Transcript TOPOGRAPHY
NORMAL CORNEA •The highest diopter of human eye,about 43D at corneal apex •Average radius curvature of 7.8 mm •Is not absolutely transparent,scatters about 10%of the incident light CORNEAL GEOGRAPHY The central zone (4mm diameter)almost spherical and called apex, responsible for the high definition vision the paracentral zone where the cornea begins to flatten The peripheral zone the limbal zone Normal cornea A normal prolate cornea is steep in center and flat in periphery the center of the normal cornea is steeper than the best fit sphere & midperiphery is flatter than reference sphere Central cornea is red on a normal elevation map and midperiphery appears blue SUPER VISION Developing new tools and extremely promising laser surgical techniques that have proven to increase the human being’s VA by reducing corneal aberrations Topographic & aberrometer linked LASIK are on the way to achieve this goal of better than normal vision Regularizing the corneal shape by means of reduction of halos,glare & other optical aberrations The influence of other dioptric surfaces (vitreous,lens,..)and interfaces still has to be ascertained Posterior corneal power Is negative and much smaller than anterior corneal power. The posterior surface reduces corneal power 6 Instruments to measure the corneal surface Keratometry Keratoscopy Computerized (topography) videokeratoscopy CORNEAL TOPOGRAPHY Wide acceptance as a clinical examination procedure with the advent of modern laser refractive surgery Measure a greater area of the cornea with a much higher number of points and produce permanent records Topographic techniques Reflection perform indirect measurement of the corneal surface Projection directly visualize the corneal surface PLACIDODISC Illuminates the cornea by sending a mire of concentric rings A videocamera captures the corneal reflex from the tear layer A computer & software perform the analysis of the data through different algorithms Keratometry and corneal topography with placidodisc systems were originally invented to measure anterior corneal curvature The problem in the placidodisc system is that cannot perform a slit scan topography TOPOGRAPHY •corneal topography plays a critical role in refractive surgery decisions •conventional axial & tangential topography are not enough to demonstrate a healthy cornea •can not decide any type of laser refractive surgery based only on surface topographic evaluation 12 ORBSCAN Combines both slit scan and placido images to give a very good composite picture from topographic analysis 14 15 Artifacts of topography Shadows on the cornea from large eyelashes or trichiasis Ptosis or non-sufficient eye opening Irregularities of the tear film layer (dry eye) Too short working distance of the small placidodisc cone Incomplete or distorted image (pathology) ORBSCAN II Is a fully integrated multidimensional diagnostic system Acquires over 9000 data points in 1.5 seconds to meticulously map the entire corneal surface Analyze elevation & curvature measurement on both the anterior & posterior surfaces of the cornea 17 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 18 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 A three dimensional slitscan topographic Orbscan detect the abnormalities on the posterior surface of the cornea 21 ORBSCAN Orbscan I only slit scan topography Orbscan II the placidodisc added in orbscan I 23 ORBSCAN II Is based on slit scanning technology in addition to traditional placido-based technique Placidodisc improve the accuracy of the curvature measurements and give information on axial keratometric readings 24 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 25 Elevation Orbscan measure elevation Elevation is important the only complete scaller measure of surface shape Both slope & curvature can be mathematically derived from a single elevation map ORBSCAN Slit beam scanners and triangulation are used to derive the actual spatial location of thousands of points on the surface Each beam sweep across the cornea gives information on corneal elevation or height from the anterior corneal surface , posterior surface & iris 27 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 28 Topography quad map The upper left : anterior float The upper right : posterior float The lower left : keratometric pattern The lower right : pachymetry map 30 31 32 33 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 35 POWER MAP = AXIAL MAP A familiar sagital map from placido system The mean power map determines the location of a surface abnormality Normal astigmatism appears a classic bowtie 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 37 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 38 39 40 41 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 43 Pachymetry Ultrasound provides a reading of corneal thickness from bowman’s membrane to descemet’s membrane Orbscan provides a reading from the precorneal tearfilm to the endothelium,slightly thicker readings can be expected 44 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 46 PACHYMETRY Provides thickness information the cornea from limbus to limbus The relationship between pachymetry readings can be looked,100 micron should be a cut-off criteria 47 pachymetry Central pachymetry compare to peripheral readings considered abnormal if : -the peripheral readings are not at least 20 micron thicker The thinnest reading less than 30 micron thinner PACHYMETRY MAP Thinnest In point <470 micron pathological corneas, thinnest point is often displaced inferotemporal Difference of >100 microns from the thinnest point to the values at 7mm optical zone 49 ELEVATION MAPS The anterior elevation map ;the top left hand map The posterior elevation map ;the top right hand map Slit scanning provides elevation data , and also can create a 3D interpretation of the cornea 50 ELEVATION MAP The green colour is referred as reference sphere (at sea level ) The warmer colours are above this level and the cooler colours are below 51 52 53 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 55 ELEVATION DATA The difference between the highest and lowest points is a potential keratoconus indicator if over 100 microns (Rousch criteria) 56 DIAGNOSTIC CRITERIA Power map changes Posterior elevation maps Pachymetry Composite/integrated topography information 57 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 58 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 ELEVATION MAPS A ratio can be calculated between the posterior and anterior surfaces ,which gives an indication of the relative difference in curvature between the two maps 60 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 61 62 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 63 POSTERIOR ELEVATION MAP Increased forward shift of the posterior corneal surface is common after myopic LASIK and correlates with the residual corneal thickness 64 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 67 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 68 69 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 70 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 72 Three step rule One abnormal map ; perform with caution Two abnormal map ; with concern Three abnormal map ;contraindicated 74 75 76 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 77 78 79 Abnormal tear film Can significantly distort the readings The significant change in surface quality and validity of the dry eye 81