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From the editor

Intraocular lens power accuracy: How are we doing?

Mamalis, Nick MD

Journal of Cataract & Refractive Surgery: January 2003 - Volume 29 - Issue 1 - p 1-3
doi: 10.1016/S0886-3350(02)02011-4
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We have met the enemy and he is us.

Pogo

Cataract surgery has evolved remarkably since the advent of phacoemulsification. The use of foldable intraocular lenses (IOLs) combined with clear corneal incisions has led to sutureless, small-incision cataract surgery.

As cataract techniques and IOL technology have improved, the incidence of serious complications after modern cataract surgery has decreased. The outcome goals have also evolved from simply removing a cloudy lens and implanting an IOL to improve vision to a refractive surgical technique. One of the last remaining problems in cataract surgery is accurate IOL measurements to ensure positive refractive outcomes.

Refractive outcome results of cataract surgery have markedly improved. The decrease in wound size with a trend toward sutureless clear corneal incisions has led to a decrease in postoperative astigmatism. Elimination of sutures with the construction of a small, watertight, clear corneal wound has helped create an astigmatically neutral technique for removing cataracts. In addition, improved quality control in the manufacture of IOLs has almost eliminated mislabeled or mispowered IOLs as a potential source of postoperative refractive errors.

Nevertheless, in surveys of members of the American Society of Cataract and Refractive Surgery and the European Society of Cataract and Refractive Surgeons about the complications of foldable IOLs that require explantation or secondary intervention, incorrect IOL power is the most common indication.1 For the 3 most commonly used IOLs (3-piece acrylic, 3-piece silicone, and 1-piece acrylic), incorrect power is the leading indication for removing or exchanging an IOL. These trends have been consistent over the past 4 years.

One essential step in avoiding postoperative refractive errors is accurate IOL measurements. The major components of calculating IOL power are axial length (AL), keratometry measurements, and IOL calculation formulas. The measurement with the largest potentialfor error in calculating IOL power is AL. Traditionally, ALs have been measured by contact A-scan ultrasonography. Potentially more accurate ways of measuring AL using ultrasound include immersion A-scan techniques. Recently, new technology using laser interferometry has been developed to improve the accuracy of AL measurements.

A recent study by Connors and coauthors2 compares contact ultrasonography and partial coherence interferometry using the IOLMaster (Zeiss Humphrey Systems) in 111 eyes. The laser interferometer provided significantly better results, with a decreased mean absolute refractive error postoperatively and an increase in the percentage of eyes within ±0.5 diopters (D) (61.2% versus 42.3%) and ±1.0 D (87.4% versus 77.5%) of the predicted refraction. The authors conclude that the IOLMaster is more accurate and reproducible than contact ultrasound in providing accurate AL measurements. A study by Packer et al.3 compares partial coherence interferometry and immersion ultrasound in 50 eyes. The AL measurements with the 2 techniques were highly correlated. Ninety-two percent of eyes were within ±0.5 D and 100% were within ±1.0 D postoperatively. The authors conclude that in addition to the high correlation between the AL measurements, both instruments had a high level of accuracy. Thus, either technique provides highly accurate AL measurements and has advantages over traditional contact ultrasound techniques by eliminating some inaccuracies in the contact measurement technique. Laser interferometry may also have the advantage of eliminating the need for a water bath to provide accurate measurements.

In this issue, Nemeth and coauthors (pages 85–88) compare measurements by partial coherence interferometry (IOLMaster) with those by standard ultrasound for IOL power calculations. They found that the measurements with the laser interferometer technique were highly reproducible in AL and anterior chamber depth (ACD). The AL and ACD values were significantly larger with the IOLMaster than with the standard contact ultrasound measurements. The corneal refractive power values using keratometry and measurements by the IOLMaster were also highly correlated. Thus, the precise measurements necessary for IOL calculations were relatively easy to obtain with the laser interferometer and had the advantage of a noncontact method. The only disadvantage was that the IOLMaster was unable to obtain measurements in patients with dense cataract; successful AL measurements were obtained in only 82% of the patients.

Newer techniques for providing accurate measurements for calculating IOL power are being developed. Kriechbaum et al. (pages 89–94) compare standard measurements using the IOLMaster and those obtained with a laboratory prototype version of partial coherence interferometry. They found that ACD measurements using these 2 techniques correlated well in phakic eyes but had large differences without correlation in pseudophakic eyes. This may have potential application in calculating the IOL constant, which is partially determined by the effective position of the IOL after cataract surgery. In fact, modern formulas to calculate IOL power have attempted to improve their accuracy by replacing a standard IOL constant with one that varies based on AL or corneal curvatures. Having an accurate way of measuring preoperative variables such as ACD in addition to the AL may be important in using relatively new formulas, such as the Holiday II,4 for IOL calculations.

Norrby and coauthors (pages 95–99) also looked at the use of 2 A-scan ultrasound machines in measuring ACD and lens thickness, especially using the lens haptic plane for IOL power calculation. They found large random errors in both ACD and lens thickness measurements with these 2 machines, which decreased their value as predictors of postoperative IOL position in formulas that used them. The authors suggest that the differences were large enough to require separate formula constants for each piece of equipment. They recommend an agreed upon standard in the universal calibration procedure for instruments intended for AL and ACD measurements.

The formula used to calculate the IOL power is also an important issue, especially in relatively short or relatively long eyes. Norrby and coauthors (pages 100–105) used 2 A-scan ultrasound machines to measure AL and compared the postoperative refractions using the 2 machines with several IOL calculation formulas. They found a trend toward undercorrection of short eyes and overcorrection of long eyes. Even when the use of personalized formula constants to reduce this error was taken into account, there was a similar trend. The authors recommend transforming the AL scale to reduce these errors and improve overall refractive outcomes. Thus, not only are the techniques used to obtain data to calculate IOL power important, the actual formulas and modification of the formulas are important, especially in relatively long or short eyes.

An area that is growing in importance is the calculation of IOL power in eyes that have had refractive surgery.5 The increasing popularity of laser techniques, starting with radial keratotomy (RK), has created a large group of patients who have had corneal refractive procedures and are now entering the age at which cataracts occur. Chen et al. (pages 65–70) retrospectively evaluated 24 eyes that had cataract surgery after RK and compared the final post-cataract refraction with the target refraction used when selecting the IOL. They found that using standardized methods and aiming for plano postoperatively would have resulted in a hyperopic refraction in most cases. The choice of an IOL targeted at myopia reduced the frequency of hyperopia but did not eliminate it. The accuracy of IOL power calculations could be improved in RK patients if myopia were targeted as the postcataract surgical refractive error and the flatter calculated K measurement were used to determine the IOL power.

In conclusion, it is now possible to significantly reduce the chance of a postoperative refractive surprise after cataract surgery with IOL implantation. New technologies to improve measurements necessary for the calculation of IOL powers are available, as are new formulas and customization techniques to reduce postoperative refractive errors in long and short eyes and eyes of normal AL. As the number of patients having refractive surgery increases, ophthalmologists must continue to improve methods of calculating IOL power in these patients. Modern cataract surgery has evolved into a truly refractive technique and it is incumbent on us to continue to refine our techniques to provide a satisfactory refractive outcome, which is presently in demand by patients and surgeons.

References

1. Mamalis N. Complications of foldable IOLs requiring explantation or secondary intervention—2001 survey update. J Cataract Refract Surg 2002; 28:2193-2201
2. Connors R, Boseman P, Olson RJ. Accuracy and reproducibility of biometry using partial coherence interferometry. J Cataract Refract Surg 2002; 28:235-238
3. Packer M, Fine IH, Hoffman RS, et al. Immersion A-scan compared with partial coherence interferometry: outcomes analysis. J Cataract Refract Surg 2002; 28:239-242
4. Hoffer K. Clinical results using the Holiday II intraocular lens power formula. J Cataract Refract Surg 2000; 26:1233-1237
5. Hoffer K. Calculating intraocular lens power after refractive corneal surgery [editorial]. Arch Ophthalmol 2002; 120:500-501
© 2003 by Lippincott Williams & Wilkins, Inc.