Lenticules: Picking Up the SLAK

Many ideas that emerged during the 1960s have not withstood the test of time, from Beatles haircuts to avocado bathrooms and smoking on planes. One innovation from the decade, however, is just coming into its own: allogeneic stromal lenticule transplantation (SLAK).

“The term SLAK derives from José Barraquer’s work where he described the procedure of stromal transplantation, which he called stromal keratophakia. His outcomes weren’t too bad, considering the technology he had available to him, though there were complications,” said Jod Mehta PhD.

“One was a complication in the lenticule creation. Then there was the cutting accuracy of the microkeratome that he used, which was very variable. The cryopreservation techniques he used were also outdated. Of course, now, in 2026, we have much better technology available.”

That improved technology can be seen in improved stromal keratophakia procedures, Professor Mehta said. As before, lenticules can be created using donated corneal tissue, but now with the aid of femtosecond lasers (VisuMax 800 and Ziemer platforms) and corneal lenticule extraction (KLEx).

Prof Mehta also reported that it is now possible to create myopic and hyperopic lenticules as well. Donor tissues can undergo decellularisation with sodium dodecyl sulphate to remove keratocytes, and cross-linking can be included for increased structural robustness.

“This is a topic we’ve investigated over 15 years, and we did extensive proof-of-concept studies in rabbits, which showed it was possible to re-implant these lenticules after they were frozen,” Prof Mehta said. “In non-human primate studies, we could actually reverse the refractive error in these animals following lenticule creation and re-implantation.”

Multiple indications, multiple data centres

There are now four primary indications for SLAK, according to Prof Mehta. These are: tectonic, i.e., a glued patch graft in corneal microperforation; therapeutic usage in keratoconus and ectasia, as well as stromal expansion in cross-linking for thin and ultra-thin corneas; refractive treatment for hyperopia, aphakia, and presbyopia; and as a scaffold in drug delivery systems and tissue engineering.

Using the presbyopia indication as an example, Prof Mehta described research published since 2017, including four papers and 120 eyes, conducted mainly in India. A multicentre study based in Europe was also included, using an allograft presbyopic inlay.

“The procedure in the multicentre study involved inserting a 1.0 mm fluorescein-stained lenticule into a femtosecond pocket,” Prof Mehta said. “We saw an improvement in uncorrected visual acuity from 0.6 on average all the way up to 0.2.”

SLAK’s efficacy can also be demonstrated in therapeutic uses, including ectasia. Prof Mehta described how SLAK uses both a plano-shaped refractive inlay and a hyperopic, negative-shaped lenticule, something he said is “currently only available on the VisuMax femtosecond laser in order to create a hyperopic refractive outcome.

“In 10 cases of keratoconus in patients that were contact lens intolerant, with curvature ranging between 55 D-CCT and 70 D-CCT, lenticules were obtained from corneal donor tissue suitable for penetrating keratoplasty and deep anterior lamellar keratoplasty. In 70% of postoperative eyes, corrected distance visual acuity reached 20/100, up from 20/200 preoperative best-corrected visual acuity.”

Banking on eye banking

SLAK has come a long way since the 1960s, yet further advancements are coming. Prof Mehta reported that bioengineered construct nanoparticles containing recombinant nerve growth factor (NGF) were sent to Singapore, where they were implanted into a rabbit eye through a pocket created using a femtosecond laser.

“Other studies have looked at other drugs as well. Some are looking at NGF, one has been using silver nanoparticles, and another is examining using antibiotics. This is quite a nascent field; none yet have been translated into patients,” Prof Mehta said.

The most ‘avant-garde’ area of SLAK is eye banking and lenticule storage. In Singapore, Prof Mehta has worked to set up an eye bank in cooperation with Cordlife, a consumer healthcare and stem cell banking company. The clinic will ensure a steady supply of lenticules for autologous and potentially allogeneic use, an initiative he’s working on with the Venice Eye Bank.

“This will require some work with the regulatory authorities because we need to get consent from donors, as these people are still alive. It’s probably going to be easier for tectonic and therapeutic uses as opposed to refractive uses,” Prof Mehta said.

“SLAK has reached more than 8 million procedures with different femtosecond lasers. The creation of these refractive human stromal lenticules offers a unique opportunity to use this stromal tissue for other applications such as tectonic, therapeutic, and refractive lenticule implantation.”

Prof Mehta presented at the EuCornea annual congress in Porto, Portugal.

Jod Mehta MBBS, FRCOphth, FRCS(Ed), FAMS, PhD is a senior consultant at the Singapore Eye Centre, Singapore. jodmehta@gmail.com