Skip Navigation LinksHome > June 2012 - Volume 89 - Issue 6 > Toric Orthokeratology for Highly Astigmatic Children
Optometry & Vision Science:
doi: 10.1097/OPX.0b013e318257c20f
Original Articles

Toric Orthokeratology for Highly Astigmatic Children

Chen, Chia Chi*; Cheung, Sin Wan; Cho, Pauline

Free Access
Article Outline
Collapse Box

Author Information




School of Optometry, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR, China.

Received October 3, 2011; accepted February 15, 2012.

Chia Chi Chen School of Optometry The Hong Kong Polytechnic University Kowloon, Hong Kong SAR, China e-mail:

Collapse Box


Purpose. To determine the efficacy of toric orthokeratology (ortho-k) in correcting myopia and astigmatism in myopic children with moderate to high astigmatism.

Methods. Asymptomatic subjects aged 6 to 12 years with myopia of 0.50 to 5.00 D and astigmatism of 1.25 to 3.50 D of axes 180 ± 20° were fitted with Menicon Z Night Toric Lens (NKL Contactlenzen B.V., Emmen, The Netherlands). Data collection was performed at baseline and 1 night, 1 week, and 1 month after the commencement of lens wear. The results from the right eye or the eye with higher astigmatism were reported.

Results. The first lens fit success rate was 95%. Two subjects had to be refitted due to lens decentration and inadequate central clearance after one overnight lens wear and were successfully fitted with a second pair of lenses. Toric ortho-k significantly reduced the manifest myopia from 2.53 ± 1.31 D to 0.41 ± 0.43 D and astigmatism from 1.91 ± 0.64 D to 0.40 ± 0.39 D (paired t-tests, p < 0.02) after 1 month of lens wear. The unaided visual acuity (logMAR) was 0.11 ± 0.13 after 1 month of lens wear. No significant lens binding, corneal staining, or other adverse events were observed during this period of lens wear.

Conclusions. This toric lens design lens, with a first lens fit success rate of 95%, was effective in correcting low-moderate myopic children who had moderate-high astigmatism. It has the potential to be used in myopic control studies for myopic children who have high astigmatism.

Myopia is a common ocular disorder especially in Asian countries such as Hong Kong, Singapore, Korea, Taiwan, and China.16 Orthokeratology (ortho-k) uses specially designed rigid contact lenses to temporarily reduce myopia.711 Recent reports have shown ortho-k to be effective in slowing the progression of myopia in children.10,12,13 The rate of axial elongation of the eyeball in children wearing ortho-k lenses was reported to be at least 40% slower compared with those wearing spectacles10 or soft contact lenses.12

Most myopic children are also astigmatic.14,15 Kleinstein et al.15 reported that the prevalence of astigmatism was 33.6% in Asian children aged 5 to 17 years. In their article, astigmatism was defined as at least 1.00 D difference between the two principal meridians. Although it has been shown that spherical design ortho-k lenses are effective in correcting low-moderate myopia, they cannot adequately reduce moderate refractive astigmatism1618 and hence ortho-k for myopia control is not indicated for children with refractive or corneal astigmatism more than 1.50 D.

The most common problem with spherical ortho-k lenses on patients with a significant amount of corneal astigmatism is poor lens centration which can lead to induced astigmatism and poor vision.17,19 Toric reverse geometry designs have been developed to improve lens centration as well as for astigmatic correction. However, apart from some case reports20,21 and some conference abstracts,22,23 to our knowledge, there are no published research reports on the efficacy of toric ortho-k for correcting astigmatism or for myopia control in moderate to high astigmatic children.

In our case reports,20,21 good lens centration, using toric ortho-k lenses, was obtained in all the three children, aged 10 to 13 years, who had high refractive astigmatism. While significant reductions of refractive myopia and refractive astigmatism were observed in two of the subjects, only a modest response was found in the other subject. Yet, despite the differences in response, during the 1-year ortho-k lens wear, all the three subjects did not show any significant increase in axial length. The results suggested the potential of myopia control on myopes with significant amount of refractive astigmatism with this lens design.

The Toric Orthokeratology-Slowing Eyeball Elongation (TO-SEE) study is a 2-year longitudinal study investigating the efficacy of toric ortho-k for myopic and astigmatic reduction in myopic children who had moderate to high astigmatism and for myopia control. This report presents the clinical performance of the toric ortho-k lens used for the correction of myopia and astigmatism in these subjects after 1 month of lens wear.

Back to Top | Article Outline


Study Design

Subject recruitment was conducted between April 2008 and December 2009. Eligible subjects were given a comprehensive vision examination at the beginning of the study. A pair of ortho-k lenses was prescribed based on the manifest refraction and the corneal topography. Lens performance was reviewed after 1 night, 1 week, and 1 month after commencement of lens wear. All procedures followed the Declaration of Helsinki, and the protocol was reviewed and approved by the Departmental Research Committee of the School of Optometry of The Hong Kong Polytechnic University. Informed consent was obtained from the parents of each subject before commencement of the study.

Back to Top | Article Outline

The subjects were between 6 and 12 years and had manifest myopia of 0.50 to 5.00 D (inclusive) and refractive astigmatism of 1.25 to 3.50 D of axes 180 ± 20° (Table 1). All subjects had unremarkable ocular health and did not have any ocular or general health problems which could affect the normal development of refractive status of the eye. They had no contraindication for ortho-k lens wear and did not have any previous myopia control treatment. Each eye had a best-corrected visual acuity (VA) of 0.10 logMAR or better.

Table 1
Table 1
Image Tools

The subjects and parents were trained to insert, remove, and care for the lenses. Subjects and their parents were also taught about the possibility of lens binding (adherence) on eye opening and how to remove a bound lens properly. Table 2 shows the grading scale of lens binding given to the subjects. The subjects were required to wear the lenses for 8 to 10 h every night unless instructed otherwise by the examiner and to attend aftercare visits which were scheduled after first overnight, 1 week, 1 month, (each visit within 2 h after waking up in the morning), and every 3 months after commencing lens wear. Unscheduled visits were also arranged when necessary in case of adverse events. They were also required to record their lens wearing and lens removal time as well as the incidence and severity of lens binding in the ortho-k diary provided.

Table 2
Table 2
Image Tools

Any subjects who presented with significant adverse events or failed to comply with the prescribed procedures despite reminders (three times) were required to withdraw from the study. All prescribed lenses and solutions had to be returned to the examiner upon completion or withdrawal from the study.

Back to Top | Article Outline
Sample Size

For the main myopia control study, the sample size required was determined to be 40 to achieve a power of 80%.

Back to Top | Article Outline
Lenses and Solutions

Lenses used were the Menicon Z Night Toric Reverse Geometry Lens (RGL) (NKL Contactlenzen B.V., Emmen, The Netherlands). The Menicon Z night toric lens is a peripheral toric ortho-k lens design (both sagittal height and tangent) with spherical back optic and reverse zones. The back optic zone diameter is 6.0 mm, center thickness is 0.24 mm, and the available diameters are 10.2, 10.6, and 11.0 mm. The back vertex power of the lens is plano. Each lens has three fenestrations at 120° intervals in the area of the reverse curve. All lenses are made in Menicon Z material [ISO Dk 163 × 10−11 (cm2/sec) (ml O2/(ml · mm Hg)) ISO 9913-1]. All lenses used in this study were replaced every 12 months.

The lens parameters were determined with the NKL Easy Fit Software (version VIP 2006, NKL Contactlenzen B.V., Emmen, The Netherlands). Pertinent data, including the manifest refractive error, the horizontal visible iris diameter (HVID), and four corneal topographic maps, were required for the computer program to calculate the initial lens parameters for the eye. The software calculates the back optic zone radius according to the refractive power and simulated keratometry reading. HVID was used to determine the lens diameter (about 90% of the HVID) and the software also determined the tangent angle and the lens height based on the corneal topography. If an ordered lens produced an adverse response24 such as displacement, smiley face, or frowny face topographic pattern after one night of lens wear, a new lens was ordered for the eye using the software.

The lens care system prescribed included Menicon O2 Care cleaner, MeniCare Plus, Progent A+B (Menicon, Nagoya, Japan), Alcon Tears Naturale Free (Alcon Laboratories, Fort Worth, TX), and Bausch & Lomb sensitive eye saline (Bausch & Lomb, Rochester, NY). Subjects were required to rinse the lenses thoroughly with saline after rubbing with Menicon O2 daily cleaner, disinfect their lenses in MeniCare Plus, and rinse their lenses again with saline before lens insertion. Artificial tears were used before lens insertion to minimize the formation of air bubbles and before lens removal to loosen the lenses. Protein removal was performed weekly. All accessories were disinfected every week, and all bottle solutions and lens cases were replaced every month. Tap water and suction were not allowed for lens handling to minimize the risk of infection. All lenses and solutions used were complimentary to ensure that all subjects used the same solutions and complied with the replacement schedule.

Back to Top | Article Outline

All examinations were carried out by the examiner (CC). The right eye was always measured first followed by the left eye at each visit.

Clinical performance of the lenses during the first month of lens wear, in terms of the anterior ocular health, VA, and subjective refraction, was assessed.

Back to Top | Article Outline
Orthokeratology Lens Fitting

At the delivery visit, the ortho-k lens fitting assessment was first performed by assessing the fluorescein pattern with the lens in situ with the slitlamp. The targeted ideal fluorescein pattern was characterized by a central zone of light touch (3.0 to 3.5 mm diameter), surrounded by a wide deep doughnut-shaped tear reservoir, a zone of peripheral light touch, and peripheral clearance, with lens movement of 1 to 2 mm on blink. Lenses demonstrating the described fluorescein pattern for an acceptable fit were delivered. If the corneal response showed a bull's eye pattern25 at the first overnight visit, the subject would continue lens wear. First, fit success rate was determined by the percentage of subjects who achieved satisfactory fitting and continued lens wear with the first pair of lenses after first overnight visit. If the cornea showed a poor response (e.g., displacement, smiley face, and frowny face topographic pattern), lens wear was ceased and a new lens with adjusted parameters, as suggested by the Easy Fit Software (based on topographical response of first lens), for each eye was ordered and the above procedures repeated. Subjects who could not achieve satisfactory fits despite repeated modifications (three pairs of lenses) were terminated from the study. For an eye with myopia not more than 4.00 D, the target reduction would be the amount of myopia of the eye. For an eye with myopia 4.25 to 5.00 D, the first lens ordered was targeted for 4.00 D reduction.

Back to Top | Article Outline
VA and Subjective Refraction

At each visit, the LCD logMAR VA chart in the same examination room was used when taking the entrance VA and subjective refraction. Distance subjective monocular refractive error for each eye was determined at every visit using trial frame and trial lenses. High (close to 100% contrast) and low contrast (10% contrast) unaided VA (UVA) and best-corrected VA were taken with the ETDRS charts (Precision Vision, La Salle, IL) at cycloplegic visits. The procedures for VA measurement were reported in a previous study.11

Back to Top | Article Outline
Anterior Ocular Health

Anterior ocular health assessment was performed at each visit using Topcon TRC-NW6S photographic slitlamp (Topcon, Tokyo, Japan) after VA assessment. Corneal staining, taking into account type, depth, and extent, was graded from 0 to 4 using Efron grading scales.26 The location (superior, inferior, nasal, temporal, and central) of the corneal staining was also recorded. Presence of lens binding and corneal pigmented arc was assessed at every visit after commencement of lens wear.

Back to Top | Article Outline
Corneal Topography

After slitlamp biomicroscopy, corneal topography was measured using the Medmont E300 corneal topographer (version 3.9.3, Medmont, Camberwell, Australia). All subjects were asked to blink normally to avoid the disruption of tear film, open the eyes wide after the last blink, and fixate on the internal target during the image acquisitions. Images were automatically captured and four images, each with score higher than 98, were accepted. The subtractive maps between pre- and post-lens wear were used to determine lens centration and size of treatment zone (tangential maps) and the amount of corneal flattening (refractive maps).

Back to Top | Article Outline
Outcome Measures

The primary outcomes for the clinical performance of the toric ortho-k lenses were the first lens fit success rate and the amount of myopic and astigmatic reductions in subjective refraction. The amount of myopic reduction at each subsequent visit was determined by subtracting the residual myopia from baseline myopia. Changes in astigmatism were determined by comparing changes in refractive astigmatism as well as power vectors: J0 = (−C/2) cos (2θ) and J45 = (−C/2) sin (2θ), where C denotes the amount of astigmatism at axis θ and J0 and J45 are the horizontal or vertical and oblique components of astigmatism, respectively.27 Changes in corneal toricity were also analyzed and presented.

Back to Top | Article Outline
Data Analysis

Statistical package used was SPSS version 18 (SPSS, Chicago, IL). Because the distribution of data were not significantly different from normal (Kolmogorov-Smirnov tests, p > 0.05), parametric tests were used to compare myopia, astigmatism, J0 and J45, and VA data between baseline and the subsequent visits. Repeated measures analysis of variance (ANOVA) was used to study the change in the parameters after ortho-k lens wear and paired t-tests, with Bonferroni correction, were used for post hoc analysis. Data from the right eye, if the two eyes had the same amount of refractive astigmatism, or the eye with higher refractive astigmatism were analyzed in this report.

Back to Top | Article Outline



A total of 83 subjects were screened and 43 subjects (22 males and 21 females), who satisfied the recruitment criteria (Table 1), were enrolled and fitted with the toric lenses. The mean ± SD age was 9.4 ± 1.4 years, and Table 3 presents the demographic data and the baseline characteristics of the subjects. Fig. 1 presents a flowchart of subject recruitment and progress of the subjects.

Table 3
Table 3
Image Tools
Figure 1
Figure 1
Image Tools
Back to Top | Article Outline
First Fit Success Rate

Lens fittings with the first pair of lenses were satisfactory for all subjects at the delivery visit. At the first overnight visit, only two subjects were refitted with a second pair of lenses due to poor lens decentration and inadequate central clearance. The first lens fit success rate without the use of trial lenses was 95%.

Back to Top | Article Outline
Visual Acuity

Mean ± SD UVA was 0.37 ± 0.24 at the first overnight and improved to 0.11 ± 0.13 logMAR after 1 month of lens wear (paired t-test, p < 0.001) (Fig. 2). UVA at 1 month was significantly different from baseline best-corrected VA (0.00 ± 0.07 logMAR) (paired t-test, p < 0.001). Best-corrected VA at the 1-month visit (0.00 ± 0.04 logMAR) was not significantly different from that at baseline (paired t-test, p = 0.628).

Figure 2
Figure 2
Image Tools
Back to Top | Article Outline
Changes in Refractive Errors and Corneal Toricity

Fig. 3 shows the changes in myopia and refractive astigmatism during the 1-month ortho-k lens wear. There were significant changes in myopia and refractive astigmatism after 1 month of lens wear (repeated measures ANOVA, p < 0.05). The mean ± SD baseline myopia was 2.53 ± 1.31 D. Myopia was significantly reduced to 1.33 ± 0.80 D (42% reduction) at the first overnight visit and to 0.41 ± 0.43 D (81% reduction) at the 1-month visit (paired t-tests, p < 0.001) (Fig. 3).

Figure 3
Figure 3
Image Tools

Refractive astigmatism (mean ± SD) reduced from 1.91 ± 0.64 D to 0.88 ± 0.59 D (54% reduction) and to 0.40 ± 0.39 D (79% reduction) at the first overnight and 1-month visits, respectively (paired t-tests, p < 0.001) (Fig. 3). Corneal toricity reduced from 2.30 ± 0.51 D at baseline to 2.01 ± 0.61 D (13%) at the first overnight visit and to 1.28 ± 0.53 D (44%) at 1-month visit (paired t-tests, p < 0.001). Fig. 4 shows changes in both refractive astigmatism and corneal toricity, J0 and J45 before and after ortho-k treatment. Significant reductions at subsequent visits were observed for J0 of both refractive astigmatism and corneal toricity (repeated measures ANOVA, p < 0.001). No significant changes were observed for J45 of both refractive astigmatism and corneal toricity over the 1-month period (repeated measures ANOVA, p = 0.168).

Figure 4
Figure 4
Image Tools
Back to Top | Article Outline
Effect on Anterior Ocular Health

At the first overnight visit, corneal staining was observed in 23% of subjects, only in one of the five corneal zone in each eye [central (5%), inferior (9%), superior (2%), nasal (2%), and temporal (5%)]. No significant corneal staining (all staining < grade 2) was observed in the subsequent visits during the 1-month lens wear, although mild (grade 1) staining was observed at different corneal locations in some subjects at different visits.

Dimple veiling was observed in 70% of the subjects (30/43) at the first overnight visit. No dimple veiling was observed at subsequent visits. During the 1-month lens wear, no pigmented arc was observed and no adverse events were noted in any subject.

Back to Top | Article Outline
Lens Binding

Grade 1 lens binding was reported by 14% of subjects at the first overnight visit and by about 5% of subjects before they removed their lenses at the 1-week visit. No lens binding was reported at the 1-month visit.

Back to Top | Article Outline


Toric design ortho-k can be used not just for reduction of refractive astigmatism but also for improving lens centration on toric corneas. In this study, no subjects had been excluded because of poor lens centration. Currently, a few manufacturers (e.g., NKL Contactlenzen B.V., Emmen, The Netherlands; Paragon Vision Sciences, Mesa, AZ) have developed toric RGL design (toric reverse and/or alignment zones). No trial lenses were used in this study. The NKL Easy Fit Software (NKL Contactlenzen B.V., Emmen, The Netherlands) allows empirical lens ordering, hence reducing chair time which would otherwise be needed for trial lens fitting, and practitioners do not need to store or maintain a large set of trial lenses. This is ideal for those who are concerned with cross-contamination or transmission of prion disease from trial lenses.28 The first lens fit success rate with Night Toric RGL was 95%, which was better than the rate with trial lens fitting reported in our clinic (73.5%).17 The Easy Fit Software also allows practitioners to modify parameters as they see fit, which is an added advantage to experienced practitioners.

Our results show that toric ortho-k could reduce myopia by 81% and refractive astigmatism by 79% after 1 month of lens wear with one pair of lenses. The mean ± SD myopia reduction after 1 month of lens wear in our subjects was 2.03 ± 1.26 D (81%), whereas previous studies have reported reduction in spherical equivalent of 1.50 to 3.50 D.10,29,30 While these previous studies have reported no significant change in refractive astigmatism with spherical ortho-k lens wear, our subjects achieved 79% reduction in refractive astigmatism with the toric design lenses after 1 month of lens wear. At the 1-month visit, mean ± SD UVA was 0.11 ± 0.13 logMAR which was satisfactory to all subjects. However, the UVA was significantly poorer compared with baseline best-corrected VA due to significant residual myopia (0.41 ± 0.43 D) and/or refractive astigmatism (0.40 ± 0.39 D). Twelve of the subjects had residual myopia or refractive astigmatism >0.75 D due to underresponding at the 1-month visit. Reordering of lenses with higher targets were made for these subjects as they did not achieve the endpoint criteria (i.e., myopia >0.75 D or UVA worse than 0.18 logMAR) to continue in the myopia control study.

Corneal staining associated with ortho-k lens wear is a common finding, and mechanical trauma and hypoxia have been proposed to be likely causes.17,31,32 Mild corneal staining of about 40% had been reported after first overnight wear of ortho-k lenses.17,32 In this study, grade 1 cornea staining was observed in only 23% of subjects at the first overnight visit. In agreement with previous reports,12,31 none of the subjects presented any adverse events that required them to cease lens wear or seek medical intervention. The low incidence of staining observed in this study may be due to improved lens centration with the toric design ortho-k, the lens material used, and the lower incidence of lens binding associated with the use of lens fenestrations.

Dimple veiling occurs when air bubbles are trapped between the lens and the cornea. The lens mechanically compresses the bubbles which indent the corneal epithelium, producing transient depressions on the corneal surface, observed as dimple veiling in the fluorescein-stained eye.33 Most of the “staining ” usually recovers after 1 to 2 h of lens removal.34 In this study, dimple veiling was observed in 70% of the subjects at the first overnight visit. This was likely because of the requirement for subjects to return for this visit without removing their lenses. Air bubbles may have been trapped during blinking with the lenses in situ on the way to our clinic. The presence of fenestrations on the lenses used may also facilitate trapping of bubbles behind the lenses on blinking. Five of the subjects forgot the instruction and attended the first overnight visit after removing their lenses and no dimple veiling was observed in their eyes. For about 10% of the subjects, some distortions of the topographical mire image during measurements of corneal topography were observed and the situation was resolved by the use of artificial tears. Subjects were required to return with their lenses in situ at the first overnight visit to allow assessment of lens binding.

For the 40 ineligible subjects at the screening visit, 18 subjects failed the refractive criteria and 22 subjects were excluded due to inability to comply with test procedures (Fig. 2). The latter included those who refused to have cycloplegic refraction, wear ortho-k lenses (i.e., parents wanted ortho-k but not the subjects themselves), attend the frequent aftercare consultations, and those (i.e., both the subject and their parent) who failed lens insertion and removal (IR) training after multiple attempts. In this study, all subjects had to learn IR using their fingers. No suction (lens) holder was used or prescribed as this item has been reported to be frequently and heavily contaminated.35,36 All subjects were taught IR and if they failed to demonstrate safe IR after multiple attempts (up to five), especially for those younger than 9 years, their parents were taught how to do it for them. Each IR attempt lasted for about 30 min. All parents were taught how to recenter a dislocated lens. Lens and accessory care procedures were taught to both subjects and parents. If care procedures were carried out by the subject, the parent had to agree to monitor compliance. These steps were taken to ensure safe ortho-k lens wear. At the 1-month visit, more than 70% of IR and care procedures were performed by the subjects themselves.

Our preliminary data showed that Night Toric RGL can be used safely, with stringent aftercare and instructions, and effectively for reductions of myopia and astigmatism in children. However, further investigation is warranted to address the issues of long-term safety and efficacy of myopia control and the results will be available after the completion of this 2-year study.

Chia Chi Chen

School of Optometry

The Hong Kong Polytechnic University

Kowloon, Hong Kong SAR, China


Back to Top | Article Outline

We thank Menicon (Menicon, Nagoya, Japan), Bausch & Lomb (Bausch & Lomb, Rochester, NY), and Alcon (Alcon, Fort Worth, TX) for providing complimentary solutions to our subjects. This clinical study was registered at (NCT00978692).

This work is supported by a Collaborative Research Agreement between The Hong Kong Polytechnic University (PolyU) and Menicon, Japan (ZG13). The facilities used are supported by the Niche Area Funding (J-BB7P) from PolyU.

Back to Top | Article Outline


1. Lam SY, Goh SH. The incidence of refractive errors among school children in Hong Kong and its relations with the optical components. Clin Exp Optom 1991;74:97–103.

2. Zhao J, Pan X, Sui R, Munoz SR, Sperduto RD, Ellwein LB. Refractive error study in children: results from Shunyi district, China. Am J Ophthalmol 2000;129:427–35.

3. Lin LL, Shih YF, Hsiao CK, Chen CJ, Lee LA, Hung PT. Epidemiologic study of the prevalence and severity of myopia among schoolchildren in Taiwan in 2000. J Formos Med Assoc 2001;100:684–91.

4. Zhao J, Mao J, Luo R, Li F, Munoz SR, Ellwein LB. The progression of refractive error in school-age children: Shunyi district, China. Am J Ophthalmol 2002;134:735–43.

5. Lam CS, Goldschmidt E, Edwards MH. Prevalence of myopia in local and international schools in Hong Kong. Optom Vis Sci 2004;81:317–22.

6. Lin LL, Shih YF, Hsiao CK, Chen CJ. Prevalence of myopia in Taiwanese schoolchildren: 1983 to 2000. Ann Acad Med Singapore 2004;33:27–33.

7. Dave T, Ruston D. Current trends in modern orthokeratology. Ophthalmic Physiol Opt 1998;18:224–33.

8. Nichols JJ, Marsich MM, Nguyen M, Barr JT, Bullimore MA. Overnight orthokeratology. Optom Vis Sci 2000;77:252–9.

9. Caroline PJ. Contemporary orthokeratology. Cont Lens Anterior Eye 2001;24:41–6.

10. Cho P, Cheung SW, Edwards M. The longitudinal orthokeratology research in children (LORIC) in Hong Kong: a pilot study on refractive changes and myopic control. Curr Eye Res 2005;30:71–80.

11. Cheung SW, Cho P, Chui WS, Woo GC. Refractive error and visual acuity changes in orthokeratology patients. Optom Vis Sci 2007;84:410–6.

12. Walline JJ, Rah MJ, Jones LA. The Children's Overnight Orthokeratology Investigation (COOKI) pilot study. Optom Vis Sci 2004;81:407–13.

13. Kakita T, Hiraoka T, Oshika T. Influence of overnight orthokeratology on axial elongation in childhood myopia. Invest Ophthalmol Vis Sci 2011;52:2170–4.

14. Fan DS, Rao SK, Cheung EY, Islam M, Chew S, Lam DS. Astigmatism in Chinese preschool children: prevalence, change, and effect on refractive development. Br J Ophthalmol 2004;88:938–41.

15. Kleinstein RN, Jones LA, Hullett S, Kwon S, Lee RJ, Friedman NE, Manny RE, Mutti DO, Yu JA, Zadnik K. Refractive error and ethnicity in children. Arch Ophthalmol 2003;121:1141–7.

16. Cheung SW, Cho P. Subjective and objective assessments of the effect of orthokeratology—a cross-sectional study. Curr Eye Res 2004;28:121–7.

17. Chan B, Cho P, Cheung SW. Orthokeratology practice in children in a university clinic in Hong Kong. Clin Exp Optom 2008;91:453–60.

18. Cheung SW, Cho P, Chan B. Astigmatic changes in orthokeratology. Optom Vis Sci 2009;86:1352–8.

19. Mountford J, Pesudovs K. An analysis of the astigmatic changes induced by accelerated orthokeratology. Clin Exp Optom 2002;85:284–93.

20. Chen C, Cho P. Toric orthokeratology for high myopic and astigmatic subjects for myopic control. Clin Exp Optom 2012;95:103–8.

21. Chan B, Cho P, de Vecht A. Toric orthokeratology: a case report. Clin Exp Optom 2009;92:387–91.

22. Beerten R, Christie C, Sprater N, Ludwig F. Improving orthokeratology results in astigmatism. Poster presented at the Global Orthokeratology Symposium, Chicago, IL, July 29–31, 2005.

23. Baertschi M. Short and long term success with correction of high astigmatism in OK. Poster presented at the Global Orthokeratology Symposium, Chicago, IL, July 29–31, 2005.

24. Mountford J. Orthokeratology. In: Phillips AJ, Speedwell L, eds. Contact Lenses, 4th ed. Boston, MA: Butterworth-Heinemann; 1997:653–92.

25. Mountford J, Noack D. Corneal topography and orthokeratology: post-fit assessment. Contact Lens Spectrum 2002;17:6.

26. Efron N, Morgan PB, Katsara SS. Validation of grading scales for contact lens complications. Ophthalmic Physiol Opt 2001;21:17–29.

27. Thibos LN, Wheeler W, Horner D. Power vectors: an application of Fourier analysis to the description and statistical analysis of refractive error. Optom Vis Sci 1997;74:367–75.

28. Hogan RN. Potential for transmission of prion disease by contact lenses: an assessment of risk. Eye Contact Lens 2003;29:S44–8.

29. Alharbi A, Swarbrick HA. The effects of overnight orthokeratology lens wear on corneal thickness. Invest Ophthalmol Vis Sci 2003;44:2518–23.

30. Sorbara L, Fonn D, Simpson T, Lu F, Kort R. Reduction of myopia from corneal refractive therapy. Optom Vis Sci 2005;82:512–8.

31. Rah MJ, Jackson JM, Jones LA, Marsden HJ, Bailey MD, Barr JT. Overnight orthokeratology: preliminary results of the Lenses and Overnight Orthokeratology (LOOK) study. Optom Vis Sci 2002;79:598–605.

32. Tahhan N, Du Toit R, Papas E, Chung H, La Hood D, Holden AB. Comparison of reverse-geometry lens designs for overnight orthokeratology. Optom Vis Sci 2003;80:796–804.

33. McMahon T. Dellen and dimple veiling. Contact Lens Spectrum 2002;17:52.

34. Dave T. Advanced orthokeratology. Part 3: aftercare and problem solving. Optician 2002;224:32–9.

35. Boost MV, Cho P. Microbial flora of tears of orthokeratology patients, and microbial contamination of contact lenses and contact lens accessories. Optom Vis Sci 2005;82:451–8.

36. Cho P, Cheng SY, Chan WY, Yip WK. Soft contact lens cleaning: rub or no-rub? Ophthalmic Physiol Opt 2009;29:49–57.

toric design; orthokeratology; astigmatism; myopia; myopia control

© 2012 American Academy of Optometry


Search for Similar Articles
You may search for similar articles that contain these same keywords or you may modify the keyword list to augment your search.