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LASIK, SMILE and LASIK plus CXL produce varying corneal changes in low vs. high myopia
Researchers test biomechanical effects of SMILE, LASIK and cross-linking.
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LASIK alone reduced biomechanics in the treatment of high myopia, whereas LASIK with concurrent corneal cross-linking increased corneal stromal rigidity, according to results of an ex vivo study presented at the European Society of Cataract and Refractive Surgeons meeting in Barcelona.
Small incision lenticule extraction and LASIK yielded similar biomechanical changes in the treatment of low myopia, according to the study.
George Asimellis, PhD, presented the study results for fellow author A. John Kanellopoulos, MD, who was subsequently interviewed by Ocular Surgery News.
“As expected, SMILE appeared to affect biomechanics less than LASIK, but less than we expected. Specifically, for the –3 D of myopia that we used as a low myopia benchmark, the LASIK corneas appeared to be slightly biomechanically stronger than the SMILE corneas. We explained this by the fact that the SMILE –3 D correction includes an additional 15 µm tissue removal for the lenticule formation, and therefore, it corresponds more to a –4.5 D correction with LASIK,” Kanellopoulos said.
“The –8 D myopes appeared ‘stronger’ after SMILE than LASIK despite the tissue removal difference. We also compared the data to a similar study we had concluded recently comparing only standalone LASIK to LASIK combined with CXL for again low and high myopia ex vivo,” he said. “When looking at both studies, we have documented that, compared to control eyes, SMILE, LASIK and LASIK/CXL result in a change of the biomechanical behavior of human corneas when studied ex vivo. It appears that LASIK affects them the most, but SMILE is close behind, and LASIK/CXL appears to reintroduce the full biomechanical strength, at least in this model of corneas undergoing an incisional laser refractive procedure.”
“If we correlate the data from this study and a previous study where we compared LASIK to LASIK/CXL, it is quite compelling the fact that LASIK/CXL appears to double in biomechanical strength in high myopic corrections attempted ex vivo and thus also surpassing significantly the biomechanical strength in the corneas that received the same amount of correction ex vivo with the SMILE procedure,” he said.
Study design and methods
The study included 30 human donor corneas. Five corneas were randomly assigned to each of six groups: 3 D SMILE (group A), 3 D LASIK (group B), 3 D LASIK with cross-linking (group C), 8 D SMILE (group D), 8 D LASIK (group E) and 8 D LASIK with cross-linking (group F).
A control group included four untreated corneas from a previous study.
Investigators performed transverse biaxial resistance tensile strength measurements. Specimens also underwent enzymatic digestion resistance testing.
Primary outcome measures were stress-strain and Young’s modulus.
“The biomechanical strength of the corneas studied was measured in the bidirectional traction device. It is therefore an extrapolation to use these data in an in vivo environment,” Kanellopoulos said. “In this ex vivo model, we tried to go a step further from traditional models used to measure biomechanical strength of the cornea where a slice of cornea tissue was stretched to establish its rigidity. With the technology used in the study, we used a bidirectional of much higher sensitivity stretching device to establish the biomechanical properties of an excised corneal button that had undergone SMILE, LASIK and/or LASIK/CXL. It is with knowledge that we extrapolate these data to generalize for their potential application in an in vivo environment.”
Results and conclusions
For low myopia, biomechanical shear modulus at 10% strain was 4.2 MPa in group A, 3.9 MPa in group B and 6.8 MPa in group C.
For high myopia, biomechanical shear modulus at 10% strain was 3.7 MPa in group D, 2.9 MPa in group E and 4.5 MPa in group F.
For enzymatic digestion, time to dissolution was 195 minutes in group A, 189 minutes in group B and 320 minutes in group C. Time to dissolution was 155 minutes in group D, 129 minutes in group E and 175 minutes in group F.
Biomechanical strength and enzymatic digestion were statistically significant between SMILE and LASIK in the high myopia groups and between LASIK plus cross-linking, SMILE and LASIK in both the low and high myopia groups.
“Interestingly enough, when we looked at the –8 D model of myopic correction, the SMILE corneas appeared to be slightly better than the LASIK corneas despite the fact that the SMILE flap was at 130 µm vs. 110 µm for the LASIK flap, and the fact that here also an additional 15 µm were removed, thus making it at par with the LASIK ablation of –9 D or –9.5 D. We think these findings are significant and correspond with common thinking that SMILE utilizing a very small vertical dissection in the anterior cornea may offer higher biomechanical stability in eyes in comparison to LASIK, although we found that these differences were not as large as we initially anticipated,” Kanellopoulos said.
Further observations
Assessing biomechanical changes in the cornea is difficult because the cornea does not act as a single tissue, Kanellopoulos said.
“It’s part of a system, and its biomechanical response heavily relies on the response of the rest of the eye — for instance, the sclera and the interior components of the eye, iris, vitreous cavity, aqueous humor, etc., so its biomechanical response is a result of many different principles such as the biomechanical response of the sclera, the elasticity of the sclera, eye pressure, outflow facility of each individual eye, etc.,” he said.
Kanellopoulos said that combined LASIK and cross-linking may be suited for young myopic patients with higher risk for corneal ectasia and/or keratoconus or a family history of keratoconus.
“We would certainly not treat any established or mild keratoconus eyes with LASIK despite these findings,” he said.
“We are used to considering collagen cross-linking for ectasia and it is quite a significant intervention, but there are several studies and many from our group that show that LASIK/CXL ... may be a very simple additional intervention with a routine LASIK procedure, thus establishing a better biomechanical response in these corneas, especially when treating younger patients with higher myopia or even in patients in an endemic environment for keratoconus or patients who had a relative with known keratoconus,” Kanellopoulos said. – by Matt Hasson
- For more information:
- A. John Kanellopoulos, MD, director of the LaserVision Clinical and Research Institute in Athens, Greece, and Clinical Professor of Ophthalmology at NYU School of Medicine, can be reached at 115 E. 61st St., New York, NY 10065; email: ajk@brilliantvision.com.
Disclosure: Kanellopoulos reports he is a consultant for Alcon, Allergan, Avedro, Carl Zeiss Meditec, IPS Surgical and KeraMed.