Intraocular Lens Power Calculations
H. John Shammas. Thorofare, NJ: Slack Inc., 2004. Pages: 240. Price: $94.95. ISBN 1–55642–652–6.
This text is directed at educating the eye care professional and the advanced technician in the measurement of the eye and the subsequent calculation of intraocular lens (IOL) power. Some of the preeminent researchers in this field are contributors, including Wolfgang Haigis, PhD; Kenneth Hoffer, MD; Thomas Olsen, MD; Shane Dunne, PhD; and others.
This book has relevance for the practicing optometrist. As we move to the next generation of cataract patients, cataract surgery is being done earlier and the outcome demands are growing. It is no longer acceptable to achieve 20/20 with ±1.50 diopters of correction. Patients are demanding refractive surgery level outcomes. In fact, clear lens extraction is now being promoted as a refractive correction for the moderate to high hyperope and the myope with thin or flat corneas. The refractive expectations for clear lens extraction are obviously very high.
Measurement of the eye, optical biometry, is the first step in determining implant power. The biometers discussed include contact and immersion A-scan, partial coherence tomography, and B-mode guided biometry. It is obvious from the discussion that no one technique is perfect, with each having its strengths and weaknesses. The authors make the case that contact A-scan, the most widely used biometer, is the most operator dependent and most likely to introduce errors into the measurement of the axial length.
The second step is to take all the variables including; axial length, anterior chamber depth, corneal curvature, IOL style, predicted IOL position, desired refractive outcome, and surgeon technique to calculate the power of the IOL. The text offers an excellent review of the different historical and current formulas for calculating IOL power. New formulas by Haigis and Olsen are presented. This review brings an understanding of all the variables that need to be considered for accurate refractive outcomes. Failure to accurately consider all the variables and to use the correct formulas can introduce significant errors into the IOL calculation. It is obvious, however, that with current biometers and newer formulas it is realistic to expect outcomes in the range of ±0.25 diopters for most patients.
An important chapter of contemporary interest is the chapter titled “Intraocular lens power calculations after corneal refractive surgery.” As refractive surgery patients age, more and more of them will begin to develop cataracts. Most high volume cataract surgeons have already performed surgery on many of these patients, who obviously have high refractive expectations. The change in anterior corneal curvature and the resultant corneal power change from refractive surgery cause the standard power calculation formulas to error dramatically. This chapter discusses two methods to accurately calculate the IOL power. One requires knowledge of the preoperative K’s and the refractive change achieved, and the second uses a hard contact lens to determine corneal power. This points to the need for every eye care practitioner to make sure that their corneal refractive surgery patients are given a permanent copy of their preoperative and postoperative keratometry and refractive readings.
Cataract surgery is a vision and a refractive procedure. Along with removing the lens opacities, cataract surgery can very accurately resolve refractive errors and significantly improve a patient’s quality of life. Today, there is no reason why the optometrist should not prescribe and expect specific refractive outcomes for his or her patients who have cataract surgery.
Optometrists who are involved in practices that do cataract surgery or that are about to offer phakic IOL’s and clear lens extractions will find this text to be an extremely valuable reference. For the practicing optometrists who want a better understanding of what they should expect from cataract surgery and clear lens extraction, this text is well worth the time.
Louis J. Phillips
Sightline Laser Eye Center, LLC