Upload
pabita-dhungel
View
293
Download
6
Tags:
Embed Size (px)
Citation preview
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
01/03/15 2
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
01/03/15 3
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
01/03/15 4
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
01/03/15 5
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
01/03/15 6
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
01/03/15 7
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.
01/03/15 8
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
01/03/15 9
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
01/03/15 11
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
01/03/15 13
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
01/03/15 14
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
01/03/15 15
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
01/03/15 16
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
01/03/15 17
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
01/03/15 18
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
01/03/15 19
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.
01/03/15 20
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
01/03/15 21
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
01/03/15 22
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
01/03/15 23
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
01/03/15 24
Corneal RGP CL Two Fitting Philosophies
1. Apical bearing – OZ bears on cone2. Apical clearance
01/03/15 25
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
01/03/15 26
Apical bearing (Flat fit)Compress the cone
Corneal flattening / Spherization
Superior visual performance
Disadvantage ??Hastens the rate of
corneal scarring (Sub-\bowman’s stroma)
01/03/15 27
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
01/03/15 28Text missing???????????
Apical clearance• Advantage
– Less role on corneal scarring – Well tolerated by atopic eye disease
• Disadvantage – Optical • Flare/monocular diplopia
– OZD is only 4 mm
01/03/15 29
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
01/03/15 30
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
01/03/15 32
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.
01/03/15 33
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.
01/03/15 34
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.
01/03/15 35
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
01/03/15 36
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
01/03/15 37
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
01/03/15 38