arises for an offset of ΔLS=2 mm. This angle is detected without problems by the focusing optical unit and can be processed in the UVL-LVC system.
The axis of symmetry 580 of the eye 510 is also depicted in
Even though the observations shown here were made for a first variant of the focusing optical unit, they also apply to a focusing optical unit according to the second variant. In this case, the curved surface is identical to the curved focal field (with RF=RS).
In the discussed configuration, the focusing optical unit (in both variants) is therefore particularly well suited to identify reflections such as the first Purkinje image and/or the vertex, and hence advantageously allows an improvement in the centration of the patient's eye with respect to the UVL-LVC system.
In UVL-LVC systems, laser pulses are typically designed approximately as low-order supergaussians, from which the pulse ablation shape 620, 620* can then be calculated. By way of example, the latter can be implemented from a given pulse shape with the aid of the blow-off model. The geometry loss is modeled as a cosine function for an infinitesimal surface element dA 625, 625*. The following applies: cos(α)=dA/dA′, with the angle of incidence a (angle of incidence with respect to the local surface normal on the cornea 615) and the projected infinitesimal surface element dA′635, 635*. In comparison with solutions according to the prior art (to the left in
The two upper graphs in
The two lower graphs in
Even though the observations shown here were made for a first variant of the focusing optical unit, they also apply to a focusing optical unit according to the second variant. In this case, the curved surface is identical to the curved focal field (with RF=RS).
The calculations apply to the laser pulse maximum fluence (“peak fluence”). A spatial extent of the laser pulse (or of the laser radiation) was not taken into account for the calculation of the geometry and Fresnel losses. This is an approximation which improves as the pulse diameter decreases (ratio of pulse diameter to the diameter of the optical zone). Attention is drawn to the fact that taking account of the relationships shown below once again benefits the solution according to the invention. As representative values in all cases, 160 mJ/cm2 was taken for the “peak fluence” (F0) and 48 mJ/cm2 was taken for the stroma threshold ablation fluence (“threshold fluence”). The refractive index of the stroma was taken to be n=1.377.
A spherical 5 D (dpt) correction in the case of a 4 mm pupil radius was considered as a case study. Attention is drawn to the fact that optical zones are usually located up to 6 mm, and often therebeyond, and, with transition zones of 1.5 mm, even lead to pupil radius coordinates closer to 4.5 mm (9 mm overall ablation diameter). Typically, hyperopic eyes (labeled as “more hyperopic like eyes”) exhibit on average larger corneal radii of curvature and require steepening, that is to say a reduction, of the corneal radii of curvature for correcting the visual defect (labeled by “hyperopia correction”). The opposite applies to myopic eyes (labeled “more myopic like eyes” and “myopia correction”).
The curves in the diagram of
The line labeled “FOSdT (with FLCSdT)” represents the deviation of the effective etch rate from the normalized target etch rate for a UVL-LVC system with a fluence loss compensation according to the prior art. This emerges from the calculated loss function and a typical compensation function according to the prior art, which does not consider the Fresnel losses but contains the cosine-dependent projection of the surface elements (cos(a) in
The line labeled “FOSdT (with FLCsdT+Fresnel)” represents the deviation for a system according to the prior art with a fluence loss compensation according to the prior art if the Fresnel losses are additionally considered. Essentially, it is possible to identify a shift of the function to the left, that is to say to smaller corneal radii of curvature (or “upward”, depending on the point of view). This is due to the fact that the Fresnel losses for unpolarized excimer laser pulses vary only slightly with the angle of incidence (cf. work point in
The line labeled “FO (with FLC)” represents the deviation of the effective etch rate from the normalized target etch rate for a UVL-LVC system with a focusing optical unit according to the invention (FO) and a compensation according to the invention (FLC). The latter considers the optical geometry (in this case, focal field curvatures RS of 12 mm and a working distance Δ of 40 mm) of the focusing optical unit according to the invention for the compensation function. The curve once again arises from the calculated loss function and the compensation function according to the invention. The latter also considers the Fresnel losses in addition to the geometry losses (which are low in comparison with FOSdT) and uses the preoperative corneal radius of curvature for the compensation. The variation of this function over the corneal radii of curvature is significantly reduced in relation to a focusing optical unit according to the prior art. This leads to a significantly improved predictability or to a reduction in the variation of the refractive result, as will still be explained below.
The line labeled “FO (with FLCSdT)” represents the deviation of the effective etch rate from the normalized target etch rate for a UVL-LVC system with a focusing optical unit according to the invention (FO), which arises if use were to be made of the above-described compensation function according to the prior art (FLCSdT). Even in this case, the deviations of the effective etch rates for the arrangement according to the invention would still be approximately one order of magnitude smaller than in the case of the prior art. This is substantially based on the above-described “more good-natured” curve of the geometry losses over the pupil coordinates (see
Finally, the line labeled “FOSdT (with FLC)” represents the deviation of the effective etch rate from the normalized target etch rate for a UVL-LVC system with a focusing optical unit according to the prior art (FOSdT) if the compensation function according to the invention FLC were to be applied. The profile of the curve shows that the compensation function according to the invention FLC already leads to significant improvement in the predictability of the refractive result.
Even though the observations shown here were made for a first variant of the focusing optical unit, they also apply to a focusing optical unit according to the second variant. In this case, the curved surface is identical to the curved focal field (with RF=RS).
The intention is now to explain why the concept according to the invention is particularly advantageous in relation to an improved predictability of the refractive results.
The curve of the percentage deviations of the etch rates for a UVL-LVC system with a focusing optical unit according to the prior art, shown in
For prior art systems, the keratometry can now be taken into account in the nomogram as follows: Let the mean value of the keratometry in the considered hyperopia group for the nomogram correction be RC=8.25 mm. If a hyperopic patient with a keratometry deviating therefrom and assumed to be RC=7.25 mm is now treated, it is possible to read the difference in the ablation rate that would not be corrected from the diagram in
The non-optimal energy compensation, which quite fundamentally is due to the ablation geometry of
One could object that it is possible to apply an exact energy correction to UVL-LVC systems according to the prior art. This is fundamentally true but not implementable in practice. Ideally, the current corneal shape at the pupil position during the ablation would be determined for the next laser pulse. However, this cannot be done with current technology (processing speed, control speed), and would be accompanied by other limitations and problems. Alternatively, attempts could be made to take account of the current corneal shape during the pulse sorting process. This would represent a great advance and is categorically doable from the view of the sorting algorithms (sorting of the pulses for etch optimization). However, a subsequent and necessary thermal sort would then be impossible without undoing the previously considered improvement (resorting of the pulses). There is currently no prospect of a physical and mathematical method for combining sort and thermal sort (“simultaneously”). Therefore, the minimizing of the energy correction by the optical unit according to the invention and the consideration of the K-values offers an improvement of the predictability of the results, a reduction in the variation and, moreover, also an improvement of the nomograms as these have to correct smaller deviations per se.
A feature of the planning unit according to the invention is that the remaining aberrations and hence the spot variations in the focal field are measured or physically modeled for the calculation of the planning data and are made available to the sorting algorithm. These data can be used to determine the accurate ablation-effective fluence distribution of the pulses in the focal field as a function of the focal field position, and hence to take this into account when sorting the pulses (see
Furthermore, the UVL-LVC system comprises what is known as an “alignment beam laser” 780. The latter serves to adjust the optical system and/or carry out an alignment with respect to the eye. The laser beam of the alignment beam laser follows the laser beams 770 of the excimer laser 720 on the cornea 715 and its focus. The “alignment beam laser” 780 is situated in the base unit of the UVL-LVC system 705.
Attention should be drawn to the fact that one or more deflectors 740 may also be embodied as beam splitters. This allows integration of other components, for example detectors for the detection of the collected corneal reflections or an observation camera. Various arrangements immediately evident to an expert but not plotted in
In contrast to the exemplary embodiment shown in
The aforementioned features of the invention, which are explained in various exemplary embodiments, can be used not only in the combinations specified in an exemplary manner but also in other combinations or on their own, without departing from the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2020 206 423.7 | May 2020 | DE | national |
| 10 2020 208 676.1 | Jul 2020 | DE | national |
This application is a National Phase entry of PCT Application No. PCT/EP2021/063609 filed May 21, 2021, which application claims the benefit of priority to DE Application No. 10 2020 208 676.1 filed Jul. 10, 2020, and DE Application No. 10 2020 206 423.7 filed May 24, 2020, the entire disclosures of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/063609 | 5/21/2021 | WO |
