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Samantha Bradford, Eric Mikula, Dongyul Chai, Tibor Juhasz, Donald J Brown, James V Jester; Corneal Crosslinking and Mechanical Stiffening using Non Linear Magic. Invest. Ophthalmol. Vis. Sci. 201657(12):.
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© ARVO (1962-2015); The Authors (2016-present)
Multiphoton, non linear magic (NLM), has been widely used in engineering to activate photoinitiators and fabricate micro-scale structures. The purpose of this study was to test the hypothesis that NLM can be used to photoactivate riboflavin within the cornea to produce collagen crosslinking (CXL) and mechanical stiffening with femtosecond laser precision.
An NLM delivery device using a variable numerical aperture objective with controllable focal plane adjustment was built and connected to a Chameleon femtosecond laser delivering 860 mW of 760 nm light to the focal plane. Ex vivo rabbit eyes (55) were then saturated with 0.5% riboflavin for 30 minutes and scanned at various speeds, depths and line separation over a 4-5 mm area in the central cornea. Effectiveness of NLM crosslinking was then assessed by evaluating crosslinking-induced corneal collagen auto fluorescence (CAF) in the region of 400-450 nm using the Zeiss 510 Meta Detector. To determine the effect on mechanical tissue stiffening, corneas were removed from the eye and subjected to mechanical indentation testing using a 1mm diameter probe and force transducer. NLM crosslinking was also compared to UVA induced crosslinking using the standard Dresden protocol.
Using the 0.1 NA objective settings, NLM line scans produced CAF cross-sections measuring 5.8 μm by 78.5 μm (FWHM). This produced a 10,000 fold greater CXL volume in the cornea compared to a standard 1.0 NA lens and by varying the NA of the device a range from 53-1800μm3 (NA = 0.25-0.1, respectively) could be achieved. Using a scan speed of 5.4mm/s and a line separation of 2μm, 4mm diameter NLM CXL induced a 2.0-2.5 fold increase in corneal stiffness, which was significantly higher than control corneas (p<0.05), but less than the 3.6 fold increase produced by 8mm diameter UVA CXL. Importantly, these effects on corneal stiffness were produced using scanning times of 10 minutes for NLM CXL compared to 30 minutes for UVA CXL.
This is the first report demonstrating significant mechanical stiffening of intact corneas using NLM CXL. The ability to precisely control the area and depth of corneal stiffening using laser guided NLM CXL may have widespread applications beyond treating corneal ectasia.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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