Optics of RGP contact lens

Optics of RGP contact lenses

Presenter : Pabita Dhungel B.optometry

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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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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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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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Choice for RGP??????????Better VA- astigmats & irregular astigmatsOnly for some conditions – keratoconus , traumatised

corneas , post grafts etcBetter oxygen transmissibility and better retro lens

tear flow suitable for higher Rx

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Choice for RGP???????Safer for extended-wear than hydrophilic lenses For patient non- compliant with cleaning and

disinfectant procedures, no time to careFor patient who requires steroids and glaucoma

drugs because no absorption as in hydrophilicIn certain specialized area - orthokeratology

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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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Tear Lens power with RGPTear 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 relationshipIf 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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Decentration Induced PrismWhen 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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Diagram for decentration

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Flat, Aligned and Steep RGP FitsFor 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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Diagram of Flat, Aligned and Steep RGP Fits

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Tear Lens Power with Rigid LensesAssumptions:• 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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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 = 7.85mmFSTears power = +42.802548 (BOZR = 7.85mm)∆ = +42.802548 – (+43.076923)

= - 0.274375 D

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Contd…Flattening produces a – 0.274375D effectTo 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 = 7.75mmFSTears power = +43.354839 (BOZR = 7.75mm)∆ =+43.354839 – (+43.076923) = +.277916DSteepening produces a +0.277916 D effect

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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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Neutralisation of AstigmatismCornea/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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Spherical Cornea: Spherical RGPThe 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

Spherical Cornea: Toric RGPIn 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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Astigmatic Cornea: Spherical RGPThe front surface of the tear lens is ‘sphericalized’ by

the back surface of the lensThe 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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Neutralisation of corneal astigmatismAssuming K readings of 8.00 mm and 7.60 mmand the following refractive indices: ncornea = 1.376,ntears = 1.336Corneal powers in air:D1 =(n’-n)/r1 = (1.376-1.000)/ 0.008D1 = 47.00DD2 = (n’-n)/r2 = (1.376 – 1.000)/0.0076D2 = 49.47 D Corneal astigmatism = D2 – D1 =2.47 D

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Contd…Corneal power under tears:D1 = (1.376 – 1.336)/ 0.008 D1 = 5.00DD2 = (1.376 – 1.336)/ 0.0076D2 = 5.26 DCorneal astigmatism = D2 – D1 = 0.26 D

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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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RGP lens : KeratoconusKeratoconus 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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Corneal RGP CL Two Fitting Philosophies

1. Apical bearing – OZ bears on cone2. Apical clearance

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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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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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Apical clearanceSmall diameter & thin

lenses (USA)TD of 6.00 mm to 8.00

mmBOZR – 5.00mm to 7.5

mm With Two flatter

peripheral curves

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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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3 point touchAlso 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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3 point touch

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Base of the cone – Apex – Base of the cone 180 degrees apart

3 point touchThings to avoidPeripheral fit too tight

causing sealing off the tear exchange behind optic zone

Excessive movement that causes discomfort and corneal scarring

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RGP lens : penetrating keratoplastyPenetrating 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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RGP lens : penetrating keratoplastyTypically 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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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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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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Contd...The procedure an d effect of the number of incisions

usually 4, 8 or 16 equally spacedIncision depth is usually 90-95% of the previously

measured central corneal thicknessThe 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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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