Carlos G. Arce, MD

1
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
cgarce_at_terracom.br Financial Disclosure Medical
DirectorGalilei RD Consultant, Ziemer Group AG,
Port, Switzerland Consultant Territory Manager
for Latin America, Vista Optics Limited, Widnes,
UK Consultant, MarkEnnovy Personalized Care,
Madrid, Spain Author does not have financial
interest in the commercialization of equipments
or IOLs mentioned
2
Myopic LASIK
Purpose To study the spherical aberration (SA)
and shape profile of normal corneas and that
underwent myopic or hyperopic corneal
LASIK. Setting Private Eye Clinic, Campinas,
SP, Brazil Methods Total corneal SA and
eccentricity (?2) index were assessed with the
Galilei (Ziemer Ophthalmic Systems AG, Port,
Switzerland) dual Scheimpflug-Placido 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.
Anterior ?2
Spherical Aberration
Posterior ?2
Hyperopic LASIK
Results Normal corneas had SA0.24 0.07 µm
(-0.18 0.08 D), anterior ?20.20 0.16 and
posterior ?20.25 0.16. Corneas that underwent
myopic LASIK had SA 0.67 0.19 µm (-0.50 0.14
D) (Plt0.005) anterior ?2 -1.01 0.33
(Plt0.0005), and posterior ?2 0.39 0.18
(Plt0.05). Corneas with hyperopic LASIK had SA
-0.66 0.13 µm (0.40 0.11 D) (Plt0.005) anterior
?2 1.54 0.22 (Plt0.0005) and posterior ?2
0.41 0.13 (Plt0.05),
Anterior ?2
Spherical Aberration
Posterior ?2
3
Normal Cornea Without Refractive Surgery

• Normal corneal surface is from sphere (?20) to
  elliptical prolate (0lt?2lt1)
• Total corneal SA is positive (µm) or negative
  (D) SA0.24 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 ?2 0.10
SA 0.28 µm SA -0.22 D
4
Cornea with Post-Myopic Refractive Surgery

• Anterior surface becomes oblate with negative ?2
• Total corneal SA is high positive (µm) or
  negative (D) SA0.67 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 ?2 -1.44
SA 0.94 µm SA -0.72 D
5
Cornea with Post-Hyperopic Refractive Surgery

• Anterior surface becomes hyper-prolate with high
  positive ?2
• 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 ?2 1.83
SA -0.81 µm SA 0.62 D
6

• Galilei measures the total corneal wave front
• Spherical aberration (SA) is linked to contrast
  sensitivity
• SA0 gives sharpness of vision
• Larger SA gives depth of focus (multifocal
  cornea)

Reasoning for Custom Selection of IOL

• When the cornea becomes more prolate
• Q factor and ?2 increase
• Positive SA reduces
• Around ?2 0.55 then SA 0
• ?2 gt 0.60 then SA is already negative
• When cornea becomes less prolate or oblate
• Q factor and ?2 reduce
• Positive SA increases

7

• 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
• marginal degeneration
• Total corneal SA after myopic
• refractive surgery is more positive
• Flatter and more oblate corneas
• seem to have larger positive SA

IOL SA Total Corneal SA Total eye SA
after cataract extraction

• Glasser Campbell. Vision Res, 1998 38 (2)
  209
• Artal et al. J. Opt. Soc. Am. A. Feb 2002

V. H. Coma
V. Trefoil
H. Trefoil
Spherical Aberration
V. Quatrefoil
H. Quatrefoil
8

• Conclusions
• SA and eccentricity factor ?2 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

• Standard Sph (SA 0.18 µm)
• PhysIOL (SA -0.11 µm)
• AcrySof IQ (SA -0.20 µm)
• Tecnis (SA -0.27 µm)
• SofPort AO (SA 0 µm)

central rays focus beyond outer rays
central rays focus in front of outer rays
All rays are focused at same point
Modified from Koch et al 2009