NCLE - General Knowledge Flashcards
Scleral (Haptic) Lens
- 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
Daily Wear, Extended Wear, or Prolonged Wear
- 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.
Gas Permeable Lens
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.
PMMA– Polymethylmethacrylate (Rigid Lens)
- Hard transparent plastic
- typically 8–10 mm in diameter
- developed in 1947 by Kevin Touhy
- "conventional hard lenses"
- Consistent vision,
- spherical lenses can correct for up to 3.00D of corneal astigmatism,
- easily fabricated
- Hydrophobic, absorbs only 1.5% water,
- poor gas permeability requires "pumping action" of the tear layer for adequate oxygen exchange,
- corneal edema/spectacle blur common
CAB– Cellulose Acetate Butyrate (Rigid Lens)
- 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
Silicone–Acrylate (rigid lens)
- The next generation of rigid gas perm contact lens materials
- Material Composition: Silicone (35%) and PMMA (65%)
- Developed in 1979 by Syntex Opthalmics
clinically by the following names:
- Polycon I and II
- Boston Lenses I and II
- Menicon O2
- Optocryl 60
- B&L Gas Perm
- 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.
Fluorocarbon and Fluorocarbon Silicone Acrylate (rigid lens)
- 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
- 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.
- 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.
Overall benefits of new higher DK Fluorocarbon lenses compared with Soft Hydrogel Materials
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
What year did the FDA become involved in regulating contact lenses?
Bausch and Lomb, Inc. (1971)
First hydrogel lens approved in the US
What year was the first clinical marketing of soft silicone lenses?
contains two curves, a primary base curve and a flatter peripheral curve. \nthe junction of the two curves is generally blended to permit greater comfort. \nthe 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\ngenerally 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. \n\nthe 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. \nIt is also known as the central posterior curve (CPC) or the primary base curve.\nThe base curve is desgined to conform to the optic zone of the cornea and is measured in millimeters of radius of an arc, or in diopters.
Radius of curvature
the distance from the geometric center of the circle to its periphery where the line is drawn. \nAs the radius of curvature becomes longer the circle gets larger.\nAs the circle gets larger, its curve gets flatter.\nAs the curve gets flatter the power in diopters become less.
Diopter to Millimeter Conversion
sagital depth or height
the distance between a flat surface and the back surface of the central portion of a lens. \nA greater sagital depth produces greater vaulting of the lens and in effect would be steeper.\nIt is sometimes referred to as sagittal vault.
Loosening and tightening of a lens
As the diameter gets greater, the sagital depth or vaulting increases.\nThe greater the sagital depth, the tighter the fit of the lens. \n\nThe 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.
Posterior Apecal Radius
This term is generally used in reference to spin cast soft lenses.\nThe 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.
Steeper Base Curve
Occurs when the posterior radius of curvature is decreased (8.4 to 8.1mm).
Flatter Base Curve
Occurs when the posterior radius of curvature is increased (8.1 to 8.4 mm).
Lens Diameter of Chord Diameter
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.
Prism Ballasted Lens
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.
Back Surface Power
Effective power of a lens when measured from the back surface
Toric Lens or Toroid Lens
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
Front surface toric lens
A lens in which the anterior surface has two different radii and the posterior surface is spherical.
Back surface toric lens
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)
Spin–cast soft lens
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.
Lathe Cut Soft Lens
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.
The first successful contact lenses which were originally molded to conform to the shape of the cornea were known as _________ lenses.
The majority of soft lenses fit today are:
A liquid coming in contact with a hydrophillic lens material with a relatively small wetting angle would be likely to...
spread evenly over the surface
A contact lens with a diameter larger than that of the cornea is called a ________ lens.
The term "sagittal value" refers to...
the vaulting effect of the contact lens
The CPC of a rigid contact lens is held constant while the diameter is increased. This would result in...
a tighter fit
HEMA is an abbreviation commonly used to describe...
It actually stands for hydroxyethyl methacrylate
The chief reason to utilize a ballasted lens is...
to inhibit lens rotation
The central portion of the back surface of the contact lens is called...
the central posterior curve
the primary base curve
the base curve
The distance between a flat surface and the back surface of the central portion of a lens is the...
A contact lens design which utilizes a lenticular bowl is typically employed with...
higher plus lenses
A contact lens in which the anterior surface contains two different radii of curvature and the posterior surface is spherical is called a ________ lens.
front surface toric
When the radius of curvature is held constant while the diameter is increased, the vault of the lens will be...
5 layers of the cornea
Epithelium (highly regenerative
Stroma (90% of thickness)
cornea is about 0.5mm thick at the center
tear film (function)
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.
layers of precorneal tear film
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.
B.U.T. or TBUT
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.
contact lens fit "on K"
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
contact lens fit steeper than K
lacrimal lens will have plus power
tear lens is thicker in the center and thinner on the edges
contact lens fit flatter than K
tear lens is thicker on the edges and thinner in the center
lacrimal lens will contain minus power
the lipid layer functions primarily to...
prevent evaporation of the tear film
The test which assesses the QUANTITY of tears normally produced is...
The test which assesses the QUALITY of the tear film is...
BUT or TBUT
The primary function of a keratometer is to...
measure the curvature of the cornea
When a spectacle lens prescription exceeds ______ diopters, vertex distance must be compensated when determining the power of a contact lens...
vertex distance formula
(Millimetres moved x Power x Power)/1000 = Dioptric Change
The transition zone between the sclera and the cornea is called...
The precorneal tear film provides...
1. a lubricant to the cornea
2. a smooth optical surface for the cornea
3. nutrients to the cornea
The normal cornea is in a constant state of dehydration also known as deturgescence. The layer of the cornea most responsible for maintaining this function is...
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.
With the rule astigmatism
the verticle meridian has the steeper curve
the lens will pull up or down when blinking
against the rule astigmatism
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.
It is possible to assess the quality of the peripheral curves of a rigid contact lens through the use of...
A flourescein pattern with a rigid contact lens in place shows a narrow band horizontally, with a concentration of flourescein under the lens both inferiorly and superiorly. The type of astigmatism represented by this pattern is...
with the rule astigmatism
The lens diameter, optic zone width, and peripheral curve width may be verified simultaneously through the use of...
A dynamic flourescein pattern is best illuminated by...
cobalt blue light
When a keratometer is used to help evaluate the fit of a soft contact lens, distorted mires would most often indicate that the lens is...
A radiuscope is designed to...
1. inspect surface quality
2. measure radius of curvature
The primary source of oxygen for the cornea is...
Concentric circles reflected off a patients cornea can be evaluated using a ____________.
When a rigid lens shows apical touch, this would indicate a...
A rigid lens showing a band–shaped area of touch on the flattest corneal meridian indicates...
"On K" astigmatic fit
A well fit soft contact lens will demonstrate...
3 point touch
An "ideal" flourescein pattern for a spherical lens on a spherical cornea would show...
an even distribution of flourescein with added thickness under the peripheral curves
Tear Film – Three Layers
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.
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 Epithelial Edema
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.
Giant Papillary Conjunctivitis
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.
Corneal Epithelial Staining Patterns
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.
Epithelial Defects Induced by Direct Trauma
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 (General Characteristics)
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.
Slit Lamp (Diffuse Illumination)
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.
Slit Lamp (Direct Focal Illumination)
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.
Slit Lamp (Indirect Illumination)
The observer focuses the microscope on an area immediately adjacent to the illuminated position. It is particularly valuable for studying the iris for pathology.
Slit Lamp (Retro Illumination)
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.
Slit Lamp (Specular Reflection)
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.
Slit Lamp (Sclerotic Scatter)
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.
Coblat Blue Filter (Slit Lamp)
A cobalt blue filter is often used after the instillation of flourescein to observe staining patterns and lens fit.
White Filter (slit lamp)
A white filter is used to decrease ultraviolet rays and minimally decrease light intensity. It is used primarily for routine examination.
Neutral Density Filter (slit lamp)
Decreases light intesity by 10%. Used to examine eyelids and conjunctiva. Useful for photophobic patients.
Green Filter/ Red Free (slit lamp)
Makes red or brown objects blacker. Useful for observing blood vessels.
After about four hours of wear, your patient complains that his eyes feel "hot". This may indicate...
too tight of a lens
The slit lamp illumination most commonly used for detecting corneal edema is...
sclerotic scatter and the naked eye
During a slit lamp evaluation, a hazzy cornea is an indication of...
Focusing the beam of the slit lamp directly on the limbus and observing the cornea without the use of the microscope is an excellent way to observe...
diffuse epithelial edema
Superficial punctate staining at the corneal apex combined with edema in this region might indicate...
a steep fit
During a slit lamp evaluation certain areas of the cornea are observed which appear dark, retain their configuration during blinking, and do not stain. These areas most likely represent...
The average pH of the human tear is...
Punctate staining is generally the result of...
inadequate tear exchange
Prolonged corneal edema can result in...
excessive spectacle blur
Stippling can result when when there is...
inadequate tear exchange
An arc shaped stain can be caused by...
a poor edge that is not well rounded
Corneal vascularization is most likely to occur with...
A slit lamp is focused on one of the deeper structures of the eye such as the lens or iris while the microscope focuses on the more anterior structures in the reflected light. This illumination is known as...
Following the installation of flourescein it is sometimes useful to use the ____________ filter of the slit lamp to observe staining patterns.
The invasion of new blood vessels into the cornea is known as...
When using flourescein to aid in the evaluation of the fit of a rigid lens fit, a special filter must be used with...
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.
Posterior Toric Lenses (Stabilization Technique for Toric Lenses)
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.
Aspheric Lens Surface (Stabilization Technique for Toric Contact Lenses)
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.
Toric Lens Rotation
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.
Aphakia (Patient Selection)
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 (Treatment With Contact Lenses)
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.
Soper Keratoconus Diagnostic Fitting Set
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.
Soft Lenses (Keratoconus)
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.
Piggyback Lenses (Keratoconus)
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.
Extending the Range of the Keratometer
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.
Contact lenses for the correction of aphakia have certain advantages over spectacle lenses. These include...
less peripheral aberration
increased visual field
A conical shaped cornea can best be fit with contact lenses through the use of...
Bitoric lenses are prescribed when...
there is at least 1.50D of corneal astigmatism accompanied by a significant amount of residual astigmatism
Early stages of keratoconus may be detected through the use of the ______________.
The hyperflange lens design is useful in fitting...
high minus lenses
A patient who has been diagnosed as having keratoconus may present which of the following during a slit lamp examination?
thinning of the corneal apex
When fitting the keratoconus patient, which lens styles could be considered?
When fitting a keratoconus patient, the lens should...
align the apex
How well a soft toric lens performs on the cornea depends on several factors. These would include...
tightness of the lid
shape of the cornea
where the lids are positioned
A spherical rigid gas permeable lens fit on the cornea with a signifcant amount of with the rule astigmatism will show touch...
along the horizontal meridian
It is possible to extend the range of a keratometer to 61.00D through the use of an auxillary trial lens with a power of...
When the power of a bifocal lens gradually changes from the central area of the lens to the periphery it is called...
an aspherical lens
It is possible to inhibit the rotation of a rigid bifocal contact lens by...
truncating the lens
using a prism ballast
a double slab off technique
Residual astigmatism can be corrected through the use of...
Presbyopia may be corrected through the use of single vision contact lenses by placing the distance vision Rx in the dominant eye and near vision Rx in the other eye. This is called...
The radiuscope is designed to measure the radius of curvature of the anterior and posterior surfaces of rigid lenses.