Fitting RGP and Orthokeratology lenses - Day 1

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Slide 163

It is perhaps easiest to split the lenses into three areas for evaluation. We will begin with the center of the lens. This area of the lens needs to gently align the center or apex of the cornea. It should have about the same color of tear film beneath it as the cornea does where there is no lens. Explain that this is the color of the normal thickness of tear film. While this is a good guideline, make sure that you don’t get too hung up on the colors. Remember, this is a simple fitting process! If the green is a little brighter or a little darker, assure them that this is fine as long as the needs of the lens are met: does provide vision, doesn’t stain the cornea; goes up and down, doesn’t go side to side.

Slide 167

In this schematic, it should be obvious that the area in the center of the lens is very bright. This indicates that there is a great deal of distance between the back of the lens and the front of the cornea. Know that if there is a bright green area in the center of the lens that is surrounded by a tighter mid-periphery, then it would indicate that this area is too far from the cornea and needs to be flattened. Discuss how to solve a situation in which the sagittal depth of the lens is too great.

Slide 168

In this photo, it should be obvious that the area in the center of the lens is very bright. This indicates that there is a great deal of distance between the back of the lens and the front of the cornea.

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In this photo, the center of the lens is too close to the surface of the cornea and doesn’t allow tears to collect there. While the patient may not have problems with this initially, it is important that the contact lens professional guards against this for their patients. This is one of the areas that we want to avoid--potential staining. In the long run, this patient would have problems if they continued to wear these lenses. Know how this situation can be remedied: increasing the sagittal depth of the lens to cornea relationship.

Slide 170

In this photo, the center of the lens is too close to the surface of the cornea and doesn’t allow tears to collect there. While the patient may not have problems with this initially, it is important that the contact lens professional guards against this for their patients. This is one of the areas that we want to avoid--potential staining. In the long run, this patient would have problems if they continued to wear these lenses.

Slide 172

While the center of the lens needs to provide vision, the mid-periphery of the lens needs to provide the positioning. Note the area in the photo that shows the lens coming closest to the cornea (along the horizontal meridian). Remember that it is the horizontal meridian that is flattest in with-the-rule corneas and we expect that the lens will touch here most closely. This allows the lens to move up and down, but keeps it from moving side to side.

Slide 174

When the middle of the lens is too tight, it will seal off the tear film in the center of the lens and inhibit the exchange of oxygen. A tight middle will also limit the movement of the lens and may cause rubbing of the cornea. In other words, it’s simply not the best situation! Know how flattening the center of the lens will loosen this area of the lens and allow greater movement.

Slide 175

When the middle of the lens is too tight, it will seal off the tear film in the center of the lens and inhibit the exchange of oxygen. A tight middle will also limit the movement of the lens and may cause rubbing of the cornea. In other words, it’s simply not the best situation! Discuss how flattening the center of the lens will loosen this area of the lens and allow greater movement. WORKBOOK #12

Slide 176

When there is not enough interaction between the back side of the lens and the cornea, there will not be the opportunity to hold the lens in place. When this happens, the lens can slip from side to side and movement will not be consistent.

Slide 177

When there is not enough interaction between the back side of the lens and the cornea, there will not be the opportunity to hold the lens in place. When this happens, the lens can slip from side to side and movement will not be consistent.

Slide 179

Edge lift is an area in which the lens manufacturers have spent considerable time and energy. They have developed comfortable, uniform, productive edge lifts. This is an area that is important to pay attention to, however, one which doesn’t need to be altered much by the contact lens professional. Since the edge contains the tear reservoir, there should be an adequate amount of edge lift in order to keep the lens comfortable on the eye.

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Fitting RGP and Orthokeratology lenses Chris Eksteen Charl Laäs

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Thought of the Day “Every morning in Africa, a gazelle wakes up. It knows it must run faster than the fastest lion or it will be killed. Every morning a lion wakes up. It knows it must outrun the slowest gazelle or it will starve to death. It doesn't matter whether you are a lion or a gazelle. When the sun comes up, you better start running."

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Day 1 Interpreting corneal topography maps Understanding different topography maps Diagnosing corneal pathology using topography maps Fitting contact lenses using the corneal topographer New technology OCT Interpreting Fluorescein patterns Practical

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Corneal Shape

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Corneal Shape Prolate Surface Normal cornea Formed by rotation of ellipse about major axis Steepest at apex Flatter towards periphery

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Corneal Shape Oblate Surfaces Myopic post refractive / ortho-k cornea Formed by rotation of ellipse about minor axis Flattest at apex Steeper towards periphery

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Eccentricity

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Eccentricity The Ellipse

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Eccentricity The Ellipse Defined as the locus of points, the sum of whose distances from two fixed points, called foci, is a constant

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Eccentricity The Ellipse When all the focal points P are plotted the points will form an ellipse

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Eccentricity The Ellipse The sum of the distances from any point P to the two foci is equal to the length of the major axis.

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Eccentricity (0<e<1) e=0 : Circle e=1 : Straight line

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Corneal Eccentricity (e) The rate at which the cornea flattens from the central area to the peripheral area Average corneal e : 0.45 (Guillon et al 1986) e >1 – pathological cornea e <0 – corneal surgery / refractive procedures

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Corneal Eccentricity Corneal e value is vital to the success of RGP and Ortho-K fitting Low e (<0.35) – less peripheral flattening – flattens the lens-cornea relation High e (>0.68) – rapid peripheral flattening – steepens the lens-cornea fitting relation

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Corneal Eccentricity To achieve the same lens-cornea relation A steeper BC would be used on a low e cornea A flatter BC would be used on a high e cornea

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Corneal Eccentricity from the Oculus Topographer

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Corneal Eccentricity Eccentricity conversions e = Eccentricity p = Shape Factor p = 1-e2

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Keratometer

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Keratometry Records the image size reflected off a known-sized object The radius of curvature of the cornea is calculated, given the object size and distance from image to object The object, 2 separate mires at distinct distances, reflects off a 3.2mm central zone on the cornea.

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Keratometry The calculation of corneal radius assumes the cornea to be a sphere with a refractive index of 1.3375

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Javal-Schiotz Keratometer Mire Alignment

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Javal-Schiotz Keratometer Mire misalignment and off axis

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Bausch and Lomb Keratometer Mire alignment

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Bausch and Lomb Keratometer Mire misalignment and off axis

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Photokeratoscope

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Photokeratoscope Based on Placido disc Rings projected onto the cornea and examiner observes the reflection The first Catoptric image This lead to modern methods, the corneal topographer.

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Photokeratoscope Placido rings projected onto cornea

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Photokeratoscope Focusing mechanism

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Topographer

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Corneal Topography The Mechanics Computerized corneal topography represents a significant advance in the measurement of corneal curvature over Keratometer Keratometer measures 4 data points within the cornea’s 3 – 4 mm Oculus topographer evaluates 8 000 – 22 000 points across the entire corneal surface

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Corneal Topography

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Calibrate Topographer! When first set up and thereafter once a month. Activate the function "Save CalibrationTable" in the "Miscellaneous" menu. Perform the measurement. Answer "Save Calibration Table?" with [OK].

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Taking measurements Adjust the height of the table to allow the combination forehead and chin rest to hold the patient’s head comfortably but firmly. Adjust the chin rest to allow the patient’s eyes to be at the level of the black ring on the head rest. Inform the patient to focus on the yellow fixation LED (in the center of the Placido system).

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Taking measurements

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Taking measurements The patient should be asked to Blink their eyes Open as wide as possible Measurement is carried out automatically if the Keratograph is in the released position (space bar) If the system does not release (for example pronounced keratoconus), the measurement can be forced by pressing the ENTER key

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Taking measurements Patient’s nose and forehead may cast shadows on the ring system. These areas are inaccessible for measurement, making interpolation Seen as black dots or white areas in the colour topographic image

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Taking measurements Minimize overshadowing of the ring system by turning the patients head more to the side. The nose must be turned as far away from the Keratograph as possible However, it is essential that the patient still keeps his eyes fixed on the yellow fixation LED.

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Topography Maps

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Understanding Corneal Maps Patterns observed in colour coded topographic maps

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Understanding Corneal Maps Image map Raw image of placido rings Shows Corneal pathology Tear film abnormalities

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Image Map

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Overview Display

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Overview Display

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Overview Display

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Overview Display

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Overview Display

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Colour Map Large

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Overview Display

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Colour scale options Most commonly used – 2D contour map Scale is colour coded with steep areas represented by warm colours and the converse

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Color Scale Stepping

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Absolute Scale Assigns the same colour to a given dioptric interval of corneal curvature. Disadvantage steps are in large increments some detail will go missing

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Understanding Corneal Maps Absolute Scale

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Relative Scale The dioptric range assigned to each colour is smaller compared to absolute map Advanatge Shows a more detailed description of the surface Disadvantage the colours of the 2 different maps cannot be compared directly

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Understanding Corneal Maps Relative Scale

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Understanding Corneal Maps Absolute scale vs. Relative scale

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Color Scale Stepping

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Tangential Maps Measures the curvature at a point on the cornea in a meridianal direction relative to the other points on a particular ring

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Tangential Maps In the tangential presentation mode irregularities in corneal geometry appear more pronounced.

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Sagittal Maps Measures the curvature at a point on cornea in axial direction relative to the center

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Sagittal Maps More representative of the influence of the cornea on the patient’s visual acuity. Gives a global view of the corneal curvature

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Elevation/Height Maps Shows the measured height from which the corneal curvature varies from a computer generated reference sphere, which best fits the measured corneal topography This map is most useful in predicting fluorescein patterns with RGP lenses.

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Elevation/Height Maps

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Elevation/Height Maps

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Elevation/Height Maps

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Difference Maps Mathematical subtractions of two user-selected maps Helpful in Ortho-K fitting for monitoring pre and post fit corneal changes.

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Refractive Maps Compensates for spherical aberrations as well as the aspheric contour of the cornea Used to evaluate visual performance in post refractive surgery and post OrthoK patients.

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Indices The large number of measurement data reduced to small number of characteristic values Used to interpret corneal surface at a glance.

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Indices ISV - Index of surface variance Deviation of individual corneal radii from the mean value Elevated in all types of irregularity of the corneal surface.

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Indices IVA- Index of vertical asymmetry Degree of symmetry of the corneal radii with respect to the horizontal meridian Elevated in cases of oblique astigmatism, keratoconus and limbal ectasias.

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Indices KI – Keratoconus index Elevated especially in Keratoconus

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Indices CKI – Center Keratoconus index Elevated especially in central Keratoconus

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Indices RMin Smallest radius of curvature in the entire field of measurement Elevated in Keratoconus

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Indices IHA – Index of height asymmetry Degree of symmetry of height data with respect to horizontal meridian Analogous to IVA, sometimes more sensitive

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Indices IHD– Index of height decentration Calculated from a Fourier analysis of height values Index gives the degree of decentration in vertical direction Elevated in Keratoconus

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Indices ABR– Aberration coefficient Calculated from a Zernike analysis 0 = no abnormal corneal aberrations As value increases past 1, the abnormal corneal aberrations increase

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Indices KKS– Keratoconus stage Classifies Keratoconus according to Amsler’s classical staging schema

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Indices AA– Analyzed area Area of corneal surface measured as a percentage

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Progress of Indices Lists the indices from all examinations performed on each eye Indices from old and new examinations can be readily compared for each eye

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Decentration of Indices

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Indices - normal

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Indices - abnormal

Slide 82

Fourier Analysis French physicist Jean Baptiste Joseph Fourier (1768-1830) A mathematical method permitting decomposition of any periodical function in terms of trigonometric sine and cosine functions.

Slide 83

Fourier Analysis First harmonic, is a sine wave whose period is equal to that of the wave being analysed. The period of the second harmonic is half that of the first (thus giving two sine waves), Period of the third harmonic is a third as long as that of the first (giving three sine waves)

Slide 84

Spherical Equivalent Zero order wave component in the form of the arithmetic mean of all radii of each individual ring. Spherical component of the radii of each ring. Approximate calculation of the eccentricity of the cornea.

Slide 85

Decentration First-order wave sine wave achieving a min and a max over a given radius ring. Measure of the tilt between the videokeratoscope optical axis and the corneal optical vertex. The min and max values of each radius are shown as white (minimum) and black(maximum) circles.

Slide 86

Regular Astigmatism Second-order regular sine wave of double frequency Achieves two minima and two maxima over a radius ring. The axis position for each zone is represented by red and blue circles.

Slide 87

Regular Astigmatism Central and peripheral astigmatism are indicated by red and blue lines. A red line signifies a steep meridian Blue line signifies a flat meridian. Keratoconus is often associated with a rotation of the astigmatic axis from the center to the periphery,

Slide 88

Regular Astigmatism The magnitude and axis position of central astigmatism obtained by Fourier analysis are in closer agreement with subjective refraction than are the central radii measured by an integrated Keratograph. This is due to the fact that any decentration or major corneal aberration (e.g. trefoil or four-lobed defect) influences the amplitude and axis position in keratometer measurements.

Slide 89

Irregularities Remaining wave components add up to give the irregularities of the corneal ring under measurement The “Irregularities” field contains only wave components that cannot be corrected by means of a sphere, cylinder or prism.

Slide 90

Irregularities Normal cornea arithmetic mean of all irregularities is <0.030 mm for sagittal curvature (<0.141 for tangential curvature). An inverse relationship exists between the degree of irregularity and the best corrected visual acuity. If these are large, the patient will only achieve good visual acuity with rigid contact lenses.

Slide 91

Combining Fourier Maps One option is to display the “Decentration“ and “Irregularities“ components separately It is also possible to add the spherical equivalent to each of these components. Use the “Settings” menu to add or remove the spherical equivalent.

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Decentration and Spherical equivalent Adding the spherical equivalent to the decentration component often provides an intuitive display of the conus position. The cone apex is more readily discerned than in the original image.

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Irregularities and spherical equivalent Combining the irregular and the spherical components often makes it easier to detect three or four-lobed defects than would a display of irregular components alone.

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Fourier Indices Calculations based on the individual components provide indices Indices permit a quick statistical characterization of the corneal surface

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Fourier Analysis - normal

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Fourier Analysis - abnormal

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Zernike Analysis Dutch physicist and Nobel prize winner Frits Zernike (1888-1966) Mathematical representation of the deviations of a real wave front from an ideal one by means of a sum of polynomials.

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Zernike Analysis Sperical Wavefront

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Sperical Wavefront

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‘Real’ Wavefront

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Zernike Analysis Maps the difference between the measured and referenced wavefront planes

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Zernike Analysis Individual modes of Zernike polynomials up to the 4th order

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Zernike Polynomial Designations Z 0,0 height constant, average height of surface Z 1,±1 tilt (+1 in x-direction, -1 in y-direction) Z 2,0 focus, resp., surface in the shape of a conic section Z 2,±2 astigmatism Z 3,±1 coma Z 3,±3 trefoil Z 4,0 spherical aberration Z 4, ±2 higher (4th) order astigmatism Z 4, ±4 four-lobed defect Z 5, ±1 higher (5th)order coma Z 5, ±3 higher (5th) order trefoil Z 5, ±5 five-lobed defect Z 6,0 higher (6th) order spherical aberration Z 6, ±2 higher (6th) order astigmatism Z 6, ±4 higher (6th) order four-lobed defect Z 6, ±6 six-lobed defect

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Zernike Analysis Zernike analysis on measured height data. Calculates for each Zernike polynomial a coefficient which describes the contribution of that polynomial to the height data

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Zernike Analysis - normal

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Zernike Analysis

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Keratoconus Zernike Profile

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Keratoconus Zernike Profile

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Practical Examples

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The ‘Normal’ Cornea

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The ‘Normal’ Cornea

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The ‘Normal’ Cornea

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Regular Astigmatism

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Regular Astigmatism

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Irregular Astigmatism

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Soft Contact Lens Wear

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Corneal Warpage

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Corneal Warpage

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Keratoconus Corneal Rubbing

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Corneal Rubbing

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Post Radial Keratotomy

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Post Radial Keratotomy

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Post PRK

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Post LASIK

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Post LASIK

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Orthokeratology

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Orthokeratology

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Orthokeratology - Hyperopia Tangential

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Orthokeratology - Hyperopia Sagittal

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Penetrating Injury

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Penetrating Injury

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Penetrating Keratoplasty

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Penetrating Keratoplasty

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Keratoglobus

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Keratoconus Nipple Cone

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Keratoconus Oval Cone

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Keratoconus Globus Cone

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Keratoconus – Fourier Display

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Keratoconus – Fourier Display Spherical equivalent Min radius of curvature is steeper than normal Sagittal curvature less than 6.93 Tangential curvature less than 6.87 Eccentricity may exceed 0.85.

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Keratoconus – Fourier Display Decentration + spherical Direction of decentration with keratoconus is usually vertical. Healthy eye decentration is normally horisontal Degree of decentration is usually greater than 0.45 mm.

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Keratoconus – Fourier Display Regular astigmatism Normal astigmatic eye the axis runs in a straight line Keratoconus the axis infrequently undergoes a rotation from the center to the periphery Irregular astigmatism normally shows a spiral appearance.

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Keratoconus – Fourier Display Trefoil images (irregularities)

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Keratoconus – Fourier Display Three and four-leafed shape (irregularities plus spherical component)

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Iatrogenic Keratoconus

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Iatrogenic Keratoconus

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Pellucid Marginal Degeneration

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Pellucid Marginal Degeneration

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Pellucid Marginal Degeneration

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Pterygium Pre op

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Pterygium Post op

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Radial Keratotomy and LASIK

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Fluorescein Pattern Interpretation

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The Basic Fit Alignment / on K

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The Basic Fit Alignment / on K

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The Basic Fit Steep Fit

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The Basic Fit Steep Fit

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The Basic Fit Steep Fit

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The Basic Fit Flat Fit

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The Basic Fit Flat Fit

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The Basic Fit Flat Fit

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Different Fluorescein Zones

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Fluorescein Pattern Interpretation

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Centre Zone

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Lens Center - Aligned

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Lens Center - Aligned

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Lens Center - Steep

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Lens Center - Steep

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Lens Center - Flat

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Lens Center - Flat

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Mid Periphery Zone

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Lens Mid Periphery - Aligned

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Lens Mid Periphery - Aligned

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Lens Mid Periphery - Steep

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Lens Mid Periphery - Steep

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Lens Mid Periphery - Flat

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Lens Mid Periphery - Flat

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Edge

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Edge width / Periphery

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Edge Lift - Ideal

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Edge Lift - Ideal

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Edge Lift – Too Narrow

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Edge Lift – Too Wide

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Lens Movement

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Ideal Movement The lens should follow the lid with the blink and then slowly glide into its final resting position.

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Too Much Movement If the lens moves from place to place Steepen the base curve or Enlarge the diameter to stabilise the fit.

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No Lens Movement If the lens does not move freely on the blink, there will be no tear exchange

Tags: topography contact lenses orthokeratology rgp cornea

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