A study recently published in Eye and Vision evaluated the impact of corneal refractive therapy on axial length (AL) elongation using orthokeratology (ortho-K) lenses with different back optical zone diameters (BOZDs).
Give me some background first.
As the global prevalence of myopia continues to be of concern, identifying effective management approaches is of keen interest to researchers and clinicians.
Although the exact mechanism of myopia control for ortho-K remains unclear, it has been widely accepted as a compelling intervention to reduce myopia progression and axial elongation.
Further, ortho-k’s mechanism for slowing myopia progression has been hypothesized to involve changes in relative peripheral defocus and higher-order aberrations (HOAs).
Go on…
A recent short-term study suggested that ortho-k lenses designed with a smaller BOZD could cause greater peripheral defocus, increased HOAs, and a smaller treatment zone (TZ) size.
Further, two long-term follow-up studies demonstrated that smaller BOZD could effectively slow axial elongation.
Which leads us to …?
These findings motivated the research team to identify the variable (i.e., HOA, TZ size, or peripheral defocus) that best predicts myopia progression and to find the ideal cut-off value of that parameter to improve myopia control for ortho-K wearers.
Now, talk about the study.
In this prospective cohort study, 301 participants (aged 7-14 years) were required to wear ortho-K lenses a minimum of 8 consecutive hours per night for at least 6 days per week.
The cohort was randomly assigned into two groups and fitted with either 5-mm or 6-mm BOZD ortho-K lenses of the same type in both eyes.
Tell me more.
Patients underwent examinations over the course of 1 year, and those who attended all follow-up appointments were included in the analysis.
Investigators obtained ocular biometric parameters at baseline, 1-, 3-, 6-, 9-, and 12-month follow-up visits.
The ocular biometric parameters included:
- AL
- Corneal topography
- HOAs
- Zernike defocus coefficient
Anything else?
Researchers also compared differences in HOAs, TZ size, and Zernike defocus coefficient with different BOZDs to identify the optimal parameter for predicting AL.
How did they analyze the data?
Researchers utilized multivariate regression analyses to understand the association between AL change and ocular biometric parameters.
Also, receiver operating characteristic (ROC) curve analysis was used to determine the best diagnostic value for AL change in ocular biometric parameters.
Findings?
The mean AL change in the 5-mm group (0.13±0.18 mm) was less compared to the 6-mm group (0.27±0.15 mm) at the 12-month visit.
Additionally, the TZ size and decentration were smaller, while the Zernike defocus coefficient and HOAs were higher in the 5-mm group (all P<0.05).
What about other parameters?
The research team also reported that older age and smaller TZ size were protective factors against AL elongation in multiple regression analyses.
Based on the ROC curve analysis, when predicting AL elongation, the TZ diameter was the optimal variable to consider.
A TZ diameter at the 1-month visit of less than 3.82 mm was the cut-off value to predict changes in AL of less than 0.2 mm per year.
Expert opinion?
According to the study authors, “We also calculated the proportion of AL elongation, and the results showed that 38.3% of patients in the 6-mm group and 59.3% of patients in the 5-mm group had axial growth of 0.2 mm or less in one year, indicating that a higher proportion of patients with 5-mm BOZD ortho-K could achieve superior control effects.”
Tie it all together for me.
These findings demonstrate the efficacy of a small BOZD, as the 5-mm group showed 0.14 mm (51.8%) less axial elongation than the 6-mm group.
Further, decreasing the BOZD (i.e., the 5-mm BOZD group) resulted in a smaller TZ size, higher Zernike defocus coefficient—meaning steeper peripheral defocus, and increased HOA, suggesting more effective myopia control.
Finally, TZ size was the best predictor of AL elongation, and having a TZ diameter of less than 3.82 mm was associated with an AL elongation of less than 0.2 mm per year.