7/24/2014 ORBSCAN. Dr. HARITHA

Transcription

1 ORBSCAN Dr. HARITHA Cornea is the most powerful refractive element, contributing 43D of total 60D power of eye. Anterior surface elliptical, HD-11.5mm VD-10.6mm. Posterior surface- circular,avg D- 11.5mm Variable in curvature with central steepening and peripheral flattening. Optical zones of cornea Central zone 4mm, spherical symmetrical, radius of curvature does not vary by >1D. Refraction differs by <.25D. Paracentral zone 4-8mm. Flatter than central but spherical. Peripheral zone 8-11mm. Flatter and asherical. The avg corneal thickness is 550mm. With temp 590, nasal 610, inferior 630 and superior 640. Thinnest site is.9mm from visual axis in inferotemporal quadrant. Central corneal thickness of 250mm is considered to ensure mechanical stability to cornea. 1

2 Terminology Curvature rate of change of tangent vector to the curve with respect to arc length of the curve. Is inversely proportional to radius. Ant surface 7.8mm Post surface 6.5mm Planes of intersection. Meridional plane or Tangent plane a plane through the center of the cornea. specified in polar coordinates. Angles increasing in counter clockwise manner. Semimeridia are defined from the center outwards and cover angular positions from degrees. Sagittal plane a transverse or sagittal plane of intersection at a surface point is perpendicular to the meridional plane through that point and contains the surface normal. It completes the three dimensional description. 2

3 Axial distance distance from a point on a curve to the reference axis along the surface normal at that point. Da = (n-1)/d Da is axial diopter, d is axial distance. n is keratometric index of refraction(1.3375), 1 is refractive index of air. History. Surface height actual elevation of the corneal surface relative to a reference. Reference axis center of topographic display and thus the meridional planes in which axial shape and curvature are determined father christopher scheiner compared images reflected from cornea to those of glass balls henry goode target disc with concentric circles antonio placido hand held keratoscope with light & dark coloured bands with a central aperture. Javal keratoscope. Gullstrand photokeratoscope rowsey et al computerized improvement 3

4 Keratometer. Instrumentation Uses illuminated object mires reflected from cornea, which acts as a convex mirror. Uses 4 reflected points.radius of curvature along that meredian is determined by measuring the distance between 2 points of a pair.2.88mm for 50D to 4mm for 36D. Image is virtual, minified and upright. Keratometric index Draw backs Does not describe the actual power or curvature of the entire central cornea. Loses accuracy over 50D. Cannot describe corneal asymmetry or ashpericity. Yeilds measurement that reflects the avg power of two semimeridians along any given meridian. Computerized videokeratography. Use placido based topography. 4

5 Draw backs. Based on algorithms for spherical surfaces. Corneal curvature is measured at the intersection of each ring with radial meredians evenly spaced around the cornea. No. of meredians measured yeilding a total no. of measured points ranging from 828(visioptic) to 7000 TMS. Central regions require high degree of subpixel resolution to detect a.25d change. Induced astigmatism secondary to misalignment or decentration. Intra observer and interobserver variability errors, alignment errors, focussing errors or errors of calibration. Elevation based topography. RASTERSTEOGRAPHY regular pattern of lines or grids of known geometry onto the corneal surface. Uses flourescein to stain the tear film. Does not require intact epithelium. The accuracy was.7d and reproducibility was.03 (SD) in imaging the calibrated sphere within a defined zone in space. COMPUTERIZED CORNEAL TOPOGRAPHY Eye sys ring videokeratoscope. 3 sec processing time. Color coded contour maps with numerical data. Analyzes data points. 5

6 TMS- 1 topographic modelling system. Analyzes 7000 data points. Corneal coverage is.02 mm -.11mm. Accuracy of.10d. Has laser alignment system. Has refractive surgery planning programme. Laser interferometry rarely used. Uses phase shifting interferometry and laser holographic interferometry. Projects lines of known geometry onto the corneal surface and uses a thin membrane to reflect the projection as the image is captured by camera and analyzed by computer. Pancorneal slit scan method orbscan. Measuring the dimensions of a slit scanning beam (3D) projected on the cornea orbscan 2 with a placido disc attachment to obtain curvature measurements. Orbscan 2z with 3.12 software is integrated with a shack- hartman aberrometer in zyoptix work station. ORBSCAN IS MULTIDIMENSIONAL. Measures multiple ocular surfaces. ant & post corneal surface ant surface of iris ant surface of lens. Integrates multiple technologies reflective slit scan ultra sound 6

7 Displays multiple & complete mathematical surfaces. curvature power elevation thickness During data acquisition, the placido disc is illuminated and subsequently 40 slits ie 20 slits from either side are projected at 45 degrees to the instrument axis, which are 12.5mm high and.3mm wide in a total time of 2.1 secs. 7

8 TOPOGRAPHICAL INDICES Simulated keratometry (SimK) - obtained from the greatest mean dioptric power analyzing along each meredian the mean dioptric power. Once the greatest value has been obtained SimK1, the mean value of 90 degree meredian is calculated SimK2. Minimum keratometric value MinK- Lowest value obtained along each meredian after analyzing the mean dioptric along each meredian. Surface regularity index (SRI) represents the local fluctuation of the central corneal dioptric power calculated on an optical zone of 4.5mm. Surface asymmetry index (SAI) represents the sum of differences of corneal dioptric power between corresponding points 180 degrees from each other calculated over the entire surface. The dioptric power of normal surface is distributed symmetrically. Keratoconus predictability index (KPI) Considered to be a numerical estimator of keratoconus which spans from zero, when there are no topographical characteristics of keratoconus present, upto 100% when all the topographical characteristics relative to the keratoconus are evident. 8

9 CURVATURE MAPS Topographic scales Absolute direct comparision of two maps, measured in steps of.5d. Do not show subtle change of curvature. Normalised maps different colour scales assigned to each map. Dioptric change is much smaller than absolute scale and are more detailed. But cannot be compared and have to be interpreted based on keratometric values from different scales. Axial cuvature map shows the radially averaged curvature. Meredional maps show the curvature measured along meredional planes. Axial & meredional maps do not display power Curvature maps and elevation maps are exactly opposite. ELEVATION MAPS Elevation map displays the height of cornea relative to reference surfaces. Sphere is the simplest reference surface as it has no unique symmetrical axis. The sphere moves and changes size until it best fits the data surface with in the specified fit zone. 9

10 Elevation topology 4 types of reference sphere alignments- floating center pinned apex,includes both center & pinned Floating alignment imposes no additional constraint on reference sphere. normal cornea is prolate shape, hence central hill Regular astigmatic cornea is toric, a saddle shape Is seen, rising in one direction and falling in Perpendicular direction. Central elevation topology is determined by the balance of central prolateness and toricity. Also depends on the fit zone. Large fit zone 10mm prolate central hill. Small fit zone 5mm toric central saddle.can display the small central anomalies. 10

11 COLOUR CONTOUR Regular patterns round, oval, superior steepening, inferior steepening. Astigmatic symmetric bowtie, asymmetric bowtie. irregular. no pattern, orthogonal. Green as reference colour Red hot colour ; high, steep, sharp, shallow, thin & focussed. Blue cool colour ; low, level, l flat, deep, thick & aberrated. If 4 or more coloured interval appear with in central 3 mm of elevation map then the cornea is deemed abnormal. Power is the property of surface shape, refractive index and illumination incidence. Optical power is measured in diopters and is inversely proportional to focal length. AIR AQUEOUS CORNEAL POWER

12 short focal length high power long focal length low power ORBSCAN PACHYMETRY The normal pachymetry pattern is smoothly varying and has a local minimum, temporal and slightly inferior to the visual axis. OD (n = 35) OS (n = 35) N Ultra sound is widely accepted method of measuring pachymetry. Orbscan produces thicker pachymetry readings by 30mc mt. takes tear film also into consideration. Orscan devices are incorperated with acoustic factor reducing the pachymetry by 8%. Post LASIK orbscan underestimates pachymetry. Diurnal variation of corneal thickness. Thicker in the morning. Stabilize only after 3hrs. 12

13 Anterior elevation Posterior elevation Abnormal anterior elevation and mean curvature Classic axis-based keratoconus pattern is an asymmetric and bent bow-tie. Elevation maps show a symmetric high cone. Mean curvature measures local sphericity. Mean curvature filters out astigmatism and emphasizes corneal anomalies. Keratoconus is cone-shaped (prolate). Extreme toricity cone. Mean curvature Thickness axial curvature Meredional curvature KERATOCONUS Rabonowitz criteria 1. Central K > 47.2D. 2. I S values over 1.2 D. 3. SRAX ( index quantifying irregular astigmatism) > 21grades. 4. Difference in central curvature of both eyes >.92D. 13

14 Forme fruste indices on quad map Inferior displacement of the steepest point of the cornea on anterior float. Posterior float >.50mcmt is referred to as posterior ectasia and is the earliest sign. A frown pattern is a bad sign. Overall corneal dioptric power > 50D. Inferior thinning of the cornea corresponding to posterior corneal ectasia supports FFKC. Keratoconus (anterior and posterior) first appears on the posterior corneal surface. Some people have posterior cones with no apparent anterior surface nor visual manifestation. Some people are pre-disposed to injury triggered and iatrogenically triggered posterior keratoconus. Anterior cornea Posterior cornea 14

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18 Contact lens wear induces warpage. Irregular astigmatism. L f di l t Loss of radial symmetry. Reversal of the normal corneal topographic pattern. 15 weeks for RGP lenses, 5 wks for soft lens. 18

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22 Penetrating keratoplasty Post op astigmatism before and after suture removal. Topography of corneal button by using artificial AC. Diurnal changes after PKP 2/3 have central flattening, 1/3 have steepening. 22

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26 Corneal ectasia after refractive surgery Useful in detecting post LASIK bulge or anterior shift of posterior cornea. Wang and coauthors directly related to residual bed thickness [RBT]. Surface based PRK with > RBT was seen to produce a similar ant shift of post cornea 40 mc gm. DRAW BACKS Radial keratotomy helpful in correcting the residual power after after an incomplete RK. 3 topographic patterns after RK - round or oval, band shaped, dumbell shaped. Placido disc image is posteriorly located. Accuracy of pixelated images is not proved. In clinically significant corneal haze there is underestimation of corneal thickness. 26

27 thank you. 27