Introduction
Laser-assisted in situ keratomileusis (LASIK) is one of the most commonly used procedures currently to reduce the dependency or need for glasses.1 Newer LASIK procedures attempt to correct the refractive error as well as adjust the higher order aberrations (HOA).2 These procedures may use either the topography of the cornea or wavefront of the eye. For topography guided lasik, anterior topography of the cornea is usually derived from Scheimpflug imaging systems. This can be a viable option for eyes with regular myopia and astigmatism too.3
The epithelium covers the true irregularities of the stroma.4,5 We had recently introduced a new method, where optical coherence tomography (OCT) was used to map the curvature and aberrations of the Bowman’s layer.4,5 We hypothesized that the topography of the Bowman’s layer could deliver a better treatment option for patients so that there was a more effective control over the higher order aberrations.4,5 In this study, we designed a prospective treatment in LASIK patients to evaluate the effect of Bowman’s topography in a current method of LASIK. One eye was chosen at random to undergo wavefront optimized LASIK (conventional) using keratometry from Scheimpflug imaging. The fellow eye of the same patient underwent Bowman assisted in-situ keratomileusis (BASIK). In BASIK, the same wavefront optimized treatment plan was used except for keratometry, which was obtained from imaging of the Bowman’s layer with OCT.
Methods
This was a prospective, interventional, and single-blinded study, which was approved by the Narayana Nethralaya Ethics committee, Bangalore. Written informed consent was obtained from the patient prior to the procedure. The study adhered to the tenets of declaration of Helsinki. Inclusion criteria included stable refraction (less than -12D with astigmatism less than -3D) for a one year period (change less than 0.25D). Patients with central corneal thickness (CCT) < 480 µm or history any known ocular pathology and any prior ocular surgery or trauma, pregnancy or breast feeding were excluded from the study. In all the eyes, the expected residual stromal thickness was > 250 µm.
The baseline tests included the assessment of uncorrected (UDVA) and corrected (CDVA) distance visual acuity. Corneal tomography was assessed with Pentacam™ (OCULUS Optikgerate Gmbh, Wetzlar, Germany). All eyes underwent OCT imaging with RTVue™ (Optovue Inc., CA, USA). Keratometry (flat and steep axis) of the anterior surface was obtained from Pentacam. In addition, keratometry of the anterior surface and Bowman’s layer was calculated from OCT scans.4,5 One eye of each patient underwent wavefront optimized LASIK (Alcon Laboratories, Ft Worth, Texas, USA) using keratometry from Pentacam. The other eye underwent BASIK using keratometry of the Bowman’s layer. Procedure (LASIK or BASIK) was randomized to each eye. A single surgeon performed all the surgeries under topical anesthesia using 0.5% proparacaine hydrochloride instilled in the eye. The WaveLight FS200 femtosecond laser and WaveLight EX500 excimer laser was used for the entire procedure. The flap had a 9.0 mm diameter, 110 µm thickness, with optical zone of 6.0 mm.
Ocular aberrometry was also performed pre and postoperatively with Visual adaptive optics (VAO) analyzer. Zernike polynomials up to order 6 were used over a fixed analyses diameter of 4.5 mm. Contrast sensitivity was assessed with VAO. Three different contrasts were tested: 50%, 75% and 100% contrast. At each contrast, the patient reported the maximum cycles per degree, which he or she could distinguish clearly. Refractive error, Tomography, contrast sensitivity and aberrometry were performed pre and postoperatively (1, 3 and 6 month). UDVA and CDVA were assessed preoperatively and 6 months post operatively.
Statistics
All variables were tested for normality of distribution with Shapiro-Wilk test. Mean±standard deviation was calculated. The p-value was adjusted with Bonferroni correction. A p-value < 0.05 was considered statistically significant. All statistical calculations were performed using MedCalc v18.5 software.
Results
Statistically, vertical coma, horizontal coma, oblique trefoil, vertical trefoil and spherical aberration (SA) were similar between the LASIK and BASIK eyes. Preoperative contrast sensitivity (cycles/degree) at different luminance were also similar between the two procedures.
On the basis of mean curvature (average of K1 and K2), Pentacam value was ~1D and ~1.35D steeper than OCT anterior and Bowman’s surface, respectively (p<0.0001). At 6 months, compared to preoperative, 54.4% and 70.2% of the LASIK eyes had postoperative UDVA of 20/20 or better. Further, 98.3% of the eyes gained or had no change in Snellen lines on CDVA. Compared to preoperative, 54.4% (p=1.0) and 70.2% (p=1.0) of the BASIK eyes had postoperative UDVA of 20/20 or better. Further, 98.2% (p=1.0) of the eyes gained or had no change in Snellen lines on CDVA. Thus, both groups had similar outcomes.
Both groups had similar contrast sensitivity preoperatively (p>0.05). Further, they had significant improvement in contrast sensitivity after surgery, which was stable up to 6-month follow-up (p<0.001). In both groups, aberrations were relatively unchanged from 1 to 6-month follow-up. In LASIK, vertical coma, SA and root mean square (RMS) of higher order aberrations (HOA) increased significantly after surgery (p<0.05). Particularly, mean SA increased from 0.019 µm to 0.057 µm, a difference of 0.038 µm. In BASIK eyes as well, vertical coma, SA and RMS HOA increased significantly after surgery (p<0.05). Mean SA increased from 0.004 µm to 0.049 µm, a difference of 0.045 µm. Statistically, LASIK and BASIK had similar change in aberrations as a result of surgery. However, a very interesting trend was noted in SA. The increase in mean SA was 18.42% more in BASIK eyes than in LASIK eyes. Thus, a small adjustment in keratometry appeared to cause a differential change in SA only between LASIK and BASIK eyes.
Discussion
Since epithelium smoothens out the stromal irregularities, planning of customized refractive treatments can benefit with bowman’s topography.4,5 In LASIK and BASIK eyes, keratometry from Pentacam and OCT were used. Both LASIK and BASIK had similar visual, refractive and contrast sensitivity outcomes up to 6-month follow-up. However, the trends with ocular aberrations, particularly SA, were noteworthy.
An interesting study investigated the correlation between the refractive state of the eye and ocular spherical aberration in emmetropic eyes using adaptive optics.6 When positive or negative SA was induced, emmetropic eyes became myopic or hyperopic, respectively.6 The study concluded that controlling the induction of positive spherical aberration after LASIK is important to ensure optical quality of night vision and reduce presbyopia symptoms.6
The most notable effect of BASIK treatment was the relative change in mean ocular SA. BASIK eyes had a 18.42% greater increase in mean ocular SA compared to LASIK. Since keratometry was the only modifiable parameter, its potential use to modulate ocular SA was a very interesting finding. Refractive indices of air and epithelium are really different. This difference is very small between epithelium and stroma. Hence, finer changes in ocular SA may be induced by using the topography of the Bowman’s layer. Oblique trefoil of LASIK eyes was significantly different from the BASIK eyes at 6 months. This difference was too small to be clinically significant. Visual acuity and refractive outcomes were excellent in both groups. These results show the selective effect of Bowman’s keratometry on ocular aberrations only. LASIK leaves the internal aberrations to their preoperative levels.7 Thus, these are unlikely to change even in this study.
Preoperatively, Bowman’s keratometry was flatter than anterior OCT and Pentacam keratometry. Thus, use of lower keratometry in BASIK eyes compared to LASIK eyes probably led to the specific ocular SA changes.. A limitation of the study was that only 16 meridians from OCT (RTVue) were available for topography reconstruction as compared to 50 from Pentacam. In conclusion, visual, refractive, contrast and most aberrometric outcomes were comparable between LASIK and BASIK groups. Only mean ocular SA tended to be slightly greater in the BASIK group.
References
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