Axial length elongation after OK lens wear
Cho et al.  reported in 2005 that AL elongation of myopia could be controlled by orthokeratology. At the end of the two-year study, axial elongation of OK lens group reduced by nearly 50% compared with that of single-vision glasses group. The similar results have been reported by numerous studies [7,8,9]. The treatment outcome was also remarkable even in high myopia children with partial reduction OK lens . In former researches, the changes of AL after OK lens wear were 0.16 ± 0.17 mm  and 0.22 ± 0.15 mm  for one year, 0.29 ± 0.27 mm , 0.31 ± 0.27 mm , 0.36 ± 0.24 mm , 0.37 ± 0.27 mm  and 0.39 ± 0.27 mm  for two years, and 0.99 ± 0.47 mm  for five years. In this study, axial elongation was 0.18 ± 0.17 mm one year after OK lens wear, which was consistent with the result of previous studies. In contrast, the treatment effects were more remarkable in some researches with AL changes of one year were 0.07 ± 0.21 mm  and 0.08 ± 0.15 mm .
These two researches were contralateral comparison studies in which myopic anisometropes were recruited. The monocular myopia or the more myopic eyes were fitted with OK lenses only. The changes in AL of the contralateral eyes after one year were 0.36 ± 0.23  mm and 0.39 ± 0.32 mm  respectively. This means that the reduction rates of AL elongation by OK lens reached to 79 and 81%, while in other studies, the reduction rates were 30% ~ 63% [6,7,8, 10,11,12,13]. The significant therapeutic effect may be attributed to not only the change of peripheral retinal defocusing pattern of the treated eye but also the change of relative relationship between the peripheral retinal defocusing patterns of the two eyes. Further exploration is needed to find and confirm the reasons.
Factors relevant to AL elongation after OK lens wear
Axial length elongation after OK lens wear was reported to be significantly correlated with baseline age [11,12,13, 17,18,19,20], initial refractive error [19, 21], CCT and post treatment relative peripheral refractive power , summed corneal power change from the central to the mid-peripheral cornea , the magnitude of corneal relative peripheral power change , and pupil diameter . There were also contradictory findings. Some reported that there was no significant correlation between the change of AL and initial age of OK lens wear [21, 22], nor the baseline refractive error [11,12,13, 18, 22]. In this study, we found that there were significant correlations between axial elongation and initial age of OK lens wear and baseline myopic spherical equivalent. Sampling error was the possible reason for different results. Nevertheless, some characteristics of OK lens treatment could be drawn from the results of these researches.
Children whose parents had higher myopia had greater AL increase in our research. It indicated the influence of genetic factors on the progression of myopia. Santodomingo-Rubido et al.  found that parental refraction had no significant correlations with children’s AL change. Nevertheless, lower levels of parental myopia were associated with smaller increases in children’s AL. In our study, we divided parental myopia into grades according to the degree of parental myopia. In the study of Santodomingo-Rubido et al. , the analysis was based on the raw data. Although the results of significance test were different, the trends of correlation were similar.
The law of myopia development after OK lens wear is in consistent with that of natural progression of myopia in children. Myopia development and axial elongation were faster in younger children [24,25,26] and those who had myopic parents [25, 27]. However, the children with higher base myopia had faster myopia progression , which was in contrast with that after OK lenses wear [19, 21]. The causation may be that to treat higher myopia the central corneal power should be reduced more, thus mid-peripheral corneal power will increase more and may induce more peripheral retina myopia defocus. Moreover, is it possible to get steeper mid-peripheral corneal power through lens design, so as to obtain more significant therapeutic effect? A short-term study of different compression factors found that no significant changes in eye parameters were found between the different design lenses . However, OK lens of smaller back optic zone diameter achieved better curative effect [29, 30]. Further investigations will be need to make OK lens more effective in myopia control.
Since the younger children with myopic parents are prone to have axial elongation after OK lens treatment, more aggressive or combined treatment should be considered. For example, combined orthokeratology and 0.01% atropine solution could achieve better therapeutic effect [31, 32], especially in children with low initial myopia .
Possible causes of negative axial length elongation
Axial length reduction after OK lens wear is an interesting phenomenon. It is common but rarely noticed. In some studies, overall AL increased but some cases decreased [9,10,11,12,13,14,15, 17,18,19, 21, 22, 33]. Moreover, in some studies, even the overall means decreased [16, 34]. Again, a few studies have investigated on the AL shortening issue. We made some comparative studies on the cases of positive and negative axial growth. We found that compared with patients with increased AL, patients with decreased AL had significantly older age, higher baseline myopia and higher baseline corneal power. That was in correspondence with the law who were less axial elongation after OK lens wear. In the studies that found AL decreased, the subjects were older children (10.8 ~ 17.0 years)  or young adults (25.62 ± 3.57 years)  whose progression of AL were slow or stopped. The AL shortening effect of the OK lens would not be concealed by the natural growth of the eyeball. Moreover, it was more obvious in young adults whose eyeball growth stopped .
The possible reasons of negative growth of AL after OK lens wear have not been addressed in most previous studies. They were only discussed in some researches and often thought to be an illusion of thinning of central cornea and thickening of choroid [9, 14,15,16, 21]. CCT thinned about 10 μm [35,36,37,38] and choroid thickened about 20 μm after OK lens wear [38,39,40]. We found that CCT became thinner by 5.95 ± 14.23 μm, and for the cases of AL decreased, CCT thinned 9.44 ± 13.28 μm. But AL decreased 0.06 ± 0.04 mm. Although choroidal thickness was not measured at the same time, the amount of AL reduction did not seem to match the amount of choroidal thickening and CCT thinning according to previous studies. González-Mesa et al.  found that AL decreased by 75 μm and 160 μm after OK lens wear for 15 days and 12 months in young adults. CCT value was subtracted from AL value to correct for the influence of CCT on AL change. If the decrease of AL was only due to the thickening of choroid, such large amount of choroidal thickening has never been reported. There were cases of AL decrease even after one month of OK lens cessation, when ocular biometric parameters all regressed to the baseline . In addition, there seems to be evidence to support this in a study that axial shortening could not be fully explained by central corneal thinning and choroidal thickening .
Further studies are needed to confirm whether there are other causes of axial shortening other than corneal thinning and choroidal thickening. Some studies reported that ACD decreased significantly after OK lens wear [20, 28, 34]. We found that ACD decreased 0.034 ± 0.075 mm (P < 0.001). ACD increased with age in children who were not treated by OK lens . In addition, some researchers found that posterior cornea flattened significantly [34, 42]. We hypothesize that the changes of anterior segment structure after OK lens wear may indicate the change of the eyeball shape, e.g., it may change from prolate to oblate. This causes the sagittal axis of the eyeball, which is commonly referred to as the ocular axis, to become shorter. However, this change may be masked by the fast growth of eyeball. Therefore, it appears more in children with slow axial elongation and adults whose eyeball growth has been stopped. As far as we know, this is the first paper that targeted research was done to understand the characteristics of cases with negative axial growth after OK lens wear. It mentions the possible role of OK lens in regulating the shape of eyeball. Further investigations are required to prove the hypothesis and elucidate the underlying mechanism.
Limitation of this study
Lack of sufficient and direct evidence to support the viewpoint is the most notably deficiency of this paper. As a retrospective study, we could only analyze the existing previous data. Prospective design researches that include comprehensive data and evidence of ocular morphological changes are needed to confirm the hypothesis. The second deficiency of this paper is that there was no control group. Although it was only a correlation study, if we could do some correlation analysis in the control group, we could compare the development trend of myopia under different treatment measures. The third deficiency of this paper is that the sample size was a little small. Because several different brands of OK lenses were used at the refractive clinic at the same time, only the patients wore one brand of OK lens were selected for research.