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Duke Elder: Optics (Alice) > Chapter 12: contact lenses > Flashcards

Chapter 12: contact lenses Flashcards

1
Q

What is the definition of aniseikonia?

A

inability to fuse images of unequal size, producing symptom of diplopia

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2
Q

What are 3 advantages of contact lenses as an alternative to spectacles?

A
  1. safer for sports, don’t break/fog/ spattered in rain, cosmetics
  2. reduce or eliminate aberrations in spectacles for high refractive errors
  3. reduce aniseikonia associated with anisometropia and high degrees of astigmatism
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3
Q

What are 2 uses of contact refracting lenses in ophthalmology?

A
  1. diagnostically to view fundus
  2. to view trabecular meshwork
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4
Q

What are 3 examples of non-refracting contact lenses used in ophthalmology?

A
  1. bandage contact lenses (precorneal membranes) - ocular surface disorders, healing + relieving pain
  2. painted contact lens - small/unsighly blind eye to / artificial iris in aniridia
  3. contact lens in attached electrode to perform ERG
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5
Q

How can cylindrical refractive errors be corrected by contact lenses?

A

lenses in which the front surface, back surface or both are toric can be used

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6
Q

What are 2 ways to prevent torsion of a toric contact lens?

A
  1. incporating an up to 2.00D base down prism to weight the lower pole of the lens
  2. removing lower 0.5-1mm of lens (truncation) to allow to sit on edge of lower eyelid
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7
Q

What defines the posterior surface of the optical zone of a contact lens?

A

posterior central curvature - aka base curve

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8
Q

What property must the posterior surface of a contact lens importantly have?

A

should conform closely to the aspheric surface of the cornea to ensure a correct fit

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9
Q

How can a contact lens be made to be a close fit for the aspheric cornea?

A

by encircling the optical zone with 1-2 concentric zones of increasing radius of curvature to produce bicurve or tricurve lens; junctions between the zones made smooth by process called blending

(computer does it now)

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10
Q

What 2 properties determine how closely a contact lens fits to the surface of the cornea?

A
  1. base curve
  2. diameter of contact lens
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11
Q

How does the size of corneal contacts diameter relate to the cornea?

A

lenses have a smaller diameter

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12
Q

How do scleral (haptic) contact lenses work?

A

have a peripheral rim which is supported by the sclera

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13
Q

What provides oxygen to the corneal surface?

A

precorneal tear film

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14
Q

How can contact lenses made of gas impermeable materials facilitate the circulation of tears behind the lens?

A

may incorporate fenestrations, slots or grooves to facilitate the circulation of tears behind the lens

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15
Q

Why do contact lenses used to correct high refractive errors cause problems?

A
  • greater thickness and weight
  • upper eyelid grips thick upper edge of high power minus (concave) lens- cause it to ride high
  • weight of high power (convex) plus lens causes it to drop to lower position
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16
Q

How can the upper eyelid gripping the thick upper edge of a high power concave lens be countered?

A

peripheral bevel

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17
Q

How can the weight of a high power convex lens causing it to drop lower be countered?

A

minus peripheral carrier portion which tends to be lifted by the upper lid

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18
Q

What is the refractive index of the precorneal tear film vs the cornea?

A

almost equal -
tear film: 1.333
cornea: 1.3375

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19
Q

What is the role of the refractive power of the tear film?

A

it neutralises corneal surface irregularity and the refractive power is effectivel that of the tear film-air interface

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20
Q

What is the tear lens?

A

tear film between the posterior surface of a contact lens and the anterior surface of the cornea

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21
Q

What is the power of the tear lens?

A

if it has uniform thickness is has plano power

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22
Q

What is the effect of a steeper base curve of a contact lens?

A

increases axial height of the tear lens, makes it more strongly positiive (converse makes it more negative)

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23
Q

What does the tear lens allow in corneal astigmatism?

A

allows spherical contact lens to neutralise the corneal astigmatism (base curve is same as corneal surface curvature in flattest meridian, so where cornea is steeper tear lens is thicker)

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24
Q

How should prescriptions for contact lenses be expresses in corneal astigmatism and why?

A

using negative cylindrical powers because only the spherical component need be prescribed

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25
Q

Why can soft contact lenses neutralise only a smaller degree of astigmatism (1.00D) compared with rigid?

A

they tend to adopt the shape of the cornea (so precorneal tear film doesn’t fill the space to carry out more refraction in flatter parts of cornea)

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26
Q

What is the only way to neutralise astigmatism from the crystalline lens/ implanted IOL?

A

front surface toric contact lens

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27
Q

What are 5 differences between contact lenses and spectacles?

A
  1. field of view
  2. optical aberration
  3. accommodation and convergence
  4. prisms
  5. tint
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28
Q

What is the field of view of a contact lens vs spectacle lens?

A

contact lens much better as moves with eye + no distortions when looking through periphery of spectacles

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29
Q

What can happen in a dilated pupil with a rigid contact lens?

A

may acuse halo effect due to refraction through peripheral zone of the lens or adjacent tear film

30
Q

What causes significantly reduced field of view in hypermetropic patients with spectacles?

A

lens periphery has prismatic effect with the base towards the visual axis

31
Q

What is the effect on field of view in myopic patients?

A

spectacles increase the field of view because of prismatic effect with the base away from the visual axis

32
Q

How does aniseikonia compare with contact lenses vs spectacles?

A

reduced in contact lenses

33
Q

How do myopic and hypermetropic lenses influence aniseikonia?

A

myopic: image magnification
hypermetropic: image minification

34
Q

What reduces oblique aberration with contact lenses?

A

contact lens fitting minimises lens movement on blinking and ensures it remains almost centred in all positions of gaze

35
Q

What happens when a person wearing spectacle lenses which are centred for distance converges for near vision?

A

induce prismatic effect

36
Q

Is there prismatic effect on convergence for near vision in distance contact lens wear?

A

no

37
Q

How do myopic spectacles reduce the amount of convergence and accommodation required for near vision?

A

they have a base-in prismatic effect

38
Q

What problem can arise with a change from spectacles to contact lenses in presbyopic myopes?

A

loss of base-in prismatic effect in spectacles that reduces convergence and accommodation for near vision may cause eye strain

39
Q

What benefit against anisometropic spectacles do contacts confer?

A

elimination of unequal prismatic effect

40
Q

How much prism power can be incoporated into a corneal contact lens without making it too thick to be practical?

A

up to 3 dioptres of prism power

41
Q

What is the position of a contact lens with a prism incorporated and why?

A

always base down as the weight of the prism rotates the contact lens

42
Q

Is horizontal prismatic correction possible with contact lenses?

A

no - always rotates to be base down

43
Q

Can contact lenses both contain prisms?

A

no - limited to one lens only (max 3 prism dioptres and can only be base down as weight rotates the lens)

44
Q

How can scleral lenses incorporate prisms?

A

allow incorporation of vertical or horizontal prism up to 6 prism dioptres divided between two lenses

45
Q

What is the maximum prism power that scleral lenses can allow?

A

up to 6 PD

46
Q

What are 2 reasons contact lenses may incorporate tint?

A
  1. slight blue tint - more visible for handling and retrieval
  2. green/blue/brown to change appearance of iris
47
Q

What are 3 groups of patients who need to achieve optical correction for more than one object distance?

A
  1. presbyopic
  2. pseudophakic (after IOL insertion)
  3. aphakic
48
Q

What are 3 contact lens options for patients without accommodation?

A
  1. wearing of spectacles over contact lenses
  2. bifocal contact lenses
  3. monovision
49
Q

What is monovision?

A

fitting one eye (usually the one with better vision) with a distance contact lens and fellow eye with a lens which corrects the near vision

50
Q

What are 2 disadvantages of monovision?

A
  1. patients must learn to adapt to having to concentrate on the clearer image from one eye
  2. binocularity and stereopsis are diminished
51
Q

What are 4 types of bifocal / multifocal contact lens designs available?

A
  1. annular
  2. aspheric
  3. segmental
  4. diffractive
52
Q

What is an annular bifocal contact lens?

A
  • has central zone which usually corrects for distance, surrounded by an annular zone for near.
  • in down gaze lens rises relative to cornea, placing near portion in front of visual axis
53
Q

What are 2 disadvantages of annular contact lenses?

A
  1. light from near portion of contact lens produces **superimposed out of focus image **when focuses on distacne image which is clear + vice versa. blurred image reduces quality of the clear one.
  2. peripheral (annular) portion of lens is not as effective when pupil diameter small
54
Q

How does a wearer overcome the blurred superimposed images in annular bifocal lenses?

A

learns to concentrate on the better focuses image

55
Q

How does an aspheric multifocal contact lens work?

A

central part corrects for distance and there is gradual transition in power to peripheral portion which corrects for near

56
Q

What are 2 disadvantages of aspheric multifocal contact lenses?

A
  1. only small amount of total light entering eye through the contact lens is in focus on the retina
  2. light focused on retina must compete with blurred image from light passing through other parts of contact lens
57
Q

How do segmental bifocal contact lenses work?

A

incorporate near addition over the lower portion of the lens; eye looks through distance portion in primary position; in down gaze, lens rises relative to cornea placing near portion in front of visual axis

58
Q

What must be done to prevent segmental lenses from rotation (to ensure near portion is in lower segment)?

A

truncation or ballasting with base-down prism

59
Q

How do difractive bifocal lenses work?

A

have concentric diffraction rings on their posterior surface designed to focus equal amounts of light from distant and near objects

60
Q

What is a disadvantage of diffractive bifocal lenses?

A

Image is less bright than with a single-fcus contact lens, may be a problem in dim illumination

61
Q

What is caused by keratoconus?

A

increasing myopia and irregular astigmatism

62
Q

What are 2 ways mild keratoconus can be managed?

A
  1. spectacle correction
  2. often progresses requiring rigid contact lenses (historically haptic lenses used)
63
Q

What one treatment option may be required in severe keratoconus?

A

corneal grafting

64
Q

What one treatment option may be required in severe keratoconus?

A

corneal grafting

65
Q

What are 4 short term problems associated with contact lens wear?

A

**1. excessive movement **on cornea if posterior surface too flat
2. upper eyelid during blinking leads to lens flattening causing temporary fluctuation in acuity
3. power of soft lens refers to in saline at room temp; changes occur when in use
4. dry atmosphere in aircraft can cause blurred vision due to evaporation

66
Q

What are 4 changes that occur when a contact lens is in use vs room temperature suspended in saline, altering refractive power?

A
  1. moulds to surface curvaure of cornea
  2. evaporation of water increases refractive index increasing negative power
  3. increase in temp increases curvature increasing negative power
67
Q

What is a key long term problem due to contact lenses?

A

corneal warpage

68
Q

Over what time frame does corneal warpage improve after removal of the lens?

A

hours to days

69
Q

Which type of contact lenses cause more a problem with warpage?

A

rigid lenses - more pronounced and longer duration

70
Q

What are 2 negative effects of corneal warpage?

A
  1. spectacles no longer compensate
  2. unable to perform refractive surgery or biometry (estimation of IOL power), prescribe glasses until changes stabilise
71
Q

What happens when a person wearing spectacle lenses which are centred for distance converges for near vision?

A

induce prismatic effect

72
Q

What is the effect of a steeper base curve of a contact lens?

A

increases axial height of the tear lens, makes it more strongly positiive (converse makes it more negative)

Duke Elder: Optics (Alice) (19 decks)

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