Corneal topography measurements and digital images of the anterior eye and adnexae were captured for 100 young adult subjects. Subjects were recruited for this experiment primarily from the students and staff of the Queensland University of Technology. All subjects had normal ocular health, were free of any ocular disease or systemic disease or syndrome that might have altered anterior eye appearance, and had no history of ocular or eyelid surgery or trauma. Each subject underwent initial slit lamp examination to rule out anterior eye disease. Wearers of rigid gas-permeable (RGP) contact lenses were excluded from the study. Nine subjects wore soft contact lenses part time, and these subjects were instructed not to wear their lenses on the day of testing. Subjects’ ages ranged from 18 to 35 years and averaged 24 ± 4 years. Fifty-nine of the 100 subjects were women. Eighty of the 100 subjects were white, and 20 were East Asian. All subjects had best-corrected visual acuity of 6/7.5 or better in the measured eye.
Subjects exhibited a normal range of refractive errors; the mean best-sphere refraction was −1.13 ± 1.85 D (range, +0.63 D to –8.13 D). The mean astigmatic refractive error was −0.32 ± 0.58 (range, 0 D to −2.75 D). To allow for statistical analysis of the subjective refraction results, each subject’s refractive error was broken down into the power vectors M (best sphere), J0 (astigmatism 90/180), and J45 (astigmatism 45/135).
38 Approval from the university human research ethics committee was obtained before the study, and informed consent was obtained from all the subjects. All subjects were treated in accordance with the tenets of the Declaration of Helsinki.
Corneal topography
39 40 and palpebral fissure
41 measures have a tendency to exhibit a high degree of symmetry between right and left eyes. For this reason, only the right eye of each subject was used for all measurements. To minimize any short-term effects on corneal topography as a result of eyelid pressure,
9 42 all measurements were taken in the morning, and subjects were asked to refrain from substantial amounts of close work before testing.
The corneal topography of each subject was measured with a videokeratoscope (Medmont E300; Medmont Pty. Ltd., Victoria, Australia) built on the Placido disk principle and exhibiting highly accurate and repeatable measurements on inanimate test surfaces
43 and highly repeatable measurements on human corneas.
44 The corneal topography of each subject was measured with a technique that allows central and peripheral corneal topography data to be captured and then subsequently combines these data to produce one large, extended corneal topography map for each subject. This method for measuring corneal topography, described in detail elsewhere,
45 46 evaluates a much larger area of the cornea than standard techniques (approximately 30% increase of topography map dimensions). Subjects exhibiting poor correlation between central and peripheral corneal topographical data (i.e., subjects showing greater than a ±0.2-mm difference in axial curvature between actual data and that predicted by a 2nd-order polynomial function at the junction between central and peripheral corneal data in more than 40 semi-meridians) were excluded from further analysis.
46 Of the 100 subjects participating, eight subjects were excluded from subsequent analyses because of poor correlation between central and peripheral corneal topography maps.
Each extended corneal topography map was rotated to make the corneal geometric center the reference axis for the maps; corneal height, axial power, and axial radius of curvature were analyzed in detail.
46 This provided a range of parameters to describe the corneal topography of the central and peripheral cornea, including the best-fitting corneal axial power spherocylinder (defined by power vectors M, J0, and J45) and the best-fitting conic section (defined by apical radius
r o and asphericity parameter
Q). Each corneal axial power map was then classified according to the amount of astigmatism present in the central 4-mm diameter as either central spherical (<0.75 DC) or central astigmatic (>0.75 DC). Axial power maps were also classified according to the central corneal cylinder axis as having either WTR central axis (central corneal cylinder axis 30°–150°;
n = 67), against-the-rule central axis (ATR; central corneal cylinder axis 60°–120°;
n = 11), or oblique central axis (OBL; central cylinder axis 30°–60° or 120°–150°;
n = 12).
47 The most common central corneal cylinder axis was WTR.
Figure 1displays a frequency histogram of the subjects’ central corneal cylinder axes.
Each subject’s central videokeratoscope image was also analyzed to determine the diameter of the cornea. Customized software allowed the user to locate 16 points at the corneal edge. An ellipse was then fitted to the 16 points defined as the edge of the cornea according to an orthogonal least squares fitting procedure.
48 This corneal ellipse is defined by its major or longest diameter (
A), minor or shortest diameter (
B), and theta (i.e., the angle between the major diameter “
A” and the horizontal).
After the measurement of corneal topography, high-resolution digital images (3072 × 2068 pixels) of the right eye in the frontal plane were captured for each subject in primary gaze (0°), 20° downgaze, and 40° downgaze. All digital images were captured with a 6.3 mega pixel digital SLR camera (Canon 300D; Canon Inc., Tokyo, Japan) and a 100-mm macro lens. Digital images from four subjects were excluded completely from analysis because of poor image quality. Each digital image was analyzed with customized software to determine a wide range of biometric measures of the palpebral fissure and the anterior eye for each subject in each of the three different angles of vertical gaze. A detailed description of the method of image capture and analysis is provided elsewhere.
49 Measures of the palpebral aperture’s vertical and horizontal dimensions, angle of the palpebral fissure, and contour of the upper and lower eyelid were calculated for each subject. Eyelid contour was quantified by fitting a polynomial function of the form
Y =
Ax 2 +
Bx +
C to the upper and lower eyelids.
50 In this polynomial, the coefficient
A refers to the curvature of the eyelid,
B refers to the tilt or angle of the eyelid, and the constant
C refers to the height of the eyelid above or below the corneal geometric center.