John Mountford

An analysis of the changes in corneal shape and refractive error induced by accelerated orthokeratology

Abstract The lack of predictability in orthokeratology has always been seen as one of the major drawbacks of the procedure. Being able to assess the likely degree of myopia reduction would be a valuable clinical tool in that those patients presenting for orthokeratology who would not be viable candidates could be advised against proceeding with a course of treatment. In this study the pre- and post-orthokeratology refraction, corneal eccentricity, keratometry, and apical corneal power changes were measured to see whether a correlation exists between the pre- and post-treatment corneal shape and refractive changes that could be used as a predictive tool. A good correlation was found between refractive change and corneal eccentricity change ( r 2 = 0.83), apical corneal power change and corneal eccentricity change ( r 2 = 0.84), and refractive change and apical corneal power change ( r 2 = 0.91). A poorer correlation was found between keratometry change and refractive, apical corneal power, and eccentricity change. The shape changes induced in the cornea by orthokeratology were also studied by topographical analysis. The final corneal shape is typically that of a central spherical zone (4.00–5.00 mm chord) surrounded by a mid-peripheral steep zone (5.00–7.50 mm chord) that tends to flatten in curvature as the periphery (8.00 mm chord) is approached.

The performance of four different corneal topographers on normal human corneas and its impact on orthokeratology lens fitting.

Purpose. To evaluate the performances of Humphrey Atlas 991, Orbscan II, Dicon CT200, Medmont E300 on young Chinese adults. Methods. Three sets of corneal topography measurements were obtained from each topographer from 22 subjects—two sets by the same examiner and one set by another examiner on the same day. Results. There were no significant within-examiner and between-examiner differences for any of the parameters tested for each topographer. However, only the repeatability and reproducibility (of apical radius[Ro], eccentricity, and elevation) of the Humphrey and Medmont were good. There was no statistically significant between-topographer difference in Ro, but significant differences in eccentricity and elevation values were found. The number of repeated readings that should be taken for a precision of 2 &mgr;m (elevation) were 12 for the Humphrey and 2 for the Medmont. Conclusions. The performance of both the Humphrey and the Medmont was very good. Roand eccentricity values of different topographers cannot be used interchangeably, but the agreement in elevation values was good for these topographers. The number of repeated readings required for maximum precision varies with the topographer used, and they are not interchangeable.

Peripheral refraction in orthokeratology patients.

Purpose. The purpose of this study is to measure refraction across the horizontal central visual field in orthokeratology patients before and during treatment. Methods. Refractions were measured out to 34° eccentricity in both temporal and nasal visual fields using a free-space autorefractor (Shin-Nippon SRW5000) for the right eyes of four consecutively presenting myopic adult patients. Measurements were made before orthokeratology treatment and during the course of treatment (usually 1 week and 2 weeks into treatment). Refractions were converted into mean sphere (M), 90° to 180° astigmatism (J180), and 45° to 135° astigmatism (J45) components. Results. Before treatment, subjects had either a relatively constant mean sphere refraction across the field or a relative hypermetropia in the periphery as compared with the central refraction. As a result of treatment, myopia decreased but at reduced rate out into the periphery. Most patients had little change in mean sphere at 30° to 34°. In all patients, the refraction pattern altered little after the first week. Conclusion. Orthokeratology can correct myopia over the central ± 10° of the visual field but produces only minor changes at field angles larger than 30°. If converting relative peripheral hypermetropia to relative peripheral myopia is a good way of limiting the axial elongation that leads to myopia, orthokeratology is an excellent option for achieving this.

Predicting Success with Orthokeratology Lens Wear: A Retrospective Analysis of Ocular Characteristics

Purpose Orthokeratology procedures suffer from lack of predictability in the response of individuals. To identify factors contributing to this, we have retrospectively studied a range of ocular parameters in patients with varying outcomes from orthokeratology lens wear. Experimental Design. Three groups were studied: an experimental group (9 subjects wearing Contex OK-3 design orthokeratology contact lenses), and 2 control groups [10 rigid gas permeable (RGP) contact lens wearers and 10 non-contact lens wearers]. Three categories were identified among the orthokeratology group: those responding well, moderately, or poorly to orthokeratology lens wear. Measurements included subjective refraction, intraocular distances, corneal thickness, ocular rigidity, and epithelial fragility. Results. When comparing the three orthokeratology categories, there was no significant difference for central and peripheral epithelial fragility and ocular rigidity. There was also no significant difference for any of the biometric characteristics measured. The prefitting spherical equivalent power was found to be significantly different between categories of responders (p = 0.0228), with the poor responders having the highest initial level of myopia. None of the measured characteristics differed significantly among the orthokeratology group and the two control groups. Conclusions. In this pilot study, the successof orthokeratology lens wear was not related to ocularbiomechanical or biometric attributes, but it was related to prefitting refractive error.

Retention and regression of orthokeratology with time

Abstract The exact changes that occur with accelerated orthokeratology over time have not been fully described to date. This paper attempts to answer the questions of the degree of retention and regression of the effect over a period of 8 to 9 hours post-wear. A retrospective study of 48 patients who had undergone orthokeratology for the correction of mild to moderate myopia was performed, with the subjects presenting for review at intervals of approximately 7, 30 and 90 days. The lenses had been worn overnight for a mean time of 9.27 ± 0.38 hours, with a mean post-wear interval of 8.52 ± 0.57 hours between initial and final assessment. A paired student t- test was used to compare the post-wear apical corneal power (ACP) changes and the retention of the induced effect over the period of time. The regression of the orthokeratology effect was analyzed by non-parametric means (chi-square analysis) due to the non-normal distribution of the results. The change in apical corneal power showed a statistically significant difference between 7 and 30 days ( p = 0.014), 7 and 90 days ( p = 0.0002), and a lower significance at 30 vs 90 days ( p = 0.06). Retention of the effect was also significant for 7 vs 30 days ( p = 0.001) and 7 vs 90 days ( p vs 90 days ( p = 0.06). Regression for degrees of 0.75 D or less showed variances in significance (chi-square) of 7 vs 30 ( p = 0.08), 7 vs 90 ( p = 0.003) and 30 vs 90 ( p = 0.026). For changes of 0.50 D or less regression, the results were 7 vs 30 ( p = 0.12), 7 vs 90 ( p = 0.0012) and 30 vs 90 ( p = 0.06). The results indicate that the major change in refraction occurs within the first 30 days, and that there is a significant increase in the degree of retention over the first 90 days of treatment. This is reflected in the degree of regression, which appears to stabilize between 0.50 to 0.75 D per day, by the 90-day period. The effects of regression should be taken into account prior to initiating a course of treatment with orthokeratology.

Do fenestrations affect the performance of orthokeratology lenses

Purpose. To investigate the effect of lens fenestrations on the performance of orthokeratology lenses. Methods. Twenty-two subjects (aged 11 to 31 years) were fitted with identical reverse geometry orthokeratology lenses in the two eyes. One eye was randomly designated to wear a lens with three 0.20 mm fenestrations at 120° intervals placed at the junction of the reverse and alignment curves. The lens for the other eye was not fenestrated. Subjects were reviewed at 1 week, 1 month, 3 months, 6 months, and 1 year after fitting. Data were collected on refraction, visual performance, incidences and severity of corneal staining, lens binding, and corneal pigmented arc. Results. Fifteen subjects achieved full correction in OU. There were no statistically significant differences in refractive and corneal changes, visual performance, incidences and severity of corneal staining and corneal pigmented arc formation between the two eyes. The incidence of lens binding was consistently higher in eyes wearing non-fenestrated lenses but was not statistically significant. Severity of lens binding, however, was statistically significant at the 1-, 3-, 6- and 12-month visits, with the non-fenestrated lenses causing more severe binding. Conclusions. The addition of fenestrations to orthokeratology lenses has no effect on the efficacy of the treatment or outcomes in low to moderate myopic subjects. However, the severity of lens binding is reduced. Fenestrating reverse geometry lenses may be of assistance in cases where lens binding is a problem.