Innovative Eye Care’s Wakefield Street practice was the first Optometry practice in the world to own a Pentacam AXL Wave. One of the incredible uses of this piece of equipment is wavefront aberrometry. The Pentacam AXL Wave’s wavefront aberrometry revolutionises the management of hyperopia, myopia, astigmatism, keratoconus, and contact lens design.

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What is aberration?

Figure 1. Ray diagram showing rays of light passing through curved lens

When light rays pass through a curved lens, like the lenses in your glasses, they ideally concentrate and point towards a single point called the focal point (see image 1.) This is how clear vision is achieved; your eye focuses light so that the focal point lies on the retina (layer of the eye that detects light.) In the real world, not all light rays behave perfectly. Some of them don't reach the focal point as expected; instead, they go off course and land at different spots. This phenomenon is known as optical aberration, where light doesn't focus perfectly, leading to vision problems or blurriness.1

Figure 2. Ray Diagram showing rays of light aberrating (focusing on a different focal point)

Aberration of the Eye: Our eyes are not perfect optical systems, so all eyes have some degree of aberration. Damaged or diseased eyes with abnormally shaped corneas or lenses can have far more complicated and bothersome optical aberration.

What is wavefront aberrometry?

Wavefront aberrometry is how we measure the amount of aberration occurring in an eye (optical system.) Devices called aberrometers are used to measure the total deviation of light rays when they enter the eye.

What is a wavefront? - Many rays of light traveling alongside each other into an eye is referred to as a wavefront. The total aberration of an eye can be determined by measuring how much aberration occurs in this wavefront. The measured wavefront aberration can be quantified mathematically, by breaking it down into a series of aberration patterns. These mathematical aberration shapes are called Zernike polynomials.²

The wavefront shapes can be simple and complex. Simple shapes are known as lower-order aberrations, the type of aberration that can cause blurring of vision. This aberration can be fixed with spectacles and contact lenses.¹ Complex wavefront shapes are known as higher-order aberrations. Examples of these aberrations are coma and trefoil. Higher-order aberrations are responsible for common visual symptoms like glare, haloes, poor night vision and double vision.³ They are more severe in conditions like keratoconus, pterygium, and corneal graft, and cannot be totally improved with conventional optical correction like lower-order aberrations can be.^4,5

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Lower-order aberrations (defocus and astigmatism) and higher-order aberrations (coma) forming the total wavefront.

How does wavefront aberrometry work?

A Hartmann-Shack wavefront sensor detects the deviation of light at hundred of points through the pupil.² Coupled with this measurement of the total eye aberration, the Pentacam AXL Wave also uses Scheimpflug imaging to measure the shape of the front and back surface of the cornea. Both total eye aberration and corneal tomography combined can identify the source of aberration as either corneal or internal.

Visual performance of a patient’s optical system (eye) based on aberrometry readings, divided into corneal and internal components.

How can aberrometry benefit me?

Now that Innovative Eye Care is armed with the Pentacam AXL Wave, we can completely individualise the management of our patients. The ultimate aim is to correct all aberrations, creating a perfect optical system for a singular point of focus. In conditions such as keratoconus, which are known to cause large amounts of higher-order aberration, we can now measure and correct these aberrations with wavefront-guided scleral contact lenses. Furthermore, in some cases we can utilise aberrations to create a multifocal effect (multiple points of focus), or even control myopia progression in children (peripheral retinal focus confusion).⁶

References

1) Maeda, N., 2009. “Clinical applications of wavefront aberrometry - a review.” Clinical and Experimental Ophthalmology,37(1): pp.118-129.

2) Salés, C. and E. Manche (2015). “Comparison of ocular aberrations measured by a Fourier-based Hartmann-Shack and Zernike-based Tscherning aberrometer before and after laser in Situ keratomileusis.” J Cataract Refract Surg 41(1): 1820-1825.

3) Ohlendorf, A., et al., 2020. “Advancing Digital Workflows for Refractive Error Measurements.” Journal of Clinical Medicine, 9(1): pp.2205.

4) Jinabhai, A. (2019). “Custominsedaberration-controlling corrections for keratoconic patients using contact lenses.” Clin Exp Optom 103(1): 31-43.

5) Kumar, P., et al. (2019). “Do visual performance and optical quality vary across different contact lens correction modalities in keratoconus?” Contact Lens and Anterior Eye, S1367-0484(20):30052-30057.

6) Hoy, L., 2019. “Demystifying Corneal Reshaping With Ortho-K.” Lecture for CCLSA Victoria.