Ultrastructure of Picosecond Laser Intrastromal Photodisruption

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Abstract
To investigate the ultrastructure of the corneal stroma after picosecond intrastromal photodisruption with a neodymium-doped yttrium-lithium-fluoride (Nd:YLF) laser. We performed picosecond intrastromal photodisruption on six human eye-bank eyes using a lamellar technique. Thirty picosecond pulses at 1000 Hz and 20 to 25 mJ per pulse were placed in an expanding spiral pattern, the pulses separated by 15 microns. Three layers were placed in the anterior stroma, separated from each other by 15 microns. In addition, intrastromal radial and arcuate incisions were generated in two living rabbit eyes in a plane perpendicular to the corneal surface. After the procedure, the corneas were processed for scanning and transmission electron microscopy. Scanning electron microscopy of the eye-bank eyes demonstrated multiple, coalescing intrastromal cavities forming a layer oriented parallel to the corneal surface. These cavities had smooth inner walls. Transmission electron microscopy demonstrated tissue loss surrounding some cavities, with the terminated ends of collagen fibrils clearly evident. Other cavities were formed by separation of lamellae, with little evidence of tissue loss. A pseudomembrane was present along the margin of some cavities. Although there was occasional underlying tissue disruption along the border of a cavity, there was no evidence of thermal damage or tissue necrosis. The perpendicular photodisruptions demonstrated intrastromal cleavage of corneal collagen similar to diamond-knife incisions, with the exception of intact overlying Bowman's and epithelial layers. Intrastromal photodisruption with a Nd:YLF picosecond laser induced no thermal necrosis or coagulative change in the region of tissue interaction. Lamellar intrastromal photodisruption demonstrated both tissue loss and lamellar separation when performed with the current treatment parameters, possibly limiting ablation efficiency and predictability.