special prescribing considerations.ppt chapter 3 dispensing
TRANSCRIPT
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OPTM 2072FALL 2011
HIGH MYOPES AND HYPEROPES
Special Prescribing Considerations
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Review of Ophthalmic Optics Prism questions
Prentice’s RuleDatum centre distance = 70mm (A + DBL)Lens size = 52 mmPD = 66 mmA +DBL – PD / 2 = decentrationMinimum uncut lens size =
Lens size + 2(decentration)
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Considerations and Objectives
Vertex DistanceFrame Selection, centration/decentration
Field of View
Lens material Refractive index Aberrations
Form Full and reduced aperture lenses
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Vertex Distance
Distance from the corneal apex to the visual point of the lens
Include a VD in any lens Rx if the power of any meridian is +/- 5.00 D and above.
VD changes the “effective” power of the lens All lenses become more + if moved away
from the eyeOpposite occurs when lenses moved closer to
eye
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Vertex Distance
You have 3 options1) Ensure that the chosen frame sits at the
prescribed VD (use nose pad pliers)2) Choose another frame that sits at the
prescribed VD3) Chose a frame that sits at a different VD,
but alter the power of the lenses accordingly
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Vertex Distance
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Vertex Distance
Measure using mm rulerConsult a computer chart or graphIf VD is decreased: F = F old/ 1- (dFold)If VD is increased: F = F new/ 1+ (dF new)d is in metresToric prescriptions should be compensated for in
each meridianDon’t forget: VD changes will also affect
spectacle magnification Increase VD = Magnifying for + lenses Decrease VD = Minifying for - lenses
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Frame Selection for Centration and Decentration
Why do we want to center a lens on the patients’ visual axis? Prescription is most effective Reduces unwanted prismatic effects Reduces the possibility of the formation of ghost images
Decentration: Any displacement, horizontal and/or vertical of the centration point from the OC
Correct centration is important in simple as well as complex Rx’s
However, higher powered Rx can have larger consequences
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Decentration in High Powered Lenses
Decentration is only necessary when the IPD and the box and the frame size (A + DBL) are not the same.
Only exception is if decentration is used to produce prescribed prism
Try to keep decentration at a minumum by choosing a frame close to the PD of the patient
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High Myopia- Lens Material and Form
Primary consideration is Edge Thickness Decentration inwards will show large temporal thickness
Use higher refractive index materials Don’t forget there is still glass N= 1.90 glass (Zeiss Lantal)
Aspheric surfacesAR coatingsBe Careful! Low Abbe Values will cause TCA
(Transverse Chromatic Aberration)Use higher Abbe Value materialsUse Best Form designs
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Abbe Values and Index of Refraction
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High Myopia- frame and fitting
Fit as close to the eye as possible (less VD keeps Visual points close to OCs and minimizes TCA)
Minimize horizontal and vertical decentration and pantoscopic tilt
Small frame and eye size with wider bridgesThicker eye wires and rims to hold thicker edgesBe careful of nose pads and arms because ET
can obstructReal field of view is greater than the apparent
field of view
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High Myopia- Reduced Aperture Lenses
This lens has reduced edge thicknesses using smaller lens apertures
Super Lenti (Norville) Decreased ET- aspheric Decreased Bottle Bottom
appearance -11.00 D and above Must have Monocular PD
Vertical and horizontal centration data
Fit with zero Panto Tilt
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Advantages of the Super Lenti
Good VAGood Field of VisionControlled peripheral aberrationsGood edge thickness and weightAllows for wide range of frame selectionReasonable cosmesisRemoval of the minification of the face seen
with full aperture lenses
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High Myopia- Lentilux
Aspheric, Single Vision Rodenstock lensAvailable up to -24.00 DClaims same advantages as the Super LentiEdge Thickness will not exceed 4.50 mm
(even at -24.00)Made of high index glass material (which
reduces the thickness)
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High Myopia- Lenticular
Edge thickness “flattened”You’ll see a step where the margin and
aperture meetMargins are convex or planoAperture shape can be round or oval.
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High Myopia- Frame Selection
Thicker eye wires and rims to hold thicker edges
Be careful of nose pads and arms because ET can obstruct
Go with smaller eye sizes with wider bridgesReal field of view is greater than the
apparent field of view
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Presbyopia
Problem with lens availabilityOnly available bifocal and multifocals are full
aperture lensesMany high myopes delay the need for reading
additionCan push glasses down nose, increasing VDNorville Solid 30mm bifocal (1.701 glass, up to -
12.00)Essilor Panamic Lineis (1.74 Resin, up to -20.00)Zeiss Tital Gradal 3 (1.706 glass, 1.80 glass, up
to -20.00)
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High Hypermetropia
What kind of problems?Nasal Edge ThicknessCentre ThicknessOverall WeightOblique performance with off axis view
Ring Scotoma Jack in the box effect
MagnificationRestricted Field of ViewCentration and prescribed prism
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High Hyperopes
Use Asphericity- dramatically improves optical performance
Polynomial Designs are higher order aspheric surface lenses that are ellipsoidal and flexes back on itself at larger diameters
Since polynomial designs introduced, lenticular lenses not used
Aphakia – Use UV 400 filter/coating
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High Hyperopes: Polynomial Designs
No visible dividing lineGood mean oblique power in off-axis viewinReduced distortionSlightly thinnerIncreased field of viewReduction in Jack in the box effectFlatterLess magnificationLess TCA
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High Hyperopes: Prescribing Points
Pantoscopic Tilt of trial frameVD of the trial frameConsider choosing frame first before doing the
refractionWhen selecting a frame, use same considerations
as the high myope. Maximise the Field of View Reduce the convergence demand Reduce the retinal image size Reduce distortion and chromatic aberration Vertical centration and pantoscopic tilt should match: 1
degree to 2mm below the pt’s pupil
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Presbyopia and High Hyperopes
Same problem as high myopesAvailability more in bifocals than PAL