Adding donor stroma is new frontier for hyperopia, presbyopia, keratoconus treatment
In 1949, José I Barraquer MD, the father of modern refractive surgery, formulated a simple, yet profound, law: if tissue subtraction from the centre of the cornea corrects myopia, then equal amounts of tissue addition should correct hyperopia of a similar magnitude.
Ronald R Krueger MD
“So, after 70 years, why are we still removing corneal tissue when treating hyperopia?” asked Ronald R Krueger MD, delivering the Barraquer Lecture at the American Academy of Ophthalmology 2018 Annual Meeting in Chicago, USA.
It’s mostly because the precision of excimer lasers simplified hyperopia correction by tissue subtraction, Dr Krueger said. However, the donut-shaped tissue removed leaves the cornea with an uneven surface, which tends to regress as the epithelium thickens to fill in the curves, and creates an extreme hyperprolate corneal shape profile that distorts vision at higher corrections.
Femtosecond lasers are now making possible additive approaches that are as precise as LASIK and may be more suitable for treating higher hyperopia and other conditions, said Dr Krueger, who recently left the Cleveland Clinic to head the Truhlsen Eye Institute at the University of Nebraska Medical Center, Omaha, USA. The techniques involve implanting donor stroma under corneal flaps or in pockets for treating hyperopia, presbyopia and keratoconus. Combined with new methods for preserving, preparing and banking stromal tissue, these could soon make lenticular implantation practical for routine clinical use.
SMILE CREATES POSSIBILITIES
The rise of small-incision lenticule extraction (SMILE®) over the past decade sparked development of additive refractive surgery by generating a large supply of lenticules, which would otherwise be discarded (see above). More than one million SMILE procedures have been performed since 2009, and the procedure is rapidly gaining in popularity. A method for eye-banking SMILE lenticules, involving serotyping, infection testing and storage at -198°C, has been developed by Sri Ganesh MD, Jayanagar, Bangalore, India, for later hyperopia correcting implantation.
An early study implanting eye-banked SMILE lenticules into 22 moderate-to-high hyperopic eyes dropped the mean spherical equivalent refraction from +5.4D to +0.65D, with an average best-corrected visual acuity of 20/26 at 12 months after the surgery (Ganesh S et al. Cornea 2014;33:1355-1362). No loss of best-corrected vision or regression was seen at 12 months, likely because adding tissue leaves the corneal surface with a more regular aspheric profile than the steep to flat profile of hyperopic LASIK, thus, leading to a stable, uniform epithelial thickness, Dr Krueger said. The mean corneal Q value did increase from -0.32 to -0.82, but this is less than the extreme hyperprolate shape seen with hyperopic LASIK corrections of this magnitude.
No adverse reactions or rejection was observed after steroids were discontinued three months following the surgery. Dr Krueger believes the risk of immunologic response may be limited in part because the implanted stroma is less responsive to the host immune system. Furthermore, studies have shown that autogenic implants and xenogeneic grafts that have been decellularised generate no inflammatory cell infiltrates or fibrotic reaction in the cornea (Liu YC et al. Sci Rep. 2018 Jan 30;8(1):1831).
Refractive lenticular implantation (RELIMP) is the name of another approach, popularised by Dr Osama Ibraham from Egypt, that cuts a small myopic SMILE crater into the cornea of a hyperopic eye to facilitate centration and refractive targeting of a higher power SMILE lenticule. For example, placing a +13.75 lenticule into a -2.25 sphere -4.00 cyl SMILE crater brings a +12.00 sphere +4.00 cyl aphakic patient to a stable, near plano refraction throughout the first postoperative year, and beyond.
Beyond hyperopia, similar tissue addition techniques have been used to treat keratoconus, inserting a planar or donut-shaped lenticule into a pocket to thicken and centrally flatten the cornea, and for presbyopia by adding a centrally placed 1.0mm-to-2.0mm lenticule, improving near vision while maintaining distance correction with no haze or inflammatory response, as seen with some synthetic implants, Dr Krueger added.
In a yet unpublished Turkish study of 17 presbyopic eyes and 28 hyperopic eyes, implanting shaped, stromal tissue from a commercial source resulted in a mean central power increase of +2.7D for the correction of presbyopia and a mean uniform power increase of +6.0D for the correction of moderate to high hyperopia (personal communication Aylin Kilic MD and Michael Mrochen PhD). The commercial sources may cut many lenticules from a single donor and sterilise, process, package and store them at room temperature, Dr Krueger said. The surgeon can later open the packaging and shape them with a compact industrial excimer laser as needed.
LIKE – A NEW TECHNIQUE
Dr Krueger and others are beginning clinical investigation of a technique popularised by Dr Theo Seiler, called LIKE (lenticular implantation keratoplasty). This involves shaping donor corneal tissue with Bowman’s layer using a lenticule cavity unit, which defines the precise shape profile and power. The implanted lenticule power is targeted to be greater, and placed under the large LASIK flap of moderate-to-high hyperopia eyes, making it possible to later lift the flap for a myopic or customised ablation enhancement. In the first 12 eyes treated for up to +8.5D in Europe and India, five have so far received a laser ablation one-to-three months after implantation, and one required a replacement implant.
Of the first nine, no eye lost more than one line and three gained two lines at six months. Four had slight, temporary lenticular haze, less than typically seen with LASIK, Dr Krueger said. LIKE-shaped lenticules are also being shaped for treating keratoconus by placing the lenticule in a corneal stromal pocket.
“High hyperopia, presbyopia and keratoconus are just a few of the errors that will implement this technology in the future. Refractive eye-banking will be the emerging, new market and partnership that brings this technology to our field,”
Dr Krueger concluded.