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Refractive Surgery | Apr 2010

P-Curve Presbyopic LASIK

A new concept in refractive surgery.

The concept of presbyopic LASIK (presby-LASIK) is fascinating: to create a corneal shape through excimer laser corneal ablation that corrects presbyopia symptoms and provides patients with excellent UCVA for far and for near without the use of glasses or contact lenses. When presby-LASIK is attempted, several visual factors are considered, including offering patients the best visual acuity for far and near. We aim to provide high quality vision— not merely Snellen acuity but visual quality that enables patients to perform daily tasks without prosthesis and in the absence of the halos, glare, and similar negative phenomena that are typical of multifocality. The lesson we learned from IOLs is that visual symptoms are related to the shape of the lens and its optical characteristics.

Many pioneers have laid a strong foundation for presby- LASIK over the past decade. Although progress in this field started slowly, it is accelerating with the establishment of scientific principles by surgeons such as Telandro, Avalos, Alió, Seiler, and our own research group at the Istituto Laser Microchirurgia Oculare in Brescia, Italy.1-5

Several unique techniques have been proposed, each one with its own set of parameters and performed using different excimer lasers. The multifocal corneal patterns investigated include central-near and peripheral-far, central- far and peripheral-near, and paracentral steep area presby-LASIK. The principles of each algorithm may be based on the dioptric power of refractive error and presbyopia- correction calculation, corneal asphericity quotient (Q-value) and higher-order spherical aberrations changes, or optical and transition zone manipulation.

ASPHERICITY
The Q value of a surface describes its deviation from a perfect spherical surface. Seiler et al5 introduced the Q value as a way to obtain better visual performance using a myopic astigmatic ablation. The human cornea is an aspheric prolate ellipsoid, and the crystalline lens and retinal surfaces are aspheric. There is no optical reason to perform a flat surface treatment on the cornea or to implant a multistep lens without considering physiologic ocular asphericity.

In the nonpresbyopic eye, negative spherical aberrations (induced as a result of accommodation)6 increase the depth of focus and in turn enhance near vision. With age and ensuing presbyopic symptoms, the lens loses the ability to induce such negative spherical aberrations. We believe that inducing negative spherical aberrations is a major factor in correcting presbyopia through presby-LASIK. However, inducing negative spherical aberrations through Q value customization alone may not be as accurate as we had hoped because Q value is a shape factor that does not consider the size of a surface. Patients have different corneal sizes and different curvature radii, which means that the effect of a Q value in one eye may yield slightly different results in another (Figure 1).7 Therefore, the Q-value is just one component of achieving presbyopia correction through inducing negative spherical aberrations.

P-CURVE TECHNIQUE
We have studied an ablation algorithm for correction of presbyopia that is able to (1) manage the Q value and predetermine a certain value (ie, P factor; variation of Q toward the negative side), and (2) redistribute the aberrations, especially spherical aberration, and determine a range for ablation. This algorithm not only creates quality UCVA but also eliminates halos, glare, and night vision problems.

Several factors including Q value, P factor, spherical aberrations, and other higher-order aberrations are taken into account to produce a customized ablation profile for each patient. The algorithm follows the patterns of asphericity and aberration range that are known to be present during accommodation in the young eye. Thus far, postoperative data using the Pcurve (Figures 2 and 3) is encouraging at 1 year, and survey results have shown high patient satisfaction.

We also established the Pinelli binocular index (PBI) to determine the relationship between binocular far and near vision in pre- and postoperative P-curve patients and detect and manage monocular and binocular (far and near) vision. This is theoretically perfect at the value of 1.0 (20/20 and J1); however, patients also seem to accept 0.9 (20/20 and J2 or 20/20- and J1; Figures 4 and 5).

CONCLUSION
P-curve presby-LASIK targets emmetropia in both eyes. One surgeon (RP) has performed this bilateral technique in 420 consecutive eyes using the same excimer laser and microkeratome. At 1 year postoperative, the retreatment rate was only 3.09%, and most patients were satisfied with J2 or J3 near vision without any problem for distance vision. It is our hope that the P-curve algorithm will continue the evolution of presby- LASIK and help surgeons who are skeptical8 to consider it as a standard of care for the presbyopic patient in the near future. Obviously more data and results are necessary, but presby-LASIK may become the standard algorithm for many surgeons around the world.

David Cacciatore, OD, works in the Department of Research and Development at the Istituto Laser Microchirurgia Oculare, Brescia, Italy. Mr. Cacciatore states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +39 030 24 28 343; e-mail: cacciatore@ilmo.it.

David Cacciatore, OD, works in the Department of Research and Development at the Istituto Laser Microchirurgia Oculare, Brescia, Italy. Mr. Cacciatore states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +39 030 24 28 343; e-mail: cacciatore@ilmo.it.

Hytham El-Shawaf, MSc, is an Assistant Researcher at the Research Institute of Ophthalmology, Giza, Egypt, and a fellow of the Istituto Laser Microchirurgia Oculare, Brescia, Italy. Dr. El-Shawaf states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +2 012 36 050 37.

Roberto Pinelli, MD, is the Scientific Director of Istituto Laser Microchirurgia Oculare, Brescia, Italy. Dr. Pinelli states that he has no financial interest in the products or companies mentioned. He is a member of the CRST Europe Editorial Board and may be reached at tel: +39 030 24 28 343; e-mail: pinelli@ilmo.it.

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