Do Advances in Biometry Matter?
Why is it important to pursue the development of advanced biometry? Despite recent advances in technology, current IOL power calculation methods are limited by a lack of large-scale clinical experience in specific situations and by the assumptions inherent to calculation methods. The prediction of IOL power in atypical eyes—long and short eyes, steep and flat corneas, or any combination of these—and in eyes after refractive surgery is still challenging. Steady growth in the amount of refractive surgeries performed worldwide has led to a significant increase in the number of patients who have had refractive surgery and now require cataract extraction. For these reasons, further advances in the fields of biometry and IOL power calculation are still needed.
Sometimes, when asked to provide assistance in IOL power calculation for patients with a history of keratorefractive surgery, I have noticed that surgeons often blame the occurrence of refractive surprises on the previous corneal refractive surgery itself—as if the procedure was comparable to some parasite signal blurring TV reception. In reality, we are paying the price for earlier simplifications in IOL power calculations. These formulas were designed to compensate for the impossibility to predict with 100% success an important factor: the effective lens position (ELP), or the final position of the IOL in the anterior segment after cataract surgery. For example, in most modified IOL power calculation formulas, a flat cornea will drive the ELP closer to the corneal plane. Previously myopic eyes that have benefitted from LASIK correction have flatter corneas but larger anatomical chamber depth, which make the inclusion of the latter parameter of importance in such situations.
Another well-known problem after corneal refractive surgery is obtaining the correct keratometric values; the clinical history method is still considered the most accurate way to estimate the true keratometric power after refractive surgery. In my mind, this sounds like a confession of our inability to properly measure the keratometric power, again due to assumptions made in earlier keratometric methods.
A growing number of eyes with a history of corneal refractive surgery now need cataract surgery, and we have an obligation to provide the best possible biometry to meet our patients’ increasingly demanding expectations for postoperative spectacle independence. We now have access to various IOLs that are optimized to provide the best optical quality, for example aspheric and toric IOLs, but we are also challenged by calculating IOL power for special clinical situations such as after refractive surgery, as well as in pediatric cataract surgery, IOL exchange, eyes after penetrating keratoplasty, and triple procedures. Additionally, cataract surgery itself has evolved significantly since the advent of phacoemulsification. Various technical refinements may influence the refractive outcome. With the recent introduction of laser cataract surgery, there is no doubt that the new level of surgical precision attained in the design, placement, and sizing of the incisions will demand increased accuracy of biometry.
The means for surgeons to pursue this quest for excellence include refinements in IOL power formulas and improved measurement technologies.
REFINEMENTS IN IOL POWER FORMULAS
All current IOL power calculation formulas contain some approximations, and eye models make simplifying assumptions about the optics of the eye. Hence, they provide imperfect prediction of IOL power.
Gullstrand was the first to create a schematic eye model.1 Although it simplified the cornea, the vitreous, and the aqueous humor, this model was especially suitable for the computation of IOL power, as it also included the anterior and posterior surfaces of the crystalline lens.
Many IOL power formulas have been proposed since Fyodorov introduced the first theoretical formula.2 His and other early formulas were based on geometrical optics using axial length and keratometry. Modern theoretical formulas (third-generation and higher) are more complete in the manner in which the postoperative anterior chamber depth (or ELP) is calculated. The main challenge of highly accurate IOL power calculation is the ability to correctly estimate the ELP. The Haigis formula uses three constants to predict postoperative ELP; alternatively, the Olsen formula uses a regression formula consisting of the following preoperative parameters: axial length, lens thickness, corneal height, and preoperative anterior chamber depth. It is reasonable to postulate that strategies for ELP prediction do not play a large role in the prediction of IOL power in average (medium-sized) eyes. On the other hand, ELP may be of crucial importance in atypical eyes.
To improve results with the newest generation of formulas, surgeons must optimize their personal constants. This requires each surgeon to pay constant attention to his or her refractive results. The possibility of IOL shifts in the early postoperative period should be investigated and may be taken into account. Also, understanding the origin of power calculation errors is obviously beneficial.
IMPROVED MEASUREMENT TECHNOLOGIES
The introduction of partial coherence interferometry has improved the accuracy of ocular biometry. Time-consuming immersion techniques have been shown to be more accurate than standard applanation techniques for ultrasound biometry. Ultrasound B-scans should be performed in eyes in which the axial length is greater than 25 mm to eliminate the possibility of a staphyloma. Estimating anterior segment dimensions prior to surgery may also help.
Proper corneal power determination is mandatory for successful IOL calculation. Standard keratometry relies on assessing the corneal power by making assumptions regarding the curvature of the posterior corneal surface. Newer instruments such as the Orbscan (Bausch + Lomb, Rochester, New York) and the Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany) offer the ability to measure the real power of the cornea and provide new readouts such as corneal wavefront and total corneal power. This is of particular importance in situations such as IOL power calculation after refractive surgery and in patients with irregular corneas or keratoconus.
CONCLUSION
The justifications for pursuing better IOL power calculations are obvious, and there are different ways to achieve this goal. Progress in microsurgery, sophisticated new measurement techniques, and improvement in IOL power calculation formulas will offer excellent chances to restore vision and attain the desired refraction in cataract patients.
Damien Gatinel, MD, is an Assistant Professor and Head of the Anterior Segment and Refractive Surgery Department at the Rothschild Ophthalmology Foundation, Paris. Dr. Gatinel is a member of the CRST Europe Editorial Board. He states that he has no financial interest in the material presented in this article. Dr. Gatinel may be reached at tel: +33 1 48 03 64 82; email: gatinel@gmail.com.
- Pernow B.The first three Nordic Nobel laureates in physiology or medicine.Sven Med Tidskr.1997;1(1):147-168.
- Retzlaff J.A new intraocular lens calculation formula.J Am Intraocul Implant Soc.1980;6(2):148-152.
TAKE-HOME MESSAGE
• In most modified IOL power calculation formulas, a flat cornea will drive the ELP closer to the corneal plane.
• The clinical history method is considered the most accurate way to estimate the true keratometric power after refractive surgery; however, the main challenge of a highly accurate IOL power calculation is the ability to correctly estimate the ELP
• The introduction of partial coherence interferometry has improved the accuracy of ocular biometry.
Ready to Claim Your Credits?
You have attempts to pass this post-test. Take your time and review carefully before submitting.
Good luck!
