NCLE - General Knowledge Flashcards

• 1st successful contact lens
• made out of gas permeable material
• very difficult to tolerate
• used when other types fail in such cases as severe keratoconus, severe lid problems, and great amounts of astigmatism.

• Most common design for rigid and gas perm lenses
• the diameter of the lens is smaller than that of the cornea
• corneal diameters range from: 10.5–12.5mm while the diameter of the lens can range from: 7.4–9.2mm

• typical of soft contact lenses
• bridge the limbus and lie partially on the conjunctiva tissue overlying the sclera and adjacent to the cornea

• Determined by the amount of gas permeability of the material
• Approved by the FDA for 30 days of consecutive wear
• FDA recommends that they be worn no longer than 7 days in a row.

Permits the passage of oxygen and carbon dioxide through the lens

• used to cover an unsightly blind eye
• enhance cosmetic appearance

Used over the cornea to protect it from external influences which permits the healing of underlying corneal disorders

Made of materials which, if not specially treated, will repel water

Made out of materials which will naturally absorb water

The angle that a bead of water makes with the surface of a given material, the smaller the wetting angle the greater the wetting ability.

• Hard transparent plastic
• typically 8–10 mm in diameter
• developed in 1947 by Kevin Touhy
• "conventional hard lenses"
• Advantages:
  • Consistent vision,
  • durable,
  • spherical lenses can correct for up to 3.00D of corneal astigmatism,
  • easily fabricated
• Disadvantages:
  • Hydrophobic, absorbs only 1.5% water,
  • uncomfortable,
  • poor gas permeability requires "pumping action" of the tear layer for adequate oxygen exchange,
  • corneal edema/spectacle blur common

• Developed in 1938 by Eastman Kodak as a photographic material
• First used as a CL material in 1974
• Wets easier than PMMA, permitting better tear flow under it
• Much better gas perm than PMMA
• Advantages: Increased comfort, longer wearing time, less edema/spectacle blur, elimination of edge flare because it can be fitted larger (about 9 mm), better for sports, can be fitted tighter so there is less chance of loss, very tough and difficult to break, greater gas perm, lower wetting angle.
• Disadvantages: Poor shape retention (can warp after 1 year of wearing time or can change after hydration and steepen after several weeks), thicker lenses can be used to reduce chance of warpage but it increases lens awareness due to increased center and edge thickness

• The next generation of rigid gas perm contact lens materials
• Material Composition: Silicone (35%) and PMMA (65%)
• Developed in 1979 by Syntex Opthalmics
• Known clinically by the following names:
  • Polycon I and II
  • Boston Lenses I and II
  • Menicon O2
  • Optocryl 60
  • Paraperm
  • B&L Gas Perm
  • Prinicple
• Advantage over CAB: Since it is a more stable material it can be made thinner. This makes the lens more comfortable and allows for greater oxygen transmission.

• highest gas permeability of all lenses.
• may be made of either fluoropolymer or fluoropolymer and silicone combined.
• Trade names:
  • Advent (Allergan/3M) 100 DK
  • Fluoperm 30 (Paragon) 30 DK
  • Fluoperm 60 (Paragon) 60 DK
  • Fluoperm 92 (Paragon) 92 DK
  • Boston Equalens (Polymer) 71 DK
  • Boston Equacurve (Polymer) 71 DK
  • Boston Equalens (Polymer) 45 DK
  • Fluorex (GT Laboratories) 70 DK
  • Quantum I (B & L) 9 DK
  • Quantum II (B & L) 210 DK
  • Boston EXPFS (Polymer) 203 DK

The oxygen permeability of a contact lens is characterized by its Dk value.

• Dk is defined as the diffusion coefficient (D) multiplied by the solubility constant (k).
• The higher the DK value the better the oxygen permeability.

• Approved by the FDA in 1971 and manufactured and marketed by Bausch and Lomb
• Advantages:
  • Greater comfort,
  • rapid adaptation,
  • spectacle blur uncommon,
  • ok to wear intermittenly,
  • minimal lens loss,
  • minimal overwear reaction,
  • less flare and photophobia,
  • corneal protection,
  • offers an alternative for rigid lens drop outs,
  • excellent cosmetic lens (difficult to see),
  • better for infants and children.
• Disadvantages:
• spherical lenses will not significantly correct for corneal astigmatism
• poor vision can result if lenses are not fitted properly or when they become dehydrated under low humidity conditions such as in a heated or air conditioned rooms or when there is inadequate tear flow.
• lack of durability– average life span of daily wear lenses about two years
• material is prone to protein build up
• impossible to modify and difficult to verify
• disinfection regiment must be strictly adhered to and can be costly.

greater oxygen permeability increases the potential for extended wear performance \nLarger tear layer allow greater exchange of fluid with each blink which results in improved venting of debris \nsince they are more durable than soft hydrogel lenses they require less freqent replacement \nthe larger lens sizes tend to mask greater degrees of corneal astigmatism without requiring a special toric lens \nlarger gas permeability rigid lenses are easier to insert and remove \nthey are more economical to maintain than hydrogel lenses\ndue to the general absence of porosity, there is less risk of corneal infection

1968

First hydrogel lens approved in the US

1970's

contains two curves, a primary base curve and a flatter peripheral curve. The junction of the two curves is generally blended to permit greater comfort. The total diameter of the lens is equal to the diameter of the optic zone plus the peripheral curve widths.

contains two peripheral curves \nthe intermediate curve may be very narrow. Generally tricurve lenses are relatively large (9.5mm or greater) with an optic zone of 6.5–7.5mm, just large enough to clear the maximum pupil diameter. The peripheral curves are slightly flatter than the base curves by 0.4–0.8mm, or 2–4 diopters, and contain a width of approx. 1.3mm. The intermediate curves of a standard tricurve lens are 1mm flatter than the base curve.

The base curve of the central portion of the back surface of a contact lens. It is also known as the central posterior curve (CPC) or the primary base curve. The base curve is designed to conform to the optic zone of the cornea and is measured in millimeters of radius of an arc, or in diopters.

the distance from the geometric center of the circle to its periphery where the line is drawn. As the radius of curvature becomes longer the circle gets larger. As the circle gets larger, its curve gets flatter. As the curve gets flatter the power in diopters become less.

337.5/Diopters

the distance between a flat surface and the back surface of the central portion of a lens. A greater sagital depth produces greater vaulting of the lens and in effect would be steeper. It is sometimes referred to as sagittal vault.

As the diameter gets greater, the sagital depth or vaulting increases.The greater the sagital depth, the tighter the fit of the lens. The shorter the radius of curvature, the greater the sagital depth; the longer the radius, the shorter the sagital depth.

the central portion of a contact lens which contains the refractive power and generally corresponds to the central corneal cap.

This term is generally used in reference to spin cast soft lenses.The curvature of the posterior surface of the lens changes with the refractive power. The radius of curvature is measured at the apex of the posterior surface.

Occurs when the posterior radius of curvature is decreased (8.4 to 8.1mm).

Occurs when the posterior radius of curvature is increased (8.1 to 8.4 mm).

The width of a lens or the measurement from one edge of the lens to the opposite edge.

The width of the CPC, PPC, or IPC

Separation between the anterior and posterior surface at the geometeric center of the lens. \nMinus lenses are thinner, plus lenses are thicker.

Lens with a heavier base which becomes oriented inferiorly or downward when the lens is worn.

A ballasted lens which utilizes a prism wedge designed to weight the lens.

A lens which has been cut off, usually 0.5–1.5mm along its lower edge, to form a horizontal base. Double truncations, along the top and bottom portion of the lens is sometimes done to help improve stabilization.

Effective power of a lens when measured from the back surface

Lenses with different radii of curvature in each meridian which are used to correct astigmatism.

The meridians of shortest and longest radii which differ by 90 degrees

A lens in which the anterior surface has two different radii and the posterior surface is spherical.

A lens in which the posterior surface has two different radii and the anterior surface is spherical.

A lens in which both the posterior and anterior surfaces contain two different radii.

A lens design generally used in higher plus powers which consists of a central optic zone and a surrounding non–optic peripheral or "carrier" portion. \n(moon shaped, thicker in the center thinner on the sides)

A method of manufacturing soft contact lenses whereby a liquid material is revolved in a mold at a controlled speed and temperature which produces the desired curvature, design, and power.

A method of manufacturing soft contact lenses in which a machine lathe is used to grind lens designs, size, and power.

A spectacle or contact lens prescription which is expressed only as a sphere. \nTo calculate the sphere equivalent, algebraically add half the cylinder power to the sphere.

haptic

semi–scleral

spread evenly over the surface

haptic

the vaulting effect of the contact lens

a tighter fit

soft lenses

It actually stands for hydroxyethyl methacrylate

to inhibit lens rotation

the central posterior curve

the primary base curve

the base curve

sagittal depth

higher plus lenses

front surface toric

increased

Epithelium (highly regenerative

Bowman's membrane

Stroma (90% of thickness)

Desemet's membrane

Endothelium

cornea is about 0.5mm thick at the center

its function: maintaining the optical quality of the cornea as well as the health of the cornea and conjunctiva. it passes lysozyme (an antibacterial enzyme) that inhibits bacterial proliferation.

when contacts are worn the tear film provides oxygen exchange as the lens is moved. the tear film can be shaped into a liquid lens with significant refractive power by the front surface of the cornea and the back surface of a rigid contact lens.

Lipid layer: top or outer layer; consists of fatty material which forms a thin layer over the whole surface of the tear film; it functions primarily to prevent evaporation of the tear film; this layer is produced by the meibomian glands located in the upper and lower eyelids.

Aqueous layer: middle layer; consists of 98% water and accounts for most of the thickness; it contains ions and other molecules such as sodium and potassium along with a concentration of protein; produced by lacrimal glands which are located in the palbebral conjunctiva or the temporal portion of the upper cul de sac.

Mucoid layer: the bottom or innermost layer; located immediately against the corneal and conjunctival epithelial cells; produced by the goblet cells located in the conjunctiva; functions to convert the hydrophobic epithelial layer of the cornea to a hydrophillic surface; without this layer the tear film would break up very rapidly resulting in drying and corneal damage.

stands for "break–up time" of tear film; assesses the quality of the tear film and is performed with flouriscein and a slit lamp.

hort break up time may indicate a problem with the mucoid layer; average break–up time is 20–25 seconds; a BUT of less than 10 seconds presents a potential problem.

assesses the quantity of tears normally produce; performed with a strip of filter paper placed under each lid and kept there for approx. five minutes; normally there would be about 15mm or more of moistened strip; patients over 40 will find 10mm to 15mm of moistened strip.

Reddish–purple stain that colors degenerating and dead epithelial cells; may be used either as a paper strip or a unit dose.

the distance from the front of the cornea to the ocular surface of the lens is called the vertex distance; as the relative position of the focal point changes the effective power of the lens is changed.

the changes only become significant for contact lenses when the lens powers exceed + or – 4.00 D

used primarily to measure the curvature of the cornea

it measures about 2–4mm of the corneal cap\nthe corneal cap itself is about 4–5mm in diameter

an attachment to the keratometer which is a movable light designed to pinpoint the specific location of the corneal cap to be measured by the keratometer.

instrument that assess the regularity of the cornea

concentric circle are reflected off the corneal surface can indicate the presence of corneal or irregular astigmatism.

the base curve of the lens parallels the curvature of the cornea

the power of the tear lens is plano

when there is astigmatism a lens fits "on K" parallels the flatter of the two corneal meridians

lacrimal lens will have plus power

tear lens is thicker in the center and thinner on the edges

tear lens is thicker on the edges and thinner in the center

lacrimal lens will contain minus power

prevent evaporation of the tear film

Schirmer

BUT or TBUT

measure the curvature of the cornea

4.00D

(Millimetres moved x Power x Power)/1000 = Dioptric Change

limbal area

1. a lubricant to the cornea

2. a smooth optical surface for the cornea

3. nutrients to the cornea

the endothelium

Flourescein is a common dye or stain whcih can be used to help analyze the fit of a rigid contact lens.

When it mixes with the tear film it will glow in the presence of ultraviolet light or cobalt blue light.

It allows the fitter to examine the shape and flow of the tear layer between the back surface of the lens and the front surface of the cornea. It is available in solution form or impregnated strips.

the verticle meridian has the steeper curve

the lens will pull up or down when blinking

the horizontal meridian has the steeper curvature

the lens will move from left to right when blinking

Spherical soft lenses are often fit even when a relatively low amount of astigmatism is present. Spherical Equivalent derived by algebraically adding half the cyl power to the sphere.

shadowgraph

with the rule astigmatism

measuring magnifier

cobalt blue light

too steep

1. inspect surface quality

2. measure radius of curvature

tears

placido's disc

flat fit

"On K" astigmatic fit

3 point touch

good centration

adequate movement

an even distribution of flourescein with added thickness under the peripheral curves

Lipid layer: a fatty material produced by the meibomian glands which forms a very thin layer over the entire surface of the tear film. It functions primarily to prevent rapid evaporation which would result in dry areas on the cornea and subsequent discomfort and corneal damage.

The Aqueous Layer : The middle layer of the tear film and consists of 98% water. However, it also contains ions and other molecules such as sodium and potassium along with a concentration of protein.

The Mucoid Layer: functions to convert the hydrophobic epithelial layer of the cornea to a hydrophillic surface. It is the innermost layer of the tear film and located immediately against the corneal and conjunctival epithelial cells.

The cornea is the anterior refracting surface of the eye. It consists of transparent tissue and is devoid of blood vessels.

The average corneal thickness is about 0.52 mm at the center and increases to about 0.65mm at the limbal area and it is composed of 5 distinct layers.

A normal cornea is in a continual state of partial dehydration which also known as deturgescence.

AKA Palpebra

The conjunctiva is the loose tissue covering the sclera and inside the lids.

The bulbar conjunctiva covers the sclera

The palpebral conjunctiva is that portion which lines the inner surface of the upper and lower eyelids.

The limbus is the transtition zone between the cornea and sclera. It is approx. 1mm wide and the cornea is dependent upon it for receiving part of its nutrients.

Corneal swelling

This condition appears as a grey cornea and may be best observed with a slit lamp utilizing sclerotic scatter illumination and the unaided eye.

Rated on a scale of 1–4, 4 being the worst.

Progressive corneal hypoxia can lead to the rupture of certain epithelial cell membranes which in turn can result in the formation of microcysts.

Microcysts are the result of fluid which has accumulated in the spaces caused by the rupture of these cells. Microcystic edema can be best seen by utilizing a slit lamp with retro–illumination.

Flourescein can also be used to outline intact microcysts and stain the punctate areas where microcysts have ruptured.

Corneal striae are linear opacities in the cornea.

Corneal striae are best seen either with retroillumination or with direct illumination.

They are found in up to 50% of soft lens wearers but are very rare in rigid contact lens wearers. The reason for this is not completely understood.

GPC is a nonspecific response to a conjunctival insult. It results in the formation of papillae on the palpebral conjunctiva.

A papillae is a small conjunctial elevation. It is seen most frequently in soft contact lens wear and is rarely seen with hard lenses.

The invasion of blood vessels into the cornea is known as corneal vascularization.

The loss of corneal epithelial cells is one of the most common adverse side effects of contact lens wear. Although this can be detected through the use of a slit lamp, the installation of flourescein dye can indicate the precise nature and location of specific corneal epithelial defects.

Flourescein acts either by pooling in the area of the defect or by staining the underlying exposed basement membrane of Bowman's layer.

Corneal epithelial damage can result in the discrete loss of a few epithelial cells to deeper craterlike lesions with cell loss to the level of Bowman's membrane. Epithelial damage can be caused either by direct trauma or by defective tear film distribution over the cornea.

1. A poorly edged or damaged lens \n2. An excessively flat lens \n3. Foreign particles such as dust or cosmetics which lodge beneath the lens. \n4. Improper insertion, removal, and reentering techniques\n5. Poor cleaning habits \n6. Mucus build up generally due to the dry storage of hard lenses.

The slit lamp or the biomicroscope is an instrument designed primarily to observe the transparent structures of the human eye under a magnification of from 10 to 50 times.

Its two principle parts include a lamp equipped with an optical system designed to project a slit of light upon the eye, and a stereomicroscope which is mounted horizontally for direct viewing of the patients eye.

The slit lamp may be adjusted to project a variety of light beams. By varying the light beam and the viewing postion, it is possible to improve the view of the various structures of the eye.

A wide beam of light is directed obliquely at the cornea with no attempt to focus the light. It provides a good overall picture of the cornea but no fine details can be seen. It is used primarily for a general survery of the eye.

The microscope and the beam of light are focused on the same area. There are three types of direct focal illumination:

1. optic section: used to see all the layers of the cornea. It is possible to determine distortions in the corneal contour and the depth of foreign bodies.

2. parallelepiped: provides a broader view of the anterior and posterior corneal surfaces. It is used to help assess any surface irregularities which may exist and to examine the endothelium. It is often used with flourescein in helping to determine the fit of the contact lens.

3. conical beam: it is the most sensitive method for observing flare or relucency in normal aqueous humor in the anterior chamber. Using high magnification it resembles light penetrating fog.

The observer focuses the microscope on an area immediately adjacent to the illuminated position. It is particularly valuable for studying the iris for pathology.

The light is focused on the deeper structures such as the iris, lens, or retina while the microscope is focused to study the more anterior structures in the reflected light.

Most typically, the light is reflected from the iris in order to study the cornea. It can be useful for the examination of corneal edema and to view blood vessels that have invaded the cornea. It can also be used to study deposits on Descemet's membrane.

The illuminating arm and the microscope are positioned such that the beam of light, when reflected from the corneal surfaces, will pass through one of the oculars of the microscope. At this point the angle of incidence of the light will be equal to the angle of reflection.

It is used for observing elevation and depressions of the anterior surface of the cornea of a soft contact lens. It may also be used to observe lipid or calcium deposits on contact lenses and their general wetting condition.

This form of illumination can also be used to observe the pre–corneal tear film to include mucus and meiobomian secretions.

A broad beam of light is focused a the temporal limbus so it transilluminates through to the nasal limbus. The microscope is then focused sharply on the cornea. Sclerotic scatter and the unaided eye is used to detect the presence of corneal edema which appears as a foggy patch of cotton.

A cobalt blue filter is often used after the instillation of flourescein to observe staining patterns and lens fit.

A white filter is used to decrease ultraviolet rays and minimally decrease light intensity. It is used primarily for routine examination.

Decreases light intesity by 10%. Used to examine eyelids and conjunctiva. Useful for photophobic patients.

Makes red or brown objects blacker. Useful for observing blood vessels.

too tight of a lens

sclerotic scatter and the naked eye

corneal edema

diffuse epithelial edema

a steep fit

dry spots

7.4

inadequate tear exchange

photophobia

limbal injection

excessive spectacle blur

inadequate tear exchange

a poor edge that is not well rounded

PMMA lenses

retro illumination

cobalt blue

neovascularization

fluorocarbon

This is one of the most common stabilizing techniques.

A prism of between 1 and 1.50D is ground into the base of the lens. However greater amounts of prism may be needed for patients with particularly tight lids, flat corneas, or oblique axis astigmatism. The lens will tend to rotate so that the base of the prism is oriented inferiorly. The added thickness of the lens along the prism base can reduce oxygen permeability through the portion of the lens resulting in possible hypoxia disturbances in the inferior zone of the cornea.

When a lens is truncated, a portion of it is sectioned off. It is usually 0.50 to 1.5mm on the lower edge of the lens. The truncation will serve to stabilize the lens when the lower flat edge comes to lie adjacent to the lower eyelid margin.

When a lens is truncated its diameter is effectively reduced which results in a looser fit. To compensate for this the base curves of truncated lenses are generally made somewhat steeper.

A back toric surface can be used as a lens stabilizing technique. When the shape of the posterior contact lens surface closely parallels that of the cornea lens rotation can be minimized.

An aspheric surface can aid in lens–axis stabilization by adding drag to the motion of the lens. It is generally used in combination with truncation or prism ballast since it is only minimally effective by itself.

Because it is necessary to inhibit lens rotation, toric lenses will typically come with reference markings so the fitter can determine how the lens is oriented on the cornea. These markings may be circles or lines located at the six–o–clock or three and nine–o–clock meridians.

The expression LARS stands for left add, right subtract. If the bottom of the lens is rotated to the fitters left, the appropriate number of degrees is added to the prescribed axis. If it is rotated to the right the appropriate number of degrees is subtracted.

Lens rotation can measured using a slit lamp equipped with a protractor. The use of trial lenses is especially important when fitting toric lenses.

Certain guidelines apply for the selection of potential aphakic lens patients. They must be able to handle the lens either on a daily or weekly basis and possess the ability to properly care for the lenses. Like any contact lens candidate there needs to be sufficient tear film as well as an abscence of any serious corneal disease.

A degenerative hereditary condition of the cornea. It results in a progressive thinning of the central or paracentral area of the cornea and is accompanied by irregular astigmatism. In more advanced cases the cornea can form a buldge or a cone which is often located near or just below its center. In other cases a diffuse thinning of the cornea can result in a sagging cone otherwise known as Keratoglobus.

Keratoconus may be successfully treated with contact lenses. The purpose of the lens is to cover the irregular astigmatism created by the distorted anterior surface of the cornea.

The tear layer found between the back surface of the contact lens and the front surface of the cornea serves to fill in the corneal irregularities thereby providing a smooth optical surface.

Rigid lenses are far more effective at accomplishing this purpose than are soft lenses. Contact lenses do not retard the progression of the disease nor do they provide a cure, although the patient may experience long periods of natural remission.

These lenses are designed with a steep base curve to accomodate the steep central cone area with a much flatter peripheral curve to rest on the surrounding cornea. It is essential to use a trial set containing base curves of 48 to 60D when fitting these lenses. The lens diameters range from 7.5 to 9.5mm.

A good fitting lens would show the following: apical clearance with circulation of tears between the apex of the cornea and the back of the lens, good centration, and some movement of the lens with blinking.

They are useful for the patient who cannot tolerate rigid lenses. They are fit with a relatively flat base curve 8.1 to 8.4mm and a fairly large diameter 13 to 14mm in order to provide lens stability.

While soft lenses don't normally mask astigmatism, they have been shown to reduce a significant amount so that overcorrection with spectacle lenses become effective.

These are used when the patient cannot tolerate rigid lenses and when the use of auxillary spectacles needs to be avoided. A soft lens of about 14mm in diameter is placed on the cornea. A rigid lens is placed over it which may ride freely or be placed in a depression in the soft lens designed to hold the rigid lens in place. The diameter of the rigid lens usually ranges from 8.5 to 9.5mm.

The range of the keratometer may be extended in the steeper range by placing a +1.25D lens over the aperture. It may be extended in the flatter range by placing a –1.00 lens over the aperature.

less peripheral aberration

less magnification

increased visual field

trial lenses

there is at least 1.50D of corneal astigmatism accompanied by a significant amount of residual astigmatism

keratometer

high minus lenses

thinning of the corneal apex

Aspheric

Soper

align the apex

tightness of the lid

shape of the cornea

where the lids are positioned

lid shape

along the horizontal meridian

b) +1.25

an aspherical lens

truncating the lens

using a prism ballast

a double slab off technique

toric lenses

monovision

The radiuscope is designed to measure the radius of curvature of the anterior and posterior surfaces of rigid lenses.