Comparison of residual stromal bed and flap thickness in primary and repeat laser in situ keratomileusis in myopic patients

Results of laser enhancement for residual myopia after primary laser in situ keratomileusis

PURPOSE:

The purpose of the study is to evaluate and analyze the results and outcomes after laser enhancement for residual myopia after primary laser in situ keratomileusis (LASIK).

MATERIALS AND METHODS:

This retrospective interventional consecutive case series clinical study was performed on 112 consecutive eyes (82 patients) that had undergone primary LASIK before the enhancement procedure. The study was done in the Refractive Surgery Unit in Yemen Magrabi Hospital between 2006 and 2014. The retreatment was for residual myopia with or without astigmatism. Either the original flap was lifted or surface ablation was performed. Parameters evaluated were uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (VA), spherical equivalent (SE) refraction, corneal topography, and pachymetry. Complications after laser enhancement were also evaluated.

RESULTS:

Mean age of the study group was 26.72 ± 6.89 years (range from 18 to 44 years). Males accounted for 37/82 (45.1%) and females for 45/82 (54.9%). The right eye was treated in 67/112 (59.8%) and the left eye in 45/112 (40.2%). Before primary LASIK, the mean SE (MSE) was −5.78 ± 1.89 D. Before enhancement, the MSE was −1.32 ± 0.61 D (range −3.25 D to −0.50 D), and none of the eyes had an UCVA of 20/40 or better. Twelve months after retreatment, the percentage of eyes having UCVA of 20/40 or better increased to 67.9% (76 of 112). There were no vision-threatening complications seen.

CONCLUSION:

Retreatment or enhancement after LASIK surgery by lifting the original flap or surface ablation is a safe and effective method for the treatment of regressed or undercorrected myopia. The risk of postoperative complications is very minimal.

Meta-analysis of Pentacam vs. ultrasound pachymetry in central corneal thickness measurement in normal, post-LASIK or PRK, and keratoconic or keratoconus-suspect eyes

BACKGROUND

The aim of this meta-analysis is to evaluate the central corneal thickness (CCT) measurement differences between Pentacam (Oculus Inc., Germany) and Ultrasound Pachymetry (USP) in normal (unoperated eyes , myopic and astigmatic eyes without corneal disease or topographic irregularity), after laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK), and keratoconic or keratoconus suspected eyes. We assess whether Pentacam and USP have similar CCT differences in normal, thinner corneas after LASIK or PRK procedures, and kerotoconic or keratoconus suspected eyes.

METHODS

Data sources, including PubMed, Medline, EMBASE, and Cochrane Central Registry of Controlled Trials on the Cochrane Library, were searched to find the relevant studies. Primary outcome measures were CCT measurement between Pentacam and USP. Three groups of eyes were analyzed: normal; LASIK or PRK eyes; and keratoconus suspected or keratoconic eyes.

RESULTS

Nineteen studies describing 1,908 eyes were enrolled in the normal group. Pentacam results were 1.47 μm ,95 % confidence interval (CI) -2.32 to 5.27, higher than USP without statistically significant difference (P = 0.45). Nine studies with total 539 eyes were included in the corneas after LASIK or PRK. The mean difference in the CCT measurement with Pentacam and ultrasound pachymetry was 1.03 μm, with the 95 % CI -3.36 to 5.42, there was no statistically difference (P = 0.64). Four studies with a total of 185 eyes were included in the keratoconic eyes or keratoconus-suspect group, however,the mean difference was -6.33 μm (95 % CI -9.17 to-3.49), which was statistically different between Pentacam and ultrasound pachymetry in the CCT measurement (P < 0.0001).

CONCLUSIONS

Pentacam offers similar CCT results to ultrasound pachymetry in normal eyes, thinner corneas after LASIK or PRK procedures. However, in keratoconic or keratoconus-suspect eyes, Pentacam slightly underestimates the central corneal thickness than does ultrasound pachymetry, which may result from the difficulty in fixation of keratoconic eyes, misalignment of Pentacam and the variation of ultrasonic velocity due to the histological deformation.

Evaluation of corneal thickness using a Scheimpflug-Placido disk corneal analyzer and comparison with ultrasound pachymetry in eyes after laser in situ keratomileusis

PURPOSE

To evaluate the repeatability and reproducibility of corneal thickness measurements in post-laser in situ keratomileusis (LASIK) eyes using a rotating Scheimpflug camera combined with a Placido disk corneal topographer (Sirius) and compare the results with those of ultrasound (US) pachymetry.

SETTING

Eye Hospital of Wenzhou Medical College, Wenzhou, China.

DESIGN

Comparative evaluation of a diagnostic test or technology.

METHODS

Patients were examined 3 times with the Scheimpflug-Placido topographer by 2 examiners. The central pupil corneal thickness (CTpupil), apical corneal thickness (CTapex), and thinnest corneal thickness (CTthinnest) were recorded. After noncontact examinations, US pachymetry was used to obtain the central corneal thickness (CCT).

RESULTS

The Scheimpflug-Placido topographer showed high intraoperator repeatability as indicated by a test-retest repeatability of less than 8.5 μm for CTpupil, CTapex, and CTthinnest, The coefficients of variation (CoV) were less than 0.7%, and the intraclass correlation coefficient was higher than 0.99. Excellent results were also obtained for interoperator reproducibility. All CoVs were less than 0.5%. The 95% limits of agreement between the Scheimpflug-Placido measurement and the US pachymetry measurements were narrow (-16.62 to 12.44 μm for CTpupil versus US pachymetry CCT; -17.49 to 12.16 μm for CTapex versus US pachymetry CCT; -18.59 to 10.90 μm for CTthinnest versus US pachymetry CCT).

CONCLUSIONS

The Scheimpflug-Placido topographer showed excellent intraoperator repeatability and interoperator reproducibility of CTpupil, CTapex, and CTthinnest measurements in post-LASIK eyes. The CCT measurements obtained using the device were in high agreement with those obtained by US pachymetry, suggesting that the 2 devices are interchangeable.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

A comprehensive comparison of central corneal thickness measurement

PURPOSE

To assess the repeatability and reproducibility of central corneal thickness (CCT) measurements by high-resolution rotating Scheimpflug imaging (Pentacam, Oculus) and Fourier-domain optical coherence tomography (RTvue-100, Optovue) after laser in situ keratomileusis (LASIK) and to compare the agreement with ultrasound pachymetry (USP).

METHODS

Forty-seven eyes of 47 patients after LASIK were included in the study. The first examiner took two successive Pentacam and RTvue CCT measurements, and this was repeated once again by the second examiner to assess intraobserver and interobserver repeatability and reproducibility. After performing non-contact examinations, the corneas were measured by USP to compare the level of agreement among the three devices.

RESULTS

All Pentacamcenter, Pentacamapex, Pentacamthinnest, and RTvue CCT measurements demonstrated high intraobserver repeatability, with respective precision (1.96 within-subject standard deviation) and intraclass correlation coefficients of 7.52, 7.43, 7.55, and 3.81 μm and 0.985, 0.986, 0.986, and 0.997; interobserver repeatability results were similar. All coefficients of variation were low: <1% for all measures. Compared with Pentacam and USP measurements, the RTvue measurement significantly underestimated CCT by a mean of 10.52 to 15.28 μm (p < 0.001) and 9.17 μm (p < 0.001), respectively. The agreement of USP with Pentacam and RTvue by Bland-Altman analysis spanned over 30 μm. The agreement of Pentacam with RTvue spanned approximate 20 μm.

CONCLUSIONS

Both Pentacam imaging and RTvue Fourier-domain optical coherence tomography provide reliable and interchangeable measurement of CCT in post-LASIK corneas. However, they cannot be considered to be clinically interchangeable with USP.

Repeatability of intraoperative central corneal and residual stromal thickness measurement using a handheld ultrasound pachymeter

PURPOSE

To determine and compare the repeatability of intraoperative central corneal thickness (CCT) and residual stromal bed thickness measurements using a handheld ultrasound (US) pachymeter.

SETTING

London Vision Clinic, London, United Kingdom.

DESIGN

Comparative evaluation of a diagnostic test or technology.

METHODS

This study comprised eyes that had laser in situ keratomileusis retreatment by flap lift in which handheld US pachymetry (Corneo-Gage Plus 50 MHz) had been performed intraoperatively. In each case, 5 consecutive measurements were obtained centrally immediately before and after the flap was lifted. The within-eye repeatability was calculated as the standard deviation of the 5 repeated measurements for the CCT measurements and the central residual stromal thickness (RST) measurements.

RESULTS

The study evaluated 134 eyes (79 patients). The mean CCT was 467 μm ± 40 (SD) (range 393 to 577 μm). The repeatability of CCT measurements was 6.83 μm, the coefficient of repeatability was 13.40 μm, and the coefficient of variation (CoV) was 1.46%. The mean central RST was 335 ± 46 μm (range 259 to 465 μm). The repeatability of central RST measurements was 4.91 μm, the coefficient of repeatability was 9.62 μm, and the CoV was 1.46%.

CONCLUSION

The repeatability of intraoperative handheld US pachymetry was similar between measurements of CCT and measurements of central RST; the CoV was 1.46% in both cases.

Reasons for not performing LASIK in refractive surgery candidates in Yemen

PURPOSE

To determine and analyze the reasons that laser in situ keratomileusis (LASIK) was not performed in patients who requested refractive surgical correction of their refractive errors.

METHODS

A retrospective observational study was performed in Yemen Magrabi Hospital between June 2005 and October 2006; the case records of 1596 new consecutive patients who presented for refractive surgery at our refractive surgery unit were reviewed. Data on the ocular status, refractive problems, and investigations performed were analyzed. The reasons for not performing LASIK in the cases that were rejected were recorded and analyzed.

RESULTS

A total of 1191 patients (74.6%) were advised to have LASIK of the 1596 patients examined. LASIK was not advised in 405 patients (25.4%). The most common reasons for rejecting LASIK were suboptimal central corneal thickness (25.9%), high myopia >-11.00 D (17.0%), keratoconus (15.5%), cataract (11.4%), and suspicious corneal topography (forme fruste keratoconus) (9.4%).

CONCLUSIONS

Patients who request refractive surgery have a variety of problems and attention must be given to these patients. Suboptimal corneal thickness, high amount of refractive error, and keratoconus were found to be the leading causes of not performing LASIK in Yemeni patients.

Comparison of fourier-domain and time-domain optical coherence tomography for assessment of corneal thickness and intersession repeatability

PURPOSE

To compare Fourier-domain optical coherence tomography (FD-OCT) with time-domain OCT (TD-OCT) for agreement in corneal pachymetry in the central (2-mm) zone and paracentral (2- to 5-mm) zone, minimum corneal thickness, and instrument intersession reliability in candidates with normal cornea.

DESIGN

Prospective comparative trial.

METHODS

The study was performed at a tertiary care ophthalmic hospital. One hundred normal candidates with no ocular pathology except mild refractive error were included. FD-OCT and TD-OCT were used for pachymetric analysis within 15 minutes of each other. Repeat testing on both machines was done on the next day. One eye from each patient was randomly selected for analysis of both comparison and reliability.

RESULTS

The mean thickness was significantly more for FD-OCT (difference being 5.8, 11.1, and 19.2 mum for central, paracentral, and minimal thickness). Highest correlation, best linear fit, and smallest range of 95% limits of agreement (LoA) was seen for central corneal thickness, followed by paracentral and minimum corneal thickness (R(2) values for the 3 best fit lines were 0.93, 0.86, and 0.58, respectively). In terms of intersession reliability, FD-OCT was better (coefficients of reliability <1.5% vs 4.5% of TD-OCT) with smaller 95% LoA range, suggesting its better performance.

CONCLUSIONS

The corneal thickness measurements between TD-OCT and FD-OCT were highly correlated but not similar. Conversion equations may be used for central and paracentral, but not for minimum, corneal thickness. Even though both the OCT instruments had good reliability, FD-OCT was better.