Understanding Eyes That Have Undergone Refractive Surgery

Dr. Luis W. Lu, M.D.

 

Looking back at the early days of refractive surgery, surgeons may now have mixed feelings about the era of radial keratotomy (RK). RK not only had relatively poor predictability but also produced eyes with unstable refractive errors. The significant diurnal fluctuations and high altitude related hyperopic shifts after surgery alone were enough to give the refractive surgeon of 30 years ago considerable clinical nightmares. Everybody wanted to do RK back then, but the minute we knew Excimer laser was coming, we stopped doing them, said Luis W, Lu, MD, Honorary Professor of Ophthalmology at CH University, School of Medicine.

What ray tracing can do Today, RK eye sand an ever-increasing number of LASIK eyes face cataract surgery more often. Fortunately, thanks to modern ophthalmology, these patients can now benefit from technologies such as ray tracing to properly buttress their situation going into phacoemulsification cataract surgery. According to Dr. Lu, ray tracing is a helpful method to analyze the problematic eyes of patients who underwent RK in the 1960s and 70s, and understand their incredible refractive abnormalities. Ray tracing, available today in wide variety, is used to determine the refractive amplitude of the eye. The eye images are digitally generated and are assessed for accuracy and repeatability, Dr. Lu said.

Although the discipline is complex, ray tracing is in fact simply a calculation method for single rays passing through an optical system, said Paul-Rolf Preussner, MD, PhD, University Eye Hospital, Mainz, Germany, in a publication presented at the 2011 European Society of Cataract & Refractive Surgeons (ESCRS) meeting. It can be a useful field for understanding eyes that have undergone refractive surgery. Eyes after corneal refractive surgery differ from normal eyes, Dr. Preussner noted in the same report. In these eyes the highest errors occur from keratometry measured corneal radii. The reason is that Keratometers are adjusted to spherical or moderately prolate aspherical corneas. But eyes after refractive surgery often have oblate aspherical cornea (such as with RK). The resulting errors can be avoided by replacing keratometry by topography, combined with an adequate algorithm included in the ray tracing that extracts corneal vertex radii together with corneal asphericity in the optical zone. The so-called corneal refractive index can also be tricky. A second source of error in eyes after refractive surgery is the above mentioned fictitious corneal refractive index, which assumes a constant ratio of anterior and posterior corneal radii, Dr. Preussner reported. But when anterior radii are modified in refractive surgery, the said ratio must change too. The solution of the problem is an additional measurement o posterior corneal curvature, e.g., by Scheimpflug or OCT techniques. Meanwhile, ray tracing software exists that allows accurate no-history IOL power calculation, even in cases like decentered radial keratotomy with residual corneal astigmatism.

Experts worldwide from Korea, Japan, and Dutch researchers are investigating the many applications and benefits of ray-tracing. Evaluating other applications in a multicenter retrospective study, Italian investigators in the July 2014 issue of the Journal of Cataract& Refractive Surgery confirmed that corneal ray tracing (performed by rotating Scheimpflug camera-Placido disk corneal topographer) can accurately measure Excimer laser surgery-induced corneal refractive changes. Managing patient expectations with the availability of modern tools in ophthalmic surgery, patient expectations have shifted as well, and clinicians today have high expectations to manage. We cannot promise patients that they are going to have 20/20 vision after the procedure, Dr. Lu said. Clinicians need to confirm first what their bottom-line impression of the keratometry is with several methods, before proceeding to phacoemulsification in cataract patients. These patients have a 20% enhancement rate, and they are not candidates for a multifocal IOL; they might certainly be candidates for a toric lens but options are limited, he added.

In current clinical practice, Dr. Lu noted the wide range of methods that can be used to procure data and analyze refractive abnormalities in eyes prior to cataract surgery, from various topographers producing outstanding images and ocular surface imaging, to Scheimpflug imaging. Indeed, compared to the past, surgeons today have a huge armamentarium even before any RK patient undergoes surgery. These days, we refrain from using analog tests unless we want to confirm something with manual keratometry, Dr. Lu said. It is the switch from analog to digital. We now have the ability to look at aberrometry, aberration indices, and irregularity indices, he said. Using topographic maps, according to Dr. Lu, clinicians can determine irregularities in the eye. Rings, reproducibility, stability, environmental and diurnal variability, dry eye and a whole lot of other issues these are all potential RK-related problems for patients operated on years ago, he said. It is also key to corroborate several topographic methods in order to ascertain keratometric accuracy for IOL calculations, with or without a toric lens implant. However, in addition to having regular and irregular astigmatic errors, these patients have a different axial length in the morning when pressure is higher compared to in the afternoon, he said. Thus, we have to be doubly vigilant of topography and axial length measurements in these patients, he
concluded.

 

References

1. Canovas C, et al. Effect of the equivalent refractive index on intraocular lens power prediction
with ray tracing after myopic laser in situ keratomileusis. J Cataract Refract Surg. 2015 May;
41(5):10301037.

2. Miyata K, et al. Use of Scheimpflug corneal anterior-posterior imaging in ray-tracing
intraocular lens power calculation. Acta Ophthalmol. 2013 Nov; 91(7):e546549.

3. Park HJ, et al. Comparison of the astigmatic power of toric intraocular lenses using three toric
calculators. Yonsei Med J. 2015 Jul; 56(4):10971105.

4. Savini G, et al. Corneal ray tracing versus simulated keratometry for estimating corneal power
changes after excimer laser surgery. J Cataract Refract Surg. 2014 July; 40(7): 11091115.

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