The effect of treatment zone diameter in hyperopic orthokeratology

Orthokeratology lenses with increased compression factor (OKIC): A 2-year longitudinal clinical trial for myopia control

PURPOSE

To investigate the effectiveness of orthokeratology (ortho-k) lenses and corneal changes with increased compression factor for myopia control over a 2-year period.

METHODS

Young participants (age: 6-<12 years), with low myopia (0.50-4.00 D) and low astigmatism (≤1.25 D), were recruited and allowed to choose to wear either single-vision spectacles or ortho-k lenses (randomly assigned to compression factor of either 0.75 or 1.75 D). Axial length and cycloplegic refraction were measured at six monthly intervals for two years by a masked examiner. The myopia control effectiveness was determined by axial elongation.

RESULTS

A significant number of control (63 %) dropped out, mainly due to concern about myopia progression (58 %). A total of 75 participants (mean age: 9.3 ± 1.0 years; control: n = 11, ortho-k [0.75 D]: n = 29, ortho-k [1.75 D]: n = 35) completed the study. Considering ortho-k groups only, the mean axial elongation of participants wearing ortho-k lenses of conventional compression factor (0.75 D) and increased compression factor (1.75 D) were 0.53 ± 0.29 and 0.35 ± 0.29 mm, respectively, over the 2-year study period. The between-group differences in corneal health were not significant at all visits.

CONCLUSION

Participants wearing ortho-k lenses of increased compression factor further slowed axial elongation by 34%, when compared with the conventional compression factor without compromising corneal health. Further investigations are warranted to confirm the potential mechanism of an increased compression factor for improved myopia control effectiveness.

Recommendations for analysis of repeated-measures designs: testing and correcting for sphericity and use of manova and mixed model analysis

PURPOSE

A common experimental design in ophthalmic research is the repeated-measures design in which at least one variable is a within-subject factor. This design is vulnerable to lack of 'sphericity' which assumes that the variances of the differences among all possible pairs of within-subject means are equal. Traditionally, this design has been analysed using a repeated-measures analysis of variance (RM-anova) but increasingly more complex methods such as multivariate anova (manova) and mixed model analysis (MMA) are being used. This article surveys current practice in the analysis of designs incorporating different factors in research articles published in three optometric journals, namely Ophthalmic and Physiological Optics (OPO), Optometry and Vision Science (OVS), and Clinical and Experimental Optometry (CXO), and provides advice to authors regarding the analysis of repeated-measures designs.

RECENT FINDINGS

Of the total sample of articles, 66% used a repeated-measures design. Of those articles using a repeated-measures design, 59% and 8% analysed the data using RM-anova or manova respectively and 33% used MMA. The use of MMA relative to RM-anova has increased significantly since 2009/10. A further search using terms to select those papers testing and correcting for sphericity ('Mauchly's test', 'Greenhouse-Geisser', 'Huynh and Feld') identified 66 articles, 62% of which were published from 2012 to the present.

SUMMARY

If the design is balanced without missing data then manova should be used rather than RM-anova as it gives better protection against lack of sphericity. If the design is unbalanced or with missing data then MMA is the method of choice. However, MMA is a more complex analysis and can be difficult to set up and run, and care should be taken first, to define appropriate models to be tested and second, to ensure that sample sizes are adequate.

Developments in the correction of presbyopia I: spectacle and contact lenses

PURPOSE

To outline the refractive problems associated with presbyopia and to review the basis and relative merits of currently-available methods for their correction, with detailed consideration of spectacle and contact lens approaches.

CONTENTS

In the developed world, most of the present population will spend roughly half their lives as presbyopes. The well-known presbyopic changes with age in amplitude of accommodation and required near addition are briefly reviewed, together with the less widely acknowledged slow drifts that occur in distance refraction. The desirability of restoring to presbyopes clear vision for objects at any distance, ideally corresponding to vergences within the range of at least 0 to -5 D, in any viewing direction, is stressed. A general outline is given of possible corrective methods. Methods which satisfy the needs of a 50 year-old may not be suitable for the 80 year-old. Corrections may involve both fixed- and variable-focus lens systems, and surgical methods which modify the optics of the cornea, replace the crystalline lens with different fixed optics, or attempt to at least partially restore active accommodation. Some more recent methods of spectacle and contact lens correction are described in more detail. Particular attention is given to recent commercially-developed spectacles in which the corrective power can be varied actively by either mechanical (liquid-filled deformable lenses or Alvarez lenses) or electrical (liquid crystal lenses) means to allow objects at different distances to be seen clearly. Contact lens corrections show less progress and are still preferred only by a minority of older patients, most of whom are early presbyopes.

SUMMARY

The rising proportion of presbyopes in the population, covering an age span of around 40 years, represents both a problem for those concerned with giving their patients the best vision possible at both far and near viewing distances and a commercial opportunity. Traditional single-vision distance and near, bifocal, and progressive spectacle lens solutions, together with contact lens modalities for presbyopic correction, are being challenged by a variety of new approaches. It remains to be seen whether the latter will receive wide acceptance in practice.

Corneal refractive therapy gas permeable lenses for the correction of hyperopia after one night of lens wear

PURPOSE

To examine the refractive and keratometric response of corneal refractive therapy (CRT) contact lenses for hyperopia (CRT) after a single night of contact lens wear with the goal of reducing hyperopia by +3.50 D.

METHOD

Twenty participants were fitted with a CRT HDS 100 contact lens, in one eye only. The back optic zone radius of the CRT lens was designed to correct 3.50 D of hyperopia. The eyes were randomly selected. The Nikon Auto Refractor was used to measure refractive error. Corneal topography and simulated K readings were measured using the Humphrey Atlas topographer. The lens was subsequently worn overnight, and the lens was assessed the next morning on awakening.

RESULTS

Refractive error changed by 1.50±0.71 D (mean ± SD, range 0.50-2.75 D) immediately after lens removal in the experimental eye. Similarly, there was a change in flat K readings of 0.58±0.62 D with a range of -0.25 to +1.87 D. These results were significantly different from the baseline measurements (P<0.0001).

CONCLUSIONS

The fitting of CRT HDS for hyperopia has a significant effect on corneal shape and refractive error. There was a moderate yet significant steepening of the cornea.