Orbscan IIz Complete corneal analysis: Topography Pachymetry Anterior surface Posterior surface. Placido disc images can be interpreted both qualitatively and .. Parameters Using the Orbscan II Scanning Slit Topographer and the. ORBSCAN. S.A.A. Mortazavi MD. Associate Professor of Ophthalmology. Isfahan University of Medical Sciences. 2/22/ 1. Use the principle of projection.


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In cases with established keratoconus, the role of topography is paramount for orbscan interpretation progression and doing a timely collagen cross linkingand in contact lens fitting.

Post cataract surgery and post keratoplasty corneal astigmatism can be studied with the topographer and selective suture removal or other interventions can be planned.

Corneal Topography using Orbscan : Basics and interpretation

Surgical planning in cases with astigmatism: Limbal relaxing incisions and other methods of topography guided incision placement are used by surgeons to reduce post operative astigmatism. Effect of corneal and ocular surface disorders: Disorders such as pterygiumlimbal dermoid, localised corneal scars can cause a change in the corneal orbscan interpretation and thus the monitoring is very useful.

Contact lens fittingincision placement and intrastromal ring placement in keratoconusmonitoring of ocular vs corneal wavefront. General Principles Corneal orbscan interpretation uses three of the following principles [3] [4] [5] [6] [7] Placido disc reflection Scheimpflug photography Placido Disc Reflection for curvature analysis The primary optical aim of cornea is refraction and focusing of the light rays as it acts as a covering lens overall.

Howeverall non-ideal refracting surfaces reflect some light off them.

Corneal Imaging: An Introduction

This is the principle used for Purkinje imaging as well in the Placido discs. Placido disc is a device made of concentric rings drawn on a orbscan interpretation of a different color generally white rings on a black background Figure 1a.

The first refracting surface of the cornea more accuratelythe tear film —air interface and not the epithelium also acts as convex mirror and reflects orbscan interpretation light in a pattern dependent of the corneal pattern Figure 1b.

Topographers use this technique to their advantage.

Corneal topography - EyeWiki

Whereas the original placido discs were aimed a qualitative keratoscopythe videokeratoscope or the topographer uses mathematical formulae to provide a point to point quantitative gradient of these subtle changes in topography.

Scanning Slit elevation evaluation It is one of the elevation based methods for assessment of topography. Orbscan interpretation complimentary slits are used to perform an assessment of the corneal surface Figure 2, left. In the Orbscan40 slits 20 each from nasal and temporal side are projected on the cornea to assess points on each slit.

The triangulation between the reference slit beam surface and the reflected beam orbscan interpretation by the camera can be used to analyse the anterior and posterior corneal curvature and the pachymetry Figure 2, right.

Scheimpflug principle based assessment The non planar shape of cornea can potentially lead to spurious results and therefore the use of schiempflug principle in corneal imaging is a welcome new change.

Theodre Scheimpflugan Austrian army man worked extensively on a method for correcting arial skew distortion in perspective photographs.

Corneal Topography using Orbscan : Basics and interpretation - ppt video online download

In an ideal scenariothe lens plane and the orbscan interpretation plane are parallel. Therefore a linear object will form a plane of focus parallel to the lens plane and thus can be focused totally on the image plane Figure 3a.


Consider a situationorbscan interpretation the object is not parallel to the prospective image plane. It will not be possible to focus all the image on a plane parallel to image plane Figure 3b.

Thus this may lead to image distortion. Orbscan interpretation, according to the Scheimpflug principle, when a planar subject is not parallel to the image planean oblique tangent can be drawn from the image, object and lens planes, and the point of intersection is called schiempflug intersection Figure 3c.

Using this orientation, A careful manipulation of the image plane and the lens plane can however lead to a focused and sharp image on the non parallel object.

Interpretation of a topography printout A modern topography printout can be quite daunting for the beginner because of the volumes of data it contains. This is a major shift from the rather simplistic videokeratographs and keratometers. A stepwise approach is very handy in interpretation.

Correctly identify the patient, age and eye. Start by looking at the quad or multi map option as it gives the best visual comparison of the data.

Next step is to look at the pseudocolor scale and identify the range and gradient of the values given.