Posterior corneal surface changes after refractive surgery

doi:10.1016/S0161-6420(01)00634-0

Ophthalmology

Volume 108, Issue 8, August 2001, Pages 1415-1422

Presented in part at the annual meeting of the American Academy of Ophthalmology, New Orleans, Louisiana, November 1998.

https://doi.org/10.1016/S0161-6420(01)00634-0 Get rights and content

Abstract

Objective

To determine the frequency of changes in posterior corneal surface after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).

Design

Nonrandomized, comparative trial.

Participants

Ninety-five eyes (71 patients) that underwent PRK (n = 45) or LASIK (n = 50).

Controls

Twenty nonsurgery eyes were used to validate the method of analysis of the posterior corneal curvature (PCC). Seventy nonsurgery eyes were used for comparisons.

Methods

Float, apex-fixed best fit corneal curvature (ABC), and posterior elevation difference were evaluated in 20 elevation topography maps at 6 zone diameters (3–7 and 10 mm) and at two time points. Corneal elevation maps before and after PRK or LASIK were analyzed by the ABC and float methods and compared with a nonsurgery group.

Main outcome measures

Posterior corneal curvature change (mm) was classified as flattening, steepening, or no change.

Results

Flattening of more than 0.12 mm was found in 22.2% of eyes (n = 10) in the PRK group, a change of ±0.12 mm was found in 53.3% of eyes (n = 24), and steepening of more than 0.12 mm was found in 24.4% of eyes (n = 11) using the float method and in 28.9% of eyes (n = 13), 35.6% of eyes (n = 16), and 35.6% eyes (n = 16), respectively, using the ABC method. In the LASIK group, the float method registered flattening in 20% of eyes (n = 10), no change in 52% of eyes (n = 26), and steepening in 28% of eyes (n = 14), whereas the ABC method registered flattening in 30% of eyes (n = 15), no change in 40% of eyes (n = 20), and steepening in 30% of eyes (n = 15). The nonsurgery group showed a similar change in PCC at two different time points similar to that of the PRK and the LASIK groups.

Conclusions

The differences observed in the PCC after PRK or LASIK were not statistically significantly different from those observed over time in the nonsurgery control group.

Section snippets

Materials and methods

Informed consent was obtained from all patients before they entered the study, which was approved by the Institutional Review Board of the Massachusetts Eye and Ear Infirmary.

Methods of topographic measurement

The mean time interval between t₁ and t₂ was 84.95 ± 40.14 days. The data are summarized in Table 1. The float method showed the lowest variation when the mean of the differences were analyzed (CV range, 1.22–3.81), followed by the ABC (CV range, 1.67–2.66) and the PED (CV range, 3.39–5.63) methods. The ABC and PED values showed a high inverse correlation between the CV and the diameter used for the topography test (ABC, r = −0.85, slope = −0.14, P = 0.02; PED, r = −0.81, slope = −0.27, P =

Discussion

Errors analyzing the central cornea have been found in Placido disk-based systems8, 9, 10, 11 as intraobserver and interobserver variability errors.12 Although the technology used in the present study has the advantage that it is not dependent on spherical assumption and the exact focus and center are of less concern,8 there have been no reports evaluating the reproducibility and accuracy of this relatively new technology. In this study, we observed that PRK (in low myopia) and LASIK (in

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Cited by (41)

Changes in posterior corneal elevation after laser in situ keratomileusis enhancement
2008, Journal of Cataract and Refractive Surgery
To evaluate changes in posterior corneal elevation using the Pentacam topographer (Oculus) in patients having laser in situ keratomileusis (LASIK) enhancement.
Private practice, Chevy Chase, Maryland, USA.
The Pentacam device was used to evaluate the changes in posterior corneal elevation above the best-fit sphere before LASIK enhancement and after LASIK enhancement in 24 eyes. The change in posterior corneal elevation in eyes for which pre-primary LASIK data were available was also evaluated.
After LASIK enhancement, the mean change in posterior corneal elevation was 5 μm. The mean posterior corneal elevation was 12 ± 7 μm before LASIK enhancement and 16 ± 6 μm after enhancement; the difference was statistically significant (P = .004). In eyes for which pre-primary LASIK data were available, the mean change in posterior corneal elevation after primary LASIK was 2 μm. The mean posterior corneal elevation was 11 ± 5 μm before LASIK enhancement and 11 ± 7 μm after enhancement.
There was a statistically significant difference in posterior corneal elevation between before LASIK enhancement and after LASIK enhancement. However, the change in posterior corneal elevation was much smaller than previously reported values and below the sensitivity of the Pentacam topographer.
Analysis of ectasia after laser in situ keratomileusis: Risk factors
2007, Journal of Cataract and Refractive Surgery
To examine a database of laser in situ keratomileusis (LASIK) procedures for preoperative and operative factors assumed to increase the risk for developing post-LASIK ectasia.
Private clinical practice.
A computer database was queried for eyes that had LASIK for myopic refractive errors with the following characteristics: preoperative corneal thickness 500 μm or less, mean keratometry greater than 47.0 diopters (D), patient age 25 years or younger, attempted correction greater than −8.0 D, refractive astigmatism not with-the-rule and greater than 2.0 D, and residual stromal bed thickness (RST) 250 μm or less. Flap thickness and RST were measured using ultrasound pachymetry. All recorded information was exported to MS Excel and analyzed for eyes that had ectasia.
Of the 9700 eyes in the database, none with the above characteristics developed ectasia over mean follow-up periods exceeding 2 years. Seven eyes had multiple risk factors without ectasia. Three eyes with abnormal preoperative topography developed ectasia.
Individual preoperative and operative factors did not in and of themselves increase the risk for ectasia. Unmeasured and unknown factors that affect the individual cornea's biomechanical stability, in combination with some suspected risk factors as well as the current inability to identify corneas at risk for developing ectatic disorders, probably account for most eyes that develop ectasia today.
Corneal changes after laser refractive surgery for myopia: Comparison of Orbscan II and Pentacam findings
2007, Journal of Cataract and Refractive Surgery
Citation Excerpt :
In their control group, Miyata et al.9 found a mean difference of 2.6 ± 5.7 μm between 2 P-CE measurements obtained 1 month apart in preoperative eyes. Significant post-LASIK increases in P-ME have been reported by Wang et al.1 and Sharma et al.6 Finally, by investigating the changes in the P-BFS, Twa et al.10 reported a steepening of 0.1 mm while Hernández-Quintela et al.5 found no significant difference between PRK, LASIK, and normal groups. All the above studies were based on Orbscan or Orbscan II measurements.
To determine the agreement between scanning slit topography (Orbscan II, Bausch & Lomb) and Scheimpflug imaging (Pentacam, Oculus) in corneal elevation, corneal curvature, and anterior chamber depth (ACD) measurements before and after laser refractive surgery for myopia and to compare the postoperative changes seen with these devices.
Noor Vision Correction Center, Tehran, Iran.
In a prospective observational case-series study, 23 consecutive myopic patients having laser refractive surgery were examined with the Orbscan II and Pentacam preoperatively and 6 weeks postoperatively. Readings of ACD, anterior (A-) and posterior (P-) best-fit sphere (BFS) size, central elevation (CE), maximum elevation (ME), axial power in the 3.0 mm zone (AX3) and 5.0 mm zone (AX5), and tangential power in these zones (TG3 and TG5, respectively) were collected and used in the analyses.
Statistically significant interdevice differences were found preoperatively for all parameters except P-TG3 (P = .014) and in operated eyes for ACD, anterior parameters of A-AX3 and A-AX5, and all posterior corneal parameters. In posterior corneal measurements, Orbscan II demonstrated significant postoperative changes in all parameters except P-AX5 (P = .004) and P-TG5 (P = .034), although none of the differences was statistically significant with the Pentacam. The devices measured similar postoperative changes in anterior curvature parameters and in P-AX5 and P-TG5; all other changes were significantly different.
Compared to the Pentacam, the Orbscan II yielded larger posterior elevation values before and after surgery and significant postoperative changes in P-CE and P-ME. Further studies can determine which device gives more accurate images of the cornea and redefine screening criteria.
May Consultation # 2
2007, Journal of Cataract and Refractive Surgery
Repeatability and Reproducibility of Posterior Corneal Curvature Measurements by Combined Scanning-Slit and Placido-Disc Topography after LASIK
2006, Ophthalmology
Citation Excerpt :
This has the least variation compared with the apex-fixed best-fit corneal curvature and posterior elevation in normal corneas.15,36 In the float method, the best-fit sphere (BFS) was allowed to float with no alignment constraints applied to the reference sphere when assessing best fit to the data surface.15,36 Throughout the study, we documented the BFS radius (in millimeters) of the posterior corneal surface using the BFS calculation option.15,36
To assess the repeatability and reproducibility of posterior corneal curvature (PCC) measurements made by combined scanning-slit/Placido-disc topography (Orbscan II) after LASIK.
Experimental instrument validation study.
We recruited 22 consecutive postmyopic LASIK patients for the repeatability study and another 50 consecutive postmyopic LASIK patients for the reproducibility study.
To analyze intrasession repeatability, 1 examiner measured 22 postmyopic LASIK eyes 10 times successively in the shortest time possible. To study intersession reproducibility, the same operator obtained measurements from another 50 eyes with stable refraction in 2 consecutive visits at the same time of the day between 6 and 9 months after myopic LASIK. We explored any association between residual stromal bed thickness and measurement variability.
Orbscan II scanning-slit PCC data, precision, within-subject coefficient of variation (CV_w), limits of agreement (LoA), and intraclass correlation coefficient (ICC).
For intrasession repeatability, precision was 0.067 mm (best-fit sphere [BFS]), 0.110 diopters (D; power within 5 mm), 0.158 D (power within 3 mm), and 0.46 (eccentricity). Repeatability was high for PCC BFS and power measurements within 3-mm and 5-mm zones (CV_w ranged from 0.5%–1.2%) but poor for eccentricity data (CV_w, 31.6%). Correspondingly, ICCs ranged from 0.89 to 0.98 for PCC BFS and power, and the ICC was 0.20 for PCC eccentricity values. For intersession reproducibility, on average, no difference in PCC measurements could be found, indicating that when there is variability, it is due to random factors. The width of the 95% LoA between sessions was clinically acceptable for BFS (0.25 mm) and power (0.4 D [within 5 mm] and 0.6 D [within 3 mm]). Similarly, ICCs indicated good intersession reliability for BFS and power (0.98, 0.96, and 0.85 for BFS, power within 5 mm, and power within 3 mm, respectively) but poor reliability for eccentricity (0.59). Repeatability and reproducibility were unrelated to stromal bed thickness.
Orbscan II provides reliable post-LASIK PCC data for symmetrical parameters (BFS and power), independent of the residual stromal bed thickness, but is unreliable for measurements that are radially asymmetrical (eccentricity). Orbscan II is useful for monitoring the PCC after LASIK once the early postoperative period is over.
Posterior Corneal Curvature Changes after Undersurface Ablation of the Flap and In-the-Bed LASIK Retreatment
2006, Ophthalmology
To analyze LASIK retreatment-induced changes in the posterior corneal curvature (PCC) with undersurface ablation of the flap (UAF) and in-the-bed techniques.
Nonrandomized, comparative, interventional study.
Forty-six eyes with a residual spherical equivalent refraction between −0.37 and −2.75 diopters (D) and astigmatism between 0.0 and −1.25 D were included prospectively. In 23 eyes, the calculated postenhancement flap thickness exceeded 150 μm using micropachymetric optical coherence tomography, whereas with further ablation of the bed, the residual bed thickness (RBT) would have been <250 μm, or <55% of the pre-LASIK central pachymetry. In another 23 eyes, RBT allowed the planned ablation for a calculated post-retreatment RBT exceeding 250 μm, >55% of the pre-LASIK central pachymetry.
Eyes with insufficient RBT for further ablation underwent UAF retreatment, whereas those with adequate RBT received conventional in-the-bed LASIK retreatment. Examinations were performed before retreatment and 3 and 6 months postoperatively. No eye was lost to follow-up.
Micropachymetry, Orbscan II scanning-slit PCC data, and visual acuity (VA).
The groups did not differ in age, intraocular pressure, or retreatment ablation depth, but the UAF eyes had a lower mean pre-retreatment RBT (270.7±25.4 μm) than conventional enhancement eyes (353.0±41.5 μm) (P = 0.001). Eyes undergoing UAF had no significant change in PCC, whereas eyes undergoing conventional retreatment had an increase in the posterior corneal power within the central 3-mm zone (P = 0.008) 3 months after retreatment. No significant changes occurred thereafter. The amount of change in posterior corneal power within the 3-mm central zone from before to after retreatment differed significantly between the groups (mean difference, 0.135 D; 95% confidence interval, 0.022–0.248 D; P = 0.02). No keratectasia developed clinically, and no retreated eye lost or gained ≥2 lines of best-corrected VA. Six months after retreatment, the efficacy and safety indices for the UAF procedure were 0.96 and 1.01, respectively, and 1 and 1.06 for conventional LASIK enhancement.
Undersurface ablation of the flap retreatment appears to have less potential for changing the posterior corneal surface than conventional LASIK enhancement and can help reduce the likelihood of retreatment-induced keratectasia.

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^☆: Supported in part by the New England Corneal Transplant Research Fund; Research to Prevent Blindness Lew R. Wasserman Merit Award, New York, New York (DTA); Massachusetts Lions Eye Research Award, Boston (DTA); the Fundacion UNAM, México (EHQ); the Asociación para Evitar la Ceguera en México (EHQ); and the Consejo Nacional de Ciencia y Tecnologı́a, México City, Mexico (grant no.: 116681 [EHQ]).

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Posterior corneal surface changes after refractive surgery☆

Abstract

Objective

Design

Participants

Controls

Methods

Main outcome measures

Results

Conclusions

Section snippets

Materials and methods

Methods of topographic measurement

Discussion

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