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Cataract Surgery | Sep 2012

Cataract Surgery After Monovision LASIK

Maintaining a monovision solution may be best, but some patients will opt for multifocal IOLs.

So many of our patients want to achieve true spectacle freedom. As a result, thousands of people over the past decade have opted for monovision LASIK in an effort to forego wearing bifocals or reading glasses for presbyopia correction. Before the advent of presbyopic laser ablation profiles, generated by tweaking the Q factor and other approaches such as PresbyMax, monovision was the only hope for phakic patients who were developing presbyopia.

Many of those patients who underwent monovision LASIK in their mid-40s are in their mid-50s today and slowly developing cataracts, especially those with high myopia, for whom there is an increased risk of developing cataract compared with the general population. Because of the LASIK boom in the late 1990s and early 2000s, cataract patients today more frequently present having had some form of refractive surgery in the past. Some of these patients may have been high hyperopes before LASIK. Especially in Asian and Indian eyes, which are the eyes I typically treat, many of these patients also show rising intraocular pressure (IOP) secondary to narrowangle glaucoma that necessitates cataract surgery.

UNDERSTANDING REFRACTIVE SURPRISES

Calculation of IOL power after monovision LASIK frequently leads to refractive surprises. The reason for this is twofold.

Reason No. 1: After refractive surgery, the surgical corneal power is underestimated.1,2 This is because the laser procedure modifies the anterior-to-posterior corneal curvature ratio.

Reason No. 2: Third-generation theoretical formulas do not accurately predict IOL position (ie, effective lens position). Formulas such as the SRK/T, Holladay 1, and Hoffer-Q use corneal curvature to predict where the IOL should be positioned. However, because the corneal curvature itself is incorrectly predicted, the effective lens position is also incorrect.

Patients who have had monovision LASIK are highly motivated to maintain spectacle independence after cataract surgery. Because they have adapted to monovision, it is advisable to offer them the same solution after cataract surgery. Alternatively, some patients opt for bilateral multifocal lenses because of the potential for minimal refractive error. In either scenario, IOL power calculation is of utmost importance; otherwise, the desired refractive outcome or bilateral minimal refractive error will not be achieved, resulting in a refractive surprise.

IOL POWER CALCULATIONS FOR POST-LASIK EYES

In our practice, we use the ASCRS online calculator (available at iolcalc.org/) or the IOLMaster 5 (Carl Zeiss Meditec) software for post-LASIK lens power calculation. In either case, IOL power is calculated with the Aramberri double-K modification of the Holladay 1 or the Haigis-L formulas.2,3

If the pre-LASIK keratometry (K) readings are known, the ASCRS calculator uses the clinical history method, the Feiz-Mannis method, and the corneal bypass method to calculate IOL power. If we know the specific change in refraction after LASIK, the Masket or modified Masket formula is used; however, in the event the patient has no prior data—which is usually the case in India—we use one of the following four approaches: Wang-Koch-Maloney, Shammas, Haigis-L, or Galilei dual-Scheimpflug analyzer (Ziemer Group).4

PREOPERATIVE ASSESSMENTS

Before cataract surgery, post-LASIK eyes should have the following work-up:

Corneal topography. Accurate corneal topography measurements allow the surgeon to place the phaco incision on the steep axis and thus minimize postoperative astigmatism—the enemy of multifocal IOLs. Corneal topography can also reasonably estimate central corneal power, which can be used in IOL power calculation formulas. Sim-K is of less relevance here. The EyeSys corneal topographer (EyeSys Vision) and the Atlas Corneal Topography System (Carl Zeiss Meditec) both have been noted in the literature to be useful tools for measuring central corneal power.5 Raytracing software provides additional information about corneal aberrations, as it can distinguish between those on the corneal surface and those caused by cataract formation.

Accurate estimation of axial length. Immersion A-scan biometry (Axis; Quantel Medical) and the IOLMaster or Lenstar (Haag-Streit) optical biometers can be used to calculate axial length. This measurement is important because errors in axial length measurement lead to errors in IOL power calculation.

Dilated fundus examination. In addition to a dilated fundus exam, optical coherence tomography (OCT) should be used to evaluate the macula. If there are problems at the macula, like an impending macular hole or dry age-related macular degeneration, we can avoid implanting a multifocal IOL.

Anterior segment OCT. Revealing the exact corneal thickness, flap thickness, and quality of the stromal bed using anterior segment OCT is helpful if the lens power calculation goes awry and the flap must be lifted to perform an enhancement.

OTHER CONSIDERATIONS

When monovision is replicated in these patients, I select a target refraction of 0.00 D in the dominant eye and -1.25 D in the contralateral eye. My monofocal lens of choice is the C-flex aspheric IOL (Rayner Intraocular Lenses Ltd.) or the Tecnis 1-Piece (Abbott Medical Optics Inc.).

If the patient selects bilateral multifocal lens implantation, I opt for an AT LISA 809M or AT LISA toric 909M IOL (Carl Zeiss Meditec) and rely on Z-Calc, the company’s online calculator (available at zcalc.meditec.zeiss. com/zcalc/#login), to determine IOL power and target refraction. This formula is based on incision placement, surgeon factor, and induced astigmatism and also provides the best axis of implantation for a toric IOL.

I use a standard surgical technique in post-LASIK eyes, starting with a diamond knife to create a biplanar 1.8-mm clear corneal incision. A typical multifocal lens implantation procedure in a post-LASIK eye can be viewed at eyetube.net/?v=dokin. What is different in post-LASIK cases, however, is that the main and sideport incisions must not be placed too close to the flap margin. Otherwise, inadvertent movement could result in elevation of the flap edge. I always use hydroxypropyl methylcellulose to coat the corneal surface—this maintains ocular hydration and improves visualization—and I perform phacoemulsification using the Ozil phaco handpiece and the Infiniti Vision System (both by Alcon Laboratories, Inc.).

After phacoemulsification, I implant the multifocal IOL using the Viscoject injector (Carl Zeiss Meditec). The plate-haptic design of the AT LISA 809M and AT LISA 909M both center extremely well and do not rotate during irrigation and aspiration or after the ophthalmic viscosurgical device is removed at the end of surgery (Figure 1).

Pearls

I have two pearls for lens implantation with the toric AT LISA. First, place only the two leading haptic tips in the bag. Only after rotating the trailing haptics over the iris and into the desired position should they be tucked under the capsular margin and into the bag. Second, if the lens is incorrectly positioned, do not rotate it in the bag, as this maneuver leads to zonular stress. Prolapse the trailing haptics out of the bag, position the lens correctly, and then tuck them back into the capsular bag. Make sure to perform refraction 1 week after surgery. If the residual refractive error is more than ±0.50 D, counsel the patient to come back in 1 month for a flap lift and enhancement. If the lens power calculation went grossly wrong, the option to exchange the lens always exists.

Cyres K. Mehta, MS(Ophth), MCh(Ophth), is the Surgical Director and Head of the International Eye Centers, Mumbai, India. Dr. Mehta states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +91 9819850971 or +91 9821322819; e-mail: cyresmehta@yahoo.com.

  1. Holladay JT. Cataract surgery in patients with previous keratorefractive surgery (RK, PRK, and LASIK). Ophthalmic Practice. 1997;15:238-244.
  2. Haigis W. Intraocular lens calculation after refractive surgery for myopia: Haigis L formula. J Cataract Refract Surg. 2008;34:1658-1663.
  3. Wang L, Hill W, Koch D. Evaluation of intraocular lens power prediction methods using the American Society of Cataract and Refractive Surgeons Post-Keratorefractive Intraocular Lens Power calculator. J Cataract Refract Surg. 2010;36:1466-1473.
  4. Savini G, Hoffer K, Carbonelli M, Barboni P. Intraocular lens power calculation after myopic excimer laser surgery: Clinical comparison of published methods. J Cataract Refract Surg. 2010;36:1455-1465.
  5. Hamde AM, Wang L, Misra M, Koch DD. A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology. 2002;109:651-658.

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