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Good morning everyone. I am Urusha Maharjan here to present on topic fitting philosophies and assessment of spherical RGP lenses. I would like to thank DR. Sanjeev bhattarai for his kind guidance.
They have inherent rigidity similar to PMMA, but somehow due to their O2 permeability they have become popular by the name semisoft lenses • Made up of polymers e.g. silicone resin, polystyrene, polysulfone copolymer and butyl styrene ( Phillips )
low levels of astigmatism left unco rrected in hydroph iliclenses but are correct ed bythe tear len s with an RGPfit
Decentration down in or down out because they lack fulcrum at 3 & 9 o’clock to guide lens vertically during blinking
The size of the pupil in both bright and dim illumination should be measured. The dilated pupil size is important
Certainly there are patients who would benefit from lid attachment designs, such as individuals with low upper lids or those requiring a large overall diameter (i.e., patients with a large pupil diameter or athletes).
No lid bumping because upper lid tucks the upper margin
Impingement: to strike or dash especially with a sharp collision
plus power lenses should be up to 0.5 mm larger than average to help maintain good
A lens with a BOZD that is too small and decentres on the cornea may cause visual problems. centration.
Lenses with a high BVP (greater than ±8.00D) are made larger than average to allow for adequate lenticulation of the front peripheral design.
If the palpebral aperture is significantly smaller than average, the lens total diameter should also be reduced
Spherical lenses may be bi-curve, tri-curve or multi-curve and with each diff erent BOZR the peripheral curve design can result in a constant axial edge lift or constant axial edge clearance design.
Under 0.75D (<0.15mm) – flattest keratometer reading to 0.05mm steeper
0.50-1.00D (0.1-0.2mm) – 0.05-0.10mm steeper than the flattest keratometer reading
1.00-2.00D (0.2-0.4mm) – steeper than the flattest k-reading by approx 1/3rd of the difference b/w the principal meridians
Over 2.00D(>0.4mm) – As above, but an aspheric or toroidal back optic zone should be considered.
The goal of RGP lens centration is to ensure that the visual axis remains within the back optic zone diameter (BOZD) for as long as possible to optimise visual acuity and avoid fl are
As the cornea flattens, a larger BOZD should be considered to maintain alignment over the cornea.
They should have a total diameter of at least 1.4mm less than the horizontal visible iris diameter (HVID) to facilitate tear exchange under the lens and help optimise the alignment of the lens fi t.
We must …
Lens ordering Needs to specify: Lens design – front surface or back surface BVP, center thickness, Material, Tint etc.
Myopic shift Base down prism induced
Calculate the power of each of the tear lens back surface meridians in air, using the K readings as the radii:
The use of a slit lamp to view the fluorescein fit provides a better overall assessment than a Bur ton lamp
As well as allowing metabolic and tear debris to be removed from underneath the lens, the RGP lens must move to enable oxygen exchange due to the tear pump. Unlike soft lens fi tting, there is a signifi cant exchange of oxygen underneath an RGP lens during the blinking cycle.
The first two are difficult, if not impossible, to measure due to the speed and coverage of the lid. The amount of movement that can be most easily measured is the post-blink recentration of the lens. The amount of post-blink lens movement will depend on the nature of the lens fitting.
Likewise, one would expect that a high plus lens may demonstrate a flatter-than-actual base curve fitting relationship because the thick center would attenuate the light more.
The three zones that are assessed when looking at the lens in the static position are the central, mid peripheral and peripheral
Adequate centration (no limbal overlap, BOZD centred over pupil) • TD approx 1.4mm smaller than the HVID • 1-1.5mm smooth movement with each blink • Lens alignment over most of the corneal sur face, or alignment along the flatter meridian in the case of spherical fits on toric corneas • Narrow band of edge clearance
If the lens total diameter is about 9.60 mm or larger and the upper lid crosses the cornea in a normal position, it is likely that the lid will cover the superior part of the lens when the patient looks in a primary gaze direction. This may be an advantage for some patients as the level of comfort may be increased.
Smooth movement for comfort and aslo for removal of tear debris
Good centration is due to a balance of forces acting on the lens (surface tension at the lens edge, a centred parabolic pressure profile under the lens (Hayashi and Fatt, 1980) (relative to atmospheric pressure, positive centrally and slightly negative at the very edge as proved by a concavemeniscus there), gravity, viscous resistance, lid pressure (see Carney et al., 1996) and the lid-lens pre-lens tear film meniscus.
HOA= high order astigmatism
Fitting Philosophies and Assessment of Spherical RGP lenses
and Assessment of
Presenter: Urusha Maharjan
Bachelor of Optometry
Introduction to RGP
• Rigid Gas Permeable
• Hard lens that allows the passage of oxygen in substantial amount for
the normal corneal metabolism.
• 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.
RGPs over SCLs
• High astigmats
• Few special conditions like keratoconus, traumatized corneas, post-
• 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
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
• 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
• 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
• Social wearers do not make ideal RGP patients.
• Special occupational or sports involved patients.
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
• As their unaided acuity is poorer they cannot function without some
form of visual correction.
• 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
• Anticipated costs involved for the lenses and maintainance should be
1. Corneal radius of curvature
-for selecting the BOZR for the trial lens fitting.
-usually made with a keratometer or video based keratoscope.
2. Corneal diameter
-lens total diameter (TD)
-horizontal visible iris diameter is used as a guide to the corneal
-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
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.
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.
Forces affecting lens
• 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
• 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.
• Essential for lens centration
• Greater the lens circumference of the meniscus, the better the lens
Lid force and position
• Upper lid covers small portion of lens holding the lens in cornea and
• For some patients the lower lid is too high to rest
• 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.
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
Slight superior positioning (decentration) with adequate pupillary
Smooth vertical movement with each blink (~1-2mm)
No lid bumping
Lid control of lens must be maintained through all phases of blink
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
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
Fit steeper than lid attachment fitted when upper lid is at/ above
Small steep apical clearance will have better fit on ATR astigmatism
and no lens binding occurs and minimizes lens induced changes in
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
• 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
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
Selection of first trial lens
• 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
Astigmatism and BOZR
Amount of corneal
• 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
• Between 0.00-0.05
steeper than flattest K
• Fit close to flattest K
• Back surface toric are
to be considered
Selection of first trial lens
2.5-3.5 mm added to pupil diameter
Better measured in dim illumination
• Peripheral curve
•Prevent edge indentation on movement
•Aiding lens removal
Initial Trial BC selection
1. Flattest K + Steep K +0.1
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”)
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
• 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
The basic rules of thumb in making alterations to lens parameters are
• 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.
Trial lens fitting (patient preparation)
• Describe the sensation- feeling of F.B, adaptation time.
• Practitioner confidence- confident handling of RGP lenses, explain why
• 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
Any excessive tearing results in an inaccurate static or dynamic
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
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
B) Over refraction
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.
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
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
• 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
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
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
Neutralization of corneal astigmatism
• Assuming K readings of 8.00 mm and 7.60 mm and the following
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
• 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
• Astigmatism (in situ) / astigmatism (in air)
= 0.26/ 2.47
● Rule of Thumb
Approximately 90% of corneal astigmatism is neutralized by a spherical
• 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
• BVP of spherical RGP trial lens:
-2.00 D Sph
• Tears: Front surface
• FF Tears = n’- n
• FF Tears = 1. 336 – 1.000
• FF Tears = +43.0769
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
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
Some claim that 2.00 D is a more realistic upper limit.
Dynamic fit assessment
• Adaptation time is generally 20-30 mins
• Patient initially looks in primary gaze position
• Use burton lamp and/ or biomicroscope with white light
• Alter patient’s direction of gaze as required
• Decentration- lens location on the cornea and degree of
decentration assessed by HVID, VVID
• Stability- good if consistent, acceptable movement and position of
• Movement after blinking- Amount, type, speed, direction
• Movement with lateral gaze
• Lower lid influence
• Upper lid influence
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
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.
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.
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:
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
The post-blink movement may be as much as 3.0 mm.
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
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
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.
Static fitting assessment
• Enables the practitioner to determine the relationship between the
lens back surface and the anterior corneal surface
• 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
Static fitting pattern
• Corelates dynamic fitting characteristics
• Assessed to determine changes in fit over time
• Evaluated in primary gaze position
• Lens centred on the cornea
• No lid influence
• Fluorescein and cobalt blue light
• Assess the tear layer thickness
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
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
• 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.
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.
Optimal fitting characteristics
a) Static fitting
• Minimal central clearance
• Light mid peripheral contact zone
• Optimal edge lift
• Average edge clearance
b) Dynamic fitting
• Centered (± 0.5mm)
• Superior lid coverage- level of comfort may increase
• Movement (important for long term successful wear)
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
b) Dynamic fitting
• Centered (± 0.5mm)
• superior lid coverage
• Movement –vertical <1mm movement
- fast speed but mostly smooth
- for large corneal toricity, rocky about the flatter meridian
Loose fitting characteristics
a) Static fitting
• Excessive central touch zone
• Flat mid peripheral zone
• Excessive edge width
• Excessive edge clearance
b) Dynamic fitting
• Decentered (> ± 0.5mm)
• High riding, low riding
•>2.0 mm movement
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
- Should not occupy more than 1/3rd of lens circumference
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
• The three different types of contact lens fitting philosophies in
I. Apical Clearance,
II. Apical Bearing or
III. Three Point Touch; Most widely accepted one
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
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.
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
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.
Three point touch or divided support
The lens bearing is shared between the apex and the mid peripheral
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
But, as the cone advances, the lens edge may stand off with pooling
and air bubbles under the peripheral edge.
Need to specify:
• Lens design
- front surface
- back surface
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.
• 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.
• Essential Contact Lens Practice; The vision care institute of J&J
• Previous presentations