What Intraocular Lens Should I Use in the Postkeratorefractive Patient?
Warren E.
Hill,
MD, FACS
Since the introduction of radial keratotomy in the United States in 1978, several additional forms of keratorefractive surgery have become popular. David Harmon of Market Scope estimates that approximately 1.3 million LASIK procedures will be performed in North America this year. A predictable consequence of such a wide acceptance of keratorefractive surgery over the last 2 decades is that these patients are now coming to cataract surgery in ever increasing numbers.
In the beginning, no one anticipated that unwanted side effects of keratorefractive surgery would include an inability to measure the central corneal power with standard equipment, across the board formula inaccuracies, and the generation of higher-order aberrations (Figure 9-1).
Figure 9-1. Zeiss Humphrey ATLAS topographer axial map of an eye with prior radial keratotomy. These eyes are often multifocal and frequently show a dramatic increase in higher-order aberrations, such as positive spherical aberration, Z(4,0).
In general, the postkeratorefractive eye can be divided into 3 categories: radial keratotomy, hyperopic laser in situ keratomileusis (LASIK)/photorefractive keratotomy (PRK), and myopic LASIK/PRK. Selecting the correct IOL has three basic components that are carried out differently than would be for a regular IOL power calculation: an estimation of central corneal power, a formula-corrected calculation of IOL power, and selecting the most appropriate IOL type. Combinations of incisional and ablative forms of keratorefractive surgery are beyond the scope of this paper.
Radial Keratotomy
Radial keratotomy can be approached in a straightforward manner. Because incisional refractive surgery typically exerts an influence on both the anterior and posterior central corneal radii, this allows for a specific form of direct measurement. The 2 methods that we have found to work best are the average of the 1 mm, 2 mm, and 3 mm power values from the Numerical View of the Zeiss Humphrey ATLAS topographer (Carl Zeiss Meditec, Jena, Germany) (Figure 9-2) and the adjusted effective refractive power (EffRPadj) from the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System.1
Figure 9-2. Zeiss Humphrey ATLAS Numerical View power values from an eye with prior hyperopic LASIK. The average of the 1 mm, 2 mm, 3 mm, and 4 mm powers are used in place of keratometry for IOL power calculations.
When using a 2-variable, third-generation theoretic formula (Holladay I, SRK/T, or Hoffer Q), the mathematical artifact produced by a very flat central cornea must be removed. Two-variable formulas use only the central corneal power and axial length to estimate the postoperative position of the intraocular lens (IOL). Without some form of special correction, a very low corneal power will often result in the formula assuming that the IOL will be in a more anterior position and call for less IOL power. This effective lens position error will result in unanticipated hyperopia. This effective lens position correction is typically carried out by applying an Aramberri double K method correction or by using the Holladay II formula, which is contained within the Holladay IOL Consultant (Jack T. Holladay, Houston, Tex).1.2
Eyes with prior radial keratotomy typically will also have increased higher-order aberrations. While it is not currently possible to correct all of these with an IOL alone, it is possible to offset an elevation in the anterior corneal spherical aberration, Z(4,0). For this, the TECNIS IOL (Advanced Medical Optics, Santa Ana, Calif) with –0.275 µm of negative spherical aberration or the Alcon SN60WF (Fort Worth, Tex) with –0.200 µm of negative spherical aberration can be useful choices. If possible, the surgeon should measure the anterior corneal spherical aberration prior to selecting the most appropriate IOL.
Hyperopic Laser In Situ Keratomileusis
Eyes that have undergone prior hyperopic LASIK can be treated in much the same way as those with prior radial keratotomy. This is because the ablation pattern is outside the central cornea and typically influences both the anterior and posterior central radii. If the amount of hyperopic LASIK is less than +4.00 D, the average of the 1 mm, 2 mm, and 3 mm power values from the Numerical View of the Zeiss Humphrey ATLAS topographer or the EffRPadj from the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System can be used. For higher powers, Wang, Jackson, and Koch have shown that a small additional correction may be needed.3
Again, if using a 2-variable, third-generation formula, an Aramberri double K method correction should be carried out or the Holladay II formula should be used so that the IOL power calculation formula does not miscalculate the effective lens position.
Eyes with prior hyperopic LASIK typically do not show increased spherical aberration because the central cornea has been steepened. If possible, a Z(4,0) value for the anterior cornea should be obtained prior to deciding on which IOL to use. If the Z(4,0) value is low, then a conventional spherical IOL or an aspheric IOL without negative spherical aberration correction, such as the Bausch & Lomb LI61AO (Rochester, NY), would be a reasonable choice. If the measured anterior corneal spherical aberration is close to a median value for the normal population (+0.275 µm) then an IOL such as the Alcon SN60WF would be a good choice. In the rare instance that the anterior cornea spherical aberration Z(4,0) value is markedly elevated, then the AMO TECNIS lens would be appropriate.
Myopic Laser In Situ Keratomileusis
Selecting the correct IOL for eyes that have undergone prior myopic LASIK is the most difficult of the 3 common forms of keratorefractive surgery. This is because the anterior radius of the cornea is primarily changed and, at present, there is no consistently reliable methodology available to directly measure the central corneal power.
For estimating the central corneal power, there have been a number of methods proposed, all of which have a limited accuracy. Some are purely historical (Holladay’s historical method), some are a combination of historical and objective methods (Masket, Feiz and Mannis, corneal bypass, central corneal power adjustment), while others are purely objective (modified Maloney, Pentacam HEK).4-7
What we have found is that the IOL power for the myopic LASIK patient is best determined if calculated by several different techniques, looking for a correlation between historical, historical-objective, and objective methodologies. A summary of the most popular techniques can be found on the Internet at http://www.doctor-hill.com/iol-main/keratorefractive.htm.
The 2 methods that we have found to be the most reliable are Holladay’s historical method (provided that the pre-LASIK data is accurate and the post-LASIK refractive information is obtained 4 months to 6 months afterwards) and the Holladay Equivalent K feature of the Oculus Pentacam (Lynwood, Wash) (Figure 9-3).
Figure 9-3. Holladay Equivalent K feature of the Oculus Pentacam from an eye with prior myopic LASIK. This method uses data points from both the anterior and posterior corneal radii within a 4-mm zone to calculate a net central corneal power and then converts this information into keratometric diopters.
Again, if using a 2-variable, third-generation formula, an Aramberri double K method correction should be carried out or the Holladay II formula should be used to preclude an unanticipated hyperopic result.
Eyes with prior myopic LASIK will often show an increase in spherical aberration because the central cornea has been flattened. If possible, a Z(4,0) value for the anterior cornea should be obtained prior to deciding which IOL to use. If the Z(4,0) value is close to a median value for the normal population then the Alcon SN60WF would be a good choice. If the anterior corneal Z(4,0) value is elevated, then the AMO TECNIS lens would be more appropriate.
Patients undergoing phacoemulsification with IOL implantation following all forms of keratorefractive surgery will continue to be a challenge until a reliable methodology has been firmly established.
References
1. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double K method. J Cataract Refract Surg. 2003;29(11):2063-2068.
2. Koch D, Wang L. Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg. 2003;29(11):2039-2042.
3. Wang L, Jackson DW, Koch, DD. Methods of estimating corneal refractive power after hyperopic laser in situ keratomileusis. J Cataract Refract Surg. 2002;28:954-961.
4. Masket S, Masket SE. Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation. J Cataract Refract Surg. 2006;32(3):430-434.
5. Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculations methods in eyes that have undergone LASIK. Ophthalmology. 2004;111(10):1825-1831.
6. Holladay JT. Consultations in refractive surgery. Refract Corneal Surg. 1989;5:203.
7. Walter KA, Gagnon MR, Hoopes PC, Dickenson PJ. Accurate intraocular lens power calculation after myopic laser in situ keratomileusis bypassing corneal power. J Cataract Refract Surg. 2006;32(3):425-429.