Manual and software‐based measurements of treatment zone parameters and characteristics in children with slow and fast axial elongation in orthokeratology

Abstract

To compare the treatment zone (TZ) measurements obtained using manual and software-based methods in orthokeratology (ortho-k) subjects and explore the TZ characteristics of children with slow and fast axial elongation after ortho-k. Methods: Data from 69 subjects (aged 7 to <13 years old), who participated in three 24-month longitudinal orthokeratology studies, showing fast (>0.27 mm, n = 38) and slow (<0.09 mm, n = 31) axial elongation, were retrieved. The TZ after ortho-k was defined as the central flattened area enclosed by points with no refractive power change. TZ parameters, including decentration, size, width of the peripheral steepened zone (PSZ), central and peripheral refractive power changes and peripheral rate of power change, were determined manually and using pythonbased software. TZ parameters were compared between measurement methods and between groups.Results: Almost all TZ parameters measured manually and with the aid of software were significantly different (p < 0.05). Differences in decentration, size and the PSZ width were not clinically significant, but differences (0.45 to 0.92 D) in refractive power change in the PSZ were significant, although intraclass coefficients (0.95 to 0.98) indicated excellent agreement between methods. Significantly greater TZ decentration, smaller TZ size and greater inferior rate of power change (relative to the TZ centre) were observed in slow progressors using both methods, suggesting a potential role of TZ in regulating myopia progression in ortho-k. Conclusion:TZ measurements using manual and software-based methods differed significantly and cannot be used interchangeably. The combination of TZ decentration, TZ size and peripheral rate of power change may affect myopia control effect in ortho-k.

Investigation of the relationship between contact lens design parameters and refractive changes in Ortho-K

“…Users cannot take account of any such smoothing when analyzing corneal measurements, as they have no access to proprietary algorithms. This phenomenon explains the differences in corneal and refractive changes reported by researchers when investigating Ortho-K effects using different devices ( Guo et al., 2022 ). Dedicated topography software packages smooth the output data in different ways, depending on the instrument, to improve the usability of the instrument but without explaining the methodology to the end user.…”

Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study: A 2‐year randomised clinical trial

“…Corneal topography (Medmont E300, http://medmont.com) was assessed before cycloplegia. Four topographical maps with similar contour and differences within 0.25 D in central K‐readings were obtained 8 . Subtractive tangential maps (before and after ortho‐k treatment) were selected to evaluate TZ decentration and other TZ parameters, because it provides the most accurate measurement of localised changes after ortho‐k 8 .…”

“…Four topographical maps with similar contour and differences within 0.25 D in central K‐readings were obtained 8 . Subtractive tangential maps (before and after ortho‐k treatment) were selected to evaluate TZ decentration and other TZ parameters, because it provides the most accurate measurement of localised changes after ortho‐k 8 . Pupil size was estimated from the topography measurement under dark illumination (18.8 lux), but was recorded as the photopic size because of the intrinsic lighting of the Medmont instrument 6 …”

“…The best topographical maps were selected at each data collection visit, according to the criteria previously presented 8 . Tangential curvature data were exported from the Medmont device and imported into customised software for analysis (Figure 1).…”

Effect of treatment zone decentration on axial length growth after orthokeratology

“…All measurements were conducted by the same technician. The decentration distance and direction were calculated according to the method of Guo et al (2022) : (1) decentration distance: the TZ was defined as the area enclosed by points with 0 diopter changes on the tangential subtractive topography maps, and the tolerance of each zero point at the edge of the TZ was ± 0.10 D. Briefly, the refractive power change was obtained by hovering the cursor above each reference point and the measurement shown on the image was recorded. As shown in Figure 1A , the area surrounded by the red circle is the TZ, line segments AB and CD were the horizontal and vertical diameters of the TZ, respectively.…”