Contact lenses

Question 1

Base curve

Answer 1

curvature of the central posterior surface of the lens, which is adjacent to the cornea, described by its radius of curvature (mm)

Question 2

Diameter (chord diameter) for soft and RGP

Answer 2

soft contact lenses ranges from 13 mm to 15 mm, whereas that of rigid gas-permeable (RGP) lenses ranges from 9 mm to 10 mm

Question 3

Peripheral curves

Answer 3

Secondary curves of the posterior lens surface away from the center, nearer the lens edge. These curves are flatter than the central posterior “base” curve to approximate the normal

Question 4

Sagittal depth or vault

Answer 4

distance between the center of the posterior surface to the plane of the edges of the lens

Question 5

If the diameter of the lens is held […], the sagittal depth […] as the base curve radius […]

Answer 5

If the diameter of the lens is held constant, the sagittal depth DECREASES as the base curve radius INCREASES

Question 6

A […] wetting angle means water will spread over the surface, […] surface wettability

Answer 6

A LOW wetting angle means water will spread over the surface, INCREASING surface wettability

Question 7

for irregular astigmatism

Answer 7

RGP, hybrid, scleral

Question 8

CL and spectacle is how many mm in front of eye

Answer 8

3 mm and 12 mm

Question 9

Prentice rule

Answer 9

as the eyes look off-axis, through parts of the spectacle lenses farther from their optical centers, the eyes encounter unequal prism

Question 10

magnification in aphakic

Answer 10

aphakic spectacle lens magnifies the retinal image about 25% larger than it would be in an emmetropic eye of the same length, whereas a contact lens magnifies it about 7%

Question 11

accommodation

Answer 11

Compared with spectacles, contact lenses increase the accommodative demand for myopic eyes and decrease it for hyperopic eyes in proportion to the size of the refractive error. Thus, contact lenses eliminate the accommodative advantage enjoyed by those with spectacle-corrected myopia and the disadvantage experienced by those with spectacle-corrected hyperopia

Question 12

spectacles, vergence and prism

Answer 12

Myopic spectacle lenses induce base-in prisms for near objects, following the Prentice rule, so that the eyes do not have to turn in as much to look at the near object. Hyperopic spectacles increase the convergence needed for fusion by inducing base-out prisms.

Question 13

do not correct lenticular astigmatism

Answer 13

RGP

Question 14

Dk

Answer 14

refers to the oxygen permeability of a lens material

Question 15

D

Answer 15

is the diffusion coefficient for oxygen movement in the material

Question 16

Dk/t

Answer 16

refers to the oxygen transmissibility of a lens, depending on its material and central thickness (t)

Question 17

desirable parameters

Answer 17

high Dk/t and a low wetting angle

Question 18

Soft contact lenses - materials

Answer 18

are made of a hydrogel polymer, hydroxyethylmethacrylate or, more often now, a silicone hydrogel

Question 19

Hydrogels have […] oxygen permeability when they have […] water content

Answer 19

Hydrogels have MORE oxygen permeability when they have HIGHER water content, but the higher-water-content lenses tend to cause dryness of the cornea if they are made thin, and they may form deposits and require frequent replacement.

Question 20

Modifying the hydrogels yielded “silicone hydrogels,” - advantages

Answer 20

achieve their oxygen permeability with less water content, using pores induced by the presence of silicon atoms, rather than high water content

Question 21

Soft CL - advantages

Answer 21

shorter adaptation, comfortable, different types, easier to fit, cheaper

Question 22

RGP - advantages

Answer 22

clear vision, correction of regular and irregular, ease of handling, stability and durability, ease of care

Question 23

addition of fluorine in RGP

Answer 23

increases oxygen permeability and encourages the coating of the lens with mucin, which improves wettability

Question 24

RGP - disadvantages

Answer 24

initial discomfort, longer period of adaptation, greater knowledge and effort required for fitting, and the greater cost

Question 25

People who use contacts only sporadically are generally more comfortable with

Answer 25

a soft lens

Question 26

A soft or rigid lens can be made to fit tighter by

Answer 26

by either choosing a smaller radius to steepen the base curve or by increasing the lens chord diameter without changing the radius

Question 27

loose fit

Answer 27

excessive movement, poor centration, lens-edge standoff, blurred mires after a blink, fluctuating vision, continuing lens awareness, air bubbles under the lens

Question 28

tight fit

Answer 28

no movement, centered lens, “digging in” of lens edge, clear mires when blink, good vision initially, initial comfort but increased awareness with continued use, limbal-scleral injection at lens edge

Question 29

In a good fit, the lens will move

Answer 29

about 1 mm with upward gaze or blink, or with gentle pressure on the lower eyelid

Question 30

apical clearance

Answer 30

(steeper than K), when the base curve has a steeper fit, with radius shorter than that of the cornea

Question 31

apical bearing

Answer 31

(flatter than K), when the base curve is flatter than the cornea

Question 32

The most common type of RGP lens fit is

Answer 32

an apical alignment fit with the upper edge of the lens under the upper eyelid. This “lid attachment” fit allows the lens to move with each blink, enhances tear exchange, and decreases lens sensation, because the upper eyelid does not strike the lens edge with each blink.

Question 33

central or interpalpebral fit is

Answer 33

achieved when the lens rests between the upper and lower eyelids. The diameter of the lens is smaller than with a lid-attachment fit. There may be greater lens sensation, because the eyelid strikes the lens edge with each blink. This type of fit may be preferable for patients who have high upper eyelids, hyperopia, or Asian eyes

Question 34

RGP - SAM and FAP

Answer 34

SAM (steeper add minus) and FAP (flatter add plus). Tear lenses created by rigid contact lenses with base curves that are steeper than K (smaller radius of curvature) have plus power, whereas tear lenses formed by base curves that are flatter than K (larger radius of curvature) have minus power

Question 35

astigmatism <1 D

Answer 35

spherical soft or RGP

Question 36

astigmatism 1-2 D

Answer 36

toric soft or spherical RGP

Question 37

astigmatism 2-3 D

Answer 37

custom toric soft or spherical RGP

Question 38

astigmatism >3 D

Answer 38

custom toric soft or toric RGP

Question 39

To adjust the prescription for lens rotation

Answer 39

follow the LARS rule (left add; right subtract)

Question 40

There are 2 types of bifocal lenses

Answer 40

alternating vision lenses and simultaneous vision lenses

Question 41

intralimbic contact lenses

Answer 41

Larger RGP contact lenses with larger optical zones (diameters >11 mm) are available for keratoconus and post-transplant fitting

Question 42

hybrid contact lens

Answer 42

has a rigid center and a soft skirt. The aim is to provide the good vision of an RGP lens and the comfort of a soft lens.

Question 43

Piggyback lens systems

Answer 43

involve the fitting of a soft contact lens with an RGP lens fitted over it. This system may allow comfort benefits like those offered by hybrid lenses, with a greater choice of contact lens parameters, if the combination of lenses allows sufficient gas permeability to avoid hypoxia.

Question 44

Gas-Permeable Scleral Contact Lenses

Answer 44

has a central optic that vaults over the cornea and a peripheral haptic that rests on the scleral surface (Fig 5-15). The shape of the posterior optic surface is chosen to minimize the volume of the fluid compartment while avoiding corneal contact

Question 45

Gas-Permeable Scleral Contact Lenses - indications

Answer 45

(1) correcting abnormal regular and irregular astigmatism in eyes that cannot be fit with corneal contact lenses, and (2) managing ocular surface diseases that benefit from the constant presence of a protective, lubricating layer of oxygenated tears, such as pellucid degeneration, Terrien marginal degeneration, keratoconus, Ehlers–Danlos syndrome, elevated corneal scars, and astigmatism following penetrating keratoplasty. These lenses can be helpful with complications of Stevens–Johnson syndrome, graft-vs-host disease, tear layer disorders, and ocular cicatricial pemphigoid, protecting the fragile epithelium of these corneas from the abrasive effects of keratinized eyelid margins associated with distichiasis and trichiasis and from exposure to air

Question 46

Semi-scleral lenses and scleral - diameters

Answer 46

Semi-scleral lenses, extending not as far onto the sclera, have diameters 15–18 mm; scleral lenses have larger diameters, 18.1–24 mm

Question 47

Therapeutic Use of Contact Lenses

Answer 47

disposable plano soft lens of high oxygen permeability, a somewhat tighter fit maybe be preferable, as lens movement could further injure the healing epithelium. Some fitters prefer high-water-content lenses, but high oxygen permeability is usually the chief factor in lens selection.

Question 48

Contact lens overwear - causes

Answer 48

Hypoxia, lactate accumulation, and impaired carbon dioxide efflux

Question 49

Sattler veil

Answer 49

Central epithelial edema (Sattler veil) may present after many hours of wear, more commonly with hard contact lenses.

Question 50

Microcystic epitheliopathy - how long may persist

Answer 50

takes up to 6 weeks following discontinuation of contact lens wear for the cysts to resolve

Question 51

Corneal neovascularization - which CL

Answer 51

Superficial pannus is rarely associated with RGP contact lens wear, but is encountered more frequently in patients who use soft lenses either as extended wear or daily wear with less frequent replacement

Question 52

Corneal neovascularization - treatment

Answer 52

Refitting with lenses of higher-oxygen-permeability material or with a looser fit, requiring fewer hours of lens wear per day, or switching to disposable lenses can prevent further progression

Question 53

Deep stromal neovascularization - which CL

Answer 53

associated with extended-wear contact lenses, especially in aphakia. This condition is not usually symptomatic unless there is secondary lipid deposition.

Question 54

Punctate keratitis - causes

Answer 54

poor contact lens fit, toxic reaction to contact lens solutions, or dry eye

Question 55

Corneal warpage - which CL

Answer 55

both soft and RGP lenses, but it is more commonly associated with hard lenses

Question 56

Ptosis - causes

Answer 56

dehiscence of the levator aponeurosis has been associated with long-term use of RGP lenses

Question 57

3-o’clock and 9-o’clock staining - causes

Answer 57

may be observed in RGP contact lens users, especially with interpalpebral fit, and is probably related to poor wetting. Paralimbal staining is characteristic of low-riding lenses and is associated with an abortive reflex blink pattern, insufficient lens movement, inadequate tear meniscus, and a thick peripheral lens profile

Question 58

Allergic reactions - type of hypersensitivity

Answer 58

IV delayed hypersensitivity response

Question 59

Giant papillary conjunctivitis - diameter

Answer 59

> 0.3 mm) of the superior tarsal conjunctiva due to disruption of the anchoring septae

Question 60

Giant papillary conjunctivitis - how long persist

Answer 60

The tarsal conjunctival hyperemia and thickening may resolve in several weeks, but papillae or dome-shaped scars on the superior tarsus can persist for years

Question 61

Sterile infiltrates - where

Answer 61

peripheral cornea, younger patients

Question 62

Sterile infiltrates - characteristics

Answer 62

peripheral cornea, younger patients, Often there is more than one spot, relatively small

Question 63

CL for dry eye

Answer 63

Some soft lenses are marketed for dry eye patients; these lenses often have lower water content, more thickness, and/or better wettability. Scleral lenses are being used for some cases of severe dry eye.