Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2001-07-02
2003-07-08
Lateef, Marvin M. (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
Reexamination Certificate
active
06588903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and a device configuration for determining the corneal thickness of an eye. In particular, this invention relates to a method and a device configuration for determining the corneal thickness of a human eye by means of light rays, first light rays being projected onto and into the cornea, and second light rays, reflected by the cornea, being registered for determining the corneal thickness.
2. Description of the Related Art
Besides the use of ultrasound for determining a human eye's corneal thickness averaged over a large range, pachymetry attachments for slit lamps are being used today to determine the corneal thickness by means of light rays. By means of a pivotable, plane-parallel plate in the path of the examination beam of a slit lamp microscope, such an attachment generates two offset half images of the light section with the cornea. By pivoting the plane-parallel plate, the offset light sections can be made congruent, and a measured value corresponding to the pivoting can be read for the cornea. Such a measurement takes place only at one point of the cornea, requires manual skill, and is difficult to reproduce since the position of the measurement point is not defined. The drawback of these methods for determining the corneal thickness is, in particular, that they are not suitable for determining corneal thickness with a high area resolution, i.e. they do not make possible determination of the corneal thickness area-wise, or in a way that completely covers the area. Consequently local deviations of the corneal thickness cannot be registered by means of these methods, which is extremely risky, and therefore disadvantageous, in particular for surgical procedures (e.g. refractive surgery).
Described in U.S. Pat. No. 5,512,965 is a device and a method based on a modified slit lamp, and by means of which a three-dimensional print-out of the cornea surface and of the respective local corneal thickness can be generated. The device according to U.S. Pat. No. 5,512,965 comprises a modified slit lamp, the projection slit of which is curved for improvement of the depth of focus, a television camera with associated lens systems, electronic circuitry for selecting and quantifying television pictures taken, as well as a control mechanism for moving the light section generated by the slit lamp. According to U.S. Pat. No. 5,512,965, a multiplicity of digitally coded pictures of the recorded light section are evaluated by means of software, measurements of individual light sections being put together via reference marks of the eye (limbus and Purkinje images of the exit pupil and highlights of the slit projector) for overall measurement. According to the method described in U.S. Pat. No. 5,512,965, the patient with the eye to be measured must focus his gaze on a target object so that eye movements that are too great can be avoided during the measurement. According to U.S. Pat. No. 5,512,965, the focus of the television camera during the measurement is updated to the cornea with an additional expense. Since the refraction of the beams, incident in the cornea, of the light section is known to depend upon the local surface inclination of the cornea, the inclination of the cornea must be measured with an additional measuring effort in order to determine the corneal thickness, in the method according to U.S. Pat. No. 5,512,965. The method according to U.S. Pat. No. 5,512,965 requires moreover a complex calibration with each measurement since the angle of illumination, the viewing angle as well as the object spacing continually change. The method according to U.S. Pat. No. 5,512,965 requires above and beyond this a complex alignment of the patient with additional marks. Owing to the high expense in technical devices, the method according to U.S. Pat. No. 5,512,965 is implemented only in the form of a standing apparatus, which has the drawback that the method is not usable for patients lying down, for example in the operating room.
The significance in ophthalmology of an area-wise corneal measurement with high resolution is clear, especially for surgical procedures on the cornea. In particular where the cornea is cut at a predefined depth or where the cornea is penetrated to a predefined depth, precise knowledge of the local corneal thickness is important. Examples of such surgical procedures are radial keratotomy, LASIK (laser in situ keratomileusis) and ALK (automated lamellar keratoplasty). Whereas in the radial keratotomy, thin superficial slits are cut into the cornea, in LASIK and ALK thin layers of the cornea are cut in flattened state of the cornea. From a medical point of view, therefore, it is extremely important to know the relevant corneal thickness prior to the operative procedure. Only with knowledge of the corneal thickness can it be ensured that an incision is not too deep and that no complications arise.
BRIEF SUMMARY OF THE INVENTION
It is an object of this invention to propose a method and a device configuration for determining the corneal thickness of an eye which at least do not have certain of the above-described drawbacks of the state of the art, and which make possible, in particular, a determination of the relevant corneal thickness of a human eye area-wise with a high area resolution. It is a further object of the present invention to reduce the expense in technical devices for determination of the corneal thickness of an eye, in order to make possible, in particular, the achievement of hand-held devices for area-wise corneal thickness measurement with a high area resolution.
These objects are achieved, according to the present invention, through the elements of the independent claims. Further preferred embodiments follow moreover from the dependent claims and from the specification.
In particular, these objects are achieved by means of the invention in that a contact element is placed on at least one contact area of the cornea, and light waves are projected through the contact element and into the cornea by a light source. Light waves, which are reflected by the cornea and the contact element, can then be registered by registering means, and a simple measurement of the thickness of the cornea is made accessible on the basis of the defined measurement conditions created by the contact element. Determination of the surface inclination of the cornea, or the curved design of projection slits to meet the Scheimpflug condition, or an autofocus system for the recording means, for example, become unnecessary. Moreover the contact element makes it possible to prevent more easily undesired eye movements and deformations of the eye, for example through eye focusing, during the measurement.
In a preferred embodiment variant, the light waves, which are emitted by the light source at least in points in a two-dimensional measuring area of the contact area of the cornea on which the contact element is placed, are projected onto and into the cornea, and the corneal thickness is determined at one or more places in the measuring area. The corneal thickness can thereby be determined with a high area resolution, for example with complete coverage within the measuring area.
In a preferred embodiment variant, a contact element is used which has a predefined thickness, and the thickness of the contact element is co-used for determination of the corneal thickness. With known local thickness of the contact element using optical methods, which are based on specular reflection at the boundary surfaces of the contact element and of the cornea, and in which the thickness of the contact element can be co-established on the basis of light waves reflected by the contact element, no knowledge is required of the incidence angle of light rays at the boundary surfaces, and only the refraction indexes of the contact element and of the cornea must be known. This embodiment variant thus has the advantage that the time, effort and expense for determining the local surface inclination of the cornea in the known meth
Lateef Marvin M.
Oliff & Berridg,e PLC
Sanders John R.
SIS AG Surgical Instruments Systems
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