Wavefront: Concepts and Insights

Raymond Applegate OD, PhD, a noted vision science researcher who has been involved with wavefront technology from the earliest days, began the session by reviewing the optical rationale of customised ablation and giving a preview of future applications of the technology.

Prior to the advent of wavefront sensing, the goal of vision correction was to reduce or eliminate errors of sphere and cylinder. While these corrections, whether by glasses, contact lenses, intraocular lenses or refractive surgery, certainly improved patients’ vision, they were not the ideal solution.

Wavefront sensing made people aware of higher order aberrations of vision not affected by simple correction of cylinder and sphere. It also revealed the extent to which some refractive surgical approaches were actually inducing significant new aberrations. In essence, wavefront sensing showed higher order aberrations of the eye in a way that hadn’t been done before, and also indicated what an ideal correction should be.

"What we are talking about is improving the optics of the eye by removing aberrations, thereby increasing the contrast and spatial detail of the retinal image. What does this mean? It means driving on a dark rainy night and being able to see someone walking along in black t-shirt and blue jeans."

Maximum MTF by pupil size (1,3,7,9 mm pupil)	PSF-Contrast diminishes as pupil size decreases

Wavefront sensing indicates what an ideal correction can be, based on the optical limits of the eye. The physiology of the eye, with densely packed foveolar cones approximately two microns in diameter, creates a fundamental limit to human acuity of something on the order of 20/8.

The importance of pupil size

Human vision is a diffraction limited system. This means that the higher the contrast, the crisper the edges of the retinal image. This is also why pupil size is such an important factor in any vision correction strategy that aims to reduce higher order aberrations. The larger the pupil is in a diffraction limited system, the greater the effect of higher order aberrations on visual acuity.

"The importance of pupil size is one of the biggest take-home messages making it down to the clinical level. All of the effects of higher order aberrations are pupil size-dependent."

The effect of pupil size on image contrast detail can easily be demonstrated by two optical measurements - the modulation transfer function (MTF) and the point spread function (PSF).

The MTF measurement indicates how much contrast is lost due to the limits of a given optical system. The procedure involved taking an object of a given spatial frequency, e.g. an image of black and white bars in a sinusoidal pattern, and imaging this through a lens. Contrast decreases by a certain percentage as the test image is made finer and finer. The MTF indicates the percentage of contrast reduction.

Contrast diminishes as pupil size decreases because of diffraction. Diffraction occurs at the edge of an aperture, causing light to spread out. These effects can be seen by observing how an optical system images a single point of light, a measurement known as the point spread function. The PSF increases as the pupil size decreases. The net result is a blurring of vision, even in a 20/20 corrected eye.

"At a 4.0mm pupil size, normal eyes with best corrected sphere and cylinder in place become aberration limited, not diffraction limited. This is the area we are trying to work on with CustomCornea®. The goal is to improve visual acuity beyond that provided by correction of sphere and cylinder by minimising the effect of higher order aberrations."

Before attempting to accomplish the perfect optical correction of all aberrations, the first goal should be to reduce the amount of aberrations being induced by current modes of refractive surgery. Customised ablation systems offer the potential to minimise the amount of induced aberrations over all pupil sizes.

Wavefront results continue to improve

"Where are we now? Is refractive surgery decreasing aberrations and improving visual resolution? It depends on which surgery you are doing and what techniques you are using. Is refractive surgery getting better? Absolutely! When I began doing wavefront research in the late 1980s, I realised that refractive surgery was increasing the aberrations of the eye and was causing a loss of best-corrected acuity. I’m happy to say now that we are beginning to minimise that loss."

He cited the initial FDA study results in which conventional LADAR™ treatment induced a 123% increase in spherical aberrations. The results with the first iteration of the Custom Cornea® cut this number in half. Subsequent adjustments in the treatment algorithm reduced the incidence of induced aberrations to 36%. The most recent data based on further modifications to the algorithm revealed a rate of induced aberrations of only 1%.

"We’re getting there. Some 30% of eyes treated with the system have less spherical aberration than before surgery. Importantly, more eyes saw 20/12 than with conventional LASIK, and 95% had 20/20 or better. No other FDA results that I am aware of compare with this."

The first requirement for correcting the higher order aberrations is to quantify the existing aberration structure of the whole eye. The next step is to design compensating optics which are compatible with the eye.

Corneal topography does not provide enough information to design an ideal correction. It does provide useful clinical data on the shape of the corneal surface. But there are more optics to the eye, namely the crystalline lens and the spacing between the optical components. This is where wavefront sensing comes in.

The LADARVision® system employs a Hartmann-Shack wavefront sensor to measure the difference between an ideal wavefront and the actual wavefront as a function of location within the pupil. It employs a linear lenslet array that samples the slope of the wavefront coming from several hundred locations, forming an image on a charged couple device. The software then reconstructs the entire wavefront, creating a map of the higher order aberrations. This measurement both demonstrates any errors while also defining the ideal compensating wavefront correction

"We used to not pay much attention to higher order aberrations because we couldn’t fix them. Now we are in a position where we can fix them. However, each fix has to be made at very specific location. That is the importance of accurate registration and tracking, both of which the LADAR™ system provides."

Increasing clinical experience with wavefront-guided custom ablation has also made clear the need for new metrics of optical performance. Traditional dioptric measurements are simply not adequate, as they do not reflect quality of vision. Raw wavefront data provided by the root mean square (RMS) numbers also fail to reflect how well people are seeing.

New metrics for the new technology

Dr Applegate discussed several new metrics that could help to better measure quality of vision before and after refractive surgery. One of these, the Rayleigh criteria, measures the refraction of the pupil with a wavefront error less than a fixed criterion. This helps to account for at least some of the variance seen in standard visual acuity measurements.

Another metric, the Marechal criteria, may offer more utility. It measures an aperture opening slowly from the centre of the pupil outwards until the total wavefront error within that aperture meets a fixed criterion. This provides useful data on the ratio of error size to total pupil size.

Finally, a third metric, this one more theoretical at present, would take a measurement of optical transfer function and combine it with the neural transfer function. This would take into consideration the contribution of the other essential component of vision, the neural system, in the evaluation of visual acuity.

"We are beginning to develop metrics of optical quality which will help teach us which optical aberrations to work on and which to avoid. In the future we will design optical corrections that routinely correct to 20/15 for –2.0 D to –6.0 D with less than 2.0 D of astigmatism. Our more immediate goal is the elimination of spherical cylinder error without inducing new errors. We are getting close to reaching this goal with the CustomCornea® approach."

customcornea® success factors

Deepak Chitkara FRCOphth, DO, a consultant ophthalmic surgeon at the Rosen Eye Clinic, Manchester, UK has been using the LADARVision® correction system since 1995. He shared a number of useful clinical pearls for getting the most from the current system as well as offering suggestions for preparing to utilise the latest wavefront innovations for custom ablation.

"It is all right to talk about this wonderful technology which will improve our successes with wavefront. But if we are not using our present technology to its best advantage, all those efforts in using new technology will be wasted," he commented.

Many of the features of the standard LADARVision® system make it the ideal platform for going to the next generation of treatment, wavefront-guided ablation. These features include the capability of performing large optic zone procedures, as well as the built-in tracking, registration and cyclotorsion capabilities.

The LADARVision® is capable of correcting optic zones as large as 8.0mm. It is now recognised that correcting a small area in a large scotopic pupil will produce diffraction effects with degradation of the image quality. The diffraction effects do not affect the image quality nearly as much when using a large optic zone in a large pupil.

"In your preoperative planning, one of the most important measures you can make is pupillometry. How you do pupillometry is a critical choice these days. Some of the best pupillometers measure binocular pupil size in a dynamic way. But having measured the pupil sizes, there is no point in ignoring it. You must make sure that the zone size you decide to treat matches the maximum measured pupil size," he emphasised.

It is also important to remember that the actual functional optic zone achieved with the laser system may actually be smaller than the nominal correction programmed into the laser.
Therefore, it is essential to get as big an optic zone as possible.
Blend zones have become another component of laser refractive surgery. However, these are mechanical rather than optical. Therefore they should not be considered when matching optic zone with pupil size, he advised.

"Postoperative halo and glare continue to be the leading complaints among LASIK patients. Yet the subjective appreciation of halo and glare is unpredictable. I have seen lots of patients with larger pupils who have been treated with smaller optical zones and yet do not complain of night-time symptoms. On the other hand, I’ve also seen patients we’ve treated with large optic zones and smaller pupils, yet they come back complaining of night vision problems. The brain still has a lot to answer for."

While treating as large an optic zone as possible is an important objective, the surgeon must also keep in mind the depth of the remaining tissue. It is now widely accepted that minimum stromal depth should be greater than 250 microns.
The LADARVision® 4000, is generally used with a 6.5 mm optical zone, when treating myopia, although other optical zones can be used. However, considering all the factors involved it is desirable to take advantage of the 6.5 mm optical zones possible with the LADAR™ system.

The LADARTracker® is another key feature of the system. This is a very high-speed infrared closed loop tracking system that measures the eye position 4,000 times per second.

The LADARTracker® system tracks the margin of the dilated pupil. The dilated pupil provides an absolute reference point that is stable during surgery. It analyses the position of the pupil with an infrared signal, analyses the return from that signal, and moves the tracker mirrors constantly to keep up with the position of the eye. This means that it can provide a space-stabilised image of the eye to the laser through the graphic user interface.

"As far as the laser is concerned, it is seeing a static eye. That is particularly important with small beam flying spot laser systems, where each laser spot has to be precisely placed and overlapped with other spots."

The LADARVision® system takes a snap shot image of the eye, allowing for better centration of the treatment. The system has built in rings that can be aligned with the limbus and the pupil. This assures that treatment is based on the centre of the pupil.

This registration becomes even more important with the new wavefront technology. Another feature of the LADAR™ system, the cyclotorsion marking, provides precise x, y and cyclotorsion orientation to guide the wavefront treatment accurately. These markings consist of two dots made on the sclera near the limbus at three and nine o’clock with gentian dye.

"Marking the patient is essential, even in conventional treatment. It improves accuracy, is good practice, and gets you ready for the next step – wavefront-guided treatment."

Dr Chitkara conducted a small study comparing the results of conventional LASIK in 50 patients who were unmarked and 50 patients who received cyclotorsion marking. The three-month follow-up results showed several advantages in favour of marking. Some 81% of marked patients were within 0.5 D of the predicted correction, compared with 66% of those who were not marked. Similarly, 88% of marked group achieved 20/20 or better, compared with 70% of unmarked. Cylindrical treatment results were also better in marked patients.

None of the patients who received the cyclotorsion marks required re-treatment, while several patients in the unmarked group did. While some 8% of patients in the unmarked group lost at least one line of vision, only half that many lost a line in the marked group.

The hinge mask is another useful feature of the LADAR™ system. This can be used to cover and protect the hinge from the ablation if required. For example, if the ablation zone overlaps the hinge, this feature could be employed to reduce the chance for an unwanted visual outcome.

Dr Chitkara shared a number of pearls for improving results with the LADAR™ system. He emphasised the importance of the accuracy of preoperative refraction, saying it was important to check and re-check optometry results. He also reminded surgeons to be aware of the chance of fixation errors that might occur if the patient is not fixating on the blinking light during the procedure.

On the practical side, it is also important never to put instruments in the line of the laser, keeping in mind that the tracker is monitoring the pupil in four different positions.

There are other factors over which the surgeon has little control. These include the biomechanics of laser flaps. In the future femtosecond technology may provide the ability to get better flaps, further improving visual results.

"Whatever you do, do it consistently all the time. Standardise your process. Monitor your results constantly and adjust your nomograms. This will improve your results and will also be good preparation for performing wavefront-based treatment," Dr Chitkara recommended.

patient selection for customised ablations

One obvious way to improve outcomes in refractive surgery while minimising untoward results is through careful patient selection.
But who is a good candidate? Wavefront sensing can both help to determine which patients are the best candidates for LASIK and then provide the essential data needed to guide the treatment, noted Luca Vigo MD, Carones Ophthalmology Centre, Milan, Italy.

"The LADARWave® system can help us in two ways - as a diagnostic tool and as a therapeutic tool. It can help us to make a diagnosis by analysing aberrations in order to better understand the quality and quantitative aspects of patients’ vision. It can also help us to select the best candidates for custom ablation."

The patient work-up forms the basis for the clinical decision on how best to proceed. The data to make the decision comes from a triad of sources - refractometry (both manifest and cycloplegic), pupillometry and wavefront aberrometry. Each of these provides essential information for determining who is likely to benefit from customised ablation and who is not.

Dr Vigo noted that the impact of defocus error is another very important parameter when considering customised ablation. But he said it would be very difficult to put a cut-off rate on this parameter alone when deciding who might be a good candidate, especially considering the differences between virgin eyes and those that have undergone a previous laser refractive treatment.

He provided a series of clinical cases illustrating how the various diagnostic parameters can be interpreted to determine who should be considered for custom ablation.

For example, a patient with virgin eyes, low myopia, low RMS values and very high defocus impact could be treated with standard ablation with a large optical zone and get a good visual result. However, custom ablation might be considered in another patient with low refractive error, higher RMS and a lower degree of defocus impact. Still, another case with high refractive error in a pre-treated patient, +6.85, and 5% defocus impact, would be a candidate for custom ablation provided there is enough tissue to work with.

Pupil diameter has become established as a key parameter in determining who might benefit most from customised ablation. Mesopic pupil measurement is precise and reliable and is probably the most important value to screen for candidates for custom ablation, he stressed.

A patient with small pupil diameter, up to 5.5mm to 6.0mm, with low RMS values, and a small amount of required correction could be safely treated with standard ablation with a large optical zone. Similarly, a low myope with medium astigmatism, a smaller pupil and low RMS values would probably also do well with a standard ablation.

However, customised ablation would be the more appropriate choice in the case of patient with a small pupil diameter, and low RMS values, needing a high amount of correction. This is because there would be a significant risk of increasing aberrations with the standard approach, he explained.

A patient presenting with small pupil diameter, high RMS values and -8.25 D, with -2.95 D of astigmatism would also be a candidate for the custom approach, again because of the ability of wavefront guidance to decrease higher order aberrations.

Dr Vigo said he would also consider using the custom ablation treatment in a patient with a larger pupil, above 6.0mm, with low RMS values. The rationale in that case would be to minimise the risk of inducing or increasing the amount of higher order aberrations with larger pupils.

"When using the aberrometer we look at the high order aberration values. In the case of virgin eyes, when we check the patient we need this data to decide the treatment and to understand the poor vision of the patient. In the case of treated eyes, we use this approach to better understand the visual complaint and to evaluate the possibility of a custom re-treatment."

He cites the case of a previously treated patient who was almost plano but complained of night halos. He had large pupils, high RMS values and a high level of spherical aberrations. In that case, the wavefront data helped explain why this patient was complaining.

"In the future, as clinical experience with the LADARWave® system confirms its utility for diagnosis and treatment of refractive errors, I expect all patients will be candidates for custom ablation, absent any obvious contraindications," he concluded.

clinical significance of customcornea® measurements

Deepak Chitkara FRCOphth, DO, was one of the first to use the new LADARWave® system in Europe. He reviewed what wavefront measurement signifies in the clinical setting and discussed how some of the different parameters affect treatment.

Showing effects of aberrations on images

"We know now that measuring vision by the Snellen acuity chart is not enough. We really have to look at optical quality. We have to be able to measure it, try to correct it and improve it. What we are talking about are very fine details. Little measurements matter a lot," he emphasised.

There are three main measures of optical quality. Metrics that measure the shape of the image are defined by point spread function. Metrics that measure the loss of contrast are described by the modulation transfer function. However, the most fundamental measurement of optical quality is wavefront aberration. It is also the most useful measurement when doing customised ablations.

Wavefront measurement does nothing less than simulate what the retinal image looks like under the influence of aberrations. One way wavefront measurement is described is by the root mean square value (RMS), a measure of the difference between the ideal wavefront and the actual distorted wavefront. By definition, RMS will always be positive; if it is zero it indicates a flat wavefront and a perfect optical system, he explained.

"An important thing to remember about RMS values is that they are all relative. You cannot compare them, unless you specify the measured zone. You cannot compare the RMS value of a 5.0mm optical zone with a 4.0mm optical zone," he commented.
Zernike polynomials are mathematical expressions which offer an increasingly familiar way to describe wavefront measurements. Zernike polynomials consist of Zernike terms, with each term corresponding to a specific geometric pattern of the aberration. These are quantified and arranged in order of complexity.

"Until now, we’ve been dealing with first and second order aberrations - defocus and astigmatism. Now we have to think about the third, fourth and fifth order aberrations. And we have to get used to thinking of the shapes of these orders," he noted.

For example, in a three-dimensional representation, myopia forms a bowl shape, cylinder is like a saddle shape, coma is like a bump and dip, trefoil is like a Napoleon’s hat and spherical aberration is like a sombrero. Quadrafoil is sometimes described as resembling a plant-stand.

The new wavefront tools also offer the ability to simulate the effects of these aberrations on vision. This confirms that a defocus myopia with a relatively modest RMS value of 0.99 micron will give a very blurred image. The higher order spherical aberration is very disruptive to vision. Although the patient may be able to see all the way down the Snellen chart, the quality of that vision is very poor. Coma has similar, but worse effects on visual quality. Secondary astigmatism is one of the worst aberrations, with a major impact on the quality of vision even at low RMS values.

The LADARWave® CustomCornea® system measures all these parameters up to the sixth order very accurately. The system takes five measurements and creates a composite image from the three most similar images. The measurements can be displayed in various forms - two dimensionally, with an extrapolated power map, or three dimensionally to display all or only selected Zernike terms. This data can be studied and used to plan treatment and guide the custom ablation to correct the aberrations.

Patient preparation for wavefront treatment

Francesco Carones MD, Medical Director of the Carones Ophthalmology Center, Milan, Italy shared his opinions on the LADARVision® platform and what it might offer patients as it becomes established in Europe. He stressed the value of careful patient evaluation and wavefront measurement before considering custom ablation

"To take full advantage of what LADARVision® can offer, we first need to select a good candidate. We then need to have a very good wavefront measurement and a way to match what we have measured to what we are going to treat with the surgery. Finally, we need to make sure we deliver the treatment to the intended area of the cornea," he said.

Slit-lamp image shows a large epithelial in-growth at the hinge side of the flap, inducing 2.5 D astigmatism, as disclosed at corneal topography (Right)

The same cornea after scraping of the epithelial in-growth shows at corneal topography (Right) an almost complete disappearance of astigmatism.

Surgeons entering the custom ablation arena can expect to encounter two categories of patients; those who have previously undergone refractive surgery and those presenting for the first time. The requirements for wavefront measurement are different for both groups.

"You will want to pay special attention in pre-treated eyes. There may be some conditions where the wavefront may be quite distorted by factors having nothing to do with refraction. For example, you may encounter the presence of debris or epithelium and melting in the interface. These can distort the wavefront and give erroneous readings," he warned.

He showed an example of an eye with a severe epithelial in-growth at the hinge of LASIK flap. That in-growth accounted for 2.5 D of astigmatism. Simply removing the in-growth by mechanical means dramatically reduced the degree of aberration.
The wavefront measurement may be done for diagnostic purposes to guide the surgery and for postoperative follow-up. He concentrated his remarks on the second application, noting the importance of accurate wavefront acquisition and centration when planning surgery.

centration

The LADARWave® system takes all of these requirements into consideration. It establishes centration by first acquiring a snap shot of the normal eye under normal lighting conditions. Working at the console, the surgeon aligns two reticules, limbal and pupil, which will keep the measurement of wavefront on the centre of the pupil.

The system then measures the wavefront on the dilated eye in order to have widest measurement area. Ink marks are previously applied for subsequent cyclotorsion tracking.

"I like to wait five minutes after making the marks in order for the tear film to stabilise. This helps to avoid false wavefront readings. It is very important to make sure tear film does not interfere with this measurement. Looking at the Hartmann-Shack image can help assure that this exam has taken place under optimal conditions," he advised.

Having measured the wavefront, it is then essential to match it to the treatment. This is where registration - the process of precisely matching the wavefront measurement to the ablation pattern - comes in. Registration provides for fine control of the complex treatment during the procedure. Lack of registration can lead to misalignment, which could produce major refractive errors. This becomes even more important when correcting higher order aberrations.

Centration and registration are not enough to guarantee optimal visual results with customised wavefront guided ablation. Excellent tracking - maintaining the ablation pattern exactly on the intended area of the cornea - is absolutely crucial. All of these parameters need to be integrated in order to avoid suboptimal results.

capture-match-treat

"If we measure or capture the wavefront but cannot align and register the profile to the correct position of the eye, we’ll have a mismatch between measurement and treatment. If we cannot track the eye properly we will have the laser ablating spots other than where they are intended. If we stabilise the eye with a fast and effective tracker and register what we measure to what we treat, we will have no misplacement errors. The laser spots fall where intended, producing optimal visual results," he explained.

There is a learning curve in preparing patients for custom ablation, as with any other procedure. The three steps - capture, match and treat - are crucial. Right now, the LADARVision® system is the only one on the market that provides all of these features, he emphasised.

European outlook for refractive surgery

Wavefront-based refractive surgery is only now entering the mainstream in Europe. Joseph Colin MD, Chairman of the Department of Ophthalmology, University of Bordeaux, France, described the current status of the new technology in Europe and elaborated on considerations for performing customised ablation for surgeons considering adding wavefront tools to their repertoires.

He began by noting the considerable difference between the European and US markets. While both regions have similar populations, LASIK and related procedures are much more established in the US than in Europe. A recent marketing survey indicates that there are now some 1,600 excimer lasers available to refractive surgeons in the US, while only 750 lasers are available in Europe.

A recent ESCRS survey revealed that LASIK is the most common refractive procedure performed in Europe. However, unlike the US, PRK is still common and LASEK is gaining ground. European surgeons are also far more likely to implant phakic IOLs for vision correction than are US surgeons.

Practice patterns vary widely throughout the continent. For example, in France, PRK still accounts for 40% of laser ablation procedures. In Italy, the situation is even more striking, with PRK accounting for 75% of procedures. In contrast, LASIK is the more popular procedure in Germany, Spain and the UK.

The numbers of procedures performed also varies widely from country to country. Spanish surgeons performed 200,000 cases in 2001, compared with approximately 100,000 cases performed by French surgeons. German and Italian surgeons performed similar amounts of refractive surgery as their French counterparts. The number of surgeries performed in smaller countries including Sweden and the UK is far lower.

The ESCRS survey also showed important differences in how surgeons from different nations approach patients. When asked how they would treat a 30-year old patient with -3.0 D myopia, 25% of the ESCRS surgeons said they would counsel the patient to do nothing. A similar number would opt for PRK, while 33% would offer LASIK.

If that same hypothetical 30-year old presented with -7.0 D of myopia, LASIK became the procedure of choice. Increasing the amount of myopia to -12 D led almost half of the doctors to recommend phakic IOLs - quite a different scenario from the US.

European patients might end up taking advantage of the open borders of the EU, considering the wide range in prices charged for refractive surgery. Patients in France will be charged a base rate of E750 and E1000 for laser treatment. In Portugal they will pay considerably more, while in Germany the price is somewhere between the two.

Most of those polled in the survey did not predict a lot of growth in demand for refractive surgery. French and Italian surgeons predicted that the market would remain flat, while German, Spanish and British surgeons predicted modest growth on the order of 1% or 2%.

"The ESCRS survey indicated that many European surgeons consider wavefront a promising but unproven technology. Some surgeons say they are very interested and excited by the technology, but one third still say they don’t know enough about it. We have a lot of work to do to explain what wavefront is and its potential for improving vision," Dr Colin commented.

It will be important to explain the advantages of custom ablation to interested surgeons. They should understand that the wavefront-guided approach will induce fewer halos and provide better quality of night vision, with less tissue ablation. There is still also plenty of discussion of where to perform wavefront-guided ablation - on the cornea surface as with LASEK or inside the stroma with LASIK, he added.

He offered a few suggestions for surgeons who are ready to catch the next wave of technology. First, anyone considering upgrading to customised ablation should be very experienced and confident with the conventional ablation procedures. An experienced surgical team is another essential element in the transition. It is also vitally important to have a system in place for tracking results and learning from them.

"European surgeons remain cautious about wavefront technology. Nonetheless, I believe that wavefront-guided custom ablation treatment has the potential to provide new energy and enthusiasm for laser refractive surgery in Europe on the part of both surgeons and patients."