Journal of Refractive Surgery

Original Article 

LASIK Versus Photorefractive Keratectomy for High Myopic (> 3 Diopter) Astigmatism

Toam Katz, MD; Lars Wagenfeld, MD; Peter Galambos, MD; Benedikt große Darrelmann, MD; Gisbert Richard, MD; Stephan Johannes Linke, MD

Abstract

PURPOSE:

To compare the efficacy, safety, predictability, and vector analysis indices of LASIK and photorefractive keratectomy (PRK) for correction of high cylinder of greater than 3 diopters (D) in myopic eyes.

METHODS:

The efficacy, safety, and predictability of LASIK or PRK performed in 114 consecutive randomly selected myopic eyes with an astigmatism of greater than 3 D were retrospectively analyzed at the 2- to 6-month follow-up visits. Vector analysis of the cylindrical correction was compared between the treatment groups.

RESULTS:

A total of 57 eyes receiving PRK and 57 eyes receiving LASIK of 114 refractive surgery candidates were enrolled in the study. No statistically significant difference in efficacy [efficacy index = 0.76 (±0.32) for PRK vs 0.74 (±0.19) for LASIK (P = .82)], safety [safety index = 1.10 (±0.26) for PRK vs 1.01 (±0.17) for LASIK (P = .121)], or predictability [achieved astigmatism < 1 D in 39% of PRK- and 54% of LASIK-treated eyes, and < 2 D in 88% of PRK- and 89% of LASIK-treated eyes (P = .218)] was demonstrated. Using Alpins vector analysis, the surgically induced astigmatism and difference vector were not significantly different between the surgery methods, whereas the correction index showed a slight and significant advantage of LASIK over PRK (1.25 for PRK and 1.06 for LASIK, P < .001).

CONCLUSIONS:

LASIK and PRK are comparably safe, effective, and predictable procedures for excimer laser correction of high astigmatism of greater than 3 D in myopic eyes. Predictability of the correction of the cylindrical component is lower than that of the spherical equivalent.

[J Refract Surg. 2013;29(12):824–831.]

From the Department of Ophthalmology, University Medical Center Hamburg-Eppendorf (TK, LW, PG, BGD, GR, SJL); and Care-Vision (TK, SJL), Hamburg, Germany.

The authors have no financial or proprietary interest in the materials presented herein.

Drs. Katz and Wagenfeld contributed equally to this work and should be considered as equal first authors.

The authors thank the staff and patients of CARE Vision for their support in establishing the anonymized refractive data collection (Hamburg Refractive Data Base) and Vasyl Druchkiv for his support in the statistical analysis.

AUTHOR CONTRIBUTIONS

Study concept and design (BGD, PG, TK, SJL, GR, LW); data collection (BGD, TK, SJL); analysis and interpretation of data (PG, TK, SJL, GR, LW); drafting of the manuscript (PG, TK, LW); critical revision of the manuscript (BGD, TK, SJL, GR); statistical expertise (TK); administrative, technical, or material support (TK, LW); supervision (TK, GR)

Correspondence: Lars Wagenfeld, MD, University Medical Center Hamburg-Eppendorf, Department of Ophthalmology, Martinistr. 52, 20246 Hamburg, Germany. E-mail: l.wagenfeld@uke.de

Received: January 14, 2013
Accepted: August 05, 2013
Posted Online: November 05, 2013

Abstract

PURPOSE:

To compare the efficacy, safety, predictability, and vector analysis indices of LASIK and photorefractive keratectomy (PRK) for correction of high cylinder of greater than 3 diopters (D) in myopic eyes.

METHODS:

The efficacy, safety, and predictability of LASIK or PRK performed in 114 consecutive randomly selected myopic eyes with an astigmatism of greater than 3 D were retrospectively analyzed at the 2- to 6-month follow-up visits. Vector analysis of the cylindrical correction was compared between the treatment groups.

RESULTS:

A total of 57 eyes receiving PRK and 57 eyes receiving LASIK of 114 refractive surgery candidates were enrolled in the study. No statistically significant difference in efficacy [efficacy index = 0.76 (±0.32) for PRK vs 0.74 (±0.19) for LASIK (P = .82)], safety [safety index = 1.10 (±0.26) for PRK vs 1.01 (±0.17) for LASIK (P = .121)], or predictability [achieved astigmatism < 1 D in 39% of PRK- and 54% of LASIK-treated eyes, and < 2 D in 88% of PRK- and 89% of LASIK-treated eyes (P = .218)] was demonstrated. Using Alpins vector analysis, the surgically induced astigmatism and difference vector were not significantly different between the surgery methods, whereas the correction index showed a slight and significant advantage of LASIK over PRK (1.25 for PRK and 1.06 for LASIK, P < .001).

CONCLUSIONS:

LASIK and PRK are comparably safe, effective, and predictable procedures for excimer laser correction of high astigmatism of greater than 3 D in myopic eyes. Predictability of the correction of the cylindrical component is lower than that of the spherical equivalent.

[J Refract Surg. 2013;29(12):824–831.]

From the Department of Ophthalmology, University Medical Center Hamburg-Eppendorf (TK, LW, PG, BGD, GR, SJL); and Care-Vision (TK, SJL), Hamburg, Germany.

The authors have no financial or proprietary interest in the materials presented herein.

Drs. Katz and Wagenfeld contributed equally to this work and should be considered as equal first authors.

The authors thank the staff and patients of CARE Vision for their support in establishing the anonymized refractive data collection (Hamburg Refractive Data Base) and Vasyl Druchkiv for his support in the statistical analysis.

AUTHOR CONTRIBUTIONS

Study concept and design (BGD, PG, TK, SJL, GR, LW); data collection (BGD, TK, SJL); analysis and interpretation of data (PG, TK, SJL, GR, LW); drafting of the manuscript (PG, TK, LW); critical revision of the manuscript (BGD, TK, SJL, GR); statistical expertise (TK); administrative, technical, or material support (TK, LW); supervision (TK, GR)

Correspondence: Lars Wagenfeld, MD, University Medical Center Hamburg-Eppendorf, Department of Ophthalmology, Martinistr. 52, 20246 Hamburg, Germany. E-mail: l.wagenfeld@uke.de

Received: January 14, 2013
Accepted: August 05, 2013
Posted Online: November 05, 2013

The prevalence of astigmatism of greater than 1 diopter (D) in the general population is reported to be between 32% and 56%.1,2 It is known that higher degrees of astigmatism are more common in eyes with a higher level of ametropia.3–5

Patients with high astigmatism pose a particular challenge in refractive laser surgery because the treatment has a lower predictability and, possibly, stability. Laser ablation of high astigmatism is technically more demanding due to the alignment of the elliptic ablation axis and the compensation for cyclorotation and coupling effect of the sphere component.6

Numerous studies have established the efficacy, safety, and predictability of PRK and LASIK in the treatment of myopia with low to moderate astigmatism,7–9 but to the best of our knowledge no other study has compared the two methodologies for the treatment of myopic eyes with high astigmatism greater than 3 D, including vector analysis. In this retrospective analysis, we present the safety, efficacy, predictability, and vector analysis parameters for myopia with astigmatism of greater than 3 D.

Patients and Methods

Patient Population

This was a retrospective observational case-control study based on 57 LASIK and 57 PRK surgical procedures for myopic patients with high astigmatism (greater than 3 D) performed in a group of private refractive surgery centers (Care Vision) in Germany and Austria between April 2006 and August 2010.

The inclusion criteria were: LASIK and PRK for myopia with a cylindrical component of greater than 3 D, no previous refractive surgery and follow-up time of 60 to 170 days with full refractive data available. For patients in whom both eyes fulfilled the defined ametropia, only one randomly selected eye per patient was included. All patients gave their informed consent in accordance with the tenets of the Declaration of Helsinki. The retrospective analysis of the data was approved by the local ethics committee (No. 2882).

Clinical Examination

Complete ophthalmic examinations were performed, which included manifest and cycloplegic refraction, optical corneal tomography, non-contact tonometry, mesopic pupillometry, and anterior and posterior segment biomicroscopy. Soft/hard contact lenses were discontinued 5 days to 2 weeks before examination. Patients with previous ocular surgery, signs of ocular disease, history of ocular herpes, uveitis, ocular involvement of connective tissue disorders or autoimmune disease, topographical signs of keratoconus, myopic spherical equivalent (SE) of greater than 11 D, expected residual stromal bed below 300 μm, and expected postoperative keratometry values of greater than 50 or less than 34 D were excluded from surgery. The decision to perform LASIK or PRK was taken by the surgeon according to the expected ectasia risk and patients’ wish. Patients with topographic signs of keratoconus or suspected keratoconus were excluded from treatment.

Surgical Technique

The LASIK procedure included flap preparation using an automated microkeratome (M2; Moria, Antony, France) and a single-use 90-μm head.

For PRK, an 8.5-mm diameter sponge was soaked in 20% alcohol and placed on the cornea for 25 seconds. The epithelial flap was peeled off using an epithelial microhoe. The corneal bed was gently wiped to remove possible residual epithelium. After all PRK procedures, a sponge (9-mm diameter) soaked with 0.02% mitomycin C was applied to the stroma according to refractive error: 30 seconds for myopia/hyperopia less than 3 D, 60 seconds for myopia between 3 and 6 D, and 90 seconds for myopia greater than 6 D (SE). A drop of 0.4% oxybuprocaine/hydrochloric acid and ofloxacin was then instilled and a therapeutic contact lens inserted. The patients were advised to use sufficient analgesia if needed (1 g paracemtamol and 60 mg codeine).

Excimer Ablation

In all eyes, the excimer ablation was performed with Allegretto 200/400 excimer laser platforms (Wave-Light, Erlangen, Germany) under constant eye tracking, using a “wavefront optimized” condition. The optical zone was selected depending on the mesopic pupil diameter. The “WaveLight myopic nomogram” recommended by the manufacturer was implemented. The treatments were performed by 19 surgeons in 9 centers.

Follow-up Examinations

All eyes were checked 30 minutes after the procedure. In case of PRK, this control was performed to ensure that the contact lens was in position and to remind the patients to follow the postoperative protocol. Patients receiving LASIK were examined after 1 day and 1 week, and those receiving PRK after 4 days when the contact lens was removed. Follow-up visits were scheduled after 1, 3, and 12 months. For eyes that needed a second treatment, only the last results of the first treatment were included.

Examinations performed at 1 month or more included uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), manifest/automated refraction, and corneal topography. Visual acuity was measured using a Snellen chart and a decimal scale and presented in logMAR.

Although 301 eyes with myopia and astigmatism of greater than 3 D received LASIK and 231 eyes received PRK, only eyes with all follow-up data available regarding manifest refraction were included in the study.

Complete datasets with postoperative manifest refraction in the defined 60 to 170 days follow-up interval were documented for 75 eyes of 57 patients receiving LASIK and 83 eyes of 57 patients receiving PRK. Because only 1 eye per patient was randomly selected, we included 57 eyes per group in this study.

Efficacy and Safety

The efficacy was calculated as the efficacy index, defined as UDVA postoperatively/CDVA preoperatively. The safety was calculated as the safety index, defined as CDVA postoperatively/CDVA preoperatively. The predictability was defined as the difference between the achieved and the attempted refractive change for each eye. Good predictability was considered as postoperative refraction within ±0.5 D of the target refraction.

Vector Analysis

Vector analysis was performed by the Alpins method.10–13 The target refraction for all treatments was emmetropia. For the vector analysis, we calculated the target-induced astigmatism (TIA) and the surgically induced astigmatism (SIA) from the preoperative and postoperative subjective refraction.14–17 The TIA is the vectorial difference between the zero target postoperative cylinder vector and the preoperative astigmatic correction vector, usually being 0. The SIA is defined as the difference between the postoperative and preoperative astigmatic vectors. The difference vector is the vector difference between TIA and SIA. The correction index is the ratio of the magnitude of SIA and TIA and the index of success is given as the proportion of difference vector and TIA. Flattening index shows a correlate for the removed astigmatism.10–13

These parameters provide a more precise understanding of the astigmatic changes.14,15

Adverse Events

All adverse events were documented, treated according to standard protocols, and followed. Adverse events were classified as severe, moderate, or mild.

Severe adverse events included corneal ectasia after LASIK and infectious keratitis with permanent scarring. Moderate adverse events included intraoperative flap complications, macrostria, central and dense diffuse lamellar keratitis, pericentral epithelial ingrowth needing surgical removal, and suspected infectious corneal infiltrates that disappeared within a few days following treatment with local antibiotics. All other adverse events were defined as mild adverse events. Small epithelial erosion or mildly dry eye was not counted as an adverse event.

Statistical Analysis

Results are presented as mean ± standard deviation (SD) or as median and quartiles. SPSS 17.0 (SPSS Inc., Chicago, IL) was used to analyze the possible relations between the efficacy index, safety index, and predictability between the two subgroups. Age and treatment parameters were considered as continuous variables. The differences between the groups were tested with a t test.

Because the efficacy index and safety index were not normally distributed, we used a non-parametric Mann–Whitney test to analyze the efficacy index differences. A P value of less than .05 was considered statistically significant.

The predictability of correcting the SE was calculated using the deviation of the achieved refraction change (postoperative refraction SE – preoperative refraction SE) from the SE of the CDVA before treatment. The deviations were divided into three groups: less than ±0.5 D, less than ±1.0 D, or greater than ±1.0 D from target.

The predictability for the correction of the cylindrical component was calculated using the deviation of the achieved cylindrical change (postoperative cylindrical value – preoperative cylindrical value) from the cylindrical value of the CDVA before treatment. The deviations were divided into three groups: less than ±0.5 D, less than ±1.0 D, or greater than ±1.0 D from target.

The differences in predictability for SE and cylindrical component between the groups were calculated and statistically tested using a Pearson chi-square test. Scatter plots were performed for all subgroups with the above-mentioned predictability levels.

To analyze the cylindrical correction further, a vector analysis was performed. The TIA, SIA, and difference vector were analyzed showing their scatter distribution incorporating magnitude in diopters and axes. The additional parameters as described above were compared using the Kruskal–Wallis test.

Results

Demographics

This study included 114 eyes of 114 patients: 57 in the PRK group and 57 in the LASIK group. Both groups showed a similar age and sex distribution and comparable mean time to follow-up (LASIK: 118 ± 26 days, PRK: 111 ± 20 days). The demographic and refraction data are summarized in Table 1.

Demographic Data and Preoperative and Postoperative Refraction of the Treated Myopic Eyes

Table 1:

Demographic Data and Preoperative and Postoperative Refraction of the Treated Myopic Eyes

The preoperative subjective cylindrical values were similar, with the subjective spherical value lower in the LASIK compared to the PRK group. A mild over-correction of spherical component was seen in both groups. The postoperative subjective astigmatism was −1.23 ± 0.51 D in the LASIK group and −1.29 ± 0.67 D in the PRK group (P = .28).

Adverse Events

Table A (available in the online version of this article) presents an overview of adverse events for both treatment modalities. No statistically significant differences were found between PRK and LASIK.

Efficacy

The efficacy was reasonable and similar in both groups with an efficacy index of 0.76 ± 0.32 in the PRK group and 0.74 ± 0.19 in the LASIK group (P = .82) (Table 2). Preoperative CDVA of better than 20/40 and postoperative UDVA of better than 20/40 are shown in Figure 1.

Efficacy and Safety Index of LASIK and PRK Treatment

Table 2:

Efficacy and Safety Index of LASIK and PRK Treatment

Percentages of various preoperative corrected distance visual acuity (CDVA) and postoperative uncorrected visual acuity (UDVA) for the (A) LASIK and (B) photorefractive keratectomy (PRK) group.

Figure 1.

Percentages of various preoperative corrected distance visual acuity (CDVA) and postoperative uncorrected visual acuity (UDVA) for the (A) LASIK and (B) photorefractive keratectomy (PRK) group.

Safety

The safety of the treatment was high and again similar in both groups, with a safety index of 1.10 ± 0.26 in the PRK group and 1.01 ± 0.17 in the LASIK group (P = .12) (Table 3). The percentage of eyes showing no change or gaining up to two lines of CDVA was 92.8% for PRK and 96.4% for LASIK.

Predictability of Postoperative Refraction (SE) for LASIK and PRKa

Table 3:

Predictability of Postoperative Refraction (SE) for LASIK and PRK

Predictability

In the PRK group, 54% of the eyes were within ±0.5 D and 86% were within ±1.0 D from target (emmetropia) postoperatively. In the LASIK group, these values were 67% within ±0.5 D and 93% within ±1.0 D. This difference was not statistically significant (P = .117) (Table B, [available in the online version of this article], Figure 2).

Predictability of postoperative spherical refraction for the (A) LASIK and (B) photorefractive keratectomy (PRK) group.

Figure 2.

Predictability of postoperative spherical refraction for the (A) LASIK and (B) photorefractive keratectomy (PRK) group.

Vector Analysis

The median TIA was 3.75 D × 26° in the PRK group and 3.75 D × 27° in the LASIK group. The median SIA was 4.73 D × 20° in the PRK group and 3.96 D × 34° in the LASIK group. The difference vector was 1.25 D × 97° in the PRK group and 1.0 D × 121° in the LASIK group. There were no statistically significant differences between these two groups regarding the magnitude and angle for TIA and difference vector, but the P value was .001 for the magnitude of SIA and 0.04 for the angle of SIA. The statistics of each component of the vector analysis are shown in Table B and the scatter diagram using the Alpins method is shown in Figure 3. The correction index showed a statistically significant advantage of LASIK over PRK (1.25 for PRK and 1.06 for LASIK, P < .001). Flattening index was comparable (0.86 for PRK and 0.78 for LASIK, P = .44). The index of success (ideally = 0) was −0.33 in the PRK group and 0.29 in the LASIK group, although these values were not significant (P = .35).

Vectorial display of the target-induced astigmatism (TIA), surgically induced astigmatism (SIA), and difference vector (DV), with summated vector mean of the group. PRK = photorefractive keratectomy

Figure 3.

Vectorial display of the target-induced astigmatism (TIA), surgically induced astigmatism (SIA), and difference vector (DV), with summated vector mean of the group. PRK = photorefractive keratectomy

Discussion

Studies comparing the outcome of LASIK and PRK mainly concentrate on low to moderate (< 3 D) astigmatic corrections. Several studies have documented good safety and efficacy for PRK and LASIK in myopic patients with moderate or low astigmatism, with a slight advantage of LASIK over PRK initially that resolves after 3 months.7–9,16

Patients with high astigmatism represent a challenging population. Efficacy is usually below that for correction of mild-to-moderate myopic astigmatism. Although two recent studies on correcting high myopic astigmatism report a high efficacy for 97.1% of patients with a postoperative UDVA of better than 20/40 with astigmatism of 4 to 7 D (mean −4.73 ± 0.89 D)17 and 96% of patients with UDVA of 20/25 or better with astigmatism greater than 2 D (76% of the eyes were within ±0.50 D and 92% were within ±1 D of the attempted astigmatic correction),18 our study and other studies reveal reduced efficacy. We showed a UDVA of better than 20/40 in 77.2% for PRK and 91.2% for LASIK, whereas Shen et al.19 found a UDVA of 20/40 or better in 84% of the eyes treated with photoastigmatic refractive keratectomy (PARK) with a preoperative mean astigmatism of 1.64 ± 0.86 D at the 6-month follow-up. Taylor et al.20 showed a UDVA of better than 20/40 in 72% of the PARK candidates with astigmatism of 6 D or less, and Tabin et al.21 found a UDVA of better than 20/40 in 71% of the PARK candidates with astigmatism of 0.75 to 4.25 D. However, except for our study, none of these studies included a vector analysis.

Among the intraoperative factors that affect the efficacy of refractive surgery are laser beam decentration, corneal surface inhomogeneities, astigmatic axis shift between vertical and horizontal position, intraoperative cyclotorsion, and poor patient fixation. Onclinx et al. described a shift in the postoperative axis when the patient’s chair was rotated 90°, and concluded that technical conditions rather than wound healing processes were responsible for the induced astigmatism.22 Postoperative factors may include wound-healing variations, such as irregular epithelial thickening and hyperplasia of epithelium. These induced astigmatisms seem to occur less after LASIK surgery23 and are often only temporary.

We found no statistically significant difference in the predictability of the SE between PRK (86% ± 1.0 D) and LASIK (93% ± 1.0 D) (P = .117). The predictability of the cylindrical value correction showed a trend toward overcorrection for both procedures and was less predictable than the SE correction. Achieved postoperative astigmatism was ±1 D in 39% of PRK-treated eyes compared to 54% of LASIK-treated eyes. There was a residual astigmatism within ±2 D (P = .218) in 88% of PRK-treated eyes and 89% of LASIK-treated eyes.

Vector analysis is a helpful tool in the evaluation of the effectiveness of astigmatic correction.24 In our study, the Alpins vector analysis revealed no statistically significant differences of TIA and difference vector between LASIK and PRK. The correction index (ideally = 1) was 1.25 for PRK and 1.06 for LASIK, documenting a slight overcorrection in both groups with a statistically significant difference (P < .001). This finding may also be a result of the relatively short follow-up period, because several studies have shown that LASIK has advantages over PRK in the short term that resolve after several months.23

Tabin et al. have shown that SIA increases with the amount of myopic treatment.21 Several studies have revealed that PRK induces an astigmatic shift toward with-the-rule astigmatism,19,22,23 which is also supported by our data. The SIA was 4.73 D × 20° for PRK and 3.96 D × 34° for LASIK, demonstrating significant differences for magnitude and angle (P < .001 and .04, respectively) with a shift toward with-the-rule astigmatism in the PRK group. We observed a trend toward overcorrection for the cylindrical component, but a slight undercorrrection for the SE.

There are several drawbacks of this study comparing LASIK and PRK in high myopic astigmatism. First is the lack of randomization in the recruitment of patients (LASIK vs PRK). Patients with topographic signs of, or suspected, keratoconus were excluded from treatment. The decision tree for laser refractive surgery is based on the “refractive surgery consensus,” which includes parameters that are recruited from internationally accepted standards with regard to pachymetric, topographic, and tomographic cut-off values. The two main reasons for performing PRK were thin corneas (< 500 μm but no suspicion of keratoconus) and anamnesis with a higher risk of trauma (eg, boxing, soldier, or police). An inherent bias when making the decision to perform LASIK or PRK based on common internationally accepted standards cannot be avoided in any study that compares LASIK and PRK for a similar correction. Second, the follow-up time (69 to 132 days) may not include the final healing process after PRK, and therefore limits the comparability of both groups. Third, the retrospective design of the study has an inherent high dropout rate. Fourth, the high number of centers and surgeons involved limits the results, even if all surgeons followed the same protocol and used the same laser platforms. Fifth, although the ideal study design would be to include only those who do not wear contact lenses, this approach is clinically not feasible because a high percentage of myopic individuals with high astigmatism do wear rigid gas permeable lenses. Therefore, our study and other studies included contact lens wearers with a rigid gas permeable lens-free interval of 2 weeks.

In this study, LASIK and PRK showed comparable safety, efficacy, and predictability for laser correction of high astigmatism (greater than 3 D) in myopic eyes. Predictability of the correction of the cylindrical component was lower than that for the SE. Vector analysis of the cylindrical correction showed a trend toward overcorrection for the cylindrical component for both procedures, with a statistically significant higher over-correction in PRK, at least in this follow-up period, and a trend toward undercorrection for the SE for both procedures. Future prospective randomized studies with a larger cohort and longer follow-up time should be performed and should also aim at optimizing nomograms for treating high cylindrical refractive errors.

References

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Demographic Data and Preoperative and Postoperative Refraction of the Treated Myopic Eyes

LASIK PRK Total

Variable No. % No. % No. %
Eyes 57 50 57 50 114 100
  Malea 26 47.46 20 57.58 46 100
  Femalea 31 52.54 37 49.06 68 66.67
Patients 57 100 57 100 114 100

Range Mean (± SD) Range Mean (± SD) Range Mean (± SD)

Age (y)b 20–59 37.0 (9.7) 19–52 37.84 (9.43) 19–59 36.11 (9.57)
Days after surgeryc 63–169 118 (26) 62–162 111 (20) 63–169 118 (27)

Preoperative Postoperative Preoperative Postoperative Preoperative Postoperative

SEd −8.63/−1.63 −3.82 (1.68) −2.38/1.88 0.07 (0.69) −8.38/−1.63 −4.72 (2.08) −2.38/1.50 −0.15 (0.77) −8.63/1.63 −4.27 (1.94) −2.38/1.88 −0.04 (0.73)
Subjective sphe −6.75/0.00 −1.89 (1.72) −2.00/2.75 0.67 (0.77) −6.75/0.00 −2.82 (2.06) −1.75/2.75 0.49 (0.93) −6.75/0.00 −2.35 (1.94) −2.00/2.75 0.58 (0.85)
Subjective cylf −5.75/−3.25 −3.86 (0.57) −3.75/0.00 −1.23 (0.51) −5.75/−3.25 −3.81 (0.63) −2.50/−0.25 −1.29 (0.67) −5.75/−3.25 −3.84 (0.6) −3.75/0.00 −1.25 (0.69)
UDVA 0.01/0.60 0.14 (0.14) 0.20/1.00 0.68 (0.18) 0.01/0.40 0.09 (0.1) 0.20/1.20 0.66 (0.24) 0.01/0.60 0.12 (0.12) 0.2/1.20 0.67 (0.21)
UDVA logMAR 0.22/2.17 1.07 (0.49) 0.00/0.70 0.18 (0.14) 0.40/2.17 1.34 (0.54) −0.08/0.70 0.22 (0.18) 0.22/2.17 1.2 (0.53) −0.08/0.70 0.2 (0.16)
CDVA 0.50/1.20 0.94 (0.17) 0.50/1.25 0.94 (0.17) 0.40/1.20 0.9 (0.17) 0.56/1.50 0.97 (0.21) 0.40/1.20 0.92 (0.17) 0.50/1.50 0.96 (0.19)
CDVA logMAR −0.08/0.30 0.03 (0.09) −0.10/0.30 0.04 (0.08) −0.08/0.40 0.05 (0.09) −0.18/0.25 0.02 (0.1) −0.08/0.40 0.04 (0.09) −0.18/0.30 0.03 (0.09)

Efficacy and Safety Index of LASIK and PRK Treatment

Variable PRK LASIK Pa Total
Efficacy
  No. of eyes 57 57 .818 114
  Median (Q25–Q75) 0.7 (0.55–0.92) 0.73 (0.64–0.82) 0.73 (0.62–0.86)
  Mean ± SD 0.76 ± 0.32 0.74 ± 0.19 0.19 ± 114
Safety
  No. of eyes 57 57 .121 114
  Median (Q25–Q75) 1.01 (0.91–1.27) 1.0 (0.92–1.07) 1.0 (0.92–1.12)
  Mean ± SD 1.1 ± 0.26 1.01 ± 0.17 1.06 ± 0.22

Predictability of Postoperative Refraction (SE) for LASIK and PRKa

Procedure Undercorrection Overcorrection


< 1.0 1.0 to 0.5 ±0.5 0.51 to 1.0 > 1.0 Total
PRK
  No. 7 10 31 8 1 57
  % 12 18 54 14 2 100
LASIK
  No. 1 6 38 9 3 57
  % 2 11 67 16 5 100

10.3928/1081597X-20131029-03

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