Eyeworld

EW REFRACTIVE 66 December 2017 side as she would Intacs segments in a keratoconus patient. She has also used CAIRS as a single, almost fully circular, 330-degree segment. She has found that like Intacs, the allogenic segments result in good regularization of the corneal topog- raphy while also correcting some myopic refraction and improving uncorrected and spectacle-corrected distance visual acuity. It also spreads out biomechanical forces by flat- tening the cornea. For patients still at risk for keratoconus progression, she will perform CAIRS and then crosslink. "When you use allogenic tissue, the complications with synthetic materials are done away with, and therefore, a huge advantage is that you could use this in patients with corneas of any thickness unlike Intacs or other artificial segments where the cornea needs to have a certain minimum thickness in the zone of implantation," she said. As for the preservation aspect of corneal lenticules for later use in procedures like these, Ganesh et al. described a technique of cryopreser- vation in 2014 that seems to provide safe long-term storage. 5 From an inlay to an onlay Allotex is still in a research and engineering phase, with Dr. Muller saying they expect to start multi- center clinical trials in Europe in the first quarter of 2018. "We haven't focused on clinicals because we know it's going to work. There's too much data out there to show it works," he said, citing pub- lished safety and/or efficacy research for epikeratophakia, intracorneal inlays, and corneal allografts, in- cluding the positive outcomes of Dr. Jacob's PEARL technique. "The key feature is with respect to safety, the jury is in. It's hard to dispute 60+ years and hundreds of publications that show that human allograft tissue works fine for cre- ating [corneal] shapes," Dr. Muller said. "If you look at the more limit- ed but compelling data that prop- erly shaped human tissue gives a properly shaped result … you look at what we're doing, what the Allotex team has done in the past, you make a pretty compelling case that we're going to do it again." While the Allotex procedure could mimic other intracorneal inlay procedures where a thin fem- tosecond flap is made, the allograft centered over the pupil, and the flap smoothed back down, Dr. Muller said they're moving toward a less invasive onlay procedure. A small divot of the patient's corneal epithe- lium would be removed and the thin allograft lenticule would be placed on top of Bowman's membrane, tucked a bit under the epithelium, and within 6–8 hours, the epitheli- um would cover back over it. "Now you have a procedure where you didn't do any surgery, just a little corneal insult," he said. "If I want to take it out, I don't have to do anything except reach in with forceps. We know of one doctor who was able to slide it out at the slit lamp with the patient not knowing it came out. It's a completely differ- ent offering to the patient." In addition to technology and surgical techniques coming of age to make corneal allografts more feasible than before, Dr. Muller said steriliza- tion and storage methods by Lions VisionGift Vision Research Labora- tory has created an economical way to store sterile, ready-to-use tissue at room temperature without the need for reconstitution. Dr. Muller said Allotex is refin- ing the process engineering to take a cornea and make 150 of these but- ton allografts out of it. At this point, Dr. Muller said he envisions stock amounts of the button lenticules of about 3 mm in diameter and 25 µm in central thickness being stored by physicians offering this procedure. Dr. Muller said this donor tissue can be used to make lenticules of about 5.5 mm in diameter with a central thickness of 50 to 100 µm for treatment of hyperopia. Custom lenticules could also be fashioned for patients with astigmatism. PEARL works, Dr. Muller said, because you're taking a clinical laser on a clinical eye to remove a set amount of tissue and then taking a piece of that lenticule and putting it on another eye. To develop a laser system and process that allows you to take bits and pieces of a donor cornea, accurately measure and shape them, requires "millions of dollars worth of engineering to turn it from a cool idea into a real product." The challenge with inlays and onlays—both allogenic and syn- thetic—remains patient neuroadap- tation. "In a sense, it's a modified monovision," Dr. Muller said. "What patient population tolerates this? We don't know that number." However, the beauty of these inlays—and the synthetic variety—is their removability. "It becomes the holy grail, a semi-permanent contact lens that can be removed if you don't like it or removed and replaced if the refraction changes. I think that's the key message," Dr. Muller said. "Just because refractive error/ wearing corrective lenses is looked upon as a 'nuisance' and non-med- ical condition doesn't mean we cannot strive to develop and offer better techniques in correcting it," Dr. Negrin said. "I am all for using donor corneal tissue for this pur- pose provided there is no scarcity of donor tissue, and we are able to reliably provide the service." EW References 1. Ratner BD, et al. Biomaterials Science, Third Edition: An Introduction to Materials in Medi- cine. Academic Press. 2012 Dec 31;931. 2. McDonald MB, et al. The nationwide study of epikeratophakia for myopia. Am J Ophthal- mol. 1987;103:375–83. 3. Pradhan KR, et al. Femtosecond laser-as- sisted keyhole endokeratophakia: correction of hyperopia by implantation of an allogenic lenticule obtained by SMILE from a myopic donor. J Refract Surg. 2013;29:777–82. 4. Jacob S, et al. Preliminary evidence of suc- cessful near vision enhancement with a new technique: PrEsbyopic Allogenic Refractive Lenticule (PEARL) corneal inlay using a SMILE lenticule. J Refract Surg. 2017;33:224–29. 5. Ganesh S, et al. Cryopreservation of ex- tracted corneal lenticules after small incision lenticule extraction for potential use in human subjects. Cornea. 2014;33:1355–62. Editors' note: Dr. Muller has financial interests with Allotex. Dr. Jacob has financial interests with instruments she uses to perform CAIRS. Dr. Negrin has no financial interests related to her comments. Contact information Jacob: dr_soosanj@hotmail.com Muller: info@allotex.com Negrin: asnreis@gmail.com Is it continued from page 64 A: 330-degree CAIRS segment seen implanted in keratoconic eye; B: preop and postop improvement seen in all parameters Source (all): Soosan Jacob, MD

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