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Corneal topography orbscan S.A.A MORTAZAVI MD Associate professor of ophthalmology Feiz hospital 1390 2.

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Presentation on theme: "Corneal topography orbscan S.A.A MORTAZAVI MD Associate professor of ophthalmology Feiz hospital 1390 2."— Presentation transcript:

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2 Corneal topography orbscan S.A.A MORTAZAVI MD Associate professor of ophthalmology Feiz hospital

3 ORBSCAN SYSTEM Use the principle of projection Use the principle of projection Forty scanning slit beams (20 from the left and 20 from the right with up to 240 data points per slit ) to scan the cornea and measure independently the X,Y & Z locations Forty scanning slit beams (20 from the left and 20 from the right with up to 240 data points per slit ) to scan the cornea and measure independently the X,Y & Z locations 3

4 Orbscan imaging Forty slit images are acquired in two 0.7 second periods Forty slit images are acquired in two 0.7 second periods Each of the 40 slit images triangulates one slice of ocular surface Each of the 40 slit images triangulates one slice of ocular surface Distance between data slices average 250 microns Distance between data slices average 250 microns

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6 ORBSCAN Orbscan I only slit scan topography Orbscan I only slit scan topography Orbscan II the placidodisc added in orbscan I Orbscan II the placidodisc added in orbscan I

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8 ORBSCAN The images used to construct the anterior corneal surface,posterior corneal surface,anterior iris and anterior lens surfaces The images used to construct the anterior corneal surface,posterior corneal surface,anterior iris and anterior lens surfaces Data regarding the corneal pachymetry and anterior chamber depth Data regarding the corneal pachymetry and anterior chamber depth 8

9 Elevation Orbscan measure elevation Orbscan measure elevation Elevation is important the only complete scaler measure of surface shape Elevation is important the only complete scaler measure of surface shape Both slope & curvature can be mathematically derived from a single elevation map Both slope & curvature can be mathematically derived from a single elevation map

10 BEST FIT SPHERE (BFS) The computer calculates a hypothetical sphere that matches as close as possible to the actual corneal shape being measured The computer calculates a hypothetical sphere that matches as close as possible to the actual corneal shape being measured Compares the real surface to the hypothetical sphere showing areas above the surface of the sphere in warm colours and areas below the surface in cool colours Compares the real surface to the hypothetical sphere showing areas above the surface of the sphere in warm colours and areas below the surface in cool colours 10

11 Topography quad map The upper left : anterior float The upper left : anterior float The upper right : posterior float The upper right : posterior float The lower left : keratometric pattern The lower left : keratometric pattern The lower right : pachymetry map The lower right : pachymetry map

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15 NORMAL BAND SCALE Highlights the abnormal areas in the cornea in orange to red colors Highlights the abnormal areas in the cornea in orange to red colors The normal areas are all shown in green The normal areas are all shown in green Helpful in generalized screening in preoperative examination Helpful in generalized screening in preoperative examination

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17 AXIAL MAP Provides detailed keratometric information across the diameter of the cornea Provides detailed keratometric information across the diameter of the cornea K readings are between certain values the cornea must be neither too steep nor too flat K readings are between certain values the cornea must be neither too steep nor too flat 17

18 AXIAL MAP To create a good quality corneal flap in LASIK if either extremes (too steep or too flat) is the case, this can lead to surgical flap complications To create a good quality corneal flap in LASIK if either extremes (too steep or too flat) is the case, this can lead to surgical flap complications K readings of more than 48 D are an indication of potential keratoconus K readings of more than 48 D are an indication of potential keratoconus 18

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21 Middle box Keratometric readings Keratometric readings White to white distance in mm White to white distance in mm Angle kappa readings Angle kappa readings The thinnest point of cornea The thinnest point of cornea irregularity within the central 3 mm & 5 mm irregularity within the central 3 mm & 5 mm

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23 PACHYMETRY MAP The orbscan measures thickness from the tear film layer to descemet’s membrane and is thicker than that obtained with ultrasound The orbscan measures thickness from the tear film layer to descemet’s membrane and is thicker than that obtained with ultrasound Adjustment factor (acoustic factor),the default setting is 92% Adjustment factor (acoustic factor),the default setting is 92% Provides a reading showing the thinnest point of the cornea that may not necessarily be the central reading Provides a reading showing the thinnest point of the cornea that may not necessarily be the central reading

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25 PACHYMETRY MAP Thinnest point <470 micron Thinnest point <470 micron In pathological corneas, thinnest point is often displaced inferotemporal In pathological corneas, thinnest point is often displaced inferotemporal Difference of >100 microns from the thinnest point to the values at 7mm optical zone Difference of >100 microns from the thinnest point to the values at 7mm optical zone 25

26 ELEVATION MAP The green colour is referred as refrence sphere (at sea level ) The green colour is referred as refrence sphere (at sea level ) The warmer colours are above this level and the cooler colours are below The warmer colours are above this level and the cooler colours are below 26

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29 ANTERIOR ELEVATION MAP Looking at a proper scale in the cornea, can see height differences Looking at a proper scale in the cornea, can see height differences Compare the height of the actual cornea to a best fit sphere Compare the height of the actual cornea to a best fit sphere

30 Posterior map The highest elevation value as a keratoconus indicator or at least as a screen for patients may be at risk of developing keratectasia The highest elevation value as a keratoconus indicator or at least as a screen for patients may be at risk of developing keratectasia 55 D elevation as an absolute cut off 55 D elevation as an absolute cut off 30

31 ELEVATION DATA The difference between the highest and lowest points is a potential keratoconus indicator if over 100 microns (Rousch criteria) The difference between the highest and lowest points is a potential keratoconus indicator if over 100 microns (Rousch criteria) 31

32 DIAGNOSTIC CRITERIA Power map changes Power map changes Posterior elevation maps Posterior elevation maps Pachymetry Pachymetry Composite/integrated topography information Composite/integrated topography information 32

33 POWER MAP Mean corneal power >45D Mean corneal power >45D In addition to steep corneal curvatures-the bowtie or broken bowtie appearance indicative of early keratoconus In addition to steep corneal curvatures-the bowtie or broken bowtie appearance indicative of early keratoconus Central corneal asymetry a change within central 3mm optical zone of the cornea of more than 3D Central corneal asymetry a change within central 3mm optical zone of the cornea of more than 3D 33

34 Irregularity in central cornea Greater than 1.5 D in 3 mm zone and Greater than 1.5 D in 3 mm zone and greater than 2.0 D in 5 mm zone is considered abnormal and cause for concern greater than 2.0 D in 5 mm zone is considered abnormal and cause for concern

35 POSTERIOR ELEVATION MAP Many surgeons think the first sign of keratoconus appears on the posterior surface of the cornea Many surgeons think the first sign of keratoconus appears on the posterior surface of the cornea 3.13% of population screened for laser surgery had posterior ectasia criteria by orbscan, despite having axial topography classified as normal 3.13% of population screened for laser surgery had posterior ectasia criteria by orbscan, despite having axial topography classified as normal 35

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37 POSTERIOR ELEVATION MAP The most common reference surface for viewing elevation maps is the best fit sphere The most common reference surface for viewing elevation maps is the best fit sphere A best fit sphere (BFS) >55D on the posterior profile, indicative of posterior ectasia A best fit sphere (BFS) >55D on the posterior profile, indicative of posterior ectasia 37

38 Posterior float difference Greater than 50 micron generally accepted as abnormal Greater than 50 micron generally accepted as abnormal In corneas thinner than normal over 40 as abnormal In corneas thinner than normal over 40 as abnormal

39 Posterior elevation map The location of the steepest part of the posterior float should be relatively central, but is a more concern it be located away from the center and in an area of corneal thinning The location of the steepest part of the posterior float should be relatively central, but is a more concern it be located away from the center and in an area of corneal thinning Posterior float difference;40 to 50 microns seems to be the maximum difference Posterior float difference;40 to 50 microns seems to be the maximum difference

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41 Correlation of signs of the highest point Highest point on the posterior elevation coincides with the highest point of anterior elevation, the thinnest on pachymetry and the point of steepest curvature on the power map Highest point on the posterior elevation coincides with the highest point of anterior elevation, the thinnest on pachymetry and the point of steepest curvature on the power map 41

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43 Although high posterior elevation and ratio between two elevation maps rarely used as exclusion criteria alone, but by considering these together, more conclusive information can be obtained Although high posterior elevation and ratio between two elevation maps rarely used as exclusion criteria alone, but by considering these together, more conclusive information can be obtained 43

44 Risks of ectasia indices Number of abnormal maps Posterior float difference > mm & 5mm irregularity Peripheral thickness changes Astigmatism variance between eyes Steep k’s –mean power map

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46 Three step rule One abnormal map ; perform with caution One abnormal map ; perform with caution Two abnormal map ; with concern Two abnormal map ; with concern Three abnormal map ;contraindicated Three abnormal map ;contraindicated

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50 Composite/integrated information Similarly between anterior & posterior profiles a forward bending of areas shown above the BFS and association with the thinnest point on the cornea Similarly between anterior & posterior profiles a forward bending of areas shown above the BFS and association with the thinnest point on the cornea Inferotemporal displacement of the highest point Inferotemporal displacement of the highest point 50

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53 Abnormal tear film Can significantly distort the readings Can significantly distort the readings The significant change in surface quality and validity of the dry eye The significant change in surface quality and validity of the dry eye

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