Minfeng Chen Sijun Zhao ^* Lulu Peng ^* Yu Rong ^* Chengwei Zhu ^* Fan Lu ^* Xinjie Mao ^*

• Affiliated Eye Hospital to Wenzhou Medical University, Wenzhou, China

The study analysed the changes in corneal surface shape after replacing orthokeratology lenses carrying a small base curve (BC) diameter. Methods: In this retrospective study, about 54 right eyes belonging to 54 myopic children who insisted on wearing an orthokeratology (ortho-k BC 6.0 mm) lens for more than 12 months, then replaced the second ortho-k (BC 6.0 mm or BC 5.0 mm) lens about 12 months. The children were categorized into two groups based on the design of the BC of the replaced ortho-k lens 6.0/5.0 and 6.0/6.0.The ratio of axial length (AL) elongation in the 6.0/5.0 group was significantly less than in the 6.0/6.0 group (-0.015 ± 0.014mm/M vs. -0.005 ± 0.012 mm/M, t = -2.672, P = 0.010). After replacing the BC 5.0 ortho-k lens the optical zone (8.19±2.60 mm 2 vs 9.64±1.57 mm 2 , t=-2.345, p=0.023), reverse zone (31.64±5.80 mm 2 vs 34.86±4.61 mm 2 , t=-2.169, p=0.035) and treatment zone (17.16±3.94 mm 2 vs 22.96±2.59 mm 2 , t=-6.049, p<0.001) were all smaller than these in 6.0/6.0 group. In 6.0/5.0 group the optical zone (wearing more than one months as first: 11.16±2.12 mm 2 , the last inspection before replacing lens as before: 10.87±1.90 mm 2 ), reverse curve zone (first: 22.03±3.11 mm 2 , before: 26.24±5.06 mm 2 ) and treatment zone (first: 35.97±5.54 mm 2 , before: 37.11±6.04 mm 2 ) were all greater than these after replacing ortho-k lens, all P<0.001. Conclusions: Replacing an ortho-k lens with a smaller BC resulted in a larger decrease in the mean central corneal zone and a substantial increase in the paracentral corneal zone. Additionally, the ortho-k lens with smaller BC does not increase the decentration distance, and contributes to effective myopia control.