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.

Relationship between corneal topographical changes and subjective myopic reduction in overnight orthokeratology: a retrospective study.

BACKGROUND: To investigate the relationship between the change in the manifest refractive error (DeltaM), the change in apical corneal power (DeltaACP) and initial corneal asphericity (Q) in overnight orthokeratology (ortho-K). METHODS: One hundred and twenty-eight clinical records of children undergoing ortho-K from a university optometry clinic were reviewed. The refractive and topographical data at baseline and at two-week visit of 58 patients who fulfilled the inclusion criteria were retrieved and analysed. RESULTS: Significant differences (p < 0.001) between the change in manifest refractive error and changes in the apical corneal power or the maximum change in corneal power (DeltaMCP) within the treatment zone were found. Linear regression analysis was used to describe the change in manifest refractive error and the change in apical corneal power, and the change in manifest refractive error and the maximum change in corneal power, with the equations: DeltaM = 0.91DeltaACP + 0.57 (r = 0.78, p < 0.001) and DeltaM = 0.93DeltaMCP + 0.01 (r = 0.79, p < 0.001) respectively. On average, the change in apical corneal power underestimated the change in manifest refractive error by 0.34 +/- 0.57 D; whereas on average, the maximum change in corneal power overestimated the change in manifest refractive error by 0.23 +/- 0.57 D (paired-t-tests, p < 0.001). A low but significant correlation between initial corneal asphericity and the change in manifest refractive error (Spearman r = -0.33, p = 0.01) was observed. CONCLUSIONS: The change in apical corneal power underestimates the change in manifest refractive error in ortho-K, whereas the maximum change in corneal power overestimates this parameter. Compared with retinoscopy and autorefraction, the change in apical corneal power is still useful for estimation of the change in manifest refractive error. Although the maximum change in corneal power appears to give a closer estimation of the change in manifest refractive error than the change in apical corneal power, there is no advantage in the use of maximum corneal power (manually located) instead of apical corneal power (a default given by the topographer) to estimate the change in manifest refractive error, as there is no significant difference in the estimations by either parameter. Initial corneal asphericity measured by the Medmont E300 corneal topographer has limited usage in predicting the change in manifest refractive error in overnight ortho-K.

The validity of the Jessen formula in overnight orthokeratology: a retrospective study.

PURPOSE: To investigate the validity of the Jessen formula, with a compression factor of 0.75, in determining the back optic zone radius (BOZR) of an orthokeratology (ortho-k) lens for myopic reduction [i.e. BOZR = flattest K- (target reduction + 0.75)]. METHODS: One hundred and twenty-three consecutive ortho-k patient files from the Optometry Clinic of The Hong Kong Polytechnic University were reviewed. Pertinent data at the preliminary visit and at the morning visit after 2 weeks of lens wear for 63 patients who fulfilled the inclusion criteria were retrieved for analysis. All patients were either fitted with DreimLens (DreimLens Taiwan, Taiwan Macro Vision Group, Taiwan, China) or eLens (E&E Optics Ltd., Hong Kong SAR, China) designs. Only data from the right eye were analysed. The validity of the Jessen formula was evaluated by comparing the equation of the plot of myopic reduction attempted (based on the Jessen formula) and myopic reduction achieved (based on subjective refraction). RESULTS: The Jessen formula was found to underestimate the intended target of myopic reduction following ortho-k. The results suggested that the formula should be revised to BOZR = flattest K- (1.23 target reduction + 1.27). CONCLUSION: If the intention is to overcorrect the ortho-k patient by 0.75 D to allow for regression during the daytime, the Jessen formula with a compression factor of 0.75 is not valid to determine the BOZR of the ortho-k lens designs.

Development of a switched integrator amplifier for high-accuracy optical measurements.

In the field of low flux optical measurements, the development and use of large area silicon detectors is becoming more frequent. The current/voltage conversion of their photocurrent presents a set of problems for traditional transimpedance amplifiers. The switched integration principle overcomes these limitations. We describe the development of a fully characterized current-voltage amplifier using the switched integrator technique. Two distinct systems have been developed in parallel at the United Kingdom's National Physical Laboratory (NPL) and Czech Metrology Institute (CMI) laboratories. We present the circuit theory and best practice in the design and construction of switched integrators. In conclusion the results achieved and future developments are discussed.

Good clinical practice in orthokeratology.

Overnight orthokeratology is becoming more and more popular especially in the Asia-Pacific region where the treatment is primarily used for myopic control in young children. Risk of complications in contact lens wear increases during overnight wear and may further increase when the treatment is used on children. The aim of this paper is to provide a comprehensive guideline for practitioners to improve their orthokeratology practice and minimize unnecessary or preventable complications. The fundamental requirement for starting an orthokeratology practice is to have proper education in the area and to equip the practice appropriately. Overnight trial fitting is recommended to confirm the physiological response prior to commencement of the treatment. Practitioners should provide adequate information, both oral and written, to patients before and after the commencement of treatment to avoid any legal dilemmas and to improve patients' compliance. Costs for the treatment should be transparent and provision of an emergency contact number is a must. Patients should be regularly recalled for aftercare visits and all communication with patients should be properly documented. In this paper, patient selection and the clinical procedures were discussed and a standard of practice in orthokeratology proposed. We believe that the key to providing a safe orthokeratology practice is to continually update knowledge in the field, and to practice to the highest professional standards.

Visual performance after overnight orthokeratology.

PURPOSE: To investigate visual performance after overnight orthokeratology in terms of changes from baseline values, regression of the orthokeratology effect over time, and evaluation of the best-corrected vision after treatment. In particular, to evaluate any residual visual deficits over the duration of a day due to the abnormal corneal topography induced by orthokeratology treatment. METHOD: One eye of each of six subjects was fitted with custom designed BE orthokeratology lenses (Capricornia, Brisbane, Australia), with the fellow eye acting as a control. Unaided vision, subjective sphero-cylindrical refraction, high contrast high luminance visual acuity, low contrast high luminance visual acuity, high contrast low luminance visual acuity and letter contrast sensitivity were measured at baseline and after one night (Day 1) and eight nights (Day 8) of lens wear. Except for baseline, data were collected after overnight lens wear immediately after lens removal, and again 3, 6 and 9h after lens removal. At each time point throughout the day, the visual performance measures were evaluated with the initial refraction of the day (the 0h refraction) and also using the optimum subjective refraction at each measurement time. This method was used to evaluate the practical visual performance to be expected after orthokeratology treatment and the residual visual deficits arising from any induced corneal changes after correction of defocus. RESULTS: As expected, orthokeratology lens wear significantly changed unaided vision and refraction from baseline. However, it did not significantly affect visual acuity in different contrast conditions, or contrast sensitivity. The spherical component of refraction was the only parameter to exhibit regression over each day (p=0.021), with more stability demonstrated on Day 8 than Day 1 (p=0.012). There were no statistically significant changes of best-corrected acuity from baseline in the differing contrast and luminance conditions. CONCLUSION: Apart from the predicted improvements in unaided vision and reduction of the myopic refractive error, orthokeratology treatment was not found to significantly change any other aspects of visual acuity and contrast sensitivity. All visual performance measures exhibited stability over a 9-h period. Spherical refractive error changed significantly on Day 1 but became stable after a week of treatment. These results indicate that the corneal topography changes induced by orthokeratology do not induce changes in aberrations that are large enough to significantly diminish visual performance.

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 degrees 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 degrees to 180 degrees astigmatism (J180), and 45 degrees to 135 degrees 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 degrees to 34 degrees . In all patients, the refraction pattern altered little after the first week. CONCLUSION: Orthokeratology can correct myopia over the central +/- 10 degrees of the visual field but produces only minor changes at field angles larger than 30 degrees . 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.

Incidence of corneal pigmented arc and factors associated with its appearance in orthokeratology.

PURPOSE: To determine the incidence of the corneal pigmented arc in orthokeratology (ortho-k) lens wearers over 12 months of lens wear and the factors associated with its appearance. METHOD: Thirty-five ortho-k subjects were recruited; refractive and corneal changes after lens wear (single-lens protocol) were monitored over 12 months. The incidence of the pigmented arc after 3, 6 and 12 months of lens wear was determined. RESULTS: The incidence of corneal pigmented arc was 17% (27%), 49% (49%) and 90% (93%) after 3, 6 and 12 months lens wear respectively in the left and right eyes. For subjects with arcs observed in the left eye within the first 6 months of lens wear, the mean +/- S.D. period of lens wear before initial detection of the arc was 14 +/- 7.4 weeks, and no correlation was found between this factor and the baseline spherical and cylindrical refractive errors (i.e. refractive sphere and cylinder, respectively), spherical equivalent refractive error (SERE), the target myopia reduction, the amount of refractive sphere (or SERE) reduction and changes in central and peripheral corneal curvatures after 6 months of lens wear. Baseline refractive sphere, baseline SERE, target, amount of myopia reduction, and change in central corneal curvature were significantly larger (p < 0.05) in those subjects with pigmented arcs after about 6 months of lens wear. The intensity of the observed pigmented arcs after about 6 months of lens wear was significantly related to the time when it was first observed (p = 0.003). Significant correlation was also found between the intensity of the arcs and the following parameters: baseline refractive sphere and SERE, target, change in central corneal curvature, and amount of myopia reduction (p < 0.006). After about 12 months of lens wear, the intensity of observed arcs was significantly related to the baseline refractive sphere, SERE and the target (p < 0.006). CONCLUSION: The incidence of ortho-k-associated pigmented arc increases from 17% after 3 months of lens wear to over 90% after 12 months of lens wear. The intensity of the arc is related to the time when the arc first appeared. Both the incidence and the intensity of the arc are related to the period of lens wear, baseline refractive sphere, SERE and the target.

Orthokeratology for myopic children: wolf in sheep's clothing?

Orthokeratology attempts to reduce myopia by remoulding the corneal shape with contact lenses. A recent resurgence is predicated on new contact lens designs with a prefigured back contact surface and higher oxygen transmissibility. This Clinical Controversy presents an analysis of the risk factors associated with orthokeratology and its suitability for children, followed by commentaries from specialists who have an interest in the method. Some state that there is a lack of data on relative risks of corneal infection and that there is a need for large-scale randomized controlled studies; however, opinion is expressed by others that orthokeratology is a clinically safe procedure using modern lenses. It is noted that the physiological and biophysical bases of orthokeratology are virtually unknown, and further research on the human cornea is indicated to scientifically establish the safety of orthokeratology. Prospective patients, and their parents in the case of children, should be fully informed of the risks.

Is fluorescein pattern analysis a valid method of assessing the accuracy of reverse geometry lenses for orthokeratology?

BACKGROUND: The aims of this study were to investigate the relative frequencies of correct identifications of variations in the fit of conventional rigid gas permeable (RGP) lenses and reverse geometry lenses (RGL) from fluorescein pattern analysis by orthokeratology (ortho-k) practitioners and non-ortho-k practitioners and to determine whether fluorescein pattern analysis is sensitive for assessing ortho-k lens fittings. METHODS: Slides of fluorescein patterns of different lens fittings were shown to the practitioners, who were asked to identify the ideal, flatter, flattest, steeper and steepest lens fittings. RESULTS: Observed frequencies of correct identifications of most of the conventional RGP lens fittingss were not significantly different from the expected frequencies for both groups of practitioners. The observed frequencies of correct identifications of all of the RGL fittings were either not significantly different or were lower than the expected frequencies. CONCLUSION: The relative frequencies of correct identifications of fluorescein patterns of both conventional RGP lens and RGL fittings by experienced ortho-k practitioners were not different from those by non-experienced ortho-k practitioners. Practitioners from the two groups were not always able to diagnose conventional RGP lens and RGL fittings adequately from fluorescein pattern analysis alone. Fluorescein pattern analysis alone may not be sufficiently sensitive for assessing ortho-k lens fitting.

An analysis of the astigmatic changes induced by accelerated orthokeratology.

PURPOSE: The change in corneal astigmatism induced by reverse geometry lenses for orthokeratology has not been described previously. This study examines the efficacy of accelerated orthokeratology for reducing astigmatism and whether this varies with the degree of pre-existing astigmatism. METHOD: Twenty-three randomly chosen eyes exhibiting 0.50 D to 1.75 D pre-fitting with-the-rule astigmatism were retrospectively analysed. Astigmatism was measured by simulated keratometry and corneal topography before and at the completion of a course of orthokeratology. The change in astigmatism measured by keratometry was calculated by two vector analysis techniques: the Bailey-Carney method, which was designed for contact lens-induced corneal shape changes, and the Alpins method, which was designed for surgically-induced corneal shape changes. The change in astigmatism measured by corneal topography was calculated by the EyeSys Version 3.2 software. RESULTS: Most patients (20/23) had some reduction of astigmatism but orthokeratology is incapable of a total elimination of pre-fit astigmatism. Alpins vector analysis showed that an increased efficacy of 60 to 80 per cent would be required to eliminate astigmatism. All three methods found a 50 per cent mean reduction in astigmatism from the pre-fit level. Topographical analysis indicates that the reduction in astigmatism occurs mainly over the central 2.00 mm chord. There is a very poor correlation between the pre-and post-wear corneal astigmatism at the 2.00 mm chord (R(2) = 0.11, p = 0.04) and the predictability of the final astigmatic axis is also poor (angle of error = 1.22 +/- 27.35). CONCLUSIONS: Accelerated orthokeratology seems more successful than conventional orthokeratology at reducing with-the-rule astigmatism. However, it reduces pre-existing astigmatism by an average of only 50 per cent and it does not do so reliably either for magnitude or direction. These results provide two useful patient selection criteria for orthokeratology. They are: assuming 0.50 D to 0.75 D of astigmatism is a satisfactory outcome, orthokeratology can be expected to be successful for pre-fitting astigmatism of up to 1.00 D to 1.50 D; and the greater the pre-existing astigmatism, the less likely orthokeratology is to be successful.

Corneal iron ring associated with orthokeratology lens wear.

A healthy 27-year-old female with a history of daily wear soft contact lenses requested orthokeratology (ortho-k) treatment and was fitted with a pair of reverse geometry lenses. Two weeks later, a faint brown pigmented arc, with fuzzy margins, extending from four to eight o'clock at the inferior midperipheral part of the corneal surface was observed in both corneas. The patient was asymptomatic and happy with the results of the ortho-k treatment, and topographical maps showed a sharp and slightly decentered bull's eye pattern that indicated a clinically acceptable ortho-k effect for both eyes. The rings extended to a near-complete ring in both eyes and became denser and more significant as lens wear continued. The rings were similar in appearance to Fleischer's rings seen in keratoconic eyes and coincided with the margin of the bull's eye observed in the topographical maps. The patient continued to be asymptomatic with unaided visual acuity of 20/20(+1) (OD) and 20/25(-1) (OS). Apart from the presence of the iron rings bilaterally, the corneal integrity and topography were stable and clinically unremarkable; the patient was allowed to continue wearing the lenses with regular aftercare scheduled on a 3-month basis. Possible etiologies of the formation of iron rings are discussed.

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 R(o), but significant differences in eccentricity and elevation values were found. The number of repeated readings that should be taken for a precision of 2 microm (elevation) were 12 for the Humphrey and 2 for the Medmont. CONCLUSIONS: The performance of both the Humphrey and the Medmont was very good. R(o) and 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.