Optical low coherence reflectometry for noncontact measurements of flap thickness during laser in situ keratomileusis

doi:10.1016/S0161-6420(02)01016-3

Ophthalmology

Volume 109, Issue 5, May 2002, Pages 973-978

https://doi.org/10.1016/S0161-6420(02)01016-3 Get rights and content

Abstract

Objective

There is growing evidence that iatrogenic keratectasia after laser in situ keratomileusis (LASIK) for high corrections occurs more frequently than initially assumed, and that it may result from larger variation in flap thickness.

Design

Consecutive noncomparative case series

Participants

Thirty-four patients who underwent LASIK for myopia and astigmatism (first treatment group) and 10 patients who received re-LASIK (retreatment group).

Methods

Central corneal thickness and thickness of the lamella during LASIK were determined by optical low coherence reflectometry (OLCR) and contact ultrasound pachymetry.

Main outcome measures

Thickness of the flap and its standard deviation, as well as its correlation with age, sphere, cylinder, corneal thickness, intraocular pressure, and corneal refractive power (K-readings).

Results

The mean flap thickness of the first treatment group determined by OLCR was 130 ± 29 μm; the 95 percentile was 169 μm and the 5 percentile was 86 μm. The flap thickness was not correlated with any of the investigated demographic or refractive parameters. The mean flap thickness of the retreatment group was 152 ± 14 μm; the 95 percentile was 175 μm and the 5 percentile was 137 μm. Thus, the flap thickness of the retreatment group was significantly thicker compared with the first treatment group (P < 0.001).

Conclusions

Optical low coherence reflectometry (OLCR) was shown to be an appropriate alternative to ultrasonic preoperative and intraoperative corneal pachymetry in laser assisted in situ keratomileusis. The lack of correlation between achieved flap thickness and preoperative clinical data, such as corneal thickness, corneal curvature, intraocular pressure, and refraction, emphasizes the importance of measuring flap thickness and corneal bed thickness during surgery.

Section snippets

Materials and methods

A first treatment group consisted of 32 patients with 34 eyes that underwent standard LASIK for myopia and astigmatism (spherical equivalent −10 diopters and less). The central corneal thickness was measured immediately before surgery by means of optical low coherence reflectometry (OLCR) and ultrasound pachymetry. After lifting the flap, the thickness of the residual stroma before photoablation was measured again, this time by OLCR only. None of the eyes had previous ophthalmic surgery or

Results

The microkeratome cuts did not cause any complications; neither did the relifts of the flaps in the retreatment group. In this group, laser treatment was not performed in two of the four eyes scheduled for residual refraction correction due to an insufficient corneal thickness of less than 250 μm.

Figure 2 represents the correlation between the corneal thickness measured with the OLCR and the one measured with ultrasonic pachymetry. The slope of the fitted linear regression is 1.006 with an

Discussion

The key finding of this study is that the flap thickness does not correlate with the spherical and cylindrical refraction, with K readings, or with intraocular pressure. This lack of correlation implies an unpredictability that impinges on the planning of the surgery as well as on the likelihood of postoperative complications since there are no parameters for predicting the flap thickness before keratomileusis. As a consequence, both the flap thickness and the residual stromal thickness must be

Acknowledgements

The authors thank Dr. K. Jost from the University of Zurich, Department of Ophthalmology, Zurich, Switzerland, for his help.

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Comparative evaluation of a diagnostic test or technology.
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.
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An ideal microkeratome must produce regular and repeatable flap cuts of predetermined thickness, which is determined by the distance of the blade from the fixed microkeratome’s head.7 However, a literature review indicated that almost all mechanical-oriented microkeratomes yield significant flap-thickness variations,8–15 mainly due to the quality and the entry angle of the blade, the translation and oscillation rate, the consistency across the cornea, the suction-ring pressure setting and suction duration, the cutting properties, room humidity, the preoperative CCT, and the corneal diameter.1,2,4–13 The purpose of this study was to evaluate the predictability of the Carriazo pendular 130 μm head microkeratome.
To assess flap-thickness predictability with a pendular microkeratome (130 μm head).
Eye Institute of Thrace, Democritus University of Thrace, Alexandroupolis, Greece.
Clinical trials.
The study comprised 263 eyes (132 patients). Laser in situ keratomileusis was performed using the 130 μm head of the Carriazo pendular microkeratome; right eyes were treated first. Ultrasound pachymetry and topography were used for central corneal thickness (CCT) and keratometry (K) measurements. Evaluation included flap thickness, flap diameter, and flap shape.
The mean flap thickness was 125 μm ± 22 (SD) (range 74 to 187 μm) in right eyes and 112 ± 21 μm (range 61 to 190 μm) in left eyes. Flap thickness was significantly correlated with preoperative CCT (r = 0.271; P<.001) but not with K values or the manifest refraction spherical equivalent (P>.15). Right eyes had thicker flaps than left eyes (P<.001); both were significantly below the 130 μm head thickness (mean flap thickness 119.2 ± 22.8 μm; P<.001). The mean achieved flap diameter was 9.2 mm using the 9.0 ring and 9.8 mm using the 10.0 mm ring. Flap-thickness stabilization and convergence between right eyes and left eyes occurred after 100 consecutive flap cuts.
Flap-thickness predictability was influenced by preoperative CCT only. All cuts were significantly thinner than the head thickness regardless of the suction ring size. Second surgical eyes had thinner flaps, possibly from blade deterioration from the first cut. Approximately 100 flaps were required as a learning curve.
No author has a financial or proprietary interest in any material or method mentioned.
Optical coherence tomography - development, principles, applications
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This paper presents a review of the development of optical coherence tomography (OCT), its principles and important applications. Basic OCT systems are described and the physical foundations of OCT signal properties and signal recording systems are reviewed. Recent examples of OCT applications in ophthalmology, cardiology, gastroenterology and dermatology outline the relevance of this advanced imaging modality in the medical field.
Diese Arbeit gibt einen aktuellen Überblick über Entwicklung, Grundlagen und Anwendungen der Optischen Kohärenz-Tomographie. Die wichtigsten OCT- Systeme und die physikalischen Grundlagen der OCT- Signale werden zusammenhängend dargestellt. Die große Bedeutung dieses neuen Abbildungsverfahrens in der Medizin wird an Beispielen aus der Ophthalmologie, Kardiologie, Gastroenterologie und Dermatologie demonstriert.
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All instruments used in this study are based mainly on noncontact laser interferometry to assess axial ocular dimensions. The Lenstar is based on the OLCR11,12 powered with an SLD.13–15 The IOLMaster uses the well-known principle of partial coherence interferometry in a dual-beam configuration to measure the axial eye length, and it is operated with a multimode laser diode.3,16,17
To evaluate a new high-resolution noncontact biometer (Lenstar; Haag-Streit AG, Koeniz, Switzerland) using optical low-coherence reflectometry and to compare the clinical measurements with those obtained from the IOLMaster (Carl Zeiss, Jena, Germany) and the Pachμmeter (Haag-Streit AG).
Exploratory evaluation of diagnostic technology and nonrandomized, prospective clinical trial.
Eighty subjects (144 eyes) aged 20 to 90 years with cataractous, pseudophakic, aphakic, silicon oil–filled, or normal eyes.
Measurements of axial length (AL), anterior chamber depth (ACD), central corneal thickness (CCT), corneal radius (R1 [flattest radius of corneal curvature] and R2 [steep radius, 90° apart from R1]), and axis of the flattest radius (Ax1) obtained with the Lenstar were compared with those obtained with the IOLMaster or Pachμmeter. The results were evaluated using Bland-Altman analyses. The differences between both methods were assessed using the paired t test, and its correlation was evaluated by Pearson coefficient.
Axial length, CCT, ACD, R1, R2, and Ax1.
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Measurements with the new Lenstar correlated well with those with the IOLMaster and Pachμmeter in cataractous, pseudophakic, aphakic, silicon oil–filled, and normal eyes. It is an accurate, fast instrument that provides additional information of interest to any cataract or refractive surgeon.
Proprietary or commercial disclosure may be found after the references.
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²: Dr. Wälti is employed by Haag-Streit, Köniz, Switzerland.

¹: Dr. Seiler is a scientific consultant of Schwind, Kleinostheim, Germany.

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Article for CME CreditOptical low coherence reflectometry for noncontact measurements of flap thickness during laser in situ keratomileusis

Abstract

Objective

Design

Participants

Methods

Main outcome measures

Results

Conclusions

Section snippets

Materials and methods

Results

Discussion

Acknowledgements

Ophthalmology

J Cataract Refract Surg

J Cataract Refract Surg

Am J Ophthalmol

Ophthalmology

J Cataract Refract Surg

Iatrogenic keratectasia after laser in situ keratomileusis

J Refract Surg

Progressive Keratektasie nach Laser-in-situ-keratomileusis (LASIK)

Klin Monatsbl Augenheilkd

Article for CME Credit
Optical low coherence reflectometry for noncontact measurements of flap thickness during laser in situ keratomileusis