Posterior corneal elevation changes during 12 month of overnight orthokeratology

Abstract

Aims

the aim of this study to investigate the elevation changes in posterior corneal surface after 12 months of orthokeratology (ortho-k) treatment.

Methods

In this retrospective chart review, medical records of 37 Chinese children who wore ortho-k lenses over 12 months were reviewed. The data of only right eye were analyzed. Variables including the flat and steep keratometry of anterior and posterior corneal principal meridians, central corneal thickness (CCT), posterior thinnest elevation of cornea (PTE), posterior central elevation of cornea (PCE) and posterior mean elevation of cornea (PME) were measured by Pentacam. Variables including anterior chamber depth (ACD), lens thickness (CLT) and ocular axis length (AL) were measured by optical biometry. All variables differences between baseline and 12 months after ortho-k treatment were assessed by statistical analyses.

Results

The average age of all subjects was 10.70 ± 1.75 years (range 8–15 years old). The baseline spherical equivalent (SE) was −3.26 ± 1.52 D (−0.50D to −5.00D). Both flat and steep keratometry of anterior corneal surface and CCT were significantly decreased after 12 month follow up during ortho-k treatment (both P < 0.000). Both flat and steep keratometry of posterior corneal surface were not significantly different after 12 month follow up compared with that of baseline (P = 0.426, 0.134 respectively). PCE, PTE and PME were not significantly changed over 12 months of ortho-k treatment (P = 0.051, 0.952 and 0.197 respectively). The ACD was significantly decreased in 12 month follow up during ortho-k treatment (P = 0.001). The CLT and the AL were significantly increased during this period (both P < 0.000).

Conclusion

Although the anterior corneal surface was significantly changed by ortho-k lens, the posterior corneal surface did not show any changes during 12 months follow up. Simultaneously, The ACD, CLT and AL were significantly changed during this period.

1. Introduction

Myopia is the common ocular disorder in adolescents and viewed as a major public health concern in the worldwide. There is a high prevalence of myopia about 80%–90% in young adults in East Asia, and an accompanying high prevalence of high myopia in young adults (10–20%). This lead to an increase in low vision and blindness due to pathological myopia [1]. Several clinical control approaches including bifocal or multifocal spectacle lenses, orthokeratology (ortho-k) lenses, multifocal contact lenses and low dose atropines have been well developed [2]. Notably, there are a number of prospective clinical trials which have demonstrated that ortho-k tends to slow the progression of myopia by 40–60% [[3], [4], [5]].

Ortho-k treatment lead to the redistribution of the anterior corneal epithelial tissues and make anterior cornea curvature flat in central and steep in mid-peripheral part [2]. But the changes of the posterior cornea surface have not been well understood. Recently, posterior corneal elevation maps have strong clinical interest for eye care practitioners to evaluate the shape of the cornea for indications of potential ectatic signs of keratoconus [6]. Recent studies since 2009 in the Middle East and Asia use videokeratography to estimate the prevalence of keratoconus ranging from 0.9 to 3.3% [7]. Due to huge number of children underwent ortho-k treatment in China; it is possible that a few of keratoconus patients existed among these children. As a result, whether ortho-k treatment changed the posterior corneal elevation and then covered up abnormally high posterior corneal elevation of subclinical keratoconus need further explored. Only a handful of studies have investigated the posterior corneal change after ortho-k wearing so far. António Queirós et al. monitored the posterior cornea elevation in 62 myopic eyes with orthokeratology treatment over 12 months. They found that the posterior central elevation (PCE) and posterior nasal/temporal elevation (at 2.5 mm point from center of the cornea) were not significantly changed during ortho-k treatment [8]. However, parameters including posterior thinnest elevation (PTE) and posterior mean elevation (PME) were more commonly variables for evaluating the possible ectasia of cornea or subclinical keratoconus before corneal refractive surgery [9]. In this study, these variables were first measured for children during 12 months follow up after ortho-k treatment. Additionally, anterior segment variables including ACD and CLT were also involved in this study.

2. Methods

2.1. Subjects and inclusion criteria

The clinical records of 37 subjects who underwent ortho-k treatment at the department of Ophthalmology of the third hospital of Hebei Medical University were retrospectively analyzed. Patients with myopia between −1.00D and −5.00D and astigmatism below −1.50D were included. Comprehensive eye examinations were performed prior to ortho-k lens fitting. The refractive error of all patients was fully corrected such that the monocular best corrected visual acuity was better than 6/7.5 on a Snellen eye chart which was achieved after one month of ortho-k wearing. Surface regularity and center of the treatment zone (less than 0.5 mm of decentralization) were selected for inclusion in the sample. The definition of the magnitude of treatment zone decentralization proposed by previous studies was used in this study [10]. Decentralization of treatment zone was not considered as ideal fitting for ortho-k lens. Some patients often have poor vision acuity and feel uncomfortable during the time. So we excluded these patients from this study. A corneal topographer (Medmont E300) was used to collect the corneal surface data and the tangential power map of corneal topography after 1 month of ortho-k lens wear was used for analysis. A computer program (Python) was used to measure the distance of the ortho-k lens decentration as reported by previous studies [10,11]. Consent was obtained from both the patient and the patient’s parents in order to use their medical records for study purposes. The study was conducted in accordance with the tenets of the Declaration of Helsinki and approved by the institutional Ethical Committee Review Board of the third hospital of Hebei Medical University. All patients underwent routine examination at 1, 3, 6, 9 and 12 months follow up on time. Only the right eye data were used for statistical analysis.

2.2. Orthokeratology lens characteristics

Subjects in the ortho-k group were fitted with ortho-k lens with reverse-geometry design (Alpha Company, Japan) in both eyes. The lenses were supplied in Boston EM material [nominal Dk 104 100 × 10–11 (cm2·mL O2)/(s·mL·hPa)] with a central thickness of 0.22 mm. After ortho-k was removed from the cornea, all measurements across study were performed in 2 h. This protocol refer to previous study [8].

2.3. Data collection

All corneal parameters were obtained by the Pentacam（Oculus, Wetzlar, Germany）. Scans classified as ‘OK’ by the instrument were considered. The reference best-fit sphere (BFS) was defined in the central 8.0-mm region of the cornea. The central corneal thickness (CCT), anterior flat and steep keratometry (A-FK, A-SK), posterior flat and steep keratometry (P-FK, P-SK), posterior central elevation (PCE)、posterior thinnest elevation (PTE) and posterior mean elevation (PME) were determined. In detail, PME was calculated as the mean value of 17 points in corneal posterior surface. These 17 points included in the central 4-mm zone are as follows: 1 point at the center, 8 points at 1 mm distance from the center along 0°, 45°, 90°, 135°, 180°, 225°,270°and 315°meridians (0°defined as the horizontal semi-meridian on the right, and rotating counterclockwise in both eyes), and the other 8 points at 2 mm distance from the center along the identical semi-meridians (details in Fig. 1A and B). Anterior chamber depth (ACD; defined as the distance from the center of the corneal endothelial to the anterior surface of the lens capsule), crystalline length thickness (CLT) and axial length (AL) were measured with a commercially available optical biometry (OA-2000,Tomey, Nagoya, Japan) at baseline and at the 12 months follow up. These measurements refer to previous studies [9,12].

Fig. 1.

This figure is the back corneal surface elevation map. Part A displayed the position of PTE (posterior thinnest elevation) and PCE (posterior central elevation). Part B displayed that 17 points were selected for calculation of PME (posterior mean elevation).

2.4. Statistical analyses

Data were analyzed using SPSS 22.0 (SPSS Inc., Chicago, IL, USA) and Python 3.7.4. The Shapiro-Wilk Test (SW) test was used to determine the distribution of all variables at baseline and at the 12-month follow-up (P > 0.05). Result showed that all variables except for PTE (P = 0.013) and P-FK (P = 0.014) were normally distributed (P > 0.05). Paired samples T-test and Wilcoxon Mann-Whitney Test were used to compare variables at baseline and at 12 months after ortho-k treatment for normal and non-normal distributed variables, respectively. P value < 0.05 was considered to indicate a statistically significant difference. Effect size analysis (Cohen’s d parameter) was used to justify the sample size.

3. Results

37 right eyes of 37 patients (14 female and 23 male), mean age of 10.70 ± 1.75 years (range 8–15 years) were included in the study. Mean baseline spherical equivalent by subjective refraction was −3.26 ± 1.52D. The baseline demographic data of all subjects included in this study was showed in Table 1.

Table 1.

Patients and study eye characteristics.

Demographic characteristics (n = 37 patients)	No. (%) or mean ± SD
Age (year)	10.70 ± 1.75
Gender (Female/male)	14(38%)/23(62%)
Ocular characteristics (n = 37 eyes)
Sphere (D)	−3.12 ± 1.47
Cylinder (D)	−0.28 ± 0.43
SER (D)	−3.26 ± 1.52
Axial length (mm)	25.01 ± 0.84
Intraocular pressure (mmHg)	15.57 ± 2.59

SD= Standard Deviation，SER = Spherical Equivalent Refractive Error.

3.1. Anterior and posterior topographic corneal parameters

First, The Shapiro-Wilk test was used to analyze the distribution of all variables at baseline and at 12-month follow-up. PTE and PFK were non-normal distributed and analyzed by Wilcoxon Mann-Whitney Testas shown in Table 2. All other variables were normally distributed and analyzed by paired sample t-test as shown in Table 3.

Table 2.

Wilcoxon Mann-Whitney Test for PTE and P-FK changes between baseline and that of 12 months with ortho-k lens wear.

Variables	M(P25,P75)	Estimated Difference, Baseline vs. 12 month (95%CI)	Wilcoxon Mann-Whitney Test
			z value	p value
PTE(B) (mm)	1 (-1,2.5)	0 (-2.0,2.0)	−0.060	0.952
PTE(Y) (mm)	1 (-3,4)
P-FK(B) (mm)	−6 (-6.1,-5.9)	0 (-0.1,0.1)	−0.797	0.426
P-FK(Y) (mm)	−5.9 (-6.1,-5.8)

Table 3.

Paired Samples T-Test for all normal distributed variables changes between baseline and that of 12 months with ortho-k lens wear.

Variables	Baseline	12 months	p value	Cohen's d
PCE(μm)	0.27 ± 3.35	−0.49 ± 3.67	0.051	0.332
PME (μm)	0.76 ± 0.85	0.88 ± 0.74	0.197	0.216
A-FK(mm)	42.33 ± 1.16	40.71 ± 1.26	0.000*	2.049
A-SK(mm)	43.87 ± 1.15	42.4 ± 1.33	0.000*	1.425
P-SK(mm)	−6.37 ± 0.20	−6.4 ± 0.25	0.134	0.252
CCT (mm)	0.56 ± 0.03	0.55 ± 0.03	0.002*	0.562
ACD (mm)	3.26 ± 0.16	3.2 ± 0.18	0.001*	0.611
CLT (mm)	3.43 ± 0.15	3.48 ± 0.16	0.000*	0.973
AL (mm)	24.89 ± 0.78	25.12 ± 0.69	0.000*	0.93

PME = posterior mean elevation, PCE = posterior central elevation, A-FK = anterior flat curvature，A-SK = anterior steep curvature, P-SK: Posterior steep curvature, CCT = central corneal thickness, ACD = anterior segment depth, CLT=Crystalline lens thickness, AL = Axial length.*p < 0.05 = statistically significant.

PTE = posterior thinnest elevation, P-FK = posterior flat keratometry, M = median. B = baseline, Y = 1 year follow up.

On the anterior cornea surface, the flat and steep keratometry were significantly decreased at 12 months after ortho-k treatment compared with that of baseline (both p = 0.000). The central cornea thickness (CCT) was also significantly decreased compared with that of baseline (P = 0.002). On the back corneal surface, the posterior flat and steep keratometry have not shown significantly changed over 12 months follow up (both P = 0.426 and 0.134 respectively). In terms of the posterior corneal elevation, posterior central elevation (PCE), posterior thinnest elevation (PTE) and posterior mean elevation (PME) did not showed significantly different between baseline and 12 months after ortho-k treatment (P = 0.051, 0.952, 0.197 respectively). (As shown in Tables 2 and 3).

3.2. Anterior segments and axial length

To observe ACD and CLT changes accompanied by AL elongation during ortho-k treatment, a paired sample t-test was used for analysis. ACD was significantly decreased after 12 months of ortho-k treatment compared with the baseline values (P = 0.001). But CLT was significantly increased during this period (P = 0.000). Similarly, the axial length was also significantly increased at 12 months follow up (P = 0.000) (Table 3).

Additionally, the effect size analysis was calculated for justified sample size. The Cohen’s d parameters were >0.5 for ACD and CCT (medium effect); and >0.8 for AFK, ASK, CLT and AL (large effect). This result was consisted with that of paired sample t-test above, which indicated that the sample size is fitful in this study.

4. Discussion

Recently, youth myopia is rapidly becoming a common disease in the world. Prevalence of myopia was more than 60% among Chinese school students and increased with grade in a non-linear manner [13]. Right now, ortho-k has become one of the popular treatment options to control the progression of myopia in China. There have been a number of prospective and retrospective studies, which have demonstrated that ortho-k tends to slow the progression of myopia by 40–60% using AL measurement [14,15].

Anterior corneal curvature changes during ortho-k treatment were well explored in previous studies [16,17]. In current study, the flat K and the steep K were significantly flatter after ortho-k treatment compared with baseline, which consisted with many previous studies [16,17]. The CCT became significantly thinner after ortho-k wearing over 12 months, which also consisted with a few of previous reports [8,18].

Keratoconus is a clinical non inflammatory corneal thinning with abnormal posterior ectasia and abnormal corneal thickness distribution [19]. It has been suggested that an increase in posterior elevation may be the earliest sign of subclinical keratoconus [20]. Pentacam is considered to be the most sensitive device for detecting the early form of keratoconus through several parameters for evaluating posterior corneal elevation, which is regarded as the gold standard in discrimination suspect subclinical keratoconus from normal corneas [[21], [22], [23]]. The abnormally high posterior cornea elevation can be occurred prior to any topographic change on the anterior corneal surface. Therefore, whether the ortho-k lens interferes with the posterior cornea elevation is worryingly.

A handful of previous studies reported how the posterior corneal curvature changes during ortho-k treatment. Jeong Ho Yoon et al. observed the posterior corneal shape of 18 adult subjects with low myopia and astigmatism fitted with ortho-k over 14 days by corneal topography (Medmont E−300). The result indicated that the posterior corneal apical radius of curvature was not significant changes during 14 days of overnight ortho-k. They concluded that Ortho-k refractive effect is achieved primarily through remodeling of the anterior corneal layers, without overall corneal bending [24]. However, Medmont E−300 is a Placido-based corneal topography for anterior corneal surface curvature evaluation. The posterior corneal apical radius in that study was calculated using an in-house program based on the anterior corneal ellipse curve and corneal thickness. Next, the real posterior corneal elevation during ortho-k treatment was first measured with pentacam by Queiros et al. in 2011. They recruited 20 adult patients (20 eyes) and analyzed the posterior corneal elevation changes in the center of the cornea and 4 points to each side of the horizontal meridian at intervals of 1 mm during ortho-k treatment over at least 3months. The result indicates that the back corneal surface does not suffer significant changes after ortho-k treatment [25]. Subsequently, Queiros et al. further analyzed posterior central elevation (PCE), posterior nasal elevation at 2.5 mm from center of the cornea and posterior temporal elevation at 2.5 mm from center of the cornea during ortho-k treatment over 12 months. The result indicated no evidence of change in central or peripheral posterior elevation values in 62 eyes of Asian children during orhto-k treatment [8]. In current study, result confirmed that PCE was not changed during ortho-k treatment over 12 months, which was consisted with Queiros et al. studies. Next, this study first measured the posterior thinnest elevation (PTE), which is of great significance to evaluate the progress of keratoconus. Extra high PTE is the most typical early manifestation of keratoconus [19]. The result showed that PTE was not significant changed during ortho-k treatment over 12 months, Except for PTE and PCE, Posterior mean elevation (PME) was another common parameter for evaluating postoperative corneal stability with good repeatability and reliability [26]. The current study also first evaluated PME with mean value of 17 points in a 4 mm diameter concentric circle and proved no change between baseline and 12 months after ortho-k treatment. Through evaluating posterior corneal elevation changes by PCE, PTE and PME measurement, this study confirmed that ortho-K wearing did not disturb the early diagnosis of subclinical keratoconus.

With respect to anterior segment changes, previous studies have reached conflicting conclusions. Sin wan Cheung et al. observed 37 subjects with ortho-k treatment, and found the anterior segments including ACD and CLT remained unchanged in 6 months follow up [27]. Subsequently, Sin Wan Cheung et al. finished 2 year observation of ortho-k treatment in 78 subjects. They concluded ACD was significantly reduced in six months, but both ACD and CLT were not obviously changed at 2 years follow up [28]. However, Ana Gonzalez-Mesa et al. found that one year of orhtho-k wearing reduces central ACD and AL in 36 subjects study [29]. Recently Yanwei Chen et al. reported a prospective study that demonstrated the ACD was decreased and CLT was thicker in ortho-k group compared with single vision spectacles group over 6 months period [30]. That study consisted with the result of current study. Additionally, The CLT was found to be significantly increased in this study also consisted with our previous findings [31].

As a result, the current study suggested that the decrease of ACD may be primarily caused by lens thickness increased given the posterior shape of the cornea was not changed during ortho-k treatment. As expected, the AL was also significantly increased after 12 months follow up of ortho-k treatment, but the extent of AL elongation was consisted with previous studies, which reported a slower elongation of AL with ortho-k wearing compared with myopia with single-vision spectacles [15,32,33].Briefly, the current study observed whole anterior segment changes and AL changes. It is helpful to better understand the mechanism of ortho-k treatment for myopia control.

In conclusion, this study showed that posterior cornea curvature during ortho-k treatment kept stable through 12 months observation. The result gives eye specialist confidence that application of ortho-k lens did not disturb the judgment of early sign for subclinical keratoconus. However, a major limitation of this study is the short term follow up period of the patients and a small sample size. This was a retrospective study and had no control group with spectacles for comparison. Additionally, a larger sample size, longer period of time and calculation of the posterior elevation in a wider area should be performed in the future to better understand the changes of the posterior corneal surface in ortho-k wear.

Declaration of competing interest

All authors (Zengying Wang, Zuocheng Wang, Yifei Meng, Pengfei Wang, Shaniya Yibulayin Bohua Jiang，Xufei Bian, Tianya Gao, Zhipeng yan*) declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled“Posterior corneal elevation changes during 12 month of overnight orthokeratology” (HELIYON-D-22-27652R1).

Funding statement

Zhipeng yan, Yifei Meng, Sufang,Qie, Zengying Wang, Pengfei Wang was supported by Natural Science Foundation of Hebei Province in China (Grant numbers H2022206225).

Additional information

No additional information is available for this paper.