Surgery – Instruments – Light application
Reexamination Certificate
2001-04-23
2003-09-02
Manuel, George (Department: 3737)
Surgery
Instruments
Light application
Reexamination Certificate
active
06613041
ABSTRACT:
BACKGROUND
The invention concerns a method and a device for determining the surface form or shape of biological tissue.
The exact knowledge of the topology of biological tissues is in many instances indispensable, e.g., for carrying out operations of the tissue surface. The corneal surface of the human eye is cited as an example. Since the cornea has a power of refraction of above 40 diopters, it is decisive in the refraction of the light falling into the eye and thus participates in the seeing process. The power of refraction thereby is primarily a function of the form of the corneal surface and in particular of its curvature. Modern methods of correcting ametropias therefore aim to alter the corneal form by the removal of corneal tissue with the aid of a laser. Therefore, the prerequisite for a purposeful working of the cornea is the exact knowledge of the form of its outer surface. This is currently determined before and several days after the correction of the ametropia with the aid of optical methods in which the measured values are not appreciably influenced by the statistical and involuntary movements of the eye on account of the rapidity of these methods.
A known method of measuring the corneal form, which is used before or after an ametropic operation or also in order to adapt contact lenses, is based on the use of so-called keratometers, in which the reflection of concentric rings (so-called placido rings) on the tear film that moistens the cornea is recorded with a camera and evaluated. An illuminating device is placed in front of the eye and a disk with circular slits concentric to each other is arranged in front of the device and in whose center a camera is set up. The light reflected from the tear film and recorded by the camera in the form of a ring pattern distorted by the curvature of the cornea is compared in order to determine the specific characteristics of the corneal form to be measured with a given corneal form of a standard eye with a corneal radius of 7.8 mm. In order to reconstruct the surface form of the particular cornea, the user first manually determines the center of the rings, usually approximately 20, with the aid of cross hairs. 180 meridians are then placed through the center over the cornea with an interval of 1° each. The interval of the intersections of the meridians with the rings increases with the growing radius of the rings up to values of approximately 300 &mgr;m. Altogether, 180 (meridians)×20 (rings)×2 (intersections)=7200 data points result from which the curvature of the cornea can be calculated. This known method and this known device have the disadvantage that due to the concentric arrangement of the illuminating device and of the camera, no data can be recorded in a surface of the center with a diameter of at least 1.5 mm. However, measurements are especially important particularly in this area. Furthermore, erroneous measurements of a corneal form cannot be avoided which form deviates greater than is customary from the form of a standard eye, such as, e.g., in the case of a central flattening. In addition, the number of 7200 data points is insufficient in some instances for the interpolation necessary to determine the corneal topology. This number of data points effectively represents an upper limit since the meridians cannot be divided at an angular interval of less than 1° on account of their finite width of line.
Since the previously described method and the corresponding device do not permit any monitoring during the removal process, erroneous corrections are recorded relatively frequently, especially in the case of high ametropias above −6 diopters. These erroneous corrections can be evaluated by the user or the operator statistically to prepare so-called “nomograms” that aid in preventing the erroneous corrections in the means in subsequent operative incisions; however, this solution is not adequate.
Moreover, the industrially established so-called strip-projection method for the optical measuring of surfaces of very different types of lifeless materials is known that permits a reliable, contactless and rapid detection of measured values. The basic idea of the strip-projection technique resides in the uniting of measuring-technology possibilities of geometrical-optical triangulation with those of classic interferometry. The mathematical connections are presented in detail in the annex. This method and the corresponding device are particularly suited for detecting rapid events since only a single photograph is necessary. In this method a suitable strip pattern is first projected onto the surface to be measured. The strips are generated by interferometry or by the representation of a suitable structure (grid, etched structure in glass, LCD matrix, micromirror). The light diffusely scattered from the surface in the form of a strip pattern distorted by the surface form of the cornea is detected at an angle a to the direction of projection or irradiation and evaluated by suitable algorithms. The required Fourier transformations that were previously time-intensive no longer constitute an appreciable delay on account of new computer possibilities.
However, the evaluation of the strip patterns becomes problematic given a relatively weak contrast of the detected strip pattern. Phase-measuring errors occasionally occur that make themselves noticeable in jumps in the surface. As is known, contrast-elevating measures consist in vapor-depositing strongly scattering layers on the object or in the addition of fluorescent dyes. The latter has been suggested in particular in ophthalmology, e.g., by Windecker at al. (Applied Optics 43, 3644 ff., 1995) who suggested enriching the tear film in front of the cornea with fluorescein in order to determine the form of the corneal surface in a strip-projection method. In this method, blue light filtered with a filter out of white light is guided onto the cornea, whereupon the tear film located in front and enriched with fluorescein emits green light as a consequence of the excitation. U.S. Pat. No. 5,406,342 teaches a similar method (and a corresponding device) in which the superpositioning of two partial patterns projected from two directions onto the cornea provided with fluorescent liquid results in the production of a moire pattern that can be evaluated. The fluorescent radiation emitted by the liquid film is combined after having passed through an optical filter by the successive recording of two half images with a video camera and evaluated by specially developed algorithms. The projection of the radiation from two directions helps, in addition to the filter, to avoid the detection of the direct reflex that is produced at the location on the cornea whose surface normal divides the angle between the direction of radiation and the direction of observation into two equally large angles and always appears when the detection unit is sensitive to the wavelength projecting the pattern.
Other methods and devices for determining the corneal topography in which a fluorescent agent is applied onto the eye are known from U.S. Pat. No. 4,995,716; U.S. Pat. No. 4,761,071 and U.S. Pat. No. 5,159,361.
These known methods and devices have the disadvantage that the tear film always exhibits locally and individually different thicknesses so that conclusions about the surface of the cornea can not be reliably drawn from measuring it. Since the fluorescent agent continues to be distributed in the tear film and thus supplies scattered light from the entire thickness of the tear film, the measuring accuracy can not be greater than the film thickness, that amounts up to 200 &mgr;m. Furthermore, the liquid would penetrate into the corneal tissue if the epithelial layer on the cornea were not present or folded back out of the radiation path, which would result in a widening of the depth resolution. Moreover, in such an instance the surface form of the cornea would change since it swells up. Thus, an intact epithelial layer is necessary for the use of this known method or this known device;
Bioshape AG
Dority & Manning P.A.
Manuel George
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