IOL Intraocular Lens Power Calculations 2-Variable Formulas Prior Keratorefractive Surgery RK ALK PRK LASIK IOL Intraocular Lens Power Calculations 2-Variable Formulas Prior Keratorefractive Surgery RK ALK PRK LASIK Eye Cataract Surgery Eyes
2008-09-13The accuracy of two IOL power formulas (SRK-T and SRK-II) in order to improve the accuracy of predicative IOL powers for patients with cataract and high myopia. METHODS: (1) Patient's axial lengths of the eye, K-readings and A-constants were input to the SRK-T and SRK-II formulas. The differences in predicative refraction between the two formulas were compared. (2) Eight six patients (130 eyes) with cataract and high myopia underwent phacoemulsification and posterior chamber IOL implantation were studied. These patients were randomly divided into two groups and received IOL whose power was determined by either SRK-T or SRK-II. Three months after IOL implantation, the postoperative refractions were recorded and compared with predictive refractions. Patients were followed-up for 3 to 24 months. RESULTS: (1) When K-reading was 43.00 D, there was no statistically significant difference between the results from these two formulas (P 0.05) and there was no correlation between the difference and the axial lengths (P 0.05). In eyes with K-reading at 39.00 D, 41.00 D, 45.00 D, or 47.00 D, there was statistically significant difference between the results from these two formulas (P 0.05). The difference between the two formulas was in proportional to the axial length (P 0.05). (2) The mean error between the postoperative refraction and predictive refraction determined by SRK-T and SRK-II formula was 0.08 D and -0.79 D, respectively. The difference between these two formulas was statistically significant (P 0.05). CONCLUSION: Selection of IOL power formula should consider the axial length, K-reading and other related factors. SRK-T formula is appropriate for high myopia eyes with axial length longer than 26.0 mm. In this group, SRK-T formula is much more accurate than the SRK-II formula. Calculating IOL Power After Refractive Surgery Conventional IOL-power formulas inaccurately predict refractive error in patients who undergo cataract surgery after laser refractive surgery. Specifically, patients who have had myopic laser refractive treatments tend to experi- ence a hyperopic surprise after cataract surgery versus a myopic surprise in those who previously underwent hy- peropic laser refractive surgery. 1 Determining the necessary IOL power for emmetropia after cataract surgery in these cases depends upon three factors: (1) corneal power, (2) axial length, and (3) the effective lens position (ELP) of the IOL within the eye. The axial length is measured directly and, therefore, introduces minimal error in the absence of unusual circumstances (eg, posterior staphyloma). However, conventional methods for determining the corneal power and ELP make nu- merous assumptions. compared the IOL power predicted by each method to the postoperative refractive outcome. They assessed four IOL formulas (SRK/T, Hoffer Q, and Holladay 1 and 2). The five methods are described with results for the SRK/T formula. Clinical history. This method requires corneal power measurements prior to laser refractive surgery (K pre ) and a record of the change in manifest refraction at the corneal plane induced by the surgery ( Feiz-Mannis method. The post-LASIK corneal curva- ture is not required. The IOL power is calculated with the pre-LASIK corneal power and axial length measurements, and it is adjusted by the change in spherical equivalent postoperative refraction at the spectacle plane multi- plied by 0.7 (to vertex-correct for the IOLs position).
Okay, Mrs. Smith had LASIK several years ago for -8.00 D of myopia and is now
scheduled to undergo cataract surgery. You have carefully gathered together
all of the historical information needed and have even measured the corneal
power its exact center with one of the newer, very sophisticated topographers.
The measured and historical numbers all line up and you are confident that
your estimation of central corneal power is as accurate as possible. Using
the SRK/T formula, you calculate for an IOL power that seems appropriate.
Her surgery goes well, but to your great surprise, the first post-operative week Mrs. Smith has a spherical equivalent of +2.00 D.
You go back and re-calculate everything, confirm the axial length and still arrive at the same IOL power. What happened?
Until recently, our attention for the post-keratorefractive eye has mainly been focused on accurately determining the central corneal power. It is now becoming more widely understood that a flattened central cornea not only renders keratometry inaccurate, but also causes problems with many IOL power calculation formulas that were previously old and trusted friends.
Third generation, 2-variable formulas such as SRK/T, assume that the anterior and posterior segments of the eye are mostly proportional and
use a combination of axial length, and keratometric corneal power, to
estimate the postoperative location of the IOL, known as the effective
lens position (ELPo). If the central corneal power is very low, as we
see following keratorefractive surgery, the formula assumes that the
anterior chamber is shallow.
Why is this important? It turns out that in a two lens system such as
the eye (cornea and lens) the power of the intraocular lens can be
thought of as something of a relative figure. For example, if a +21.00
D IOL placed within the capsular bag produces emmetropia, with only a
0.50 mm posterior displacement, that same lens has an effective power
of +20.00 D. If that same lens is moved anterior by only 0.50 mm, it
then would have an effective power of +22.00 D. Try holding your
fingers 0.50 mm apart and the it's easy to see why even a small
miscalculation can lead to problems.
With a low central corneal power the formula makes the assumption that
the IOL following cataract surgery will end up sitting closer to the
cornea than normal and call for less power. And the flatter the cornea,
the bigger a problem this becomes. Where the IOL power calculation
formula thinks the lens will sit inside the eye can have a profound
effect on the power that it recommends.
Unless a correction is made for this situation, the artifact of
centrally flattened Ks following keratorefractive surgery will have
these formulas incorrectly assume a falsely shallow post-operative
ELPo. The end result is that without a special correction, 2-variable
formulas following LASIK, PRK and RK will recommend less IOL power than
is actually required. This is a second, and little recognized, source
of unanticipated post-operative hyperopia following keratorefractive
surgery for myopia.
So, what do we do?
IOL power calculations following any form of keratorefractive surgery
are best carried out using the Holladay 2 formula (contained within the
Holladay IOL Consultant). If your office does not have this software
package, a trial version can be downloaded from the Internet at:
Lacking the Holladay 2 formula, instead you can use the "double K
method" in conjunction with the SRK/T, Hoffer Q or Holladay 1 formulas
as described in:
Aramberri J. Intraocular lens power calculation after
corneal refractive surgery:
Double K method. J Cataract Refract Surg 2003; 29(11): 2063-2068.
View this article at: http://ascrs.org/publications/jcrs/absnov03.html along
with the companion editorial piece, which has a very nice "double K method" nomogram
for the Hoffer Q, SRK/T and Holladay 1 formulas:
Koch, D., Wang I. Calculating IOL power in eyes that
have had refractive surgery.
J Cataract Refract Surg 2003 29(11) 2039-2042.
This article, and its Hoffer Q, SRK/T and Holladay 1 formula correction tables can be viewed at:
http://ascrs.org/publications/jcrs/editnov03.html. Both are excellent articles and well worth the time it takes to go through them.
A basic correction table for the Aramberri "double K method" can be viewed for
each of the following formulas based on the above article By Drs. Koch & Wang.
By making this additional correction, the accuracy of your refractive outcomes in the setting of cataract surgery following keratorefractive surgery should be significantly improved.
2008-09-13
LASIK Surgeon San Clemente California The 11 eyes studied by Wang et al 3 found that, with this method of IOL calculation, the mean spherical error was -0.78 1.40D (range, -3.44 to 2.00D). Feiz et al 1 proposed a nomogram to adjust the IOL power values when a pre- operative change in refractive error is known but when preoperative keratometry values are not available. 3 Adjusted effective refractive power (EffRP adj ). This method, described by Wang et al, 3 requires a knowledge of a change in refractive error ( MR) induced by LASIK and a measurement of postoperative corneal power using the Eyesys Corneal Analysis System (Eyesys Technologies, Inc., Houston, TX), which provides an EffRP in the Holladay Diagnostic Summary. The EffRP adj = EffRP - (0.15 X MR). Using the EffRP adj as the corneal power value with the SRK/T formula for their series of 11 eyes, the investigators found that the mean spherical error was 0.88 0.81D (range, -0.13 to 2.12D). Contact lens overrefraction (CLOR). In eyes with a visu- al acuity of 20/70 or better (eg, can be accurately refracted), the plano hard contact lens of a known base curve that induces no change in refraction has the same dioptric power as the anterior corneal surface. 5 Using this technique to estimate corneal power, Wang et al 3 found in their series that the mean spherical error was 4.26 3.13D (range, 0.09 to 8.54D). Haigis 5 has demonstrated that this relatively high error is due to alterations from the Gullstrand relationship between the anterior and posterior corneal curvatures after refractive surgery. Haigis proposed formulas based on theo- retical models to correct for these changes using the CLOR method, but the investigators formulas have yet to be test- ed in a clinical series. Maloney method (personal communication, Robert K. Maloney, MD, and Ling Wang, MD, October 2002). Central corneal power is determined with the Humphrey Atlas Topography System (Carl Zeiss Meditec Inc., Dublin, CA) and modified using the formula: corneal power = (central topographic power X [376/337.5]) - 4.9. Using this method, Wang et al 3 found in their series a mean spherical error of 1.94 1.00D (range, 0.64 to 3.40D). Sonego-Krone et al 2 evaluated a direct method to measure corneal power after myopic LASIK using the Orbscan II topographer (Bausch Lomb, Rochester, NY). The investigators noted the limitation of other topogra- phy and manual keratometry systems that measure only the anterior corneal curvature and considered the advan- tages of the combined slit-scanning and Placido-disk cor- neal videokeratographer (Orbscan II), which is capable of taking into account corneal thickness and posterior cor- neal curvature. The investigators examined corneal power before and after myopic LASIK in 26 eyes using the Orbscan II and the clinical history method. They found the best agreement with clinical history to be from the 2-mm total mean power calculation (difference from re- fractive = 0.07 0.62D) and the 4-mm total optical power calculation (difference from refractive = -0.08 0.53). This method has not been used in a clinical study to test its validity as an estimate of corneal power. E L P ELP is related to anterior chamber depth and has a re- lationship with corneal shape (eg, hyperopic eyes tend to have shallower anterior chambers with a more anterior ELP. Myopic eyes tend to have deeper anterior chambers and a more posterior ELP). Numerous IOL formulas, in- cluding SRK/T, Hoffer Q, and Holladay 1 and 2, use cor- neal power in their calculation of ELP, thereby com- pounding the effect of changes in corneal shape after refractive surgery on IOL power calculation. 3,4 Aramberri 4 studied a series of nine eyes with a history of laser refractive surgery that underwent cataract sur- gery and compared the refractive outcome predicted by the standard SRK/T formula to a double-K modified SRK/T. 4 The modified formula used the prerefractive sur- gery keratometric value to determine the ELP and the postrefractive surgery corneal power (determined by the clinical history method) to calculate IOL power by the ELP is related to anterior chamber depth and has a relationship with corneal shape (eg, hyperopic eyes tend to have shallower anterior chambers with a more anterior ELP). PEER REVIEW 40 I CATARACT REFRACTIVE SURGERY TODAY I 2008-09-13T14:17:04-07:00 vergence formula. Using the standard formula, the mean spherical error was 1.82 0.73D (range, 0.96 to 3.19D). Using the double-K method, the mean spherical error was reduced to 0.13 0.62D (range, -0.56 to 1.47D). 4 Wang et al 3 applied the double-K method to each for- mula tested in their series of 11 eyes. They used the Feiz- Mannis approach that included only pre-LASIK kerato- metric measurements in the IOL calculation, thereby avoiding the problem with the ELP. The results for the three other methods of determining corneal power using the double-K SRK/T formula are: a mean spherical error of -1.02 1.13D (range, -3.25 to 0.97D) with the clinical history method; a mean spherical error of -0.98 0.72D (range -2.09 to 0.01D) with the EffRP adj method; and a mean spherical error 0.45 0.51D (range, -0.45 to 1.13D) with the Maloney method. Wang et al 3 proposed a modified Maloney equation to reduce the risk of a hyperopic outcome such that K = (central topographic power X Although the Aramberri double-K method improves accuracy, many IOL calculation formulas do not readily allow for the entry of two different K values. Ladas and Stark described a method to utilize standard IOL for- mulas to determine an outcome that is very similar to the clinical history double-K method or the Feiz-Mannis method. Using a given IOL formula, the surgeon inserts the prerefractive surgery corneal power. He then inserts the change in manifest refraction at the spectacle plane as the target refraction for emmetropia. Using the SRK/T formula, investigators found that this method produced essentially the same results as those present- ed by Aramberri. As data have emerged to evaluate methods of IOL power calculation after refractive surgery, studies have shown that the assumptions underlying conventional methods based on the Gullstrand eye produce errors both in corneal power and in the estimation of the ELP. The best method for a given patient depends upon the available clinical history and instrumentation. A perfect method would require no clinical history and would accurately estimate both the cor- neal power and ELP based solely on measurements taken after refractive surgery. For now, the studies presented here- in provide a useful framework for approaching the challenge presented by each patient with a cataract after refractive surgery.
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