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Ablation of flap and bed can be effective in high myopes
For patients with thin corneas, ablating the underside of the LASIK flap can spare stromal tissue.
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LASIK, combined lamellar corneal surgery with excimer laser photoablation, has been widely performed as the procedure of choice for correcting refractive errors. Although it is a safe and effective procedure overall, in some circumstances the stromal bed is not thick enough to qualify for ablation of high myopia. An increasing number of reports describe keratectasia after conventional LASIK surgery.
The anterior flap after LASIK does not contribute to the biomechanics of the cornea; the stress is supported by the residual thickness of the stromal bed only. Therefore, the question arises of a minimal thickness of the residual stroma related to the upper limit of myopia correction by LASIK.
Based on biomechanical consideration, Seiler et al recommended a residual corneal thickness of over 250 µm after LASIK to prevent corneal ectasia in a normal cornea, and Barraquer has suggested a 300-µm thickness of stress-bearing cornea.
Based on these reports and our extensive experience with lamellar refractive surgery, we strongly believe a minimal thickness of 300 µm of stress-bearing corneal stroma is necessary to prevent keratectasia after LASIK.
We have developed a novel technique for LASIK in eyes with high myopia with insufficient corneal thickness for conventional LASIK ablation. With the aid of intraoperative pachymetry, the actual stromal bed thickness was evaluated, and in those eyes deemed suitable, simultaneous LASIK — both stromal bed and undersurface of the flap — was performed to extend the range of myopia correction by means of LASIK.
We included 43 eyes of patients who had high myopia with insufficient corneal thickness for conventional LASIK ablation (the remaining stromal bed thickness was less than 300 µm after conventional LASIK). Age range was between 20 and 51 years (mean ± SD: 29.6 ± 7.9 years).
| Important steps in simultaneous LASIK
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Simultaneous surgical procedures
The center of the entrance pupil was carefully marked with a Gulani LASIK marker (Figure 1a) and a Slade LASIK spatula on the corneal top, inked with a gentian violet solution (Figure 1b). The peripheral cornea was also marked with a Gulani LASIK marker as a routine procedure (Figure 1c).
The hinged anterior corneal flap was temporarily elevated, and the thickness of the actual residual stromal bed was measured several times using a pachymeter (Figure 1d).
We calculated each amount of the laser ablation in stromal bed and undersurface of the flap out of the total attempted amount. We use the formula F = S(S+T) + F(S) , where F = full correction amount, S(S+T) = stromal portion (spherical correction + toric correction) and F(S) = flap portion (additional spherical correction).
Toric ablation for astigmatism was performed only on the stromal bed, and the ablation on the undersurface of the flap was only for residual spherical ablation power. The application of this formula did not lead to a change in toric ablations axis on the stromal portion. The stromal ablation pattern (spherical with or without toric ablation) and the undersurface of the flap ablation pattern (spherical ablation) were identical to conventional LASIK. (Figure 1e).
All these eyes received laser ablation without violating the posterior 300 µm of remaining stromal bed after stromal portion laser ablation. After ablating on the stromal bed portion with laser, the flap was then reflected out on a specially designed rigid contact lens and carefully extended with a wet, soft Merocel sponge.
We used the specially ordered light-blue rigid contact lens (8.8-mm diameter, 6.5-mm front base curve, 9-mm hind base curve) to support the flap (Figure 1f). Therefore, the surgeon had better control over centration of the ablation of the undersurface of the flap and ultimately increased the maximal visual improvement. Adequate exposure of the flap stroma was achieved by requesting the patient to look downward. After accurate centration of the helium-neon laser aiming beam on the marked visual axis, the photoablation was performed under operating microscope surveillance (Figure 1g).
The laser-treated corneal flap was then folded back onto the cornea with a cannula. The interface was irrigated to remove debris with saline. The flap was applied with Johnston LASIK flap applicator to remove any fluid from the interface to enhance the seating of the flap to the stromal bed, and then painted into position with a wet, soft Merocel sponge. The flap was then allowed to settle for at least 3 minutes to ensure good adhesion between the flap and the stromal bed (Figure 1h).
Results
Mean spherical equivalent (SE) refraction before LASIK was –8.741 ± 2.32 D (range –12.5 to –3.75 D). After LASIK treatment, mean SE refraction was –0.128 ± 0.657 D (range –1.75 to +1.25 D) at 1 month, –0.183 ± 0.631 D (range –1.5 to +1.25 D) at 3 months and –0.128 ± 0.58 D (range –1.5 to +1.25 D) at 6 months. Postop refraction was statistically significantly lower than the pre-treatment values at all intervals (P < .0001,="" figure="" 2).="">
Before treatment, 26 of the 43 eyes had best corrected visual acuity of 20/20 or better, and 42 achieved 20/30 or better. Following treatment, uncorrected visual acuity of 20/20 was achieved in 15 eyes and 20/30 in 35 eyes at 1 month, 16 eyes and 36 eyes at 3 months and 16 eyes and 34 eyes at 6 months, respectively.
The achieved SE refraction of 25 of treated eyes was within ± 0.5 D of the targeted SE refraction; 39 eyes were within ± 1 D of the targeted SE refraction at 1 month.
Twenty-seven of treated eyes were within ± 0.5 D of the targeted SE refraction; 37 eyes were within ± 1 D at 3 months, and 29 eyes were within ± 0.5 D of the targeted SE refraction and 39 eyes were within ± 1 D at 6 months (Figure 3).
During surgery and the follow-up period, no vision-threatening complications occurred. Fine microstriae were present in one of 43 eyes. Interface debris was present in one eye after LASIK. During the follow-up period, serial tangential videokeratography examinations could not detect any sign of keratectasia.
If we could “save” the stroma, with at least 300 µm left by using a flap when the posterior stromal thickness was not within the currently accepted range for conventional LASIK surgery for high myopia, it might mean the difference between being a good candidate for correction or a high surgical risk. When ablating the undersurface of the flap, special care must be taken not to ablate through the disc into Bowman’s layer; otherwise, with wrinkling of Bowman’s layer, significant loss of BCVA could occur.
Maldonado reported that undersurface ablation of the flap for LASIK re-treatment for low residual refractive errors in eyes with an SE residual refraction between –0.75 and –3.25 D and astigmatism between zero and –1.5 D after LASIK with sufficient flap stroma seems to be effective and may prevent future keratectasia.
Our results indicate that simultaneous LASIK treatment appears to be safe. In our series of patients there were no serious complications and no eyes losing two or more lines of BCVA. No vision-threatening complications occurred. The predictability of simultaneous LASIK was acceptably good; 37 of treated eyes were within ± 1 D of the targeted SE refraction at 3 months; 39 eyes were within ± 1 D at 6 months.
In addition, the stability of the correction was also satisfactory, with two eyes changing more than 1 D throughout the postop period, and four eyes changing between 0.5 and 1 D. In our study, 34 eyes had UCVA of 20/30 or better at 6 months. This percentage reportedly ranges from 80% to 85.1% after conventional LASIK. As expected, no keratectasia or suspicious central corneal steepening could be detected by serial tangential videokeratography.
Simultaneous LASIK is a new treatment method that effectively reduces myopia with or without astigmatism in eyes with insufficient posterior stroma. Because the procedure did not involve further ablation of the residual posterior stroma less than the 300 µm that supplies tectonic integrity to the cornea, the risk of future keratectasia as a consequence of treatment in these eyes with thin cornea may be greatly reduced.
For Your Information:
- Jae-Ho Kim, MD, PhD, can be reached at The 21C Eye Hospital, Seoul Paik Hospital, Inje University, 85, 2-Ga, Jeo-Dong, Jung-Gu, Seoul, Korea, 100-032; (82) 2-2270-0236; fax: (82) 2-2266-6718; e-mail: kimjheye@ijnc.inje.ac.kr.
- Myung-Jin Joo, MD, PhD, can be reached at The 21C Eye Hospital, Seoul Paik Hospital, Inje University, 85, 2-Ga, Jeo-Dong, Jung-Gu, Seoul, Korea, 100-032; (82) 2-2270-0082; fax: (82) 2-2266-6159; e-mail: mjjoo@ijnc.inje.ac.kr.
Reference:
- Maldonado MJ. Undersurface ablation of the flap for laser in situ keratomileusis re-treatment. Ophthalmology. 2002;109:1453-1464.