Effect of high add orthokeratology lens designs on corneal hysteresis and ocular aberrations

Authors : Langis Michaud(1); Patrick Simard(1); Rémy Marcotte-Collard(1)

(1) Optometry, Ecole d'Optom de l'Univ de Montreal, Montreal, Quebec, Canada

Date: June 2020

Source: ARVO 2020 Abstracts - video presentation

Orthokeratology lenses designed to create a high add do not alter corneal hysteresis (rigidity) in up to 1 month of wear but increase higher order aberrations, mainly coma and spherical aberration.

OK lenses designed to target higher add power do not alter corneal rigidity and were found by participants to not induce daytime haloes or glare.

• Meeting abstract so not fully peer-reviewed
• Statistics are not included in the abstract or to presentation to define significance of findings - consequently, despite likely obvious change in measurements in some cases (ocular aberrations) reported outcomes should be considered as observational
• Pupil diameter used for measuring higher order aberrations is not defined
  • Ocular aberrations alter as a function of pupil diameter, which in the absence of knowing pupil diameter makes it difficult to interpret possible influence of the presented values
• No comparison to standard OK lens designs to determine differences
• Long term studies needed to ascertain whether customised high add designs create a greater myopia control effect than standard OK lens designs as speculated in the poster conclusions

Customised orthokeratology (OK) lenses specifically designed to create a high add power (plus power around the treatment zone) were fit to 64 Canadian participants who were followed at 1 week and 1 month. The average 'add power' induced by the OK lens was highest in the nasal quadrant at +9.6D and lowest in the temporal and superior quadrants at +9.2D.

In his presentation, the lead author suggests that corneal hysteresis (a measure of corneal viscoelasticity) is an important measure because in theory corneas with higher hysteresis (greater viscoelasticity) should respond better to OK due to the cornea absorbing more of the pressure forces generated by the OK lens. He also suggested that this may explain why Asian corneas, which have been reported to an average have higher corneal hysteresis than Caucasian corneas, typically show a greater response to OK than Caucasian eyes. It should also be kept in mind, however, that Asian eyelids have been reported to provide higher pressure to the anterior corneal surface, which could similarly explain this greater response from OK outcome by virtue of Asian eyelids amplifying the pressure forces generated by the OK lens.

Corneal hysteresis was found to not change from baseline at either post lens wear visit. OK is reported to primarily alter refractive power by altering the anterior corneal epithelium thickness profile by thinning its profile centrally (1-3) and thickening peripherally (2,3). There is some evidence to support the corneal stroma thickening in the periphery in response to OK lens wear to correct myopia, but not change centrally (1). In the study reported here, corneal hysteresis was measured centrally, where stromal thickness is not altered by OK, so no change to corneal hysteresis should be expected unless the structure of the stroma is altered by OK. The lack of change to corneal hysteresis reported by the authors therefore suggests that customised OK lens designs are providing similar central corneal biomechanical changes to standard OK lens designs. It also fails to confirm the theoretical concept that corneas with higher viscoelaaticity, as measured by corneal hysteresis, will respond better to OK lens wear.

The reported results from the current study show that total ocular aberrations increased from 0.167±0.074μm at baseline to 0.499±0.239μm at 1 week and 0.576±0.296μm at 1 month. The authors attributed most change to coma followed by spherical aberration. On a clinical note, participants did not report perception of haloes during the day, which perhaps could be expected from targeting a high add design.

The authors concluded that customisation of OK lens design increases the level of positive aberrations which may contribute to a better myopia control. However, while this may well be the case it needs to be kept in mind that all OK lenses increase spherical aberration and no comparison was made in this study, so it cannot be concluded that customised lenses may offer better myopia control effect than standard OK lenses. Also, there is little evidence to support increase to spherical aberration as beneficial towards slowing myopia progression. Read more on this topic in Understanding spherical aberration and The effect of orthokeratology on ocular aberrations.

• High add customised OK designs:
  • Do not alter corneal hysteresis (rigidity)
  • Increase total higher order aberrations, coma and spherical aberration
  • Were found by participants to not induce daytime haloes or glare
• Authors report that customization of the design helps to increase the level of positive aberrations and then may contribute to a better myopia control
  • Long term studies measuring change to axial eye length are needed to determine whether this is actually the case

Title: Corneal biomechanical changes and high order aberrations variations following the short term wear of a customized orthokeratology lenses

Purpose: To evaluate the short term impact of a customized OK lens wear on biomechanical properties of the cornea and HOAs.

Methods: All patients newly fitted in OK lenses at Universite de Montreal (June-Sept. 2019) were recruited. In addition to regular baseline testing, corneal hysteresis (ORA, Reichert, US) and aberrometry (iTrace, Clarion, US) were conducted, before fitting, then repeated at 1 week and 1 month after delivery of the lenses. The additional testing were performed before cycloplegia and always at the same time of the day (+ 1h00). OK lenses were designed through RGP Designer software and customized to the ocular parameter of each participant. Both eyes were tested but only one, randomly selected, was kept for analysis (ANOVA -repeated measures).

Results: 64 participants were recruited (37 M, 27 F; 53% Caucasian , 47% Asian, Average age 11.7 + 0.7 years old, -5.75 + 1.97D, AL 24.63 +1.15mm ) and completed the study. Based on topography analysis taken after 1 week, customized OK lenses generated an add power of +9.6 + 0.45 D on the nasal quadrant, +9.2 + 3.9 D temporally, 9.2 + 3.1 superiorly and 9.4 + 2.1 inferiorly. Corneal hysteresis (CH) varied from 11.5 + 2.1 mm Hg to 11.3 + 2.0 mm Hg (1 week) and 11.4 + 0.7 mm Hg after 1 month. These modifications were not found statistically significant (p>0.05- ANOVA repeated measures). Total high order aberrations (HOA) were evaluated at 0.167 + 0.0739 mm at baseline, significantly increasing up to 0.499 + 0.239 um (1 week = +199%) and 0.576 + 0.296 (1 month = +244%; - p<0.05). Comas are mostly responsible for this varation (0.165 to 0.522 and 0.566 um) followed by positive spherical aberration (0.06 to 0.285 and 0.251). Each of these variations were found statistically significant as well. However, participants did not report perception of disturbing haloes and glare during the day.

Conclusions: Short term customized OK lens wear, with very high add power (+9D) has no impact on the corneal hysteresis, but is associated with a significant variation of the overall HOA, more specifically of coma and positive spherical aberrations, the latter being a known factor acting against myopia evolution. Customization of the design helps to increase the level of positive aberrations and then may contribute to a better myopia control.

Abstract link is here