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Friday / August 5.
HomemiophthalmologyBeyond the Sphere and Cylinder

Beyond the Sphere and Cylinder

Being particularly attentive to higher order aberrations when refracting and performing cataract surgery can help optimise a patient’s visual quality.

Higher order aberrations (HOAs) are complex vision errors that create difficulty seeing at night, glare, halos, blurring, starbursts patterns and diplopia. These HOAs, known as spherical aberration, coma and trefoil, can exist in normal eyes as well as those that have had surgery or are diseased.

Wavefront technology is a major advance in the field of ophthalmic optics that allows us to measure and treat higher order aberrations as well as the more familiar lower order aberrations (normally measured when refracting patients and expressed in sphere and cylinder).

The goal of reducing higher order aberrations is to maximise image quality and contrast sensitivity – perhaps even beyond that which was once considered ‘normal’.

Wavefront technology is a major advance in the field of ophthalmic optics

The most readily available clinical way to measure higher order aberrations is the Hartmann-Shack aberrometer. An example of such an instrument is the Abbott VISX Wavescan (Figure 1). Hartmann-Shack can have a very high resolution, which is directly related to the number of measuring points. A light source from the Wavescan is focused onto the retina. The retina, acting as a concave mirror, reflects the light. As the reflected rays pass through the optical components of the eye they become warped, or aberrated and exit the pupil in this imperfect state. One hundred and eighty tiny lenses (lenslets) focus this imperfect wavefront onto a screen for analysis. The analysis compares the detected wavefront to an ideal wavefront allowing quantification.

Higher order aberration minimisation can be population-based or individual-based. That is, we can use wavefront data from a population and use that as the goal for our patients undergoing optical correction. Another way is to use wavefront measurements of each individual eye and customise the wavefront treatment according to the measured aberrations.

Cataract Surgery

When cataract surgery is performed with standard intraocular lenses (IOLs) we add spherical aberration to the eye. This is because the crystalline lens, which is removed, normally cancels out at least part of the positive spherical aberration existing in the cornea. It is for this reason that manufacturers of IOLs now offer the surgeon a variety of aspheric designs to implant in the eyes of patients undergoing cataract (or refractive lens) surgery.

It has been shown that spherical aberration of the crystalline lens increases from negative to positive as we age.1 Spherical aberration of the entire eye is nearly zero in the late teens. This occurs because the negative spherical aberration of the crystalline lens neutralises the positive spherical aberration of the cornea. While there are concerns that reducing spherical aberration reduces depth of focus, simple ray diagrams demonstrate that this could not be the case.

An aspheric lens brings light to a single point of focus in an image plane, whereas a spherical lens forms a blur circle of ‘least confusion’ in the same plane. The aspheric lens gives best focus in the image plane. However, 0.50 dioptres in front or behind that image plane, the pencil of light has a certain diameter but it is not greater with an aspheric lens than with a spherical one.

For any given diameter of the pupil there is no difference in the depth of focus between the two lens types. At worst, there is a greater ‘drop off’ in clarity relative to their optimal potential with aspheric lenses once the eye is defocused with refractive error.

Negative spherical aberration can actually provide some added power with pupillary constriction. That’s because the central curvature of a lens with negative spherical aberration is relatively convex compared to the periphery. Whereas a lens which has positive spherical aberration is flatter centrally, causing less refraction to take place and making near images more difficult to focus on. Positive spherical aberration makes it harder to see at near as the pupil constricts.

We see this with hypermetropic LASIK too. The negative spherical aberration, which is induced with central steepening of the cornea affords even a presbyopic eye much better near vision than expected from a near zero refraction for distance. Presbyopic LASIK exaggerates this effect by inducing prolateness of the cornea (making it much steeper near the vertex of the cornea and flatter peripherally). The dilemma is that both types of spherical aberration result in halo formation around the image.

Zero Spherical Aberration the Target

As a result, zero spherical aberration appears to be the best target for most eyes, whether it’s with a laser refractive procedure or cataract surgery. Some negative spherical aberration may help near vision in presbyopes but the best distance clarity will be compromised.

A reduction in spherical aberration following cataract surgery has been shown to translate into better contrast sensitivity.2,3,4 The beneficial effect of such advancements in IOL design would be masked by higher order aberrations that can be induced by incising the cornea. Modern small incision surgery minimises the induction of such aberrations. We must remember that our visual system includes a neurological component that is malleable. The contrast sensitivity, which has been demonstrated with the Tecnis aspheric lens, continues to improve from three months to one year as adaptation takes place.5 Of course, there are other higher order aberrations affecting optic systems such as coma, trefoil and secondary astigmatism but spherical aberration is on axis and rotationally symmetrical making it particularly amenable to manipulation using
aspheric IOLs.

Some experts argue that there may be a worsening of aberrations if the implanted IOL should be slightly decentred or tilted.6,7 However, with current IOL design and capsulorhexis construction, clinical decentration has been shown to be < 0.09mm and tilt is < 3.2 degrees.8

Contrast Sensitivity

Numerous studies have demonstrated a positive effect on contrast sensitivity in using aspheric IOLs in cataract surgery. Some have even shown a positive effect on visual acuity.9

At present, the default IOL used in my cataract patients is the Tecnis Aspheric ZCB00 from Abbott Medical Optics. This is a one-piece lens with an aspheric optic correcting 0.27 micrometres of spherical aberration in attempt to reduce corneal spherical aberration to zero. Of course, the exact effective correction depends on the amount of positive spherical aberration in the individual’s cornea though the average corneal SA has been shown to be + 0.27 microns and the optimal total SA to be close to zero or slightly minus.10 Naturally, the closer the postoperative result is to emmetropia, the greater the benefit of aberration correction. Now this is even more attainable with the recent availability of toric corrections on most IOL platforms, including the Tecnis Toric.

The Tecnis has many of the other features which make an IOL attractive to surgeons, such as hydrophobic acrylic composition, allowing it to be folded and inserted through a micro incision, three point fixation for stable centration and refractive outcomes, and an edge design minimising posterior capsular opacification. The Tecnis lens also has the highest ABBE number of available IOLs, which means it has the least amount of chromatic aberration, further contributing to overall quality of visual outcomes.

Small Incision Lenses

With the evolution of small incision lenses that accurately correct spherical and cylindrical refractive error and also reduce aberrations, surgeons have the opportunity to further optimise the quality of postoperative vision. This refers to vision that is excellent not only in optimal light conditions but even under low light or in poor contrast situations. Given that we are now implanting IOLs into younger, active patients, this is particularly important.

For decades, contact lenses have been known to induce spherical aberration due to their steep curvatures, however, is there the potential to optimise vision by incorporating wavefront guided optical modifications into contact lenses?

As already stated, there is great variation in the degree of spherical aberration in a population though it is nearer to zero in people aged around 15 to 20 years than when they are older. Furthermore, spherical aberration is not the dominant higher order aberration in most eyes. Coma and trefoil have much larger magnitudes. Yet to correct these non-radially symmetric aberrations you
would need to have rotationally stable contact lenses made in a way that is not radially symmetric.

Rigid Gas Permeable Lenses

Rigid Gas Permeable (RGP) lenses are known to improve acuity in eyes with corneas that have pathology or have been highly modified by crude refractive surgery. RGP lenses can improve vision in such eyes far beyond what is possible with standard sphere and cylinder correction. However, this is not because the RGP lens is manufactured with wavefront modification but because it’s front surface forms a new, regular refracting surface and the tear film fills in the difference between the cornea and the back surface of the lens. But RGP lenses are mobile over the cornea and are designed to move with each blink. They are difficult to stabilise over an eye.

Soft Lenses

Soft lenses, on the other hand, are held in position on the cornea by a set of forces due to the fact that they deform to take on the shape of the cornea. The deformation generates radial stress in the lens and gravity assists in centering the lens at a position of equilibrium. A prism ballast, as used in toric contact lenses, provides additional rotational stability, usually within five degrees between blinks.

Impact of Rotation

Does this small degree of rotation negate any beneficial effect a wavefront-designed contact lens will have on higher order aberrations?

It has been shown that coma is particularly tolerant to misrotation of a correcting surface. A visual benefit is likely to be present with misrotation of up to 60 degrees. Astigmatism correction with misrotation of up to 30 degrees is still of benefit and trefoil correction is of value with misrotation of up to 20 degrees. Rotations from the ideal any more than this will provide a retinal image poorer than leaving the aberrations uncorrected.

Therefore, a contact lens designed to correct more than just spherical aberration needs to be soft, prism ballasted and wavefront modified –preferably modified after fitting and settling, as some distortion would result with squeezing of the soft lens onto the cornea by the eyelid. Manufacturing technology is available to incorporate a wavefront-derived surface onto a soft contact lens but a business model is necessary to make production of such lenses, likely to be demanded in a disposable format, feasible.

In the near future, we may see referring optometrists take a greater interest in the type of IOLs implanted in their patients. Perhaps they may even measure the wavefront error in the cornea of their patients to assist in the selection of the optimal IOL, as together; we pursue the best possible vision for our patients.

Dr. Con Moshegov is the principal surgeon at Perfect Vision Laser Correction. He trained in Sydney and spent two years undergoing an advanced Fellowship in Cornea, External Diseases and Refractive Surgery in the United Kingdom. Dr. Moshegov now specialises in refractive and cataract surgery.

References

1. Glasser A, Campbell MCW. Presbyopia and the optical changes in the human crystalline lens with age. Vision Res. 1998; 38: 209-29.

2. Belluci R, Scialdone A, Buratto L, et al. Visual acuity and contrast sensitivity comparison between TECNIS and AcrySof SA60AT intraocular lenses: a multicentre study. J Cataract Refract Surg. 2005; 31:712-7.

3. Kershner RM. Retinal image contrast and functional visual performance with aspheric, silicone and acrylic intraocular lenses. Prospective evaluation. J Cataract Refract Surg. 2003; 29:1684-94.

4. Packer M, Fine IH, Hoffman RS, Piers PA. Improved functional vision with a modified prolate intraocular lens. J Cataract Refract Surg. 2004; 30: 986-92.

5. Artal P, Chen L, Fernandez EJ, et al. Neural compensation of the eye’s optical aberrations. J Vis. 2004; 4-281-7.

6. Buhren J, Kohnen T. Application of wavefront analysis in clinical and scientific settings. From irregular astigmatism to aberrations of a higher order-part 1. Basic Principles. Ophthalmology. 2007; 104: 909-23.

7. Rocha KN, Soriano ES, Chamon W et al. Spherical aberration and depth of focus in eyes implanted with aspheric and spherical intraocular lenses: a prospective randomised study. Ophthalmology. 2007: 114: 2050-4.

8. Mester U, Sauer T, Kaymak H. Decentration and tilt of a single-piece aspheric intraocular lens compared with the lens position in young phakic eyes. J Cataract Refract Surg. 2009; 35(3):485-90.

9. Martinez Palmer A, Palacin Miranda B, Castilla Cespedes M, et al. [Spherical aberration influence in visual function after cataract surgery: prospective randomized trial.] Arch Soc Esp Oftalmol. 2005; 80(2): 71-8. Spanish language.

10. Douglas D. Koch MD and Li Wang MD. Custom optimization of intraocular lens asphericity. Trans Am Ophthalmol Soc 2007;105:36-42. http://saavision.com/

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