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

ORBSCAN SYSTEM 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

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

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

ORBSCAN 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

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

BEST FIT SPHERE (BFS) 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

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

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

AXIAL MAP 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

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 K readings of more than 48 D are an indication of potential keratoconus

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

PACHYMETRY MAP 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% Provides a reading showing the thinnest point of the cornea that may not necessarily be the central reading

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

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

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

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 55 D elevation as an absolute cut off

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

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

POWER MAP Mean corneal power >45D 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

Irregularity in central cornea 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

POSTERIOR ELEVATION MAP 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

POSTERIOR ELEVATION MAP 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

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

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 Posterior float difference;40 to 50 microns seems to be the maximum difference

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

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

Risks of ectasia indices Number of abnormal maps Posterior float difference >0.050 3mm & 5mm irregularity Peripheral thickness changes Astigmatism variance between eyes Steep k’s –mean power map

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

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 Inferotemporal displacement of the highest point

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