Owing to the efficacious control of myopia progression in adolescents, there is a gradual incremental application of orthokeratology, which has been chosen by more than 1.5 million adolescents in China [17]. In recent years, many scholars have noticed the axial shortening of partial subjects at the initial stage of orthokeratology, which may explain why many studies have shown that the axial growth rate in the initial stage is much slower than that in the subsequent stage [18–21]. Some studies have indicated that the average axial length of the eye has a negative growth in the initial stage [8–15]. Therefore, it is necessary to clarify the pattern of axial shortening in the entire period of orthokeratology because neglecting the initial axial shortening and axial rebound after discontinued wear may cause overestimation of the control effect [14].
According to our study, the AL of the subjects with axial shortening started to grow after one month of wearing OK lenses and then did not return to baseline until the 7th month and began to exceed baseline at nearly the 13th month. With regard to the degree of axial shortening, the data of other studies can be seen in Table 2[8–13, 15]. Overall, the amount of axial shortening after one month of wearing in the ALS group of our study (0.08±0.04 mm) was between the results of these studies. As for the quite different results of those studies, on the one hand, is the inconsistent time of exam, and on the other hand is the nonnegligible individual differences, to be more specific, whether the axial length is shortened or not and the degree of shortening depends on the comparison between the causes of shortening and the growth of the axial length.
Table 2
Degree of axial shortening according to some studies.
Study
|
Axial shortening(mm)
|
Time
|
Number of subjects
|
Country or area
|
Age(y)
|
Inclusion criteria
|
Instrument
|
Gardner. et al. [8]
|
0.04
|
1 M
|
9
|
USA
|
11-15
|
-1.00~-4.00D
|
Lenstar
|
Ana González-Mesa. et al. [9]
|
0.157
|
1 M
|
34
|
Spain
|
18-30
|
-0.50~-4.50D
|
IOL-Master
|
António Queirós. et al. [10]
|
approximately 0.02
|
1W
|
62
|
Asian
|
5-19
|
-1.00~-8.00D
|
IOL-Master
|
Lau, Jason K. et al. [11]
|
approximately 0.25
|
1W
|
25
|
Hong Kong, China
|
6-10
|
-0.50~-4.00D
|
Lenstar
|
Lau, Jason K. et al. [12]
|
0.26± 0.41
|
1W
|
58
|
Hong Kong, China
|
6-10
|
-0.50~-4.00D
|
Lenstar
|
Michael J Lipson. et al. [13]
|
0.01±0.53
|
1Y
|
97
|
USA
|
7-14
|
-1.00~-6.50D
|
A- scan
|
Helen A. Swarbrick. et al. [15]
|
0.04±0.08
|
3 M
|
26
|
Australia
|
10-17
|
-1.00~-5.50D
|
IOL-Master
|
Regarding the reasons for the shortening of the AL, the main views by scholars are central corneal thinning combined with choroidal thickening [11,14−16]. Some data about central corneal thinning and choroidal thickening are listed in Table 3[12,14,22−25]. Central corneal thinning was epithelial in origin, whereas mid-peripheral thickening was primarily stromal. The nature of the epithelial cellular changes underlying central epithelial thinning induced by orthokeratology remains obscure, although some possibilities have been revealed in the literature [23, 25]. Axial length collected by A-scan or partial coherence interferometry (e.g., IOL-Master) is likely to be influenced by choroidal thickness because A-scan ultrasonography is an acoustic method in which axial length is defined as the distance between corneal anterior surface and vitreous-retina reflection peak and devices based on partial coherence interferometry, e.g., the IOL-Master, defines the axial length as the distance between anterior cornea and retinal pigment epithelium (RPE) [14]. Whether the OK lens will cause a tiny transshape of the eyeball needs further study, so strictly speaking, the change in AL mentioned in this study is actually based on the value measured by IOL-Master.
Table 3
Data about central corneal thinning and choroidal thickening according to some studies
Study
|
central corneal thinning (mm)
|
subfoveal choroid thickening (mm)
|
Time
|
Number of subjects
|
Country or area
|
Age(y)
|
Inclusion criteria
|
Lau, Jason K. et al. [12]
|
0.009±0.004
|
0.009±0.001
|
1W
|
58
|
Hong Kong, China
|
6-10
|
-0.50~-4.00D
|
Zhouyue Li. et al. [22]
|
-0.01±0.01
|
approximately 0.16
|
1 M
|
29
|
China
|
8-15
|
-1.00~-4.00D
|
Alharbi and Swarbrick. [23]
|
0.016 ± 0.003
|
NA
|
1 M
|
18
|
Australia
|
22-29
|
-1.25~-4.00D
|
Wan-Qing Jin. et al. [24]
|
NA
|
0.006±0.007
|
3 M
|
30
|
China
|
9-14
|
-1.00~-6.00D
|
Wook Kyum Kim. et al. [25]
|
0.006±0.005
|
NA
|
2 M
|
36
|
China
|
7-25
|
-0.50~-5.00D
|
Zhi Chen. et al. [14]
|
NA
|
0.022±0.025
|
3W
|
39
|
China
|
7-17
|
-1.00~-5.50D
|
According to the study of Lau [11], after the first week of lens wear, central corneal thinning (9 ± 4 µm) and choroid thickening (9 ± 12 µm) contributed to approximately 70% of the axial shortening (26 ± 41 µm). Moreover, the phenomenon of axial shortening is relatively underestimated in our opinion because most previous studies mixed subjects with shortened AL and those without shortened AL. Combined with the obvious axial shortening collected in the ALS group in this study (-0.08±0.04 mm), it can be claimed that central corneal thinning and choroid thickening do not provide entire explanations of axial shortening. Therefore, the mechanism of axial shortening needs further study.
There was an obvious rebound in AL after discontinued wear of the lenses (wash-out period) in both groups, which corresponds with the recent discovery by Swarbrick [15] and Zhouyue Li [21]. This means that the data about AL after orthokeratology becomes relatively true only after the washout period. In other words, if we use the AL data measured without wash-out period as the cutoff point of the experiment about axial elongation after orthokeratology, we will likely overestimate the control effect of the OK-lens especially in the research between orthokeratology group and group without orthokeratology(e.g., blank control group, glasses, atropine, etc.).
Other researchers have not observed that if OK lenses were re-worn after the washout period, the phenomenon of axial shortening would reappear, similar to the initial phenomenon. This means that axial shortening does repeat in some subjects; in addition, the observation of this phenomenon can now close the loop in the timeline. The differenrent size of the sample regarding re-wear between the two groups in this study may be because subjects with better control effect are more likely to continue, but the reduced sample size did not affect the repeating axial shortening. In the ALS group, the amount of axial shortening after re-wear for one month (20-22 M) was less than that of the first time (0-2 M). In this regard, we think it is caused by the remaining influence of the OK lens even after a one-month washout period In other words, the longer washout period required in clinical research needs further study.
It has been suggested that myopia control with OK lenses is influenced by a number of factors, including patient age and sex, age at onset, degree of myopia, and various anatomic features, including corneal power and shape, anterior chamber depth, iris color, pupil diameter, corneal relative peripheral power change and choroidal thickness [21,26−29]. Although the effect of OK lenses is worth affirming, all these factors remind us that the mechanism by which orthokeratology might control myopia is complex and influenced by individual differences. Therefore, it is very important to predict the development of myopia as soon as possible and to filter OK-lens wearers who may still undergo rapid myopia progression.
The comparative study of the ALS group and NALS group showed significantly slower growth in AL, SE and spherical refractive error in subjects with axial shortening. Because the final data are measured after the washout period, the accuracy of the results can be certain. Under such circumstances, we can conclude that axial shortening after one month of wearing as the indicator of the ideal control effect of myopia and subsequently predict myopia progression.
Through a multiple linear regression model, our study showed a significant correlation between axial change after one month of wearing OK lenses and changes in both AL and SE. In other words, the more axial shortening at the beginning, the slower the progression of myopia; the more the AL increases after one month of wearing OK lenses, the faster myopia will progress. Although the degree of shortening itself is related to the speed of axial growth, it does not affect the correlation. In the clinic, if we encounter patients with axial shortening after the first month of orthokeratology, we can confidently instruct them to continue wearing OK lenses. In another situation, if we encounter patients with rapid growth of AL after the first month of orthokeratology, whether to combine AL with other methods, such as low-concentration atropine, is worth further study [30, 31]. In addition, compared with several other prediction methods, AL is a routine examination, which means obvious operability and feasibility.
We also found that older age at baseline was correlated with a lower increase in AL, which matches previous studies [26, 29]. Regarding the relationship between SE and the progression of myopia, some studies reported slow progression with higher baseline myopia [19, 32], and some reported lower baseline myopia [33], whereas more studies showed that the rate of progression was not significantly associated with baseline myopia [20,34−36]. In our study, we did not find a significant correlation between them.
Although this paper is the first to study axial shortening and propose the relationship between axial shortening and myopia control, including AL and SE, there are still some deficiencies in this study. Although the samples were selected continuously, this study still cannot answer the question about the proportion of patients with shortened AL due to the exclusion of some of the subjects with shortened AL and the heavy workload. However, in line with the research of Swarbricks and Zhi Chen [14, 15, 23], 19%-50% of the patients with OK lenses showed axial shortening, which means that this phenomenon should not be ignored. In addition, this study cannot exclude influencing factors such as parents' situation and pupil size, which are difficult to avoid completely in retrospective studies. Finally, we still cannot completely reveal the reason for axial shortening, which is worthy of further study.