The concept of presbyopic LASIK (presby-LASIK) is fascinating: to create a corneal shape through excimer laser ablation that corrects presbyopic symptoms and provides patients with excellent UCVA for far and near without the need for glasses or contact lenses. When attempting presby- LASIK, we have several goals, including offering patients the best visual acuity for far and near. We aim to provide not merely high-quality Snellen acuity, but also visual quality that enables patients to perform daily tasks without spectacles and without the halos, glare, decreased contrast sensitivity, and other 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. Progress in this field 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
Each of several unique proposed techniques has its own set of parameters and is performed using different excimer lasers. The multifocal corneal patterns investigated include central-near and peripheral-far, central-far and peripheral-near, and paracentral steeparea presby-LASIK. The principles of each algorithm may be based on the dioptric power of refractive error and presbyopia-correcting calculation, corneal asphericity quotient (Q value) and higher-order spherical aberrations, or manipulation of the optical and transition zones.
The Q value of a surface describes its deviation from a perfect sphere or its level of spherical aberration. Seiler et al5 introduced the Q value as a way for the surgeon to obtain better visual performance for the patient with 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 multistepped lens without considering physiologic ocular asphericity.
In the nonpresbyopic eye, negative spherical aberrations (caused by 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 produce 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 the Q value is a shape factor that does not consider the size of a surface. Patients have different corneal sizes and different radii of curvature, which means that the effect of a Q value on any one eye may yield slightly different results in another (Figure 1).7 Therefore, the Q value is just one component of achieving presbyopic correction through inducing negative spherical aberrations.
P CURVE TECHNIQUE
We have studied an ablation algorithm for the correction of presbyopia that is able to manage the Q value (asphericity) and predetermine the value needed (ie, P factor variation shifting the Q value toward the negative side). It can also redistribute the aberrations, especially spherical aberration, and determine a range for ablation. We find this algorithm not only creates quality UCVA but also eliminates halos, glare, and night vision problems.
Factors that are taken into account to produce a customized ablation profile for each patient include Q value, P factor, sphere, and other higher-order aberrations. The algorithm follows the patterns of asphericity and aberration range that are known to be present during accommodation in the young eye. Thus far, 1-year postoperative data using the P curve (Figures 2 and 3) are encouraging, and survey results have shown patients’ high degree of satisfaction.
We also established the Pinelli binocular index to determine the relationship between binocular far and near vision in pre- and postoperative P curve patients, which helps in quantifying the gain the patient experiences. This is theoretically perfect at the value of 1.0 (20/20 and J1), but patients also seem to accept 0.9 (20/20 and J2 or 20/20- and J1 (Figures 4 and 5).
P curve presby-LASIK targets emmetropia in both eyes and increases the depth of focus by increasing the negative spherical aberration. One author (RP) has performed this bilateral technique on 420 consecutive eyes using the same excimer laser and microkeratome. One year postoperatively, the retreatment rate was only 3.09%, and most patients were satisfied with J2 or J3 near vision and had no problems with distance vision. It is our hope that the P curve algorithm will continue the evolution of presby-LASIK and help surgeons who are skeptical to consider it for presbyopic patients in the near future. Although more data and results are needed, 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. Dr. Cacciatore may be reached at +39 030 24 28 343; firstname.lastname@example.org.
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. Mr. El-Shawaf may be reached at +2 012 36 050 37.
Roberto Pinelli, MD, is the scientific director of Istituto Laser Microchirurgia Oculare, Brescia, Italy. Dr. Pinelli is a member of the CRST Europe editorial board and may be reached at +39 030 24 28 343; email@example.com.
- Telandro A.Pseudo-accommodative cornea:a new concept for correction of presbyopia.J Refract Surg.2004;20:S714-S717.
- Avalos G,Silva A.Presbyopia LASIK-the PARM technique.In:Agarwal A,ed.Presbyopia,a Surgical Textbook.Thorofare,NJ:Slack Inc;2004:139-146.
- Alió JL,Chaubard JJ,Caliz A, et al.Correction of presbyopia by Technovision central multifocal LASIK (presbyLASIK).J Refract Surg.2006;22:453-460.
- Pinelli R,Ortiz D,Simonetto A,et al.Correction of presbyopia in hyperopia with a center-distance,paracentral-near technique using the Technolas 217z platform. J Refract Surg.2008;24:494-500.
- Koller T,Iseli HP,Hafezi F,et al.Q-factor customized ablation profile for the correction of myopic astigmatism.J Cataract Refract Surg.2006;32(4):584-589.
- Ninomiya S,Fujikado T,Kuroda T,et al.Changes of ocular aberration with accommodation.Am J Ophthalmol. 2002;134(6):924-926.
- Tuan KM,Chernyak D.Corneal asphericity and visual function after wavefront-guided LASIK.Optom Vis Sci.2006;83(8):605- 610.
- Bond WI,Gordon M,Kezirian GM,et al.An update on presby-LASIK.Cataract & Refractive Surgery Today. October 2009;9(10):21-22.