The document explains various types of toric RGP (rigid gas permeable) contact lenses, including front surface, back surface, bitoric, and peripheral toric designs. It discusses the importance of stabilization for lens rotation, fitting considerations based on corneal characteristics, and the effects of lens design on vision quality. Additionally, it outlines the challenges faced when fitting toric lenses on corneas with astigmatism and emphasizes the need for appropriate materials and dimensions to ensure optimal fitting and performance.

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.

14. 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.

15. 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.

16. 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

18. 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

19. 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

20. FITTING REQUIREMENTS LENS REQUIREMENT

21. 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

22. • 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.

23. • 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.

24. 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.

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

26. BITORIC LENS FITTING

27.  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.

28. 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.

29. 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.

30. LENS ORDERING

31. TORIC RGP LENSES ADVANTAGES AND DISADVANTAGES
DISADVANTAGES
ADVANTAGES

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

33. 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

34. 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