Orbscan Indices

Keratoconus, rare, self-limiting Ectasia of the cornea with progressive thinning & steepening associated with hereditary predisposition ,onset in teenage years, progressing for ~10y,90% bilateral (2nd eye delayed ~5y) decrease in vision (myopic astig.,diplopia, distortion, …) Correction with spectacles, later rigid CL, corneal transplant (20%) (Source: Center for Keratoconus; text edited)

Definition of Keratoconus (Conical Cornea)
They may have normal keratoscopy in Early Keratoconus, Moderate keratoconus
Clinical criteria
–refractive stability
–crisp endpoint refraction
–Family history of Keratoconus
Topography signs please note that
..Each of the presented indices alone usually will not make the diagnosis
..However if one or more are suspicious, have the patient return for a 6-month repeat Orbscan to measure progression.
Topography signs

  1. –Pachymetry
  2. –Power Map
  3. –Posterior Elevation
  4. –Cumulative Information


  • < 470 μm
  • difference of > 100 micron from thinnest point to 7 mm OZ values

–Power Map

mean keratometric power > 45 D
  • bow-tie / broken bow-tie appearance
  • difference of > 3 D within 3 Mm Zone From Superior To Inferior

–Posterior Elevation

  • posterior BFS > 55 D
  • highest point on posterior elevation > 50 micron
above BFSTopography signs
  • bent / warped cornea (symmetry of anterior and posterior
Elevations inferotemporal displacement of highest point on
–anterior elevation
–posterior elevation)
  • highest point on posterior elevation coincides with–highest point on anterior elevation –point of maximal curvature–thinnest point

Inferior-temporal displacement highest point on posterior BSF coincides with highest point on anterior, maximal curvature and thinnest point
Pathology pointers

-Pattern Recognition

  • Look for related changes on other map
  • Isolated findings have limited value

Three Step Rule

  • One abnormal map: Caution !
  • Two Abnormal maps: Concern !!
  • Three Abnormal maps: Contraindication !!!

Caution: One abnormal map

One abnormal map does not usually indicate form fruste Keratoconus but requires patient education or having the patient return to monitor changes in 6-12 months

Concern: Two abnormal maps

Two abnormal maps may indicated early keratoconus or if it is the posterior float

that is abnormal with a slightly thinner cornea (i.e. <500microns) may still indicate keratoconus de pending on other variables listed below. If it does not indicate form

fruste Keratoconus but there are two abnormal maps, surface ablation would likely

be a better procedure than LASIK for this patient.

Contraindication: Three abnormal maps

Three or more abnormal maps is a contraindication to corneal surgery and often indicates a high risk of post-LASIK ectasia.

Irregularity Index at 3 mm and 5 mm ZoneRed Flags-if the irregularity is greater than 1.5 D in 3 mm zone-if irregularity is greater than 2 D in 5 mm zone“It does not indicate formfrustekeratoconus in and of itself but raises suspicion to look for other signs such as the number of abnormal maps or a posterior float difference > 0.050 mm (50 microns). Irregularity at 3 and 5 may simply indicate the presence of HOA and not anything pathological so this index should only be used in conjunction with other findings to diagnose keratoconus.”

Red Flags:
-Any elevation differences from the best fit sphere in the posterior float map > +/-50 μm raises

-Significant inferior thinning matching the steepest posterior apex when > +/-40 μm (and in some cases as low as 35 with other corresponding abnormal maps) for early form fruste keratoconus.

“Keep in mind it is not necessary to have a thin cornea to confirm a high risk of ectasia.”
-If the thinnest point on the cornea is more than 30 microns thinner than the central pachymetry -If the thinnest point is outside the central 5 mm of the cornea

-If the peripheral (usually inferior) is not 20 microns steeper than the central pachymetry this raises suspicion for FFkeratoconus increased risk of post-LASIK ectasia, since the peripheral cornea should show significant steepening from the central cornea.

“Since keratoconusis known to be an asymmetric condition (CLEK study), one eye usually progresses faster than the other eye.”
Red Flag:
Difference of > 1D in Orbscan measured cylinder between eyes and/or increasing cylinder

Scheimpflug Imaging

These differ from slit scanning systems primarily in image acquisition since the cameras rotates around a meridian point to capture and display thin “slices” of tissue, these slices have a common point, and any error caused by movement of the eye during the 1.5-second capturing process can be almost eliminated by registering the slices. Thus, this echnology gives a very accurate picture of the anterior segment.


The Holladay Report on the Pentacam includes standard topography complete with an axial power map.The tangential curvature map is slightly more sensitive regarding corneal curvature; it is really a local radius-ofcurvature map, and it provides the best representation of the detail of the corneal surface.


Cataracts have variable densities, components, and volumes, which are difficult to identify within the limitations of two dimensions. The Pentacam’s Scheimpflug camera offers 3D lens densitometry via the PNS software. The PNS sampling technology allows surgeons to evaluate the individual components of the cataract in three dimensions and determine the relative density of the nucleus and epinucleus. They may then modify and set their preferred phaco settings

linked to the cataract’s grade. Ultimately, surgeons may be able to establish default phaco settings that they initiate at the start of surgery based on the PNS grade.


Screening II is a new software package available on the Pentacam/Oculyzer.This software combines corneal pachymetric evaluation and elevation-based mapping in one display. Any reference surface can be used, the best surface is one that easily and understandably
conveys clinically relevant information, such as astigmatism
and areas of ectasia. Based on these criteria, we
chose a best-fit sphere for the topographic reference


Lascia una risposta

Your email address will not be published. Required fields are marked *