Correlation of central and peripheral corneal thickness in healthy corneas

https://doi.org/10.1016/j.clae.2011.07.004Get rights and content

Abstract

Purpose

To study the thickness profile of the normal cornea in order to establish any correlation between central and peripheral points.

Methods

Sixty-seven eyes of 40 patients were subjected to central corneal thickness measurement (CCT) with an ultrasound pachymeter (UP) and corneal thickness mapping with the Oculus Pentacam. The corneal apex thickness (CAT), pupil centre thickness (recorded as CCT and corresponded to CCT of UP) and thickness at the thinnest location (CTL) were obtained and compared with each other. Corneal thickness data at 3 mm and 7 mm temporally, nasally, superiorly and inferiorly from the corneal apex were obtained. The mean corneal thickness values along the 2, 4, 6, 8 and 10 mm diameter concentric circles, with the CTL as the centre, were also obtained. The above data at different points were statistically correlated.

Results

There was no significant difference between CCT readings measured by UP and Pentacam (P = 0.721). There was high positive correlation between the CAT values and the thickness at 3 mm (R  0.845, P < 0.001) and at 7 mm points (R  0.654, P < 0.001). A gradual increase in thickness was noted from the centre to the periphery with a high positive correlation between the CTL values and the mean thickness at the circles of 2, 4, 6, 8 and 10 mm (R  0.635, P < 0.001).

Conclusion

The results suggest that central corneal thickness can serve as a good guide for predicting peripheral thickness. For surgical procedures specifically undertaken at mid-peripheral and peripheral zones, the actual measurements at the site of surgery may confer some advantage.

Introduction

Morphometric data on central corneal thickness (CCT) has assumed considerable clinical importance in relation to refractive [1] and non refractive corneal surgery and glaucoma [2], [3]. The depth of incisions and of ablations in refractive surgery and the thickness of the cut in lamellar corneal surgery, all require fairly precise measurement of corneal thickness. Some incisions are made at the mid-periphery (arcuate incisions) [4], at the periphery (limbal relaxing incisions) [5] or from the edge of the optical zone to the periphery as in radial keratotomy [6]. In manual preparation of donor material for the DSEK procedure the depth of the initial peripheral incision in the donor sclerocorneal disc can be estimated by the CCT. By implication, the CCT data is used as a surrogate for peripheral corneal thickness (PCT) in some instances.

Ultrasound pachymetry has been reported to give very accurate, clinically reliable and reproducible data [7], [8], but determination of the actual points to apply the probe is operator dependent and can lead to inaccuracies when mapping the corneal thickness. The advent of devices such as the Scheimpflug rotating camera (Pentacam; Oculus, Inc., Wetzlar, Germany), slit-scanning optical pachymetry (Orbscan; Bausch & Lomb, Rochester, New York, USA) and high speed optical coherence tomography (OCT; Carl Zeiss Meditec, Inc., CA, USA) has allowed accurate mapping of corneal thickness at multiple points on the cornea [9]. The Pentacam for example collects information from up to 25,000 data points [10]. Using such data we have been able to correlate CCT to the PCT and establish a comprehensive thickness profile of the normal cornea. This information is reported in this paper.

Section snippets

Methods

This prospective clinical study enrolled 67 eyes of 40 patients. Inclusion criteria were healthy corneas with a best corrected visual acuity of 6/6 or better. The following exclusion criteria were applied: amblyopia, previous history of ocular disease or trauma, previous ocular surgery including refractive surgery and contact lens wear. Thirteen eyes of 13 patients were thus excluded. The study was performed according to the Tenets of the Declaration of Helsinki. We certify that all applicable

Results

Of the 40 patients studied 24 were males (60%) and 16 were females (40%). The mean age was 38.65 years ± 14.58 (range 19–76 years). Keratometry readings (K) measured in dioptres (D) were as following: flat meridian (43.12 ± 1.43 D, range 40.3–47.4 D), steep meridian (44.18 ± 1.54 D, range 41.3–49.3 D) and average K (43.65 ± 1.44 D, range 41.1–48.4 D).

Discussion

Corneal thickness measurements are essential in the work up of patients for laser or incisional refractive surgery [1], assessment of glaucoma [14] and monitoring certain corneal diseases such as keratoconus [15], [16] and Fuchs endothelial dystrophy [17]. For all of the above indications, reliance is placed on measurement of the central corneal thickness by manual placement of the ultrasound probe. Clinically this has proven to be reasonable and reliable. Relatively newer instruments have

Conflict of interest

None of the authors have any financial or other conflict of interest.

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