Artefact-free topography based scleral-asymmetry

Abstract

Purpose: To present a three-dimensional non-parametric method for detecting scleral asymmetry using corneoscleral topography data that are free of edge-effect artefacts.

Methods: The study included 88 participants aged 23 to 65 years (37.7±9.7), 47 women and 41 men. The eye topography data were exported from the Eye Surface Profiler software in MATLAB binary data container format then processed by custom built MATLAB codes entirely independent from the profiler software. Scleral asymmetry was determined initially from the unprocessed topography before being determined again after removing the edge-effect noise. Topography data were levelled around the limbus, then edge-effect was eliminated using a robust statistical moving median technique. In addition to comparing raw elevation data, scleral elevation was also compared through fitting a sphere to every single scleral surface and determining the relative elevation from the best-fit sphere reference surface.

Results: When considering the averaged raw topography elevation data in the scleral section of the eye at radius 8 mm, the average raw elevations of the right eyes' sclera were -1.5±1.77, -1.87±2.12, -1.36±1.82 and -1.57±1.87 mm. In the left eyes at the same radius the average raw elevations were -1.62±1.78, -1.82±2.07, -1.28±1.76 and -1.68±1.93 mm. While, when considering the average raw elevation of the sclera after removing the edge effect, the average raw elevations of the right eyes were -3.71±0.25, -4.06±0.23, -3.95±0.19 and -3.95±0.23 mm. In the left eyes at the same radius the average raw elevations were -3.71±0.19, -3.97±0.22, -3.96±0.19 and -3.96±0.18 mm in the nasal, temporal, superior and inferior sides respectively. Maximum raw elevation asymmetry in the averaged scleral raw elevation was 1.6647±0.9015 mm in right eyes and 1.0358±0.6842 mm in left eyes, both detected at -38° to the nasal side. Best-fit sphere-based relative elevation showed that sclera is more elevated in three main meridians at angles -40°, 76°, and 170° in right eyes and -40°, 76°, and 170° in left eyes, all measured from the nasal meridian. Maximum recorded relative elevation asymmetries were 0.0844±0.0355 mm and 0.068±0.0607 mm at angular positions 76° and 63.5° for right and left eyes in turn.

Conclusions: It is not possible to use corneoscleral topography data to predict the scleral shape without considering a method of removing the edge-effect from the topography data. The nasal side of the sclera is higher than the temporal side, therefore, rotationally symmetric scleral contact lenses are more likely to be translated towards the temporal side. The scleral shape is best described by levelled raw elevation rather than relative elevation.

Fig 1. Average raw elevation maps for right and left eyes.

Black contour lines represent the standard deviation of the raw elevation data.

Fig 2

Average eyes’ raw elevation as measured by ESP with the origin at the corneal apex, (a) Right eyes temporal-nasal, (b) Left eyes nasal-temporal, (c) Right eyes inferior-superior, (d) Left eyes inferior-superior.

Fig 3

Edge effect detection example for the right eye of a 43 years old female participant, (a) Inferior meridian where two edges were detected, (b) Superior meridian where one edge was detected. The digital image of the eye as captured by the ESP was projected onto the eye surface for display purposes.

Fig 4. The moving median algorithm used in detecting the edge-effect.

Fig 5

Scleral raw elevation and relative elevation as determined after levelling the eyes and eliminating the edge effect; (a) Raw elevation (right eye); (b) Raw elevation (left eye). The polar plot in the middle of subplots (a) and (a) shows the scleral raw elevation asymmetry in polar coordinates scaled 5 times their values for display purposes. The thick black line is the average asymmetry and the thin black lines are the standard deviation added and subtracted to the mean values. The red line is pointing to the angle where the asymmetry was a maximum. (c) Relative elevation (right eye); (b) Relative elevation (left eye). Elevation reference for both right and left eyes were best-fitted spheres whose radii were determined by minimising the least squares fitting error. The polar plot on the middle of subplots (c) and (d) shows the scleral relative elevation asymmetry in polar coordinates scaled 40 times their values with their standard deviation scaled up to 10 times for display purposes.

Fig 6

Average eyes’ raw elevation as determined after removing the edge-effect with the origin at the corneal apex, (a) Right eyes nasal side, (b) Left eyes nasal side, (c) Right eyes temporal side, (d) Left eyes temporal side, (e) Right eyes superior side, (f) Left eyes superior side, (g) Right eyes inferior side, and (h) Left eyes inferior side.

Fig 7

Asymmetry significance around the sclera, the value 1.0 indicates positive test decision and 0.0 indicates a negative test decision, however the significance (p-value) was presented in red (a) Raw elevation asymmetry, right eyes, (b) Raw elevation asymmetry, left eyes, (c) Relative elevation asymmetry, right eyes, (d) Relative elevation asymmetry, left eyes.

Fig 8. Right eye of a 33 years old male participant divided into three sections; corneal surface, scleral ring and artefact ring called the ‘edge effect’.

The digital image of the eye as captured by the ESP was projected onto the eye surface for display purposes.