
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.
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 midperiphery, 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.
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.
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 avoidpotential 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.
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 avoidpotential staining. In the long run, this patient would have problems if they continued to wear these lenses.
While the center of the lens needs to provide vision, the midperiphery 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 withtherule 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.
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.
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
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.
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.
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.
Fitting RGP and Orthokeratology lenses Chris Eksteen Charl Laäs
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."
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
Corneal Shape
Corneal Shape Prolate Surface Normal cornea Formed by rotation of ellipse about major axis Steepest at apex Flatter towards periphery
Corneal Shape Oblate Surfaces Myopic post refractive / orthok cornea Formed by rotation of ellipse about minor axis Flattest at apex Steeper towards periphery
Eccentricity
Eccentricity The Ellipse
Eccentricity The Ellipse Defined as the locus of points, the sum of whose distances from two fixed points, called foci, is a constant
Eccentricity The Ellipse When all the focal points P are plotted the points will form an ellipse
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.
Eccentricity (0<e<1) e=0 : Circle e=1 : Straight line
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
Corneal Eccentricity Corneal e value is vital to the success of RGP and OrthoK fitting Low e (<0.35) – less peripheral flattening – flattens the lenscornea relation High e (>0.68) – rapid peripheral flattening – steepens the lenscornea fitting relation
Corneal Eccentricity To achieve the same lenscornea relation A steeper BC would be used on a low e cornea A flatter BC would be used on a high e cornea
Corneal Eccentricity from the Oculus Topographer
Corneal Eccentricity Eccentricity conversions e = Eccentricity p = Shape Factor p = 1e2
Keratometer
Keratometry Records the image size reflected off a knownsized 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.
Keratometry The calculation of corneal radius assumes the cornea to be a sphere with a refractive index of 1.3375
JavalSchiotz Keratometer Mire Alignment
JavalSchiotz Keratometer Mire misalignment and off axis
Bausch and Lomb Keratometer Mire alignment
Bausch and Lomb Keratometer Mire misalignment and off axis
Photokeratoscope
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.
Photokeratoscope Placido rings projected onto cornea
Photokeratoscope Focusing mechanism
Topographer
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
Corneal Topography
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].
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).
Taking measurements
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
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
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.
Topography Maps
Understanding Corneal Maps Patterns observed in colour coded topographic maps
Understanding Corneal Maps Image map Raw image of placido rings Shows Corneal pathology Tear film abnormalities
Image Map
Overview Display
Overview Display
Overview Display
Overview Display
Overview Display
Colour Map Large
Overview Display
Colour scale options Most commonly used – 2D contour map Scale is colour coded with steep areas represented by warm colours and the converse
Color Scale Stepping
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
Understanding Corneal Maps Absolute Scale
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
Understanding Corneal Maps Relative Scale
Understanding Corneal Maps Absolute scale vs. Relative scale
Color Scale Stepping
Tangential Maps Measures the curvature at a point on the cornea in a meridianal direction relative to the other points on a particular ring
Tangential Maps In the tangential presentation mode irregularities in corneal geometry appear more pronounced.
Sagittal Maps Measures the curvature at a point on cornea in axial direction relative to the center
Sagittal Maps More representative of the influence of the cornea on the patient’s visual acuity. Gives a global view of the corneal curvature
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.
Elevation/Height Maps
Elevation/Height Maps
Elevation/Height Maps
Difference Maps Mathematical subtractions of two userselected maps Helpful in OrthoK fitting for monitoring pre and post fit corneal changes.
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.
Indices The large number of measurement data reduced to small number of characteristic values Used to interpret corneal surface at a glance.
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.
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.
Indices KI – Keratoconus index Elevated especially in Keratoconus
Indices CKI – Center Keratoconus index Elevated especially in central Keratoconus
Indices RMin Smallest radius of curvature in the entire field of measurement Elevated in Keratoconus
Indices IHA – Index of height asymmetry Degree of symmetry of height data with respect to horizontal meridian Analogous to IVA, sometimes more sensitive
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
Indices ABR– Aberration coefficient Calculated from a Zernike analysis 0 = no abnormal corneal aberrations As value increases past 1, the abnormal corneal aberrations increase
Indices KKS– Keratoconus stage Classifies Keratoconus according to Amsler’s classical staging schema
Indices AA– Analyzed area Area of corneal surface measured as a percentage
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
Decentration of Indices
Indices  normal
Indices  abnormal
Fourier Analysis French physicist Jean Baptiste Joseph Fourier (17681830) A mathematical method permitting decomposition of any periodical function in terms of trigonometric sine and cosine functions.
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)
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.
Decentration Firstorder 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.
Regular Astigmatism Secondorder 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.
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,
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 fourlobed defect) influences the amplitude and axis position in keratometer measurements.
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.
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.
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.
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.
Irregularities and spherical equivalent Combining the irregular and the spherical components often makes it easier to detect three or fourlobed defects than would a display of irregular components alone.
Fourier Indices Calculations based on the individual components provide indices Indices permit a quick statistical characterization of the corneal surface
Fourier Analysis  normal
Fourier Analysis  abnormal
Zernike Analysis Dutch physicist and Nobel prize winner Frits Zernike (18881966) Mathematical representation of the deviations of a real wave front from an ideal one by means of a sum of polynomials.
Zernike Analysis Sperical Wavefront
Sperical Wavefront
‘Real’ Wavefront
Zernike Analysis Maps the difference between the measured and referenced wavefront planes
Zernike Analysis Individual modes of Zernike polynomials up to the 4th order
Zernike Polynomial Designations Z 0,0 height constant, average height of surface Z 1,±1 tilt (+1 in xdirection, 1 in ydirection) 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 fourlobed defect Z 5, ±1 higher (5th)order coma Z 5, ±3 higher (5th) order trefoil Z 5, ±5 fivelobed defect Z 6,0 higher (6th) order spherical aberration Z 6, ±2 higher (6th) order astigmatism Z 6, ±4 higher (6th) order fourlobed defect Z 6, ±6 sixlobed defect
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
Zernike Analysis  normal
Zernike Analysis
Keratoconus Zernike Profile
Keratoconus Zernike Profile
Practical Examples
The ‘Normal’ Cornea
The ‘Normal’ Cornea
The ‘Normal’ Cornea
Regular Astigmatism
Regular Astigmatism
Irregular Astigmatism
Soft Contact Lens Wear
Corneal Warpage
Corneal Warpage
Keratoconus Corneal Rubbing
Corneal Rubbing
Post Radial Keratotomy
Post Radial Keratotomy
Post PRK
Post LASIK
Post LASIK
Orthokeratology
Orthokeratology
Orthokeratology  Hyperopia Tangential
Orthokeratology  Hyperopia Sagittal
Penetrating Injury
Penetrating Injury
Penetrating Keratoplasty
Penetrating Keratoplasty
Keratoglobus
Keratoconus Nipple Cone
Keratoconus Oval Cone
Keratoconus Globus Cone
Keratoconus – Fourier Display
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.
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.
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.
Keratoconus – Fourier Display Trefoil images (irregularities)
Keratoconus – Fourier Display Three and fourleafed shape (irregularities plus spherical component)
Iatrogenic Keratoconus
Iatrogenic Keratoconus
Pellucid Marginal Degeneration
Pellucid Marginal Degeneration
Pellucid Marginal Degeneration
Pterygium Pre op
Pterygium Post op
Radial Keratotomy and LASIK
Fluorescein Pattern Interpretation
The Basic Fit Alignment / on K
The Basic Fit Alignment / on K
The Basic Fit Steep Fit
The Basic Fit Steep Fit
The Basic Fit Steep Fit
The Basic Fit Flat Fit
The Basic Fit Flat Fit
The Basic Fit Flat Fit
Different Fluorescein Zones
Fluorescein Pattern Interpretation
Centre Zone
Lens Center  Aligned
Lens Center  Aligned
Lens Center  Steep
Lens Center  Steep
Lens Center  Flat
Lens Center  Flat
Mid Periphery Zone
Lens Mid Periphery  Aligned
Lens Mid Periphery  Aligned
Lens Mid Periphery  Steep
Lens Mid Periphery  Steep
Lens Mid Periphery  Flat
Lens Mid Periphery  Flat
Edge
Edge width / Periphery
Edge Lift  Ideal
Edge Lift  Ideal
Edge Lift – Too Narrow
Edge Lift – Too Wide
Lens Movement
Ideal Movement The lens should follow the lid with the blink and then slowly glide into its final resting position.
Too Much Movement If the lens moves from place to place Steepen the base curve or Enlarge the diameter to stabilise the fit.
No Lens Movement If the lens does not move freely on the blink, there will be no tear exchange
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