Millions of patients around the world have undergone refractive surgery to attain independence from spectacles and contact lenses. But as this population ages, these patients face the possibility of cataract extraction. Performing cataract surgery on a post-LASIK patient with extremely high expectations poses new challenges for the surgeon.
Calculating IOL power in eyes that have previously undergone refractive surgery is imprecise and may result in unplanned postoperative refractive errors for a variety of reasons. Errors such as inaccurate corneal curvature measurements, measured keratometric values that are higher than the actual power, a difference between the visual axis and the center of the cornea, and incorrect anterior chamber depth or IOL position can create inaccuracies in some IOL power formulas.
Juxtaposed to the complexity of surgical planning, you have a patient who underwent elective surgery to avoid wearing corrective lenses and most likely expects to achieve excellent UCVA again. Various intraoperative technologies can help surgeons satisfy this most demanding set of patients, one of which is wavefront aberrometry. The ORange Intraoperative Wavefront Aberrrometer (WaveTec Vision Systems, Inc., Aliso Viejo, California) enables real-time IOL power calculations in eyes that have previously undergone refractive surgery. Visual outcomes and pseudophakic IOL power can be confirmed and aphakic IOL power calculated before leaving the operating room.
RETROSPECTIVE ANALYSIS
Kerry Assil, MD; Dan B. Tran, MD; Robert J. Weinstock, MD; William F. Wiley, MD; and I conducted a retrospective analysis of cataract surgery outcomes in 29 eyes that had previously undergone refractive surgery (five hyperopic and 24 myopic LASIK). The average axial length was 25.1 mm, and the average keratometry value was 42.8 D; the mean age of patients was 61 years.
Following cataract surgery and IOL implantation, an intraoperative wavefront aberrometer was used to perform aphakic and pseudophakic measurements. Depending on the refractive measurement, IOL exchange was carried out. In nine cases, limbal relaxing incisions (LRIs) were performed in conjunction with cataract surgery. Each surgeon used his standard method for IOL power calculation, including the ASCRS Web site tool for power calculation after refractive surgery. Surgeons did not follow the ORange recommendation in all cases, relying on their professional experience.
We found much greater precision with ORange than with standard aberrometry. The mean absolute value prediction error (MAVPE; 1 month postoperative spherical equivalent [SE] minus predicted postoperative SE for IOL power implanted) was 0.72 D with the standard method and 0.48 D with the ORange. Standard deviation was ±0.59 D with the standard method versus ±0.356 D with the ORange. The percentage of patients with less than 0.50 D of error was 44.83% versus 55.17% in the standard and the ORange groups, respectively, and likewise the percentage of patients with less than 1.00 D of error was 68.97% versus 93.20% in the two groups, respectively. The maximum error in the standard method group was 2.38 D versus 1.52 D in the ORange group. ORange and standard method MAVPE differed by less than 0.25 D in 16 cases and by less than 0.50 D in 24 cases. In the four cases in which the difference was greater than 0.50 D, ORange had the lower MAVPE. We believe these differences are due to ORange's ability to measure the true refractive power of the cornea, whereas traditional predictive measurements, including historical data and contact lens over-refraction, cannot measure the cornea's true refractive power.
CONCLUSION
Intraoperative wavefront aberrometry is capable of measuring the refraction of the aphakic eye through the postrefractive surgery cornea. In these corneas, a 1.00 D shift in corneal power changes the IOL power exponentially, further increasing the importance of a correct measurement.
The ORange system enables on-the-table refractive measurements. It guides LRIs and enhancements and ensures correct placement of toric IOLs, limiting refractive surprises in eyes after refractive surgery and the need for postoperative IOL exchange. Using an intraoperative aberrometer provides me with refractive confidence in the pseudophakic refraction at the end of a case.
Eric D. Donnenfeld, MD, is a trustee of Dartmouth Medical School in Hanover, New Hampshire, and a partner in Ophthalmic Consultants of Long Island in Rockville Centre, New York. He is a member of CRST Europe's Global Advisory Board and states that he is a consultant to Abbott Medical Optics Inc. and WaveTec Vision Systems, Inc. Dr. Donnenfeld may be reached at tel: +1 516 766 2519; e-mail: eddoph@aol.com.
