CW-IR laser thermokeratoplasty: Refractive changes and analysis by optical coherence tomography and light microscopy

Purpose: Laserthermokeratoplasty is currently performed with flash lamp-pumped holmium lasers. Due to their well-defined thermal effect, cw-emitting laser sources are more appropriate for the induction of corneal collagen shrinkage. Refractive changes achieved using such laser sources with various irradiation modalities are reported. To correlate the refractive changes to the extent of the coagulations and to visualize local changes in thickness and radius of curvature of the cornea, optical low coherence tomography (OCT) was performed prior to and after coagulation and compared to histology. Methods: An IR laser diode (SDL) emitting up to 300 mW at 1.87 μm was used. The laser light was coupled to a 400 μm low-OH quartz fiber and the cornea irradiated in non contact mode either directly or by focusing into the stroma. Single lesions as well as limbus-to-limbus cross sections were scanned with OCT at an axial and lateral resolution of 20 μm. Histologic evaluation of the scanned areas was performed by polarization microscopy after Sirius-Red staining. Results: Maximum refractive changes could be achieved with the focusing device and an irradiation power of about 200 mW. The histology shows more details than the OCT-images, however, the lateral and axial extent of the coagulations were found to correlate well. Local changes in the corneal thickness around the coagulation spots are best analyzed by OCT, since it avoids histologic preparation artifacts. Conclusion: Continuous irradiation with laser diodes provides reproducible and homogeneous coagulations giving suitable refractive changes. OCT provides a non-invasive method of immediate control of the axial extent of the lesions, as well as of changes in thickness and curvature of the cornea. Besides topographic information provided by this method, it enables an invivo follow-up of the long-term postoperative changes of the corneal tissue.