When planning keratorefractive surgery, can we identify and target an ideal postoperative corneal shape that will perform best for all patients in all situations? Certainly, the answer to this question is complex. The optical performance envelope of the visual system is amazing in that it must function well over an enormous spectrum of illuminative conditions. With a dynamic pupillary aperture, a constantly changing constellation of rays participates in the stimulation of the retina. Additionally, chromatic aberration means that what might be best for one wavelength of light might not be best for others.CURRENT EXCIMER LASERS
Although current excimer systems are all capable of delivering very high-quality acuity postoperatively, there are clearly limits to the amount of manipulation that the visual system will tolerate. In general, most surgeons believe that the central corneal power should not be reduced below 35.00D nor steepened beyond 49.00D. The latter case is particularly prone to problems related to epithelial breakdown of the central cornea from poor wetting by the tear film. Higher-diopter corrections introduce a greater transition between centrally treated and peripherally untreated corneal tissue and can result in spherical aberration.
Wavefront-guided ablations have successfully reduced the induction of spherical aberration by creating a less oblate postoperative shape. The ability to adjust the wavefront treatment's optical zone diameter may have great importance in reducing dysphotopsia as the pupil dilates. In a study examining patients treated with the Technolas 217z Zyoptix System for Personalized Vision Correction (Bausch & Lomb, Rochester, NY), spherical aberration increased as the pupil aperture grew in patients with a 6-mm treatment zone.1 This mydriasis-related increase in spherical aberration, however, was largely eliminated by the use of a 7-mm treatment zone. Other laser systems such as the Allegretto Wave (Wavelight Laser Technologie AG, Erlangen, Germany) and the MEL-80 (Carl Zeiss Meditec AG, Jena, Germany; not available in the US) use an aspheric ablation profile to maintain a more prolate corneal shape and reduce the induction of spherical aberration. Excellent clinical results have been achieved with both of these nonwavefront-guided systems.2,3THE SCOTOPIC PUPIL
The effect of scotopic pupil size on dysphotopsia with any given ablation size is a highly controversial matter.4 Most experienced surgeons consider a scotopic pupil that is larger than the planned ablation zone to be a risk factor for dysphotopsia, but others claim it has no correlation. Part of the controversy may stem from the fact that scotopic pupil size is often incorrectly measured. Devices such as the Procyon infrared pupillometer (Keeler Instruments Inc., Broomall, PA) provide highly accurate measurements of pupil size under true scotopic conditions (Figure 1).
It is generally accepted that larger corrections are much more likely to produce postoperative symptoms than smaller ones. I have seen many patients with scotopic pupils larger than their ablation zones who have absolutely no night-vision complaints. Conversely, I have also seen well-centered ablations that are larger than the scotopic pupil in patients who have significant dysphotopsia. I tend to be much more willing to perform surgery in the context of a pupil-optical zone mismatch in low corrections, particularly with minimal astigmatism. I have used several laser platforms, and I find that, when the optical zone is at least 6.5mm and the ablation is well centered, night-vision complaints are quite rare, even in traditional LASIK. With customized wavefront-guided treatments at the 6.5-mm zone or larger, night-vision complaints have been rare as well.CONCLUSION
All corneal-ablation planning represents a compromise of many competing factors, including ablation depth, effective optical zone size, treatment time, asphericity, prolate or oblate shape, and thermal effects. The ideal corneal shape for any given patient is the one that provides the best quality of vision under the greatest range of visual conditions for the longest portion of the patient's lifetime. Current ablation strategies meet this task admirably, and future refinements will enhance our ability to improve upon this performance.
Steven J. Dell, MD, is Director of Refractive and Corneal Surgery at Texan Eye Care in Austin, Texas. He is a consultant to Bausch & Lomb, and his research is partially funded by Carl Zeiss Meditec Inc. Dr. Dell may be reached at (512) 327-7000; email@example.com. MacRae SM. Effect of customized pupil size, optical-zone size, and refractive error on the magnitude of spherical aberration induced by LASIK. Paper presented at: The ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery; April 13, 2003; San Francisco, CA.
2. Chotiner B. Efficacy of the Wavelight Allegretto excimer laser in a clinical setting for LASIK. Paper presented at: The ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery; June 1, 2002; Philadelphia, PA.
3. Goes FJ. LASIK for myopia and myopic astigmatism using the MEL-80 laser: 1 month and 1 year follow-up. Paper presented at: The ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery; May 4, 2004; San Diego, CA.
4. Dell SJ. Pupil testing and its clinical significance. In: Probst LE, ed. LASIK: Advances, Controversies, and Custom. Thorofare, NJ: Slack Inc.; 2003: 15-22.
Shachar Tauber, MD
At present, there does not seem to be an ideal corneal shape. It is important to realize that the tear film, lens shape and position, axial length, and corneal shape all contribute to the complete optical function of an eye. Based on the typical ocular anatomy of the tear film; the power, clarity, and anatomic dimensions of the average lens; and the usual overall length of the eye, a prolate corneal shape is ideal. Because the eye's internal structure varies in the general population, however, a single corneal profile cannot be universally ideal. The degree of “prolateness” is individual, and certain individuals may actually benefit from an oblate cornea.THE TRANSITION ZONE
The refractive knee is an optical term describing the region of the cornea where there is a transition from the corrected central ablation zone after myopic LASIK to the untreated peripheral cornea. The region is highly variable, determined by the specifics of the ablation zone, the parameter of the LASIK flap, perhaps the location of the LASIK hinge, and other factors not yet identified.
In attempting to determine if there is an upper limit to the contour change from the center of the ablation to the 10-mm optical zone, one must consider many issues. Optics will be highly dependent on how the epithelium smoothes over the peripheral ablation. Corneal wound healing and flap biomechanics are unique to each eye and, to some degree, each surgeon. Certainly, it is important that the transition zone be as smooth as the surgical plan will allow. One must bear in mind that, the farther out the transition zone, the deeper the overall ablation. This tradeoff must take into account the risk of ectasia in lamellar refractive surgery.THE SCOTOPIC PUPIL
With the advent of wavefront-guided excimer laser ablation, the role of the pupil as a stand-alone factor has become a scientific question. Pupil size may be a minor concern at most in wavefront-guided treatments using a true 6.5-mm ablation zone.INDIVIDUALITY
To provide patients with the best quality and quantity of vision possible, surgeons must continue to tailor the treatment to the individual. Defining a one-size-fits-all first optical surface (eg, the cornea) may limit ophthalmologists' ability to treat the highly variable and complex imaging system known as the human eye.
Shachar Tauber, MD, is Director of Refractive Surgery and Ophthalmic Research at St. John's Hospital and Clinic in Springfield, Missouri. He is on the Speakers Bureau of Alcon Laboratories, Inc., but states that he holds no direct financial interest in the products and companies mentioned herein. Dr. Tauber may be reached at (417) 820-9723; firstname.lastname@example.org.Karl G. Stonecipher, MD;
Joachim Loffler, MD; and
Guy M. Kezirian, MD, FACS
The search for the perfect corneal shape is the Holy Grail of refractive surgery. Attempts to model human optics often fail to consider the complex and diverse conditions under which people see. Few models consider the effects of aging or the impact of the tear film and ocular surface conditions. Moreover, binocular integration and issues such as monovision are difficult to incorporate in any model. The interactions of the aspheric cornea and aspheric crystalline lens are complex, especially if one considers the different functional conditions of distance and near vision and the effects of different pupil sizes.
Because the human optical system is so difficult to model, ophthalmologists must rely on clinical imaging technologies such as topography and aberrometry to evaluate the effects of surgery. Attempts to model ablation profiles in theoretical eyes have been published,1,2 but their real-life applicability is limited. Although progress in improving ablation profiles continues, the effects of any changes are often evaluated based on subjective clinical feedback and psychomotor testing, which at best can be difficult to quantify.3 Furthermore, clinical feedback sometimes conflicts with the assumption that aberration-free vision may not provide the best optics for all situations: some aberrations (especially coma) may help in certain conditions, for example, by allowing early presbyopes to delay their need for reading glasses.4AT ISSUE
Where does the quest for the ideal corneal shape stand? The answer must consider the following issues.
The eye is not a static structure. The ideal corneal shape may be different for patients of various age who have different needs. Accommodation
Accommodation induces significant aberrations and does so inconsistently. Coma, spherical aberration, and significant changes in astigmatism have been observed with accommodation. A defined focal point for those lens systems is needed. The ideal corneal shape should work well for all focal points.Chromatics
Chromatic aberration can induce more than 1.00D of aberration under normal conditions. Clinically, ophthalmologists take advantage of this phenomenon in the red-green test. Functionally, the ideal corneal shape may have to anticipate focal points across the visible spectrum.ASPHERICITY
Recent work has convincingly demonstrated that excimer ablations altering the prolate asphericity of the cornea increase spherical aberration and decrease mesopic visual function.5,6 Conversely, prospective studies with newer laser platforms have demonstrated that maintaining the natural prolate shape permits refractive correction without increasing night-driving complaints.7
To allow the corneal curvature (and therefore its power) to be altered while preserving the relative ratio of central-to-peripheral curvature (positive asphericity) requires wider, deeper ablations that remove more tissue. There is generally enough corneal tissue available to perform so-called prolate ablations of up to approximately 6.00D. Beyond that point, the tissue demands exceed supply, and spherical aberration is induced.
In a normal population of healthy eyes, Kiely et al8 found a mean value for asphericity of -0.26 ±0.18 and a mean value for the central radius of curvature of 7.72 ±0.27mm (Figure 2). They also pointed out that there was no tendency for the asphericity value to indicate more asphericity in a particular meridian. The values for radius of curvature, however, did show a trend toward having large (or flat) values in the horizontal meridian (clinically known as with-the-rule astigmatism), but this relationship, too, changes with time and age. Asphericity only compensates for spherical aberration (C12, C24). It does not take into account other higher-order aberrations.CONCLUSION
From the aforementioned concepts, it is clear that corneal asphericity is important for visual quality and that, if asphericity is affected, patients are most intolerant of mesopic conditions as opposed to photopic conditions, when their pupils are smaller. The ideal asphericity depends on the focal length of the eye and is prolate for most patients.
At present, one thing is certain: technology will only improve. Surgeons' understanding of the perfect optical system in theory or in fact will change. It is the quest, however, that makes refractive surgery interesting.
Guy M. Kezirian, MD, FACS, is President of Surgivision Consultants, Inc., in Scottsdale, Arizona. His company runs the US regulatory trials for the Wavelight Allegretto Wave excimer laser, but Dr. Kezirian states that he holds no direct financial interest in the other products or companies mentioned herein. He may be reached at (480) 664-1800; email@example.com.
Joachim Loffler, MD, is Director, New Wave Consulting, Erlangen, Germany. He is a consultant to Wavelight Laser Technologie AG but states that he holds no direct financial interest in the products and other companies mentioned herein. Dr. Loffler may be reached at firstname.lastname@example.org.
Karl G. Stonecipher, MD, is Director of Refractive Surgery at Southeastern Eye Center in Greensboro, North Carolina. Dr. Stonecipher is a consultant for Intralase Corp. and has received travel grants from Wavelight Laser Technologie AG, but he states that the holds no direct financial interest in the products or other companies mentioned herein. Dr. Stonecipher may be reached at (800) 632-0428; email@example.com. Mrochen M, Donitzky C, Wullner C, Loffler J. Wavefront optimized ablation profiles: theoretical background. J Cataract Refract Surg. 2004;20(suppl):550-554.
2. Manns F, Ho A, Parel JM, Culbertson W. Ablation profiles for wavefront-guided correction of myopia and primary spherical aberration. J Cataract Refract Surg. 2002; 28:766-774.
3. Kezirian GM, Eydelman MB, Drum B. Systemic evaluation of outcomes. Paper presented at: The 5th International Congress on Wavefront Sensing and Optimized Refractive Correction; February 21-23, 2004; Whistler, Canada.
4. Chalita MR, Krueger RR. Correlation of aberrations with visual acuity and symptoms. Opthalmol Clin N Am. 2004;17:135-142.
5. Holladay JT, Janes JA. Topographic changes in corneal asphericity and effective optical zone after laser in situ keratomileusis. J Cataract Refract Surg. 2002;28:942-947.
6. Boxer Wachler BS, Huynh VN, El-Shiaty AF, Goldberg D. Evaluation of corneal functional optical zone after laser in situ keratomileusis. J Cataract Refract Surg. 2002;28:948-953.
7. Kezirian GK, Stonecipher KG. Subjective assessment of mesopic visual function after LASIK. Opthalmol Clin N Am. 2004;17:211-224.
8. Kiely PM, Smith G, Carney LG. The mean shape of the human cornea. Optica Acta. 1982;29:1027-1040. Please note that this article calculates ideal asphericity to -0.46 ±0.01 for 0 to -10.00D.