OPTM 2072FALL 2011

HIGH MYOPES AND HYPEROPES

Special Prescribing Considerations

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)

Considerations and Objectives

Vertex DistanceFrame Selection, centration/decentration

Field of View

Lens material Refractive index Aberrations

Form Full and reduced aperture lenses

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

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

Vertex Distance

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

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

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

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

Abbe Values and Index of Refraction

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

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

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

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)

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.

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

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)

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

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

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

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

Presbyopia and High Hyperopes

Same problem as high myopesAvailability more in bifocals than PAL