front 1 Scleral (Haptic) Lens | back 1
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front 2 Corneal Lens | back 2
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front 3 Semi–Scleral Lens | back 3
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front 4 Daily Wear, Extended Wear, or Prolonged Wear | back 4
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front 5 Gas Permeable Lens | back 5 Permits the passage of oxygen and carbon dioxide through the lens |
front 6 Cosmetic Lens | back 6
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front 7 Bandage Lens | back 7 Used over the cornea to protect it from external influences which permits the healing of underlying corneal disorders |
front 8 Hydrophobic Lenses | back 8 Made of materials which, if not specially treated, will repel water |
front 9 Hydrophilic Lenses | back 9 Made out of materials which will naturally absorb water |
front 10 Wetting Angle | back 10 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. |
front 11 PMMA– Polymethylmethacrylate (Rigid Lens) | back 11
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front 12 CAB– Cellulose Acetate Butyrate (Rigid Lens) | back 12
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front 13 Silicone–Acrylate (rigid lens) | back 13
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front 14 Fluorocarbon and Fluorocarbon Silicone Acrylate (rigid lens) | back 14
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front 15 DK value | back 15 The oxygen permeability of a contact lens is characterized by its Dk value.
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front 16 HEMA– Hydroxyethylmethacrylate | back 16
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front 17 Overall benefits of new higher DK Fluorocarbon lenses compared with Soft Hydrogel Materials | back 17 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 |
front 18 What year did the FDA become involved in regulating contact lenses? | back 18 1968 |
front 19 Bausch and Lomb, Inc. (1971) | back 19 First hydrogel lens approved in the US |
front 20 What year was the first clinical marketing of soft silicone lenses? | back 20 1970's |
front 21 Bicurve lens | back 21 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. |
front 22 Tricurve lens | back 22 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. |
front 23 Base Curve | back 23 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. |
front 24 Radius of curvature | back 24 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. |
front 25 Diopter to Millimeter Conversion | back 25 337.5/Diopters |
front 26 sagital depth or height | back 26 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. |
front 27 Loosening and tightening of a lens | back 27 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. |
front 28 Optic zone | back 28 the central portion of a contact lens which contains the refractive power and generally corresponds to the central corneal cap. |
front 29 Posterior Apecal Radius | back 29 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. |
front 30 Steeper Base Curve | back 30 Occurs when the posterior radius of curvature is decreased (8.4 to 8.1mm). |
front 31 Flatter Base Curve | back 31 Occurs when the posterior radius of curvature is increased (8.1 to 8.4 mm). |
front 32 Lens Diameter of Chord Diameter | back 32 The width of a lens or the measurement from one edge of the lens to the opposite edge. |
front 33 Curve Widths | back 33 The width of the CPC, PPC, or IPC |
front 34 Central Thickness | back 34 Separation between the anterior and posterior surface at the geometeric center of the lens. \nMinus lenses are thinner, plus lenses are thicker. |
front 35 Ballasted Lens | back 35 Lens with a heavier base which becomes oriented inferiorly or downward when the lens is worn. |
front 36 Prism Ballasted Lens | back 36 A ballasted lens which utilizes a prism wedge designed to weight the lens. |
front 37 Truncated Lens | back 37 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. |
front 38 Back Surface Power | back 38 Effective power of a lens when measured from the back surface |
front 39 Toric Lens or Toroid Lens | back 39 Lenses with different radii of curvature in each meridian which are used to correct astigmatism. |
front 40 Principle Meridians | back 40 The meridians of shortest and longest radii which differ by 90 degrees |
front 41 Front surface toric lens | back 41 A lens in which the anterior surface has two different radii and the posterior surface is spherical. |
front 42 Back surface toric lens | back 42 A lens in which the posterior surface has two different radii and the anterior surface is spherical. |
front 43 Bitoric lens | back 43 A lens in which both the posterior and anterior surfaces contain two different radii. |
front 44 Lenticular Bowl | back 44 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) |
front 45 Spin–cast soft lens | back 45 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. |
front 46 Lathe Cut Soft Lens | back 46 A method of manufacturing soft contact lenses in which a machine lathe is used to grind lens designs, size, and power. |
front 47 Spherical Equivalent | back 47 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. |
front 48 The first successful contact lenses which were originally molded to conform to the shape of the cornea were known as _________ lenses. | back 48 haptic |
front 49 The majority of soft lenses fit today are: | back 49 semi–scleral |
front 50 A liquid coming in contact with a hydrophillic lens material with a relatively small wetting angle would be likely to... | back 50 spread evenly over the surface |
front 51 A contact lens with a diameter larger than that of the cornea is called a ________ lens. | back 51 haptic |
front 52 The term "sagittal value" refers to... | back 52 the vaulting effect of the contact lens |
front 53 The CPC of a rigid contact lens is held constant while the diameter is increased. This would result in... | back 53 a tighter fit |
front 54 HEMA is an abbreviation commonly used to describe... | back 54 soft lenses It actually stands for hydroxyethyl methacrylate |
front 55 The chief reason to utilize a ballasted lens is... | back 55 to inhibit lens rotation |
front 56 The central portion of the back surface of the contact lens is called... | back 56 the central posterior curve the primary base curve the base curve |
front 57 The distance between a flat surface and the back surface of the central portion of a lens is the... | back 57 sagittal depth |
front 58 A contact lens design which utilizes a lenticular bowl is typically employed with... | back 58 higher plus lenses |
front 59 A contact lens in which the anterior surface contains two different radii of curvature and the posterior surface is spherical is called a ________ lens. | back 59 front surface toric |
front 60 When the radius of curvature is held constant while the diameter is increased, the vault of the lens will be... | back 60 increased |
front 61 5 layers of the cornea | back 61 Epithelium (highly regenerative Bowman's membrane Stroma (90% of thickness) Desemet's membrane Endothelium cornea is about 0.5mm thick at the center |
front 62 tear film (function) | back 62 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. |
front 63 layers of precorneal tear film | back 63 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. |
front 64 B.U.T. or TBUT | back 64 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. |
front 65 Schirmer Test | back 65 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. |
front 66 Rose Bengal | back 66 Reddish–purple stain that colors degenerating and dead epithelial cells; may be used either as a paper strip or a unit dose. |
front 67 vertex distance | back 67 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 |
front 68 keratometer | back 68 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 |
front 69 topogometer | back 69 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. |
front 70 placido's disc | back 70 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. |
front 71 contact lens fit "on K" | back 71 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 |
front 72 contact lens fit steeper than K | back 72 lacrimal lens will have plus power tear lens is thicker in the center and thinner on the edges |
front 73 contact lens fit flatter than K | back 73 tear lens is thicker on the edges and thinner in the center lacrimal lens will contain minus power |
front 74 the lipid layer functions primarily to... | back 74 prevent evaporation of the tear film |
front 75 The test which assesses the QUANTITY of tears normally produced is... | back 75 Schirmer |
front 76 The test which assesses the QUALITY of the tear film is... | back 76 BUT or TBUT |
front 77 The primary function of a keratometer is to... | back 77 measure the curvature of the cornea |
front 78 When a spectacle lens prescription exceeds ______ diopters, vertex distance must be compensated when determining the power of a contact lens... | back 78 4.00D |
front 79 vertex distance formula | back 79 (Millimetres moved x Power x Power)/1000 = Dioptric Change |
front 80 The transition zone between the sclera and the cornea is called... | back 80 limbal area |
front 81 The precorneal tear film provides... | back 81 1. a lubricant to the cornea 2. a smooth optical surface for the cornea 3. nutrients to the cornea |
front 82 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... | back 82 the endothelium |
front 83 Flourescein | back 83 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. |
front 84 With the rule astigmatism | back 84 the verticle meridian has the steeper curve the lens will pull up or down when blinking |
front 85 against the rule astigmatism | back 85 the horizontal meridian has the steeper curvature the lens will move from left to right when blinking |
front 86 Spherical Equivalent | back 86 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. |
front 87 It is possible to assess the quality of the peripheral curves of a rigid contact lens through the use of... | back 87 shadowgraph |
front 88 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... | back 88 with the rule astigmatism |
front 89 The lens diameter, optic zone width, and peripheral curve width may be verified simultaneously through the use of... | back 89 measuring magnifier |
front 90 A dynamic flourescein pattern is best illuminated by... | back 90 cobalt blue light |
front 91 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... | back 91 too steep |
front 92 A radiuscope is designed to... | back 92 1. inspect surface quality 2. measure radius of curvature |
front 93 The primary source of oxygen for the cornea is... | back 93 tears |
front 94 Concentric circles reflected off a patients cornea can be evaluated using a ____________. | back 94 placido's disc |
front 95 When a rigid lens shows apical touch, this would indicate a... | back 95 flat fit |
front 96 A rigid lens showing a band–shaped area of touch on the flattest corneal meridian indicates... | back 96 "On K" astigmatic fit |
front 97 A well fit soft contact lens will demonstrate... | back 97 3 point touch good centration adequate movement |
front 98 An "ideal" flourescein pattern for a spherical lens on a spherical cornea would show... | back 98 an even distribution of flourescein with added thickness under the peripheral curves |
front 99 Tear Film – Three Layers | back 99 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. |
front 100 The cornea | back 100 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. |
front 101 Eyelids | back 101 AKA Palpebra |
front 102 Conjunctiva | back 102 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. |
front 103 Limbus | back 103 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. |
front 104 Corneal Edema | back 104 Corneal swelling |
front 105 Corneal Epithelial Edema | back 105 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. |
front 106 Microcystic Edema | back 106 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. |
front 107 Corneal Striae | back 107 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. |
front 108 Giant Papillary Conjunctivitis | back 108 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. |
front 109 Corneal Vascularization | back 109 The invasion of blood vessels into the cornea is known as corneal vascularization. |
front 110 Corneal Epithelial Staining Patterns | back 110 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. |
front 111 Epithelial Defects Induced by Direct Trauma | back 111 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. |
front 112 The Slit Lamp (General Characteristics) | back 112 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. |
front 113 Slit Lamp (Diffuse Illumination) | back 113 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. |
front 114 Slit Lamp (Direct Focal Illumination) | back 114 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. |
front 115 Slit Lamp (Indirect Illumination) | back 115 The observer focuses the microscope on an area immediately adjacent to the illuminated position. It is particularly valuable for studying the iris for pathology. |
front 116 Slit Lamp (Retro Illumination) | back 116 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. |
front 117 Slit Lamp (Specular Reflection) | back 117 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. |
front 118 Slit Lamp (Sclerotic Scatter) | back 118 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. |
front 119 Coblat Blue Filter (Slit Lamp) | back 119 A cobalt blue filter is often used after the instillation of flourescein to observe staining patterns and lens fit. |
front 120 White Filter (slit lamp) | back 120 A white filter is used to decrease ultraviolet rays and minimally decrease light intensity. It is used primarily for routine examination. |
front 121 Neutral Density Filter (slit lamp) | back 121 Decreases light intesity by 10%. Used to examine eyelids and conjunctiva. Useful for photophobic patients. |
front 122 Green Filter/ Red Free (slit lamp) | back 122 Makes red or brown objects blacker. Useful for observing blood vessels. |
front 123 After about four hours of wear, your patient complains that his eyes feel "hot". This may indicate... | back 123 too tight of a lens |
front 124 The slit lamp illumination most commonly used for detecting corneal edema is... | back 124 sclerotic scatter and the naked eye |
front 125 During a slit lamp evaluation, a hazzy cornea is an indication of... | back 125 corneal edema |
front 126 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... | back 126 diffuse epithelial edema |
front 127 Superficial punctate staining at the corneal apex combined with edema in this region might indicate... | back 127 a steep fit |
front 128 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... | back 128 dry spots |
front 129 The average pH of the human tear is... | back 129 7.4 |
front 130 Punctate staining is generally the result of... | back 130 inadequate tear exchange |
front 131 Prolonged corneal edema can result in... | back 131 photophobia limbal injection excessive spectacle blur |
front 132 Stippling can result when when there is... | back 132 inadequate tear exchange |
front 133 An arc shaped stain can be caused by... | back 133 a poor edge that is not well rounded |
front 134 Corneal vascularization is most likely to occur with... | back 134 PMMA lenses |
front 135 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... | back 135 retro illumination |
front 136 Following the installation of flourescein it is sometimes useful to use the ____________ filter of the slit lamp to observe staining patterns. | back 136 cobalt blue |
front 137 The invasion of new blood vessels into the cornea is known as... | back 137 neovascularization |
front 138 When using flourescein to aid in the evaluation of the fit of a rigid lens fit, a special filter must be used with... | back 138 fluorocarbon |
front 139 Prism Ballast | back 139 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. |
front 140 Truncation | back 140 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. |
front 141 Posterior Toric Lenses (Stabilization Technique for Toric Lenses) | back 141 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. |
front 142 Aspheric Lens Surface (Stabilization Technique for Toric Contact Lenses) | back 142 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. |
front 143 Toric Lens Rotation | back 143 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. |
front 144 Aphakia (Patient Selection) | back 144 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. |
front 145 Keratoconus | back 145 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. |
front 146 Keratoconus (Treatment With Contact Lenses) | back 146 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. |
front 147 Soper Keratoconus Diagnostic Fitting Set | back 147 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. |
front 148 Soft Lenses (Keratoconus) | back 148 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. |
front 149 Piggyback Lenses (Keratoconus) | back 149 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. |
front 150 Extending the Range of the Keratometer | back 150 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. |
front 151 Contact lenses for the correction of aphakia have certain advantages over spectacle lenses. These include... | back 151 less peripheral aberration less magnification increased visual field |
front 152 A conical shaped cornea can best be fit with contact lenses through the use of... | back 152 trial lenses |
front 153 Bitoric lenses are prescribed when... | back 153 there is at least 1.50D of corneal astigmatism accompanied by a significant amount of residual astigmatism |
front 154 Early stages of keratoconus may be detected through the use of the ______________. | back 154 keratometer |
front 155 The hyperflange lens design is useful in fitting... | back 155 high minus lenses |
front 156 A patient who has been diagnosed as having keratoconus may present which of the following during a slit lamp examination? | back 156 thinning of the corneal apex |
front 157 When fitting the keratoconus patient, which lens styles could be considered? | back 157 Aspheric Soper |
front 158 When fitting a keratoconus patient, the lens should... | back 158 align the apex |
front 159 How well a soft toric lens performs on the cornea depends on several factors. These would include... | back 159 tightness of the lid shape of the cornea where the lids are positioned lid shape |
front 160 A spherical rigid gas permeable lens fit on the cornea with a signifcant amount of with the rule astigmatism will show touch... | back 160 along the horizontal meridian |
front 161 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... | back 161 b) +1.25 |
front 162 When the power of a bifocal lens gradually changes from the central area of the lens to the periphery it is called... | back 162 an aspherical lens |
front 163 It is possible to inhibit the rotation of a rigid bifocal contact lens by... | back 163 truncating the lens using a prism ballast a double slab off technique |
front 164 Residual astigmatism can be corrected through the use of... | back 164 toric lenses |
front 165 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... | back 165 monovision |
front 166 Radiuscope | back 166 The radiuscope is designed to measure the radius of curvature of the anterior and posterior surfaces of rigid lenses. |