Contact lenses Flashcards
Base curve
curvature of the central posterior surface of the lens, which is adjacent to the cornea, described by its radius of curvature (mm)
Diameter (chord diameter) for soft and RGP
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
Peripheral curves
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
Sagittal depth or vault
distance between the center of the posterior surface to the plane of the edges of the lens
If the diameter of the lens is held […], the sagittal depth […] as the base curve radius […]
If the diameter of the lens is held constant, the sagittal depth DECREASES as the base curve radius INCREASES
A […] wetting angle means water will spread over the surface, […] surface wettability
A LOW wetting angle means water will spread over the surface, INCREASING surface wettability
for irregular astigmatism
RGP, hybrid, scleral
CL and spectacle is how many mm in front of eye
3 mm and 12 mm
Prentice rule
as the eyes look off-axis, through parts of the spectacle lenses farther from their optical centers, the eyes encounter unequal prism
magnification in aphakic
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%
accommodation
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
spectacles, vergence and prism
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.
do not correct lenticular astigmatism
RGP
Dk
refers to the oxygen permeability of a lens material
D
is the diffusion coefficient for oxygen movement in the material
Dk/t
refers to the oxygen transmissibility of a lens, depending on its material and central thickness (t)
desirable parameters
high Dk/t and a low wetting angle
Soft contact lenses - materials
are made of a hydrogel polymer, hydroxyethylmethacrylate or, more often now, a silicone hydrogel
Hydrogels have […] oxygen permeability when they have […] water content
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.
Modifying the hydrogels yielded “silicone hydrogels,” - advantages
achieve their oxygen permeability with less water content, using pores induced by the presence of silicon atoms, rather than high water content
Soft CL - advantages
shorter adaptation, comfortable, different types, easier to fit, cheaper
RGP - advantages
clear vision, correction of regular and irregular, ease of handling, stability and durability, ease of care
addition of fluorine in RGP
increases oxygen permeability and encourages the coating of the lens with mucin, which improves wettability
RGP - disadvantages
initial discomfort, longer period of adaptation, greater knowledge and effort required for fitting, and the greater cost
People who use contacts only sporadically are generally more comfortable with
a soft lens
A soft or rigid lens can be made to fit tighter by
by either choosing a smaller radius to steepen the base curve or by increasing the lens chord diameter without changing the radius
loose fit
excessive movement, poor centration, lens-edge standoff, blurred mires after a blink, fluctuating vision, continuing lens awareness, air bubbles under the lens
tight fit
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
In a good fit, the lens will move
about 1 mm with upward gaze or blink, or with gentle pressure on the lower eyelid
apical clearance
(steeper than K), when the base curve has a steeper fit, with radius shorter than that of the cornea
apical bearing
(flatter than K), when the base curve is flatter than the cornea
The most common type of RGP lens fit is
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.
central or interpalpebral fit is
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
RGP - SAM and FAP
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
astigmatism <1 D
spherical soft or RGP
astigmatism 1-2 D
toric soft or spherical RGP
astigmatism 2-3 D
custom toric soft or spherical RGP
astigmatism >3 D
custom toric soft or toric RGP
To adjust the prescription for lens rotation
follow the LARS rule (left add; right subtract)
There are 2 types of bifocal lenses
alternating vision lenses and simultaneous vision lenses
intralimbic contact lenses
Larger RGP contact lenses with larger optical zones (diameters >11 mm) are available for keratoconus and post-transplant fitting
hybrid contact lens
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.
Piggyback lens systems
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.
Gas-Permeable Scleral Contact Lenses
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
Gas-Permeable Scleral Contact Lenses - indications
(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
Semi-scleral lenses and scleral - diameters
Semi-scleral lenses, extending not as far onto the sclera, have diameters 15–18 mm; scleral lenses have larger diameters, 18.1–24 mm
Therapeutic Use of Contact Lenses
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.
Contact lens overwear - causes
Hypoxia, lactate accumulation, and impaired carbon dioxide efflux
Sattler veil
Central epithelial edema (Sattler veil) may present after many hours of wear, more commonly with hard contact lenses.
Microcystic epitheliopathy - how long may persist
takes up to 6 weeks following discontinuation of contact lens wear for the cysts to resolve
Corneal neovascularization - which CL
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
Corneal neovascularization - treatment
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
Deep stromal neovascularization - which CL
associated with extended-wear contact lenses, especially in aphakia. This condition is not usually symptomatic unless there is secondary lipid deposition.
Punctate keratitis - causes
poor contact lens fit, toxic reaction to contact lens solutions, or dry eye
Corneal warpage - which CL
both soft and RGP lenses, but it is more commonly associated with hard lenses
Ptosis - causes
dehiscence of the levator aponeurosis has been associated with long-term use of RGP lenses
3-o’clock and 9-o’clock staining - causes
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
Allergic reactions - type of hypersensitivity
IV delayed hypersensitivity response
Giant papillary conjunctivitis - diameter
> 0.3 mm) of the superior tarsal conjunctiva due to disruption of the anchoring septae
Giant papillary conjunctivitis - how long persist
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
Sterile infiltrates - where
peripheral cornea, younger patients
Sterile infiltrates - characteristics
peripheral cornea, younger patients, Often there is more than one spot, relatively small
CL for dry eye
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.
