7. Investigaciones y estudios económicos
7.1. Proyectos contratados con instituciones
INTRODUCTION
IOL power calculation is difficult for eyes which have undergone a previous corneal surgery. The problem occurs not only after radial keratotomy or T cut or Arc cut for astigmatism, but also after LASIK or FemtoLASIK, or after corneal graft. Even in some cases of keratoconus, scar, or traumatic cornea with a clear visual axis. The problem is always the same, what is the real kératometric central value, and which is the value the patient uses.
Then in an oblate cornea, IOL power can be underestimated and overestimated in an prolate cornea.
JAVAL keratometer, does not measure the central keratometry, inside the 3 mm OZ (optical zone) and has not to be used for K evaluation. Classical topograph does not measure the very central keratometry, but can give an estimation by a mathematical extrapolation. This
extrapolation can be useful, but some errors can occur with it, in cases of central islands or irregular astigmatism for instance.
POST CORNEAL SURGERY SHAPE
There are two type of errors, the more frequent is an overestimation of the central keratometry power, after a myopic treatment. In this case the cornea becomes oblate;
it means that the central keratometry power is less than the peripheral. But the very central cornea power is less than the paracentral cornea. In resulting the IOL power is underestimated with the SRKT formula P = A- 2.5 L -0.9 K.
P (power lens in diopter), A (IOL constant, often 118), L (axial length), K mean keratometry and the pseudophakic eye becomes hyperopic.
Figs 27.1 and 27.2: Irregular astigmatism
In this case there is a paracentral inferior scar on the cornea
The difference map power of two points check near the visual axis is about 6.12 D !
Fig. 27.3: Spherical, oblate, prolate cornea
On this slide we can see that Q value aphericity describe the variation of cornea power from the center to the periphery. In this case for a calibration measurement we are using a perfect 42 D sphere, the result is that the corneal power is the same at any point, then Q = 0. that’s the reference. In case of a spherical cornea Q = 0. after a laser treatment Q is modified. After myopic treatment the cornea becomes oblate and Q > 0 (blue curve on the slide).
After a hyperopic treatment the cornea becomes hyperprolate Q < 0 (red curve on the slide).
Recent topograph like the Pentacam (oculus) comprehensive eye scanner measured the real central keratometric value. But this evaluation is on the vertex normal, that it means centered on the light reflex rather than on the center of the pupil, so it does not take true measurements of what area the patient is using to see (visual axis). It will be dramatic with a large angle kappa.
Another advantage: the pentacam is able to eliminate any error caused by ocular movement during the image’s capture.
On this slide we can see the angle kappa between the center of the pupil with the cros mark and the visual axis (patient fixation) with the center point (the middle of the first ring of the placido disk). In cases of important angle kappa more than 200µ, you have not to consider the corneal power on the center of the pupil but on the centre of the first ring.
FORMULAS FOR IOL CALCULATION
After a corneal refractive surgery no machines, therefore no formula can give by themselves an exact keratometric value for IOL power calculation.
We never enough insist about the importance of a correct evaluation of the axial length. For eyes more than 26 mm, or less than 20 mm, IOL master is necessary.
We have to analyze some of them.
a. Patient data is well known before and after surgery1 we can use the well known Holladay “double K method”
For myopic eyes K = Kb - ARA For hyperopic eyes K = Kb + ARA Kb (keratometry before refractive surgery) ARA (amount of reduction of ametropia) b. ARA is known but not the Kb
We can use linear regression formulas, like Feiz-Mannis, RA Laktani, Masket5-7
Feiz-Mannis
Myopic eye P underestimation = -0.231 + (0.595 x ARA) Hyperopic eye P overestimation = +0.751 – (0.862 x ARA)
TOPOGRAPHIC METHOD
From Dr Borasio,3 this calculation uses the pentacam, the anterior and posterior curvature of the cornea and the central pachymetry.
The second point is a modification of the post curvature of the cornea, after refractive surgery. Pentacam can also calculate this variation. In virgin cornea the power ratio between anterior and posterior cornea is 82%, 18%, then the posterior cornea power can be neglected, that’s not the same thing after excimer. This machine is also very useful in case of central islands.
Another advantage: The pentacam is able to eliminate any error caused by ocular movement during the image’s capture. The pentacam’s camera rotates around the visual axis of the eye 360° while taking 100 slices. Although the process takes less than 2 seconds, the eye can move during that time. Each central point (the thinnest) of every slice are registered for the reconstruction of the three-dimensional image. That what the orbscan is not able to Fig. 27.4: Angle kappa
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Fig. 27.5: The pentacam. 1- Corneal thickness, 2- Tangential curvature (front) 3- True net power, 4- Elevation back
do.Then the machine give the EKR equivalent K reading, which is matched with the posterior K value to minor a little bit the EKR, as Dr Holladay explains.4 We can use also the Besst software of Dr Barosio. The pupil size is also important, and has to be 4.00 -4.50 mm to gives a best data of EKR.
EXAMPLES FOR TOPOGRAPHIC METHOD After RK Radial Keratometry
Corneal refractive power after RK was best described by averaging the topographic data of the central 3.00 mm area, Dr Awwad says,2 because central area is often irregular, and it’s not possible to extract a single value of
Figs 27.6 and 27.7: After RK incisions, topographic profile is modified
K. That’s called the ACCP for average central corneal power.
In this example of a four RK incisions, we can estimate the variation of corneal power between the very center and the paracentral region in using the difference value between the center of the first ring and point of the edge of the pupil. In this example 40.98 D from the edge of the pupil to 37.28 in the very center. Δ = 3.71 diopter. IOL calculation For L = 24.0 mm and A = 118, and SRKT formula, P= 21.12 for K 40.98, instead of P = 24.44 for K 37.28.
Without the topographic method the hyperopic shift of IOL power will be + 3.32 D.
After LASIK
We can use the same technique of the ACCP, but in case of LASIK, generally the shape of the central cornea are more regular, and we can choice easily the good value on the power map, and on the visual axis in case of Angle Kappa.
In this case we have to take 31.5 D instead of 37.3 as the topograph suggests on this example of major treatment, after a LASIK of 8 diopter for myopia.
In this case the topographic method show a difference power of 5.75 diopter.
We can see than the potential error of overestimation the K value in myopic eye can induce a big hyperopic shift. That the same thing after a hyperopic treatment with the risk of a myopic shift about the IOL calculation power.
This topographic method is useful in any cases, even if previous K and ARA are unknown. Measure the central corneal power, on the visual axis, measure axial length with the IOL master, and use SRKT or an other one, that’s all.
Figs 27.9 and 27.10: Post LASIK case 2
Fig. 27.8: Post LASIK case 1
CONCLUSION
After a corneal refractive or not refractive surgery no machines, therefore no formula can give by themselves a exact keratometric value for IOL power calculation.
Pentacam is the best of, but we have to use the keratometric value on the visual axis instead of the vertex or the pupil center, especially in cases of major angle kappa (more than 200µ), or irregular astigmatism. Nevertheless, in all cases, we have to advise our patients than a mistake in the IOL power calculation can occur sometimes, and we do not recommend multifocal IOL in these difficult cases.
REFERENCES
1. Albou-Ganem C. Calcul d’implants. Ophthalmologies, 2007;1:240-44.
2. Awwad ST. Intraocular lens power calculation after radial keratotomy: estimating the refractive corneal power. J Cataract Refract Surg 2007;33:1045-49.
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3. Borasio E. Estimation of true corneal power after kerato-refractive surgery in eyes requiring cataract surgery: BESSt formula. J Cataract Refract Surg 2006;32(12):2004-14.
4. Holladay JT. IOL calculations after refractive surgery with the pentacam. Cataract and refractive surgery today Europe, summer 2007.
5. Mackool RJ. Intraocular lens power calculation after LASIK:
aphakic refraction technique. J Cataract Refract Surg 2006;32:435-36.
6. Masket S. Simple regression formula for intraocular lens power after excimer laser photoablation. J Cataract Refract Surg 2006;32:430-36.
7. Preussner PR. Topography-based intraocular lens power selection. J Cataract Refract Surg 2005;31:525-33.