Journal of Refractive Surgery

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NEW LASIK TECHNIQUES 

Combined Flap Undersurface and Bed LASIK for High Myopia

Li Wei Li, MD; Wang Qin Mei, MD; Fang Xue Jun, MD

Abstract

ABSTRACT

PURPOSE: To discuss the predictability, stability, and safety of LASIK for high myopia using laser treatment on both the flap undersurface and bed (both-sided LASIK).

METHODS: One hundred twenty-one eyes of 63 patients underwent flap undersurface and bed LASIK for high myopia. Postoperative uncorrected visual acuity (UCVA), refraction, and topography were evaluated up to 12 months. The safety and injury indices in both-sided LASIK were compared with that of conventional LASIK.

RESULTS: In both-sided LASIK, the largest diopter correction on the undersurface of the corneal flap was -6.00 diopters (D), correlating to an ablation of 44.62 µm. The mean thickness of residual corneal stroma bed was 283.87 ± 19.83 µm after the laser treatment. Postoperative UCVA was close to the preoperative best spectacle-corrected visual acuity (BSCVA). The mean BSCVA was significantly better (P<.05) than preoperatively at each time point (1 week, 1, 3, and 6 months, and 1 year) after surgery. Postoperative corneal topography was normal in all cases. The safety and injury indices for conventional LASIK in both-sided LASIK eyes were calculated by programming the excimer laser as though conventional treatments would be performed to calculate the theoretical bed ablation. The safety factor of both-sided LASIK was higher than that calculated for conventional LASIK (P<.05); the injury index was lower than that calculated for conventional LASIK (P<.05).

CONCLUSIONS: Surgery on high myopia by bothsided LASIK is safe and effective. [J Refract Surg. 2005;21(Suppl):S606-S609.]

Abstract

ABSTRACT

PURPOSE: To discuss the predictability, stability, and safety of LASIK for high myopia using laser treatment on both the flap undersurface and bed (both-sided LASIK).

METHODS: One hundred twenty-one eyes of 63 patients underwent flap undersurface and bed LASIK for high myopia. Postoperative uncorrected visual acuity (UCVA), refraction, and topography were evaluated up to 12 months. The safety and injury indices in both-sided LASIK were compared with that of conventional LASIK.

RESULTS: In both-sided LASIK, the largest diopter correction on the undersurface of the corneal flap was -6.00 diopters (D), correlating to an ablation of 44.62 µm. The mean thickness of residual corneal stroma bed was 283.87 ± 19.83 µm after the laser treatment. Postoperative UCVA was close to the preoperative best spectacle-corrected visual acuity (BSCVA). The mean BSCVA was significantly better (P<.05) than preoperatively at each time point (1 week, 1, 3, and 6 months, and 1 year) after surgery. Postoperative corneal topography was normal in all cases. The safety and injury indices for conventional LASIK in both-sided LASIK eyes were calculated by programming the excimer laser as though conventional treatments would be performed to calculate the theoretical bed ablation. The safety factor of both-sided LASIK was higher than that calculated for conventional LASIK (P<.05); the injury index was lower than that calculated for conventional LASIK (P<.05).

CONCLUSIONS: Surgery on high myopia by bothsided LASIK is safe and effective. [J Refract Surg. 2005;21(Suppl):S606-S609.]

Multiple studies have demonstrated that LASIK is safe and effective.12 Surgeons differ, however, in I their approach to performing LASIK for high myopia. In this study, LASIK was performed on the flap undersurface and bed, and its predictability, stability, and safety were analyzed.

PATIENTS AND METHODS

Between September and November 2003, 121 eyes of 63 patients (20 men [38 eyes] and 43 women [83 eyes]) underwent LASIK treatment on both the flap undersurface and bed (both-sided LASIK). Mean patient age was 28.61?7.53 years (range: 18 to 48 years). Preoperative examination included manifest and cycloplegic refraction, corneal topography and pachymetry, measurement of axial length, and a fundus examination. Mean preoperative refractive error was -11. 65 ± 2. 21 diopters (D) (range: -7.25 to -17.25 D). Mean preoperative central corneal pachymetry was 509.41 ± 29.42 pm (range: 467 to 588 µm).

The both-sided LASIK surgical technique involved separating the laser ablation with the NIDEK EC-5000 CXII excimer laser (NIDEK, Gamagori, Japan) into two steps based on the patient's refractive error and corneal thickness to ensure that at least 250 pm would remain in the stromal bed following laser ablation. The corneal flap was created with the NIDEK MK-2000 microkeratome with the 130-?p? head. The flap was reflected and placed on a flap mat (Fig). The first step in the laser treatment involved ablating the undersurface of the flap. Centration of the flap treatment was facilitated by marking the visual axis prior to creation of the flap and centering the reticule on the corneal mark. The treatments on the flap stroma had an optical zone between 3.0 and 4.5 mm with a transition zone of 4.5 to 6.0 mm. The second step involved ablation of the corneal stromal bed similar to conventional LASIK. In this step, the minimum optical zone was 5.0 or 5.5 mm and the transition zone diameter was between 7.0 and 8.0 mm.

The safety and injury indices for conventional LASIK in both-sided LASIK eyes were calculated by programming the excimer laser as though conventional treatments (ie, all of the treatment in the stromal bed) would be performed to calculate the theoretical bed ablation. In this study, the safety index was defined as the thickness of residual corneal stroma bed/central corneal pachymetry X 100%. The injury index was defined as the ablation depth on corneal stroma bed/central corneal pachymetry X 100%. Statistical analysis of the acuity and refractive data was performed with SPSS 11.5 statistical software (SPSS, Chicago, Ill).

RESULTS

On postoperative day 1, all flaps were normal and transparent by slit-lamp examination. The mean optical zone and transition zone diameters of the flap treatments were 3.54±0.61 mm and 5.18±0.27 mm, respectively. The mean optical zone and transition zone diameters of the stromal bed ablations were 5.63±7.68 mm and 7.77±0.29 mm, respectively. The degree of dioptric correction ablated onto the corneal flap was between -6.00 and -2.00 D (mean: -4.38 ± 0.96 D). The flap stroma ablation depth ranged from 15.10 to 44.62 µm (mean: 27.26±9.22 µm). The corneal stroma bed ablation depth ranged from 44.1 to 138.4 pm (mean: 93.45 ± 19.90 µm). The remaining corneal stromal bed thickness following ablation ranged from 252.8 to 330.6 µm.

Table 1 compares both-sided LASIK to calculated values as if conventional LASIK were to be performed on the same eye. Compared to a conventional LASIK procedure with all of the treatment in the corneal stromal bed, both-sided LASIK uses larger optical and transition zone diameters. In both-sided LASIK, corneal stromal bed ablation depth is significantly less than the calculated depth for conventional LASIK (P<.01), resulting in the mean total corneal thickness being 28.90 ± 16.40 pm thicker in both-sided LASIK than the calculated values for conventional LASIK. Also, in comparing both-sided LASIK to conventional LASIK, the safety index is larger (P<.01) and the injury index is less (P<.01).

The mean UCVA after both-sided LASIK was approximately 0.85 and significantly better than preoperative at each time point (P<.01) (Table 2). Mean BSCVA after both-sided LASIK was nearly identical to preoperative BSCVA at every time point after surgery (Table 3). The refractive outcomes with both-sided LASIK for high myopia were excellent and stabilized by 3-month follow-up. At 1 week postoperatively, the mean manifest refractive spherical aberration (MRSA) and cylinder were -0.35±1.79 D and -0.68±1.30D, respectively. At 1 month, 3 months, 6 months, and 1 year postoperatively, the mean MRSA and cylinder were -0.67±1.09 D and 0.59±0.90 D, -0.67±1.06 D and -0.72±0.80 D, -0.83±1.35 D and -0.46±0.66 D, and -0.53±1.09 D and -0.53±0.66 D, respectively.

Figure. Ablation of the flap bed.

Figure. Ablation of the flap bed.

Postoperative corneal topographies were regular and similar to our conventional LASIK patients for high myopia at each time point for every eye in the study. In terms of complications, cornea edema occurred 3 days postoperatively in one (0.83%) eye. In this same eye, the interface developed haze by 1 month but disappeared by 3 months, at which time the UCVA was 0.5.

DISCUSSION

Our study demonstrates that both-sided LASIK is safe and effective in high myopia with an improved safety and injury index when compared theoretically to conventional LASIK. The fact that postoperative topographies remain regular demonstrate that this technique does not induce significant irregular astigmatism.

One of the main benefits of LASIK is that the corneal epithelium is maintained, allowing fast visual recovery. Our study shows that up to 6.00 D of myopic refractive error can safely and effectively be treated on the undersurface of the corneal flap with a depth up to 44.62 pm. Six diopters of treatment performed on the flap represents approximately 35% of the total myopia in the highest myope in our study. By performing this treatment on the flap rather than the bed, more residual stromal tissue is maintained, theoretically decreasing the risk for ectasia. Clinical risk factors for ectasia after LASIK include patient age, preoperative intraocular pressure, corneal thickness, ablation depth, ablation diameter, diopter correction, and postoperative residual corneal thickness.

Table

TABLE 1Both-sided LASIK Compared to Calculated Conventional LASIK on the Same EyeTABLE 2Comparison of Pre- and Postoperative UCVA After Both-sided LASIKTABLE 3Comparison of Pre- and Postoperative BSCVA After Both-sided LASIK

TABLE 1

Both-sided LASIK Compared to Calculated Conventional LASIK on the Same Eye

TABLE 2

Comparison of Pre- and Postoperative UCVA After Both-sided LASIK

TABLE 3

Comparison of Pre- and Postoperative BSCVA After Both-sided LASIK

The residual stromal thickness after LASIK is what maintains corneal integrity and prevents ectasia. Experts suggest that residual stromal bed thickness should be between 250 and 325 µm after LASIK. Others maintain that at least 50% of the original central corneal thickness should remain in the residual bed. We recommend a residual postoperative corneal bed of at least 280 pm and a residual bed thickness >50% of the preoperative thickness.3"8 The safety and injury indices of both-sided LASIK are significantly lower than conventional LASIK, implying a lower risk of iatrogenic keratoconus with both-sided LASIK.

Laser in situ keratomileusis can destabilize the cornea by decreasing its tensile strength, which can lead to progressive weakening and irregular corneal astigmatism. Some patients manifest early irregular astigmatism following LASIK. Fortunately, this irregular astigmatism is typically due to temporary corneal edema that resolves as the flap thins, rather than corneal instability.913 We believe both-sided LASIK may speed up the resolution process of the cornea edema compared to conventional LASIK because the resulting flap is thinner.

In recent years, laser subepithelial keratomileusis (LASEK) was developed in an attempt to decrease pain and haze that occurred after photorefractive keratectomy while avoiding the potential risks of ectasia where too little residual corneal bed remains. Unfortunately, some patients still develop significant pain and haze after LASEK. Because LASEK does not routinely result in excellent outcomes for these high myopes because of haze, we believe both-sided LASIK is an excellent alternative for patients who would have a calculated residual corneal thickness <280 pm or <50% of the original central corneal thickness with conventional LASIK. Because both-sided LASIK results in less attenuation of the remaining stromal thickness, it will lead to more stable long-term results than conventional LASIK in high myopic patients with borderline corneal thickness.

REFERENCES

1. Chen Y et al. Excimer laser in situ keratomileusis for high myopia. Transactions of Beijing Medical Institute. 1 99 7; 2 9 : 3 99 - 402.

2. Knorz MC, Liermann A, Wieisinger B, Seiberth V, Liesenhoff H. Laser in situ keratomileusis (LASIK) for correction of myopia [German]. Ophthalmologe. 1997;94:775-779.

3. Liu L et al. Modern Ocular Surgery. Beijing, China: Beijing Military Medical Publication; 1995:215.

4. Li ZX, Xie LX, Hu LJ. Experimental study of the influence of cornea flap and residual cornea bed on corneal intensity after LASIK [Chinese]. Zhonghua Yan Ke Za Zhi. 2003;39:150-155.

5. Seitz B, Torres F, Langenbucher A, Behrens A, Suarez E. Posterior corneal curvature changes after myopic laser in situ keratomileusis. Ophthalmology. 2001;108:666-672.

6. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg. 1998;14:312-317.

7. Probst LE, Machat JJ. Mathematics of laser in situ keratomileusis for high myopia. J Cataract Refract Surg. 1998;24:190-195.

8. J oo CK, Kim TG. Corneal ectasia detected after laser in situ keratomileusis for correction of less than -12 diopters of myopia. J Cataract Refract Surg. 2000;26:292-295.

9. Argento C, Cosentino MJ, Tytiun A, Rap etti G, Zarate J. Corneal ectasia after laser in situ keratomileusis. / Cataract Refract Surg. 2001;27:1440-1448.

10. Maguen E, Zorapapel NC, Zieske JD, Ninomiya Y, Sado Y, Kenney MC, Ljubimov AV. Extracellular matrix and matrix metalloproteinase changes in human corneas after complicated laser-assisted in situ keratomileusis (LASIK). Cornea. 2002;21:95-100.

11. McLeod SD, Kisla TA, Caro NC, McMahon TT. Iatrogenic keratoconus: corneal ectasia following laser in situ keratomileusis for myopia. Arch Ophthalmol. 2000;118:282-284.

12. Baek T, Lee K, Kagaya F, Tomidokoro A, Amano S, Oshika T. Factors affecting the forward shift of posterior corneal surface after laser in situ keratomileusis. Ophthalmology. 2001;108:317320.

13. Pallikaris IG, Kymionis GD, Astyrakakis NI. Corneal ectasia induced by laser in situ keratomileusis. J Cataract Refract Surg. 2001;27:1796-1802.

TABLE 1

Both-sided LASIK Compared to Calculated Conventional LASIK on the Same Eye

TABLE 2

Comparison of Pre- and Postoperative UCVA After Both-sided LASIK

TABLE 3

Comparison of Pre- and Postoperative BSCVA After Both-sided LASIK

10.3928/1081-597X-20050902-09

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