Laser-based corneal refractive surgery using a traditional monofocal approach has a long track record of success; for 2 decades, it has provided good postoperative distance UCVA to millions of people with a high degree of patient satisfaction.¹ A variety of ablation techniques have been described, including PRK, LASIK, and other variants. These approaches generally correct vision only for distance; in presbyopic patients, reading or computer glasses may still be needed postoperatively to see at near and intermediate distances, respectively.

Recently, interest has grown in developing a multifocal approach to corneal refractive surgery, thereby providing presbyopic patients with greater spectacle independence postoperatively. Efforts to date have centered on creating an aspheric corneal surface to focus light from distance and near on the fovea. An empirically based, noncustomized approach has shown greater efficacy in hyperopic patients than in myopes.² In this most widely investigated multifocal LASIK technique, a hyperopic distance treatment is performed and a supercentral near addition is superimposed.

Although this multifocal approach shows promise, some questions remain. How does the surgeon determine how much near add is appropriate for each patient? What is the optimum size for the distance and near optical zones and for the transition zone between them? How does the surgeon take into account and treat existing higher-order aberrations (HOAs), and how does he avoid inducing aberrations? Lastly, what procedure can be offered to myopic patients, as the above-mentioned approach works best for hyperopes?

NONEMPIRICAL APPROACH
My colleagues and I have described³ and patented⁴ a nonempirical approach for the correction of presbyopia. This technique is customized based on the needs of each patient. An analytical model is used to establish the correct treatment modality, and this allows the clinician to predict the effect of the near add on the patient's distance vision. In this technique, the individual's pupil dynamics are analyzed, and this information is combined with measurement of the corneal and total-eye wavefront aberrations to provide multifocality while optimizing visual quality.

Wyatt and colleagues⁵ showed that the pupil in the normal eye is not centered within the circle described by the limbus. In dark-adapted eyes, the pupil is located a mean 0.25 mm nasal and 0.14 mm superior to the limbus center. In photopic conditions, the pupil moves even more nasal (0.28 mm) and superior (0.18 mm). There is a tendency for right and left eyes of an individual to show mirror symmetry of shape, but there is variability in pupil response within and across patients. Also, there is no relationship between variation in pupil diameter and the degree of pupil shift from dark to light conditions.

Taking these individualized shifts in pupil position into account, the near add in a multifocal corneal ablation must be placed so that it is centered over the photopic pupil, as most reading occurs in a well-lit environment. Because the pupil position shifts when conditions change from dark to light, the near add must be properly placed. If a multifocal treatment has concentrically placed distance and near optical zones, the near add will be displaced with reference to the photopic pupil. Even if the distance and near zones are displaced so as to account for some empirical mean shift in pupil location, the near zone will be decentered in some patients due to inter-individual variability.

INDIVIDUAL GEOMETRY
A nonempirical presbyopia treatment will take into account the individual's pupil geometry dynamics and use the eye's HOAs to induce a multifocal effect. The process begins by measuring the pupils in dark (distance) and light (near) conditions and calculating their placement in respect to the reference domain of the limbus (Figure 1). The center points and radii of the two pupil domains can be used for subsequent calculations.

It is important to understand that concentric and nonconcentric variations in pupil diameter affect the expression of the eye's optical wavefront. Guirao and colleagues⁶ showed that decentration of an otherwise ideal correction through rotation or translation reduced the optical benefits of the correction. Variations in pupil size and position that change the optical zone in relation to the multifocal correction effectively change the nature of the correction.

A concentric change in pupil diameter can alter the effect of HOAs in the cornea. Consider constriction of the pupil over the Zernike fourth-order term spherical aberration (Figure 2). During constriction, the brim of the familiar Mexican hat representing spherical aberration (Z 4,0) is masked, and second-order defocus (Z 2,0) increases, resulting in induced myopia.

Similarly, a nonconcentric pupillary change can also alter the effect of HOAs. Vertical coma (Z 3,-1) in a 5-mm pupil (Figure 3A) becomes myopic defocus (Z 2,0) plus cylindrical defocus (Z 2,-2) when the pupil is constricted to 3 mm, and the center point is shifted (Figure 3B).

Because pupil constriction affects the distribution of Zernike terms, and therefore changes the optical qualities of the eye (Figure 4), aspheric multifocal corneal ablations must be designed to take these alterations into account.

BASED ON PUPIL CHARACTERISTICS
The target multifocal wavefront design may be created based on the individual patient's pupil characteristics and existing HOAs (Figure 5). Using calculations based on the location and size of the distance pupil and near pupil with reference to the limbus, a target corneal profile can be generated with qualitative and quantitative precision. With the proper diagnostic information, a variety of metrics can be computed for the final resulting correction, including point spread function, modulation transfer function, and root mean square of the total HOAs. Solutions for different candidates can be designed by varying the diameter of the treatment zones or the amount of addition for near. The effect of these corrections can then be simulated using adaptive optics.

The final step in this method of nonempirical correction of presbyopia is to apply the arrived-at multifocal correction through some therapeutic device—whether this takes the form of customized corneal ablation with an excimer laser, a custom-designed contact lens, a preview lens for demonstration, or the light-adjustable optic of an implanted IOL. The result will be a customized correction of presbyopia using the optimal characteristics of the multifocal wavefront calculated from measurements of the individual's eye.

CONCLUSION
Although it is possible to model the optimal characteristics of a multifocal wavefront for the correction of presbyopia, this strategy must be evaluated in clinical trials to demonstrate efficacy. However, just as Munnerlyn and colleagues¹ pioneered myopic and hyperopic ablations 20 years ago, we believe that we have set the basis for customized multifocal wavefront correction of presbyopia in the future.

Damien Gatinel, MD, PhD, is a cataract, corneal and refractive surgery specialist. He is an Assistant Professor and Head of the Anterior Segment and Refractive Surgery Department at the Rothschild Ophthalmology Foundation, Paris. Dr. Gatinel is a member of the CRST Europe Editorial Board. He states that he holds the patent on the technique for presbyopia correction described in this article. He may be reached at tel: +33 1 48 03 64 82; e-mail: gatinel@aol.com.

1. Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg. 1988;14(1):46-52.
2. Jackson WB, Mintsioulis G, Lafontaine MD. International multifocal presbyopia clinical trial: long term outcomes. Paper presented at: annual meeting of the European Society of Cataract and Refractive Surgeons; September 11, 2007; Stockholm, Sweden.
3. Gatinel D, Malet J, Azar DT. Multifocal visual correction for presbyopia compensation using a customized non-empirical model. Paper presented at: Presbyopia International meeting; September 12, 2008; Berlin, Germany.
4. Azar DT, Gatinel D, Malet J, inventors. Massachusetts Eye & Ear Infirmary, assignee. Ocular wavefront-correction profiling. US Patent 7,341,345. March 11, 2008.
5. Wyatt HJ. The form of the human pupil. Vision Res. 1995;35(14):2021-2036.
6. Guirao A, Williams DR, Cox IG. Effect of rotation and translation on the expected benefit of an ideal method to correct the eye's higher-order aberrations. J Opt Soc Am A Opt Image Sci Vis. 2001;18(5):1003-1015.