Contact Lens Case Reports
Ortho-k and Larger Pupils
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Chen (2012) recently discussed a two-year study of children 9 to 14; 25 were fit with ortho-k lenses and 22 with single vision glasses. They were further divided into small and large pupil groups. Results showed that pupil size was not a factor with spectacles; however, axial eye growth was significantly slower in the ortho-k group with larger pupils.
These results may be explained by the unique corneal shape following ortho-k. Our August 2012 column described how the size of the central, foveal treatment zone is approximately 3mm or less with ortho-k. Beyond that the cornea begins to radically steepen, resulting in increasing plus power across the pupil. This highly aspheric geometry forms the basis for the proposed myopia controlling effects.
Figure 1. Pre- and post-ortho-k with a −2.12D correction.
Figure 2. A +1.12D add at 4mm and +2.00D add at 6mm.
Figure 3. Pre- and post-ortho-k with a −5.00D correction.
Figure 4. A +3.00D add at 4mm and +6.25D add at 6mm.
Figure 5. Pre- and post-ortho-k with a −7.25D correction.
Figure 6. A +4.00D add at 4mm and +9.50D add at 6mm.
Figures 1 and 2 demonstrate a patient pre- and post-ortho-k with a −2.12D correction. With a 4mm pupil, the amount of add power in the periphery is +1.12D; however, at 6mm the potential add power is doubled to +2.00D.
Figures 3 and 4 demonstrate a patient with a −5.00D correction. With a 4mm pupil, the amount of add power in the periphery is +3.00D. However, at 6mm the potential add power is again doubled to +6.25D.
Finally, Figures 5 and 6 show a patient with a −7.50D correction. With a 4mm pupil, the amount of add power in the periphery is +4.00D, however at 6mm the potential add power is more than doubled at +9.50D.
These cases illustrate the rather linear increase in peripheral add power between pupil diameters of 4mm and 6mm. The doubling effect of add power through larger pupils may explain the greater ortho-k effect with larger pupils. Clinically, this is most important in the lower minus powers where a smaller pupil diameter may obscure the needed add power to adequately correct a patient’s hyperopic defocus. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #206.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.