Contact lenses and low vision aids

1. CONTACT LENSES
AND LOW VISION AIDS
Dr. Zaid Azhar
Ophthalmology Resident SIH

2. Overview
 Contact Lenses
1. Contact Lenses and their types
2. Optical Properties of Contact lenses
3. Indications of Contact lenses
4. Contact lens vs Spectacle lens
5. Problems with Contact lenses
 Low Vision Aids
1. Convex Lenses
2. Galilean System

3. Contact Lenses

4. Contact Lenses
• Contact lenses are lenses/
ocular prosthetic devices
that are placed directly on
the surface of the eyes.
• They are used for a variety
of reasons – therapeutic,
cosmetic, diagnostic.

5. History of CLs
 1880s- Scleral lenses
 1940s – development of
PMMA (Hard lenses)
 1970s – development of
HEMA (Soft lenses)
 1980s – development of
RGP lenses (gas
permeable)
 1990s – development of
silicone hydrogel lenses

6. Contact Lens materials
 Can be classified as hard
or soft based on rigidity.
 Can be classified as
hydrophilic or hydrophobic
based on interaction with
water.
 Can be classified by
modality – daily wear,
extended wear and
continuous wear

7. Hard Contact Lenses (HCLs)

8. PMMA
(Polymethylmethacrylate)
 Organic plastic
compounds but hard as
glass
 Refractive Index: 1.48 -
1.50
 Pros: good clarity,
durability, lack of toxicity
 Cons: lack of oxygen
permeability can leads to
corneal hypoxia and
limbal neovasularization.

9. Cellulose Acetate Butyrate
 Used to make Rigid Gas
Permeable (RGP)
lenses.
 Pros: Slightly better
oxygen permeability,
strong and durable
 Con: Prone to warpage
(temporary change in
corneal curvature not
associated with corneal
edema. Regress after
CL is removed)

10. Soft Contact Lenses (SCLs)

11. HEMA/ Hydrogel/ Silicon-
hydrogel
 Polymer of 2-hydroxyethyl
methacrylate (HEMA), combined
with silicone
 Refractive Index: 1.43
 Water content ranges from 38%
to 60 %
 Pros: comfortable, flexible,
excellent oxygen permeability,
easier adaptability, can wear for
longer
 Cons: shorter life, less durable,
less optical quality

12. Hard vs Soft Contact Lenses

13. Contact Lens Optical
Properties

14. Base Curve
 Base
Curve/Posterior
Central Curvature:
curvature of central
posterior surface of
the lens adjacent to
cornea. Measured in
mm (radius) or
dioptric power
(reciprocal of radius)
in air.

15. Diameter
 Diameter: Width of
contact lens, typically
varies with lens
material (SCL - 13mm
to 15mm, HCL -9mm to
10mm).
 Scleral CLs have a
diameter of around
25mm.

16. Power
 Power: dioptric power of
the contact lens similar
to posterior vertex
power of spectacle lens
 calculated by Snell’s law
 D=[n2-n1]/r
 n1= Refractive Index in
air, n2= Refractive Index
CL constant
r= radius of curvature of
CL

17. Optic Zone
 Optic Zone – Area of
front surface of contact
lens that is main
contributor to refractive
power of the contact
lens

18. Peripheral curves
 Peripheral curves:
Secodary curves just
outside the base curve
at the edge of the lens.
Typically flatter than the
base curve with edges
smoothed out/blended to
increase comfort and
reduce aberrations.

19. Tear Lens
 Tear Lens – Optical
lens formed by the
tear film layer
between posterior
surface of a contact
lens and anterior
surface of cornea.

20. Tear Lens
 Soft CLs are flexible and conform
to shape of cornea so tear lens in
Soft CLs have identical front and
back surfaces making it’s power
plano.
 In Rigid CLs, tear lens can have
different anterior and posterior
surfaces thus inducing an inherent
power of their own which is then
added to the optical system of the
eye.

21. Tear Lens
 Steeper than K – plus power
 On K – no power
 Flatter than K – minus power

22. Contact Lens Indications
 Refractive error –
myopia,
hypermetropia
 Astigmaitsm
 Presbyopia
 Aphakia
 Keratoconus
 Corneal protection/
healing/ pain relief
 In ERG attached to
electrode
 Cosmesis

23. Contact Lenses vs Spectacle
Lenses

24. Field of view
 Contact lenses provide a larger field of view than spectacle
lenses.
 CLs move with the eye and ensure good vision in all gaze
positions.
 Much of peripheral distortion such as spherical abberations
are avoided by CLs.

25. Image size/ Anisometropia
 Recall - Relative Spectacle Magnification - change
in retinal image size with optical correction.
 In axial myopes, correcting lens closer to anterior
focal point produces a magnified image
 Because CLs have a shorter vertex distance image
size changes are less pronounced with CLs as
compared to spectacles.
 CLs produce a magnified image in axial myopes.
 In refractive ametropia image size approaches
emmetropic image size as lens is moved nearer
than anterior focal point.

26. Aniseikonia
 Aniseikonia- difference in retinal image size due to unequal
magnification
 Can be due to retinal pathology or anisometropia (e.g in
unilateral aphakic patient)
 Aniseikonia is reduced with CLs as they become itegral part of
optical system of the eye.
 Hence CLs can improve binocular vision in some cases.
Uncorrected Corrected with Spectacles CLs

27. Convergence
 Spectacle lenses centered for
distance induce a prismatic effect
when eyes converge for near
vision.
 Hyperopic specatcle correction
increases convergence demand
while myopic spectacle correction
increases convergences demand.
 This is because as eyes
converge, the spectacles don’t.
 CLs remain centered at all gazes
hence they reduce this effect.
 CLs decrease convergence
demand in hypermetropic eye
and increase convergence
demand in myopic eye.
A- convergence demand
increases in hyperope
spectacles
B- convergence demand
decreased in myopic

28. Accomodation
 Myopic spectacles have a
base-in prismatic effect
that reduces the amount
of convergence and
accomodation.
 Using CLs therefore
demands greater
convergence and
accomodation.

29. Accomodation
 Compared with spectacle lenses:
1. CLs increase the accomodative requirements of
myopic eye and decrease
2. CLs decrease accomodative requirements of
hypermetropic eye.
 This is because CLs eliminate accomodative advantage
of myopia correcting spectacles and accomodative
disadvantage of hypermetropia correcting spectacles.

30. Prisms
 It is possible to incorporate upto 3 dioptres of prism
power to a corneal CL and upto 6 dioptres of prism
power in a scleral lens.
 Prism is always base down because it’s weight can
rotate CL.
 Horizontal prismatic correction is not possible with
corneal CLs.

31. Optical Aberrations
 Prismatic aberrations reduced by CLs as described earlier.
 Oblique aberrations occuring when looking through non-
axial portion of lens is also minimized by CLs.

32. Keratoconus
 Certain cases of
Keratoconus may also
benefit from contact lens
use.
 Usually only rigid contact
lenses allow good vision
in such cases.
 If cornea is too steep or
scarred CLs may not be
appropriate.

33. Multifocal CL
 Presbyopic, aphakic and pseudophakic patients need to
achieve optical correction for more than 1 distance as
their accomodation is reduced or absent.
 Different CLs can be prescribed to these patients as an
alternative.
 One of these options is monovision which means fitting
one eye with a lens for distance correction and fellow eye
with a lens for near correction. In such cases, patients
must learn to adapt to concentrating on clearer image
from one eye.

34. Multifocal CL
 Multifocal and Bifocal
CLs have different
designs e.g annular
where central zone
corrects for distance and
surrouding annular
zones correct for near.

35. Multifocal CL
 For concentric/Annular CLs, patient must adapt to
concentrate on viewing through certain part of the CL to
form a clear image on the retina.
 For aspheric CLs the central part of the CL corrects for
distance and there is a gradual transition in power at the
periphery which corrects for near.
 For segmental bifocal CLs, the near addition is incorporated
in the lower portion of the lens. The eye looks through the
distance portion when in primary position.
 Diffractive bifocal lenses have concentric diffraction rings on
their posterior surface which focus equally on distance and
near objects.

36. Problems with CL useage
 Ill fitting CL – if
posterior surfce of CL
is flat it will move
excessively on cornea
and cross visual axis
 CL power is affected
by evaporation of
water, temperature,
dryness of
atmosphere.
 Corneal Warpage
 Mechanical, hypoxic,
immmune response
and suppurative
keratitis

37. Low Vision Aids (LVAs)

38. Low Vision Aids - Intro
 An optical or non-optical device that
immproves or enhances residual vision by
magnifying image of object at retinal level.
 Magnifying devices of several kinds are used
to assist poorly sighted patients in daily life.
 Most of LVAs are designed to help with
reading.
 However some LVAs help with distance vision
as well e.g watching TV, bus numbers etc

40. Low Vision Aids - Intro
 Most LVAs present the patient with
a magnified view of object by
increasing angle subtended by the
object at eye.
 Recall – angular magnification is
governed by the angle subtended at
the eye. If the angle stays constant
the apparent size of image of
objects at A, B, C or D remains

41. Magnification Power
 Magification of Power of an optical system as
described above is given by the formula:

42. Convex Lenses
 A convex lens (32D)
may be used as a
magnifying loupe.
 Object is brought within
the focal length of the
convex lens and an
erect, virtual and
magnified image is
formed.
 High powered convex
lenses mounted in
spectacles work
similarly.
 Power of such lens is

43. Convex Lenses
 Convex lens may also
be used as hand held
magnifiers or mounted
on legs as stand
magifiers.
 Paperweight magnifiers
rest on page and use a
thick plano convex lens.
 Bar shaped cylindrical
convex lenses may also
be used for reading.

44. Convex Lenses
 Object is located between
first principal focus and the
lens.
 As the object moves closer
to the lens the virtual
image enlarges.
 Hence the hand held
magnifier can be
positioned at an
appropriate distance by the
patient.
 Field of vision – increases
with size of lens,
decreased with eye-lens
distance.

45. Galilean System
 Galiliean telescope
is an optical system
that produces an
erect magnified
image.
 Composed of a
convex objective
and concave
eyepiece lens
separated by

46. Galilean System
 Works by increasing the
angle subtended by the
object at the eye.
Fe- Power of eye piece lens in dioptres
Fo – Power of objective lens in dioptres

47. Galilean System
 Pros:
1. Good enhancement of
distant vision
 Cons:
1. Restricted Field of view
2. Depth Perception distorted
3. Expensive

48. Other LVAs – Contrast
enhancement

49. Other LVAs – Relative Size
Devices

50. Other LVAs – Electronic
Magnifiers

51. Limitations of LVAs
 High magnification results in decreased field of view –
makes rapid scanning difficult
 Object has to be held close to eye
 Depth of field reduced