RGP TORIC LENS presentation slideshow.pptx

RGP TORIC

1. RGP TORIC LENS NAOBA MUTUM F.Y. M.OPTOM

2. TORIC RGP CONTACT LENS TYPES : FRONT SURFACE TORIC BACK SURFACE TORIC BITORIC PERIPHERAL TORIC

3. FRONT SURFACE TORIC LENS : A Spherical back surface with a front surface cylinder to correct residual astigmatism . Stabilization is necessary to maintain the correct cylinder axis .

4. SPHERICAL BACK SURFACE A lens with an optical zone of 7.70 mm, a total diameter of 9.20 mm and manufactured in a tricurve back surface design is an ideal initial trial lens for the majority of patients

5. BASE DOWN PRISM Base down prism produces a thickness/weight differential between the top and bottom of the lens. This assists in preventing lens rotation with each blink and in maintaining the meridional orientation of the cylinder axis. The amount of base down prism can be controlled during the production of the lens front surface. The appropriate amount of prism is defined by many variables such as the lens power, lens total diameter, and rotational force of the lids.

6. CYLINDER FRONT SURFACE CIRCULAR DESIGN The optical zone is centred. The amount of prism required for lens rotational stability is less than that needed for a truncated lens. This is because none of the prism base is removed in any secondary manufacturing steps.

7. TRUNCATED DESIGN

8. The truncation must be aligned with the lower lid. The truncation must be slightly rounded to maximize the comfort. If the edges of the truncation are not smoothed they will cause lower lid discomfort. DOUBLE TRUNCATION

9. The use of an additional truncation in the superior part of the lens may help to increase the lens rotational stability. It may be useful when the patient has a very tight upper lid or when the palpebral aperture is small.

10. BACK SURFACE TORIC LENS : The decision to fit a cornea with a back surface toric RGP lens is based on the need to maintain an acceptable physical fitting relationship between the lens and the cornea. When a spherical back surface lens is fitted to a cornea with moderate to high toricity , areas of harsh bearing can be seen in the static fluorescein pattern. Such a fitting can cause significant optical, physical and physiological problems for the patient A significant amount of corneal toricity is necessary for a back surface toric lens to function reliably. At least two dioptres of corneal toricity is required to provide resistance to lens rotation.

11. BITORIC LENS

12. PERIPHERAL TORIC LENS Toric peripheral curves are useful in cases where the peripheral corneal toricity is greater than that measured centrally .

13. OPTICS OF TORIC LENSES 1 1 2. 3 4 5 If the lens back surface is made toric it will not correct all the corneal astigmatism . For example, when a spherical, non-flexing RGP lens corrects 90% of the corneal astigmatism and the over-refraction shows just 10% of the original cylindrical component, it can be claimed that all the ocular astigmatism is corneal. If, on the same cornea, the lens is fitted with a toric back surface design, the corneal astigmatism can not be as fully corrected as it was with a spherical back surface shape. An over-refraction will contain a cylindrical component which is not physiological in origin. The cylinder is induced by making the lens back surface toric .

14. CALCULATING RESIDUAL ASTIGMATISM When working with toric RGP lenses it is always worthwhile to calculate the expected residual cylinder that would be present if a non-flexing spherical lens were on the cornea. The calculated residual astigmatism, total refractive astigmatism and corneal astigmatism are related to one another by the following formula: CRA = TRA – CA In this example, a spherical RGP lens fitted to the cornea requires a –1.75 D cylinder at an axis of 90 degrees to provide the highest quality vision.

15. INDUCED CYLINDER POWER The dioptric power of the induced cylinder can be calculated for any lens if the following values are known: • Refractive index of the lens material. • Radii of curvature of the principal meridians For any toric back surface/spherical front surface lens, the induced cylinder power may also be calculated from the lens cylinder power measured by the focimeter

16. When trial fitting, a lens that is as close as possible to the final design to be ordered should be used. One of the most important lens parameters to consider is lens total diameter . If a trial lens with an excessively large diameter is used, the final lens can have significantly different dynamic fitting characteristics when the prism is incorporated. This is due to the thickness differences and substantial weight increase that will occur with the incorporation of the prism ballast. As a general rule, smaller (8.80 mm) diameter lenses are used for corneas with steeper than average curvatures and/or smaller diameters. Larger (9.20 mm) diameter lenses are more suitable for patients with flat corneal curvatures and/or larger corneal diameters FITTING RGP TORIC CONTACT LENS

17. When fitting front surface toric RGP lenses, it is best to use the smallest possible lens total diameter that provides acceptable dynamic performance characteristics. When determining the appropriate lens diameter the practitioner must assess : • Palpebral aperture size. • Lid characteristics. • Corneal topography. • Lens power. • Lens centration

18. FITTING REQUIREMENTS LENS REQUIREMENT

19. The trial fitting assessment should include all the dynamic and static lens fitting characteristics that are important for any RGP lens. • Centration: The prism ballast lens will tend to ride low on the cornea. If a non-ballasted trial lens is used, ensure centration is acceptable, as problems such as riding excessively low will be accentuated once the prism is added to the design • Lid interaction and lens movement. If a prism ballast trial lens shows very little movement with each blink, the design needs to be altered to increase the movement. A thick, non-moving lens is likely to cause problems such as 3 and 9 o’clock staining

20. • An alignment central fitting pattern is ideal as it gives the best chance of achieving stability and good movement. • Most front surface toric lenses will decentre inferiorly. Excessive decentration beyond the limbus should be prevented as it is likely to result in discomfort and poor visual performance. • Some movement (> 0.5 mm) is needed for successful performance. The weight of the front surface toric lens may result in reduced movement. An immobile lens will, in most cases, cause significant problems such as 3 and 9 o’clock staining.

21. • For optimum vision, the front surface toric lens must be rotationally stable to ensure that the cylindrical correction is properly aligned. Variable rotation with each blink can result in substantial visual disturbance. • Since most front surface toric lenses decentre , the practitioner must give careful consideration to the degree of pupil coverage. Inadequate coverage will result in visual problems. These are most likely to be apparent at night when the pupil is dilated.

22. FITTING BACK SURFACE TORIC RGP LENSES One of the most important factors in achieving a successful RGP back surface toric fitting is the selection of the lens material. The key issues are: • Dimensional stability. The lens must maintain its shape to ensure physical compatibility between the back surface and the cornea is maintained. With some materials the degree of back surface toricity may vary over time and, as a result, the quality of vision may deteriorate with wear. • Oxygen transmissibility. As toric lenses are thicker, the Dk/t is less than an equivalent spherical lens of similar power. Selection of a lens with moderate to high oxygen permeability is necessary to ensure that the cornea’s physiological requirements are satisfied. • Optical stability. A stable lens material minimises the risk of lens warpage. An irregular change in the shape of the lens would result in a degradation of the quality of vision.

23. It can be applied when the induced cylinder can be used as the correcting power for the physiological residual astigmatism

24. BITORIC LENS FITTING

25. SPHERICAL LENSES ON TORIC CORNEAS Poor vision. This may be due to residual astigmatism, unstable fitting characteristics and lens flexure. • Poor centration. This may be due to the lack of physical compatibility between the lens and the cornea. • Lens rocking on flat meridian. This can cause greater lens awareness and visual instability. • Unstable fitting. Poor physical compatibility between the lens and the cornea will cause the lens to move excessively and to centre poorly. • Lens flexure. As the cornea becomes more toric , a spherical RGP lens is more likely to flex to conform to the corneal shape. This can reduce the quality of vision.

26. Increased bearing pressure. If the back of the RGP lens and the cornea are not physically compatible, areas of the cornea will be exposed to increased bearing pressure as the lens presses against the surface. • Corneal distortion. Continuous pressure of an RGP lens against the cornea may cause the corneal shape to alter in either a regular or irregular manner. • Spectacle blur. Any change in corneal shape may affect the refractive status of the eye. This may cause a visual problem when the patient transfers from contact lenses to spectacles. Poor blinking due to discomfort. If the lenses are uncomfortable, the patient is less likely to blink properly. This may affect the frequency of blinking and/or the completeness of each blink. • Epithelial damage. Any significant degree of physical incompatibility between the lens and the cornea may cause damage to the epithelium due to the increased bearing that is produced. • 3 and 9 o’clock staining. Poor lens centration, movement and physical incompatibility with the cornea are likely to result in an increased level of 3 and 9 o’clock staining.

27. An example of a spherical lens on a toric with-the-rule cornea, where the fluorescein pattern clearly shows excessive localized clearance and bearing - an unacceptable fitting pattern.

28. LENS ORDERING

29. TORIC RGP LENSES ADVANTAGES AND DISADVANTAGES DISADVANTAGES ADVANTAGES

30. A prism-ballasted rigid toric lens with single truncation SOME FIGURES

31. Left eye with high corneal astigmatism. Keratometer reading 8.19 mm along 176, 7.47 mm along 86. Fitting with spherical (7.70 mm) back optic zone radius reveals harsh bearing along the horizontal (flatter) meridian and poor centration Same left eye wearing an alignment fitted rigid lens using a toroidal curve of back optic zone radius 8.15 × 7.50 mm

32. A right lens with a toroidal back optic zone fitted in alignment. Keratometer reading is 8.13 mm along 160 and 7.62 mm along 70. Lens back optic zone radius 8.10 × 7.70 mm. The 8.10 meridian is marked with grease pencil and can be seen aligning well with the 160 meridian. There is no significant rotation, thus permitting accurate correction of residual astigmatism, as well as induced astigmatism, with a front surface cylinder. Same right eye as in Figure 17.5. Lens back optic zone radius 8.05 × 7.75 with 8.05 meridian marked with grease pencil. This should be located along the 160 meridian, but, as shown, this lens rotates badly, thus permitting only the accurate correction of induced astigmatism with a front surface cylinder