HARD TORIC CONTACT LENSES ASTIGMATISM DEFINITION AND OPTIC BASIS

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1 Mario Giovanzana Milano HARD 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. 1

2 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 againstthe 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. CORNEAL ASTIGMATISM Fig.2 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. 2

3 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. 3

4 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 HARD TORIC CONTACT LENSES Hard toric contact lenses are characterized by the stabilization determined by the difference between the sagittal of the shortest and longest meridian. Optic considerations: If we fit a contact lens with a toric inner surface, the system of adaptation will not be a cornealconform type. To avoid an over-correction of the astigmatism, the curvature rays of the lens will not be identical to those of the cornea. This would be determined by the greater refraction index of the lens material in respect of the tear lens. For example, if the rays of the cornea are: Flattest meridian RADIUS 8.00 mm DIOPTER at 180 degrees Narrowest meridian RADIUS 7.50 mm DIOPTER at 90 degrees and taking in account that: a REFRACTION INDEX of the tear lens of gives a POWER of D 2.75 a REFRACTION INDEX of the material of 1.46 gives a POWER of D 3.85 It is evident that because the diopter value is higher than 1.10, the stabilization of the lens trough the radius of the narrowest meridian will be incorrect. The problem is solved by using the following lens: Lens with toric inner surface, toric optic zone and peripheric curve. Whit this type of lens it is possible to correct all types of astigmatism caused by a torical cornea. Stabilization is achieved through the peripheric surface of the steeper meridian by simply changing The relationship between the two principal meridians to obtain a greater optic zone on the steeper meridian. Using the peripheric surface of the lens and exploiting the power of the tear lens we do not exceed the astigmatic correction of the narrowest meridian. A good stabilization of the lens is achieved and compensation with an external torus ( bi-toric lens ) is not necessary. 4

5 The pictures ( from 4 to 7 ) show that with a reduction of the existing flattening among the curves of the narrowest meridian, the optic zone changes from a horizontal to a vertical form ( fig.7 ). By reducing the difference between the two curves of the narrowest meridian, the saggital changes from a difference of 0 on the spherical curve to progressively higher values. Fig.4 Fig.5 5

Fig.6 Fig.7 The picture 8 shows how traditional methods adopt constant variations between both the base curve of the narrowest meridian and the peripheric curve.

6 Fig.6 Fig.7 The picture 8 shows how traditional methods adopt constant variations between both the base curve of the narrowest meridian and the peripheric curve. In this case the optic zone of the vertical meridian is 7.54 mm it will be 2.00 mm. The use of this same ratio does not permit a correct stabilization of the lens on the peripheric surface, because the dimension of the optic zone are too contained. 6

7 Fig 8 This manufacturing system is only feasible for lathing, because this method permits the development of various independent curvatures. Lathing also permits a uniform edge thickness, by eliminating differences between the two sagittals with the use of a fixed decentralising mandrel. The program about lens with toric inner surface, toric optic zone and peripheric curve, developed by me, permits the calculation of both the external and lenticular radius of the flattest meridian, and, in order to reduce the edge thickness, also gives the value of the lenticular radius of the steeper meridian. 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.9 7

$EXAMPLE OF PROGRAM APPLICATION By inserting the following data into the computer: 1. refraction index ( 1.46 ), 2. sagittal ( 1.41 ), 3. sagittal cyl ( 1.52 ), 4. central thickness ( 0.10 ),5.$

8 EXAMPLE OF PROGRAM APPLICATION By inserting the following data into the computer: 1. refraction index ( 1.46 ), 2. sagittal ( 1.41 ), 3. sagittal cyl ( 1.52 ), 4. central thickness ( 0.10 ),5. base radius ( 7.80 ), 6. power ( ), 7. external optic zone ( 8.90 ), 8. P area ( 0.60 ), 9.total diameter ( 9.30 ), 10. total thickness ( 2.20 ) we obtain the values of: 1.external radius ( 8.40 ), 2. lenticular radius ( 7.45 ), 3. T gauge ( ), 4. angle ( ), 5. technical diff ( 1.15 ), 6. lever ( ), 7. T gauge ( 6.11 ), 8. P lenticular radius ( 6.64 ) Negative lens Fig. 10 By inserting the following data into the computer: refraction index ( 1.46 ), 2. sagittal ( 1.41 ), 3. sagittal cyl ( 1.52 ), 4. junction ( 0.15 ), 5. base radius ( 7.80 ), 6. power ( ), 7. optic zone ( 7.80 ), 8. P area ( 0.60 ), 9. total diameter ( 9.30 ), 10. total thickness ( 2.20 ) we obtain the values of: 1.external radius ( 6.36 ), 2. lenticular radius ( 8.54 ), 3. T gauge ( 8.12 ), 4. central thickness ( 0.44), 5. angle ( ), 6. technical diff ( ), 7. lever ( 9.84 ), 8. T gauge ( 7.25 ), 9. P lentic. radius ( 7.21 ), 10. B power ( ) Positive lens Fig.11 8

$By inserting the following data into the computer: 1. refraction index ( 1.46 ), 2. sagittal ( 1.41 ), 3. sagittal cyl ( 1.52 ), 4. base radius ( 7.80 ), 5. power ( 2.00 ), 6. P area ( 0.60 ), 7.$

9 By inserting the following data into the computer: 1. refraction index ( 1.46 ), 2. sagittal ( 1.41 ), 3. sagittal cyl ( 1.52 ), 4. base radius ( 7.80 ), 5. power ( 2.00 ), 6. P area ( 0.60 ), 7. total diameter ( 9.30 ), 8. total thickness ( 2.20 ) we obtain the values of: 1. external radius ( 7.62 ), 2. T gauge ( 9.57 ), 3. central thickness ( 0.25 ), 4. T gauge ( 6.40 ), 5. P lenticular radius ( 6.73 ) Spherical lens Fig.12 LENS WITH BASE-CURVE IN THE TORIC PERIPHERIC CURVED SPHERICAL INNER OPTIC ZONE This type of lens is utilized to improve the positioning of a spherical lens on a toric cornea. It is normally used when the cornea is only slightly toric in the central zone but this toricity tends to increase towards the peripheric zone. This type of lens must include a deeper sagittal on the narrowest meridian, in order to obtain good stabilization. It is evident that in a lens of this type the sagittal of the narrowest meridian ( cylinder ) is of a higher value than the sagittal of the flattest meridian. The thickness at the edge in the vertical meridian will be reduced to utilize a fixed decentralizing mandrel. 9

MULTIFOCAL CONTACT LENSES INTRODUCTION

Mario Giovanzana Milano, 15 th december 00 MULTIFOCAL CONTACT LENSES INTRODUCTION In my professional experience over the years I ve developed both support in the production of contact lense as well their