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.

Limbus misrepresentation in parametric eye models

“…The eye surface data was processed by custom-built MATLAB codes entirely independent from the built-in ESP software digital signal processing (DSP) algorithms. Firstly, each eye surface was freed of edge-effect artefacts following Abass’s three-dimensional non-parametric method [ 46 ], Fig 2A and 2B . Three-dimensional surfaces were fitted to the ESP topographical data using the spherical, conic and biconic techniques outlined in the introduction.…”

“… Left eye of a 25 year old male subject where the eye’s front image was projected on the surface for displaying purpose (a) as scanned by the ESP, where the limbus was detected in three-dimensions [ 8 ] (b) after being processed to split cornea, sclera and the edge effect artefacts following Abass’s method. Two edge detection strategies were used simultaneously to cut the edge of the eye’s surface data at the border between the authentic eye surface and the artificial boundaries which result from interference of tears, eyelid edges or lashes [ 46 ] (c) corneal conic fit up to 5mm, (d) whole corneal conic fit, (e) sclera fitted to a sphere, (f) Fitted limbus as constructed by the intersection of the fitted surfaces representing the cornea and the sclera. …”

“…The eye surface data was processed by custom-built MATLAB codes entirely independent from the built-in ESP software digital signal processing (DSP) algorithms. Firstly, each eye surface was freed of edge-effect artefacts following Abass's threedimensional non-parametric method [46], Fig 2A and 2B. Three-dimensional surfaces were Left eye of a 25 year old male subject where the eye's front image was projected on the surface for displaying purpose (a) as scanned by the ESP, where the limbus was detected in three-dimensions [8] (b) after being processed to split cornea, sclera and the edge effect artefacts following Abass's method.…”

Which feature influences on-eye power change of soft toric contact lenses: Design or corneal shape?

“…Nevertheless, the study followed the tenets of the Helsinki Declaration of 1964 (revised in 2013). Written informed consent was also provided in the primary data collection by all participants for the use of their de-identified data in scientific research [31].…”

“…When the limbus was detected; the scleral topography data was first processed through an edge-effect elimination process where topographical artefacts caused by the eyelash's interference or tear pooling were removed using the technique introduced by Abass et al [31]. Once the scleral topographical data is cleared of measurement artefacts, it was then fitted to a sphere using the least squares error method, minimising the fitting error Err for every point i of the n points as described in Eq 1…”