Stromal thickness in the normal cornea: three-dimensional display with artemis very high-frequency digital ultrasound

Abstract

Purpose: To characterize the stromal thickness profile in a population of normal eyes.

Methods: Stromal thickness profile was measured in vivo by Artemis very high-frequency digital ultrasound scanning (ArcScan, Morrison, Colo) across the central 10-mm corneal diameter on 110 normal eyes. Maps of the average, standard deviation, minimum, maximum, and range of stromal thickness were plotted. The average location of the thinnest stroma was found. The cross-sectional hemi-meridional stromal thickness profile was calculated using annular averaging. The absolute stromal thickness progression relative to the thinnest point was calculated using annular averaging as well as for 8 hemi-meridians individually.

Results: The mean stromal thickness at the corneal vertex and at the thinnest point were 465.4+/-36.9 mum and 461.8+/-37.3 mum, respectively. The thinnest stroma was displaced on average 0.17+/-0.31 mm inferiorly and 0.33+/-0.40 mm temporally from the corneal vertex. The average absolute stromal thickness progression from the thinnest point could be described by the quadratic equation: stromal thickness = 6.411 x radius(2) + 2.444 x radius (R(2) = 0.999). Absolute stromal thickness progression was independent of stromal thickness at the thinnest point. The increase in hemi-meridional absolute stromal thickness progression was greatest superiorly and lowest temporally.

Conclusions: Three-dimensional thickness mapping of the corneal stroma and stromal thickness progression in a population of normal eyes represent a normative data set, which may help in early diagnosis of corneal abnormalities such as keratoconus and pellucid marginal degeneration. Absolute stromal thickness progression was found to be independent of stromal thickness.

Figure 1

Topographical map of the descriptive statistics of stromal thickness centered on the corneal vertex for the population. The color scale represents the stromal thickness in microns. A Cartesian 1-mm grid is superimposed with the origin at the corneal vertex.

Figure 2

Stromal thickness maps of 15 randomly selected eyes each plotted with an individual color scale representing the stromal thickness in microns. A Cartesian 1-mm grid is superimposed with the origin at the corneal vertex. The epithelial thickness profiles of the same 15 eyes have been published previously.

Figure 3

Average location of the thinnest stroma within the central 5 mm of the cornea. The red dot represents the average location of the thinnest stroma for all eyes tested. The thick red ellipse represents one standard deviation in the x- and y-directions and the thin red ellipse represents two standard deviations in the x- and y-directions. A Cartesian 1-mm grid is superimposed with the origin at the corneal vertex. Positive x values represent the nasal stroma and negative values represent the temporal stroma. Positive y values represent the superior stroma and negative values represent the inferior stroma.

Figure 4

Topographic map of the average and standard deviation of stromal thickness for the population centered on the thinnest point. The color scale represents the stromal thickness in microns. A Cartesian 1-mm grid is superimposed with the origin at the thinnest point. Both maps include all eyes with left eyes mirrored (positive x-values represent the nasal stroma and negative values represent the temporal stroma).

Figure 5

Cross-sectional hemi-meridional average stromal thickness profile (μm) for 110 eyes using mirrored left eye symmetry. The data points represent the average stromal thickness of all data within an annulus of a given radius. The x axis is the radial distance (mm) from the location of the thinnest point. The thick blue line represents the average stromal thickness profile. The thin blue lines represent one standard deviation less than and one standard deviation greater than the average stromal thickness.

Figure 6

Average absolute stromal thickness progression (μm) with reference to the thinnest point for 110 eyes using mirrored left eye symmetry. The data points represent the difference between the average stromal thickness of all data within an annulus of a given radius and the thinnest stromal thickness. The x axis is the radial distance (mm) from the location of the thinnest point. The thick blue line represents the absolute stromal thickness progression. The thin blue lines represent one standard deviation less than and one standard deviation greater than the absolute stromal thickness progression.

Figure 7

Average absolute stromal thickness progression (μm) with reference to the thinnest point for 110 eyes using mirrored left eye symmetry grouped as thin, average, and thick central stromal thickness. The data points represent the difference between the average stromal thickness of all data within an annulus with a given radius and the value at the thinnest point. The x axis is the radial distance (mm) from the thinnest point location. The thin stroma group consisted of patients whose stromal thickness was at least one standard deviation less than the average stromal thickness at the thinnest point and is represented by the pink line. The thick stroma group consisted of patients whose stromal thickness was at least one standard deviation greater than the average stromal thickness at the thinnest point and is represented by the blue line. The average stroma group consisted of patients whose stromal thickness was within one standard deviation of the mean and is represented by the green line.

Figure 8

Average absolute stromal thickness progression (μm) with reference to the thinnest point in eight hemi-meridians for 110 eyes using mirrored left eye symmetry. The data points represent the difference between the average stromal thickness of all data within an annulus with a given radius and the value at the thinnest point. The x axis is the radial distance (mm) from the thinnest point location. Each line represents the absolute stromal thickness progression along a given hemi-meridian, in eight hemi-meridians at 45° intervals.