Troubleshooting bifocals and Market Availability in Nepal Bifocals in Anisometropia Prismatic Effect in Bifocal Bifocal Prescription Bifocals in High Astigmatism

1. Bifocal
Troubleshooting
and Market Availability
Rabindra Adhikary
M.Optom, 1st Batch
Tilganga Institute of Ophthalmology
Pokhara University
Date of Presentation:
2nd December 2019
Topic On: Ophthalmic Dispensing

2. • In an ideal condition, a patient puts an
eyewear and sees the world well, comfortably.
• At times, however,
– Some patient immediately reject the new eye
wear
– Some may return after a few days of struggling
with their new eye wear

3. • How we handle patients’ complaints, troubleshoot the
offending eye wear and subsequently resolve the issue is an
important skill
• Expect complaints from new wearer  we are imposing
new visual world to them
– So, counsel pros and cons
– Encourage
– Empower
– Discuss options
– Give examples; beforehand

4. • If you get a returnee with complaints  now
that’s the real challenge
• First!
– Just don’t ignore them
– LISTEN
– Question them
– Again listen them

5. Start Looking
• Sphere, cylinder, and axis
• ADD power

6. Case #1
• Your Prescription:
OD: +1.00/-0.50 X 90
OS: +0.75/-0.75 X90
ADD: +2.25Ds
• Patient goes to the optical shop and says
– “I am comfortable with my distance vision, make
me glasses at near only with this power”

7. • All optical dispensers may not understand that
near power in the prescription is on top of
distance prescription

8. Prescribed prism
• Confirm power and orientation
• If prism is prescribed to correct double vision at
distance,
– Pt experiences diplopia from the segment if the extent of
diplopia are dissimilar (power of prism will be the same at
N & D)
– Soln ?
• Explain the root cause of the problem
• Two glasses
• Occlusion

9. Induced Prism in RT bifocals
• Plus power : BU
– Less pronounced in bifocals
• Minus Power : BD
– More pronounced due to additive effect

10. Case #2
• A myope with near
addition complains
that he needs to tilt
back his head to see
the letters more
comfortably

11. • So, round segments,
especially large
diameters are avoided
in myopic bifocal
prescription

12. Choose wisely

14. Anisometropia
• Adaptation to anisometropic bifocals
requires:
– User should have large tolerance to prism imbalance
– Amount of anisometropia is small
– User does not have binocular vision at near, and
prismatic imbalance is not causing diplopia
– Use a Franklin-style bifocal because we can
independently customize the segment
– Locating S above the dividing line for myopes and
below the reading region for hyperopes would reset
the prism at the dividing line and locate the optical
center of the near region at the useful spot.
– The patient still must learn to rotate their gaze
downward to achieve the addition (they would not
do this with single vision lenses to avoid prismatic
imbalance).

15. Case #3
• A patient has following prescription:
OD +1. 00 Ds ADD: +2.00Ds
OS +6.00 Ds ADD: +2.00 Ds
– If distance between DVP and NVP is 8mm
• Prismatic effect at NVP of RE = 0.8 X 1 = 0.8 pd BU
• Prismatic effect at NVP of LE = 0.8 X 6 = 4.8 pd BU
• Prismatic imbalance = 4.8 -0.8= 4.0 pd BU in LE
– So, for correction of this imbalance, incorporate
• 4.0 pd BD in LE or 4.0 pd BU in RE in distance portion

16. OC placement
• Vertical:
• Verify if OC is decentered a/c to tilt
– for 20 tilt 1mm down

17. Segment inset
• Horizontal decentration of
OC of near segment from
the distance fitting point
– Near and Distance PD
measured monocularly
Segment Inset = (Distance PD
– Near PD)/2

18. • We can also bring about the desired amount
of prism by additional decentering of segment
OC
• BO prism in high AC/A ratio as prescription
• BO prism in low AC/A ratio as training
Total Seg inset = Seg Inset + Additional inset

19. Slab-off for correcting vertical Imbalance
• most patients have difficulty fusing
an image when there is more than
a 2pd difference between the two
eyes.
• Generally, slab-off is not prescribed
with imbalances of less than 1.5pd
– Range: 1.5 pd to 6 pd
• With glass lenses, the earliest slab-
off lens was ground, to remove
base-down on the lens having the
greatest minus or weakest plus
power.

20. • Slab-off available in both glass and
plastic lenses
• Reverse slab-off lenses are molded,
or cast, with base down prism in
the lower segment area, rather
than having base up prism
generated using bi-centric grinding
• Because reverse slab-off provides
BD prism instead of BU it is always
used on the most plus, or least
minus lens in the vertical meridian
to offset excessive BU effect

21. • It should be noted that although slab-off can be used on any lens, cosmetically it
works best on a flat top bifocal due to the slab line forming a continuation of the
top of the segment.
• In addition, the wider the bifocal used, the less noticeable the slab line will be.

22. Unequal Segment
• We can use dissimilar segment
size to combat vertical
imbalance in anisometropia
• The larger segment will exert
more base down at the near
vision point than the smaller one
• So obviously the larger segment
(more base down) goes in the
eye with the least base down to
‘balance’ the prismatic effect

23. Unequal Segment: Drawbacks
• both optically and cosmetically this
is not an ideal long-term solution
for all but elderly presbyope.
– the difference in segment sizes
needed changes with the add (i.e.
as the patient gets older) and,
– for low adds, the difference in
segment sizes needed is such that it
usually looks ridiculous or is
unavailable.

24. Accommodation
• The amount of accommodation
required for an individual to see
clearly at near is determined by three
things:
– The near-viewing distance
– The power of the distance spectacle
lens prescription being worn
– The distance from the lens to the
principal planes of the eye

25. • Hyperope will need to have more
accommodative effort followed by
emmetrope and then myope for clearly seeing
the object at same distance under the same
conditions
• Effective power at
secondary principal plane

26. • So, in the cases of moderate to
high hyperopia, check the
accommodative amplitude with
correction.
– If reduced: address the issue by
increasing the add power

27. Ask the Occupation
• Before you make a final prescription of bifocals,
always ask what the patient does as occupation
because
– Add power is not always about the age
• Major Factors to be cautious about:
– working distance
– Arm Length (or person’s height)
– Working conditions like illumination, posture
– Pupil size
– Systemic condition

28. Case #4
• A monk of 62 years old comes for
a refraction. He has no power at a
distance. You show the letters in
the test conditions and he can
read N6 very well with +3.00Ds.
You prescribe the glasses, but the
monk comes after a few days
saying he can’t read Tripitaka.
– This is a common case scenario

29. He has to bring the texts too near!
• Remember you did the refraction just
in a usual way
• Every patient has their special
demand/need. Ask them what they
want to do especially with their new
glasses!
• In this case, if you had known that the
monk needed to read the letters
@80cm, you would have prescribed
him just +1.25Ds, instead of +3.00Ds.

30. • Sometimes, patients’
complaints can just
be resolved by asking
them increase the
level of room
illumination while
doing near works!

31. High Astigmatism
• A person who has high
astigmatism with an
occupation requiring
intense near work
complains with eye fatigue
with near viewing.
• You check everything but
solution remains elusive.

32. • A spectacle lens containing a large cylinder component has a
considerable difference in refractive power between its two major
meridians.
• This means that a single vision lens wearer may require a different
amount of accommodation for one meridian of the lens than for
the other when comparing the effectiveness of that lens at distance
and near.
• If the distance sphere power is also large, this effect can be even
more significant.
– Advise to get a separate pair for near viewing by modifying the original
prescription

33. Bifocals for Children
• Bifocal Fitting Line
– Lower pupilary border
• For child < 8 yrs
• Bifocal style
– Straight top, large
field (FT 35)
– Second option:
executive bifocal
> 8 yrs

34. Other Factors
• Unsatisfactory tints or coatings
• Waves or warpage
• Optical (manufacturing) defects
• Materials of different Abbe Value (carrier vs
segment)chromatic disp.
• Frame Selection
• Buyers’ remorse

35. Bifocal Markets
• Zeiss Bifocal Classic CT 25 / CT 28
– Curved top 25 mm and 28 mm diameter
– Medium refractive index
– Options: glass, plastic, (photochromic)
• Essilor Bifocal 360 FT-28
– Digital surfacing technology
– Less peripheral distortion
– Systematic crizal treatment of choice
– Eg. FT-28 Essilor Digital Bifocal 360
• Essilor ready made bifocals
– Crizal essilor lens_ECO bifocal reading spectacle +1.00 to +3.00
options_rectangular_unisex_

37. References:
– System for Ophthalmic Dispensing, third Ed, Clifford . Brooks, Irvin M.
Borish
– Modern Ophthalmic Optics (2019), Cambridge University Press. Jose
Alonso, Gomez Pedrero, Juan A Quiroga
– Perspectives in Refraction: Compensation procedures for the
anisometropic presbyope. Frank Kozol
– Ophthalmic Lenses, Ajay K Bhootra
– Troubleshooting Progressives and other Multifocals, Valerie Manso
– Clinical Optics, third Edition, Andrew R. Elkington
– Clinical Optics, Second Edition, Troy E Fannin, Theodore Grosvenor
– Clinical Optics, American Academy of Ophthalmology (2018-19)

39. • Franklin-style bifocals.
• These lenses do not have the constraint of a round intersection between the surface defining
the segment and the surface of the main lens that accommodates the segment. The straight
ledge separating the far and near regions allows for a greater flexibility in the positioning of
the optical centers. For example,the Executive bifocal from American Optical was manufactured in
such a way that the center of the segment was located at the middle point of the dividing line in
the blank. During lens processing the optical center of the far region could be displaced as needed.
With regard to prismatic effects, this lens would behave just the same as a semi-round segment
bifocal. Other lens manufacturers were making front-side Franklin-style bifocals for the last two
decades of the twentieth century, in which the location of S was base curve dependent so that the
optical center of the near region could be approximately located at the reading region (for this S
should be moved upward for plane base curves intended for negative prescriptions, and moved
down for stepper base curves intended for positive prescriptions).