Early results are encouraging using the 500 kHz femtosecond laser in a reverse-cut technique to prepare DSEK donor tissue.
Dr. Baikoff told attendees that a number of differences that distinguish this femtosecond laser from other femtosecond lasers favor use of the system for cutting a DSEK donor graft. Nevertheless, he said, initial experience using this femtosecond laser yielded disappointing results; the procedures involved cutting the cornea from the epithelial surface.
Beginning in June 2009, Dr. Baikoff introduced a novel "reverse-endocut" technique. Using this approach, he said he is obtaining excellent-quality endothelial donor tissue grafts with a predictable thickness of about 90 µm and a low rate of donor endothelial cell loss.
The femtosecond laser performs 3-D cuts with a fast pulse rate frequency (500 kHz), delivers tiny (1.5 µm) spots with very tight spacing (1.6 µm) using low energy (<0.20 µJ), and features a PMMA concave applanation cone that exerts low applanation pressure. Together these features favor reliable cutting of a high-quality DSEK graft.
"The Ziemer femtosecond laser has a pulse rate of 1,000 kHz but only does a 2-D cut, and compared with the IntraLase [Abbott Medical Optics], the VisuMax has smaller spot spacing and delivers 10 times less energy," he said. "Neither the Ziemer nor the IntraLase device has a concave applanation cone, and their applanation cones are plastic."
Cutting depth is very unpredictable from the endothelial side using a mechanical microkeratome because it depends on the thickness of the donor tissue and the quality of the microkeratome, according to Dr. Baikoff.
"This means that endothelial graft thickness is between 100 and 300 µm in the center with much thicker edges," he said. "In my private practice, I cannot afford to waste any donor material because I have no back-up tissue. Therefore, I thought using a femtosecond laser would offer a safer technique."
"I began using the 200 kHz femtosecond laser [VisuMax], but was not pleased with the quality of the cut," he added. "The 500-kHz system has a very low power impact and together with the tiny, tightly spaced spots and low applanation pressure, creates a graft with a very smooth surface without risk of perforating the cornea and losing the donor."
Initially, Dr. Baikoff said he began cutting the donor from the epithelial surface. The quality, however, was suboptimal because in performing a deep lamellar cut (~400 to 450 µm) to obtain the DSEK graft, light scattering is observed due to the depth, and light transmission is irregular through the corneal edema of the donor tissue.
Switching to the endothelial side required less energy because the technique is similar to LASIK flap creation and resulted in a much smoother cut.
Dr. Baikoff said that although applanation on the endothelium can result in damage, because the cone on the 500-kHz femtosecond laser is concave with a glass optic and exerts very low pressure (equivalent to 50 cm of water in the anterior chamber), it can be placed safely on the corneal endothelium as long as an efficient "shield" is placed between the lens and the endothelium.
"We realized that the viscoelastic was too 'viscous' and created an irregular film that led to uneven cuts and even a buttonhole," he said. "Balanced salt solution provides insufficient protection of the endothelium, however, substituting tissue culture medium [Optisol], the material in which the graft is preserved, for viscoelastic as the interface fluid yielded improved results."
Initial data from use of the reverse-endocut technique in straightforward DSEK procedures involving no additional surgery showed average endograft thickness of 89 µm with very low standard deviation. Average donor endothelial cell loss was about 15%, although Dr. Baikoff said that a colleague using this technique in the laboratory has found that the cell loss rate is only 7%.