Laser in situ keratomileusis (LASIK) has gained popularity because of its advantages over photorefractive keratectomy (PRK) such as preservation of Bowman's layer, rapid recovery, and significantly less postoperative pain.1 With the recent approval by the U.S. Food and Drug Administration of hyperopic LASIK, an increasing number of studies have begun to evaluate its safety and efficacy.1–12
Excimer lasers that deliver small “flying spot” beams in addition to accurate eye-tracking technologies are the latest advancements in the field. The purpose of this study was to analyze the results of LASIK treatment of eyes with hyperopia with the center of the laser ablation being on the coaxially sighted corneal light reflex of the fixation light.
Patients and Methods
Charts of consecutive patients who had uncomplicated primary LASIK by a single surgeon (B.B.W.) from November 2000 to January 2002 at a refractive surgery practice were reviewed. Only data for eyes that had hyperopic corrections programmed into the laser and had completed a minimum of 3 months of follow-up were enrolled. Sixty-one eyes of 41 patients (17 women and 24 men) were retrospectively analyzed. Spherical corrections were performed on 13 eyes, and astigmatic corrections were performed on 48 eyes. The mean patient age was 55.6 years ± 10.6 (SD) (range 20.8 to 70.7 years).
Standard preoperative and postoperative examinations were done on all patients, including uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), uncorrected contrast sensitivity (UCCS), best spectacle-corrected contrast sensitivity (BSCCS), slitlamp biomicroscopy, tonometry (Tono-Pen®, Medtronic Ophthalmics), pachymetry (Advent pachymeter), videokeratography (C-scan corneal topography, MedRx), manifest and cycloplegic refractions, and a dilated fundus examination. LogMAR visual acuity was measured using the CSV-1000 acuity chart (Vector Vision). Contrast sensitivity log units were measured using the CSV-1000 contrast sensitivity chart (Vector Vision) at spatial frequency of 12 cycles/degree (cpd).
Informed consent was obtained from all patients, and approval from the institutional review board of the institution was received. All patients were treated with the LADARVision 4000 excimer laser (Alcon). A uniform optical zone of 6.0 mm with a 3.0-mm blend zone was used for both spherical and cylindrical corrections. The ablation was centered on the coaxially sighted corneal light reflex (CRC) based on the study by Pande and Hillman,13 who showed that the CRC gives the optical axis of the cornea (the line joining the fixation point to the center of the curvature of the cornea) that is nearest the corneal intercept of the visual axis (VAC), the line joining the fovea to the fixation target, compared with the entrance pupil center (EPC) and geometric corneal center (GCC). Uozato and Guyton14 recommend procedures be centered on the EPC, defined as the line joining the fixation point to the center of the entrance pupil, which is also known as the line of sight.
The CRC was obtained on the LADARVision 4000 with the following technique. Patient eye photos were captured only on dilated pupils that included the red corneal light reflex of the patient fixation light (CRC) with the laser head. On the computer monitor, the green cross hair (representing the center of the ablation) was placed directly on top of the red corneal light reflex. The limbus ring was aligned per standard technique. The 4 white laser illumination lights for the surgical field had been adjusted so that they were equidistant from the red corneal light reflex. During the LASIK procedure, after the flap was lifted, the red corneal light reflex of the relatively rough stromal bed could not be visualized through the microscope oculars or on the computer monitor. However, the white illumination lights reflected off the stromal bed as visualized through the microscope oculars and on the computer monitor. On the monitor, the green cross hair was placed in the equidistant center of the 4 white illumination lights, which is the location of the CRC.
For most eyes, the Hansatome® (Bausch & Lomb Surgical) and the SKBM (Alcon) microkeratomes were used. Other microkeratomes used were the LSK (Moria) and Carriazo-Barraquer (Moria). Preoperative and postoperative BSCVA and BSCCS were analyzed, as was the postoperative deviation from target and UCVA.
A Microsoft Excel spreadsheet was initially used to enter the data, which were later transferred to and updated in Statview version 5.0.1 (SAS Institute) for statistical analysis. A paired t test was used to analyze preoperative and postoperative BSCVA and BSCCS for each eye. Regression analysis was then used to evaluate the relationship between the attempted correction and the achieved correction 3 months postoperatively. A P value of less than 0.05 was considered statistically significant.
The mean attempted correction (spherical equivalent [SE]) was + 2.73 ± 1.41 diopters (D) (range +0.38 to +6.0 D). The mean spherical correction (range + 0.9 D to +6.0 D) was +3.41 ± 1.43 D, and the maximum cylindrical correction was −5.25 D (mean −1.35 ± 1.25 D). Mean SE of the deviation from target at 3 months after LASIK was +0.25 ± 0.82 D.
Table 1 shows the available data for 3-month uncorrected binocular vision of 21 patients who had primary hyperopic LASIK. Table 2 summarizes the key efficacy and safety variables.
No visually significant complications were encountered intraoperatively. Seven eyes (11.5%) had loose epithelium noted during surgery, and 2 others (3.3%) had small epithelial defects intraoperatively. The Hansatome microkeratome was used in most of the loose epithelium cases (4 eyes), although the LSK was used in 2 eyes. The SKBM microkeratome was used in 1 case with loose epithelium and the 2 that had epithelial defects. Postoperatively, 1 eye required lifting and repositioning of the flap after 2 weeks for flap wrinkles (1.6%). Another patient had both flaps dislocated on the first postoperative day, which required repositioning and 1 nylon 10-0 suture for each flap (3.3%). The sutures were removed 2 weeks later. None of the eyes lost 2 or more lines of BSCVA. The BSCCS decreased by 4 patches in the eye that needed flap repositioning 2 weeks postoperatively. It recovered by 9 months postoperatively.
Mean achieved correction at 3 months after LASIK was +2.87 ± 1.7 D. Simple regression analysis showed good correlation between the attempted and achieved corrections, as evidenced by the adjusted R2 value of 0.64 (Figure 1). Mean deviation from target 3 months postoperatively also showed a mean undercorrection of +0.25 ± 0.82 D.
Bscva and bsccs
As seen in Table 2, none of the eyes lost 2 or more lines of BSCVA. Only 1 of the eyes lost 4 patches of BSCCS (1.6%), and 3 eyes lost 3 patches of BSCCS (4.9%).
Table 3 summarizes the BSCVA and the BSCCS pre- and postoperatively. Paired t test showed no statistical difference between the pre- and the postoperative BSCVA (P = .75). No significant changes in BSCCS were seen before and after LASIK with a paired t test (P = .55).
A minimum of 3 months of postoperative results were used in our study, based on the study of Rashad3 in which the refractive results for spherical and astigmatic hyperopia were stable by 3 months after surgery if a large ablation zone of 6.0 mm with a transition zone of 9.0 mm were used. As for visual acuity, no significant change in BSCVA was noted (Table 3) by 3 months postoperatively. Contrast sensitivity, a more sensitive measure of visual function than Snellen acuity, is important to evaluate because the visual environment comprises objects of varying contrasts. In our study, spatial frequencies of 12 cpd were used to test contrast sensitivity. According to Mutyala and coauthors,15 using the CSV 1000, 12 cpd was found to be the most sensitive frequency to detect reduction in contrast sensitivity after LASIK. As in the case of BSCVA, no significant change in the BSCCS (P = .55) was seen (Table 3).
Efficacy was determined in terms of UCVA of patients who were not corrected for monovision and who had preoperative BSCVA of 20/20 or better. Davidorf and coauthors4 report that 40% of eyes achieved 20/20 or better, 60% of eyes achieved 20/25 or better, and approximately 90% achieved 20/40 or better UCVA at their last follow-up. They also found that no eyes in the study lost more than 1 line of BSCVA postoperatively. In our study, the postoperative values for UCVA were similar, with 44.4% achieving 20/20 or better, 55.6% achieving 20/25 or better, and 88.9% achieving 20/40 or better. Moreover, no eye lost 2 or more lines of BSCVA (Table 2). In Esquenazi and Mendoza's5 study, 64% were within ±0.50 D of target at 3 months, and 73% were within ±1.0 D. In comparison, our study showed 65.6% within ±0.50 D of target and 83.6% within ±1.0 D.
Unlike the studies of Esquenazi and Mendoza5 and Reviglio and coauthors,6 we did not have any uncommon LASIK complications (ie, flap amputations, free caps, buttonholes, or infections under the flap). Our intraoperative complications were confined to 7 cases of loose epithelium and 2 small epithelial defects. Postoperatively, no infections were seen, although 1 eye had flap wrinkles that needed refloating and repositioning. Two additional eyes had displaced flaps that required 1 nylon 10-0 suture each for 2 weeks.
Rashad3 also noted that the larger corneal flap of 9 to 10 mm in diameter created with the use of the Carriazo-Barraquer microkeratome helped eliminate the operative difficulties of hyperopic LASIK, facilitating full exposure of the corneal bed for laser ablation and proper centration. The microkeratomes we used, Hansatome and SKBM, had similar advantages. In addition, we believe that the centration method we used, based on corneal reflex instead of the center of the pupil, contributed to the satisfactory results of our study.
Contrary to the views of Uozato and Guyton,14 we have adopted into clinical practice the recommendations of Pande and Hillman,13 who suggest that surgeons center the laser ablation on the CRC, not the EPC. In 1987, Uozoto and Guyton14 asserted that centering on the pupil is the proper method of centration because the photoreceptors are aimed toward the center of a normal pupil; their method has since become standard practice. Our comment is that although their rationale may be valid, there is no evidence that other methods have an adverse effect on vision because of photoreceptor orientation. Uozoto and Guyton's criticism of using the corneal light reflex for centration was that it is dependent on the direction of the gaze of the eye with respect to the position of the fixation light source. Our comment is that if the eye is gazing at the fixation light source, it is properly fixated, and thus this criticism does not apply.
In their 1993 article, Pande and Hillman13 stated that the optimal centration is the corneal intercept of the visual axis because this is the line joining the fovea to the fixation point. In clinical practice, it is not practical to locate the visual axis intercept of the cornea. In their study, they determined and compared the locations of the VAC, CRC, EPC, and GCC. They determined that the CRC was located 0.02 mm from the VAC, whereas the EPC was located 0.34 mm from the VAC, significantly farther than the CRC. They explained that centering procedures on the EPC will alter the laser optical zone placement by 10% for a 6.0 mm pupil diameter with a range of displacement up to 1.06 mm and a standard deviation up to 0.21 mm. They also explained that the EPC is not a static point because it changes location with changes in pupil diameter for the same patient. They concluded that the CRC was nearest the corneal intercept of the visual axis compared with the EPC.
Our comment is that for hyperopic patients who have a tendency toward greater distance between the EPC and VAC, and thereby the CRC (the definition of positive angle kappa), this issue is more important than it is for myopic patients.16Figures 2 and 3 illustrate this issue. This importance is compounded because hyperopic functional corneal optical zones tend to be smaller compared with myopic functional optical zones, thereby making the tolerance of decentration smaller in the former group of patients (personal data).
Studies have reported a loss of at least 2 lines of BSCVA in 1.2% to 10% of cases.1,3,5 None of our cases lost 2 or more lines of BSCVA. The high safety profile in this study compared with that in others lends support to this method of centration, although other factors such as the larger ablation zone may have contributed to the safety record. The high safety results may be attributable to the laser we used. We believe, however, that the results of this study are largely due to the nature of centration as opposed to the laser or the optical zone diameter because the same surgeon has not had a similar level of safety with similar optical zones in the past, albeit using a different laser. A prospective evaluation of these methods of centration is indicated to determine if one centration technique is superior to the other when using an advanced flying spot laser.
This study shows that primary LASIK for spherical hyperopia of up to 6.0 D with astigmatism of up to 5.25 D using a 6.0 mm optical zone with a 9.0 mm blend zone is safe and effective and maintains preoperative BSCVA and BSCCS. Our practice pattern for patients with less than 6 D of hyperopia is LASIK because of the high safety results achieved with this centration technique. For patients with greater than 6 D of hyperopia, we perform clear lens extraction with intraocular lens implantation.
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