Interpreting the iTrace Maps: A Practical Guide for Clinicians for premium IOL
- Subhabrata Bhattacharya
- 20 hours ago
- 9 min read
Updated: 13 minutes ago
Wavefront aberrometry has become an essential tool in modern cataract and refractive surgery planning. Among available devices, the iTrace Visual Function Analyzer stands out because it combines ray-tracing aberrometry with Placido-disc corneal topography.
Unlike traditional Hartmann-Shack systems, iTrace maps analyze how light actually travels through the eye by sending sequential laser beams through the pupil and tracing their exit from the retina. This method allows clinicians to separate corneal aberrations from internal aberrations, an extremely useful feature when evaluating cataract patients, premium IOL candidates, and post-refractive surgery eyes.
What the map represents:
The ray tracing map is the core technology of iTrace. Instead of sampling the wavefront simultaneously, iTrace projects 256 sequential laser beams through different pupil locations.
A narrow laser beam is sequentially projected through multiple locations across the pupil. For each ray, the instrument records three key parameters:
1. Entry location in the pupil. The precise point where the laser beam enters the eye through the pupil.
2. Return position after retinal reflection. After reaching the retina, the beam is reflected back and captured by the sensor. The position of the returning ray indicates how the eye’s optics have deviated the light path.
3. Optical path deviation. The difference between the measured ray path and the path that would occur in a perfectly aberration-free optical system. This difference in optical path can be positive or negative difference from the optical path of an aberration free optical system.
What the Colors Represent
🔴 Red bars → Positive aberration values
🔵 Blue bars → Negative aberration values
Using these measurements from hundreds of rays across the pupil, the system reconstructs the wavefront of the eye and generates a detailed map of optical aberrations.
This process effectively creates a ray-by-ray reconstruction of the eye’s optical system, allowing the device to separate:
Total ocular aberrations
Corneal aberrations
Internal (lenticular) aberrations
The result is a highly accurate representation of how light actually travels through the eye, which is particularly useful for analyzing cataract-related optical degradation and evaluating candidates for premium intraocular lenses.
Validating the data -:
All devices that are used in a clinical practice needs to be validated by the operator. Each of the biometry device will have its own validation process. The iTrace displays the validation display (image 1). In the top left corner the number of points rejected are displayed. The rejected number of points are shown in yellow (caution). This is usually when 1 to 10 points are rejected. If rejected points are more than 10, red caution will appear.
All measurements of the iTrace is done on the real pupillary plane (not the entrance or exit pupil plane). To understand the difference between real pupil, entrance and exit pupil you can follow the link here: https://www.quickguide.org/post/relationship-between-the-mesopic-pupil-size-and-diffractive-zone-of-multifocal-iol
B) Measurement should be done in both light and dark condition to mimic photopic and mesopic vision. However, the pupil of the patient must be at least 2.5 mm for a dependable measurement. In iTrace you have both pupil diameter and scan diameter displayed. During measurement, iTrace sends sequential laser rays through different locations across the pupil. However, not all rays may be usable due to:
eyelid interference
tear film instability
small pupil size
poor fixation
The system therefore calculates aberrations within the largest reliable circular zone of the pupil. That diameter becomes the scan diameter.
For evaluating premium IOL candidates, surgeons often focus on 5 mm scans, because this approximates the mesopic pupil size where symptoms like halos and glare occur.
Typical interpretation:
Corneal HOA at 5 mm | Interpretation |
<0.30 µm | Excellent candidate for multifocal/trifocal |
0.30–0.50 µm | Borderline |
>0.50 µm | Prefer monofocal |
C) Dilating the patient before measurement may lead to wrong lenticular aberration measurement. It may also lead to an error in understanding aberration values in smaller and larger pupil diameter simulating day and night conditions.
Understanding Basics - HOA/RMS/Zernike Polynomials
If you are a cataract surgeon with interest in premium IOLs, and want to use iTrace for validating patient candidacy, let us start with the WF and CT display page as this page will give you a summary, yet comprehensive understanding of the patient candidacy so far as higher order aberration (HOA) is concerned.
We will now analyze each information in the image 3.
In image 3, the internal (lens) HOA is displayed. This is measured in RMS or root mean square. The word 'no defocus; after RMS in the top row means that the values of spherical errors like myopia and hyperopia are not displayed. Information on aberrations are only useful when you know the pupil diameter that it is measured with. Thus the pupil diameter of 3.70 mm is displayed, that is the aberrations are measured at a pupil diameter of 3.70 mm.
For those of you who have already read my post on zernike polynomials (https://www.quickguide.org/post/a-simple-interpretation-of-zernike-polynomials) can easily interpret the numbers that come below the letter z in image 3a. Yes, these are depicting the higher order aberration ( 5-14). The lower order aberration of astigmatism is displayed as .149 micron (z-3). This is what you typically correct with spectacles or Toric IOL. All other aberrations (z5-14) are higher order aberrations.
Difference in HOA display in iTrace than typical mathematical display in zernike polynomials.
Earlier, I had said that the aberrations are measured by optical path difference. The red bars are positive while the blue is negative. But as a cataract surgeon, you should know that the bars are bad. Less the bars, the more the patient is qualified for a premium IOL.
The iTrace does not follow the Zernike grouping. Instead, it groups and puts more emphasis on aberrations that are asymmetric. Thus coma, trefoil rank higher in orders than spherical aberration (displayed as z-12) in image 3(a,b). Thus higher the impact of an higher order aberration on the asymmetry of an image, the higher the ranking - spherical aberration creates less distortion than coma or trefoil. Having said so, remember, all of the three - trefoil, coma and spherical aberration above the cut off value disqualifies patients from a premium IOL.
What cut off values should you follow?
For the total HOA, it is safe to follow a cut off value of .3 micron for a pupil diameter of 5 mm. Over .4 micron of HOA could be regarded as a red flag
For individual coma and trefoil values, accept values lower than .2 micron for a scan diameter of 5mm.
Like the internal (lens) the corneal and total eye aberrations are displayed in the left bottom and upper middle respectively. We will now analyze different patient maps:
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Following three cases would help you to understand the iTrace maps and how to determine the candidacy of the patient for premium IOL:
Case 1 :
(1) The HOA of the eye (top middle box) largely resembles the HOA of cornea. The coma and trefoil values for the total eye are low (below .3 microns) - 1.
(2) The coma and trefoil values are largely a result of lens aberration that will no longer be there once the cataract surgery has been performed - 2.
(3) The coma and trefoil values are lower on the cornea (3). However a significant 5th order (higher order) irregular astigmatism is visible on cornea (3) that manifests in total eye aberration. Therefore it is important for the clinician to look for dry eye in the patient.
(4) The corneal spherical aberration is normal for a 5.90 mm cornea ( .248 micron). The average corneal spherical aberration is positive .26 microns for a 6 mm cornea. Therefore any premium IOL that provides negative spherical aberration to correct for the positive corneal spherical aberration can be implanted.
Case 2:
Total HOA is .165 micron which is well below the cut off limit.
The patient also has very low individual coma, spherical aberration and trefoil values of both the total eye (1 & 2) and the cornea (3)
This together with a very low angle alpha (difference between optical axis and visual axis of eye) is low (.165 mm). Larger values of more than .3 mm are marked by yellow
- All the above points make the patient ideal for premium IOLs, say a diffractive multifocal IOL
2 . In the Case 2 image of iTrace, you can observe very high higher order (irregular) astigmatism of cornea (4). This alone should have kept the HOA of total eye quite significant, which however is not the case (Total eye HOA is .165 micron). This is because the lens (internal) has a similar amount of astigmatism (3) in negative. So a very high positive higher order astigmatism of the cornea is negated by the negative internal astigmatism of lens. Therefore the total eye irregular higher order astigmatism is less than the cut off value (2).
This brings to an observation here: what happens to the total eye irregular astigmatism once the cataract is replaced. That's when the corneal irregular astigmatism will manifest as total eye astigmatism as the lens will no longer negate the higher order astigmatism of the cornea. This is a point to consider for this patient. However, one may argue that the corneal irregular higher order astigmatism is still lower than the cut off value of .2 micron ( a value you should follow for each individual higher order aberrations).
Suggestion- Dry eye and ocular surface should be looked into this patient.
Tip - When you see low coma, and trefoil values and yet very high fifth order astigmatism in the iTrace, do not forget to rule out ocular surface disease before selecting the patient for premium IOLs. iTrace however comes now with a Tear Film analysis.
Case 3:
Before we go and analyze the image related to Case 3, we should be careful about the Scan size 3.50 mm (1). This is too low to get any realistic understanding of the HOA of the patient. The total HOA of the patient is .360 micron which is mostly a result of internal aberrations (3). But the fact that the scan size is only 3.50 mm, warrant us not to jump into any conclusion without remeasuring with a slightly higher pupil size. Care must be taken that the scan is done not in too bright room light conditions.
Suggestion: Reject the measurement and remeasure again in an appropriate room light conditions.
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Now I will provide a little bit more understanding of the Chang analysis/CT & WF map of the iTrace.
The Effective RP is the net power of the cornea based on real corneal refractive indices and measurement of the posterior cornea and taking into account the spherical aberration. Below it, the central radius/power mimics the simulated K reading of corneal topography or the manual keratometry/auto keratometry. It is thus based on the Keratometry Index of 1.3375 and does not take into account the real measurement of the posterior cornea.
A large difference between the Effective Real Power (RP) of the cornea and the central radius/power indicates a post lasik or a pathologic cornea.
In image 5, we see a difference of .73 diopters. In a normal cornea, the typical difference of the Effective Rp and simulated K readings or keratometry readings is from .50 diopter to .75 diopter. Beyond this difference, may indicate prior refractive surgery or corneal ectasia.
Note in image 5, the spherical aberration values are provided for 6.0 mm even through the pupil size and scan size are 4.52 mm and 2.90 mm, respectively. As earlier described, the spherical aberration values in such case are extrapolated from mathematical models based on Zernike polynomials.
Normal cornea is prolate in shape, meaning it is steeper in the center and flatter in the periphery. Note the steeper reading of cornea (1) in image 6 at the 0 mm as you move to 3 mm of the cornea the corneal appears flatter. For a normal prolate cornea, there is a difference of .2 to .3 diopters at 0 and 3 mm of the cornea, a fact that is taken into account to optimize a constant when one moves from the manual keratometry to the optical keratometry.
The fact that this cornea is prolate in shape, therefore throws a corneal spherical aberration of .267 micron at 6.0 mm of the cornea. This value is not real spherical aberration of the cornea, but based on a mathematical model based on zernike polynomials, as the scan size is only 2.90 mm. Therefore, the data needs to be validated per the validation guideline discussed in the outset of this article, available in the iTrace WF Verification display.
The contrast reserve principle and interpreting MTF maps from iTrace:
This is a developing article, so keep an eye on this space
