Introduction
Decentration of the ablation and other factors have contributed to inducing irregular astigmatism in a certain number of eyes treated with laser refractive surgery. Topography-guided ablations present a surgical tool to correct theses errors. In this chapter, the technique of TopoLink laser in situ keratomileusis (LASIK), based on corneal topography, will be described and illustrated by some examples.
The Technique of TopoLink
The TopoLink procedure used the Technolas 217z excimer laser (Bausch & Lomb). Laser ablation was based on the preoperative corneal topographic map obtained with the Orbscan II corneal analysis system (Bausch & Lomb). Three different maps were taken and the one featuring the least eye movement was used. Once the topography was taken, data were copied and the ablation profile was calculated using a special software called TopoLink (Version 2.9992TL; Bausch & Lomb Technolas). Input values were manifest refraction in minus-cylinder format and corneal thickness as measured by the Orbscan II. The target K value was determined by the software by subtracting the manifest sphere from the K value in the steep corneal meridian. The target K value and a preset shape factor of −0.25 defined the target asphere that we planned to achieve after LASIK. The TopoLink software basically compares the shape of the target asphere to the corneal shape actually measured. Simplified, the target shape is fitted from beneath to the actual cornea for a given planned optical zone size. The difference between the two shapes is then ablated. Any overlap between target and actual shape must thus be outside the planned optical zone, as tissue cannot be added but rather only ablated. The TopoLink software therefore represents a new and different approach, which is not based on Munnerlyn’s formula. Rather, it calculates a certain “lenticule” of corneal tissue to be removed; the scanning laser used provides the means to remove this tissue even if its shape is asymmetric or even irregular. The diameter of the planned optical zone was 6 to 7 mm. Only in those cases in which the ablation required to achieve these optical zones would have left a residual corneal stromal bed of less than 250 µm was the diameter of the planned optical zone decreased to maintain a residual stromal bed of at least 250 µm. Based on these data, TopoLink calculated a session file that basically contained information for the scanning laser regarding which ablation pattern to perform. The session file was transferred via disc and loaded into the Technolas 217z excimer laser (Bausch & Lomb) just prior to treatment.
Examples of Topolink ( )
Patient 1: Irregular Astigmatism After Penetrating Keratoplasty and Astigmatic Keratotomy
This patient had a penetrating corneal graft because of recurrent stromal herpetic keratitis in 1992. He was first referred in 1993. Manifest refraction was +0.25 sphere −6 cylinder axis 135°. Corneal astigmatism was −8 diopters (D) axis 135° and slightly asymmetric. Initially, astigmatic keratotomy (AK) was performed in 1994. After AK, manifest refraction was −2.5 sphere −4 cylinder axis 165°. Uncorrected visual acuity (UCVA) was 20/400 and best corrected visual acuity (BCVA) was 20/60. Corneal topography showed marked irregularity and axis shift ( Fig. 14.1 , upper left ). We therefore decided to perform TopoLink LASIK. Average refractive power of the cornea overlaying the entrance pupil was estimated to be 45 D. Spherical equivalent of manifest refraction was −4.5 D. We therefore selected a target K value of 40.5 D. A 5.4-mm optical zone was used, and ablation depth was 150 µm. Corneal thickness was 610 µm centrally and both the internal and external margins of the graft were well aligned with the host cornea. It is very important to check alignment prior to the lamellar cut. When alignment is poor or there is localized ectasia at the edge, corneal thickness might be reduced and the keratome cut may cause further weakening of the cornea, inducing more ectasia, or even a penetration of the anterior chamber. In this patient, alignment was perfect and the LASIK procedure performed in July 1997 was uneventful. A 160-µm flap was used. One day after TopoLink LASIK, the UCVA had improved to 20/30, and BCVA was 20/25 (correction: +0.75 sphere). After 4 months, UCVA was 20/30 and BCVA 20/25, but manifest refraction had changed slightly to +1 sphere −2.0 cylinder axis 10°. Corneal topography 4 months after TopoLink LASIK showed marked improvement of the irregularity. Some residual with-the-rule astigmatism was still present, but the irregular astigmatism that was present preoperatively had virtually disappeared, as shown by the differential map ( Fig. 14.2 ).
Patient 2: Irregular Astigmatism
This patient had irregular astigmatism owing to a peripheral scar caused by a corneal ulcer in prolonged contact lens wear in his right eye. The preoperative topographic map (see Fig. 14.2 ) shows marked asymmetry of the astigmatism. Refraction was −2.5 sphere −0.5 cylinder axis 5°. UCVA was 20/200, and spectacle-corrected visual acuity was 20/25. We performed a TopoLink LASIK. One month after surgery UCVA was 20/20 and refraction was plano. Corneal topography showed no irregularities ( Fig. 14.3 ).
Patient 3: Decentered Ablation
This 36-year-old woman had LASIK in both eyes in 1998 and was referred because of a decentered ablation. The right eye was perfect but the patient complained bitterly about permanent monocular diplopia and distorted halos in her left eye. A TopoLink LASIK was planned. The corneal topography taken prior to the TopoLink LASIK is shown in Fig. 14.4 , lower left and Fig. 14.5 , lower right. A decentered myopic ablation is visible. The ablation is decentered about 1.5 mm downwards and 1 mm temporally. We calculated a customized ablation based on the Orbscan II topographic map just described. The planned ablation pattern is shown in Fig. 14.4 , upper right; the scale is in microns. The predicted outcome of corneal topography is shown in Fig. 14.4 , lower right. The scale is in diopters again. The Hansatome (Bausch & Lomb) was used to create a new flap with a thickness of 160 µm and a diameter of 8.5 mm (8.5-mm suction ring). The ablation was centered on the center of the entrance pupil, and the eye tracker was used. Fig. 14.5 shows the preoperative and postoperative maps as well as the differential map, taken 1 day after surgery. The postoperative map (see Fig. 14.5 , upper right ) shows significantly improved centration and no residual astigmatism. The differential map ( Fig. 14.5 , left ) shows the asymmetric ablation pattern, customized to this individual eye. Visual acuity improved to 20/25 uncorrected; even more important, monocular double vision and halos were no longer visible.
