Orthokeratology has been shown to be the most effective non-pharmacological method for controlling the progression of myopia. OKs make corneal epithelial cells to migrate and redistribute and flatten the central cornea while steeping the midperipheral cornea to reduce relative peripheral hyperopia, which may slow the elongation of the axial length[1, 2]. At present, the axial length is considered to be an effective parameter for monitoring the progression of myopia[3, 4], and the effect of the orthokeratology in controlling the axial elongation is certain[5–7], but there are many factors that affect the development of the axial length, such as age, myopia degree, corneal thickness and corneal curvature. The study has analyzed multiple factors and multiple grades and found that the axial elongation is highly correlated with the initial age and myopia degree of OK wearers.
Wong et al researched the eye growth fitting curve and found that the annual growth of the axial length decreased with age, especially after 10 years old[8]; Fledelius et al found that the average elongation of the axial length was less than 0.1 mm/year between the age of 7 to 8 and less than 0.01 mm/year between the age of 10 to 12, during their researches, axial length was about 16 mm at birth, grew to 23 mm at 3 years old, and 24 mm at adulthood[9]. The above studies have proved that the growth rate of the axial length slows down with the age. This study found that the axial elongation rate of the older children (≥ 13 years old) is slower than that of the younger children (8–10 years old). Studies at home and abroad support that the initial age of OK wearers is negatively correlated with the axial elongation[10–15] and consider that the effect of age may be due to the slower eyeball growth of older children. Cho et al proved that OKs can significantly reduce the risk of rapid progression of myopia and younger wearer can benefit more in the long term, although younger children have a higher axial growth rate[16].
The results of current researches on the relationship between initial myopia degree and axial elongation are contradictory. A small number of studies show that there is no correlation between initial myopia degree and axial elongation[17, 18]. Kakita et al obtained a negative correlation between initial myopia degree and axial elongation, however, they compared the distribution of axial elongation in different grade of myopia degree and concluded that the axial elongation wound be slower only in the higher myopia degree[19]. A large number of studies still believe that the the axial elongation is negatively correlated with the initial myopia degree[12, 20, 21]. Wang et al considered that this was related to the possible existence of the maximum threshold of myopia degree and axial length, there is a lot of room for axial elongation in lower myopia degree, and the axial length grow fast, but once it approaches the threshold, the axial elongation naturally slows down[12]. Most studies believe that this may be related to the relative peripheral hyperopia defocus which can cause axial elongation and myopia progression[22, 23], the hypothesis is that, the higher the myopia degree, the steeper the central and peripheral cornea will be, and the less harmful effects of relative peripheral hyperopia will be on the retina which can reduce the stimulation to axial length and myopia progression[24–26]. This article compares the mean value of the axial length between baseline and the follow-up time of 1 year among the low, medium, and high myopia groups and supports that the benefit in OK wearers of the moderate and high myopia is better than that of the low myopia.
This study proves that the initial corneal thickness has no correlation with the axial elongation, which is consistent with the conclusions of the current literature[12, 27]. Lee et al concluded that the initial corneal thickness was negatively correlated with the axial elongation, but they proposed that this correlation is highly dependent on the age, and believed that the younger wearers had thinner and more flexible corneal thickness which is less likely to be affected by the cornea shape changes caused by the orthokeratology lens[11].
At present, there are few studies on the relationship between initial corneal curvature and axial elongation. Some studies believe that the initial corneal curvature has nothing to do with the axial elongation[15]. Xie Jing et al believe that the initial corneal curvature is negatively correlated with the axial elongation[28], but there is no similar report in China. In this study, the initial corneal curvature is not related to the axial elongation.
In summary, the orthokeratology lens can effectively control the elongation of the axial length and is an effective method to control the progression of myopia. The initial age and myopia degree significantly affect the axial elongation. Although the older initial age and the higher myopia degree OK wearers achieve the better effect in controlling the progression of myopia, but in the long term, the earlier the treatment for myopia, the higher benefit the children will obtain.