Calculating IOL power can be a challenge in patients who have previously had refractive surgery. The following tips from two experts can help cataract surgeons get accurate results in their post-LASIK or -PRK patients.
Measuring Central Corneal Power
One of the most important measurements is the central corneal power. It’s also one of the most difficult to get, thanks to two common errors:
Error 1. Refractive surgery changes the front curvature of the cornea, so the physiologic ratio of the front to back curvatures has changed, resulting in an error when the standardized keratometric index of refraction is used, said Jack T. Holladay, MD, clinical professor of ophthalmology at Baylor College and president of the Holladay LASIK Institute, Houston. “For example, a cornea that reads 44 D on a topographer might actually represent 49 on the front surface and –5 on the back,” said Douglas D. Koch, MD, professor and chairman of ophthalmology at Baylor.
Error 2. The keratometer and topographer sample peripherally, rather than centrally, then extrapolate the central power. After myopic refractive surgery, the peripheral sample is much steeper than the central power, causing significant overestimates of the central power. The opposite is true of hyperopic refractive surgery.
Three methods can be used to adjust those corneal power values:
Historical method. If you know the K reading taken before the refractive surgery and the change in refraction from the surgery, you can subtract the value of the refractive change from the original K reading to get the new corneal power. There are limitations: You can’t always get that data, nor can you be sure that the cause of postoperative refraction is due to the refractive surgery, Dr. Holladay explained. For instance, a nuclear sclerotic cataract may begin to develop shortly after LASIK, causing changes in the refraction.
Adjusted current keratometry method. If you don’t have the preoperative K reading, you can measure and adjust the patient’s current keratometry reading and relate it to the actual change in refraction.
Often, refractive surgeons don’t record the K reading, but they do record the patient’s refraction before and after the refractive surgery. “If the patient’s glasses before the surgery were –10, and he didn’t need glasses after the surgery, a 10-D refractive change is a good guess,” Dr. Holladay said.
With the keratometer, using the average posterior curve creates an error of about 11 percent. The peripheral sampling creates an additional error of about 14 percent, for a total error of about 25 percent. Thus, we know that the current keratometry reading will be 25 percent off of the actual change in refraction, Dr. Holladay explained.
So, if the keratometer measures 40 for the corneal power today, and you estimate a 5-D refractive change using the patient’s old glasses, you take 25 percent of that 5-D change, or 1.25 D, to adjust for the keratometer error, and subtract that from 40 to get a K reading of 38.75 D, Dr. Holladay said.
Adjusted topography method. On the EyeSys Corneal Analysis System, a display called the Holladay Diagnostic Summary gives the effRP (effective refractive power), which is the average value over the central 3 millimeters of the cornea. This reading is “very useful if you reduce it based on the amount of correction induced by LASIK or PRK,” Dr. Koch said.
Most topography units have software that measures the effRP of the cornea. Dr. Holladay explained that while the keratometer uses only two points out in the periphery, the topographer takes several measurement points up to about a millimeter from the center of the cornea, thereby reducing the error to about 15 percent. Thus, the multiplier is 0.15, Dr. Koch said.1
When using the topographer, it is important to distinguish between effRP and “sim Ks,” Dr. Holladay said. On the topographer, sim Ks simulate the keratometer and create the same 25 percent error.
If you don’t have the topography software for effRP, Dr. Koch recommends a method originally suggested by Robert K. Maloney, MD: Place the cursor in the center of your topography map, multiply that power by 1.11, then subtract 6. “We get nine of 10 eyes within a diopter of emmetropia in eyes that have undergone cataract surgery after LASIK, all using the same IOL [Alcon SA560],” Dr. Koch reported.
Measuring Axial Length
After corneal power, axial length is the most important measurement. “The IOLMaster is superior if the patient has mild to moderate cataracts,” Dr. Holladay said. “For severe cataracts, ultrasound A-scan works best.”
The IOLMaster is “10 to 20 times more precise,” Dr. Holladay said. In addition, because ultrasound measures the internal limiting membrane, you have to add an average retinal thickness to this measurement. “The A-scan adds 200 to 250 microns for that length, but the retina can range from 150 to 400 µ thick in the fovea,” Dr. Holladay said. “At 400, you have made a 150-µ error.”
The IOLMaster also measures the “optical axial length,” from the vertex of the cornea directly to the fovea. Ultrasound measures the “anatomical axial length,” from the anterior pole of the eye to the posterior pole, the deepest part of the retina—but the fovea is off to the temporal side. The difference is minimal in eyes that are 23.5 mm, but as the eye gets longer than 26.5 mm, the posterior compartment begins to bulge out and form a staphyloma. The fovea is no longer at the deepest part of the posterior pole.
“The average error between anatomic and optical is 0.8 mm, with errors ranging up to 3 mm, and every millimeter of error creates about a 2.5-D surprise,” Dr. Holladay said. “We are making 7.5-D surprises in high myopes when we measure the anatomic rather than the optical axial length.”
Other helpful measurements for calculating lens power are anterior chamber depth (ACD), white-to-white and lens thickness. The IOLMaster measures white-to-white, with the new upgrade for the Holladay 2 formula. “It does not measure lens thickness, and the ACD it gets is a little more variable than you get with ultrasound,” Dr. Holladay said. “Many people measure axial length with the IOLMaster, then measure ACD and lens thickness with ultrasound.”
Effective Lens Position
Finally, you must determine the effective lens position (ELP)—that is, where the IOL will sit within the eye. The distance from the lens to the corneal vertex is different on each patient. “This is the one variable we can’t measure or choose, so we use our IOL calculation formulas to estimate it,” Dr. Holladay noted.
The ELP is calculated differently by each of the third-generation formulas. “If the patient had a 36-D cornea after LASIK and you enter that in your calculation, [the calculation] will predict that the IOL will sit more anteriorly than in fact it will. Thus you will put in an IOL that is too weak, leading to postoperative hyperopia,” Dr. Koch explained.
If a clinician wants to use these third-generation formulas, Dr. Koch recently published tables showing how to adjust them for post-LASIK patients.2
“All of the third-generation formulas are excellent, but we use only the Holladay 2 formula,” Dr. Koch said. This formula lets you enter the patient’s K readings taken before refractive surgery. If you don’t know those values, you can check a box called “previous RK” to automatically use the value of 44 for calculating ELP. “In most instances, that guess on the part of Dr. Holladay is more accurate than putting the post-LASIK value of 35 or 38,” Dr. Koch said.
The Holladay 2 formula uses seven variables—K reading, axial length, white- to-white, ACD, lens thickness, age and refraction—to predict the lens position. The white-to-white measurement is more valuable than are ACD and lens thickness measurements in sizing the anterior segment, Dr. Holladay said. “An eye that has a small anterior segment might still have a fairly deep ACD and a very thick lens. The average is about 11.7 mm from white to white horizontally. A 13-mm white-to-white means the anterior segment is big, regardless of K readings, ACD or lens thickness.”
Despite the surgeon’s best efforts to calculate lens power in post-LASIK or PRK patients, the visual results will not equal those in patients who haven’t had the surgery. “We do warn patients of the possible inaccuracy of power calculations—and that they may need glasses, contacts, LASIK enhancement, an IOL exchange or a piggyback IOL after cataract surgery,” Dr. Koch said.
1 Hamed, A. M. et al. Ophthalmology 2002; 109(4):651–658.
2 Koch, D. D. and L. Wang. J Cataract Refract Surg 2003;29(11):2039–2042.
Dr. Holladay is the author of the Holladay 2 formula and is a consultant to Zeiss and Pharmacia. Dr. Koch is a paid consultant of Pharmacia and a paid instructor for Alcon courses.