Variation of orthokeratology lens treatment zone (VOLTZ) study

Abstract

The increasing prevalence of myopia worldwide is of public concern and requires interventions to prevent progression and long-term serious consequences. Orthokeratology (ortho-k) is one of the most frequently employed optical method to slow myopia progression (in terms of axial elongation (AE)). Although shown to be effective in retarding AE in school-aged children, the exact optical mechanism of ortho-k remains uncertain. Induced myopic defocus, increased higher order aberrations (HOAs), and increased choroidal thickness (ChT) are thought to play a role in regulating AE in children wearing ortho-k lenses. Recently, novel ortho-k lens designs have been investigated by researchers to improve the level of myopia control using ortho-k. One such design uses a smaller back optic zone diameter (BOZD), which aims to alter the corneal refractive changes and HOAs profiles by creating a smaller treatment zone (TZ). The refractive and optical changes associated with a smaller TZ are hypothesized to further reduce AE in children, compared to standard-design lenses. Before the commencement of the current study, there were no published longitudinal studies reporting the clinical outcomes of using a smaller BOZD ortho-k lens and its effect on AE in myopic children. Therefore, it was necessary to fill this research gap, by conducting a randomized clinical trial to investigate the myopia control effect (AE) of an ortho-k lens with a smaller BOZD and the associations of TZ parameters and ocular aberrations with AE. Objectives 1. To investigate the association of TZ parameters with AE in fast and slow progressing ortho-k subjects from previous myopia control studies (retrospective study) 2. To determine and compare the clinical performance and AE of 5 mm BOZD and 6 mm BOZD ortho-k lenses 3. To determine and compare refractive, biometric, and ocular aberration changes of 5 mm BOZD and 6 mm BOZD ortho-k lenses over two years Methods Eighty-two Chinese children aged 6 to < 11 years old, with myopia between -4.00 and -0.75 D, low to moderate astigmatism (≥ -2.50 D for axes 180 ± 30º and ≥ -0.50 D for other axes), less than 1.00 D anisometropia, unremarkable ocular and general health conditions, and no myopia control history were recruited and randomly assigned to wear 6 mm (6-MM group) or 5 mm (5-MM group) BOZD ortho-k lenses at a 1:1 ratio. Seventy subjects commenced lens wear (6-MM group: n = 34; 5-MM: n = 36) and were masked for the lenses used during the study period. Data was collected prior to commencing lens wear and every six months after, during the two-year study period. The primary outcome, axial length, was measured by a masked examiner. Other secondary measurements, including subjective refraction, corneal topography and pupil size, ChT, and ocular aberrations were measured by the same unmasked examiner. TZ, determined manually and using a customized software, was defined as central flattened zone enclosed by points with zero refractive changes, before and after ortho-k on tangential topographical maps. Changes in AE, refraction, TZ parameters, anterior segments, ChT, and ocular aberrations were compared between groups and over time. Results A total of 23 and 22 subjects from 6-MM and 5-MM groups, respectively, completed the two-year study. No significant differences were found in the first-fit success rate (at 1-month visit) (6-MM group: 100%; 5-MM group: 94%). Lens binding, clinically insignificant corneal staining (≤ Grade 2), sterile corneal infiltrates (6-MM group: 6; 5-MM group: 8), and microcysts (6-MM group: 12; 5-MM group: 9) were observed during the study period, with similar incidence in the two groups. Most subjects with sterile infiltrates or microcysts continued lens wear, except for a few with recurrent episodes or non-compliance with instructions. Among 31 TZ parameters, including TZ decentration, peripheral steepened zone depth, and central dioptric power change, the horizontal TZ size was found to be most likely to be correlated with AE (β = 0.130, p = 0.001) (using pooled data), because it gave the best fit of the model used in multivariate analysis. Significantly less AE and smaller horizontal TZ size were observed in the 5-MM group at all visits over two years (p ≤ 0.005). At the 24-month visit, AE and the horizontal TZ size were 0.35 ± 0.23 mm and 3.84 ± 0.39 mm in the 6-MM subjects, and 0.15 ± 0.21 mm and 2.69 ± 0.28 mm in the 5-MM subjects. Repeated measurements showed that the horizontal TZ size did not change over time after the first six months of lens wear, indicating a stabilized TZ thereafter. ChT was found to be significantly increased at the 6-month visit, using pooled data, but it was not significantly different between groups (p > 0.05). A significantly greater increase in the total root-mean-square (RMS) of HOAs, the RMS of spherical aberrations (SA) (combination of C40 and C60), and primary (C40) SA were found in the 5-MM group at all visits (p < 0.001). When using pooled data, multivariate analysis revealed a negative association between the horizontal TZ size and change in RMS HOAs (β = -0.19, p < 0.001), after adjusting for baseline data. The horizontal TZ size was also associated with the change in RMS SA (β = -0.16, p < 0.001), RMS coma (β = -0.09, p < 0.001), and primary (C40) (β = -0.17, p < 0.001) and secondary (C60) SA (β = 0.02, p < 0.001). Changes in RMS HOAs, RMS SA, and primary SA were significantly and negatively associated with AL over time (p ≤ 0.004). Conclusions Compared to standard 6 mm BOZD lenses, similar first-fit success rate and clinical performance were found with 5 mm BOZD lenses. The 5-MM group showed a smaller TZ, resulting in greater increases in RMS HOAs, RMS SA, and primary SA, which were associated with slower AE. The findings suggested that, compared to 6 mm BOZD lenses, the 5 mm BOZD lenses further retarded AE by 0.13 mm. ChT (although weak) and HOAs may play an important role in regulating myopia progression.