Journal of Refractive Surgery

Original Article 

Visual Acuity Outcomes in Eyes With Flat Corneas After PRK

David Varssano, MD; Michael Waisbourd, MD; Liza Minkev, BSc; Tzahi Sela, BSc; Meira Neudorfer, MD; Perry S. Binder, MS, MD

Abstract

PURPOSE:

To evaluate the impact of corneal curvatures less than 35 diopters (D) after photorefractive keratectomy (PRK) on visual acuity outcomes.

METHODS:

Visual acuity outcomes of 5,410 eyes that underwent PRK from January 2006 to November 2010 were retrospectively analyzed for the impact of postoperative corneal curvatures on visual outcomes. All procedures were performed on a single platform (Allegretto 200Hz excimer laser; Alcon Laboratories, Inc., Irvine, CA). Main outcome measures were postoperative corrected distance visual acuity (CDVA) and loss of CDVA.

RESULTS:

Corneas with a measured or a calculated postoperative flat meridian less than 35 D and those with a measured postoperative steep meridian less than 35 D had worse postoperative CDVA than corneas with meridians of either 35 D or more (P ⩽ .021). However, the preoperative CDVA was worse in the flatter curvatures in all comparisons performed (P ⩽ .024). Consequently, the measured or calculated meridian curvature had no effect on CDVA loss (P ⩾ .074).

CONCLUSION:

Postoperative corneal keratometry values (flat and steep meridians) less than 35 D did not have a predictive effect on the risk of losing visual acuity following myopic PRK performed on the Allegretto 200Hz excimer laser.

[J Refract Surg. 2013;29(6):384–389.]

From the Department of Ophthalmology, Tel-Aviv Medical Center, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel (DV, MW, MN); Care Vision, Tel-Aviv, Israel (LM, TS); and Gavin Herbert Eye Institute, University of California–Irvine, Irvine, California (PSB).

Drs. Minkev and Sela are employees of Care Vision. Dr. Binder is a consultant for Abbott Medical Optics, Inc. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (DV); data collection (DV, LM, TS); analysis and interpretation of data (DV, MW, MN, PSB); drafting of the manuscript (DV, MW, TS); critical revision of the manuscript (DV, LM, MN, PSB); administrative, technical, or material support (DV, TS, PSB)

Correspondence: David Varssano, MD, Department of Ophthalmology, Tel-Aviv Medical Center, 6, Weizmann St., Tel-Aviv 64236, Israel. E-mail: varssano@gmail.com.

Received: November 19, 2012
Accepted: March 05, 2013

Abstract

PURPOSE:

To evaluate the impact of corneal curvatures less than 35 diopters (D) after photorefractive keratectomy (PRK) on visual acuity outcomes.

METHODS:

Visual acuity outcomes of 5,410 eyes that underwent PRK from January 2006 to November 2010 were retrospectively analyzed for the impact of postoperative corneal curvatures on visual outcomes. All procedures were performed on a single platform (Allegretto 200Hz excimer laser; Alcon Laboratories, Inc., Irvine, CA). Main outcome measures were postoperative corrected distance visual acuity (CDVA) and loss of CDVA.

RESULTS:

Corneas with a measured or a calculated postoperative flat meridian less than 35 D and those with a measured postoperative steep meridian less than 35 D had worse postoperative CDVA than corneas with meridians of either 35 D or more (P ⩽ .021). However, the preoperative CDVA was worse in the flatter curvatures in all comparisons performed (P ⩽ .024). Consequently, the measured or calculated meridian curvature had no effect on CDVA loss (P ⩾ .074).

CONCLUSION:

Postoperative corneal keratometry values (flat and steep meridians) less than 35 D did not have a predictive effect on the risk of losing visual acuity following myopic PRK performed on the Allegretto 200Hz excimer laser.

[J Refract Surg. 2013;29(6):384–389.]

From the Department of Ophthalmology, Tel-Aviv Medical Center, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel (DV, MW, MN); Care Vision, Tel-Aviv, Israel (LM, TS); and Gavin Herbert Eye Institute, University of California–Irvine, Irvine, California (PSB).

Drs. Minkev and Sela are employees of Care Vision. Dr. Binder is a consultant for Abbott Medical Optics, Inc. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (DV); data collection (DV, LM, TS); analysis and interpretation of data (DV, MW, MN, PSB); drafting of the manuscript (DV, MW, TS); critical revision of the manuscript (DV, LM, MN, PSB); administrative, technical, or material support (DV, TS, PSB)

Correspondence: David Varssano, MD, Department of Ophthalmology, Tel-Aviv Medical Center, 6, Weizmann St., Tel-Aviv 64236, Israel. E-mail: varssano@gmail.com.

Received: November 19, 2012
Accepted: March 05, 2013

Most corneas are steeper than 35 diopters (D) in their flattest meridian,1 but corneal refractive surgery, especially for high corrections, can cause corneas to become flatter than 35 D.2

It is commonly believed that substantial flattening of the cornea during myopic refractive surgery may have a negative effect on postoperative visual acuity (VA).3 In a retrospective study of eyes undergoing LASIK between 1995 and 1999, a postoperative mean keratometry reading of less than 36 D was associated with statistically fewer eyes with an uncorrected distance visual acuity (UDVA) of 20/25 or better and statistically more eyes with a loss of two lines or more of corrected distance visual acuity (CDVA) (P.S. Binder, personal communication, February 12, 2011). Newer ablation profiles, which preserve the prolate nature of the cornea,4 are believed to provide better optical outcomes than previous generation lasers, but we failed to find publications that support this assumption.5

Marked changes within the transition zone area between the steeper, untreated peripheral cornea and the flatter, treated central area are known to have the potential for producing spherical aberration, especially in patients with large pupils.6–8 However, reports in the literature do not provide clear-cut answers regarding the effect of substantial postoperative flattened corneas on UDVA and CDVA.

The aim of this study was to evaluate VA outcomes following flattening of corneas to less than 35 D following photorefractive keratectomy (PRK).

Patients and Methods

The study was approved by the Institutional Review Board of the Tel-Aviv Medical Center, Tel-Aviv, Israel. We retrospectively analyzed the electronic medical records of all patients undergoing PRK in a refractive surgery facility (Care Vision, Tel Aviv, Israel) between January 1, 2006, and November 1, 2010. The data were routinely collected and entered into the electronic medical records database by the facility staff. All procedures were performed by eight ophthalmic surgeons who used the Allegretto 200Hz excimer laser (Alcon Laboratories, Inc., Irvine, CA) with a planned optical-zone diameter of 6.5 mm in most cases. The following data were retrieved and analyzed: patient demographics (age and gender), preoperative and postoperative CDVA, and keratometry readings obtained by corneal topography (Orbscan II; Bausch & Lomb, Rochester, NY). Inclusion criteria were: patients undergoing myopic PRK treatment, age 18 years or older, and minimal follow-up of 75 days. Exclusion criteria were: any re-treatment/enhancement or any other refractive procedure.

We conducted a search for relevant studies by means of a structured literature search of PubMed (on November 4, 2012) using the expression: “PRK or photorefractive keratectomy or LASIK or laser in situ keratomileusis” and “flat cornea or flat meridian or steep cornea or steep meridian.”9–11

Postoperative Keratometry

Postoperative measured keratometry values obtained from the Orbscan II were used. The flat and steep axes were termed “measured final flat meridian” and “measured final steep meridian,” respectively. Corneas with final measured keratometry values of 35 D or more were compared with corneas whose postoperative measured mean keratometry value was less than 35 D.

Prediction of the keratometry values following the procedure, based on preoperative data, is of clinical importance in the context of this article. Calculated postoperative keratometry values can be used by the surgeon before surgery to assess the risk of losing vision due to excessively flat corneas. For this study, predicted postoperative keratometry values were calculated following correction of the planned refractive correction to the corneal surface and applying it to the steep and flat meridians of the anterior corneal surface, as recommended by Holladay et al.12

The flat and steep calculated axes were termed “calculated final flat meridian” and “calculated final steep meridian,” respectively. Corneas with final calculated flat and/or steep keratometry readings of 35 D or more were compared with those postoperative readings that were calculated to be less than 35 D.

Statistical Analysis

Statistical analysis was performed using SPSS version 20.0 (SPSS, Inc., Chicago, IL). Pearson correlation and linear regressions were used to compare measured and calculated final meridian values. Fisher’s exact test was used to compare the proportion of eyes that lost one or more lines of CDVA. The Student’s paired and unpaired t tests were used to compare CDVA and loss of CDVA. A P value less than .05 was considered statistically significant.

Results

Data from 5,410 eyes that underwent myopic PRK were evaluated. The mean follow-up period was 326 days (standard deviation [SD]: 327, range: 75 to 1,774 days). Gender was female in 46.8% of the eyes and male in 53.2%; 49.7% were right eyes and 50.3% were left eyes. The mean age was 26.5 years (SD: 7.7 years, range: 18 to 61 years). The average keratometry reading for the cohort was 44.21 D (SD: 1.57 D, range: 38.35 to 50.40 D). Average treatment spherical equivalent was −4.56 D (SD: 1.80 D, range: −9.89 to −0.25 D). Average treatment zone was 6.474 mm (SD: 0.231 mm, range: 5.0 to 8.0 mm). Treatment spherical equivalent values were not different for treatment zones above and below that value (−4.56 ± 1.74 and −4.52 ± 2.10 D, respectively) (P < .675).

Calculated And Measured Final Meridians

The calculated final flat meridian values (n = 4,600) and measured final flat meridian values (n = 845) were well correlated (Pearson’s coefficient 0.927, P < .001) (Figure 1 ). The calculated final steep meridian values (n = 3,763) and the measured final steep meridian values (n = 831) were also well correlated (Pearson’s coefficient 0.929, P < .001) (Figure 2 ).

Calculated and measured final flat meridian values correlated well (Pearson’s coefficient 0.927, P < .001).

Figure 1. Calculated and measured final flat meridian values correlated well (Pearson’s coefficient 0.927, P < .001).

Calculated and measured final steep meridian values correlated well (Pearson’s coefficient 0.929, P < .001).

Figure 2. Calculated and measured final steep meridian values correlated well (Pearson’s coefficient 0.929, P < .001).

Following the laser procedure, the average calculated final flat meridian value was 40.03 D (SD: 2.18 D, range: 32.48 to 47.79 D). The average calculated final steep meridian value was 40.33 D (SD: 2.22 D, range: 32.22 to 48.49 D). The average measured final flat meridian value was 39.23 D (SD: 2.76 D, range: 32.10 to 46.70 D). The average measured final steep meridian value was 40.06 D (SD: 2.75 D, range: 32.40 to 48.00 D).

Preoperative and Postoperative CDVA

The preoperative logMAR CDVA was significantly worse in corneas measured or calculated to have a postoperative curvature less than 35 D in the flat meridian than in corneas expected to have a postoperative curvature of 35 D or more in the flat meridian (Table 1 ). A similar significant difference was seen when measured or calculated steep postoperative meridians were compared. Postoperative logMAR CDVA was significantly worse in corneas measured or calculated to have a postoperative curvature less than 35 D in the flat meridian than in corneas expected to have a postoperative curvature of 35 D or more in the flat meridian (Table 2 ). A similar significant difference was seen when measured steep postoperative meridians were compared. However, the difference was not significant when the calculated steep postoperative meridians were compared.

Preoperative Corrected Distance Visual Acuity

Table 1: Preoperative Corrected Distance Visual Acuity

Final Corrected Distance Visual Acuity

Table 2: Final Corrected Distance Visual Acuity

Visual Acuity Outcomes

We compared loss of VA (difference in logMAR between calculated measurements before and after PRK) (Table 3 ). Neither measured nor calculated meridians showed a significant difference.

Loss of Corrected Distance Visual Acuity

Table 3: Loss of Corrected Distance Visual Acuity

When comparing the proportion of eyes that lost one or more lines of VA, there was no difference between the less than 35 D and the 35 D or more calculated groups in the flat meridian (P = .161) or the steep meridian (P = .405). There was no difference between the less than 35 D and the 35 D or more groups in the flat meridian (P = .323) or the steep meridian (P = .201) when comparing the proportion of eyes that lost two or more lines of visual acuity.

We also looked at the measured groups. When comparing the proportion of eyes that lost one or more lines of VA, there was no difference between the less than 35 D and the 35 D or more measured groups in the flat meridian (P = .436) or the steep meridian (P = .599). There was no difference between the less than 35 D and the 35 D or more groups in the flat meridian (P = .265) or the steep meridian (P = .092) when comparing the proportion of eyes that lost two or more lines of visual acuity.

Postoperative logMAR CDVA and loss of VA are presented versus measured final flat meridian in Figures 3 and 4 , respectively. Postoperative logMAR CDVA and loss of VA are presented versus measured final steep meridian in Figures 5 and 6 , respectively.

Postoperative corrected distance visual acuity versus measured final flat corneal meridian. Worse acuity for corneas flatter than 35 diopters (D) (t test, P = .002).

Figure 3. Postoperative corrected distance visual acuity versus measured final flat corneal meridian. Worse acuity for corneas flatter than 35 diopters (D) (t test, P = .002).

Postoperative loss of corrected distance visual acuity versus measured final flat corneal meridian. No significant difference (t test, P = .594).

Figure 4. Postoperative loss of corrected distance visual acuity versus measured final flat corneal meridian. No significant difference (t test, P = .594).

Postoperative corrected distance visual acuity versus measured final steep corneal meridian. No significant difference (t test, P = .021).

Figure 5. Postoperative corrected distance visual acuity versus measured final steep corneal meridian. No significant difference (t test, P = .021).

Postoperative loss of corrected distance visual acuity versus measured final steep corneal meridian. No significant difference (t test, P = .367).

Figure 6. Postoperative loss of corrected distance visual acuity versus measured final steep corneal meridian. No significant difference (t test, P = .367).

Discussion

We investigated the VA outcomes of patients who underwent myopic PRK surgery, comparing those with measured final corneal curvatures less than 35 D using the Orbscan II system with those who had measured final curvatures of 35 D or more and found that CDVA was consistently worse in patients whose corneas were eventually flatter than 35 D than in those whose corneas were eventually steeper than 35 D. The difference was significant in all four comparisons before surgery (Table 1 ) and in three of four comparisons after surgery (Table 2 ); however, when comparing the loss of VA in the flat and steep corneas, no difference in the outcomes was demonstrated in any of the four comparisons made (Table 3 ).

We hypothesize that the preoperative and postoperative difference in VA between the groups emerged from difference in the structure of the eyes in the two groups. Some eyes that have flat postoperative keratometry values were probably more myopic prior to surgery and some may have subclinical macular pathology.13 These eyes may also have a smaller retinal image, even when the optic correction is on the corneal plane. This minification can lead to lower postoperative measured VA compared to eyes with less myopia.14 The magnitude of treatment was higher by approximately 3 D for corneas flatter than 35 D than for those 35 D or more (Table 4 ). The difference was significant when comparing flat or steep meridians, measured or calculated values.

Planned Treatment (Spherical Equivalent)

Table 4: Planned Treatment (Spherical Equivalent)

The calculated/predicted postoperative keratometry value is of practical value to the surgeon planning the procedure. The fact that calculated final flat or steep meridian values (a calculation that can be performed before surgery based on available data) are not a predictor of loss of VA is the most important result of this study.

Our review of the literature failed to yield any study relevant to ours. Therefore, we could not correlate our results with those of others.

One limitation of this study is that we did not have a long-term postoperative follow-up period and could not evaluate the rate of postoperative complications in markedly flattened corneas (eg, possible corneal ectasia), which may be evident not only in LASIK but also in PRK surgery.11,15,16

We had significantly fewer eyes with measured keratometry values compared to the calculated measurements, because not all eyes had postoperative topographies performed and tabulated in the electronic medical records system during follow-up. We used the keratometry values from the Orbscan II system, which obtains its measurements from the 3- or 5-mm diameter of the cornea and may not be comparable to keratometry as measured by a manual keratometer or other topography instrumentation.11,15,16

It is of interest to note the consistency in the comparisons of logMAR CDVA between corneas flatter than 35 D and those 35 D or more. Statistical significance was achieved in most of the comparisons, but the values were worse for corneas flatter than 35 D in all comparisons, both before and after surgery. Not all comparisons were statistically significant, partially because of the small number of corneas flatter than 35 D. Partial documentation of postoperative measurements further lowered the numbers of corneas available for comparisons based on actually measured data. Other sources of error include the accuracy of measurement with the Orbscan II, the use of data from procedures performed by multiple surgeons, and other possible differences in environmental factors throughout the study period.

This study evaluated surgical outcomes in terms of CDVA. But higher-order aberrations, which can affect postoperative acuity and visual quality, were not studied. For example, spherical aberration may be caused by the steep change within the transitional zone between the treated and untreated cornea, and large scotopic pupil size is a potential risk factor for night vision complaints.17 This possible concern should be investigated in further comparative studies.

Another limitation of the study is the small amount of eyes in the less than 35 D group. Because our study was retrospective, most procedures were done in concordance with the common belief that flat corneas have worse outcomes. This probably influenced surgeon reluctance to induce keratometry values flatter than 35 D. These small numbers have implications on the power of the study: there is a chance that a real difference exists between the groups, but the size of our cohort was not large enough to demonstrate it.

Barraquer3 and Binder (unpublished observations) had worse results in flatter corneas while using smaller optical zones, with early mechanical and excimer refractive hardware and software and significantly different laser beam applications, whereas current lasers use larger optical zones and optimized ablation patterns.

Both measured and calculated flat and steep final keratometry meridians lower than 35 D and even as low as 32.5 D were not shown to have a predictive effect on the risk of losing VA following myopic PRK. These conclusions apply solely to the laser platform used in the current study. Prospective, large scale studies, possibly with a single surgeon, should be done to improve the validity of these conclusions and apply them to other modern excimer lasers.

References

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Preoperative Corrected Distance Visual Acuity

Meridian < 35 D
35 D
P
No. Mean ± SD logMAR (CDVA) No. Mean ± SD logMAR (CDVA)
Measured final flat meridian 70 0.0452 ± 0.07234 (20/22.194) 771 0.0068 ± 0.05978 (20/20.316) .001
Measured final steep meridian 22 0.0388 ± 0.05157 (20/21.869) 805 0.0093 ± 0.06068 (20/20.433) .024
Calculated final flat meridian 45 0.0554 ± 0.07569 (20/22.721) 4,539 −0.0022 ± 0.06414 (20/19.899) .001
Calculated final steep meridian 26 0.0666 ± 0.08131 (20/23.315) 3,725 0.0005 ± 0.06735 (20/20.023) .001

Final Corrected Distance Visual Acuity

Meridian < 35 D
35 D
P
No. Mean ± SD logMAR (CDVA) No. Mean ± SD logMAR (CDVA)
Measured final flat meridian 57 0.0366 ± 0.07578 (20/21.759) 716 0.0026 ± 0.05861 (20/20.120) .002
Measured final steep meridian 18 0.0378 ± 0.09445 (20/21.819) 742 0.0044 ± 0.05927 (20/20.204) .021
Calculated final flat meridian 36 0.0361 ± 0.08091 (20/21.734) 3,895 0.0047 ± 0.06767 (20/20.218) .006
Calculated final steep meridian 21 0.0501 ± 0.09843 (20/22.446) 3,194 0.0074 ± 0.06491 (20/20.344) .061

Loss of Corrected Distance Visual Acuity

Meridian < 35 D
35 D
P
No. Mean ± SD logMAR No. Mean ± SD logMAR
Measured final flat meridian 57 −0.0077 ± 0.07277 713 −0.0029 ± 0.06436 .594
Measured final steep meridian 18 0.0101 ± 0.07593 739 −0.0039 ± 0.06448 .367
Calculated final flat meridian 36 −0.0159 ± 0.07938 3,880 0.0062 ± 0.07387 .074
Calculated final steep meridian 21 −0.0160 ± 0.09816 3,183 0.0061 ± 0.07162 .161

Planned Treatment (Spherical Equivalent)

Meridian < 35 D
35 D
P
No. Mean ± SD (D) No. Mean ± SD (D)
Measured final flat meridian 64 −7.3553 ± 1.34583 630 −4.6556 ± 1.89848 .001
Measured final steep meridian 21 −7.4329 ± 1.68020 662 −4.8120 ± 1.97992 .001
Calculated final flat meridian 42 −7.3803 ± 1.10295 3713 −4.4933 ± 1.83698 .001
Calculated final steep meridian 26 −7.8407 ± 0.99345 3737 −4.5028 ± 1.83911 .001

10.3928/1081597X-20130515-02

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