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Guang-ming G. Dai; Shape Induced Spherical Aberrations: Munnerlyn Shape versus Q-Preserved Shape. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4028.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the theoretical basis for the induction of spherical aberration for low order aberration correction in refractive surgery with the use of the Munnerlyn formula.
A theoretical foundation for the analysis of ocular aberration correction using conic section representation is developed. It enables a comprehensive study for the induction of spherical aberration with the use of the Munnerlyn formula. Two non-wavefront-guided treatment modalities, VSS Refractive and Q-preserved, are used for comparison purpose. VSS Refractive uses the Munnerlyn formula as the treatment target and the variable spot scanning and variable repetition rate technologies for laser delivery. The Q-preserved uses a modified Munnerlyn shape to keep the post-operative corneal asphericity the same as the pre-operative level. The treatment shapes for both the classical Munnerlyn formula and the Q-preserved are derived from the Baker’s equation. The induction of spherical aberration from the shape alone is formulated by means of the calculation of the optical path difference between the on-axis and off-axis rays. The cosine effect that compensates for the loss of energy due to the curved cornea and the induction of high order aberrations due to the biomechanics and healing are excluded in this study. A Monte Carlo simulation for 1000 synthetic eyes is used for comparing the induction of spherical aberration between the two surgical modalities.
A refractive surgical factor is identified that leads to a simple cubic function for the post-operative asphericity factor for the Munnerlyn-based refractive surgery. Opposite to the general belief, the Munnerlyn shape makes myopic LASIK more prolate, not oblate. Monte Carlo simulation shows that for the induction of spherical aberration from the treatment target shape, the Munnerlyn-based treatment shape induces similar amount of spherical aberration as compared to the Q-preserved treatment shape. However, the Munnerlyn-based shape is more predictable than the Q-preserved shape (R-square of 0.96 versus 0.65).
Munnerlyn shape makes myopic LASIK more prolate, not oblate. Monte Carlo simulation shows that the shape induced spherical aberration from the Munnerlyn shape is more predictable than that from the Q-preserved shape.
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