Mario Giovanzana Milano 20.06.01

SOFT

TORIC CONTACT LENSES

ASTIGMATISM DEFINITION AND OPTIC BASIS An astigmatism, according to Whevell (1817) has been defined as astigmatism or astigmatic ametropia; the ametropia is characterized by a difference in the curvature rays and therefore among the dioptric powers. Gullstrand defined the astigmatic asymmetry of the ocular refraction.

Conoide di STURM

Fig.1

GENESIS OF ASTIGMATISM The standard cause of astigmatism is the toricity of one or more ocular surfaces resulting in a curvature astigmatism. We cannot exclude the possibility of an astigmatism for oblique incidence. On the contrary, an irregular astigmatism is caused by a lack of conformity between the optic axis of the cornea and that of the crystalline lens. An astigmatism defined by index is irregular and usually an astigmatism of the crystalline lens in the incipient cataract. Of the least importance is the astigmatism caused by an inclination of the retinal screen on the optic axis.

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PLACE OF ASTIGMATISM In the majority of cases, an astigmatism can be attributed to the front surface of the cornea. In with-the-rule astigmatism, the vertical meridian of the cornea has a narrower ray and in against-the rule astigmatism, the ray will be flatter. An astigmatism can also occur on the back surface of the cornea. An astigmatism of the crystalline lens represents the most important element of residual astigmatism.

Fig.2

CORNEAL ASTIGMATISM The front corneal surface does not appear spherical. SENFF and VON HELMHOLTZ considered it to be of an ellipsoidal contour but, according to the more recent research by MATTHIESSEN, ERIKSEN, GULLSTRAND and LO CASCIO it has been shown to be distinguishable in two zones. It is the central part of optic zone of a circular form, with a diameter not greater than four millimeters and a middle point decentralized to the ophtalmometric pole and slightly toric. This is followed by a basal, annular zone which is progressively flatter towards the periphery. Because of the pressure exerted by the eyelid, the flattening of the basal zone is a little more accentuated in the vertical meridian. The back surface of the cornea is difficult to see because of it’s proximity to the front surface. ASTIGMATISM OF THE CRYSTALLINE LENS The outer surface of the crystalline lens does not have a spherical curvature but, according to the theories of NORDENSON and LO CASCIO, tends towards the paraboloid. However, as the front surface is flatter than the back surface, a rapid flattening towards the periphery is certain. The crystalline lens is approximately four millimeters thick.

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THE CORRECTION OF AN ASTIGMATISM SPHERIC LENSES Hard and soft spherical lenses are the best solution for correcting low power astigmatism because a coaxial lens (without prism or differences in thickness of the joint) assure an optimal correction and give greater wearing comfort.

Fig.3

ASPHERIC LENSES The study of aspheric lenses was started by CARTESIO more than three centuries ago and is still actual today. This study is based on the law of refraction, from which we observe that the concentration of rays parallel to the optic axis of a lens, do not, when they go out from the face of emergence, meet in a point on the axis itself, but in point of different zones. The zone closest to the optic axis is the PARAXIAL and that close to the edge is the MARGINAL. Consequently, there is a paraxial focus, a marginal focus and many intermediate focuses on the axis. The power of the different zones of a lens changes on passing from the marginal to the paraxial zone, this process is the SPHERICAL ABERRATION. The spherical aberration is so-called because it is generated by the spherical form of its surfaces. My solution was not to generate a spherical surface, but one revolving around the optic axis. Aspheric contact lenses have an aspheric surface (front) and a spherical surface (base) and are assosymmetric. The use of aspheric lenses proved effective because on correcting the spherical aberration they aid vision with better contrast and cleaner images. Spherical and aspherical lenses do not however, assure an optimal correction of astigmatism if the defect is above a certain value (e.g.:2 diopters for hard lenses; 0.75 diopters for soft lenses). With hard lenses and a corneal astigmatism above 2.00 diopters, the undercorrection of the defect is determined through the lowest refraction index of the astigmatic tear lens with respect to that of the lens material. With soft lenses, the flexible characteristics of the material do not allow the tear lens to intervene in the correction of astigmatism.

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TORIC CONTACT LENSES Toric contact lenses have been in use for many years and their use has greatly improved due to improvements in manufacturing techniques and the repeatibility of geometry. Toric lenses can be fitted in the following cases: - when spherical lenses cannot be used for optimum correction because of high values or the type of astigmatism - to improve the fitability through a physiologically more comfortable geometry. These two uses can be separated or, in some circumstances, superimposed. The basic principle of a toric lens is the alignment of the principal lens meridians to the two principal corresponding ocular meridians through different system of stabilization. Toric lenses can be manufactured from hard and soft materials. I have selected those lens types which, during my professional experience, I have been able to verify as the most apt to solve the difficulties already mentioned. HARD LENSES - lenses with a toric inner surface, optic zone and peripheric toric curve - lenses with a base-curve in the spherical inner optic zone and toric peripheric curve SOFT LENSES - lenses with prismatic stabilization - lenses with dynamic stabilization - lenses with prismatic-dynamic stabilization SOFT TORIC CONTACT LENSES Soft toric contact lenses are ideally fitted in cases of non-corneal astigmatism. They can also be an effective solution for other astigmatism. The most widely available instrument used to diagnose a corneal astigmatism is the ophtalmometer. After fitting a spherical soft lens on an astigmatic cornea the ophtalmometer shows that variations between the two principal meridians, the axis of the astigmatism, do not greatly alter. But after fitting a toric soft lens on an astigmatic cornea the difference between these two principal meridians will be greatly reduced, but because the refraction index of the lens material is higher than that of the cornea, will not disappear completely. The toric soft lens presents some problems in stabilization linked to palpebral pressure, because the eye-lid tends to seek the smallest thickness of the lens. Therefore we must pay some attention to the various system of stabilization. In a normal astigmatic cornea, where the horizontal meridian is flatter than the vertical meridian, the lens will rotate to 90 degrees if an inner toric bicurve lens without a stabilization system is fitted. The reason for this rotation can be investigated in the differences of thickness in the junction between the two principal meridians. With an identical eyelid closure axis we find that the thickness of the junction is smaller in the meridian where the radius is flatter, and greater in the steeper meridian.

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GENERATOR OF TORIC INNER SURFACE Gfeller City Crown IT – 4 CNC This kind of lathe allows to produce soft and hard contact lenses, bicurve and tricurve with combination of spherical and toric surfaces.

Fig. 5

EXAMPLE OF TORIC INNER SURFACE Picture 6 shows the internal surface of a soft toric contact lens. The central zone ( yellow pointed out ) is a toric surface, while the peripheric curve is spherical. To stabilize the contact lens in the eye it is not necessary that the peripheric curve is toric.

Fig.6 5

SOFT CONTACT LENS WITH PRISMATIC STABILIZATION This type of lens is the most widely available, because the manufacturing system is consolidated and the result on every power, ( positive or negative ) is reliable. The negative side of this lens is the displacement of the external surface in respect of the inner surface. When one single lens is to be fitted, it is advisable to use a dynamic stabilization. When the decentralization ( fig. 4 and 5 ) and the peripheric areas of stabilization have been established, my computation system gives the value of the lenticular radius in order to guarantee the required edge thickness. This system is developed with a computerized program which can be utilized for both negative and positive lenses. The values of the refraction index, sagittal, base radius, power, total diameter, total thickness, width of the two areas of stabilization, thickness at the edge and decentralization must be input. For a negative lens the central thickness must be included and for positive lens the thickness of the joint between the optic curve and the lenticular radius must be included. The program then calculates the other variables which are necessary for manufacturing this lens. EXAMPLE OF PROGRAM APPLICATION Negative lens

Fig.7 Negative lens

Fig.8

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Positive lens

Fig.9 Positive lens

Fig.10

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SOFT CONTACT LENS WITH DYNAMIC STABILIZATION This type of lens utilizes the base surface for the correction of the astigmatism and the front surface for the stabilization. Stabilization is obtained by assuring the smallest possible thickness of the junction on the closing axis of the eye-lids, and opposing a minor resistance to their pressure. By inserting the following data into the computer refraction index, sagittal, central thickness ( for negative lenses ) and junction ( for positive lenses ), base radius, power, optic zone, total diameter, total thickness, width of the area of stabilization and decentralization, the program calculates the values for optic curve, lenticular radius and total diameter. EXAMPLES OF PROGRAM APPLICATION Negative lens

Fig.11 Positive lens

Fig.12 8

SOFT CONTACT LENS WITH PRISMATIC-DYNAMIC STABILIZATION This type of lens is surely the most innovating because it takes advantage of the two previous systems of stabilization without incurring the disadvantages of the prismatic effect. By changing the width of the area P1 and P2 my computation system permits a prismatic profile of the lens, given by a difference of junction. EXAMPLE OF PROGRAM APPLICATION Negative lens

Fig. 13

Positive lens

Fig.14 9