July 1, 2021
By Dwight Akerman, OD, MBA, FAAO, FBCLA
During orthokeratology treatment, the degree of myopia is reduced by flattening the central cornea. This central flattened zone is referred to as the treatment zone (TZ). It has been hypothesized that a reduced treatment zone following OrthoK creates increased peripheral refractive power and higher spherical aberration, which may further retard myopia progression in children.
The Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study is a two-year myopia control study conducted at The Hong Kong Polytechnic University. The study aims to compare myopia retardation, in terms of axial elongation, in two groups of young children wearing OrthoK lenses of the same design but different back optic zone diameters (conventional BOZD of 6 mm vs. reduced BOZD of 5 mm). The primary outcome is axial elongation over two years. This paper presents the one-year results of the VOLTZ Study.
The researchers concluded that the two OrthoK lenses’ clinical performance was similar, indicating that a smaller BOZD (5 mm) did not affect lens performance or ocular integrity. However, a smaller BOZD led to a reduced TZ, with retardation of axial elongation by 0.13 mm compared to conventional 6 mm BOZD OrthoK lenses after one year of lens wear. Since the majority of axial length changes occurred within the first six months of lens wear, a longer study period is warranted to investigate the treatment effect over time
One-year Results of the Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study: A Prospective Randomised Clinical Trial
Biyue Guo, Sin Wan Cheung, Randy Kojima, Pauline Cho
Purpose: To present the 1-year results of the Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study, which aims to investigate the myopia control effect of orthokeratology (ortho-k) lenses with different back optic zone diameters (BOZD).
Method: Children aged 6 to <11 years, having myopia -4.00 D to -0.75 D, were randomly assigned to wear ortho-k lenses with 6 mm (6-MM group) or 5 mm (5-MM group) BOZD. Data collection included changes in refraction, vision, lens performance and binding, ocular health conditions, axial length, and characteristics of the treatment zone (TZ) area.
Results: The 1-year results of 34 and 36 subjects (right eye only) in the 6-MM and 5-MM groups, respectively, are presented. No significant differences in baseline demographics were found between the groups (p > 0.05). The first-fit success rates, based on satisfactory centration at the 1-month visit, were 100% and 94%, respectively. Horizontal TZ size was 0.92 mm and 0.72 mm smaller in the 5-MM group at the 6-month and 12-month visits, respectively (p < 0.05). At the 12-month visit, no significant between-group differences were found in the incidence of corneal staining (low grade only), lens binding, and visual performance (all p > 0.05). Axial elongation was slower in the 5-MM group (0.04 ± 0.15 mm) than the 6-MM group (0.17 ± 0.13 mm) (p = 0.001). A significant positive correlation was observed between the horizontal TZ size and axial elongation (r = 0.36, p = 0.006).
Conclusion: Clinical performance of the two ortho-k lenses was similar, indicating that a smaller BOZD (5 mm) did not affect lens performance or ocular integrity. However, a smaller BOZD led to a reduced TZ, with retardation of axial elongation by 0.13 mm compared to conventional 6 mm BOZD ortho-k lenses after one year of lens wear.
Guo, B., Cheung, S. W., Kojima, R., & Cho, P. (2021). One‐year results of the Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study: a prospective randomised clinical trial. Ophthalmic and Physiological Optics.