Fitting an Astigmatic Patient is really a challenging.Though fitting a Toric Cornea is another challenge in CL Dispensing practice.This Slide will give you a basic considerations in RGP Toric lens.

1. RGP TORIC CONTACT LENS
MS.PRIYADHARSHINI.M
LECTURER
SCHOOL OF ALLIED HEALTH SCIENCES
VINAYAKA MISSION’S RESEARCH FOUNDATION-DU

2. LEARNING OBJECTIVES
• Understand about Toric CL, RGP Toric CL
• Able to know about each RGP CL designs
• Able to understand the dispensing tips for Toric CL
• Able to understand the Fitting RGP toric lens
• Will be able to order the lens design with the suitability of Error

3. CONTACT LENS FOR ASTIGMATISM
• Toric contact lenses correct for astigmatism issues that arise from a different
curvature of the cornea or lens in eye (referred to as regular astigmatism,
corneal astigmatism or lenticular astigmatism).
• In these cases, the cornea or lens is curved so that the refraction of eye
differs between the vertical and horizontal planes.
• This causes blurry vision and trouble seeing fine details.

4. WHAT ARE TORIC CONTACT
LENSES?
• A toric lens is a contact lens that's shaped in a particular way. Standard
contact lenses have a spherical surface
• A torus, in contrast, is a geometric shape that looks like a donut.
• The shape of toric contact lenses creates different refractive, or focusing,
powers on the vertical and horizontal orientations.
• The refractive strength increases or decreases gradually as you move around
the lens[1].
1.https://coopervision.com/about-contacts/toric-contact-lenses

5. WHAT IS RGP TORIC LENS?
• RGP toric lenses are indicated for high amounts of prescription astigmatism,
high corneal toricity or when a smaller diameter or other parameter is needed.
They are not readily available in a soft lens design such as a steeper or flatter
base curve.[2]
2.https://www.2020mag.com/ce/understanding-and-working-with-toric-31D6B

6. ADVANTAGES OF RGP TORIC
CONTACT LENSES[3]
• Significant advantages over soft toric lenses or inadequately fitting spherical RGP lenses.
• As corneal astigmatism increases the fitting characteristics of a spherical RGP lens will
become less stable.
• An RGP lens can be readily manufactured to provide a suitable back surface shape to
match that of the cornea.
• Physical compatibility between the lens and the cornea is important for successful long
term wear.
3.IACLE

7. • The quality of vision is optimized by the rigidity of the material.
• The required astigmatic correction is often less with an RGP lens due to the effects of
the tear lens.
• The physiological benefits of RGP lenses over soft lenses are well known.
• These benefits result from the high level of oxygen that can be supplied to the cornea
by the highly permeable materials and the significant tear exchange that occurs with
each blink.

8. DISADVANTAGES
• Toric lenses are thicker than standard spherical lenses and can cause problems for
some patients due to:
• Increased lens awareness.
• Increased levels of 3 and 9 o’clock staining.
• Reduced oxygen permeability.

9. • Lack of uniformity can cause a transient or persistent decrease in the level of comfort.
• In some cases the production of a toric lens makes it more difficult to ensure an overall
uniform edge profile.
• Significant differences between the corneal and spectacle cylinder axes may cause a
problem with the correction of any residual astigmatism when a front surface cylinder
must be produced on the lens.
• In such cases the principal meridians on the back and front surfaces of the lens will be
misaligned.

10. VERIFICATION OF RGP TORIC LENSES
• When a new patient visits and wears RGP contacts, verify if they are toric.
• Using a lensometer, check the power.
• Then with a radiuscope check curvature.
• Inspect for lens deposits, if any
• Watch : https://www.youtube.com/watch?v=KO_KRjv2ZPk
• Watch : https://vimeo.com/203038117

11. INDICATIONS FOR RGP TORIC
CONTACT LENSES
RGP toric lenses are indicated for
• high amounts of prescription astigmatism,
• high corneal toricity
• when a smaller diameter or other parameter is needed.

12. PROBLEMS WITH RGP TORIC
LENSES[4]
The most common problems associated with toric lenses are usually related to
comfort and vision.
• Problems with visual acuity usually are constant or variable perceptions of vision.
• These occur due to improper lens positioning or errors in calculating the required
correction.
• In some instances the solution is simple
4.https://www.reviewofoptometry.com/article/a-fix-it-guide-for-toric-lens-
fits#:~:text=The%20most%20common%20problems%20associated,in%20calculating%20the%20required%20correction.

13. TROUBLESHOOTING
• Lens Awareness
• Rotational Stability
• Sphero-Cylindrical Over refraction
• Dryness and Deposits

14. PROBLEM REASON TROUBLESHOOT
lens discomfort increased lens mass and
thickness profile
refit into a thinner lens
lens discomfort deposits or defects inspect the lenses to verify and fit a new lens if necessary
Variable vision when a lens is so flat and
loose that the lid induces
unwanted lens rotation
steepening the base curve to correct this problem
Constant blur a constant misalignment of
the lens axis
place it into its proper position manually, then monitor it
for misalignment after the patient blinks

15. PROBLEM REASON TROUBLESHOOT
constant blur the lens may be too tight monitor the lens for these clinical signs of steepness:
conjunctival congestion and injection, difficult lens
removal, or induced apical corneal distortions due to
vaulting

16. MANUFACTURING OF RGP TORIC
LENSES
• The following manufacturing procedures are described in Bier and Lowther (1977).
• The manufacturer cuts a spherical back surface on a button which is then placed in a
crimper, or collar, which can apply pressure across the diameter. The pressure of the
collar produces a toric surface which can be controlled by increasing the pressure
until the desired amount of toricity is achieved.
• The button, which is in the collar, is then placed on a lathe. A spherical surface is cut
and then polished. When the pressure of the collar is relaxed the flexed button will
return to its original state which will be toric.

17. • The same technique can be applied to both the front and back surfaces of the lens
and may therefore produce a back surface only toric, a front surface only toric or a
bitoric lens.
• Toric surfaces can also be ground by using toric tools or toric lens generators. The
main disadvantage of the former technique is the large number of tools required to
cover all the possible combinations of curves which are needed.
• The disadvantage of toric generators is that they are usually limited to front surface
designs and the complexity of the curves they can generate is limited.

18. • Prism ballast is created using a mounting tool whose angular relationship to the
main axis of the tool is adjustable. By tilting the semi-finished lens button so that its
front surface is no longer perpendicular to the lathe axis, prism is generated.
• In practice, a series of fixed-tilt mounts may be employed where each mount offers
a unique amount of prism. Alternatively, a continuously adjustable mount may be
used.

19. FACTORS AFFECTING LENS
ORIENTATION[3]
• Lid torque
• Gravity
• Tear film surface tension
• Lens to cornea fitting relationship
• Lid tension
• Lid position

20. LID TORQUE
• The upper and, to a much lesser extent, lower lids can impart a significant rotational
force to the RGP lens which is counter to the desired fitting characteristic of
meridional stability.
• Some mechanism is usually required to resist or retard the rotational movement of
the lens caused by this force.

21. GRAVITY
• The gravitational force acts to move the lens toward the inferior region of the
cornea.
• In most toric lenses the increased thickness results in greater lens mass which is
more likely to be affected by gravity.
• The dynamic fitting characteristics of the lens may be adversely influenced by the
gravitational force.

22. TEAR FILM SURFACE TENSION
• The surface tension of the tear film will act to hold an RGP lens on the cornea as
well as providing some force for centration and stability of the lens.

23. LENS TO CORNEA FITTING RELATIONSHIP
• The physical fitting relationship between the lens back surface and the cornea plays
a major role in the success of RGP toric lenses.
• The relationship must ensure that a stable dynamic fitting is achieved if it is to
provide the highest possible level of vision.
• Generally, a loose fitting RGP toric lens will not be successful.

24. LID TENSION
• During a blink, the upper lid can exert a considerable force on the lens.
• This can result in a poor fitting due to misalignment of the cylindrical axis or
unacceptable vision due to an unstable fitting.

25. LID POSITION
• The positions of both the upper and lower lids can exert considerable influence on
the dynamic fitting of the lens.
• Care needs to be taken to select the most suitable type of toric lens for the patient
requiring RGP lenses.

26. TYPES[3]
• Front surface toric
• Back surface toric
• Bitoric
• Peripheral toric

27. FRONT SURFACE TORICS
• The front surface toric lens has a number of very specific design features. These are
intended to meet both the requirements for fitting a spherical lens and those that
maintain the meridional orientation required to correct the patient’s astigmatic error.
• The following are the components of Front surface toric lens design:
• Spherical back surface. The back surface design principles are similar to those
employed for standard RGP lens fittings.
• Base down prism. This is the key factor in maintaining the lens in the proper
meridional orientation to correct the astigmatism

28. • Cylindrical front surface. The visual quality is dictated by the correction of any
residual astigmatism. This is achieved by inclusion of a cylindrical correction on the
front surface of the lens.
• Circular design. The lens can be produced in a traditional circular design where
the lens back surface is rotationally symmetrical about its central axis.
• Truncated design. When the prism is unable to maintain the orientation of the
cylinder, the use of an inferior lens truncation may be employed to stabilize the
by interacting with the lower lid margin.

29. • This lens design is used when there is over 1.25D of prescription cylinder correction
with a spherical or near spherical cornea.
• This lens is ordered with a cylinder power and axis.
• To prevent the lens from rotating, the lens is ordered with a prism base down to act
as weighted ballast or stabilizer.
• A prism of usually 1.5Δ base down will keep the lens from rotating.

30. • The lens is also dotted at 6 o'clock for diagnostic purposes when dispensing and for
evaluations at later visits.
• The lab will do the dotting of the lens and should be verified when received from the
lab. Do this before dispensing.
• Use the lensometer positioning the reticle hairline at 90 degrees and move the
center of the target to the point 1.5Δ base down.
• Confirm the prescription.
• Again, confirm that the dot is at 6 o'clock when on the patient.

31. • Wait approximately 15 minutes for the lens to settle and the tear layer to stabilize.
• Remember, left-add, right-subtract (LARS) since this lens should be analyzed
similar to a soft lens.
• If the dot isn't at 6 o'clock it needs to be re-ordered with corrected cylinder axis or
with more prism ballast, or a steeper/tighter fit if it rotates too much with-the-blink[5].
5.https://www.2020mag.com/ce/understanding-and-working-with-toric-31D6B

33. • For the K reading 41 D which will be the suitable Base curve?
9.6
• For the K reading 46 D which will be the suitable Base curve?
8.8

34. EXAMPLE
• Prescription: OD -2.50-2.50x180
K Readings: OD 44.00/44.00X090
Lens design considerations: Order the prescription sphere, cylinder power
and axis from the prescription. Include with 1.5Δ ballast and dot @ 6 o'clock.
Lens ordered:
OD 44.00BC, 9.2 dia., -2.50 -2.50 x 180,
1.5Δ prism ballast, dot @6:00 Blue

35. BACK SURFACE TORIC
• This lens design is rarer than the others and is used when there is a highly toric
cornea (3.00D or more) with a spherical correction.
• 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.

36. FITTING REQUIREMENTS
• 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.
• The correspondence between the back surface shape of the lens and the toricity of the
cornea minimizes rotation.
• Smaller amounts of corneal toricity make it difficult to locate the lens with the correct
meridional orientation.
• If such a lens were to rotate on the cornea, the physical compatibility between the lens
and the corneal surface would be poor, leading to discomfort and other problems for the
wearer.

37. EXAMPLE
• Prescription: OD +4.50
K Readings: OD 41.50/46.50 @ 090
Lens design considerations: Fit to the corneal curves (on K) and order
spherical power from the prescription considering any vertex change effects.
Power vertex corrected:
+4.50 at 12 mm = +4.75 at 0 mm vertex
Lens ordered:
OD 41.50/46.50 9.2 +4.75 Blue

38. BITORIC
• A bitoric lens is required in many cases as a back surface toric/front surface sphere
lens will result in an excessive amount of residual astigmatism.
• This can be corrected by producing a front surface cylinder.

39. LENS DESIGN
• To best understand the design and principles of a bitoric lens, the back surface should
be thought of in terms of its ability to provide an acceptable physical fitting relationship
with the cornea.
• The lens front surface design then has to provide the visual correction for both the
spherical and cylindrical components.
• Successful bitoric RGP lenses must maintain a stable orientation on the cornea to allow
the front surface cylinder to provide an optimal visual correction.
• The rotational stability is usually achieved by a close physical fitting relationship
between the lens back surface and the cornea.
• If adequate lens stability cannot be obtained in that manner, the addition of prism
ballast is an option.

41. CONTACT LENS CALCULATOR
• https://coopervision.com/practitioner/tools-and-calculators/toritrack-calculator

42. PERIPHERAL TORICS
• Peripheral toric RGP lenses are modified spherical designs.
• The BOZD of the lens is spherical and only the peripheral curves are manufactured with a
toric design.
• This is a relatively simple lens to produce.
• The toric periphery has no effect on the visual correction.
• The lens can rotate on the cornea without compromising visual performance.
• Physically, the lens will have a better peripheral fitting relationship with the cornea as
long as it is rotationally stable.

43. • The peripheral toric design may be employed in cases where:
• An acceptable central fitting pattern is observed.
• The peripheral axial edge clearance along the steeper corneal meridian is
unacceptably large.
• In such cases, the lens stability may be improved by more closely matching the
peripheral corneal toricity with the peripheral curves of the lens.
• The degree of peripheral corneal toricity is generally determined by trial lens fitting.
However modern topographical corneal mapping systems may also be used as a
guide

44. • By making the peripheral curves toric, the optical zone of the lens will be elliptical in
shape. The smaller optical zone diameter will be oriented along the flatter principal
meridian of the cornea.
•

46. FEATURES
• Toric peripheral curves are useful in cases where the peripheral corneal toricity is
greater than that measured centrally.
• By producing toric peripheral curves on an RGP lens, the physical fitting relationship
between the lens and the cornea will be improved.
Improved fitting characteristics include:
• Better and more stable centration.
• Reduced localized bearing on the corneal periphery.
• Reduced risk of bubble formation at the periphery which may occur due to
excessive edge clearance.

47. COMMON FITTING PARAMETERS
• Refraction details
• Keratometer
• HVID/VVID
• Palpebral aperture
• Lens Diameter Selection
• Base Curve
• BOZR or BOZD
• Compensate Back vertex Distance
• Calculate Residual Astigmatism
• Prism Ballast
• Truncation
• Calculate Lens Power in Mandell – Moore form

48. FRONT SURFACE TORIC FITTING
1.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
• It is important to recognize that the formula will only give an approximate result which can
be used as a guide to determining the best course of action when fitting the patient. It is
advisable to apply a lens to the cornea to ascertain the sphero-cylindrical over- refraction
data.

49. • There are many sources of error in calculating the residual astigmatism. Some of
these include:
• Inaccurate keratometry calibration and/or measurement.
• Inaccurate ocular refractive information.
• Significant misalignment of the axes of the refractive and corneal cylinders.
• The measured residual astigmatism may also be significantly different from the
calculated residual astigmatism due to the flexure of the RGP lens on the cornea.

50. CRA EXAMPLE
• Spetacle Rx -3.25/-2.00 x 90
• Keratometry 7.80 @180 (43.25 D)
7.85 @ 90 (43.00 D)
• Corneal cylinder = -0.25 D x 90
• Calculated residual cyl = -1.75 x 90

51. EXERCISE
• Spetacle Rx -4.25/-1.00 x 90
• Keratometry 7.80 @180 (43.75 D)
7.85 @ 90 (43.00 D) What will be the CRA and Corneal
cylinder?
• Corneal cylinder = -0.75 D x 90
• Calculated residual cyl = -0.25 x 90

52. 2.PRISM BALLAST
• Normal lid forces acting on a spherical RGP lens often cause the inferior lens edge
to rotate in a nasal direction i.e. anticlockwise for the right eye and clockwise for the
left eye.
• To overcome normal rotational forces, the RGP lens must be designed so that its
orientation on the cornea is more stable.
• The most common method of ensuring stability is to manufacture the lens with a
prism incorporated in the design.

54. • Answer is Option 2
• Why??
• This gives ‘Clockwise Add, Anticlockwise Subtract’, or CAAS.
• A common variation of the above rule is the ‘Left Add, Right Subtract’ or LARS rule.
• This concept relies on the fact that most of the reference marks are found in the
vertical meridian on the lower part of the lens.
• Clockwise rotation of the lens causes the reference line to move left and we need to
add the rotation to the axis.
• Anticlockwise rotation causes the reference line to move right and we must subtract
the rotation from the axis.

55. EXERCISE
• Consider the following spectacle prescription: Plano/-2.00 x 80
• If the reference lines on the lens are seen to rotate by 10 degrees in a
clockwise direction, the cylinder on the eye would have an axis of
• The same if rotated Anticlockwise,what is the axis of the cylinder?
• Clockwise 90*
• Anticlockwise 70*

56. STABILISATION – TRUNCATION[7]
• When a lens is truncated, a portion of it is sectioned off.
• It is usually 0.50 to 1.5 mm on the lower edge of the lens.
• The amount sectioned off will depend on the size of the lens; larger lenses require
greater amounts than smaller lenses.
• Occasionally the upper edge is sectioned off as well resulting in a double truncated
lens.
7.https://www.opticaltraining.com/html/continuing_ed/wbt/NCLE/Advanced_Techniques/page_six.html#:~:text=Truncation%3A%
20When%20a%20lens%20is,greater%20amounts%20than%20smaller%20lenses.

57. • The truncation will serve to stabilize a lens when the lower flat edge comes to lie
adjacent to the lower eyelid margin.
• Truncation is often combined with prism ballast.
• When a lens is truncated its diameter is effectively reduced which results in a looser
fit.
• To compensate for this the base curves of truncated lenses are generally made
somewhat steeper.

58. DOUBLE SLAB-OFF LENS[7]
• This technique creates a lens which is thicker along its central body which lies along
the palpebral fissure and thinner along the inferior and superior edges which come to
lie under the upper and lower lids.
• This technique is comfortable since there is no lid impact along the inferior surface.

59. • However it does not offer as much lens stability as the truncated or posterior toric
techniques.
• Double slab-off lenses are often combined with a prism ballast to help prevent
rotation.

60. FITTING OPTIONS
• Spherical trial lens
• Prism ballast spherical lens
• Diameter 8.80 - 9.20 mm
• Acceptable static and dynamic fitting
• Assess rotation of prism base

61. 3.TRIAL FITTING
• Lens diameter between 8.80 and 9.20 mm
• Determine optimum BOZR for alignment fitting in the static assessment
• Lens as close as possible to the final design

62. 4.TRIAL FITTING ASSESSMENT
• The trial fitting assessment should include all the dynamic and static lens fitting
characteristics that are important for any RGP lens. Key factors for careful analysis
include:
• 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.

63. FIT DIAMETER OF THE LENS USEFUL FOR
TRIAL SMALL
Wide palpebral aperture
Steeper corneas
Minus power
When lenses center well
TRIAL LARGE
Normal lid margin location
Strong lid force
Flatter or larger corneas
Plus powers

64. OPTIMUM FITTING CHARACTERISTICS
• 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.

65. • 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.
• 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.

66. BITORIC FITTING-PRESCRIPTION
GUIDELINES[6]
I)POSTERIOR CENTRAL CURVE (PCC) AND POSTERIOR OPTICAL ZONE (POZ)
• Both corneal meridian should be fitted with the on K procedure
• An arbitrary initial value of 7.5 mm for the POZ is used as a reference point.
• For every 0.5 mm change in the POZ from the initial diameter of 7.5 mm, a
compensatory change in both radii of curvature of the PCC by 0.05 mm should be
made.
6.Theory and Practice of Optics and refraction by AK Khurana

67. II)LENS POWER[6]
• Ocular correction for the vertex distance is required for both corneal meridian.
• For example, a patient with spectacle refraction of
-6.25 Dsph / -4.75 Dcyl X 180* will require -5.75 Dsph / -4.00 Dcyl X 180*
• When the PCC is fitted steeper ,add minus to the spherical power only
• When the PCC is fitted flatter, add plus to the spherical power only
• For every 0.05 mm change in PCC ,a 0.25 D of sphere power should be changed

68. III)PERIPHERAL CURVES:
• This should also to be made to be toric to have a round POZ
IV)CENTRAL THICKNESS:
• Should be calculated from the spherical thickness chart,using the spherical
equivalent of the bitoric lens

69. THANK YOU