Patent Application
20170079843
The present invention pertains generally to systems and methods for removing corneal epithelium tissue. More particularly, the present invention pertains to the photoablation of corneal epithelium tissue using a laser beam. The present invention is particularly, but not exclusively, useful as a system and method for corneal epithelium removal which monitors fluorescence epithelial tissue to ensure that the correct amount of tissue is removed.
The corneal epithelium forms a protective tissue layer on the front surface of the cornea. Structurally, the tissue of the epithelium layer is relatively soft and it is in contact with the tear film of the eye. Upon removal from the eye, the epithelium can completely replace itself from limbal stem cells within a few days with little or no loss of clarity.
During some surgical procedures it is often desirable to remove the corneal epithelium or portions thereof. For example, the removal of the epithelium is a necessary step in several corneal procedures, including (but not limited to) corneal opacity or scar removal, photorefractive keratectomy for treatment of refractive errors (PRK) and the treatment of anterior basement membrane corneal dystrophy (ABMD).
Heretofore, several methods have been used to remove the corneal epithelium. In general, these methods rely on chemical and/or mechanical processes. For example, the application of ethyl alcohol is often used to loosen or sever the connections that join the epithelium to the underlying Bowman's membrane and corneal stroma. Alternatively, rotating brushes, surgical knives, and other instruments have been used to remove the epithelium using mechanical means. Usually this is done by hand, and under an operating microscope. These methods are typically performed under topical anesthesia.
The use of lasers to remove the corneal epithelium has several advantages over the chemical/mechanical techniques described above. These include improved patient comfort and decreased trauma to the underlying cornea. In addition, as compared with chemical/mechanical techniques, laser removal often results in decreased liberation of cellular contents and their associated inflammatory components. Also, laser removal provides a more exact matching of the desired and actual zone of epithelial removal. Further, laser removal typically results in a shorter time for re-epithelialization after surgery, and avoids risks associated with using toxic chemicals, such as ethyl alcohol, on the ocular surface.
On the other hand, there are certain challenges associated with the use of lasers to remove the corneal epithelium. In particular, it is sometimes difficult to accurately determine during a laser procedure, when the epithelium has been completely removed. Two factors contribute to this difficulty. For one, the thickness of the corneal epithelium varies over the corneal surface, and varies inconsistently from eye to eye. For another, the thickness of the epithelial layer is not consistent from patient to patient. Plus, the corneal epithelium is difficult to visualize, making determination of the endpoint of removal difficult to ascertain.
Because of the inherent epithelial thickness variations described above, a somewhat complex laser treatment is typically employed to remove the entire epithelial layer without disturbing the underlying tissue layers (i.e. Bowman's Membrane or the stroma). In most cases, substantial negative consequences arise if the entire epithelium layer is not removed or if the underlying tissue layers are disturbed. In particular, if too little epithelium is removed, residual epithelial tissue that is left behind can interfere with subsequent procedures. For example, if the goal is to remove all of the epithelium to create new epithelial attachments to Bowman's layer (such as with treatment of ABMD), residual epithelial tissue can cause a treatment failure. As another example, if the goal is to remove the epithelium as a component of laser refractive surgery in a PRK procedure, the residual epithelium can result in unpredictable and irregular ablation patterns, with adverse visual consequences. This is made worse by the irregular thickness of the epithelial layer, such that the subsequent refractive treatment may cause elevations and depressions in the cornea that may not be amenable to correction with current technology.
Adverse consequences can also occur when tissue is inadvertently removed beyond the epithelium (i.e. in Bowman's membrane or the stroma). For example, if the removal is performed to treat ABMD, then removing the underlying layer (called Bowman's layer) can interfere with the treatment success. On the other hand, if the removal is performed as a step in a laser refractive correction (such as PRK) then an incomplete removal of the epithelial tissue can alter the refractive correction in the underlying corneal tissue and impair the visual outcome, which may require additional treatment.
In light of the above it is an object of the present invention to provide a system and method for corneal epithelium removal which monitors epithelium tissue removal to ensure that the correct amount of epithelium tissue is removed. Another object of the present invention is to remove a corneal epithelium without leaving residual epithelial tissue that can interfere with subsequent procedures. Still another object of the present invention is to safely remove a corneal epithelium without disturbing underlying tissue layers such as Bowman's Membrane and the stroma. Yet another object of the present invention is to provide a system and method for removing a corneal epithelium that are easy to use and comparatively cost effective.
In accordance with the present invention, a laser system for removing tissue of a corneal epithelium includes a laser unit for generating a surgical laser beam. Once generated, the laser beam is directed onto target tissue in the corneal epithelium to photoablate the target tissue. During the photoablation procedure, an autofluorescent response from tissue of the epithelium is monitored to determine whether residual epithelial tissue remains.
In one aspect of the present invention, the autofluorescent response is induced by the surgical laser beam. In another aspect, an external light source, such as a light emitting diode (LED), provides light having a wavelength suitable for creating the autofluorescent response from the corneal epithelium. For the present invention, monitoring of the autofluorescent response can be accomplished by visual observation of the eye, or a sensor can be employed. In some cases, a display for presenting the autofluorescent response as an image can be employed. In any event, the autofluorescent tissue response is monitored during the laser procedure and laser unit output is stopped where an autofluorescent response is not detected or observed.
When a sensor is used to monitor the autofluorescent response, the system can include a control unit which is operationally connected to the sensor and the laser source. With this arrangement, the control unit receives an input from the sensor and provides a control signal to the laser source. When the sensor input indicates that an autofluorescent response is detected, a control signal is transmitted to the laser source to continue photoablation. Where the sensor input indicates that an autofluorescent response is not detected, a control signal is transmitted to the laser source to discontinue photoablation.
In a particular laser procedure protocol, an ablation zone in the epithelium is first identified. Next, topographical contour features on an anterior epithelial surface within the ablation zone are determined. For example, the topographical contour features can be determined using an Optical Coherence Tomography (OCT) device. Then, based on the contour features of the epithelial surface, a predetermined pathway for movement of the laser beam's focal point through tissue of the epithelium is developed. Tissue is then photoablated along the predetermined pathway. In some cases, the protocol can also include the sequential identification of another ablation zone for subsequent conduct of the protocol. Further, during the implementation of a protocol, where no autofluorescent response has been detected by the sensor, the protocol can be modified to indicate such a non-response.
In one process, the predetermined pathway is layered over the ablation zone using a sequence of an n number of raster patterns. In this process, each raster pattern is positioned in a layer at a respective predetermined elevation en from the interface between the epithelium and Bowman's membrane of the cornea. During the event, photoablation is performed in each respective raster pattern according to contour features of the epithelial surface. Upon completion of a raster pattern at a specified contour elevation, the laser beam's focal point is selectively advanced through a predetermined contour interval distance, Δe, toward the epithelium/Bowman interface for photoablation along a successive raster pattern at the next (i.e. lower) contour elevation.
In another process in accordance with the present invention, the predetermined laser point pathway is segmented with a plurality of contiguous segments. For this process, each segment includes an ablation area that is determined by topographical contour features on the epithelial surface in the ablation zone. In more geometric terms, for this process, each segment extends through the epithelium from the ablation area to the interface between the epithelium and Bowman's membrane of the cornea.
The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:
Referring initially to
For a more automated embodiment of the present invention, the system 10 can also include an imaging unit 28 which is electronically connected with the controller 12 for viewing the epithelium 20. More specifically, in this automated embodiment, a display 30 is also connected with the controller 12 for visually presenting images that are generated by the imaging unit 28.
An important aspect of the present invention is the capability of the system 10 to detect when a predetermined portion of the epithelium 20 has been completely removed without affecting other tissues of the eye 24. To do this, the present invention relies on an autofluorescent response that will result when the laser beam 26 interacts with tissue of the epithelium 20. When a sensor 32 is used for detecting the autofluorescent response, the response is transferred by the controller 12 for presentation on the display 30.
As disclosed above, the system 10 of the present invention is intended to detect an autofluorescent response that will result when tissue of the epithelium 20 is photoablated by the laser beam 26. For one embodiment of the present invention, the detection of an autofluorescent response is accomplished by direct visual observation, such as by a user (e.g. attending physician not shown) viewing the epithelium 20 through the eyepiece 16. In this case, the wavelength of the laser beam 26 must be capable of causing autofluorescent response, as well as performing the required photoablation. Accordingly, a laser beam 26 having a wavelength in the far violet or near ultraviolet wavelengths, will be required. For the other embodiment, the detection is accomplished by the sensor 32. In this case, it will be necessary to employ a Light Emitting Diode (LED) 34 that is capable of inducing the autofluorescent response. A Wood's lamp could be used.
Referring now to
With reference to
By cross-referencing
With the above in mind, and with reference to
It is to be further appreciated that a combination of the horizontal and vertical photoablation procedures disclosed here can be used together if desired. Regardless how employed, the resultant autofluorescent response is monitored and, whenever there is no such response, the conclusion is that all tissue of the epithelium 20 that was above the interface 40 has been removed. Depending on the absence of an autofluorescent response, or an indication from the imaging unit 28 that epithelial tissue remains in the zone 36, an operation of the present invention is either stopped, continued as indicated, or it is moved to another zone 36 where epithelial tissue still remains.
While the particular System and Method for Removing Corneal Epithelium as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.
