Labeling accuracy applies to lenses with powers less than 20 D, but clinical impact is unknown, researcher says.
However, the magnitude of improvement is small overall, depends on lens power, and its clinical relevance needs to be proven, said Kenneth J. Hoffer, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery.
Dr. Hoffer, clinical professor of ophthalmology, Jules Stein Eye Institute, UCLA School of Medicine, Los Angeles, presented results from a study he carried out to see if an exact-labeled lens would be beneficial. He requested and received 20 randomly selected Technomed IOLs from the company and then asked the FDA to test their powers.
Improvement in accuracy was determined by comparing the difference between the FDA-measured lens power and the conventional-labeled lens power in 0.5-D increments against the difference between the FDA-measured lens power and the manufacturer-determined and labeled exact power. For all 20 lenses, the exact power labeling offered a small, average 0.03-D increase in accuracy.
However, when the results were analyzed with the IOLs divided into two groups by power (<20 D and ≥20 D), there was an increase in accuracy of only 0.01 D for lenses >20 D, but an improvement of 0.09 D for the IOLs <20 D.
"Arguments continue about the benefit of exact-power-labeled IOLs considering the error intrinsic in any measurement technique and in our refraction of the patient postoperatively," Dr. Hoffer said. "Most surgeons would probably say that the 0.01-D increase in accuracy seen for the higher-power IOLs probably does not make a difference.
"The greater increase in accuracy for the IOLs <20 D is about one-tenth of a diopter, more remarkable, and could be significant if it were possible to eliminate other sources of IOL power errors and errors in refraction," he added.
Effect of testing conditions
In testing the IOL powers, the lenses were first hydrated at room temperature for 24 hours and then mounted in a wet cell to keep the temperature controlled at eye temperature (35° C). A substudy testing four different IOLs first at room temperature and then at body temperature showed the temperature increase was associated with an average 0.13-D increase in IOL power.
"One benefit of exact-diopter-labeling is that at least we know that lens power has been measured," Dr. Hoffer said. "However, the finding that temperature affects the results indicates that it is important to specify the testing conditions."
He noted no theoretical explanation is apparent so far to account for the observed power-related difference in accuracy (almost ten-fold). Dr. Hoffer also stated his study was the first such independent study of such lenses and was a preliminary report. He has been in contact with other laboratories that will also be measuring the powers of the exact-diopter-labeled IOLs.
His interest in performing this study was stimulated by a discussion of the Technomed IOL technology at the inaugural meeting of the IOL Power Club in San Sebastian, Spain, in September 2005. In addition to Dr. Hoffer, members of that group include Jaime Aramberri, MD (Spain); H. John Shammas, MD (United States); Wolfgang Haigis, MD (Germany); Thomas Olsen, MD (Denmark); and Sverker Norrby, PhD (Holland).