Modelling the refractive and imaging impact of multi-zone lenses utilised for myopia control in children's eyes

On-eye centration of soft contact lenses

PURPOSE

To evaluate the relative positions of modern soft contact lenses (SCLs) relative to the limbus/cornea and the pupil.

METHODS

Sixty images of the anterior eyes of 101 subjects were acquired over 10 s while participants fixated the centre of the camera lens located 33 cm in front of the eye in a well-lit (300 lux) clinic. Custom validated image analysis software was used to locate the boundaries of the contact lenses, pupils and corneas (limbus). Horizontal and vertical relative positions of the contact lens, pupil and limbus were calculated from the fitted boundaries.

RESULTS

The mean (standard deviation) pupil and corneal diameters for all subjects were 3.84 mm, (0.83) and 11.97 mm (0.48), respectively. The mean [95% confidence interval] pupil centre was located 0.28 mm [0.26, 0.30] nasally and 0.07 mm [0.05, 0.10] superiorly to the corneal centre. Consistent with clinical observations, the contact lenses centred accurately relative to the corneal centre both nasally 0.04 mm [0.01, 0.07] and inferiorly -0.01 mm [-0.06, 0.03]. However, regardless of the eye, the contact lens was significantly (p < 0.001) decentred relative to the pupil centre both temporally -0.23 mm [-0.26, -0.20] and inferiorly -0.08 mm [-0.12, -0.04]. Decentration magnitudes were significantly correlated between the right and left eyes.

CONCLUSIONS

Spherical SCLs centred well on the cornea but temporally and inferiorly from the primary line of sight (pupil centre), due to the differences in the location of the pupil and corneal centres. Contrary to some previous reports, there was no evidence that lens optics or material affected lens centration significantly.

Refractive power profiles of commercially available soft multifocal contact lenses for myopia control

PURPOSE

Lens power profiles can provide valuable insights on the imposed optical defocus and visual experience of contact lens wearers, especially in the context of myopia control. This study measured the refractive power profiles of multifocal soft contact lenses (MFCLs) currently used or that have the potential for use in myopia control using high spatial resolution aberrometry. The instrument's repeatability for determining MFCLs power profiles was also assessed.

METHOD

The power profiles of 10 MFCLs of various designs (centre-distance, centre-near and extended depth of focus) were measured using the Lambda-X NIMOEVO, a phase shifting Schlieren-based device. Power profiles were graphically expressed as measured power at each chord position and the maximum add power was calculated. The repeatability of the NIMOEVO was expressed as the within-subject standard deviation at each chord position for a subset of five MFCLs.

RESULTS

The measured distance powers differed from nominal powers for more than half of the MFCLs with a definable distance zone. There were variations in the chord position of the distance and near correction zones, rate of power transitions and calculated maximum add between the MFCLs which did not depend on lens design. For half of the MFCLs, the power profile shape was inconsistent between different nominal back vertex powers of the same design. The repeatability of the NIMOEVO was dependent on the lens design, with designs featuring faster rates of power change exhibiting worse repeatability.

CONCLUSIONS

Significant differences in MFCL power profiles were found which were not adequately represented in labelling. This is likely due to the small number of parameters used to define lens power characteristics. Eye health care practitioners should be aware of potential differences in power profiles between different MFCLs, which will impact the retinal defocus introduced during lens wear and the wearer's visual experience.

Dual-focus contact lenses for myopia control modify central retinal electrophysiology in humans

INTRODUCTION

Dual-focus contact lenses create two focal planes, one providing a clear retinal image while the other imposes myopic defocus on the retina to slow myopia progression. This study used global-flash multifocal electroretinogram (gmfERG) response amplitudes to compare central versus peripheral retinal responses under dual-focus conditions and to assess the optimal degree of myopic defocus compared with a single-vision control lens.

METHODS

Twenty participants each underwent three gmfERG trials, wearing a spectacle correction over dual-focus contact lenses with plano central power and peripheral secondary focal powers of either +2.00D, +4.00D or a plano single-vision lens. We compared amplitudes and latencies of the gmfERG direct and induced components (DC and IC) within participants, between the three different contact lens powers and at different retinal eccentricities (gmfERG ring).

RESULTS

We observed significant differences in the gmfERG responses between the single-vision and dual-focus contact lenses. Overall, DC amplitudes peaked between zero and +2.00D secondary power, while IC amplitudes were maximal between +2.00D and +4.00D. Compared with the single-vision control, the greatest increase in DC and IC amplitudes while wearing dual-focus lenses occurred within the central 10° of the retina. There was no interaction effect between gmfERG ring (eccentricity) and secondary power, and no difference in the latency of the gmfERG responses between different powers.

CONCLUSION

We found that dual-focus contact lenses with a +2.00D secondary power are close to that expected to induce the greatest increase in gmfERG responses relative to a single-vision lens. Dual-focus lenses produced the highest DC and IC response amplitudes relative to a single-vision lens in the central 10° of the retina. This suggests that dual-focus contact lenses slow myopia progression by modifying central rather than peripheral retinal activity.

Impact of zone geometry on the introduction of myopic defocus in young adult eyes wearing multi-zone lenses

PURPOSE

Multizone contact lenses control myopia progression by proposed introduction of myopic defocus. This project investigated how much of the pupil area and how many dioptres of myopic defocus are introduced by different lens zone geometries with near- and off-axis viewing.

METHODS

Ten young myopic adults (18-25 years) binocularly wore four soft contact lenses including a single vision (SV), concentric-ring dual-focus (DF), centre-distance multifocal (MF) and a RingBoost™ (RB) multi-zone design containing a combination of coaxial and non-coaxial zones. A modified aberrometer captured aberrations and pupil sizes at four target vergences between -0.25 and -4.00 D (on-axis) and across the central ±30° of the horizontal retina (off-axis). Defocus was quantified as the difference between the measured refractive state and the target vergence within each zone of a multi-zone design within the pupil and compared with that of equivalent zone areas of the SV lens. The percentage of the pupil containing myopic defocused light for each lens was calculated.

RESULTS

Defocus within the distance correction zones of multi-zone lenses was similar to that of the SV lens. When viewing on-axis at -0.25 D target vergence, on average 11% of the pupil was myopic with SV, whereas 62%, 84% and 50% of the pupil was myopic for the DF, MF and RB designs, respectively. At -4.00 D target vergence, all lenses exhibited a systematic decrease in the percentage of pupil area having myopic defocus (SV: 3%; DF: 18%; MF: 5% and RB: 26%). The off-axis proportions were similar across multi-zone lenses; however, multi-zone lenses retained approximately 1.25-3.0× more myopic defocus than the SV lens.

CONCLUSIONS

Subjects accommodated using the distance-correction zones of multi-zone lenses. Multi-zone contact lenses introduced significant myopic defocus on-axis and across the central ±30° retina. However, the magnitude and proportion of defocus were influenced by zone geometry, add power and pupil size.

Comparison of optical myopia control interventions: effect on peripheral image quality and vision

This study compares the effects on peripheral vision and image quality of four myopia control interventions: a) Perifocal spectacles/ArtOptica, b) Stellest spectacles/Essilor), c) MiyoSmart spectacles/Hoya and d) MiSight contact lenses/CooperVision. Five subjects participated with habitual or no correction as reference. Three techniques were used: 1) Hartmann-Shack sensors for wavefront errors, 2) double-pass imaging system for point-spread-functions (PSF), and 3) peripheral acuity evaluation. The results show that multiple evaluation methods are needed to fully quantify the optical effects of these myopia control interventions. Perifocal was found to make the relative peripheral refraction (RPR) more myopic in all subjects and to interact with the natural optical errors of the eye, hence showing larger variations in the effect on peripheral vision. MiSight had a smaller effect on RPR, but large effect on peripheral vision. Stellest and MiyoSmart also showed small effects on RPR but had broader double-pass PSFs for all participants, indicating reduced retinal contrast. Reduction in peripheral retinal contrast might thereby play a role in slowing myopia progression even when the peripheral refraction does not turn more myopic.