GB2129956A - Selecting contact lenses for visual correction using photokeratogram - Google Patents

Abstract

A method of selecting the contact lens using a set of trial lenses is disclosed, each of said lenses being characterized by the central radius of the optical zone, asphericity and toricity. Selection of the lens is founded upon the use of a photokeratogram to determine parameters of three rings (the three ovals shown being the images of these three rings on a cornea) pertaining to its adjusting mark, the thus obtained parameters providing for a choice of the appropriate contact lens with a high degree of accuracy, while reducing the number of repeated fittings in the patient's eye and simplifying the computation procedure. <IMAGE>

Description

SPECIFICATION A method of selecting contact lenses for visual correction This invention relates to ophthalmology, and more particularly, to a method of selecting contact lenses for visual correction.

An object of the invention is to provide a method of selecting the contact lens for visual correction such that will markedly expedite the procedure of selecting the appropriate contact lens by reducing the number of "try-ons" with trial contact lenses.

Another object of the invention is to somewhat relieve the physical discomfort sustained by the patient being fitted with a contact lens by means of trial contact lenses.

According to the invention, there is provided a method of selecting the contact lens for visual correction by using a set of trial lenses, each having an inner surface characterized by the central radius of the optical zone, asphericity and toricity of the sliding zone, wherein, according to the invention, a photokeratogram is obtained by means of a photokeratometer with known magnification of the objective and topographic angles corresponding to the adjusting mark.

The photokeratogram is used to determine the inner surface of the contact lens nearest to the cornea. To this end, the image diameters of the three rings, one of which occupies the center of the photokeratogram, the second lies closer to the middle, and the third nearer the outer edge of said photokeratogram, are determined, followed by determination of the coordinate-axial radius (R,r) for each of the diameter values allowing for the photokeratometer objective magnification and topographic angle corresponding to a given ring, then used to determine the values of the central optical zone radius and asphericity of the sliding zone on the inner surface of the nearest fitting trial contact lens, toricity of the sliding zone of the same lens being determined by the change in another diameter pertaining to one of the photokeratogram rings, whereupon the contact lens conforming to the resultant values is selected from the trial set.

The advantage of such a procedure applied to the selection of contact lenses for visual correction resides in that the number of the rings and the parameters required to choose the contact lens is substantially reduced dispensing with a complex special mathematical processing, in particular that calling for use of the computer, on the one hand, while ensuring an accurate choice of the contact lens avoiding repeated placement of trial lenses into the patient's eye on the other.

The invention is further described, by way of example only, with reference to a single drawing in which there is shown the normal photokeratogram of a patient's eye obtained wih any of the known photokeratometers having an adjusting mark in the form of concentric rings.

To accomplish the method of this invention it is necessary to provide a set of trial contact lenses.

Each contact lens of this set is characterized by the central radius Ro of the optical zone, asphericity A and toricity T of the sliding zone arranged on the inner surface. As a rule, such a set of trial contact lenses comprises about 500 pieces having the referred to values of Rot A and T within the range of 7.00 to 8.50 mm at an increment of 0.05 mm for Rot from 0.4 to 1.0 mm at an increment of 0.3 mm for A, from 0 to 0.6 mm at an increment of 0.3 mm for T. Furthermore, to accomplish the method of this invention it is likewise necessary to obtain a photokeratogram which can be done on any photokeratometer, for example, such as described in US Patent No. 3,598,478, US. Cl.

351-6, 1971, with known objective magnification and topographic angles corresponding to its adjusting mark, We do not dwell on the photokeratogram since it is well known to those skilled in the art, and seek to focus attention on selecting the photokeratometer mark required to accomplish the method of this invention.

Although the heretofore known methods of selecting the contact lens which rely upon the use of photokeratometric technique require at least seven rings of the adjusting mark, the method of this invention yields sufficient accuracy with only three rings. Hence, the appended drawing shows only three ovals observed as the images of the three rings on the cornea. To accomplish the method, one oval, generally designated by the reference numeral 1, should fall onto the center of the corneal surface, while an outermost oval 3 should be close toward the edge of the corneal surface, the oval 2 assuming the position approximately halfway between the ovals 1 and 3.

Using the obtained photokeratogram with the known objective magnification K and topographic angles a1, a, and a3 the parameters required to select the appropriate trial contact lens including the central radius of the optical zone, asphericity and toricity of the sliding zone are determined, as hereinafter described in detail. For this purpose, the photokeratogram is used to produce the straight line A-A which coincides with the direction of the major axes of the ovals. The line corresponds to the major plane section of the corneal surface.Diameters DX, D2 and D3, three ovals of the photokeratogram, one of which, as hereinbefore described, is placed in proximity to the photokeratogram center (D1), the second (D2)-closer to the m#iddle, and third (D3)-nearnr the edge of said photokeratogram are measured along line A-A. These photokeratogram rings approximately correspond to the center, middle, and edge of the part of the corneal surface adjacent the contact lens.

Then the diameter of one of the photokeratometry rings, for example, diameter d, of the oval 1, disposed in proximity to the center of the photokeratogram, is measured along line B-B lying at right angles to line A-A which coincides with the major axes of the photokeratometry ovals.

Thenceforward, the values of the coordinateaxial radii of the corneal surface F1, R2, R3 and r, are computed. Extensive research allows the conclusion that with a sufficient degree of accuracy the computation can be performed using the formulae: Rl=D1 12K Sin a1; R2=D2 12K Sin a2; R3=D3 I 2K I Sin a3; r,=d, I 2K Sin a1; where:: K is a linear magnification of the photokeratometer objective employed in the obtainment of the photokeratogram; a1, a2, a3 are the topographic angles formed by the normals to the corneal surface at points of reflection of the major ray of the beams producing images of the corresponding photokeratometry rings.

It must be understood that, to anyone skilled in the art to which this invention relates, other computation formulae will occur, however, the spirit of the method of this invention rests upon the need to ascertain the coordinate-axial radii of the corneal surface for subsecuent selection of the contact lens.

The values of topographic angles a1, a2, and a3 included in the above-mentioned formulae can be determined by a standard photokeratogram, for example, that of a spherical surface with a known radius using any known technique.

Upon completing the computation as above described, values (R,~r,) are determined, which characterize toricity T of the sliding zone on the inner surface of the trial contact lens, as well as differences (R2~R,) and (R3-R2) which characterize asphericity of the profile of the cornea section taken in the center and at the periphery, the two differences along with R1 are considered in selecting the nearest value of the central radius (Ro) of the optical zone and asphericity (A) of the sliding zone on the inner surface of the trial contact lens, such selection being conducted with specially designed tables similar to those suggested, for example, in International ophthalmology clinics, 1969, v. 9, No. 2 pp. 353-359, Contact lenses, Paul Cochet. Hugues Amiard.

Using the values of Row A and T determined with the help of the photokeratogram as above described, the trial contact lens most fitting to the patient's cornea is selected from the set. Its conformity to the corneal surface of the patient's eye is tested by the fluorescein method.

Subsequently, a copy of the trial lens with the desired refractive power is made.

Use of this invention allows considerably more expeditious fitting of the contact lens for visual correction in the patient due to a sharply reduced number of measurable parameters.

While we have described our invention with respect to the details of a preferred embodiment thereof, many changes and variations will occur to those skilled in the art upon reading this description, and such can obviously be made without departing from the scope and spirit of our invention as set forth in the appended claims.

Claims (2)

Claims

1. A method of selecting the contact lens for visual correction using sets of trial contact lenses, each having an inner surface characterized by the central radius of the optical zone, asphericity and toricity of the sliding zone, characterized in that, first a photokeratogram is obtained with a photokeratometer having pre-set values of objective magnification and topographic angles corresponding to the adjusting mark, whereupon said photokeratogram is used to determine the inner surface of the trial contact lens most fitting to the corneal surface of the patient's eye, wherefore image diameters of three rings pertaining to the adjusting mark of the photokeratometer are determined, one of said rings is situated in the central part of the photokeratogram, the second-near its middle part, and the third near the outermost edge of said photokeratogram, followed by determination of the coordinate-axial radius for each of the diameter values allowing for the photokeratometer objective magnification and topographic angle, corresponding to a given ring, then used to determine the values of the central optical zone radius and asphericity of the sliding zone on the inner surface of the nearest fitting trial contact lens, toricity of the sliding zone of the same lens being determined by the change in another diameter pertaining to one of said photokeratogram rings, whereupon the contact lens conforming to the resultant values is selected from the trial set.

2. A method of selecting the contact lens for visual correction substantially as herein described with reference to, and as shown, in the accompanying drawing.