Feature, Ophthalmic equipment & diagnostics

Redefining refractive laser surgery with an eyevatar

Eyevatar

With perfect distance vision becoming the standard of refractive laser surgery, the goal of modern techniques is to go beyond 20/20 in even more patients. Dr Chandra Bala discusses the excellent results he is achieving with technology that creates an ‘eyevatar’ for each patient and treats the eye, not the glasses.

Sydney eye surgeon Dr Chandra Bala is achieving numbers that would have been the envy of ophthalmology just a couple of decades ago. Across 200 patients who received an advanced refractive laser procedure at his practice, the fact that all achieved uncorrected distance visual acuity (UDVA) better than 20/20 was a non-event. The real headline was the proportion achieving “supervision”: 89% with ≥20/16, 51% with ≥20/12 and 8% with a remarkable 20/10.

The outcomes were so good that PersonalEYES – a 13-clinic ophthalmology network in NSW and ACT that Bala is a partner of – had to source new eye charts because patients were running out of lines to read. And with a touch up rate of less than 0.8%, it was the icing on the cake.

How he has been able to achieve this comes down to refractive surgery technology from Alcon called Wavelight Plus. The concept is also known as ray tracing-based LASIK and PersonalEYES was the nation’s first to offer the breakthrough method that measures and combines multiple datapoints with artificial intelligence to generate a multi-dimensional model of the eye.

Bala says this is a major advance over anything else he has seen in refractive surgery – allowing him to offer greater precision.

To make the calculation, the SIGHTMAP all-in-one diagnostic device captures wavefront of the entire eye, corneal topography, corneal back surface, and biometric data including corneal thickness, anterior chamber depth, lens thickness, and axial length. Image: Alcon.

“Suddenly everyone gets 20/20. There are always difficult cases; for example, those with a very high cylinder over 2.00 D of astigmatism, but even then, the quality of output in a regular clinical practice is astonishing. We’ve also corrected up to -8.00 D with -4.50 D of cylinder – we pushed this technology to its limit, and it performs well,” he says.

Bala says Wavelight Plus’ ability to generate a highly accurate “eyevatar” using multiple refractive elements/interfaces is what sets it apart.

The software uses measurements captured on Alcon’s SIGHTMAP all-in-one diagnostic device – including wavefront of the entire eye, corneal topography, corneal back surface, and biometric data including corneal thickness, anterior chamber depth, lens thickness, and axial length. After the ray-trace based algorithm in Wavelight Plus generates an individualised 3D optical eye model, it calculates a LASIK ablation profile unique to the eye.

Traditionally, Bala says the inputs for refractive surgery were much fewer. It left room for assumptions and guesswork.

“We were taking the person’s script – essentially their cylinder and sphere – and if it matches another person’s script, you basically perform the same treatment, but the fundamental problem is the eyeballs of these two individuals might be completely different,” he says.

“The script is a manifestation of the paraxial optics, which is only about the central 4mm. Once you start going beyond that, you start getting aberrations. As a surgeon, I can’t do anything about the light coming from a faraway object; all I can do is change the angle of incidence of that light at the cornea, and once it’s crossed the front surface, I can’t control the path it will head down.

“We therefore need to know before surgery what that internal path for each beam ought to be in this unique eye through a validated personalised model, then the angle of incidence can be altered to match the external and internal beam paths. Historically laser technology has not accounted for the internal element of the eye.”

A patient example of how the the ray-trace based algorithm generates an individualised eye model and calculates a unique LASIK ablation profile for the surgeon. Image: PersonalEYES.

But Wavelight Plus and SIGHTMAP can achieve this, according to Bala. When the patient’s unique eye model is generated with their biometric datapoints, the software models what the wavefront will be on the front surface of the lens after passing through the cornea, and then compares this with the measured wavefront of the entire eye – at the same location.

“It then asks, do these two match? Initially, they don’t, and then alterations are made iteratively such that the modeled starts to match the measured, and suddenly for the first time you have a real-life eye model. This eyevatar now tells you what will happen inside the eye for any beam of light hitting the cornea at about 600-700 different locations in the pupil,” Bala explains.

“Finally, the system virtually treats the cornea of the eyevatar and calculates the effect using Snellen’s law and again iteratively, over four to six cycles, determines the best plan for this eye. This is automatic, the only surgeon input arises from taking the correct measurements.”

‘It’s got everything to do with incidence of light’

To reach a final calculation with Wavelight Plus requires immense computing power. According to Bala, this is why it has taken several years to bring the concept to market.

“The technology actually came out 12 years ago, but required so much computing power because it used Snell’s Law to calculate the path for each of these beams, which took 24 hours to process,” he says.

“What’s more, the technology was taking information from multiple devices, so you had an aberrometer, topographer, axial length from the biometer, and so inter-instrument variability existed and these compounded, making a calculation harder. After the Arthur Cummings’ paper, published in Clinical Ophthalmology in 2013, it was clear that even in the difficult cases, which were the high myopes, the outcomes were far superior. We knew then it was going to be a game-changer, but it required so much computing power that it got shelved for almost a decade.”

In the intervening years, Bala says the industry has relied on various options such as photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), and small incision lenticule extraction (SMILE). All are good options and have been improved, but can be imperfect.

“The technology has been focused on how we enact change in the eye and not what change is enacted,” he says.

“The goal of refractive surgery has nothing to do with refraction, it’s got everything to do with incidence of light, and this concept has escaped us. On the other hand, cataract surgery has everything to do with refraction because we insert a new refractive element, but not in refractive surgery which is supposedly more advanced. In refractive surgery, we have gone down a path of removing a lenticule like element from the cornea because it was easier to calculate mathematically, but the problem has been we did not account for internal eye elements and ended up creating aberrations, instead of correcting existing aberrations.

“This limit exists for all pre-existing laser surgery options. Attempts have been made to correct it, but none has been made to model an individual’s eye.”

Real-world performance 

For PersonalEYES, offering Wavelight Plus forms part of a major R&D focus. Bala himself has developed a novel DMEK punch, which can potentially improve surgical control of the graft in this surgery. Meanwhile, partner Dr Andrew White is developing a glaucoma tube device, and they are currently recruiting for a contralateral myopia LASIK study comparing a new UV laser and the current infrared femtosecond laser.

“I love technology, but I don’t trust it, and so it’s integral for me to drill down and understand the nuances, and to not pretend it is a panacea for everything,” Bala says. “I like to know the original intent of a design to understand its physical and optical limits, rather than relying on marketing or heresy. We know there are assumptions in all design and as we push the limits, this will be challenged and improved.”

Today in Bala’s clinic, ray tracing-based LASIK using WaveLight Plus has been performed on around 1,500 eyes. Four hundred of those were included in a study Bala and ophthalmology registrar Dr George He published in the Journal of Cataract & Refractive Surgery in November 2023.

Involving 200 people, it is the world’s largest real-world examination of the technology.

The major findings feature in the first paragraph of this article, but ultimately all achieved better than 20/20 UDVA at three-months, approximately half of eyes achieved ≥20/12.5 UDVA and 8% achieved 20/10. The technique was also found to be safe and effective for correction of myopia with and without astigmatism.

With a low 0.8% enhancement rate and no clinically significant increase in total whole eye higher order aberrations, patients leave the clinic satisfied, but Bala often needs to remind them just how good their results are. Especially those who achieve 20/10.

“They think it’s normal and how everybody sees the world, until we explain to them that only 8% of the population can actually do what they can do,” he says.

“That line (20/10) didn’t exist on our eye chart, so we had to go out and buy new ones to put in each room so we can push to that next level.”

Bala says the technology represents “a whole new world” in refractive laser surgery. The SIGHTMAP diagnostic device – and its various capabilities – has also proven a major space- and time-saver, taking around eight minutes to measure both eyes. The data is then pre-populated into the laser suite where calculations take place.

While the industry could push such technology further by removing some still-used assumptions such as the refractive index of the cornea, lens and posterior surface of the lens, it’s a major step in the expanding era of personalised eyecare.

“Many patients are coming in and their parents have had laser eye surgery. This technology is no longer experimental, it’s now in its maturity phase where the focus is on optimisation and customisation.”

NOTE: *WaveLight Plus is the alternative equivalent trade name for INNOVEYE treatments. 

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