Femtosecond Laser Flap Creation for Laser In Situ Keratomileusis in the Setting of Previous Radial Keratotomy

Abstract

Purpose 

The aim of the study was to report the outcomes of laser in situ keratomileusis (LASIK) in subjects with previous radial keratotomy (RK) using a novel femtosecond laser setting on a proprietary femtosecond laser platform.

Design 

This was a retrospective, consecutive chart review of patients at a single private practice institution.

Methods 

The medical records of 16 eyes of 8 subjects who underwent femtosecond-assisted LASIK for consecutive hyperopia after RK were retrospectively reviewed. The preoperative characteristics, intraoperative details, and postoperative outcomes were analyzed.

Results 

All 16 eyes had successful femtosecond laser flap creation without significant intraoperative or postoperative complications. Uncorrected visual acuity significantly improved postoperatively (P = 0.0142) and remained stable through the final follow-up interval at 9 to 12 months postoperatively. None of the subjects lost any lines of best spectacle-corrected visual acuity in the postoperative period.

Conclusions 

The novel femtosecond laser technique described in this study can provide a safe and effective method for patients undergoing LASIK after previous RK. Future investigations are required to further validate the findings reported in this study.

A significant portion of patients who underwent radial keratotomy (RK) during the 1980s and 1990s developed hyperopic regression.^1–3 These patients may present many years later seeking additional refractive treatments.⁴ Previous investigators have described successful enhancement for consecutive hyperopia with suture techniques, excimer laser photoablation, and even refractive lensectomy.^5–11 While there is no clear consensus as to which refractive modality is most effective in this scenario, laser in situ keratomileusis (LASIK) in particular has been successfully reported using various mechanical microkeratome techniques.^12–14 As femtosecond laser technology has become more prevalent among refractive surgeons, there has been increasing clinical interest in determining whether this technology could be safely utilized to create flaps through existing RK incisions.¹⁵ Prior studies have shown that standard femtosecond laser settings can frequently lead to intraoperative complications including RK incision openings and loss of suction during the femtosecond laser treatment as well as postoperative complications including interface inflammation, corneal haze, and loss of best spectacle-corrected visual acuity (BSCVA).^16,17

To our knowledge, no one has ever reported a successful technique for femtosecond laser flap creation in patients undergoing LASIK after previous RK. In this study, we report a series of post-RK patients who underwent femtosecond-assisted LASIK using novel femtosecond laser settings on the Wavelight FS200 (Alcon, Fort Worth, Tex) femtosecond laser platform.

MATERIALS AND METHODS

The Southwest Retina Specialists Institutional Review Board (IORG0007600/IRB00009122) approved this retrospective, consecutive chart review of patients who received femtosecond laser-assisted LASIK using certain femtosecond settings at a single private practice institution from September 2013 through February 2014. All research components adhered to the tenets of the Declaration of Helsinki and were conducted in accordance with human research regulations and standards.

Inclusion/Exclusion Criteria and Data Collection

The operative eyes of all post-RK patients who underwent femtosecond-assisted LASIK on the Wavelight FS200 femtosecond laser and the Allegretto Wave Eye-Q 400-Hz excimer laser platforms (Alcon) using novel femtosecond laser settings during the aforementioned study interval were included. The baseline demographic data collected from each subject included sex, age, preoperative uncorrected visual acuity (UCVA), preoperative BSCVA, and preoperative manifest refraction spherical equivalent. In addition, the preoperative corneal pachymetry and corneal epithelial thickness were measured by optical coherence tomography (OCT) with the Cirrus HD-OCT (Carl Zeiss Meditec, Inc, Dublin, Calif). Ultrasonic pachymetry was used intraoperatively to measure the stromal bed thickness immediately after the flap was lifted but before the excimer laser photoablation treatment. Intraoperative details and any intraoperative or postoperative complications were recorded for each case. Each subject’s UCVA, BSCVA, and refractive measurements were taken postoperatively at 2 weeks, 2 months, 4 to 6 months, and 9 to 12 months.

Femtosecond Laser Settings

The following laser settings were programmed for the flap creation in all study subjects: bed cut energy = 1.4 μJ, bed cut spot separation = 6.0 μm, bed cut line separation = 6.0 μm, side cut energy = 0.8 μJ, side cut spot separation = 5.0 μm, and side cut line separation = 3.0 μm. The summary of these settings compared with the standard manufacturer default flap settings is detailed in Table 1. All study subjects had a 9.0-mm flap diameter with a 70-degree side cut angle. The flap depth varied according to preoperative OCT measurements of both the total central corneal thickness and the central epithelial thickness of each patient. The target flap depth was selected to be in the range of at least 100 μm thicker than the central epithelial thickness and yet thinner than that required to leave at minimum 350 μm of residual stromal bed tissue after excimer laser photoablation.

TABLE 1: Novel Femtosecond LASIK Flap Settings Compared With the Default Manufacturer Settings on the FS200 Femtosecond Laser

Statistical Analysis

The JMP 11 mathematical software package from the SAS Institute (Cary, NC) was used to perform statistical analysis to calculate means and SDs. One-way analysis of the variance was used when comparing means. Results were considered statistically significant at the α < 0.05 level.

RESULTS

There were 16 eyes of 8 subjects included in the analysis with a mean follow-up interval of 318.1 ± 70.3 days. The summary of the baseline characteristics and demographic features are presented in Table 2. There were no cases in which suction was either lost or could not be obtained, and all flaps were able to be lifted without complication. There were no cases in which the RK incisions opened during surgical manipulation of the flap during surgery. In addition, there were no cases of vertical gas breakthrough or formation of opaque bubble layer in any of the patients. The mean intended flap depth for the study population was 192 ± 13 μm. Intraoperative measurements demonstrated that the average error from the intended femtosecond flap depth was −11.9 μm ± 8.5, but there was not a statistical difference among the target depth versus the actual measured depth (P = 0.1775). No patients had postoperative refloating or repositioning of the flap. Uncorrected visual acuity significantly improved at 2 months’ follow-up (P = 0.0033), 4 to 6 months’ follow-up (P = 0.0120), and 9 to 12 months’ follow-up (P = 0.0142). Postoperative spherical equivalent averaged 0.23 ± 0.18 diopters (D), and UCVA in logMAR averaged 0.13 ± 0.10 at 9 to 12 months’ follow-up. No significant refractive shifts or regression (>1 D) was noted over the various follow-up intervals, and there were no eyes that lost 1 or more lines of BSCVA at the final postoperative follow-up visit.

TABLE 2: Baseline Features of the Study Population

DISCUSSION

This case series is the first to describe a successful femtosecond-assisted LASIK technique in the setting of previous RK on the Wavelight FS200. The efficacy and predictability of this technique in terms of visual results show outcomes comparable to previously reported studies using femtosecond LASIK in otherwise healthy eyes without history of RK where greater than 97% of all treated eyes achieved postoperative spherical equivalent within 1 D of error from the intended refractive target.¹⁸ As seen in this study and others, many post-RK patients have a thickened corneal epithelium, and standard depth femtosecond flaps can potentially lead to the formation of a flap with compromised structural integrity.¹⁹ Separating femtosecond laser stromal tissue bridges can further traumatize flaps and unintentionally separate the existing RK incisions. In our series, there were no cases of opaque bubble layer in any of the patients because the stromal tissue in the lamellar plane could be visibly seen separating during the femtosecond laser treatment. Increasing both the power per spot and the total number of spots allows greater photodisruption in the lamellar plane with less tissue bridge formation, especially where there are tighter adhesions present in the case of fibrotic RK scars. The characteristic femtosecond laser stromal tissue bridges in the lamellar plane were found to be completely absent using our power and spot separation settings, and excluding the side cut tissue bridges, the flap lifted similar to a mechanical microkeratome cut with virtually no resistance. For this reason, we intentionally left the standard default side cut femtosecond settings unchanged to avoid premature flap dislocation while transferring the patient from the femtosecond laser to the excimer laser.

The femtosecond laser settings used in this study were noted to give flap depths of approximately 12 μm (or 6.8%) thinner than anticipated. This finding is important to note when selecting the appropriate flap depth, especially in cases where the corneal epithelium tends to be thicker such as after RK. The small number of cases reported in this study are inadequately powered to determine if this trend is statistically significant.

Weaknesses of this study include its retrospective design, the lack of a control group, the small number of cases, and the relatively short postoperative follow-up period. Future investigations will be necessary to validate the femtosecond-assisted LASIK technique described in this study in subjects with previous RK. Further studies may also determine if similarly enhanced femtosecond laser settings will allow for successful flap creation in patients with corneal scars from causes other than RK such as trauma or a previous corneal ulcer.