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Topographic changes induced by flap creation
Flap creation alone changes corneal topography, a study shows. The authors suggest performing LASIK in two stages.
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LASIK and PRK each have their respective advantages. We believe that PRK and other, better surface ablation techniques are re-emerging, despite the enthusiasm for LASIK. We also believe that surface ablation will gain more credibility as laser technology and biopharmacological modulation improve. As an example, we have been working with amniotic membrane and autologous serum after PRK and achieving good results, although improvement is still needed.
Nevertheless, LASIK acceptance continues to grow due to its greater degree of refractive stability and predictability with higher corrections, rapid recovery time, less significant postoperative discomfort and decreased rate of haze complications. The complications most often described with LASIK are a result of flap creation.
Microkeratomes have been designed to perform corneal lamellar resection to achieve a supposed diameter and thickness; however, multiple factors — environmental, mechanical and thermal — can affect the quality of a corneal disc. With manual microkeratomes, advancing speed, smoothness of movement and the pressure exerted by the suction ring can modify the quality of the flap. With an automatic microkeratome cut, advancing speed and frequency of the cut are considered to have the greatest influence. In addition, the cut itself changes the topographic map due to the direct interruption of tensile strength exerted by the totality of the corneal thickness.
Considering these factors, we attempted to identify the topographic changes induced by corneal flap creation. We studied the topographic changes caused by the cut with a Moria LASIK One microkeratome, a linear manual microkeratome that creates a nasal-hinged flap, and the Moria M2, an automatic rotational microkeratome.
These are the preliminary results of a prospective study in which we evaluated microkeratome-induced topographic changes and the predictability of flap thickness comparing different equipment and different hinge positions. Final results will be published in the next few months.
Five-point pachymetry
In this report, we review the results in 25 eyes operated on using the M2 microkeratome with the head designed to create a flap of 150 µm.
Preop Orbscan (Bausch & Lomb) and Keratron Scout (Optikon) topographic images were made immediately before performing the microkeratome cut. Three sequential data acquisitions were done with the Orbscan preop and immediately prior to flap creation, and three sequential exams were acquired with the Keratron Scout when the patient was under the laser. Pachymetry was measured with the DGH ultrasonic pachymeter (DGH Technologies Inc.) in the center and in four cardinal points placed 3 mm away from the center. These five points and three ultrasound values were recorded.
Immediately after the cut and before lifting the flap, a Keratron Scout topography exam was performed. After the flap was lifted, ultrasound pachymetry of the stromal bed was acquired in the 5 positions described above (three measures per position). Follow-up was done at 1 day, 1 week and 3 months.
Preliminary results
In right eyes we found a mean topographic change of 0.91 D induced at 3 mm from corneal center in the horizontal axis (range between 0.31 and 2.75 D) and 1.32 D in the vertical axis (range between 0.17 and 2.35) (figure 1a). In the left eye, the mean topographic change induced at 3 mm was 0.95 D (range 0.32 to 2.35) in the horizontal axis and 1.68 in the vertical axis (range 0.32 to 2.35) (figure 1b).
We obtained a mean axis displacement of 27.1° in the right eye and 13.5° in the left eye, both in an inferior direction with respect to the hinge position (figures 2a and 2b). The hinge was placed in the temporal-superior area in the right eye and in the nasal-superior area in the left eye. We observed that the change persisted until 6 weeks postop. The peripheral measurements improved slightly with time, but not the central measurement.
Using the five-point pachymetry method, we found we can create a three-dimensional representation of the flap. In this way we obtained the following data: In the right eye, the mean central pachymetry was 136 µm; at 3 mm from the center in the superior area was 153.8 µm; in inferior area 147.4 µm; in temporal area 148.3 µm; and in nasal area 153.8 µm (figure 3a). In the left eye, the mean central pachymetry was 127 µm; at 3 mm from the center in the superior area was 153.8 µm; in the inferior area 147.4 µm; in the temporal area 148.3 µm; and in nasal area 153.9 µm (figure 3b).
With these preliminary results, we can demonstrate the importance of taking into account topographic changes induced by the microkeratome. In the field of customized ablation, it is especially important to use either wavefront data or topographic information to observe the cut-induced alteration prior to laser application. Therefore, for any customized lamellar surgery, we recommend a “two-step” LASIK procedure — waiting at least between 3 to 5 weeks before laser application after the microkeratome cut — to achieve better results.
For Your Information:
- Jose L. Güell, MD, PhD, can be reached at the department of corneal and refractive surgery at the Instituto de Microcirugia Ocular, c. Munner, 10-08022 Barcelona, Spain; +(93) 253 15 00; fax: +(93) 417 13 01; e-mail: guell@imo.es.