There was a significant association between the changes in signed SA and refractive error from baseline immediately after the lens removal (r = 0.603, p = 0.005, the bivariate regression is shown in Fig. 9). The larger the myopic shift (hyperopic correction), the greater the amount of negative SA induced by the CRTH® lenses.
Rigid contact lenses can be used to correct hyperopia by steepening the central cornea.11,14 However, previous studies12,13 have not unequivocally shown that steep lenses steepened the corneal curvature, perhaps because of lens fitting and/or lens design differences. In addition, because of changes in corneal curvature, an initially steep lens might not be steep after it is worn for some time. Recently, Swarbrick et al.14 reported that lenses with base curves about 0.3 mm steeper than the flattest K could successfully correct hyperopia or induce a myopic shift, about 0.4 D, after 4 hours of PMMA lens wear, but not in the Boston XO lens. In the current study, the selected base curve was 0.7 mm steeper than the flat K, and the depth of the mid-peripheral return zone was 175 μm deeper than the calculated return zone depth, which determined the final sagittal depth of the initial lens.27 The lenses were worn for a single night for about 8 to 9 hours. As a result, the cornea steepened centrally and flattened paracentrally (Figs. 2 and 3). There was a predictable increase in the defocus component, in myopic direction (Fig. 5), and refractive error became more myopic or less hyperopic (1.23 D on average) (Fig. 4), suggesting that CRTH® could be used to treat hyperopia. Furthermore, after one night of CRTH® lens wear, the optical effects did not return to baseline by 12 hours (except SA which lasted 6 h), indicating that this overnight lens wear modality may be feasible for retaining reasonable daytime vision.
Change in corneal shape alters the ocular aberration structure. After one night of hyperopic corneal reshaping, the defocus component increased because of the mostly myopic subjects enrolled (Fig. 5). As might be expected, the HOAs, including coma and SA, increased (Figs. 6 to 8) but astigmatism did not change.
SA was the major component of HOAs induced after CRTH®, increasing by a factor of 4.07. Similar to hyperopic corneal refractive surgery,15,28 signed SA shifted from positive to negative after one night of CRTH® lens wear. SA is relatively low in the population,29–31 and this low SA is believed to be largely a result of the balance of the corneal shape and the crystalline lens.32,33 It has been hypothesized that the cornea reduces overall ocular SA by its aspheric prolate shape, which is steeper centrally and flatter in the periphery,34,35 and perhaps by variation of the refractive index across the cornea.36 CRTH® corrects hyperopia by steepening the central cornea and flattening the paracentral region. This shape change alters the positive30,31,37 or negative32,38 corneal SA to be less positive or more negative after hyperopic corneal reshaping. The balance of SA between the cornea and crystalline lens was disturbed, shifting ocular SA from positive to negative. Furthermore, uneven epithelial distribution (discussed later) may also alter the refractive index across the cornea, contributing to the imbalance of the SA.
The amount of the signed SA change was associated with the refractive error change. Similar to hyperopic corneal refractive surgery15,39 and myopic nonsurgical corneal reshaping,22 the increment of SA was significantly related to the change of the refractive error immediately after the lens removal. The greater the amount of the refractive error corrected, the more the negative SA induced (Fig. 9).
Increased coma may have been caused by slight lens decentration. Topography data in this study showed the center of the treatment zone displaced 0.47 ± 0.30 mm temporally and 0.09 ± 0.27 mm inferiorly on average. This decentration outcome is comparable to the myopic corneal reshaping.40 Lid-lens interaction has been hypothesized to affect the lens centration during blinking,41 especially the forced and squeezed blinks from lens discomfort immediately after rigid lens insertion. Coma caused by lens decentration after myopic corneal reshaping has been reported,21,22,40 and it also happened after hyperopic corneal reshaping in this study. Therefore, decentration-induced coma may be difficult to completely avoid in corneal reshaping.
After one night of CRTH® lens wear, the central cornea steepened and paracentral cornea flattened (Figs. 2 and 3). We hypothesize that the compression from the junction (“knee”) between the return zone and the optic zone (Fig. 1) presses the epithelium and forces it to migrate centrally (toward the optic zone) and also squeezes the epithelium to the mid-periphery (toward the return zone). Central and mid-peripheral negative forces (the space between the lens and cornea generating capillary suction) are also hypothesized to drive the tissue toward the center and mid-periphery. The effect of lid tension42,43 through the contact lens may induce squeeze pressure on paracentral epithelial cells through the “knee” although the eyelid tension may be low during sleep. In addition, Allaire and Flack44 demonstrated that different thickness tear film profiles between a contact lens and the cornea induced different hydraulic forces or pressures on the cornea. Similarly, Mountford45 simulated a tear film profile to explain the hydraulic force underneath the myopic corneal reshaping lens. The different hydraulic forces resulting from the uneven tear film profile underneath the lens can also be assumed in hyperopic corneal reshaping. These hypotheses have been supported by a corneal morphological study46 suggesting that corneal reshaping was caused by epithelium accumulation centrally, and by histological data in cats47,48 suggesting that the epithelium was thicker centrally47,48 and thinner paracentrally.47
Full recovery is a critical clinical issue in corneal reshaping. In this study, the corneal shape and all optical parameters had returned to baseline by 28 hours, indicating that CRT® for hyperopia was reversible. This temporary effect is a drawback in nonsurgical corneal reshaping, but it may also be attractive for candidates who are concerned about the safety of corneal refractive surgery.
Diurnal variation of ocular aberrations and corneal shape might potentially have affected the outcome in this study. The contralateral eyes without lenses served as controls and no diurnal variation of aberrations was found in these eyes during this study, suggesting that the robust treatment effect was valid in experimental eyes. Although the control cornea was slightly flatter (0.18 ± 0.05 D) than baseline after one night of sleep (consistent with previous experiments49,50), this corneal flattening disappeared by 3 hours of eyes being open. In addition, the profile of the change of corneal curvature in experimental and control eyes was significantly different, i.e., the central corneal steepening and paracentral flattening in experimental eyes, and no significant location effect in control eyes (Figs. 2 and 3), also indicating an effective corneal reshaping in experimental eyes.
Our result of no diurnal variation of HOAs in control eyes, especially coma and SA, is in accord with the report of Mierdel et al.,51 who demonstrated no change of Zernike coefficients during the day in 22 eyes, except for the coefficient z4±2 (quantifying secondary astigmatism 90/180). This lack of diurnal variation in aberrations reflected relatively stable corneal shape at different corneal locations in the control eyes over time (Fig. 2).
There are a number of issues that might warrant further investigation. An example of this is the large 95% confidence intervals in the experimental eyes (Figs. 4 to 8) suggesting high treatment variability. The cause of this needs to be determined to make the clinical outcome more predictable. For instance, lens decentration might lead the “knee” to touch the central cornea, resulting in a myopic-like correction, an opposite effect. In addition, only one night of lens wear might be a source of transient variability. For example, as in myopic corneal reshaping,52 central topographic irregularities that appear as “central islands” and that occur in the earlier stage of treatment might resolve over time. A second issue is that the majority of subjects in this study were myopes, whose corneal shapes perhaps are not exactly the same as those of hyperopes,53,54 and who, theoretically, may have subtly different ocular structure. Therefore, further work is needed to clarify the treatment effect on hyperopes. A third issue relates to long-term effects beyond a single night that need to be investigated. Finally, the safety of these lenses should be evaluated in a much lager clinical trial.
After one night of CRTH® lens wear, CRT® steepens the central cornea and flattens the paracentral region, altering the ametropia by inducing a myopic shift. It therefore appears to be effective for correcting hyperopia. HOAs increased in predictable ways and the optical effects did not return to baseline by 12 hours, but did so by 28 hours. No significant diurnal variation in optical performance was found in control eyes.
This work was supported by Paragon Vision Sciences and Canadian Optometric Education Trust Fund. Lu is a recipient of the Ontario Graduate Scholarship, Ontario, Canada.
This study was presented in part at the annual meeting of American Academy of Optometry, December 2004, Tampa, FL.
None of the authors of this study have any financial or other interests/arrangements with the products/companies mentioned in the manuscript.
1. Dave T, Ruston D. Current trends in modern orthokeratology. Ophthalmic Physiol Opt 1998;18:224–33.
2. Nichols JJ, Marsich MM, Nguyen M, Barr JT, Bullimore MA. Overnight orthokeratology. Optom Vis Sci 2000;77:252–9.
3. Swarbrick HA, Wong G, O'Leary DJ. Corneal response to orthokeratology. Optom Vis Sci 1998;75:791–9.
4. Sridharan R, Swarbrick H. Corneal response to short-term orthokeratology lens wear. Optom Vis Sci 2003;80:200–6.
5. Alharbi A, Swarbrick HA. The effects of overnight orthokeratology lens wear on corneal thickness. Invest Ophthalmol Vis Sci 2003;44:2518–23.
6. 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.
7. Wang J, Fonn D, Simpson TL, Sorbara L, Kort R, Jones L. Topographical thickness of the epithelium and total cornea after overnight wear of reverse-geometry rigid contact lenses for myopia reduction. Invest Ophthalmol Vis Sci 2003;44:4742–6.
8. Sorbara L, Fonn D, Simpson T, Lu F, Kort R. Reduction of myopia from corneal refractive therapy. Optom Vis Sci 2005;82:512–18.
9. Lui WO, Edwards MH, Cho P. Contact lenses in myopia reduction—from orthofocus to accelerated orthokeratology. Cont Lens Anterior Eye 2000;23:68–76.
10. Lu F, Sorbara L, Kort R, Fonn D, Simpson T, Jones L. Topographic keratometric effects of corneal refractive therapy after one night of lens wear. Invest Ophthalmol Vis Sci 2003;44:E-abstract 3699.
11. Jessen G. Orthofocus techniques. Contacto 1962;6:200–4.
12. Sarver MD, Harris MG. Corneal lenses and “spectacle blur.” Am J Optom Arch Am Acad Optom 1967;44:502–4.
13. Hill JF, Rengstorff RH. Relationship between steeply fitted contact lens base curve and corneal curvature changes. Am J Optom Physiol Opt 1974;51:340–2.
14. Swarbrick HA, Hiew R, Kee AV, Peterson S, Tahhan N. Apical clearance rigid contact lenses induce corneal steepening. Optom Vis Sci 2004;81:427–35.
15. Llorente L, Barbero S, Merayo J, Marcos S. Total and corneal optical aberrations induced by laser in situ keratomileusis for hyperopia. J Refract Surg 2004;20:203–16.
16. Chalita MR, Krueger RR. Correlation of aberrations with visual acuity and symptoms. Ophthalmol Clin North Am 2004;17:135–42, v–vi.
17. Marcos S, Barbero S, Llorente L, Merayo-Lloves J. Optical response to LASIK surgery for myopia from total and corneal aberration measurements. Invest Ophthalmol Vis Sci 2001;42:3349–56.
18. Marcos S. Aberrations and visual performance following standard laser vision correction. J Refract Surg 2001;17:S596–601.
19. Moreno-Barriuso E, Lloves JM, Marcos S, Navarro R, Llorente L, Barbero S. Ocular aberrations before and after myopic corneal refractive surgery: LASIK-induced changes measured with laser ray tracing. Invest Ophthalmol Vis Sci 2001;42:1396–403.
20. Oshika T, Klyce SD, Applegate RA, Howland HC, El Danasoury MA. Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol 1999;127:1–7.
21. Joslin CE, Wu SM, McMahon TT, Shahidi M. Higher-order wavefront aberrations in corneal refractive therapy. Optom Vis Sci 2003;80:805–11.
22. Hiraoka T, Matsumoto Y, Okamoto F, Yamaguchi T, Hirohara Y, Mihashi T, Oshika T. Corneal higher-order aberrations induced by overnight orthokeratology. Am J Ophthalmol 2005;139:429–36.
23. Berntsen DA, Barr JT, Mitchell GL. The effect of overnight contact lens corneal reshaping on higher-order aberrations and best-corrected visual acuity. Optom Vis Sci 2005;82:490–7.
24. Applegate RA, Thibos LN, Bradley A, Marcos S, Roorda A, Salmon TO, Atchison DA. Reference axis selection: subcommittee report of the OSA Working Group to establish standards for measurement and reporting of optical aberrations of the eye. J Refract Surg 2000;16:S656–8.
25. Thibos LN, Applegate RA, Schwiegerling JT, Webb R. Standards for reporting the optical aberrations of eyes. J Refract Surg 2002;18:S652–60.
26. York D. Least-squares fitting of a straight line. Can J Phys 1966;44:1079–86.
27. Sorbara L, Lu F, Fonn D, Simpson T. Refractive and keratometric effects of corneal refractive therapy for hyperopia after one night of lens wear. Optom Vis Sci 2004;81 (Suppl):72.
28. Yoon G, Macrae S, Williams DR, Cox IG. Causes of spherical aberration induced by laser refractive surgery. J Cataract Refract Surg 2005;31:127–35.
29. Williams D, Yoon GY, Porter J, Guirao A, Hofer H, Cox I. Visual benefit of correcting higher order aberrations of the eye. J Refract Surg 2000;16:S554–9.
30. He JC, Gwiazda J, Thorn F, Held R. Wave-front aberrations in the anterior corneal surface and the whole eye. J Opt Soc Am A Opt Image Sci Vis 2003;20:1155–63.
31. Kelly JE, Mihashi T, Howland HC. Compensation of corneal horizontal/vertical astigmatism, lateral coma, and spherical aberration by internal optics of the eye. J Vis 2004;4:262–71.
32. Artal P, Guirao A. Contributions of the cornea and lens to the aberrations of the human eye. Opt Lett 1998;23:1713–15.
33. Artal P, Guirao A, Berrio E, Williams DR. Compensation of corneal aberrations by the internal optics in the human eye. J Vis 2001;1:1–8.
34. Somani S, Tuan KA, Chernyak D. Corneal asphericity and retinal image quality: a case study and simulations. J Refract Surg 2004;20:S581–5.
35. Yebra-Pimentel E, Gonzalez-Jeijome JM, Cervino A, Giraldez MJ, Gonzalez-Perez J, Parafita MA. Corneal asphericity in a young adult population. Clinical implications. Arch Soc Esp Oftalmol 2004;79:385–92.
36. Vasudevan B. Regional variation in refractive index of the bovine and human cornea. Master's Thesis. Waterloo, Canada: University of Waterloo; 2005.
37. Millodot M, Sivak J. Contribution of the cornea and lens to the spherical aberration of the eye. Vision Res 1979;19:685–7.
38. Guirao A, Redondo M, Artal P. Optical aberrations of the human cornea as a function of age. J Opt Soc Am A Opt Image Sci Vis 2000;17:1697–702.
39. Ma L, Atchison DA, Albietz JM, Lenton LM, McLennan SG. Wavefront aberrations following laser in situ keratomileusis and refractive lens exchange for hypermetropia. J Refract Surg 2004;20:307–16.
40. Yang X, Gong XM, Dai ZY, Wei L, Li SX. Topographical evaluation on decentration of orthokeratology lenses. Zhonghua Yan Ke Za Zhi 2003;39:335–8.
41. Carney LG, Mainstone JC, Carkeet A, Quinn TG, Hill RM. Rigid lens dynamics: lid effects. CLAO J 1997;23:69–77.
42. Lieberman DM, Grierson JW. The lids influence on corneal shape. Cornea 2000;19:336–42.
43. Ehrmann K, Francis I, Stapleton F. A novel instrument to quantify the tension of upper and lower eyelids. Cont Lens Anterior Eye 2001;24:65–72.
44. Allaire PE, Flack RD. Squeeze forces in contact lenses with a steep base curve radius. Am J Optom Physiol Opt 1980;57:219–27.
45. Mountford J. A model of forces acting in orthokeratology. In: Mountford J, Ruston D, Dave T, eds. Orthokeratology: Principles and Practice. Philadelphia: Butterworth-Heinemann;2004:269–302.
46. Haque S, Fonn D, Sorbara L, Simpson T. Corneal and epithelial thickness changes following one night of CRT gas permeable lens wear for hyperopia, measured with optical coherence tomography. Optom Vis Sci 2004;81 (Suppl):27.
47. Choo JD, Caroline PJ, Harlin DD, Meyers W. Morphologic changes in cat epithelium following overnight lens wear with Paragon CRT lens for corneal reshaping. Invest Ophthalmol Vis Sci 2004;45:E-abstract 1552.
48. Hughes B, Caroline PJ, Choo J, Gondo M, Bergmanson JPG. 24 hour orthok-induced epithelial alterations in cats. Optom Vis Sci 2004;81 (Suppl):73.
49. Kiely PM, Carney LG, Smith G. Diurnal variations of corneal topography and thickness. Am J Optom Physiol Opt 1982;59:976–82.
50. Cronje S, Harris WF. Short-term keratometric variation in the human eye. Optom Vis Sci 1997;74:420–4.
51. Mierdel P, Krinke HE, Pollack K, Spoerl E. Diurnal fluctuation of higher order ocular aberrations: correlation with intraocular pressure and corneal thickness. J Refract Surg 2004;20:236–42.
52. Mountford J: History and general principles. In: Mountford J, Ruston D, Dave T, eds. Orthokeratology: Principles and Practice. Philadelphia: Butterworth-Heinemann;2004:1–17.
53. Llorente L, Barbero S, Cano D, Dorronsoro C, Marcos S. Myopic versus hyperopic eyes: axial length, corneal shape and optical aberrations. J Vis 2004;4:288–98.
54. Carney LG, Mainstone JC, Henderson BA. Corneal topography and myopia. A cross-sectional study. Invest Ophthalmol Vis Sci 1997;38:311–20.