Screening of the Previous Photorefractive Surgery Using the Minimum Distance Between the Anterior Best Fit Sphere and the Center of the Anterior Corneal Surface

Lee Hoo Kim; Jae Hwan Lee; Jung Hyub Oh

doi:10.3341/jkos.2008.49.1.27

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Kim, Lee, and Oh: Screening of the Previous Photorefractive Surgery Using the Minimum Distance Between the Anterior Best Fit Sphere and the Center of the Anterior Corneal Surface

Original Article

J Korean Ophthalmol Soc 2008;49(1):27-33.

Published online: 3 August 2008

DOI: https://doi.org/10.3341/jkos.2008.49.1.27

Screening of the Previous Photorefractive Surgery Using the Minimum Distance Between the Anterior Best Fit Sphere and the Center of the Anterior Corneal Surface

Lee Hoo Kim, M.D., Jae Hwan Lee, M.D., Jung Hyub Oh, M.D., Ph.D.

Department of Ophthalmology, Inha University College of Medicine, Incheon, Korea

Address reprint requests to Jung Hyub Oh, M.D., Ph.D. Department of Ophthalmology, Inha University, Inha University Hospital #7-206 Shinheung-dong, Jung-gu, Incheon 400-103, Korea Tel: 82-32-890-2408, Fax: 82-32-890-2403, E-mail: jhoh9707@hanmail.net

Received 29 May 2007 Accepted 10 October 2007

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 develop a screening test based on the difference (Diff) between the anterior corneal surface and the anterior best‐ fit sphere in the central region of the Orbscan IIz topography as a way of detecting previous myopic photorefractive surgery.

Methods

From 1623 patients who had no refractive surgery and no corneal disease, 3132 topographies were defined as normal. From 120 patients who had Orbscan IIz topography after myopic photorefractive surgery, 238 topographies were defined as eyes that had undergone refractive surgery. The first objective was to determine the difference (Diff) between the anterior corneal surface and the anterior best‐ fit sphere in the central region. The second objective was to classify the anterior elevation map of Orbscan IIz topography.

Results

The Diff value of the center of the anterior cornea surface averaged 0.008±0.003 mm in normal eyes, and all values were over 0 mm. However, in eyes that had previous refractive surgery, the average was 0.014±0.009 mm, and all values in this group were less than 0. The specificity and sensitivity was 100 % in both groups. The spherical equivalent of the degree of myopic correction and the Diff value of the center of the anterior cornea surface showed a linear relationship. Consequently, we could derive a formula to determine the degree of myopic correction with a known Diff value of the center of the anterior corneal surface.

Conclusions

The screening test, based on the Diff value of Orbscan IIz topography, is quite useful in determining whether an eye has undergone previous myopic photorefractive surgery.

Keywords: Difference, Orbscan, Photorefractive, Screening

References

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Figure 1.

‘Diff’ value in the corneal center at anterior elevation map.

Figure 2.

‘Diff’ value in the anterior corneal center of a normal and the correction of myopia.(A) ‘Diff ’ value in the anterior corneal center in a normal.(B) ‘Diff ’ value in the anterior corneal center in a correction of myopia.(C) Percent distribution of ‘Diff ’ value in the anterior corneal center in a normal and correction of myopia.

Figure 3.

Plot diagram of the point for the correlation of ablation power for myopic correction and ‘Diff’ value in the anterior corneal center. (A) Post-operative ‘Diff ’ value in the anterior corneal center and amount (spherical) of myopic correction (r=0.8667, p<0.001). (B) Post-operative ‘Diff’ value in the anterior corneal center and amount (spherical equivalent) of myopic correction (r=0.8732, p<0.001).

Table 1.

Population characteristics and surgical parameters

Classification	Normal (N=3132)		Classification	Myopic correction (N=238)
Classification	Mean±SD^*	Median, Range	Classification	Mean±SD^*	Median, Range
Best Correction Spherical equivalent (D^†)	0.53±3.76	0.50,	Myopic correction Spherical equivalent (D^†)	-4.61±1.76	-4.25,
Best Correction Spherical equivalent (D^†)		-12.75 ∼ 8.50	Myopic correction Spherical equivalent (D^†)		-2.00 ∼ -10.50
Best Correction spherical (D^†)	0.07±2.17	0.00,	Myopic correction spherical (D^†)	)-4.30±1.74	-4.00,
		-12.00 ∼ 8.25			-1.50 ∼ -9.00
Best Correction Cylinder (D^†)	0.71±1.12	0.75	Myopic correction Cylinder (D^†)	-0.60±0.66	-0.50,
		0 ∼ 7.25			0 ∼ -3.00
Center Pachymetry	535.12±35.27	537,	Center Pachymetry (Pre-OP)	532.98±28.06	537,
		412 ∼ 723			461 ∼ 582

^* standard deviation.

^† diopter.

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