There are many methods of IOL calculations and with every generation, these get more refined and more accurate. Most surgeons have accuracy of only about 70% with only a select group achieving 90% or higher. Currently, the limit seems to be about 95% of patient achieving within 0.5 diopters of the intended target. This means, however, that 5% of patients will be a refractive miss, even if we do everything correctly.
Our guest surgeon today is Dr. Juliano Vescovi from Aracruz, Brazil who does a beautiful surgery and very exacting lens calculations. He uses the Ladas Super Formula 2.0 Artificial Intelligence (free access at IOLcalc.com), the Barrett Universal II, the Haigis, and the Hill RBF. And all of these different methods determine that +22.5 would give the closest result to plano.
The surgery proceeds beautifully with good incisions, a nice round capsulorhexis, and the IOL placed securely in the capsular bag. The patient had a normal post-op course with resolution of inflammation and a stable refraction after a few weeks. The only issue is that the patient ended up with a post-op refraction of -0.75 and we are not sure why.
Back-calculating, this means that the ideal IOL power given the same surgical course, an IOL power of +21.5 would have resulted in the plano outcome that we were seeking. How can this be explained?
The possible explanations for this discrepancy include:
- (1) our biometry apparatus is misreading this eye’s keratometry, axial length, anterior chamber depth, or other parameter
- (2) the calculation methods are all making the same error in determining the IOL power
- (3) the effective lens position (ELP) is different than anticipated due to the patient’s anatomy or healing response
The most likely situation is (3) where the patient’s healing response is causing the IOL to be more anterior in the eye than anticipated. The calculated IOL power of +22.5 would be great if the IOL was sitting just a little bit deeper in the eye. But because the IOL is sitting a little more anterior, closer to the back of the iris, this IOL power induces a mild degree of myopia.
What can we do now? If this was a monofocal IOL, we would leave this eye at -0.75 D and then use the healing response to help hone the IOL calcs of the second eye. This way we could achieve a result of plano in one eye and then -0.75 in the other. This would provide an excellent, wide range of vision.
In this patient, the IOL is a trifocal IOL which tend to provide better visual results with a plano outcome. We could do the second eye surgery and achieve a plano outcome in that eye, then, if needed, come back to this eye and use the excimer laser to perform PRK or LASIK to treat the -0.75 D of myopia.
click below to watch the surgical video of this case: