Interpreting the iTrace Maps: A Practical Guide for Clinicians for premium IOL

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 analyse 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 iTrace 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 iTrace Colours 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

How does the iTrace separate corneal aberration from internal aberration?

The iTrace separates corneal from internal aberrations by combining corneal topography (Placido disk) with ray-tracing aberrometry. Corneal aberrations are derived from the corneal surface map, while internal aberrations are calculated by subtracting corneal contributions from the total wavefront error measured through ray tracing.

iTrace combines two measurements:

• Placido corneal topography

→ gives corneal wavefront

• Ray-tracing aberrometry

→ gives total ocular wavefront

Then it subtracts:

Internal aberration = Total aberration − Corneal aberration

1. Validating the data -

Image 1: The itrace WF Verification Display guideline quickguide.org

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.

Image 1b: Points rejected will be displayed in the WF Verification display(image 1) in the top left corner below Point 1 quickguide.org

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:

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