Corneal Elevation, Power, and Astigmatism to Assess Toric Orthokeratology Lenses in Moderate-to-High Astigmats

Abstract

Objectives: Fitting philosophies for toric orthokeratology are based on elevation or corneal astigmatism, but it is unclear which is more effective. The purpose of this analysis was to further understand corneal shape and the relationship between peripheral elevation and central astigmatism in moderate-to-high astigmats.

Methods: Corneal tomography was measured three times on the right eyes of 25 moderate-to-high refractive myopic astigmatic adults. Corneal astigmatism and elevation were calculated at 4-, 6-, and 8-mm chords. Subjects were fitted with toric orthokeratology lenses following the manufacturer's guidelines based on elevation. Twenty subjects completed 10 days of wear. A masked examiner assessed movement and centration via slitlamp videos and quantified treatment zone and decentration from tangential power difference tomography maps. Correlations between variables were assessed.

Results: Average corneal astigmatism was 2.20±0.70 DC and peripheral elevation was 50.88±18.92 μm and they were strongly correlated (4 mm R2=0.96, 6 mm R2=0.92, 8 mm R2=0.86, all P<0.001). Each diopter of astigmatism equated to 25 μm of elevation at an 8-mm chord. Via slitlamp, average treatment zone area was 12.73±4.62 mm2 and 13 lenses decentered. From tomography, average treatment zone area was 7.16±2.56 mm2 and 17 were decentered. Tomography treatment zone area was negatively correlated with central corneal astigmatism (R2=0.60) and elevation at an 8-mm chord (R2=0.64, both P<0.001).

Conclusions: For tomography images, central corneal astigmatism was highly correlated with peripheral elevation and may be a more expedient measure for clinical use. Treatment area decreased as corneal astigmatism and elevation increased.

Trial registration: ClinicalTrials.gov NCT03728218.

Copyright © 2020 Contact Lens Association of Ophthalmologists.

Figures

Figure 1.

An elevation map showing the relative elevation in the vertical and horizontal meridians at an 8 mm chord. The difference between the average of the vertical and horizontal meridian is calculated.

Figure 2.

ImageJ software was used to fit a best fit ellipse to the screenshot of the slit-lamp video to calculate the size and area of the treatment zone of the lens (outlined in yellow).

Figure 3.

Keratometry (A, B) and elevation (C, D) values in both the horizontal (A, C) and vertical (B, D) meridians as measured by the Pentacam tomographer. Each individual line represents a subject. Black dashed line and values represent the average for each point. The x-axis shows the corneal location by the notation S (superior), I (inferior), N (nasal), or T (temporal). It also indicates the chord length in mm (2, 4, 6, 8).

Figure 4.

The relationship between central and corneal astigmatism and corneal elevation difference at the 4, 6, and 8 mm chords.

Figure 5.

Treatment zone decentration from tomography difference maps. The center (0,0) represents the pupil center as designated by the Pentacam software. The black dots represent the treatment zone centers.