Influence of induced decentered orthokeratology lens on ocular higher-order wavefront aberrations and contrast sensitivity function

ARTICLE

Influence of induced decentered orthokeratology lens on ocular higher-order wavefront aberrations and contrast sensitivity function Takahiro Hiraoka, MD, Toshifumi Mihashi, PhD, Chikako Okamoto, MD, Fumiki Okamoto, MD, Yoko Hirohara, PhD, Tetsuro Oshika, MD

PURPOSE: To quantitatively evaluate the effect of overnight orthokeratology lenses intentionally left decentered after 3 months of wear and assess the influence on clinical outcomes such as ocular higher-order wavefront aberrations and contrast sensitivity function. SETTING: Department of Ophthalmology, Tsukuba University Hospital, Ibaraki, Japan. METHODS: This prospective study assessed refraction, visual acuity, corneal topography, wavefront aberration, and contrast sensitivity function before and 3 months after overnight orthokeratology treatment. Decentration of the treatment zone from the center of the entrance pupil was determined using computerized videokeratography (TMS-4) and data-analysis software (MatLab). The relationship between decentration and the clinical parameters was analyzed. RESULTS: The mean age of the 23 patients (46 eyes) was 24.2 yearsG3.3 (SD) and the mean spherical equivalent refraction before treatment, 2.38G0.98 diopters. The mean magnitude of decentration (0.85G0.51 mm) was statistically significantly correlated with the amount of myopic correction (P<.05), increases in coma-like aberration (P<.01), increases in spherical-like aberration (P<.01), and reductions in contrast sensitivity function (P<.0001). Changes in contrast sensitivity function were also statistically significantly correlated with the amount of myopic correction (P<.05), changes in coma-like aberration (P<.01), and changes in spherical-like aberration (P<.01). Stepwise multiple regression analysis showed that the magnitude of decentration was the only explanatory variable related to contrast sensitivity function (P<.0001). CONCLUSION: Decentered treatment of orthokeratology resulted in decreased contrast sensitivity after treatment, showing that centration of the procedure is crucial to good outcomes. J Cataract Refract Surg 2009; 35:1918–1926 Q 2009 ASCRS and ESCRS

Orthokeratology, also called corneal reshaping or corneal refractive therapy, is a nonsurgical approach to temporarily decreasing the amount of myopic refractive error and improving uncorrected vision by the programmed application of rigid contact lenses. Modern orthokeratology using sophisticated contact lenses with a reverse-geometry design can provide faster, larger, and more predictable refractive changes1–4 than lenses used in the original method introduced in the early 1960s.5 The presumed mechanisms of myopic reduction in orthokeratology include central corneal flattening, thinning of the central corneal epithelium, and thickening of the midperipheral cornea, all of which are accomplished by wearing reverse-geometry lenses that have a central flat curve followed by a very steep reverse curve.6,7 With the advance of gas-permeable materials in the 1980s, it 1918

Q 2009 ASCRS and ESCRS Published by Elsevier Inc.

became possible for patients to wear contact lenses while sleeping, a treatment called overnight orthokeratology. Overnight orthokeratology can decrease the patient’s need to wear contact lenses or spectacles in the daytime by providing acceptable vision during these hours. Many studies report the efficacy of overnight orthokeratology, although continued lens wear is required to maintain the effect.1,2,4,8 In recent years, the procedure has gained acceptance by practitioners and patients as a treatment for the correction of myopia and will likely become a popular alternative to spectacles, daily-wear contact lenses, or corneal refractive surgery, especially in the case of low to moderate myopia. However, several studies report that orthokeratology increases higher-order aberrations (HOAs) of the cornea9,10 and the eye11–15 and decreases contrast 0886-3350/09/$dsee front matter doi:10.1016/j.jcrs.2009.06.018

DECENTERED ORTHOKERATOLOGY: EFFECT ON HOAs AND CONTRAST SENSITIVITY

sensitivity function,12,13 similar to corneal refractive surgery procedures for myopia, such as photorefractive keratectomy (PRK)16,17 and laser in situ keratomileusis (LASIK).18,19 Orthokeratology is similar to PRK and LASIK in terms of the achieved corneal shape, which shows oblate asphericity with a flat central area and increasing power toward the periphery. In PRK and LASIK, a decentered laser ablation from the center of the entrance pupil is considered to be a main cause of reduced optical quality and visual performance postoperatively.20–27 However, the effect of decentered orthokeratology treatment on optical quality and visual performance has not been studied. We performed a prospective study to evaluate the relationship between the magnitude of treatment decentration and various clinical outcomes, including ocular wavefront HOAs and contrast sensitivity function, in eyes treated by overnight orthokeratology. PATIENTS AND METHODS This prospective study comprised consecutive patients being treated by overnight orthokeratology at Tsukuba University Hospital. The hospital’s institutional review board approved the study protocol, which followed the tenets of the Declaration of Helsinki. All participants provided written informed consent after receiving a full explanation of the nature and possible consequences of the study. Inclusion criteria were a spherical equivalent (SE) refraction between –1.00 diopter (D) and –4.00 D, refractive astigmatism of 1.00 D or less, a mean keratometry (K) reading between 40.00 D and 46.25 D, corrected distance visual acuity (CDVA) of 20/20 or better, age between 20 years and 37 years, no ocular or systemic pathology, and no previous orthokeratology treatment. Patients were instructed not to wear contact lenses for at least 3 weeks before the pretreatment evaluation.

Lenses The orthokeratology lenses (DreimLens, DreimLens Corp. Ltd.) used in the study were of hexafocon A (Boston XO, Submitted: September 18, 2008. Final revision submitted: June 24, 2009. Accepted: June 30, 2009. From the Department of Ophthalmology (Hiraoka, C. Okamoto, F. Okamoto, Oshika), Institute of Clinical Medicine, University of Tsukuba, Ibaraki, and Technical Research Institute (Mihashi, Hirohara), Topcon Corp., Tokyo, Japan. Drs. Mihashi and Hirohara are employees of Topcon Corp. No other author has a financial or proprietary interest in any material or method mentioned. Corresponding author: Takahiro Hiraoka, MD, Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, 1-1-1, Tennoudai, Tsukuba, Ibaraki, 305-8575 Japan. E-mail: [email protected].

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Polymer Technology Corp.) with an oxygen permeability of 100. The lens was approved for overnight orthokeratology by the Japanese Ministry of Health, Labour and Welfare. The lens has a 4-zone reverse-geometry design that consists of a 6.0 mm wide base curve (central optical zone), 0.6 mm wide reverse curve, 1.0 mm wide alignment curve, and 0.4 mm wide peripheral curve. The lenses were selected according to the manufacturer’s fitting guidelines, which have been described in detail.13 In brief, the alignment curve was chosen based on corneal keratometry using the flatter K value. The base curve was determined as the target power plus 0.75 D. A total lens diameter of 10.0 mm was chosen for each eye. After insertion of a trial lens, the fit was assessed at the slitlamp to verify proper lens centration, a bull’s-eye fluorescein pattern showing a 3.0 to 5.0 mm wide central touch and midperipheral pooling, and adequate lens movement (ie, approximately 1.0 mm) on blinking. Lenses that were decentered superiorly were considered to be loose and those decentered inferiorly, tight. In eyes with apparent lateral decentration, the total lens diameter was increased to 10.6 mm. When the contact lens fitting was satisfactory, the final lens was ordered. After the lenses were dispensed, patients began wearing them every night while sleeping. They were asked to wear the lenses no fewer than 7 hours a night. If corneal topographic changes occurred or improvement in uncorrected distance visual acuity (UDVA) was poor after initiation of treatment, the lens design was modified. At each follow-up visit, a detailed slitlamp evaluation, including fluorescein staining, was performed to monitor ocular health and the effects of lens wear.

Patient Evaluations Examinations were performed before (baseline) and 3 months after orthokeratology treatment began. Evaluation included refraction, keratometry, UDVA and CDVA measurements, corneal topography, wavefront aberrometry, and contrast sensitivity testing. To minimize the effect of diurnal variation, all measurements were performed between 9 AM and 11 AM and patients were asked to remove the lenses from 2 to 4 hours before the examinations. Corneal topography was performed by computerized videokeratography (TMS-4, Tomey Corp.). The measurements were repeated at least 3 times; the best-focused properly aligned image was selected. Decentration was analyzed using an instantaneous power map (tangential plot) with the Klyce/Wilson scale, which displays powers from 28.00 to 65.50 D in 1.50 D intervals; this allowed the central flattened area to be discriminated from the surrounding cornea as a circular area of different power. On the map obtained at the 3-month visit, 16 points were plotted along an equalrefractive-power line surrounding the central flattened area. On the basis of these plotted points, an ellipse and its center were approximated using a purpose-designed data-analysis program written in a numerical computing environment and programming language (MatLab, The MathWorks, Inc.). The pupillary center determined by the videokeratographer’s software was used as the reference point. The magnitude of decentration was calculated as the distance between the center of an approximate ellipse and the center of the entrance pupil (Figure 1). The meridian degree of decentration from the pupillary center was also measured. To assess the reproducibility of this method, the magnitude of decentration was measured 10 times for 12 randomly selected eyes. The repeatability of the magnitude

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Figure 1. Assessment of decentration of central flattened area by overnight orthokeratology.

of decentration was determined from the standard deviation of the 10 measurements per eye. Ocular wavefront aberration data were obtained using a Hartmann-Shack wavefront aberrometer (model KR-9000 PW, Topcon Corp.).19,28 The measurement was performed in a dark room through a natural (undilated) pupil. To obtain well-focused and properly aligned Hartmann images, the measurements were repeated at least 4 times in each eye; the 3 best images were chosen and averaged. Higher-order aberrations were computed for a 4.0 mm pupil by expanding the set of orthogonal Zernike polynomials. The magnitudes of the Zernike coefficients were represented as the root mean square (RMS). The RMS of the 3rd-order Zernike coefficients C(3,3) to C(3,3) was used to denote coma-like aberration and the RMS of the 4th-order coefficients C(4,4) to C(4,4), to denote spherical-like aberration. The magnitude and orientation of the combined vector of horizontal C(3,1) and vertical C(3,1) coma aberrations were determined using a Zernike vector-analysis program based on polar coordinate expression of Zernike polynomials. This method of analysis has been described in detail.29 In brief, the magnitude of the combined coma vector is obtained by the formula qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2  2  RMSZ C13 þ C1 3 The angle of the combined coma terms is the arc tangent of coefficients of the vertical coma divided by that of the horizontal coma. This angle (expressed in degrees) is attained by the following formulas: If C1 s0  -1  0 3  1  C C3 !0 axisZtan-1 C31 @  -31    C axisZtan-1 3 þ 180 C1 O0 C13

3

If C1 Z0  -1   3 axisZ90 C !0  3-1  axisZ270 C3 O0 According to these formulas, the orientation of each combined coma vector can be represented as an angle from 0 to 360 degrees. This angle setting coincides with that of the topographic map of the videokeratographer used in the study. This analysis helps define the characteristics of a pair (horizontal and vertical) of coma aberrations.

Contrast sensitivity function was assessed by measuring contrast sensitivity and low-contrast visual acuity (LCVA) using the CSV-1000E chart and the CSV-1000LanC10% chart (both VectorVision), respectively. The measurements were performed monocularly with undilated pupils at 2.5 m with full spectacle correction. Contrast sensitivity was determined at 3, 6, 12, and 18 cycles per degree (cpd), and the area under the log contrast sensitivity function (logCS) was calculated according to the method of Applegate et al.30 In brief, the area under the logCS was determined as the integration of the fitted 3rd-order polynomials of the log contrast sensitivity units between the fixed limits of 0.48 (corresponding to 3 cpd) and 1.26 (18 cpd) on the log spatial frequency scale. The CSV-1000LanC10% chart uses the Landolt ring as the optotype under 10% low contrast. It presents 5 letters per line; a 1-line step represents a change of 0.1 logMAR units. Low-contrast visual acuity was scored by giving a credit of 0.02 logMAR units for each letter correctly identified.

Statistical Analysis Data obtained at the 3-month follow-up visit were compared with the baseline data using the paired t test. The magnitude of decentration of the orthokeratology treatment zone was analyzed in relation to the clinical outcomes by the Pearson correlation (r) test. Stepwise multiple regression analysis was performed to assess which factors significantly affected contrast sensitivity function. The dependent variables were the area under the logCS and the LCVA after treatment. The explanatory variables included patient age, amount of myopic correction, and magnitude of decentration and posttreatment HOAs (3rd- and 4th-order RMS). The amount of myopic correction was defined as the reduction in the manifest SE refraction at the 3-month visit. A P value less than 0.05 was considered statistically significant.

RESULTS Forty-six eyes of 23 consecutive patients (12 men, 11 women) were evaluated. The mean age of the patients was 24.2 yearsG3.3 (SD) (range 21 to 33 years) and the mean SE before treatment, 2.38G0.98 D (range 4.00 to 1.00 D). Table 1 shows the patients’ data at baseline and 3 months after the start of orthokeratology. At 3 months, the manifest refraction was statistically significantly lower and the UDVA statistically significantly better (both P!.0001). The changes in CDVA (PZ.4895) and in regular astigmatism were not statistically significant (PZ.1131). The treatment resulted in a significant increase in 3rd-order RMS and 4th-order RMS values (both P!.0001). After treatment, there was a statistically significant decrease in contrast sensitivity function; that is, in the area under the logCS (P!.0001) and in LCVA (PZ.0025). The mean magnitude of decentration of the orthokeratology treatment zone from the pupillary center was 0.85G0.51 mm (range 0.17 to 2.05 mm). Figures 2 to 6 show the correlations between the mean magnitude of decentration and clinical outcomes as well as the r and P values. The magnitude of decentration

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Parameter Manifest refraction (D) Mean G SD Range Regular astigmatism (D) Mean G SD Range UDVA (logMAR) Mean G SD Range CDVA (logMAR) Mean G SD Range 3rd-order RMS (mm) Mean G SD Range 4th-order RMS (mm) Mean G SD Range Area under logCS Mean G SD Range LCVA (logMAR) Mean G SD Range

Baseline

After Treatment

2.38 G 0.98 4.00 to 1.00

0.24 G 0.71* 2.75 to 1.25

0.14 G 0.23 0.75 to 0.00

0.07 G 0.20 0.75 to 0.00

0.77 G 0.31 0.22 to 1.52 0.10 G 0.07 0.30 to 0.00

0.03 G 0.16* 0.18 to 0.40 0.09 G 0.06 0.18 to 0.15

0.074 G 0.028 0.022 to 0.133

0.259 G 0.150* 0.029 to 0.758

0.038 G 0.020 0.010 to 0.101

0.134 G 0.061* 0.037 to 0.275

1.451 G 0.120 1.177 to 1.615

1.291 G 0.177* 0.834 to 1.570

0.02 G 0.09 0.20 to 0.24

0.11 G 0.14† 0.18 to 0.50

CDVA Z corrected distance visual acuity; LCVA Z low-contrast visual acuity; logCS Z log contrast sensitivity function; RMS Z root mean square; UDVA Z uncorrected distance visual acuity *P!.0001 (paired t test), baseline versus after treatment † P!.01 (paired t test), baseline versus after treatment

was significantly correlated with the amount of myopic correction and with changes in 3rd-order RMS, 4th-order RMS, area under the logCS, and LCVA. There was no significant correlation between the magnitude of decentration and changes in CDVA (rZ0.284, PZ.0567). The mean standard deviation (10 independent measurements of a single eye) of the magnitude of decentration was G0.018 mm (range 0.008 to 0.023 mm), showing sufficient reproducibility. Changes in the area under the logCS after treatment were significantly correlated with the amount of myopic correction (rZ0.462, PZ.0010), change in 3rd-order RMS (rZ0.430, PZ.0026), and change in 4th-order RMS (rZ0.418, PZ.0035). The change in LCVA at 3 months was also statistically significantly correlated with the amount of myopic correction (rZ0.308, PZ.0365), change in 3rd-order RMS (rZ0.423, PZ.0031), and change in 4th-order RMS (rZ0.425, PZ.0029). The change in ocular HOAs was significantly correlated with the amount of myopic correction (rZ0.650, P!.0001 for 3rd-order RMS; rZ0.582, P!.0001 for 4th-order RMS).

Amount of myopic correction (D)

Table 1. Clinical data at baseline and 3 months after overnight orthokeratology.

r = 0.332 P = 0.0269

5

4

3

2

1

0 0

0.5

1

1.5

2

2.5

Magnitude of decentration (mm) Figure 2. Relationship between amount of myopic correction and magnitude of decentration.

Stepwise multiple regression analysis showed that the magnitude of decentration was the only explanatory variable related to the posttreatment area under the logCS (P!.0001, r2Z0.298) and posttreatment LCVA (P!.0001, r2Z0.327). The mean meridian degree of decentration on corneal topography was 218G85 degrees (range 52 to 359 degrees) in right eyes and 272G112 degrees (21 to 359 degrees) in left eyes. For horizontal displacement, temporal decentration was observed in 34 eyes (74%). For vertical displacement, inferior decentration was observed in 34 eyes (74%). For overall displacement, decentration toward the inferotemporal quadrant was observed in 26 eyes (57%) (Figure 7). On Zernike vector analysis, the mean combined vector of horizontal and vertical coma aberrations was 0.254G0.170 mm (range 0.064 to 0.776 mm) at 215G64 degrees (range 59 to 309 degrees) in right eyes and 0.265G0.156 mm (range 0.035 to 0.579 mm) at 253G105 degrees (range 11 to 340 degrees) in left eyes. The magnitude of the combined coma vector was significantly correlated with the magnitude of topographically measured decentration (Figure 8). The angle of the combined coma vector was also significantly correlated with that of topographical decentration from the pupillary center (Figure 9). DISCUSSION In corneal refractive surgery for myopia, decentration of the ablation zone can cause visual disturbances such as loss of CDVA,21,27,31 poor contrast sensitivity,21 and monocular diplopia.22 In addition, several studies23–26 show that decentered laser treatment is associated with increases in HOAs and that this

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0.3

0.8 r = 0.467 P = 0.0009

r = 0.450 P = 0.0015

0.25

Changes in 4th-order RMS (µm)

Changes in 3rd-order RMS (µm)

0.7 0.6 0.5 0.4 0.3 0.2 0.1

0.2 0.15 0.1 0.05 0

0

-0.05

-0.1 0

0.5

1

1.5

2

0

2.5

0.5

1

1.5

2

2.5

Magnitude of decentration (mm)

Magnitude of decentration (mm) Figure 3. Relationship between changes in the 3rd-order RMS and magnitude of decentration (RMSZroot mean square).

Figure 4. Relationship between changes in the 4th-order RMS and magnitude of decentration (RMSZroot mean square).

unfavorably affects quality of vision.18,19 Decentered ablations also have a significant negative correlation with subjective patient satisfaction.32 Therefore, proper centration of the treatment over the center of the entrance pupil is one of the most important technical considerations in corneal refractive surgery.33 Similarly, orthokeratology involves flattening the central corneal curvature by contact lenses to reduce myopia. However, to our knowledge, the effect of decentered orthokeratology treatment on optical quality and visual performance has not been assessed. We performed this study to evaluate the magnitude of decentered orthokeratology treatment and its effect on clinical outcomes.

In our study, the manifest refraction was significantly reduced and UDVA was significantly improved by orthokeratology. However, there were significant increases in coma-like and spherical-like HOAs, a finding that agrees with results in previous studies.9–15 There were also significant decreases in the contrast sensitivity function. Johnson et al.34 report that orthokeratology treatment did not lead to significant changes in contrast sensitivity; however, they examined only 6 eyes for up to 8 days after commencement of treatment. In contrast, larger studies with a longer follow-up show that a decentered orthokeratology lens significantly reduces contrast sensitivity function,12,13 which agrees with our results. In our study,

Changes in low-contrast visual acuity (logMAR)

0.8 r = 0.572 P < 0.0001

0.6

0.4

0.2

0

-0.2

-0.4 0

0.5

1

1.5

2

2.5

Magnitude of decentration (mm) Figure 5. Relationship between changes in the area under the logCS and magnitude of decentration (logCSZlog contrast sensitivity function).

Figure 6. Relationship between changes in LCVA and magnitude of decentration.

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Figure 7. Decentration of treatment zone from the center of the entrance pupil. Each plotted point shows the distance and angle of the center of corneal flattened area from the pupillary center.

there was no difference in CDVA or regular astigmatism between baseline and after treatment. In the current study, the mean decentration of the orthokeratology treatment from the center of the entrance pupil was 0.85G0.51 mm. In previous studies, the mean decentration ranged from 0.33 to 0.78 mm in PRK20,21,31–33,35–39 and from 0.35 to 0.96 mm in LASIK.20,35,39,40 The decentration in our study is similar to that reported in early PRK clinical trials (0.78 mm)37 but larger than that reported in other PRK studies.20,21,31–33,35,36,38,39 Two early LASIK studies found a larger mean decentration (0.96 mm39 and 0.90 mm)20; later LASIK studies with active eye-tracking systems report much smaller values (0.35 mm35 and 0.43 mm40). Tsai and Lin35 propose that in corneal refractive surgery, ablation

0.8

Angle of combined coma vector (degree)

Magnitude of combined coma vector (µm)

0.9 r = 0.552 P < 0.0001

0.7

decentration less than 0.5 mm (mild decentration) is optimum, between 0.5 and 1.0 mm (moderate decentration) is acceptable, and greater than 1.0 mm (severe decentration) should be avoided. Based on this classification, 14 eyes (30%) in our study had mild decentration, 17 (37%) had moderate decentration, and 15 (33%) had severe decentration. In PRK and LASIK studies, the percentage of eyes with mild, moderate, and severe decentration ranges widely (mild, 18% to 85%; moderate, 13% to 59%; severe, 0% to 23%).31–33,35–38,41 In our study, the percentage of eyes with severe decentration was higher than in previous PRK and LASIK studies. Thus, a treatment zone that is severely decentered can be a serious drawback of current orthokeratology methods.

0.6 0.5 0.4 0.3 0.2 0.1

360

270

180

r = 0.535 P < 0.0001

90

0

0 0

0.5

1

1.5

2

2.5

0

180

270

360

Angle of decentration (degree)

Magnitude of decentration (mm) Figure 8. Relationship between magnitude of combined vector of horizontal and vertical coma aberrations and magnitude of decentration.

90

Figure 9. Relationship between angle of combined vector of horizontal and vertical coma aberrations and angle of decentration from the pupillary center.

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Only one previous study evaluated the amount of orthokeratology treatment decentration using corneal topography.42 In that study, the mean decentration from the pupillary center 6 months after treatment began was 0.59G0.39 mm; the percentage of eyes with decentration within G0.5 mm, from 0.5 to 1.0 mm, and of more than 1.0 mm was 48.4%, 36.9%, and 14.7%, respectively. The mean decentration and proportion of eyes with severe decentration were smaller than in our study. The authors also evaluated the relationship between decentration and total lens diameter and found that the smaller the lens, the greater the decentration. The mean decentration was 0.77G0.46 mm in eyes with 10.0 mm diameter lenses, 0.54G0.37 mm in eyes with 10.2 to 10.6 mm lenses, and 0.41G0.28 mm in eyes with 11.0 mm lenses. The mean value with the smallest diameter lens corresponds approximately to our mean decentration value. Because we used 10.0 mm diameter lenses in most cases, the large amount of decentration in our study seems to be attributable to the small diameter of the lenses used for treatment. Moreover, in the previous study, the amount of decentration depended on the initial refractive error,42 which agrees with our results. In our study, there was a relationship between decentration of the orthokeratology treatment zone and visual function. The magnitude of decentration was significantly correlated with a decrease in contrast sensitivity function but not with a decrease in CDVA. These results indicate that conventional visual acuity testing does not necessarily reflect changes in the optical quality of the eye and that contrast sensitivity function is a more sensitive tool to detect changes. We also found a significant correlation between the magnitude of decentration and increases in ocular HOAs. Furthermore, the magnitude of decentration, increase in ocular HOAs, and decrease in contrast sensitivity function induced by orthokeratology were significantly correlated with the amount of myopic correction. These results suggest that orthokeratology increases ocular HOAs and accordingly decreases contrast sensitivity function depending on the magnitude of decentered treatment and that these changes also depend on the amount of myopic correction. To our knowledge, ours is the first study of the relationship between decentered treatment, HOAs, contrast sensitivity function, and myopic correction in patients being treated by orthokeratology. Studies of corneal refractive surgery report a significant relationship between decentered treatment and HOAs23–26 and between decentered treatment and contrast sensitivity function21; however, no study has found a significant correlation between all 3 parameters. Seiler et al.17 suggest that light-scattering structures, such as scars and corneal haze, are responsible for reduced

visual performance and optical aberrations after corneal refractive surgery. In orthokeratology, however, there are usually no scars or corneal haze after treatment. This may be the reason for the significant relationship between HOAs, contrast sensitivity function, and decentered treatment in our study. We found several interrelated parameters, and simple correlation analysis showed contrast sensitivity to be significantly associated with ocular HOAs, the amount of myopic correction, and the magnitude of decentration. Therefore, we used multivariate analysis to further elucidate which factors were mainly related to contrast sensitivity function. The analysis showed magnitude of decentration to be the only variable affecting contrast sensitivity function. This indicates that good centration of the orthokeratology procedure is critical to good outcomes and that decentration decreases quality of vision. Thus, close attention must be paid to proper treatment centration. Regarding the direction of decentration, for overall displacement, decentration tended to be toward the inferotemporal quadrant. Several studies report that the temporal cornea is likely to become flatter than the nasal cornea after orthokeratology.9,43 In addition, decentration of orthokeratology treatment to the temporal side is reported to be more common in eyes with decentration greater than 0.5 mm.42 For horizontal displacement, we found temporal decentration in 74% of eyes, which is similar to results in previous orthokeratology studies, which report a tendency toward a temporal shift. Although the reason for this has not been determined, Yang et al.42 suggest that reverse-geometry lenses are inclined to drift toward the steeper quadrant. The cornea is generally asymmetric, with the temporal cornea being steeper than the nasal cornea in normal eyes43,44; thus, the lenses may shift toward the temporal side. For vertical displacement, we observed inferior decentration in 74% of eyes; there are no other reports of a tendency toward inferior decentration. Although we have no clear explanation for this, we believe Bell’s phenomenon plays a role. Contact lens design and fitting technique may also have a significant effect on the direction of decentration. Further studies of this are needed. We also evaluated the orientation and magnitude of the combined vector of horizontal coma and vertical coma aberrations. The orientation of the combined coma vector was significantly correlated with that derived from the corneal topography analysis. The magnitude of the combined coma vector also showed a significant correlation with the magnitude of the topographically calculated decentration. These findings indicate that a decentered orthokeratology treatment increases coma aberration in the same direction as the direction of decentration.

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One limitation of our study is that we measured lens decentration only once after treatment began. It would be valuable to know how the magnitude and direction of decentration change after orthokeratology. Additional studies should be performed to clarify this point. Another limitation is that we used only 1 design of orthokeratology lens. It is unclear whether orthokeratology lenses with different designs would yield similar results. Further studies with different lenses should be performed. In conclusion, decentered orthokeratology treatment significantly affects visual performance. For better quality of vision, every attempt must be made to ensure good centration of the procedure. Future refinements of the orthokeratology technique and lens design are needed to minimize decentration of the treatment and optimize the patient’s quality of vision.

15.

16.

17.

18. 19.

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First author: Takahiro Hiraoka, MD Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, Ibaraki, Japan