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Spherical Aberration and Eccentricity Factor of Normal Corneas and
Corneas That Had Underwent Refractive Surgery
Carlos G. Arce, MD
Ophthalmologist, Private Eye Clinic, Campinas, SP, Brazil
Volunteer Ophthalmologist and Researcher, Ocular Bioengineer and Refractive Surgery Sectors,
Institute of Vision, Department of Ophthalmology, Paulista School of Medicine,
Federal University of São Paulo, SP, Brazil
[email protected]
Financial Disclosure:
Medical Director–Galilei R&D Consultant, Ziemer Group AG, Port, Switzerland
Consultant & Territory Manager for Latin America, Vista Optics Limited, Widnes, UK
Consultant, Mark’Ennovy Personalized Care, Madrid, Spain
Author does not have financial interest in the commercialization of equipments or IOLs mentioned
Myopic LASIK
Purpose:
To study the spherical aberration (SA) and shape
profile of normal corneas and that underwent
myopic or hyperopic corneal LASIK.
Anterior Є2
Setting:
Private Eye Clinic, Campinas, SP, Brazil
Posterior Є2
Methods:
Total corneal SA and eccentricity (Є2) index were
assessed with the Galilei (Ziemer Ophthalmic
Systems AG, Port, Switzerland) dual ScheimpflugPlacido integrated system were assessed in 39
normal corneas (24 patients) that underwent
myopic or hyperopic LASIK. Preoperative spherical
equivalent range was -1.75 to -8.25 D in 29
eyes/19 patients, and +2.0 to +5.25 D in 10 eyes/5
patients, respectively.
Spherical Aberration
Hyperopic LASIK
Anterior Є2
Spherical Aberration
Posterior Є2
Results:
Normal corneas had SA=+0.24 ±0.07 µm (-0.18
±0.08 D), anterior Є2=+0.20 ±0.16 and posterior
Є2=+0.25 ±0.16.
Corneas that underwent myopic LASIK had SA
=+0.67 ±0.19 µm (-0.50 ±0.14 D) (P<0.005);
anterior Є2 = -1.01 ±0.33 (P<0.0005), and posterior
Є2 =+0.39 ±0.18 (P<0.05).
Corneas with hyperopic LASIK had SA -0.66 ±0.13
µm (0.40 ±0.11 D) (P<0.005); anterior Є2 = +1.54
±0.22 (P<0.0005) and posterior Є2 =+0.41 ±0.13
(P<0.05),
Normal Cornea Without Refractive Surgery
• Normal corneal surface is from sphere (Є2=0) to elliptical prolate (0<Є2<1)
• Total corneal SA is positive (µm) or negative (D): SA=+0.24 ±0.07 µm (-0.18 ±0.08 D)
• Final total eye SA depends on SA of IOL chosen.
• Target Rx for IOL calculation may be plano or little negative
Example:
Є2 = 0.10
SA = 0.28 μm
SA = -0.22 D
Cornea with Post-Myopic Refractive Surgery
• Anterior surface becomes oblate with negative Є2
• Total corneal SA is high positive (µm) or negative (D): SA=+0.67 ±0.19 µm (-0.50 ±0.14 D)
• IOL implanted should not be spherical with positive SA (µm)
• Target Rx may be plano or negative. Monovision is better accepted
Example:
Є2 = -1.44
SA = 0.94 μm
SA = -0.72 D
Cornea with Post-Hyperopic Refractive Surgery
• Anterior surface becomes hyper-prolate with high positive Є2
• Total corneal SA becomes more negative (µm) or positive (D): SA= -0.66 ±0.13 µm (0.40 ±0.11 D)
• IOL implanted should not be aspheric hyper-prolate with negative SA (µm)
• Target Rx may be plano or little positive. Traditional monovision is less tolerated
Example:
Є2 = 1.83
SA = -0.81 μm
SA = 0.62 D
Reasoning for Custom Selection of IOL
• Galilei measures the total corneal wave front
• Spherical aberration (SA) is linked to contrast sensitivity
• SA=0 gives sharpness of vision
• Larger SA gives depth of focus (multifocal cornea)
• When the cornea becomes more prolate
• Q factor and Є2 increase
• Positive SA reduces
• Around Є2 = 0.55 then SA = 0
• Є2 > 0.60 then SA is already negative
• When cornea becomes less prolate or oblate
• Q factor and Є2 reduce
• Positive SA increases
• Total SA of the eye increases with
age due to lens changes
• Total corneal SA maintains a stable
value with age in normal corneas
that do not change shape
• Total corneal SA becomes less
positive or negative in typical
keratoconus
• Total corneal SA becomes some
more positive in typical initial pellucid
IOL SA + Total Corneal SA = Total eye SA
marginal degeneration
• Glasser & Campbell. Vision Res, 1998: 38 (2); 209
• Artal et al. J. Opt. Soc. Am. A. Feb 2002
after cataract extraction
• Total corneal SA after myopic
refractive surgery is more positive
• Flatter and more oblate corneas
seem to have larger positive SA
value
V. Trefoil
V. Quatrefoil
V. & H. Coma H. Trefoil
• Total corneal SA after hyperopic
refractive surgery is more negative
Spherical
Aberration
• Steeper and more prolate corneas
seem to have larger negative SA
value
H. Quatrefoil
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Conclusions
SA and eccentricity factor Є2 have an inverse
correlation
Normal corneas and those that underwent refractive
surgery have not the same SA
Our results suggest that rational IOL selection and the
target refraction expected in IOL calculation may be
optimized with preoperative data from total corneal
wavefront derived by dual Scheimpflug –Placido
tomographic system
Spherical IOLs with positive SA seem a good option for
eyes that underwent hyperopic refractive surgery or
with typical keratoconus. Aspheric IOLs with negative
SA seem a worst option
In these eyes a plano or little positive target would be
ideal and classic monovision would not be recommended
Aspheric IOLs with negative SA seem a good option for
eyes with normal corneas and that underwent myopic
surgery
Spherical IOLs are also an option in normal corneas
when a small negative refractive target is expected
Eyes that underwent myopic LASIK seem to be ideal for
classic monovision with myopic residual refraction
•
Standard Sph (SA = +0.18 μm)
central rays
focus beyond
outer rays
•
•
•
PhysIOL (SA = -0.11 μm)
AcrySof IQ (SA = -0.20 μm)
Tecnis (SA = -0.27 μm)
central rays
focus in front of
outer rays
•
•
•
•
SofPort AO (SA = 0 μm)
Crystalens (SA =0 μm)
Rayner (SA = 0 μm)
Mediphacos (SA= 0)
All rays are
focused at
same point
Modified from Koch et al 2009