Comparison of Flap Thickness Measured with Ultrasound Subtraction Method, Direct Method, and Optical Coherence Tomography

Su Hwan Lee; Joo Eun Lee; Jae Yong Kim; Myung Joon Kim; Hung Won Tchah

doi:10.3341/jkos.2011.52.5.524

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Lee, Lee, Kim, Kim, and Tchah: Comparison of Flap Thickness Measured with Ultrasound Subtraction Method, Direct Method, and Optical Coherence Tomography

Original Article

J Korean Ophthalmol Soc 2011;52(5):524-530.

Published online: 3 August 2011

DOI: https://doi.org/10.3341/jkos.2011.52.5.524

Comparison of Flap Thickness Measured with Ultrasound Subtraction Method, Direct Method, and Optical Coherence Tomography

Su Hwan Lee, MD, Joo Eun Lee, MD, Jae Yong Kim, MD, Myung Joon Kim, MD, Hung Won Tchah, MD

Department of Ophthalmology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea

Address reprint requests to Hung Won Tchah, MD Department of Ophthalmology, Asan Medical Center #388-1 Pungnap 2-dong, Songpa-gu, Seoul 138-736, Korea Tel: 82-2-3010-3680, Fax: 82-2-470-6440, E-mail: hwtchah@amc.seoul.kr

Received 25 August 2010 Accepted 26 October 2010

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Purpose

To compare the flap thicknesses measured with the ultrasound (US) subtraction method, the direct method, and optical coherence tomography (OCT) in femtosecond laser-assisted LASIK eyes.

Methods

The present study included 65 eyes of 37 patients who underwent femtosecond laser-assisted LASIK surgery. Flap thickness was measured intraoperatively using the direct and subtraction methods with US pachymetry and post-operatively using anterior segment OCT. Flap thickness measurements were compared among the three methods.

Results

There was no significant difference between the flap thickness measured using the US subtraction method and OCT (p = 0.511). The flap thickness values obtained by these two different methods also correlated well with each other (γ= 0.820, p < 0.001). However, flap thickness measured using the US direct method was significantly different (p < 0.001) with a low agreement value compared to the other two methods. Furthermore, the US direct method showed a lower measurement success rate compared to the US subtraction method (54% vs. 95%).

Conclusions

Flap thickness measured using the US subtraction method and OCT showed different values compared to that measured using the US direct method. Because of the lower measurement success rate and agreement value, the US direct method is not capable of being used in substitute for the US subtraction method.

Keywords: Anterior segment optical coherence tomography, Femtosecond laser, Flap thickness, Ultrasound pachymetry

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Figure 1.

Postoperative flap thickness measurement with anterior segment optical coherence tomography.

Figure 2.

Scattergram showing the correlation of flap thickness measured by ultrasound (US) subtraction method, US direct method, and optical coherence tomography (OCT). (A) Correlation between US subtraction method and US direct method. (B) Correlation between US subtraction method and OCT. (C) Correlation between US direct method and OCT.

Figure 3.

Bland-Altman plots between each two measurement methods. The middle line is the mean and the lines on the side represent the upper and lower 95% limits of agreement (LoA). (A) Ultrasound (US) subtraction method and US direct method. (B) US subtraction method and optical coherence tomography (OCT). (C) US direct method and OCT.

Table 1.

Comparison of the measured flap thickness

Method	n	Flap thickness (mean ± SD, μm)	Range (μm)	p-value
US^* subtraction pachymeter	32	118.3 ± 14.4	86.3-154.8	0.005
US direct pachymeter	32	127.4 ± 14.3	103.0-158.5
US subtraction pachymeter	28	125.9 ± 16.8	96.7-167.3	0.511
OCT^†	28	124.7 ± 10.6	111.0-153.0
US direct pachymeter	19	131.4 ± 14.5	103.0-158.5	0.007
OCT	19	120.0 ± 10.3	101.0-141.0

^* US = ultrasound;

^† OCT = optical coherence tomography.

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