Graft harvesting in centralized facility found a viable option

May 15, 2006

Baltimore-Harvesting of lamellar grafts in a centralizedfacility is a feasible strategy, said Roy S. Chuck, MD, PhD. Hereported the results of a study showing that the lamellar grafttissue remained viable 2 days later when it was cut at one site,placed in cold storage, and shipped to another facility viaairmail.

Baltimore-Harvesting of lamellar grafts in a centralized facility is a feasible strategy, said Roy S. Chuck, MD, PhD. He reported the results of a study showing that the lamellar graft tissue remained viable 2 days later when it was cut at one site, placed in cold storage, and shipped to another facility via airmail.

"However, associated cost and the need to develop a new skill set to operate modern microkeratome technology may prevent some surgical facilities from offering that procedure," said Dr. Chuck, associate professor, Wilmer Eye Institute, Johns Hopkins University School of Medicine, Baltimore. "Our findings show that reliance on a centralized facility to prepare the lamellar graft is a reasonable alternative for providing more surgeons with viable tissue."

Preparation

The corneas were mounted on an artificial anterior chamber (ALTK system, Moria), pressurized in the chamber with Optisol GS to maximize endothelial protection, and mechanically de-epithelialized. Both microkeratomes were used to cut the cornea at a lamellar depth of 350 μm.

For the mechanical microkeratome-cut buttons, the thickness of the anterior and posterior grafts was measured with ultrasound pachymetry immediately after cutting. The five buttons cut with the femtosecond laser were not immediately separated into anterior and posterior sections.

Then, the specimens were replaced in the original eye bank Optisol-filled storage containers and shipped by cold storage via express airmail to Dr. Chuck's laboratory at the Wilmer Eye Institute. At Wilmer, the anterior and posterior grafts were separated (manual separation of the laser-cut tissue was performed at this time). Section thickness was measured and endothelial viability was evaluated with trypan blue and alizanin red S staining.

The results showed no significant difference in mean total pre-cut thickness between the corneas in the mechanical microkeratome and femtosecond laser groups, 559 versus 578 μm, respectively. Immediately after cutting, mean thickness of the anterior and posterior grafts in the mechanical microkeratome group was 361 and 203 μm, respectively.

"These results showed that the mechanical microkeratome performed well in cutting at the targeted depth," Dr. Chuck said.

Remeasurement

Remeasurement of the grafts cut with the mechanical microkeratome at 2 days post-cutting revealed there was significant thinning of the anterior lamellar sections accompanied by significant swelling of the posterior sections. However, comparisons between the mechanical microkeratome and femtosecond laser groups showed no significant differences for either mean anterior graft thickness, 282 versus 324 μm, or mean posterior graft thickness, 413 versus 397 μm, respectively.

In addition, the percentage of devitalized endothelial cells at that time was similarly low in the mechanical microkeratome and femtosecond laser groups, 3.4% versus 1.6%, respectively.

"We postulated that delaying separation of the anterior and posterior sections in the femtosecond laser group might reduce exposure of the stromal surfaces to the storage media and thereby limit posterior section swelling," Dr. Chuck said. "After 2 days of storage, however, there was no difference between groups in total corneal thickness or thickness of the anterior and posterior sections."

Discussing other potential pros and cons of using the femtosecond laser for lamellar graft preparation, Dr. Chuck acknowledged that it is significantly more costly to purchase and maintain than a classic mechanical microkeratome.

Furthermore, it was recently reported that posterior grafts created using the femtosecond laser contain optically imperfect ridges in the periphery.