Purpose
To analyze the long-term safety profile, visual and refractive results, and incidence of complications between sub-Bowman keratomileusis with 90- and 100-μm flaps.
Design
Prospective, randomized, comparative clinical study.
Method
A total of 385 candidates (770 eyes) underwent bilateral, single-sitting, sub-Bowman keratomileusis, with flap creation (90 or 100 μm) on IntraLase 60-kHz (Abott Medical Optics) and ablation on Technolas 217z100 (Technolas PV) . Right and left eyes were randomized to undergo 90- or 100-μm flap procedures. Preoperative and postoperative assessment included uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), refraction, and topographic analysis. All cases were followed up until 12 months after surgery. After excluding cases lost to follow-up, a final analysis of 368 patients was carried out (368 eyes in each of the 2 groups). The main outcome measures were BSCVA, UCVA, complication rates, and residual spherical equivalent refractive error.
Results
The mean preoperative values were: spherical equivalent, −6.08 ± 2.7 diopters (D; 90-μm group) and −5.99 ± 2.8 D (100-μm group; P = .7); and logarithm of the minimal angle of resolution BSCVA, 0.01 ± 0.03 (90-μm group) and 0.01 ± 0.04 (100-μm group: P = .8). Postoperative 12-month values were: spherical equivalent, −0.02 ± 0.4 D (90-μm group) and −0.01 ± 0.4 D (100-μm group; P = .8); logarithm of the minimal angle of resolution BSCVA, −0.05 ± 0.07 (90-μm group) and −0.04 ± 0.07 (100-μm group; P = .8); and logarithm of the minimal angle of resolution UCVA, 0.012 ± 0.01 (90-μm group) and 0.017 ± 0.02 (100-μm group; P = .2). No loss of BSCVA was seen in any case. The efficacy indices were 1.039 ± 0.21 (90-μm group) and 1.014 ± 0.24 (100-μm group; P = .2); safety indices were 1.163 ± 0.21 (90-μm group) and 1.158 ± 0.22 (100-μm group; P = .6); and vision difference indices were 0.09 ± 0.14 (90-μm group) and 0.10 ± 0.15 (100-μm group; P = .1). Both groups had a low but comparable incidence of diffuse lamellar keratitis and microstriae. However, the incidence of microstriae (although visually asymptomatic) was significantly higher in ablation with spherical equivalent of −9 D or more compared with lesser ablations (6.7% vs 0.8%; P < .001).
Conclusions
The 1-year follow-up of femtosecond sub-Bowman keratomileusis with 90- and 100-μm flaps suggests that both the flap options have comparable outcomes.
In recent years, there has been a significant improvement in our understanding of the alterations of the cornea after excimer laser ablation. It has been suggested that surface ablation procedures have better biomechanical outcomes compared with thick, flap-based procedures. However, traditional surface ablation is associated with slower recovery and increased risk of haze, limiting its useful in higher amounts of ablation. A hybrid approach is sub-Bowman keratomileusis. The term was proposed by Slade and associates in a comparative study between thin-flap (100 μm) laser in situ keratomileusis (LASIK) and photorefractive keratectomy. Sub-Bowman keratomileusis combines the faster visual recovery of flap-based LASIK with the biomechanical benefits of a surface ablation. A sub-Bowman flap currently can be made by either a microkeratome or a femtosecond laser, both having good results in either short- or intermediate term follow-up.
An important factor in determining the stability of a thin flap is the amount of epithelium and stroma in it. Reinstien and associates showed that the corneal epithelial thickness is approximately 53.4 ± 4.6 μm at the vertex. Therefore, a sub-Bowman flap should have a sufficient amount of anterior stromal collagen to prevent tenting, leading to microstriae formation; this is more so after deeper ablations.
It has been shown in previous studies that a femtosecond 90-μm or 100-μm flap can be created with comparable architectural quality and similar early visual results compared with thicker flaps on the same platform. Some surgeons prefer not to make the 90-μm flap because of a potential risk of a flap that is too thin, interface haze, or both. However, to the best of our knowledge, there are no comparative long-term studies (1 year or longer) of sub-Bowman keratomileusis options (90 and 100 μm) in a prospective, intereye comparative fashion. Therefore, in this study, we analyzed the long-term safety profile, visual and refractive results, and incidence of flap complications (diffuse lamellar keratitis [DLK], microstriae, or haze) and the long-term complications (ectasia or regression) in a fellow-eye comparison between 90- and 100-μm flaps.
Methods
This prospective, randomized, comparative clinical study was conducted at a tertiary care and teaching ophthalmic hospital. The initial cohort consisted of young, myopic patients who sought treatment at our refractive surgery service for bilateral femtosecond-assisted LASIK. All patients underwent a detailed LASIK assessment, including anterior segment slit-lamp microscopy, corneal topographic analysis by Orbscan IIz (Bausch & Lomb, Rochester, New York, USA), wavefront assessment by Zywave aberrometry (Bausch & Lomb), biometry, and dilated retinal indirect ophthalmoscopy. Exclusion criteria were: abnormal corneal topography suggestive of forme fruste keratoconus or corneal ectasia, minimum corneal thickness less than 480 μm (Orbscan measurement with acoustic factor of 0.94), residual bed thickness less than 300 μm at a 6.5-mm optical zone, local or systemic contraindications for LASIK, candidates who declined to undergo sub-Bowman keratomileusis (and instead opting for a 120-μm flap), and candidates who declined to be followed up for at least 12 months. A total of 385 candidates were included and underwent sub-Bowman keratomileusis. A computer-generated random number binary option sequence was used to decide the flap thickness. In this, each patient was allotted a random value by the software, where if the value was 1, the right eye was kept as 90 μm and the left eye was kept as 100 μm, and if value was 0, the right eye was kept as 100 μm and the left eye was kept as 90 μm.
All patients underwent flap creation by a 60-kHz femtosecond laser (IntraLase; Abott Medical Optics, Chicago, Illinois, USA). To prevent intersurgeon variation, all cases were performed by the same surgeon (G.P.) and same assistant (D.S.). The flap diameter was kept between 8.5 and 8.7 mm in all patients. The remaining parameters were: flap thickness, 100 or 90 μm; superior hinge; hinge angle, 55 degrees; bed energy, 0.9 μJ; spot separation, 7 μm; line separation, 7 μm; side-cut energy, 1.5 μJ; side-cut angle, 70 degrees; pocket enable, on; pocket width, 0.250 μm; pocket start depth, 220 μm; and both pocket tangent and radian spot separation, 5 μm.
Stromal ablation was performed with the Technolas 217z100 laser (Technolas PV, St Louis, Missouri, USA) with iris recognition-based Zyoptix personalized (wavefront-guided) algorithms. The stromal bed was irrigated with balanced salt solution after the excimer laser ablation to remove any debris, and then the flap was repositioned. Complications, if any, were noted. After surgery, patients received ofloxacin 0.3% eyedrops 3 times daily and dexamethasone 0.1% eyedrops 3 times daily for 1 week. Carboxymethyl cellulose drops were given 8 times daily for 1 month and were tapered to as and when required afterward.
Postoperative evaluation consisted of uncorrected visual acuity (UCVA); best spectacle-corrected visual acuity (BSCVA); refraction; and slit-lamp assessment at 1 day, 1 week, 1 month, 3 months, 6 months, and 12 months after surgery. Topography was performed at every postoperative visit beginning in month 1. Anterior segment optical coherence tomography (Visante; Carl Ziess AG, Jena, Germany) was carried out at 1 month after surgery using a flap thickness tool with manual override to analyze flap thickness. The mean flap thickness outcomes at each of the zones (from the corneal vertex, + is in positive and – is in the negative x-axis direction of the image): ± 0.5 mm, ± 1.5 mm, ± 2.5 mm, ± 3.5 mm was calculated along the 4 axes (0 degrees → 180 degrees, 45 degrees → 225 degrees, 90 degrees → 270 degrees, 135 degrees → 315 degrees), as described previously. The observer analyzing flap thickness was blinded to the attempted flap thickness. A single observer (D.A.K.) measured all flaps. Seventeen patients lost to follow-up between months 1 and 6 were excluded from our analysis.
Statistical Analysis
Because this was a bilateral eye study, the term cases denotes individual eyes and the term patient denotes each patient (2 eyes each). All outcome data were entered into a Microsoft Excel spreadsheet (Microsoft, Redmond, Washington, USA), and statistical calculations were performed using SPSS software version 16.0 for Windows (SPSS, Inc, Chicago, Illinois, USA). The 2-tailed Student t test was used to evaluate comparisons refractive and visual outcomes for parametric data and Rank Sum test was used for non parametric distributions. The P value was considered significant at less than 0.05. Standard refractive surgery reporting graphs were used to present the refractive outcome data.
Visual acuity was converted to logarithm of the minimum angle of resolution (logMAR) from the decimal notation for statistical analysis, using a conversion chart. The logMAR and decimal notations have scales in reverse direction to each other, for example 20/20 is 1.0 in Decimal and 0.0 in logMAR . All the previous literature we reviewed on the safety and efficacy indices took Decimal scores and therefore higher the index, better was the outcome. This would become reversed if we took the logMAR score for index calculation (a lower value will be better) and thus this would make it difficult for the results to be compared. The decimal notations were converted from the respective Early Treatment Diabetic Retinopathy values. logMAR is the standard method for comparing statistical data for visual acuity outcome because it converts the geometric sequence of a traditional chart to a linear scale. Thus it was used for statistical comparison between means of the visual acuities in this study too.
The decimal visual outcomes at 12 months were also used to derive three indices, first two previously used for reporting refractive surgery outcomes and a third new one:
Efficacy Index = UCV A Postop 12 months / BSCV A Preop
Visual Difference Index = ( BSCV A Postop 12 months – UCV A Postop 12 months ) / BSCV A Postop 12 months
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