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Latest-generation LASIK technology helps patients achieve best vision
A review of LASIK terminology explains where the procedure stands today.
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LASIK has been a successful procedure now for many years. The basic steps of the procedure, including a femtosecond lamellar flap and excimer laser ablation, are well understood. However, there is still a lot of confusion out there among patients — and, frankly, clinicians — about the terminology we use to describe the various LASIK platforms currently in use around the world.
Here is a refresher on the terminology and what it means:
Conventional LASIK was the first type of LASIK procedure to be offered. These treatments are based on a subjective, phoropter-derived manifest refraction. The surgeon determines the correct manifest refraction is –3 D and plugs that into the laser to perform a –3 D myopic correction. Some physicians make nomogram adjustments to account for the fact that the excimer lasers were originally designed to perform PRK. Conventional treatments, because they are based on the phoropter, are limited to 0.25 D increments, making them somewhat imprecise. We can correct 3 D, 3.25 D, 3.5 D or 3.75 D, but not 3.36 D, for example.
Wavefront-optimized LASIK is a conventional procedure with the added benefit of a built-in correction for spherical aberration. The laser algorithm is optimized by factoring in the population average for spherical aberration from a data set of wavefront exams. By attempting to counter this average induced spherical aberration and maintain the natural prolate shape of the cornea, optimization does improve upon conventional treatments. However, the treatment that is delivered is still phoropter-based. It has nothing to do with the individual patient’s preoperative wavefront or actual induced spherical aberration, so in patients with significantly more or less spherical aberration than average, an optimized procedure may be less beneficial.
Wavefront-guided LASIK, also called custom LASIK, is a treatment based on objective measurements obtained from a wavefront aberrometer. The precision of the wavefront refraction is 25 times greater than a phoropter refraction because it can measure 1/100th of a diopter (3.01 D, 3.02 D and so on, up to 3.99 D), rather than just one-quarter of a diopter. In addition, the aberrometer measures all distortions in the path of light traveling through the eye (the wavefront). This includes not just sphere and cylinder but also higher-order aberrations such as spherical aberration, coma and trefoil. In 2011, Schallhorn reported at the American Academy of Ophthalmology meeting in Orlando that 57% of eyes undergoing refractive surgery have significant higher-order aberrations, with root mean square greater than 0.3 µm.
Topography-guided LASIK combines custom and conventional approaches. Topographical information determines where to deliver the pulses to smooth out irregularities and homogenize the corneal surface, but the refractive error correction is determined by the manifest refraction from the phoropter. The rationale for topography-guided LASIK is that it makes sense to correct only the corneal aberrations on the cornea; aberrations from the lens or other internal structures might change over time and should not be changed on the cornea. The challenge is that the topography-correcting pulses influence the refraction, so additional calculations are required to avoid overcorrection or undercorrection.
Topography-integrated wavefront-guided LASIK relies on a high-resolution aberrometer to objectively measure refractive error and calculate the treatment based on the custom wavefront. In addition, coaxially obtained keratometry and topography measurements influence the pulses, particularly to prevent the cosine effect, which can otherwise reduce the effectiveness of laser pulses to the periphery.
Source: Christopher L. Blanton MD
Small-incision lenticule extraction is not a LASIK procedure. In SMILE, the cornea is reshaped by removing a lenticule of tissue rather than ablating tissue. In the U.S., SMILE can be used to treat myopia and astigmatism, but not hyperopia.
Which is better?
Wavefront-optimized (WFO) results have generally been better than previously reported conventional LASIK results; a few direct comparisons have found this to be true as well.
There is substantial evidence in the literature that wavefront-guided (WFG) LASIK produces better results than phoropter-driven ablations, whether conventional or optimized. For example, WFG ablations have been shown to improve contrast sensitivity, reduce glare and halo, and improve functional night vision.
In prospective, contralateral eye studies utilizing several different laser platforms, Manche and colleagues at Stanford reported that WFG LASIK more consistently delivers 20/12.5 and 20/16 or better visual outcomes than WFO LASIK. Moussa and colleagues also compared WFG and WFO outcomes. Four times more eyes in the WFG group achieved 20/12.5 or better vision. And Khalifa and colleagues have found that WFG results are more predictable (Figure 1) and less likely to increase higher-order aberrations compared with either WFO or SMILE procedures.
My personal experience with iDesign (Johnson & Johnson Vision) WFG procedures is similar to published results. The share of my patients achieving uncorrected visual acuity of 20/20 or better improved from 85% with conventional LASIK to 98% with WFG (Figure 2). Anecdotally, topography-integrated WFG LASIK takes this a step further. Today I am seeing many patients with results of 20/12.5 or better, although direct comparisons with the topography-integrated WFG platform are still lacking.
Laser vision correction overall has been highly perfected, and there are many good laser systems available. The latest-generation technology, such as the topography-integrated WFG iDesign Refractive Studio, provides subtle but worthwhile improvements that get us closer to the goal of helping patients achieve uncorrected vision that is as good as or better than their best corrected vision before surgery.
- References:
- Durrie DS, et al. J Refract Surg. 2010;doi:10.3928/1081597X-20090617-07.
- El-Danasoury AM. Contralateral randomized comparison of optimized prolate ablation and conventional LASIK. Presented at: American Society of Cataract and Refractive Surgery annual meeting; March 25-29, 2011; San Diego.
- He L, et al. Am J Ophthalmol. 2014;doi:10.1016/j.ajo.2014.02.037.
- Khalifa MA, et al. Int J Ophthalmol. 2017;doi:10.18240/ijo.2017.02.17.
- Khalifa MA, et al. J Refract Surg. 2017;doi:10.3928/1081597X-20170222-01.
- Lee HK, et al. J Refract Surg. 2006;doi:10.3928/1081-597X-20060901-05.
- Manche EE, et al. Stanford iDesign WFG system LASIK versus WFO LASIK study. Presented at: American Society of Cataract and Refractive Surgery annual meeting; April 13-17, 2018; Washington.
- Moussa S, et al. Eur J Ophthalmol. 2016;doi:10.5301/ejo.5000882.
- Sáles CS, et al. Ophthalmology. 2013;doi:10.1016/j.ophtha.2013.05.010.
- Schallhorn SC, et al. Ophthalmology. 2008;doi:10.1016/j.ophtha.2008.04.010.
- Schallhorn SC, et al. Ophthalmology. 2009;doi:10.1016/j.ophtha.2008.12.038.
- For more information:
- Christopher L. Blanton, MD, can be reached at Inland Eye Institute, 1900 E. Washington St., Colton, CA 92324; email: blanton007@aol.com.
Disclosure: Blanton reports he is a consultant and medical monitor for Johnson & Johnson Vision.