Optics of RGP contact lens

1. Optics of RGP contact
lenses
Presenter : Pabita Dhungel
B.optometry
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2. Presentation layout
 Introduction to contact lenses
 Why RGP lenses???
 spherical cornea: spherical RGP
 Spherical cornea : Toric RGP
 Astigmatic cornea : Spherical RGP
 Special fitting:
Keratoconus
Refractive Sx RGP fitting
PK RGP fitting
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3. Introduction
 ‘Contact lens’ is a thin transparent lens made up of
different materials like PMMA, HEMA, Silicon –
Acrylic etc
 First conceived by – Leonardo Da Vinci (1508)
 Development
1. PMMA - 1940s
2. Hydrogel CL – 1960s
3. RGP – 1970s
Source: IACLE Module 2
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4. What is RGP lens??????
RGP lenses are those lenses made up of materials
which are permeable to oxygen.
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
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5. Choice for RGP??????????
Better VA- astigmats & irregular astigmats
Only for some conditions – keratoconus , traumatised
corneas , post grafts etc
Better oxygen transmissibility and better retro lens
tear flow suitable for higher Rx
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6. Choice for RGP???????
Safer for extended-wear than hydrophilic lenses
For patient non- compliant with cleaning and
disinfectant procedures, no time to care
For patient who requires steroids and glaucoma
drugs because no absorption as in hydrophilic
In certain specialized area - orthokeratology
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7. Forces affecting lens
 Tear meniscus
- Essential for lens centration
- Greater the lens circumference of the meniscus, the
better the centration
▪ Lid force and position
- Upper lid covers small portion of the lens holding the
lens in cornea and lid
- For some patients the lower lid is too high to rest
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8. 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 decentres, the tear lens will acquire a
prismatic component in addition to the spherical or
sphero-cylindrical optics dictated by the fitting
relationship.
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9. Decentration Induced Prism
When a rigid lens decentres, 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 decentred optics
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10. Diagram for decentration
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11. Flat, Aligned and Steep RGP Fits
For steep cornea, the RGP lens will touch the tip of
the cornea with flat fitting and induce concave lens
like tear film
For aligned RGP as in case of normal corneal surface
the tear lens so formed will be aligned and will have
plane surface with nearly zero power
For flat cornea , the RGP lens will touch the two ends
of the cornea with steep fitting forming a convex tear
film
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12. Diagram of Flat, Aligned and Steep
RGP Fits
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13. Tear Lens Power with Rigid Lenses
Assumptions:
• nTears = 1.336
• nLens = 1.490
• nAir = 1.000
• r0 = 7.80 mm
– flatter = 7.85 mm
– steeper = 7.75 mm
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14. Contd…
TL front surface power (FSTears):
= (n’ – n)/r
= (1.336- 1.000)/ 0.0078
FSTears power = +43.076923 (BOZR = 7.80mm)
In flattening the BOZR by 0.005, BOZR = 7.85mm
FSTears power = +42.802548 (BOZR = 7.85mm)
∆ = +42.802548 – (+43.076923)
= - 0.274375 D
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15. Contd…
Flattening produces a – 0.274375D effect
To maintain the same back vertex power of the
system a compensating +0.274375 D must be added to
the BVPCL in air while ordering
Steepening the BOZR by 0.05mm, BOZR = 7.75mm
FSTears power = +43.354839 (BOZR = 7.75mm)
∆ =+43.354839 – (+43.076923)
 = +.277916D
Steepening produces a +0.277916 D effect
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16. Contd…
To maintain the same BVP of the system a
compensating -0.277916 D must be added to the
BVPCL (in air) when ordering
Rule of thumb:
∆0.05mm in BOZR ≈ ∆0.25 D in the BVP required to offset
∆ in tear lens power
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17. Neutralisation 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
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18. 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
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Fig:Optimal edge width and adequate
clearance

19. Spherical Cornea: Toric RGP
In case of spherical surface of cornea and toric RGP
the back surface should be spherical in nature while
the front surface is toric
These lens are prescribed in the cases where the
astigmatism is not due to corneal surface but due to
lens
E.g astigmatism induced in cases of subluxation of
lens and dislocation of IOL after cataract surgery
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20. 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.
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21. Neutralisation 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
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22. 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
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23. 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
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24. RGP lens : Keratoconus
Keratoconus is a benign,
non inflammatory,
progressive central
corneal ectasia and
thinning resulting into
high irregular myopic
astigmatism with
observable structural
changes appearing in
later stage
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25. Corneal RGP CL
 Two Fitting Philosophies
1. Apical bearing – OZ bears on cone
2. Apical clearance
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26. Apical bearing (Flat fit)
Larger diameter lenses
TD – 9.50 to 11.50 mm
Single back curve
KC cone touches central
cone apex
Lower edge stand away from
cornea
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27. Apical bearing (Flat fit)
Compress the cone
 Corneal flattening /
Spherization
Superior visual
performance
Disadvantage
??Hastens the rate of
corneal scarring
(Sub-bowman’s
stroma)
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28. Apical clearance
Small diameter & thin
lenses (USA)
TD of 6.00 mm to 8.00
mm
BOZR – 5.00mm to 7.5
mm
With Two flatter
peripheral curves
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Text missing???????????

29. Apical clearance
• Advantage
– Less role on corneal scarring
– Well tolerated by atopic eye disease
• Disadvantage
– Optical
• Flare/monocular diplopia
– OZD is only 4 mm
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30. 3 point touch
Also known as ‘divided
support’
Most weight of the lens
is on almost normal
peripheral cornea
Central cornea is
supported by slight touch
Bearing is not heavy to
cause abrasion & scarring
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31. 3 point touch
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Base of the cone – Apex – Base of the cone
180 degrees apart

32. 3 point touch
Things to avoid
Peripheral fit too tight
causing sealing off the
tear exchange behind
optic zone
Excessive movement
that causes discomfort
and corneal scarring
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33. RGP lens : penetrating keratoplasty
Penetrating keratoplasty (PK) is a surgical procedure
in which the host cornea is replaced with donor
cornea.
 Corneal graft sizes typically range from 7.5 to 8.5
mm.
 Sutures used to keep the graft in place can be
radially interrupted sutures or a single continuous
suture.
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34. RGP lens : penetrating keratoplasty
Typically we begin fitting 6 to 12 months after surgery
following removal of the sutures.
 The epithelium is intact 4 days post-operative, but
the cornea as a whole may take 18 to 24 months for
complete healing.
The fitting process can begin as early as 3 months for
some patients who require contact lenses for
functional vision
 Thus, it is best in most cases to wait at least 6
months before initiating contact lens treatment.
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35. Contd…
The main concern of post-PK fitting is to minimize
trauma to the corneal graft.
 Typically, large diameter (9.5-12.0mm) RGP lenses are
prescribed to minimize bearing on the graft-host
interface and provide improved stability and
centration.
 A large optic zone size will help to minimize glare.
RGP lenses offer excellent oxygen transmission and
have the ability to correct astigmatism and smooth
out irregular corneal surfaces.
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36. RGP lens: Radial Keratotomy
Radial (incisional)
keratotomy is a surgical
procedure for reduction
of myopia by incision
into the anterior
portion of the cornea,
avoiding a central zone
of 3-4mm diameter
No sutures or supports
are involved
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37. Contd...
The procedure an d effect of the number of incisions
usually 4, 8 or 16 equally spaced
Incision depth is usually 90-95% of the previously
measured central corneal thickness
The rigidity of the cornea is decreased such that
intraocular forces act on the cornea , causing the mid
peripheral regions to bulge forward effectively giving a
apical cap of flatter curvature than that measured
preoperatively
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38. Contd...
This flatter central curvature has less power and
results in a hypermetropic shift, hence reducing the
original myopia
After RK, the central cap is wider and needs a larger
back optic zone diameter (BOZD) to cover it and
give a lens stability
Fluorescein assessment should reveal good tear
flow beneath the lens and avoidance of undue
pressure on the mid peripheral region
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Thank you 