Reducing treatment zone diameter in orthokeratology and its effect on peripheral ocular refraction
Introduction
Orthokeratology (OK) describes the use of rigid gas permeable lenses that are specially designed to alter the anterior corneal curvature during overnight wear by the amount required to correct ametropia once the lens is removed [1]. Since first described in the 1960′s [2], OK has continued to retain a small proportion of contact lens fits for correcting myopia, largely by practitioners who specialize in contact lens fitting [3]. More recently, it has been shown that in addition to providing reliable myopia correction, OK is also effective in slowing progression of myopia in children [[4], [5], [6], [7], [8], [9], [10]]. The recent increase in prevalence of myopia worldwide [11] has consequently led to growing interest in OK amongst contact lens practitioners [12].
The mechanism by which OK slows myopia progression is not yet known. OK has been shown to alter the peripheral optics of the eye by shifting the peripheral image shell in an anterior direction relative to the shift in paraxial focus to create a myopic relative peripheral refraction (RPR) defocus [13,14]. Animal studies have shown that inducing relative peripheral myopic defocus drives development towards hyperopia [15]. OK has also been shown to induce a positive shift in spherical aberration (SA) [[16], [17], [18], [19], [20], [21]], which has the effect of increasing range of paraxial focus [22]. It has been demonstrated that changes to SA could reduce lag of accommodation [23] and through this mechanism cause the entire image shell to undergo a myopic shift [24]. Regardless of mechanism, meta-analyses report similar efficacy for slowing axial eye elongation across different OK designs [25,26], which, supported by a study reporting minimal differences to OK induced changes to RPR between three different OK designs [27], suggests that it is the generic profile of corneal topography change induced by OK that is responsible for their myopia controlling effect.
A recent study investigated the influence of pupil size on myopia control efficacy from OK by stratifying participants as either having a pupil size above or below the mean cohort value [28]. The analysis revealed greater slowing of axial eye elongation in the group with pupil diameters above the mean. The authors suggested that the larger pupil diameters allow more of the annular steepened zone (ASZ) of corneal curvature that is observed around the central treatment zone (TZ) to fall within the pupil margin, and thereby provide greater shift in relative peripheral myopia created by OK. This being the case, a reasonable hypothesis to draw from this explanation is that moving the ASZ closer to the corneal apex by reducing the TZ diameter should lead to an enlarged area of induced relative peripheral myopia, and thereby potentially provide greater myopia control efficacy. Consistent with this hypothesis, animal studies have demonstrated evidence towards a dose dependent treatment effect, with larger peripheral zones resulting in greater compensation [[29], [30], [31], [32]].
It has been shown in a retrospective analysis that smaller TZ’s can be achieved using an OK design constructed with five back surface curves compared to a different design constructed from four back surface curves [33]. However, in a stronger repeated measures design analysis, where we independently altered the back optic zone and periphery of a four curve OK design, we found no differences to OK induced changes to RPR [34]. While we did not investigate differences to TZ, the outcomes create conjecture on whether altering OK lens parameters for an individual will result in changes to TZ diameter and if so, whether this would have any differential effect on RPR.
In this study we utilized a randomized repeated measures study design to investigate whether TZ diameter can be reliably reduced by more complex modification to OK lens design. If TZ were to be reduced, a secondary purpose of this study was to investigate whether reduction to TZ caused a greater myopic shift in measured RPR.
Section snippets
Sample size calculations
PS software version 3.1.6 (Vanderbilt University, USA) was used for sample size calculations. Prior data indicated that to detect a difference of 0.65 mm in TZ diameter to a study power of 0.80 at the 5 % level of statistical significance with assumed 20 % dropout would require seventeen participants.
Participants
Seventeen participants completed the study. Data from one participant was removed from analysis due to poor visual outcome from OK resulting in data being analyzed from sixteen participants (mean
Refraction and visual acuity
Other than a significant though clinically irrelevant difference in J180 there was no significant difference in any other refractive measurements between both baseline visits (Table 1). This indicates that across the 1-week minimum washout period between each lens wearing period, refraction had returned to within clinically acceptable values of initial baseline measurements.
Both control and test lenses reduced myopic spherical equivalent (M) refraction over 7 nights of lens wear with no
Discussion
We have demonstrated, we believe for the first time following a prospective study design, that the TZ diameter induced by OK can be reduced in size by altering OK lens specifications. A previous retrospective study demonstrated that a proprietary five curve (Dreamlens, Polymer Technology, Rochester, NY) lens design also creates a smaller TZ diameter compared to that provided by a proprietary four curve (CRT, Paragon, Arizona, USA) lens design [33]. However, the strength in the current
Conclusions
In this study we have shown that TZ diameter can be reliably reduced by altering OK lens design without detrimentally effecting lens centration or refractive effect, however, in doing so we fail to reveal a significant difference in OK induced change to RPR. A potential limitation that needs to be kept in mind when considering this evaluation is large variability across RPR measurements. Longitudinal studies are needed to assess whether smaller TZ OK lens designs increase efficacy for slowing
Acknowledgements
Capricornia Contact Lens Pty Ltd, Slacks Creek, 4127, Australia, for providing the orthokeratology lenses used in this study.
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