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Topography Assisted LASIK
Mayank S Pangtey, Namrata Sharma, Rasik B Vajpayee
The concept of topography assisted customized ablations was first suggested by Seitz et al in 1998 in experimental studies. This was made possible due to the various topography based software and the technology of the flying spot excimer lasers (Fig. 23.1).
The surface aberrations from the cornea are first calculated from the corneal elevation data, which is derived from the various corneal topography measurements, and this is coupled with a standard refraction to generate customized ablation patterns through an excimer laser delivery system.
To begin with, this approach was initiated in the ‘repair procedures’, i.e. eyes who had undergone previous refractive surgeries and had corneal abnormalities. This was then done in the so-called normal eyes with the routine LASIK procedure.
Basic Principle
The basic formula for the topography-guided ablation is expressed in the following equation:
A(x, y)=C−(T[x, y]−Ttarget[x, y])
Where A(x, y) is the ablation pattern, T[x, y] is the actual corneal elevation topography, Ttarget [x, y] is
Figure 23.1. Topography assisted LASIK. Due to flying spot laser preferential aspheric treatment is
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possible (red area denotes normal curvature)
the target corneal topography one wishes to achieve, and C is the smallest constant depth needed to keep A (x, y) from becoming negative anywhere. Ablation depth A(x, y) cannot take negative values because the ablation cannot add tissue to the cornea. C= the
maximal value of (T [x, y]−Ttarget [x, y]) over the optical zone.
A tricky step in topography guided ablation is the determination of the target
topography Ttarget [x, y]. Theoretically, Ttarget [x, y] should be a parabolic surface with the right refractive power to achieve emmetropia or other postoperative refractive targets. In
practice, the spherical equivalent power of the preoperative cornea and the spherical equivalent refraction of the preoperative eye may be difficult to determine in cases with severe aberrations and poor best corrected visual acuity.
Indications
The indications of topography assisted customized ablation vary from the correction of the primary errors to the corrections of aberrations due to iatrogenic causes, following corneal trauma and keratitis (Table 23.1).
The results have been encouraging in regular astigmatism and decentered ablations but require refinement with irregular astigmatism. Though this
Table 23.1: Indications of Topography assisted customized ablations
Primary
Secondary
Iatrogenic
Previous refractive surgery
Decentration
Irregular ablations
Interface irregularities after DLK
Insufficient effective optical zone size
After other eye surgery like penetrating keratoplasty
After eye injury
After keratitis
technique is more challenging in patients with irregular astigmatism, they may benefit the most once the technique becomes more refined. This is especially true in cases of penetrating keratoplasty, and posttraumatic corneal scar with large irregular postoperative astigmatism.
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Prerequisites
1.The stability of the corneal curvature prior to surgery should be ascertained. One should wait for at least 1 year before performing topography assisted customized ablation following penetrating keratoplasty or in cases of larger corneal scars. For smaller corneal scars, this interval may be shortened provided the corneal topography is stable.
2.A high quality topography map should be generated, since it is the basis of customized ablation. A good image is a one that is acquired with wideopen lids and regular tear film. The image should be well centered, and focused and there should be no dark spots on the map. The images should be taken repeatedly, unless optimal maps are acquired. It is best to obtain 3 different maps and the one featuring the least eye movement should be used.
Topography Assisted Customized Ablations
A variety of software is available which link the topography systems to the excimer laser delivery systems in order to achieve the topography assisted customized ablation. These include the following:
Topolink (Bausch and Lomb Surgical, St.Louis Mo)
The topography is obtained on Orbscan II analysis system (Bausch &Lomb Surgical, Orbtek, Sat Lake City, UT) the data are copied and ablation profile is calculated based on Topolink software. The software determines the target keratometric value by subtracting the manifest sphere from the keratometric value in the steep corneal meridian. The target keratometric value and a preset shape factor of −0.25 is defined as the target asphere which one aims to achieve after LASIK.
The topolink software is not based on Munnerlyn’s formula; instead, it calculates a certain “lenticule” of corneal tissue to be removed The Topolink software compares the shape of the target asphere to the corneal shape actually measured. The target shape is fitted from beneath to the actual cornea for a given planned optical zone size. The difference between the target and actual shapes is then ablated.
Corneal Interactive Programmed Topographic Ablation
The corneal interactive programmed topographic ablation (CIPTA) system is an interactive software programme that links the elevation data obtained with the Orbscan II corneal topography system (Bausch and Lomb Surgical, St. Louis, Mo) and a flying-spot excimer laser (LaserScan 2000, Laser Sight, Orlando, Fla) to develop a customized ablation pattern.
The normal cornea has an aspheric surface with a more pronounced curve in the central portion and a gradual flattening at the periphery. CIPTA software is based on the altimetric topography and compares the cornea with an aconic ellipsoid with an adjustable coefficient of asphericity to preserve the patient’s physiological astigmatism and prolate shape.
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To increase the precision, at least three measurements are performed and accuracy is assumed when the height difference in the central 5 mm differs by less than 3 microns. Cyclo-torsion of the eye during topography acquisition and ablation should be avoided because it can reduce the accuracy of the astigmatic correction. In addition, there is a learning curve for planning the ablation with CIPTA, as the software requires the user to select the center of the ablation, the size of the transition zone, and the index of asphericity.
Topographic Simulated Customized Ablation (TOSCA)
In this technique the elevation maps of Technomed C-scan (Technomed GmbH, Baseweiler, Germany) and the Tomey TMS II or III (Tomey and Erlanger Tennenoloke, Germany) are used. The keratometric and the elevation maps are used to calculate the best fit sphere. A simulated ablation map is generated which is automatically transferred to the laser shot file, which is loaded onto the laser machine MEL70 Asclepion-Meditec uses a TSA (Tissue Saving Algorithm) module to TOSCA (Topography Supported Customized Ablations) software for carrying out topography-guided corrections.1 This module automatically minimizes the tissue removal when calculating the correction program (Fig. 23.2).
Figure 23.2. Topographic simulated customized ablation (TOSCA)
Custom Contoured Ablation Patterns (C-CAP)
The custom contoured ablation patterns An elevation topography map is generated with the help of a placido disc shaped topographer (such as Humphrey) or a slit scan elevation topographer (such as Orbscan). The most elevated areas of the cornea are located on the horizontal and the vertical meridians using the grid pattern. The shape, size and depth of the most elevated areas and axis of ablation (cylindrical or elliptical) are defined and the elevation is related to the best fit curvature and the elevation in microns of the zones to be ablated is calculated. The treatment of the irregular astigmatism is fed into the Visx Star
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system and the ablation performed. C-CAP method is basically used for decentered ablations.
Results of Topography Assisted Corneal Ablations
Studies have been done to evaluate the role of topography guided corneal ablations using various machines and software.
Knorz et al treated 114 patients (eyes) with myopia of −1 to 6D and astigmatism of 0 to −4D (group 1), and 89 patients (eyes) with myopia of −6.1 to −12D and astigmatism of 0 to −4D (group 2). They treated these patients on the basis of corneal topography measured with the Orbscan II system using a Keracor 217 excimer laser and the Hansatome microkeratome. In the low (high) myopia group, 96.1% (75.0%) were within 0.50D of emmetropia, and uncorrected visual acuity was 20/20 or better in 82.4% (62.5%), 20/25 or better in 98.0% (70.0%), and 20/40 or better in 100% (95.0%).2,3
Tamayo et al performed topography guided custom ablation with LASIK on 5 eyes for irregular astigmatism secondary to penetrating keratoplasty for keratoconus, prior decentered laser in situ keratomileusis, or incisional refractive surgery.4 The contoured ablation patterns (CAP) method (VISX, Inc.) was used to automatically decenter the ablation over the corneal elevation. The UCVA uncorrected visual acuity (UCVA) was better than 20/30 in all but one eye. The best corrected visual acuity (BCVA) was maintained or improved in all eyes.4
Alessio et al5 evaluated the efficacy, predictability, stability, and safety of CIPTA. Forty-two eyes of 34 subjects had CIPTA were treated for hyperopic and myopic irregular astigmatism using a corneal topography guided PRK and LASIK At a mean follow-up of 13.2 months, 26 eyes (92.8%) in the hyperopic group and 12 eyes (85.7%) in the myopic group had an UCVA superior to 20/40. Twelve hyperopic eyes (42.8%) and five myopic eyes (35.7%) had a UCVA of 20/20.
Comparison between Topography and Wavefront based Systems
As corneal topography can only measure the aberrations of the corneal surface, it cannot by itself eliminate all ocular aberrations. In comparison to wavefront technology, which measures only a few hundred points6,7 corneal topography maps the surface based on several thousand points in a reflected ring pattern. In cases in which corneal aberration is predominant and contains small-scale irregularities as scar, dystrophies, ectasia, steep central islands, corneal topography may be able to map ocular aberration with higher resolution than current wavefront sensors.
Moreover wavefront sensing devices measure the entire refractive status of the eye and cannot differentiate measured aberrations caused from the cornea or by a combination of effects of the cornea and crystalline lens. Unlike corneal topographers, they give no direct information on the corneal status of the corneal surface. In addition, there are cases in which a wavefront sensor will not work well because wavefront sensing devices must be able to send and receive light from the retinal surface without significant interference front the ocular tissue. In corneal trauma, the cornea may be compromised to the extent that it is not possible to for the incoming beam to form a reasonably wellfocused spot on the retina. When this happens, the detected spots of light may be so badly
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deformed that a good measurement of higher order aberrations is not possible. The corneal topographer has no such limitations, which can measure the corneal surface even in the presence of serious irregularity.
Limitations of Topography based Systems
There are several limitations to the accuracy and completeness of corneal topography in practice. Placido ring based devices measure the local radial slope of the cornea at discrete sampling points. Slope data is converted to height topography by an integrated operation starting from the center outward. This integration introduces the error in a cumulative fashion; therefore, height estimates becomes less reliable further from the center. The coverage of the ring projections on the cornea can be the nose and the brow. The tear film is variable over time and often there are poorly wet areas on the cornea where measurement cannot be made. These patches of missing slope data can introduce large errors into height calculations. With all these limitations, Placido disc based technology is still the most successful technology so far.
REFERENCES
1.Wygledowska-Promienska D, Zawojska I, Gierek-Ciaciura S. New generation of excimer laser—Asclepion Meditec MEL 70 G-Scan. Klin Oczna. 2000; 102(5):373–75. Polish.
2.Knorz MC, Neuhann T. Correction of myopia and astigmatism using topography-assisted laser in situ keratomileusis (TopoLink LASIK). Ophthalmologe 2000; 97(12):827–31.German.
3.Knorz MC. TopoLink LASIK. Int Ophthalmol Clin 2000; 40(3):145–49.
4.Tamayo Fernandez GE, Serrano MG. Early clinical experience using custom excimer laser ablations to treat irregular astigmatism. J Cataract Refract Surg 2000;26(10): 1442–50.
5.Alessio G, Boscia F, La Tegola MG, Sborgia C. Topographydriven excimer laser for the retreatment of decentralized myopic photorefractive keratectomy. Ophthalmology 2001; 108(9):1695–1703.
6.Liang J, William DR. Aberrations and retinal image quality of the normal human eye. J Opt Soc Am A 1997; 14(11):2873–83.
7.Liang J, Grimm B, Golez S et al. Objective measurement of wavefront aberrations of the human eye with use of a Hartmann-Shack wavefront sensor. J Opt Soc Am A 1994; 11(7):1949–57.