Special Ophthalmic Dispensing Aphakic Spetacle Dispensing Ophthalmic Optics Ophthalmic Dispensing

1. Special Ophthalmic
Dispensing
Aphakic Spetacle Dispensing
Rabindra Adhikary
M. Optometry
Semester II
Tilganga Institute of Ophthalmology, KTM
(Pokhara University)
UNIT VIII

2. Introduction
Aphakia
• Literally, “Aphakia” means
absence of lens.
• Greek word: Phak =Lentil
• Optically, “Aphakia” refers
to absence of lens from
pupillary area, so it does
not take part in
refraction.

3. Modalities of Correction
Spectacles Contact
Lens
IOL Refractive
Surgery

4. Spectacle Correction
• Optical principle is to correct the error by
convex lens of appropriate power.
• Most common method in the past.

5. Advantages of spectacle correction
Safe
Easy
Inexpensive

6. 1. Increased mage Magnification
2. Decreased Field of View
3. Presence of Ring scotoma
4. Increased lens aberrations
5. Motion of object in field of view
6. Appearance of the wearer
7. Increased ocular rotations
8. Increased convergence demand

7. Image Magnification
• Magnification of retinal image by 28%
– comparing B/L aphakia with Gullstrand’s schematic
eye of relatable parameters
• In B/L aphakia greatly increased image size
means that aphakic patients must adapt to new
size-distance relationship
• Familiar object not only appear to be much larger
, they also appear to be much closer
• In U/L aphakia corrected with spectacles, BSV
impossible due to aniseikonia and diplopia

8. • Sometimes aphakic patients achieve central
visual acuity that exceeds the best VA obtained
before surgery due to magnification of retinal
image size
• Increased magnification may permit to
prescribing weaker reading addition

9. Reducing SM
• We can reduce Spectacle magnification with
altering three parameters:
– Reduce lens thickness
– Aphakic lens with flat back surface and aspheric
front surface reduces SM and looks more
attractive
– Reduce vertex distance

10. Decreased FOV
• Base to the center prismatic effect of high plus
lens reduces the size of field of view
• With standard parameter lens of +12.50 D,
 loss of peripheral field of view - about 7%
 loss of macular field about 17 %

11. Presence of Ring scotoma
• Base in prismatic effect at the periphery of
strong plus lens causes an angular gap in
object space completely around the lens
which is known as ring scotoma
• center to the ring scotoma, wearer
has corrected field of vision

12. • Ring scotoma moves in opposite direction of
the eye movement
• So also called roving eye scotoma

13. Factors affecting size of ring scotoma
• Lens power
• Vertex distance
• Lens size
• Pupil size
• Lens thickness
• Base curve
NB:
Vertex distance and pupil size bears inverse relationship
with the size of scotoma
All others are directly proportional

14. Jack in the box phenomenon
• Object seems to jump in and out of field of
view as it moves out of and into the ring
scotoma
• This sudden disappearance and sharp
reappearance of the object in visual space
with ring scotoma is called jack in the box
phenomenon

16. How to increase FOV
1. Decrease vertex distance
2. Increasing the lens size
– Keep in mind it adds weight, thickness and SM
– Point 1 and 2 both increases the size of ring
scotoma, but this is of little concern as the
scotoma is moved further away into the periphery
3. Using aspheric front surface  reduces
power in the periphery of the lens

17. Increased lens aberration
• Aberrations of oblique astimatism, curvature of
image and distortion(pincushion distortion) are
most important in high plus lens design.
• Tscherning ellipses limits from -23.00D to + 8.00 D
so it can not cover the aphakic patients.
• Aspheric surfaces can be used to reduce such
aberrations.
• Chromatic aberration makes the edges of white
object appear rainbow colored.
• Low dispersion glass, most notably, glasses
containing fluorite can reduce chromatic
aberration

18. Increased ocular rotation
• Angle of eye turning while changing fixation
from one object point to another increases
in high plus lenses compared to
emmetropic.
– Due to prismatic effect

19. Motion of objects in FOV
• When the wearer of high plus error holds the eyes steady
and moves head towards any object, a marked reversed
motion of the field of view is experienced
– Swim effect
• can be avoided by moving the eyes from one fixation
point to another keeping the head stationary
– but this may produce distortion
• Best procedure is to turn head slowly so that head and
eye moves together

20. Appearance
• Poor cosmetic appearance
• Apparent enlargement of eye and their
unusually bulbous appearance behind thick
lens

21. Convergence demand
• Base out effect created by high plus lens during
near work increases convergence demand
• places strain on positive fusional vergence and
causes discomfort
• This effect can be reduced by placing distance
optical center closer than the patient’s distance PD
Or, by incorporating base-in prism in aphakic lens

22. Correction option in spectacle
• Single vision lens
– optimum correction is advised if single vision lens is being
prescribed
• We will discuss in detail about correction
modality in:
– Bifocals
– PALs

23. Bifocal
• Round segment (kryptok or Ultex) bifocal for high plus
lens to minimize image displacement.
• D-segment relatively wide at the top with little image
displacement
• Executive bifocal not indicated as it adds the prismatic
effect to the original distance lens.
• Increasing amount of oblique astigmatism is induced as
the visual axis move downward through the lens
– So bifocal segment of aphakic lenses should be placed as
high as possible

24. Dispensing aphakic lens
Critical lens specification and fitting includes
– Vertex distance
– Pantoscopic tilt
– Centration
– Adjustment of the spectacles
– Design
– Material
– UV protection
– Frame selection

25. Vertex distance
• To correct the ametropia properly, aphakic
lenses should be fitted at the vertex distance
used during the refraction.
– If different? Compensation should be made with
effective power formula
• Advice to place spectacles closer to eye to
reduce vertex distance.

26. Pantoscopic tilt
• In aphakia to minimize the weight & thickness,
optical center should fall at the geometric
center and the optic axis should pass through
the center of rotation by altering the vertical
position of the frame on the face.

27. Position of optical center:
• Small error in PD produce significant effect in aphakic
lens
• Monocular PD should be taken, preferably by a
method of corneal reflection to determine the
position of visual axes
Spectacle adjustment:
• Because aphakic lens are heavy, they tend to slip
down on the nose
• Small misalignment and change in position can
introduce large error

28. Design :
1. Aspheric :
• Aspheric front surface reduce the effects of aberration,
weight, thickness, magnification and size of ring scotoma by
reducing power in periphery of lens
• Steeper back surface curve
has advantage as it reduces:
a) distortion,
b) lens reflections
c) and sensitivity to the
differences in vertex distance

29. • Full- diameter aspheric lenses
– Most FD aspheric lenses for aphakia are made
with aspheric front surface.
– This construction not only reduces effects of
aberration, but also reduces lens thickness and
weight
– To keep lens thickness to a minimum, frames with
large eye sizes should be avoided.

30. 2. Lenticular lens:
• Consists of central area of prescribed lens power
(aperture) surrounded by an outside area of little or no
power (carrier)
• Decrease weight and thickness
• Reduce the lens aberration
• poor cosmetic appearance
(bull’s eye or fried egg) due to high
magnification confined to central
portion of the lens

31. Material :
• High index : thinner and lighter
• Plastic : lighter and safer
• Impact resistant
• Polycarbonate
• Trivex

32. UV protection :
• Retina receives increased amount of concentrated radiant
energy per unit area
• So it is essential to incorporate UV absorbing filter
• Brown or green tint
• Further advantage of improving the appearance

33. Frame selection :
• Sturdy and lightweight
• Round or oval shaped frames so that the effective
diameter of lens is no more than 2mm larger than eye
size
• difference in A & B size greater than 9mm should be
avoided because high plus lens tend to be thick on top
and bottom edges
– which gives apparent look of high plus lens being even
stronger

34. • Short vertex distance
• Adjustable nosepads become more necessity as they offer
advantage of versatility in allowing modification of vertical
position of the frame
• Comfort cable temples help to the glasses from slipping
down the nose