Histology and ultrastructure of picosecond laser intrastromal photorefractive keratectomy (ISPRK)

Purpose: Picosecond intrastromal ablation is currently under investigation as a new minimally invasive way of correcting refractive error. When the laser pulses are placed in an expanding spiral pattern along a lamellar plane, the technique is called intrastromal photorefractive keratectomy (ISPRK). Methods: We performed ISPRK on six human eye bank eyes. Thirty picosecond pulses at 1000 Hz and 20-25 μJ per pulse were separated by 15 microns. A total of 3 layers were placed in the anterior stroma separated by 15 microns. The eyes were then preserved and sectioned for light, scanning and transmission electron microscopy. Results: Light and scanning electron microscopy reveals that picosecond intrastromal ablation using an ISPRK pattern demonstrates multiple, coalescing intrastromal cavities oriented parallel to the corneal surface. These cavities possess a smooth appearing inner wall. Using transmission electron microscopy, we noticed tissue loss surrounding some cavities with collagen fibril termination and thinning of collagen lamella. Other cavities we formed by separation of lamella with little evidence of tissue loss. A pseudomembrane lines the edge of some cavities. Although underlying tissue disruption was occasionally seen along the border of a cavity in no case was there any evidence of thermal damage or tissue necrosis. Conclusion: Ablation and loss of tissue in ISPRK results in nonthermal microscopic corneal thinning around some cavities whereas others demonstrate only lamellar separation. Alternate patterns and energy parameters should be investigated to bring this technology to its full potential in refractive surgery.