There are more than 20 different methods of calculating the IOL power for cataract surgery. This tells me that none are perfect — after all, if one formula did the trick in all situations, then it would be the only one I'd use. Instead, I've learned that these methods are all types of estimation. Ken Hoffer and Giacomo Savini have put together a very useful spreadsheet of these formulae and have graciously provided it free of charge at www.eyelab.com for instant downloading. The Savini Schema is a way of diving the formulae into groups depending on the preoperative data available and the type of calculation.

There are three primary sources of variance in IOL power calculations: (1) axial length measurements, (2) corneal power measurements and (3) determination of the effective lens position. The axial length can be measured quite accurately, particularly with new optical devices that have largely displaced their ultrasound counterparts. The corneal power value is more difficult to determine because LASIK changes the relationship of the anterior cornea to the posterior cornea. There is also difficulty in determining the effective lens position after implantation of the IOL because it is not precisely known until after healing and capsular contraction are complete. Most of the theoretical formulae use the K value to help estimate the effective lens position, with the assumption that flat K measurements indicate a shallower anterior chamber and thus an effective lens position that is closer to the cornea, whereas steep K measurements indicate a deep anterior chamber and thus an effective lens position that is farther from the cornea. This usually holds true, but sometimes, such as in post-LASIK eyes, a formerly myopic eye with a deep anterior chamber has flat cornea measurements. The Aramberri double-K method addresses this issue by using one K measurement to determine the effective lens position and another one for the IOL power determination.

The Savini Schema shows the myriad of IOL calculation formulae for determination of cataract lens power in post-LASIK eyes. Click image for larger view

The best thing about having more than 20 formulae to calculate the IOL power in post-LASIK eyes is that it shows the effort and enthusiasm that so many people put into solving the puzzles of ophthalmology. With so many great minds actively trying to solve these riddles, it's only a matter of time before the true answer is derived.

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The clear corneal incision's dimensions are outlined to highlight the square shape.

I had just finished teaching cataract surgery to one of the UCLA ophthalmology residents who had a lot of difficulty with creating the clear corneal incision. The key point that she was missing was that the incision should be relatively square, with the tunnel length of the incision about equal to the incision width.

The square incision creates a better-sealing incision and will induce less astigmatism. Making a consistent square incision is paramount to becoming a refractive cataract surgeon. This may also be helpful in preventing endophthalmitis, which is far more

common if the incision is less than perfectly sealed. With premium lenses, such as accommodating IOLs, making the eye absolutely water-tight at the end of the surgery is critical to maintaining the proper effective lens position in the eye. Any flattening of the anterior chamber can cause an anterior displacement of the IOL and an induced myopia with loss of accommodative amplitude.

I asked the resident to make one simple New Year's resolution for the operating room: Learn how to make a square clear corneal incision.

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Because corneal nerves can be disrupted during LASIK, there can be a tendency for some eyes to become dry during the postoperative period. This can affect the healing of the eye and cause the patient discomfort and blurred vision. It is important that we keep the ocular surface healthy and primed, with a good tear film. This ensures a healthy corneal surface and a good refracting surface in order to provide the best vision.

Superficial punctate keratitis evident after LASIK.

I performed LASIK on a 30-year-old woman with a history of moderate myopia and astigmatism, and she did great. At the initial postop visits, she was better than 20/20 in each eye and she was thrilled. But during the course of her work as a computer programmer, she spent 8 hours a day staring at her computer monitors. Because she blinked less during this activity, the surface of the eye tended to dry out. She presented to me at the 1-month mark with vision decreased to 20/30 and complaints of irritated, dry eyes. Slit lamp exam showed extensive dryness of the ocular surface.

We prescribed a regimen of preservative-free artificial tears as well as oral omega-3 supplements and asked her to return in a week. The next visit she looked great: great tear film, great ocular health, and most importantly, great vision. Despite the success of your surgical procedure, the final result depends on many factors, including the tear film.

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