This document discusses the fitting of spherical rigid gas permeable (RGP) contact lenses. It covers preliminary measures like determining corneal curvature and diameter. Forces affecting lens fit like gravity and tear flow are described. Selection of the first trial lens involves choosing the appropriate back optic zone radius, diameter, and power based on factors like corneal curvature and prescription. Dynamic and static fitting criteria are provided. The lens is assessed for proper movement, centration, and vision. Neutralization of corneal astigmatism by about 90% with a spherical RGP lens is explained through an example.

1. Fitting Philosophies
and Assessment of
Spherical RGP
lenses
Presenter: Urusha Maharjan
Bachelor of Optometry
18th batch

2. Contents
• Introduction
• Preliminary measures
• Forces affecting the lens on eye
• Trial lenses
• Selection of 1st trial lens
• Trial lens fitting assessment
• Dynamic fitting
• Static fitting
• Optimal fitting characters
• Summary

3. Introduction to RGP
• Rigid Gas Permeable
• Hard lens that allows the passage of oxygen in substantial amount for
the normal corneal metabolism.
•Philips.
• A lens that with normal lid pressure, body temperature and water
content at equillibrium with the wearing environment does not mould
to the eye surface.

4. RGPs over SCLs
• High astigmats
• Few special conditions like keratoconus, traumatized corneas, post-
graft, etc.
• Narrow inter-palpebral apertures, enopthalmics
• RGP lenses perform better physiologically and there are fewer adverse
corneal reactions because there is:
- less corneal coverage
- greater oxygen permeability.
- better retrolens tearflow.
• Better deposit resistant
• Orthokeratology

5. Lens selection for astigmatism
• Rigid contact lens candidates:
• Spherical RGP
•Corneal cylinder ≤ 3.00D
•Residual astigmatism ≤ 0.75D
• Bitoric RGP
•Corneal cylinder >3.00D
• Prism ballast front surface RGP
•Low or nearly spherical corneal cylinder
•RA ≥1.00D
• Flexing spherical RGP
•Corneal cylinder is WTR and > 1.50 D and RA is ATR
•Corneal cylinder is ATR and >1.50 D and RA is WTR

7. Patient selection
High motivation
• A highly motivated patient is more likely to be successful.
• Motivation is a key factor in RGP lens wear as the initial adaptation
period will not be tolerated by wearers lacking the desire to be
successful.
• Social wearers do not make ideal RGP patients.
• Special occupational or sports involved patients.

8. Moderate to high prescription
• Patients with a moderate to high prescription tend to be more
motivated than those with a low power requirement.
• The desire for an alternative to glasses is stronger among these
patients.
• As their unaided acuity is poorer they cannot function without some
form of visual correction.

9. Corneal toricity
• Those patients who require an astigmatic visual correction are better
suited to RGP lenses as the quality of vision is superior to soft lenses.
• Patients with significant ATR or oblique corneal toricity often have less
success with spherical RGP lenses because of lens decentration and
residual astigmatism.
Financial considerations
• Anticipated costs involved for the lenses and maintainance should be
fore mentioned.

10. Preliminary measures
1. Corneal radius of curvature
-for selecting the BOZR for the trial lens fitting.
-usually made with a keratometer or video based keratoscope.

11. 2. Corneal diameter
-lens total diameter (TD)
-horizontal visible iris diameter is used as a guide to the corneal
diameter.
-can be measured by using a hand held millimetre ruler or by utilizing a
graticule in the eyepiece of a slit-lamp biomicroscope.
-As a general rule the lens total diameter will be 2.3 - 2.5 mm less than
the HVID.

12. 3. Lid characteristics
-in determining the lens total diameter
-patient maintain primary gaze.
-The tonus in the upper lid can be assessed when the lid is everted for
examination. It can be graded as loose, average or tight.
-Tight lids will pull a lens upward or may squeeze it downward
(watermelon seed effect).
-Loose (heavy, fatty) lids will displace a lens downward.

13. 4. Pupil size
-selection of the appropriate BOZD
5. Spectacle refraction
-The relationship between the refraction, corneal topography and visual
acuity will indicate the type of lens which will most suit the patient.

14. Forces affecting lens
Capillary attraction
• Lens matching corneal contour greater the force of attraction
• If flatter, capillary attraction lessened and moves more easily
• If steeper, capillary attraction lessened but lens doesn’t move
because of suction effect
Gravity
• As the center of gravity moves towards the front surface of lens ,
less support and drop more readily
• Effects of gravity lessened for lenses with negative powers, minimal
ct, steep corneal curvature and larger TD.

16. Tear meniscus
• Essential for lens centration
• Greater the lens circumference of the meniscus, the better the lens
centration
Lid force and position
• Upper lid covers small portion of lens holding the lens in cornea and
lid
• For some patients the lower lid is too high to rest
Frictional forces
• Keeps the lens stationary due to viscosity of the precorneal tear film.
If thin direct friction b/w lens and corneal epithelium.
• Variation in viscosity affect the precorneal tear film and affect the
position of the lens.

17. Fitting Philosophy
A) Alignment fitting (Lid attachment fitiing)
Fit on/ slightly flatter than patient’s flattest meridian
When 1/4th or 1/3rd of lens is tucked under the upper lid (stable)
Smooth lens movement
Normal blinking

19. Criteria:
Slight superior positioning (decentration) with adequate pupillary
coverage inferiorly
Smooth vertical movement with each blink (~1-2mm)
No lid bumping
Lid control of lens must be maintained through all phases of blink

20. A thin, even layer of fluorescein between cornea and lens with apical
alignment/ slight apical bearing (aka feathering)
Adequate peripheral lens clearance inferiorly
Less than 180 of bearing in mid periphery with fulcrum at 3 o’clock and
9 o’clock to prevent post blink nasal/temporal decentration.
No impingement on limbus in any direction

21. B) Inter-palpebral fitting
Patients who have an upper lid near the superior limbus or who have
steep corneal curvatures would typically benefit from an interpalpebral
fitting relationship.
Criteria:
Fit steeper than lid attachment fitted when upper lid is at/ above
superior limbus
Small steep apical clearance will have better fit on ATR astigmatism
and no lens binding occurs and minimizes lens induced changes in
corneal topography

23. Lens optics postioned directly over the pupil
Fluorescein pattern: slight apical clearance with bubbles and dimples
1-2 mm smooth vertical movement
Adequate edge lift for good tear exchange

24. Trial lenses
• Low and high minus- -3.00 D and –6.00 D
• Low and high plus- +2.00 D and +5.00 D
• Diameters- 9.2mm and 9.6mm of each power
• Back optic zone radius-
• 7.00 to 8.40mm in 0.1mm steps
• 7.60 to 8.00mm in 0.05mm steps

25. Trial lens selection is based on:
• Corneal topography- controls BOZR,BOZD,TD
• Corneal size- controls TD, BOZD
• HVID measured (2.3-2.5mm lesser)
• Prescription- controls BVP, TD
• Pupil size- controls BOZD
• BOZD larger than pupil in dim illumination by about 2mm
• Lid position- control aperture size (9.5mm) and TD
• Lid tonus- controls TD

26. Selection of first trial lens
A) BOZR
• Start from BOZR of 0.15mm steeper (for new practitioner) because
0.2mm flatter appears correct
• Then flatten in steps of 0.05mm until central alignment
• Central tear layer thickness(TLT)
•Central alignment (1-5 micrometer)
•Minimal apical clearance (10-20um)- optimize centration and movement
•Great apical clearance(>25um)- transitional bearing/ tight fit/ lens flex
•Distinct apical bearing(TLT=0)- discomfort due to excess lens
lag,unstable centration , rotation around corneal apex, lid margin
irritation

27. Astigmatism and BOZR
Amount of corneal
astigmatism
• 0.00-0.75 DS
• 0.50-1.00 DC
• 1.00-2.50 DC
• Over 2.50 DC
Suggested first fit BOZR
with spherical RGP lens
• Fit on flattest K
reading
• Between 0.00-0.05
steeper than flattest K
• Fit close to flattest K
between 0.05-0.10
steeper
• Back surface toric are
to be considered

28. Selection of first trial lens
• BOZD
2.5-3.5 mm added to pupil diameter
Better measured in dim illumination
• BPR
• Peripheral curve
•Tear meniscuscentration
•Tear flow
•Prevent edge indentation on movement
•Aiding lens removal

29. Initial Trial BC selection
1. Flattest K + Steep K +0.1
2
2. Corneal Astigmatism Factors
Corneal Astigmatism 9.2 mm Diameter 9.6 mm Diameter
0.00 to 0.50 Ds 0.50 D Flatter 0.75 D Flatter
0.75 to 1.25 Ds 0.25 Ds Flatter 0.50 D Flatter
1.50 to 2.00 Ds On Flat ‘K’ 0.25 D Flatter
2.25 to 2.75 Ds 0.25 D Steeper On Flat ‘K’
3.00 to 3.50 Ds 0.50 D Steeper o.25 D Steeper
(Relative To Flat “K”)

30. Selection of first trial lens
• Axial edge clearance(AEC)- range 60-90um
•For 3&9 o’clock staining- closer to cornea(40-60um AEC)
•Excessive AEC- thin tear layer punctate staining
•Larger AEC- destabilze the lens due to movement
• TD
• Larger TD- position upper edge behind upper lid
• Aspheric design
• Excess AEC- steepen PAR/reduce TD, lens with lower e value
• Little AEC- flatten PAR/ increase TD, lens with higher e value

31. The basic rules of thumb in making alterations to lens parameters are
outlined below.
• A change in BOZR of 0.05mm is equivalent to 0.25D change in power if
radius is in region 7.80mm
• A change in BOZR of 0.05mm results the BOZD be changed by 0.5mm
to maintain the same fluorescein pattern.
 For most rigid lens materials today, the realistic minimal centre
thickness is approximately 0.14mm.

32. Trial lens fitting (patient preparation)
• Describe the sensation- feeling of F.B, adaptation time.
• Practitioner confidence- confident handling of RGP lenses, explain why
discomfort
• Technique to promote adaptation
• Use viscous wetting solution, ask to close and look down for 5- 10
seconds to minimize lens dislocation due to excessive blink.
• Open the eyes and blink gently
• Look in primary gaze and superiorly for few minutes to minimize
reflex tearing.

33. Any excessive tearing results in an inaccurate static or dynamic
appearance.
Topical anesthetic can be used for initial comfort.
This is because:
• The fluorescein may be washed away too quickly, resulting in a
misleading dark pattern.
• The dynamic fitting will be more unstable with a tendency toward an
excessive amount of lens movement

34. Vision assessment
A) Lens front surface wetting
Quality of the wetting of the front surface of the lens prior to
performing an over-refraction should be checked.
• If the lens is wetting poorly, the end point of the refraction will be
uncertain due to the irregular refractive surface.
• If the trial lens wets poorly it should be removed and, before
reapplication, rewetted by rubbing wetting solution onto its surfaces.
• Using contact lens cleaner on the lens may render the surfaces more
hydrophobic.

35. B) Over refraction
- spherical
- Cylindrical
A full sphero-cylindrical over-refraction will be required if an
unacceptable level of vision is obtained with spherical lenses alone.
• The over refraction result will dictate the BVP to be ordered.
• If a significant cylinder is found, consideration of the need for toric
RGP lenses is required.

36. Trial lens fitting assessment
• Lens application
• Patient instruction, solutions and setting time
• Fit analysis
• Dynamic fitting  position and movement
• Static fitting  lens to cornea relationship
• Vision assessment
• Lens front surface wetting – to get better vision
• Over refraction- spherical and cylindrical may need to switch to toric
RGP lenses.

37. Tear Lens power with RGP
• Tear lens under a flexible lens is very thin and has no power
• Tear lens under a rigid lens depends on material rigidity and the fitting
relationship
• If a rigid lens decenters, the tear lens will acquire a prismatic
component in addition to the spherical or sphero-cylindrical optics
dictated by the fitting relationship

39. Decentration Induced Prism
• When a rigid lens decenters, and is possibly tilted by upper or lower lid
pressures, a prismatic tear lens may be induced under it.
• In higher powered lenses, any induced tear prismatic effect may be
insignificant when compared with the prism induced by the
decentered optics

40. Neutralization of Astigmatism
• Cornea/tears interface is optically insignificant
• Tear lens is sphericalized by the back surface of a spherical lens
• This results in a major reduction of corneal astigmatism with a
spherical lens

41. Neutralization of corneal astigmatism
• Assuming K readings of 8.00 mm and 7.60 mm and the following
refractive indices:
ncornea = 1.376,
ntears = 1.336
• Corneal powers in air:
D1 =(n’-n)/r1 = (1.376-1.000)/ 0.008
D1 = 47.00D
D2 = (n’-n)/r2 = (1.376 – 1.000)/0.0076
D2 = 49.47 D
• Corneal astigmatism = D2 – D1 =2.47 D

42. Contd…
• Corneal power under tears:
D1 = (1.376 – 1.336)/ 0.008
D1 = 5.00D
D2 = (1.376 – 1.336)/ 0.0076
D2 = 5.26 D
Corneal astigmatism = D2 – D1
= 0.26 D

43. Contd…
• Astigmatism (in situ) / astigmatism (in air)
= 0.26/ 2.47
= 10.64%
● Rule of Thumb
Approximately 90% of corneal astigmatism is neutralized by a spherical
RGP lens

44. Example
Given:
• Ocular Rx: –2.00 / -1.00 X 180
• K Readings:
• 7.80 mm (43.27 D) @ 180
• 7.60 mm (44.41 D) @ 90
• BOZR of spherical RGP lens: 7.80
mm
• BVP of spherical RGP trial lens:
-2.00 D Sph
• Tears: Front surface
• FF Tears = n’- n
r
• FF Tears = 1. 336 – 1.000
0.0078
• FF Tears = +43.0769

45. • Tears: Back surface, Vertical
meridian
• FBV Tears = n-n’
r
• FBV Tears = 1.000 – 1.336
0.0076
• FBV Tears = –44.2105
• Tear Lens Power: Vertical
• FV Tears = (Front + Back) Tear
Lens Powers
• FV Tears = +43.0769 + (– 44.2105)
• FV Tears = –1.1336 D
• Tears: Back surface, Horizontal
meridian
• FBH Tears = 1.000 – 1.336
0. 0078
• FBH Tears = –43.0769
• FH Tears = (Front + Back) Tear Lens
Powers
• FH Tears = +43.0769 + (– 43.0769)
• FH Tears = Plano
∴ Tear Lens BVP = Pl / –1.13 X 180
Residual Astig. = –0.13 D Cyl X 180

46. Spherical Cornea: Spherical RGP
The tear lens has no much optical role in case of spherical surface of
cornea and spherical back surface of RGP contact lens

47. Astigmatic Cornea: Spherical RGP
 The front surface of the tear lens is ‘sphericalized’ by the back surface
of the lens
 The toric interface between tear lens and cornea has its optical
effectiveness significantly reduced.
 It is usually difficult to fit spherical lenses on corneas with 3.00 D of
corneal astigmatism.
 Some claim that 2.00 D is a more realistic upper limit.

48. Dynamic fit assessment
• Adaptation time is generally 20-30 mins
• Method
• Patient initially looks in primary gaze position
• Use burton lamp and/ or biomicroscope with white light
• Alter patient’s direction of gaze as required

49. Clinical Observation
• Decentration- lens location on the cornea and degree of
decentration assessed by HVID, VVID
• Stability- good if consistent, acceptable movement and position of
rest
• Movement after blinking- Amount, type, speed, direction
• Movement with lateral gaze
• Lower lid influence
• Upper lid influence

50. Dynamic Fitting Assessment: Decentration
Excessive decentration may cause significant problems with visual
performance, limbal and conjunctival irritation and lens instability.
Patient symptomatology is often the best indicator of poor lens
centration.
The decentration of the lens is determined by comparing the relative
position of the geometrical centre of the cornea and the geometrical
centre of the lens.
The simplest method is to use the known values for the HVID and the
lens total diameter.

51. Dynamic Fitting Assessment: Stability
 important for long-term patient satisfaction.
A lens that has consistent, acceptable movement and position of rest is
most likely to perform successfully.
 In the early stages of lens adaptation, stability may be compromised
due to excess tearing.
 Judged once an adequate degree of adaptation has been achieved.

52. Dynamic Fitting Assessment: Movement After Blinking
one of the most important considerations
The lens movement is initiated by the action of the lids when blinking.
The listed components should be assessed:
•Amount
•Type
•Speed
•Direction

53. Lens Movement: Amount
The blinking action of the upper eyelid will cause the lens to move.
The three phases of lens movement are:
•With the downward motion of the lid.
•With the upward motion of the lid.
•Recentring movement following the blink.
This measurement involves assessing the highest point on the cornea
that the inferior edge of the lens has reached on eye opening and then
determining the amount that the lens moves to regain its position of
rest.
The post-blink movement may be as much as 3.0 mm.

55. Lens movement: Type
Smooth: ideal ; near to alignment
Apical rotation : during flat fit
Rocky : when fitted slightly tight in increased corneal toricity
Lid attachment: high rising positions between blinks, lens moves as
though firmly attached with lid
Two part: combination

56. Lens movement: Speed
• Rate as slow, average or fast.
Lens movement: Direction
• If rotates about the apex, whether nasal or temporal
• If oblique or diagonal, indicate start point and finish point

57. False fluorescein patterns
Selection of a steep BCR may result in poor tear exchange and a
misleading small amount of fluorescein centrally.
In certain individuals—particularly dry-eye patients—the fluorescein
will dissipate quickly and may create a “pseudoapical flat”
relationship; therefore, the pattern should be evaluated immediately
after fluorescein instillation.
A “pseudosteep” pattern has been reported in high minus fluoro-
silicone/acrylate (F-S/A) lenses.
Apparently, the edge thickness blocks the fluorescence, giving an
appearance of central pooling.

58. Static fitting assessment
• Enables the practitioner to determine the relationship between the
lens back surface and the anterior corneal surface
Fluorescein application
• Moisten strip with sterile solution
• Shake off excess- if not irritation, tearing, front surface film, altered
dynamic fitting characteristics
• For safety and ease, patient can be made to look infero-nasal and after
raising the upper lid, strip touched at supero-temporal bulbar
conjunctiva.

59. Static fitting pattern
• Corelates dynamic fitting characteristics
• Assessed to determine changes in fit over time
• Method
• Evaluated in primary gaze position
• Lens centred on the cornea
• No lid influence
• Fluorescein and cobalt blue light
• Assess the tear layer thickness

60. Static fitting assessment
A) Central Zone
• Pooling – steep(graded as slightly steep, steep, very steep)
- When BOZR is made shorter
• Alignment – BOZR is close
- thin parallel layer, feather touch
• Touch- flat (slightly flat, flat , very flat)
• Width of pooling or touch zone- measure in horizontal meridian

61. B) Mid periphery
• Contact- when BOZR is shorter
- steep fit
- light, medium, heavy
• Alignment- light bearing
• Fluorescein band adjacent to zone of pooling.
- flat fit

62. C) Periphery
• Must be considered multi- dimensionally.
• Band of fluorescein must be observed and the width and depth of the
clearance(AEC) must be classified.
• Practitioner should look if meniscus exists.
• Due to surface tension effects, the tear film forms a meniscus as long
as the axial edge clearance is not too great. The absence of a meniscus
is indicative of excessive clearance.

64. The second component of the peripheral fitting assessment involves
the depth or clearance between the lens back surface and the cornea.
As the lens periphery lifts further away from the cornea the fluorescein
will be brighter. This variation allows the practitioner to estimate the
axial edge clearance (AEC). It is possible for the clearance to be
excessive while the width of the peripheral zone is minimal.

66. Optimal fitting characteristics
a) Static fitting
• Minimal central clearance
• Light mid peripheral contact zone
• Optimal edge lift
• Average edge clearance

67. b) Dynamic fitting
• Centered (± 0.5mm)
• Stable
• Superior lid coverage- level of comfort may increase
• Movement (important for long term successful wear)
•Smooth
•Vertical (1-2mm)
•Average speed

69. Tight fitting characteristics
a) Static fitting
• Excessive central clearance
• Heavy mid peripheral contact zone
May result in deformation or warpage of corneal topography
• Narrow edge width
• Reduced edge clearance

70. b) Dynamic fitting
• Centered (± 0.5mm)
• Stable
• superior lid coverage
• Movement –vertical <1mm movement
- fast speed but mostly smooth
- for large corneal toricity, rocky about the flatter meridian

72. Loose fitting characteristics
a) Static fitting
• Excessive central touch zone
• Flat mid peripheral zone
• Excessive edge width
• Excessive edge clearance

73. b) Dynamic fitting
• Decentered (> ± 0.5mm)
• High riding, low riding
• Unstable
• Movement
•Apical rotation
•Lid attachment
•variable speed
•>2.0 mm movement

77. Fitting astigmatic cornea
• With the rule – flatter horizontal meridian
• Fluorescein picture-
• Central- elongated H or dumb-bell shape blue touch area
• Peripherally- blue touch area in hori. meri. & green stand-off in
vertical meri.
- Should not occupy more than 1/3rd of lens circumference

78. Fitting in various cases
 The irregular surface between the back of the contact lens and the
front of the cornea is filled in with tears and creates what is called a
“lacrimal lens .”
This interaction creates an optically improved surface by masking the
regular and irregular astigmatism and reducing the HOA .
RGP lenses usually rest on the apex of the cone; so to fit RGP lenses in
keratoconus, lenses that have a tri-curve or more peripheral curves are
used.

79. Fitting philosophies
• The three different types of contact lens fitting philosophies in
keratoconus:
I. Apical Clearance,
II. Apical Bearing or
III. Three Point Touch; Most widely accepted one

80. Apical clearance
There is no bearing or touch in the apical area i.e., the apical area is
clear (vault is present) and the lens bearing is directed towards the
periphery
Reduces the risk of scarring, whorl keratopathy and erosions but the
tightening at the periphery may result in sealing of tear exchange.
These are usually smaller diameter steeper lenses, centered over the
cone which is usually decentered in keratoconus which sometimes
result in lens edge in the visual axis or optic zone bifurcating the pupil
and flare or fluctuating vision.

81. Apical bearing
The optic zone of the contact lens touches or bears on the apex of the
cone resulting in good visual acuity due to flattening of steep corneal
axis
Because of the flatter fitting of the lens on the cornea, there can be
heavy bearing on the cornea resulting in corneal scarring and
intolerance over long term use.

82. Three point touch or divided support
The lens bearing is shared between the apex and the mid peripheral
cornea.
This helps in minimizing the risk of apical scarring as well as facilitates
the tear exchange.
It provides good vision, better comfort and prolonged wearing time.
An ideal fit will show a central feather touch, a peripheral edge lift of
0.5-0.7 mm.
But, as the cone advances, the lens edge may stand off with pooling
and air bubbles under the peripheral edge.

84. Lens ordering
Need to specify:
• Lens design
- front surface
- back surface
• BOZR
• BOZD
• TD
• BVP
• Material
• Tint

85. Summary
The practitioner has many parameters to choose from in deciding the
optimal fit for a rigid lens.
Although system-designed lenses are suitable for many, optimum
comfort or visual acuity should not be compromised if an ideal fit
cannot be achieved.
If careful attention is devoted to proper design and evaluation, a high
success rate with GP lenses should be expected.

86. References
• The IACLE Contact Lens Course
• Module 3: Contact Lens Fitting
• Module 2: Introduction to Contact Lenses
• Contact lenses by Anthony J. Phillips
• CET articles
• Manual of gas permeable contact lenses; Edward S. Bennett, Milton M.
Hom
• Essential Contact Lens Practice; The vision care institute of J&J
• Previous presentations
• Internet