Elsevier

Ophthalmology

Volume 106, Issue 5, 1 May 1999, Pages 977-981
Ophthalmology

Orbscan pachymetry: Implications of a repeated measures and diurnal variation analysis,

Presented at the American Academy of Optometry annual meeting, San Antonio, Texas, December 1997.
https://doi.org/10.1016/S0161-6420(99)00519-9Get rights and content

Abstract

Introduction

Corneal thickness changes reflect alterations in hydration and metabolism. Ultrasound pachymetry determinations may be adversely influenced by fluctuations in tissue hydration, whereas optical systems are apparently unaffected by these fluxes. A recently marketed, optical-based, topographic mapping system (Orbscan; Orbtek, Inc.) uses anterior and posterior corneal surface data to calculate corneal thickness.

Objective

This new instrumentation presents as a potentially useful pachymetry tool for evaluation of corneas under hydration flux or challenge (e.g., postphotorefractive keratectomy [PRK] healing studies) and was therefore evaluated for accuracy and variability.

Measurements

Three calibrated standards were measured in repeated fashion. Additionally, 1 test subject was measured 30 times in 1 day (5 measurements each at 8:00, 9:30, and 11:00 am and at 1:00, 2:30, and 4:00 pm). Corresponding measurements were made at 8:00 and 11:00 am and at 4:00 pm on 3 separate days to assess repeatability. Grouped data from 18 volunteer subjects were compared to the data of the test subject as well.

Results

Pachymetry accuracy on a calibrated standard was determined to be ±2 μm (standard deviation, n = 12). Repeated measures on the subject demonstrated a mean standard deviation of 9.08 μm for 750 thickness data points across the central 7 mm of the cornea; peripheral measurement points exhibited progressively greater variability than at the apex (analysis of variance; P < 0.0001). A plot of thickness by corneal location and time of day exhibited a diurnal pattern, with the peripheral cornea exhibiting progressively greater thickness changes than the central cornea (two-way analysis of variance; P < 0.00001). The data significantly correlated across days when all times of day were considered (r = 0.999). However, thickness values obtained at 8:00 am were significantly different across days (t test; P < 0.0002). The subject’s data correlated very well (r = 0.9996) with the grouped volunteer data.

Conclusions

These data show this system to be useful in corneal research and in clinical settings. The data confirm early morning pachymetry to be highly variable. Additionally, the data not only indicate a diurnal variation of corneal hydration over time, but also imply the presence of a diurnal-based hydration gradient across the peripheral cornea, both of which can have significance for PRK, since excimer tissue ablation effectiveness is influenced by tissue hydration.

Section snippets

Instrumentation

The Orbscan system is a computerized corneal mapping device that obtains elevation information from both the anterior and posterior corneal surfaces. It uses a calibrated video and slit-beam system to noninvasively measure three-space points on the front and rear corneal surfaces. The three-space points are used to construct mathematical representations of the true topographic surfaces (i.e., maps of z [elevation] vs. x and y [horizontal and vertical coordinates]). Therefore, corneal thickness,

Results

A calibrated surface was repeatedly scanned to assess accuracy of the estimated anterior surface elevation model. Calibrated surface data were very accurate with the mean standard deviation (SD) of 12 repeated measures equal to 2 μm over the entire measured 7-mm diameter anterior surface map (range, 1.2 μm centrally to 3.2 μm peripherally). These measurements were taken four times from three different calibrated surfaces of spherical shape (37.5 D, 47.54 D, and 54.57 D).

Figure 2 illustrates the

Discussion

The accuracy of the surface maps of the calibrated standards yielded a mean SD of 2 μm (range, 1.2 μm centrally to 3.2 μm peripherally). Presuming similar accuracy for posterior maps, a simple mathematical addition of those errors would yield pachymetry SD values of greater than 2 μm but perhaps less than 8 μm for a constructed pachymetry map. This is in general agreement with the experimental data obtained: the apical measurements had a mean SD of 5 μm (range, 3–7 μm), and the mean SD for the

Acknowledgements

The authors thank Hospital Corpsman 3rd Class (HM3) George Meyer for his exceptional support services.

References (15)

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