Myopic excimer laser ablation of the cornea tends to create an oblate cornea. Holladay et al1 report I that this loss of negative asphericity may be the predominant factor in the functional decrease in vision after LASIK and photorefractive keratectomy (PRK). The prolate nature of the cornea creates positive spherical aberration, which compensates for the negative spherical aberration of the crystalline lens.2 This balance is disrupted by the changing properties of each element, mostly the lens, with increasing age.2,3 This uncoupling of the spherical aberration of the cornea and lens can reduce visual quality as one ages.
The creation of an oblate cornea after laser ablation can also disrupt the balance between lenticular and corneal aberrations. The maintenance of the prolate corneal shape postoperatively is one method to address the potential loss in visual quality after laser ablation. However, the retention of the physiologic prolate corneal shape may not adequately address the age-related changes in the aberrations of the eye Q.T. Holladay, MD, personal communication, 2005). This observation is supported by the loss in visual quality over time in nonsurgical eyes due to age-related lens changes. Additionally, the ablation process itself creates spherical aberration on the cornea due to under-ablation in the corneal periphery, a problem endemic to conventional and possibly custom laser ablation algorithms. The induction of positive spherical aberration decreases the functional optical zone.4 An ablation strategy that retains a prolate corneal shape over the scotopic pupil, addresses induced spherical aberration, and compensates for the known age-related changes in lenticular spherical aberration should result in excellent postoperative visual quality for many years to come.
In this article, we present the first published report of the excimer laser optimized prolate ablation algorithm (OPA) using the NIDEK Advanced Vision Excimer laser platform (NAVEX; NIDEK, Gamagori, Japan). Optimized prolate ablation maintains a prolate cornea over the scotopic pupil, compensates for age-related changes, and reduces the induced spherical aberration.
Figure 1. Preoperative OPD-Scan corneal topography and wavefront analysis of the A) right and B) left eyes.
PATIENTS AND METHODS
A 20-year-old man with preoperative manifest refractions of -6.50 -1.25 X 180 and -6.50 -1 X 180 in the right and left eyes, respectively, underwent LASIK.
The baseline best spectacle-corrected visual acuity (BSCVA) was 20/20 in both eyes. Central corneal thickness was 558 pm and 559 pm for the right and left eyes, respectively. Scotopic pupil size was 7.30 mm and 7.07 mm for the right and left eyes, respectively. The right eye was treated using OPA and the left eye using conventional ablation. For the OPA treated right eye, the optical zone was 7.33 mm and the transition zone was 9.00 mm.
The patientunderwent afull ophthalmic examination to rule out any ocular diseases or prior ocular surgery that would contraind?cate LASIK. Preoperatively, the patient underwent uncorrected visual acuity (UCVA) and BSCVA manifest and cycloplegic refractions, corneal topography, and wavefront screening using the NIDEK OPD-Scan, anterior segment examination, and dilated fundus examination. Postoperatively, UCVA, BSCVA, anterior segment evaluation, and OPD-Scan analysis was performed at 1 day, 1 week, and 3 months postoperatively. Both eyes underwent treatment with NAVEX with a prolate ablation algorithm in the right eye that corrected for the decrease in energy delivered to the corneal periphery due to changes in the corneal curvature. The left eye underwent conventional ablation that did not account for the loss in effective energy in the corneal periphery. A laser keratome (IntraLase, Irvine, Calif) was used to create the keratectomy.
Postoperative outcomes are presented at 1 week and 3 months. Preoperative corneal topography and wavefront analysis for the right and left eyes are presented in Figure 1.
Preoperatively, OPD-Scan analysis of the right eye shows an instantaneous corneal topography map that is steeper in the center and flatter peripherally (Fig IA). The refractive wavefront of the right eye shown on the OPD map displays a homogenous refractive power distribution across the pupil with a mild decrease in myopia towards the periphery, which indicates negative spherical aberration of the entire eye. The OPD higher order map shows that >1.00 diopter (D) of refractive error is caused by higher order aberrations. The modulation transfer function (MTF) plots the optical performance of the eye at various spatial frequencies. The MTF shows that optical performance of the right eye is significantly reduced if both higher and lower order aberrations remain uncorrected (red curve). The Zernike graph shows the patient has 0.277 pm of higher order aberrations (see Fig IA).
Figure 2. Preoperative (top) and 1 week postoperative (bottom) corneal topography and refractive wavefront of the right eye.
Preoperatively, OPD-Scan analysis of the left eye shows an instantaneous corneal topography map that is steeper in the center and flatter peripherally (Fig IB). The OPD map of the left eye displays a homogenous refractive power distribution across the pupil with a mild decrease in myopia towards the periphery, indicating negative spherical aberration of the entire eye. The OPD higher order map shows that approximately 1.00 D of refractive error is caused by higher order aberrations (see Fig IB).
The MTF shows that optical performance in the left eye is significantly reduced if both higher and lower order aberrations remain uncorrected (red curve) (see Fig IB). The Zernike graph shows the patient has 0.253 pm of higher order aberrations (see Fig IB).
One week postoperatively, the OPA treated right eye shows an instantaneous corneal topography that is steep in the center and flat peripherally (Fig 2). The refractive map, plotting the refractive power of the cornea, shows a largely monodioptric optical zone out to 7.5 mm with an increase in power past 7.5 mm (see Fig 2). The OPD map, which presents the wavefront of the entire eye in diopters, shows a trend towards hyperopia peripherally (see Fig 2). At 3 months postoperatively, increased flattening is noted peripherally on instantaneous topography, a monodioptric zone of 7.5 mm of corneal refractive power remains, and the trend towards peripheral hyperopia increases (Fig 3).
Figure 4 plots the preoperative and 1 week postoperative results of the left eye, which was treated with conventional ablation. The cornea is flatter in the center and steeper peripherally. The refractive map shows a monodioptric optical zone out to approximately 5 mm with a significant increase in power peripherally after 5 mm (see Fig 4). The OPD map shows a trend towards myopia peripherally (see Fig 4). At 3 months postoperatively, the cornea remains flatter centrally on instantaneous topography, a monodioptric zone of corneal refractive power of 5 mm remains, and the trend towards peripheral myopia increases (Fig 5).
The oblate cornea and induced positive spherical aberration after excimer laser ablation combine to reduce visual quality after PRK or LASIK. The cornea accounts for the majority of the power of the eye, hence it is the main contributor to aberrations of the eye. The high magnitude of aberrations that might have existed in the cornea are reduced due to the prolate, aspheric shape rather than oblate or spherical shape.5 Peripheral flattening of the corneal curvature toward the periphery reduces spherical aberration to approximately 10% of what exists in spherical lenses4 and likely even less than that of an oblate lens model.5 Investigators have predicted that the best optical quality is produced when the corneal profile is a prolate ellipse.6 However, preservation of the physiologic prolate shape of the cornea or attempts at increasing the asphericity both require greater depth than conventional laser ablation. Hence preoperative corneal thickness, asphericity, and refractive error must be considered.
Figure 3. Three-month postoperative A) corneal topography and B) refractive wavefront of the right eye after OPA treatment.
Figure 4. Preoperative (top) and 1 week postoperative (bottom) corneal topography and refractive wavefront of the left eye.
One of the goals in this investigation was to make the cornea slightly more prolate than necessary to account for the known age-related changes in the crystalline lens. Aging causes a gradient refractive index in the lens, causing a change from negative to positive spherical aberration of the lens over time. Investigators have shown that young eyes have more negative or overcorrected spherical aberrations.7 As the eye ages, positive spherical aberration tends to increase. This is due to the positive spherical aberration of cornea combining with the positive spherical aberration of the lens.
The result shown here represents the early outcomes of a first attempt at creating a slightly more prolate cornea than necessary to account for changes in the crystalline lens. The corneal topography of the OPA treated eye shows that the patient did indeed become more prolate postoperatively. However, this caused an increased negative asphericity beyond what may be required for a 20-year-old patient. Clinically, the work of Holladay and Janes4 and the work of other investigators8 indicate that a trend towards oblate corneas is detrimental to visual quality. Whether the induced negative asphericity has the same effect remains to be seen as the number of patients treated with OPA and longer-term follow-up becomes available. Based on current theory and our observations, we believe similar amounts of physiologic negative and positive asphericity will have significantly different effects on vision with the former being more beneficial than the latter.
Figure 5. Three-month postoperative A) corneal topography and B) refractive wavefront of the left eye after conventional treatment.
In this study, a patient with moderate myopia and large scotopic pupils was selected for treatment. Laser in situ keratomileusis for low myopia is very successful and would represent a confounding variable as less spherical aberration is induced. Additionally, a patient with large scotopic pupils would have a greater likelihood of being symptomatic for halos and glare at night. Selecting a patient with low myopia and small physiologic pupils would have been of little value in developing an ablation algorithm that is specifically tailored to providing best photopic and scotopic visual quality.
Spherical aberration occurs, at least partly, due to a continuous loss of effective laser energy as the ablation moves from center to periphery.9 This is seen as central overcorrection and a peripheral undercorrection, resulting in positive spherical aberration. This decreases the effective optical zone after PRK or LASIK, which in turn can increase the likelihood of poor scotopic vision.4 Using proprietary ablation algorithms developed for NAVEX, the laser treatment was adjusted to compensate for this loss of effective energy and to create a prolate cornea based on corneal topography and wavefront aberrometry measured with NIDEK OPD- Scan.
As seen on postoperative topography and OPD maps, positive spherical aberration is evident in the conventionally treated left eye, but not the OPA treated right eye. Additionally, a significantly larger effective optical zone is present in the OPA treated eye compared to the conventional treatment. This refined ablation architecture is likely to maintain excellent visual quality over time as it accounts for age-related changes, maintains a prolate shape, and reduces the amount of spherical aberration induced by the conventional ablation algorithms. This study presents results of one eye using this OPA treatment and we intend to follow this patient over the long-term to see the effect over time. Additionally, the outcomes of larger numbers of patients will be presented as data become available.
1. Holladay JT, Dude ja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography. / Cataract Refract Surg. 1999;25:663-669.
2. Artal P, Guirao A, Williams DR. Aberrations of the internal ocular surfaces measured in vivo with a Hartmann-Shack sensor. Invest Ophthalmol Vis Sci Suppl. 1999;40:39.
3. Guirao A, Redondo M, Artal P. Optical aberrations of the human cornea as a function of age. / Opt Soc Am A Opt Image Sci Vis. 2000;17:1697-1702.
4. Holladay JT, Janes JA. Topographic changes in corneal asphericity and effective optical zone size following LASIK. / Cataract Refract Surg. 2002;28:942-947.
5. Charman WN. Visual optics and instrumentation. In: CronlyDillon J, ed. Optics of the Human Eye. Boca Raton, Fla: CRC Press Ine; 1991:1-26.
6. Patel S, Marshall J, Fitzke FW III. Model for predicting the optical performance of the eye in refractive surgery. Refract Corneal Surg. 1993;9:366-375.
7. Jenkins TC. Aberrations of the eye and their effects on vision, I: spherical aberration. Br J Physiol Opt. 1963;20:59-91.
8. Gatinel D, Malet J, Hoang-Xuan T, Azar DT. Analysis of customized corneal ablations: theoretical limitations of increasing negative asphericity. Invest Ophthalmol Vis Sci. 2002;43:941-948.
9. Mrochen M, Seiler T. Influence of corneal curvature on calculation of ablation patterns used in photorefractive laser surgery. J Refract Surg. 2001;17:S584-S587.