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keratometry
CS final
| Question | Answer |
|---|---|
| provides information on corneal curvature and hence on corneal astigmatism | keratometry |
| retinoscopy and autorefraction provide information on both | spherical ametropia (abnormal refractive condition) and astigmatism |
| the keratometer is an objective method for determining | curvature of the cornea, amount and direction of corneal astigmastism, quality of corneal refracting surface, stability of corneal refracting surface |
| in terms of contact lenses, keratometry is used to | fit contact lenses, detect rigid gas permeable lens flexure, to detect contact lens deposits...poor wetting..or irregularities, to monitor corneal changes, to verify base curves of rigid cl |
| we can detect and monitor various corneal diseases or conditions such as | keratoconus or irregular astigmatism |
| a corneal dystrophy where the center thins and bulges | keratoconus |
| the keratometer measures ___ ___ & determines ____ ____ | corneal curvature and determines corneal power (using assumed refractive index 1.3375) |
| the radius of curvature in each of two principal corneal meridians is found, this is achieved by measure the size of reflected | mires |
| it measure only two small areas on an annulus of the cornea's | anterior surface |
| the cornea acts as a | highly convex spherical mirror |
| the keratometer uses what properties of the cornea to measure its radius of curvature | reflective |
| what are the three essential components of the keratometer | object, magnifying device (since the image is about 3 mm), doubling systems |
| using keratometry we can get a refractive estimate in | unresponsive pts, children, amblyopes, high ametropes, pts w/media opacities |
| two types of ametropia | axial (ex: high myopia w/40 D K reading), refractive (ex: high myopia w/49 D K reading) |
| in addition to refractive, axial length, and other information, the keratometer measurements are used to calculate the _____ ______ for implantation | intraocular lens powers |
| what are two post surgeries that the keratometer is used for | post-keratoplasty; post-cataract |
| who invented modern keratometer? | helmhotz |
| who developed clinical instrument? | javal and schiotz |
| what are the three essential components of the keratometer | object, magnifying device (since the image is about 3 mm), doubling systems |
| what is the purpose of the doubling system | for focusing (uses Scheiner's disc mechanism); for measuring image size (and thus radius of curvature) |
| when the keratometer is in focus, the image will be | single |
| what are the two types of keratometers | one position, two position |
| variable doubling keratometers; since the horizontal and vertical doubling are done simultaneously, you can measure both principal meridians (they are perpendicular to each other) | one position keratometers |
| two position keratometers have how many doubling systems? | 1 |
| with a two position keratometer, what must you do to get the secondary meridian | rotate the instrument (allows measurement if two principle meridians are not perpendicular to each other) |
| does the pt wear spectacles? | no |
| what do you explain that you are going to do to pt. | measure curvature of front surface of their eye |
| focus the eyepiece on the occuluder (to retro-illuminate reticule), by first turning counterclockwise (blurs reticule) then clockwise until sharp. incorrect focusing can produce a significant errors up to ____ radius | 0.2 mm |
| is it ok for pt to blink | yes |
| the principle meridians are normally _______ to each other | perpendicular |
| these are the meridians of ______ and ______ curvature | least and greatest |
| how can you check the accuracy of the alignment | unfocus the lower right hand circle (=doubled plus sign); the horizontal line in the plus sign o f the L circle should exactly split the distance btwn the doubled horizontal lines in the plus sign of the focusing circle |
| which arms of the plus signs should overlap when focused | horizontal |
| the primary meridian is definifed as the meridian closest to | 180 |
| the vertical meridian is the | second meridian |
| one of the objectives of the keratometer is to measure the height of a single mire. but its constantly moving, so its very difficult. thus, a ___ ____ is used to double the image and adjust separation until the two images are just touching | variable prism |
| when the two images are just touching, the linear distance moved equals the | mire size (from the measured mire image height the corneal radius of curvature is calculated directly) |
| what are the two types of astigmatism | regular, irregular |
| if they have irregular astigmatism, mires appear ? principal meridians are? | distorted (clear); not perpendicular to each other |
| what are the mire like in people with regular astigmatism | not distorted, clear and principle meridians are perpendicular |
| what are the three types of regular astigmatism | with the rule, against the rule, oblique (OBL) |
| more power in vertical meridian (between 60 and 120) | with the rule |
| principle meridians lie between 30 and 60, 120 and 150 | oblique |
| more power in the hoizontal meridian (between 30 and 150) | against the rule |
| corneal astigmatism is measured by the | keratometer |
| refractive astigmatism is measured by | retinoscopy or subjective refraction |
| the two don't coincide bc of | 1. physiological lenticular astigmatism; 2. effectivity change; 3. conreal posterior surface curvature, 4. corneal refractive index variation, 5. abberations of the eye |
| astigmatism in corneal plane is not equal to astigmatism in spex plane....b/c of effectivity, there is a ___% increase in astigmatism going from corneal plane to spex | 25 |
| what is javals rule formula | At = 1.25 Ac -0.50 x 090 |
| modified | At = Ac -0.50 x 090 |
| which fits the data better | modified javals |
| what is the range of the B & L keratometer | 36-52 D |
| what is the mean (normal) corneal power | 44 D |
| to increase the range of the keratometer what can you do | place a 1.25 D trial lens in front of aperature; place a -1.00 D trial lens in front of aperature |
| placing a 1.25 trial lens in from extends the range to ???? simple add ___ D to the keratometer reading | 61 D ; 9 D |
| the -1.00 lens extends the range to _???? simply SUBTRACT __ D from the keratometer reading | 30 D; 6 D |
| what are some sources of error | improper calibration, faulty positioning of pt., lack of proper fixation of pt, forget to focus eyepiece, not center mire images, not determining correct axis, accomadative fluctuation by examiner, reduced acuity of examiner, localized corneal distortions |
| what are used to calibrate keratometers | steel balls of known size |
| one method using steel ball | measure radius of one steel ball and compare to measurement of known radius |
| better method using steel ball to calibrate | measure radii of several balls that span instrument's range. graph the mean of measured values against know radii & access correlation |
| what can happen if there is drooping of the pts eyelid | it can obscure one of the minus signs, thus examiner thinks the two minus signs are already superimposed |
Created by:
Lkauf11