Orthokeratology (ortho-k) lenses with smaller back optic zone diameters (BOZD) may result in slower axial length (AL) elongation compared with ortho-k lenses with larger BOZD, according to a study published in Contact Lens and Anterior Eye.
Investigators included 90 children in the prospective analysis and randomized them to treatment with ortho-k lenses with a 5.0 mm BOZD (mean age, 10.65 years; 28 girls; n=46) or 6.2 mm BOZD (mean age, 10.40 years; 22 girls; n=44). Participants wore the lenses for 8 hours or more per night at least 6 days per week. Patients underwent ophthalmic examination, biomicroscopy and corneal topography measurements at baseline and at 1-day, 1-week, 1-month, 6-month and 1-year follow-up visits. Relative corneal refractive power (RCRP), AL and corneal higher-order aberrations (HOA) were compared between the 2 groups.
Participants fit with 5.0 mm BOZD lenses experienced 0.04 mm AL elongation at 6 months and 0.13 mm at 1 year, which was significantly less than the AL growth noted in children fit with 6.2 mm BOZD at 6 months (0.14 mm; P <.01) and 1 year (0.28; P <.01).
Orthokeratology lenses with a BOZD of 5.0 mm were associated with a larger relative corneal refractive power (RCRP; mean, 15.71 D*mm2) distribution compared with 6.2 mm BOZD lenses (mean, 10.68 D*mm2; P <.01).
The change from baseline to year 1 differed significantly for the root mean square of total higher-order aberrations (mean difference [MD], 0.0529 vs -0.1622; P <.001), coma root mean square (MD, 0.0109 vs -0.1871; P <.001), and primary horizontal coma (MD, 0.0212 vs -0.1757; P =.0052) for children in the 5.0 mm BOZD and 6.2 BOZD cohorts, respectively.
Multiple regression analysis revealed that change in AL was associated with age (t, -3.87; P <.001), spherical equivalent refraction (SER) at baseline (r, 2.88; P <.01) and treatment zone size (t, 2.27; P =.03).
“We showed that a smaller BOZD design of ortho-k lenses improved efficacy in slowing the progression of myopia, mainly by inducing a faster corneal annulus steepening within the pupillary area and subsequently changing the distribution of myopic defocus,” according to the investigators. “More complex lens design changes, such as decreasing BOZD or changing back optic zone asphericity, which lead to an RCRP profile within the pupillary area rising to its three-quarter-peak level at a smaller distance from the apex, may be considered to enhance myopia control efficacy in younger children or fast progressors.”
Study limitations include failure to measure corneal HOA with a wavefront aberrometer and short duration of follow-up.
Li N, Lin W, Zhang K, et al. The effect of back optic zone diameter on relative corneal refractive power distribution and corneal higher-order aberrations in orthokeratology. Cont Lens Anterior Eye. Published online September 7, 2022. doi:10.1016/j.clae.2022.101755