Eye to Eye - Sponsored by Pharmacia Ophthalmology

Improving the quality of vision with a new wavefront designed IOL

AT a EuroTimes satellite symposium of the XX ESCRS Congress entitled "Improving the quality of vision
with a new wavefront designed IOL", a distinguished panel of ophthalmologists described how an improved understanding of both the physics of functional vision and the anatomical changes that take place in the human eye have contributed to the design of a revolutionary new intraocular lens.

The symposium also heard the latest results from several trials with the new lens. The moderator Thomas Neuhann MD introduced the session with a reflection on how until now IOL designers have lagged behind spectacle designers in providing people with good optical results.

"The topic of this symposium is one that has been unduly neglected. I have long been unable to understand why it is that, while we have known a great deal for decades about refined physical optics, what we actually do in surgery is use optical devices such as IOLs that are completely primitive in their optical design. After all the eye that we're dealing with in my profession is first and foremost an optical apparatus and therefore an IOL with two spherical surfaces or one plano and one spherical are probably the most primitive optical devices that can be imagined.

"We have been using eye glasses- spectacle glasses-that have been infinitely more refined for decades. Finally, we are uniting the art of surgery and basic optical physics and it gives me great pleasure to moderate this symposium where we have an distinguished faculty to talk about multiple aspects of a wavefront IOL," he said.

Wavefront technology and the cataract surgeon
Paul Rosen MD told the symposium how the same wavefront technology that has been changing the face of refractive procedures such as LASIK and PRK in recent years has now begun to make an impact on cataract surgery.

"Wavefront analysis has captured the imagination of refractive surgeons but it is only now coming into the realm of the cataract surgeon. It has made it possible to design IOLs which reduce the higher order aberrations which remain after cataract surgery and increase contrast sensitivity and improve visual performance."

The principle of wavefront analysis is that when a plane wave reflects back and out from the eye the resulting distortions of the wavefront will correspond to defects in the optical system. The distortions that are most important to vision include lower order aberrations such as defocus and cylinder and higher order aberrations such as coma and spherical aberration, the latter of which is particularly relevant to cataract surgery, Dr Rosen explained.

To measure aberrations there are three types of aberrometer, Hartmann-Shack, Tscherning, and Tracey. What they all have in common is that they compare a grid image with a perfect theoretical grid representing the perfect wavefront. The deviations may then be averaged by complex mathematical formulae which provide the root mean square value, either of each aberration individually or of the aberration profile in total.

It is then possible to classify the aberration measurements according to mathematical descriptions of baser sets, Zernike polynomials being the most popular, he said. One can also interpret the aberrations in terms of the effect they have on image formation, that is, as a point-spread function or modulation transfer function.

The point-spread function measures how well the optical system will focus parallel rays from a point source. It measures the image point in terms of its diameter and the height and distribution of light intensity. Another interpretation of the visual effect of optical aberrations is the modulation transfer function (MTF), which relates to an optical system's ability to resolve a sinusoidal grating.

"The modulation transfer function describes the relationship between the spatial frequency and modulation amplitude, that is, the change in the luminance of an image as it goes into the eye. It really describes the efficiency of an optical system and corresponds closely to subjective measurements of contrast sensitivity."

The possible applications of wavefront technology to cataract surgery only began to become evident over the last few years, as researchers learned to elucidate more precisely the contribution that changes in the crystalline lens make to the degradation of vision with age. Several recent studies have shown that while the cornea remains fairly stable as one gets older, the crystalline lens changes in terms of both its contour and refractive index.

"Wavefront errors can explain what appears to be poor visual acuity as measured by Snellen acuity or monocular diplopia in the presence of what appears to be minimal cataract, it is a very sensitive way of assessing the optical system of the eye."

When a patient has had a cataract removed, most of the remaining aberrations are those naturally present in the cornea. Corneal topography therefore provides enough information for the design of IOLs which will reduce the eye's optical aberrations.

"If you can measure these aberrations you can use them in IOL designs the aim being principally to reduce spherical aberration and improve contrast sensitivity function which is more relevant to what the patient actually sees then ordinary Snellen visual acuity. And I would really like to congratulate Pharmacia for what is a very clever yet very simple idea."

Quality of vision with a wavefront IOL
One of the important revelations that wavefront analysis has brought to cataract surgery is an increased awareness of what actually constitutes functional vision. Ophthalmologists now recognise that the optical aberration profile of the eye provides a much more useful guide to a patient's visual perception in real world conditions than standard visual acuity tests. This is borne out by subjective measurements of contrast sensitivity, Jack Holladay MD told the symposium.

"We've all learned that, when it comes to assessing vision, standard visual acuity testing only tests our ability to see small objects of high contrast. This part of vision testing is extremely important, but is not sufficient to determine visual performance for other tasks. We're so impressed when we hear these refractive outcome studies and they tell us that the patient is 20/10, but that's only one small tip at the end of the curve when it comes to the optical performance of the eye.

"Contrast Sensitivity Testing determines our ability to see larger objects at the lowest possible contrast. These additional values give us measures which would relate to how well a person would see a grey truck in a dense fog. Both Snellen Visual Acuity and Contrast Sensitivity are subjective measures of the entire visual system," Dr Holladay said.

He added that wavefront analysis, in contrast, provides an objective measure of the optical system of the eye. The wavefront profile can then be converted to a point spread function (PSF) or modulation transfer function (MTF).

The MTF objective measure of the optical system is very similar to the subjective findings of contrast sensitivity of the entire visual system. These independent measures can be very helpful diagnostically in determining whether a visual disturbance is optical or sensory, he said.

"So, in essence, you can think of wavefront aberrometry as an autorefractor that gives us objective information about the optical quality of the eye and you can think of contrast sensitivity as the patient's subjective visual performance.

"And while it is true that wavefront analysis is improving, just as autorefractors have a correlation with manifest refraction of about 70% to 80%, wavefront aberrometry is only about 80% accurate for measuring a patient's refraction and other aberrations," Dr Holladay said.

Contrast sensitivity measurements can be performed with low contrast Snellen Letters or sinusoidal gratings. Clinicians and patients prefer letters, but visual scientists prefer the sinusoidal gratings because they are "pure" and much easier to analyse mathematically.

Using either letters or gratings, determining a persons contrast threshold from small objects (20/20 or 30 cycles per degree) to large objects (20/200 or 3 cycles/degree) yields a much more comprehensive determination of the visual performance.

A number of studies have shown that contrast sensitivity gets worse with age and that this is due to an increased spherical aberration of the eye from changes in the crystalline lens, Dr Holladay noted.

"In the youthful eye, the crystalline lens has a negative spherical aberration while the cornea has a roughly equal and opposite positive spherical aberration. As a result people under 20 years of age can generally achieve a nearly perfect foveal image.

"However, with age the crystalline lens becomes progressively more positive in spherical aberration while the cornea remains unchanged. The quality of vision therefore decreases with age, beginning when we are very young," he said.

Until now, IOL manufacturers have not taken the spherical aberration of the eye into account in their lens designs. Apart from the Tecnis lens, all current IOLs have spherical surfaces, which like the aged crystalline lens increases the positive spherical aberration of the eye, he pointed out.

"A spherical IOL in a patient who has had cataract surgery has the same quality of vision as a 60 to 70 year-old without a cataract. That means that if you are 20-years-old and you take out a clear crystalline lens or cataract and put in a spherical IOL, your vision will be the same as a 60 to 70-year-old without a cataract. The difference with the Tecnis lens is that you see like the 20-year-old," Dr Holladay said.

The Tecnis lens has a modified prolate anterior surface that is designed to compensate for the positive spherical aberration of the cornea. In studies conducted to date, the clinical outcome with the lens has correlated closely with those predicted by optical theory, he pointed out.

"Wavefront spherical aberration in a patient with a Tecnis lens is almost zero whereas with spherical lens it is eight times higher. Based on this, contrast sensitivity improvement should be 39% and the improvements obtained in clinical studies have been between 38% and 40%, so they agree almost exactly. It has been rare in my life that anything has come as close in actual performance as our theoretical expectations," Dr Holladay said.

Initial clinical results comparing the Tecnis IOL and the acrylic AR40e
In the first of the comparative trials presented at the symposium, the improvement in contrast sensitivity afforded by the Tecnis lens enabled cataract patients to see as well in dim light as patients with conventional acrylic IOLs could see under full illumination, said Mark Packer MD.

In the prospective, randomised study, 21 cataract patients underwent implantation with either the Tecnis Z9000 IOL or the spherical AR-40e IOL. At three month's follow-up, eyes implanted with the Tecnis lens had a contrast sensitivity under mesopic conditions almost identical to that which eyes with a spherical lens had under photopic conditions, he noted, adding:

"Putting in the Tecnis IOL is like trading night for day for these patients because there is no statistically significant difference between the Z9000 Tecnis mesopic contrast sensitivity and the AR40e photopic contrast sensitivity. When I saw these results I just about fell out of my chair because that means that eyes I've been putting standard spherical IOLs into are seeing about the same at 85 candles per square metre as those with the Tecnis are seeing at three candles per square metre ."

Dr Packer noted that eyes implanted with the modified anterior prolate aspheric IOL achieved between 0.11 and 0.32 log units higher contrast sensitivity at all spatial frequencies under photopic conditions and between 0.13 and 0.27 log units higher contrast sensitivity at all spatial frequencies under mesopic conditions.

These differences reached statistical significance at 6, 12 and 18 cycles per degree with photopic testing, and at 1.5 and 3 cycles per degree with mesopic testing.

Comparing the data to that obtained in driving simulation studies, he pointed out that a driver with the Tecnis IOL would be expected to detect a pedestrian on the road at night 20 metres further away than would a driver with the spherical lens.

The patients in the study were aged 50 to 80 years and had a visually significant cataract in one eye with good potential acuity and no other ocular pathology.

None of the patients had high corneal cylinder preoperatively and all had a mesopic pupil size greater than 4.0 mm. Dr Packer and his associates tested best corrected contrast sensitivity using a sine wave grating FACT chart at both mesopic and photopic luminance.

The Tecnis IOL has an optic with a modified-prolate anterior surface made of UV-absorbing polysiloxane, a next-generation silicone. It has an overall length of 12 mm and an optic diameter of 6.0 mm. The IOL's design includes the prolate Z-SHARP™ Optic Technology as well as the square optic edge design of the CeeOn 911 IOL to prevent posterior capsule opacification.

"In order for patients to benefit from this lens it has to be decentred less than 0.4 mm and tilted less than 7º. Fortunately it is within our ability to achieve this with continuous curvilinear capsulorhexis and in-the-bag placement of the IOL."

Dr Packer noted that a comparison between the patients in the study and healthy subjects aged 20 to 50 years showed that contrast sensitivity was actually better in the Tecnis patients than it was in the 20 to 30 year-olds while those with a spherical IOL had a vision more comparable to the 30 to 50 year olds.

"The TECNIS IOL provides superior functional vision as measured by sine wave grating contrast sensitivity testing. Vision need not deteriorate with age," he added.

Comparative trial with the Tecnis and the acrylic SA60T
Dr Packer's findings were supported by those of an Italian trial presented at the symposium. In a study involving 30 cataract patients who underwent bilateral implantation of either the Tecnis lens or the acrylic SA60 IOL, contrast sensitivity was better by 20% in eyes with the new IOL for all testing conditions, said Robert Bellucci MD.

"The Z9000 IOL improved contrast sensitivity at all spatial frequencies above 1.5 cycles per degree under every lighting condition compared to the acrylic IOL. Improvements were even better in mesopic conditions, especially at higher frequencies and with glare. We found also that threshold central perimetry was improved."

In photopic conditions contrast sensitivity was significantly better than the acrylic lens. In fact, at 3, 6, 12 and 18 cycles/degree the values were double and more than double with the Tecnis lens. The results were similar with glare added to photopic conditions with a statistically significant difference at 3, 6, and 12 cycles per degree. Under mesopic conditions the difference was statistically significant at 3, 6 and 12 cycles /degree and with glare at 3, 6, 12 and 18 cycles/degree.

Dr Bellucci and his associates also found that the Tecnis lens improved perifoveal vision. When they performed a Humphrey visual field test they found that while there was no difference between the two lenses as regards the foveal threshold, there was a statistically significant difference in the mean defect in favour of the Tecnis. Moreover, when they divided the 30 degrees measured by the amp apparatus into central 10 degrees and a paracentral 10 to 20 degrees, the Tecnis had a significantly better luminance threshold in both areas.

The patients in the study were aged between 50 and 70 years. All had senile cataracts and no other ocular pathology. All underwent random bilateral implantation with either the Tecnis or the SA60 IOL. In all eyes the investigators attempted to make the capsulorhexis smaller than the optic of the IOL and performed a careful cortical cleanup. They enlarged the incision to 4.1 mm for IOL implantation.

Dr Bellucci noted that the optics of the two IOLs were both 6 mm in diameter and had a similar sharp edge design. However, the two lenses had different optical materials; the Tecnis a new generation silicone (polysiloxane) and the SA60 IOL a hydrophobic acrylic. The haptics of the two IOLs were different in terms of both design and material. Those of the Tecnis lens had capsular loops made of PVDF, while those of the SA60 IOL lens were made of PMMA.

Visual acuity at one month was similar for the two IOLs. The mean UCVA was 20/32 for eyes with the Tecnis lens and 20/33 for eyes with the SA60 IOL, while BCVA was 20/22 for eyes with the Tecnis and 20/22 for the SA60 IOL. With regard to PCO, the posterior capsules in eyes with the Tecnis lens were "crystal clear" in all eyes at two months with some peripheral folds. In eyes with the SA60 IOL lens there was some material on the posterior capsule in five eyes, and no peripheral folds.

"Correcting the spherical aberrations of the eye was expected to produce a significant increase in contrast sensitivity and in central threshold perimetry and this is what we found in clinical practice with the Tecnis lens."

Intra-individual comparison of Tecnis and silicone IOLs
Ulrich Mester MD told the symposium that neither the lens material nor any aspect of lens design other than the prolate anterior surface was responsible for the improved contrast sensitivity obtained by patients with Tecnis lens.

In an intra-individual comparison involving 45 patients implanted with the Tecnis IOL in one eye and the silicone SI 40 bi-convex lens in the other, eyes with the Tecnis IOL had significantly better BCVA and contrast sensitivity after three months.

"Although the eyes with the Tecnis IOL showed significantly better BCVA after three months, the improved quality of vision is more apparent when assessing low contrast visual acuity and contrast sensitivity. The difference between the IOLs was significant under photopic conditions and even more pronounced under mesopic conditions."

In addition, wavefront measurements revealed no significant spherical aberration in the eyes with the Tecnis IOL while there was significant positive spherical aberration in the eyes with the SI 40, he said.

In all patients the same surgeon performed the cataract procedures on both eyes. All patients had identical mesopic and photopic pupils and corneal spherical aberration preoperatively, he noted. Both lenses in the study were made from second-generation silicone with a high refractive index. The most important difference between the two lenses was its optic design, he said.

Dr Mester and his associates achieved similar results in another intra-individual study which compared the Tecnis IOL with the CeeOn Edge 911. The study was identical to the comparative trial with the SI 40 lens he noted. They found that three months after surgery the average spherical aberration was 0.08 mm for the CeeOn Edge 911 and almost zero for the Tecnis IOL. The mesopic and scotopic contrast sensitivity were also significantly better with the new lens, he said.

"The spherical aberration of the eye after cataract surgery can be eliminated by modifying the anterior surface of the IOL. The Tecnis Z 9000 lens compensates for the positive spherical aberration of the older eye and this leads to a significant improvement in contrast sensitivity and mesopic visual quality."