A larger treatment zone (TZ) decentration is associated with a larger summed relative corneal refractive power (RCRP) in the central cornea, according to a study published in BMC Ophthalmology. This association may explain why TZ decentration is beneficial in slowing myopia progression in children undergoing orthokeratology treatment.
Researchers collected data from 400 children with myopia (mean age 12 years, 217 boys) wearing orthokeratology lenses (n=200) or single-vision spectacles (n=200). The inclusion criteria included a spherical equivalent (SE) of -1.00D to -6.00 D, with-the-rule cylinder less than 1.5 D, and a monocular acuity better than 20/20. Patients with a history of strabismus, ocular surface disease, ocular surgery, or contact lens wear were excluded from the analysis.
The investigators fit participants in the orthokeratology group with a spherical four-zone orthokeratology lens with a total lens diameter ranging from 10.2 mm to 11 mm. They instructed the children to wear their lenses for more than 8 hours per night and conducted follow-ups at 1-day, 1-week, 2-week, and 1-month intervals following the baseline visit. The team continued to monitor patients at least once per 3 months thereafter. They measured axial length and corneal topography, and used axial corneal maps to determine RCRP.
The investigators found that children fit with orthokeratology lenses experienced significantly less axial length growth (ALG) over 1 year compared with spectacle wearers (0.1 ± 0.15 mm vs 0.32 ± 0.17 mm, P <.001). ALG was negatively associated with summed RCRP within a 2 mm central corneal radius and with the magnitude of TZ centration (P <.01), but not direction (P =.905).
Although clinicians have traditionally been instructed to fit orthokeratology lenses on the corneal geometric center, “a larger TZ decentration was associated with a larger summed RCRP in the central cornea. It may be one of the possible reasons why TZ decentration is beneficial to retarding myopia progression,” according to the report.
Study limitations included a single center design and no direct measurements on retinal defocus, accommodative response, and pupil size.
Lin W, Gu T, Bi H, Du B, Zhang B, Wei R. The treatment zone decentration and corneal refractive profile changes in children undergoing orthokeratology treatment. BMC Ophthalmol. Published online April 18, 2022. doi:10.1186/s12886-022-02396-w