Anterior Segment Cataract
Figure 1: True Vertex Power K versus Measured Power (=3375/Rant) of the Cornea for Model Eyes
45
44 43 42
41 40 39
38 37
35
34 36
34 35 36 37 38 39 40 K meas (D) The radius measurement error is assumed to be zero. Rant = anterior corneal radius in mm.
Figure 2: Keratometer Index Error in Corneal Power (Total Power P and Vertex Power K) versus Change in Refraction Caused by Simulated Laser Vision Correction for Model Eyes
1.4
1.2 1.0 0.8
0.6 0.4
0.0 0.2
-12
y = -0.107x R2
= 0.998 -10 -8 Total power P -6 -4 Change in refraction (D) Vertex power K
For definitions of ‘total’ and ‘vertex’ powers, see appendix of Haigis, 2003.15 measurement error is assumed to be zero.
Figure 3: Intraocular Lens Formula Error 0.0 The radius -2 0
y = -0.122x R2
= 0.998
However, this ratio is deliberately altered by refractive surgery. Consequently, the keratometer index is no longer constant and will lead to meaningless K values in these cases.
The IOL formula error, eventually, is typical for the ‘American formulas’ (Hoffer Q, Holladay-1, SRK/T), all of which use Ks to predict the effective lens position (ELP). Including Ks, namely the corneal radii, in the ELP prediction makes sense because they represent to a certain degree the geometry of the anterior segment: flat radii are usually associated with smaller, steep radii with larger ELPs. This concept has already been used by Fyodorov and Kolonko10
in the first IOL power
formula; however, after refractive surgery, the Ks no longer represent the eye’s geometry as is the case in the cornea’s untouched state. The ‘new’ flat corneal radii are falsely linked to shallow ELPs, causing a hyperopic refractive shift in patients after laser surgery for myopia and a myopic shift after preceding LVC for hyperopia.
Magnitude of Error Contributions -1.0
If allowance is not made for these error sources, the overall effect is a hyperopic deviation in eyes after LVC for myopia, which can be up to 3D, and a typically smaller myopic shift in eyes after refractive surgery for hyperopia.
-2.0 HAI SRK2 myop5 post LASIK SRKT HOLL myop10 post LASIK Virtual anterior shift of lens position (following Haigis, 200316 ) caused by the use of
post-refractive Ks for the effective lens position (ELP) prediction in different intraocular lens power formulas, calculated for different pre-existing myopia (-5 and -10 D). The radius measurement error is assumed to be zero. HAI = Haigis; HOFF = Hoffer Q; HOLL = Holladay-1.
HOFF
The radius error depends on the measurement area of the keratometry instrument and on the optical zone affected by the laser. Typical diameters of keratometer measurement areas are 3.4 mm for the Haag-Streit11
and 2.5 mm for the Zeiss12 41 42 43 44 45
y = 1.51 x - 6.626 R2
= 1.000
measurement principle applied, it is not possible to take the measurements at the optical axis, but rather a little peripherally. Depending on the optical zone of the ablation, in the periphery there is a good chance of measuring a steeper radius than is actually effective at the centre (e.g. after preceding laser surgery for myopia). Therefore, the corneal power will be overestimated, the IOL power underestimated and the patient left hyperopic. This radius measurement error is relevant when patients have had laser surgery for myopia. However, after laser vision correction (LVC) for hyperopia this should not be a problem, although evidence demonstrating this is lacking.
The keratometer index error stems from the fact that in classical keratometry or corneal topography the corneal power is derived from a measurement of the anterior corneal surface alone without knowledge of the properties of the posterior corneal surface. This works only if there is a fixed ratio between anterior and posterior corneal curvatures because only then is it possible to derive the posterior radius from the anterior radius or the posterior surface power from the anterior. This ratio is reflected in the keratometer index (KI), which is used to convert the measured anterior corneal radius Rant into the refractive power K of the cornea according to the following:
K=KI / Rant
keratometers. With the
development of modern lasers in the last few years the optical zones have become larger, so the radius error is considered to be minor,13,14 only a few tenths of a diopter.
Unfortunately, there is no instrument to directly measure corneal power (‘Ks’) in diopters. Keratometry or topography derive corneal power from the radius of corneal curvature. Due to the
22
myopic model eyes were constructed from the Gullstrand eye by extending the vitreous length and keeping the anterior ocular segment constant. Laser refractive surgery was
EUROPEAN OPHTHALMIC REVIEW
The individual contributions of the keratometer index and the IOL formula errors can be assessed from model calculations. In previous publications15,16
Virtual change in ELP [mm]
Error in corneal power (D)
K true (D)
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