Purpose: To objectively measure the corneal treatment spots in vivo using very high-frequency ultrasound (VHFU) after non-contact laser thermal keratoplasty (LTK) to better understand the variability and regression of refractive outcomes.
Methods: In an institutional setting, VHFU was performed on 128 spots (8 eyes of 4 patients) using an immersion scanning technique 1 to 2 years after LTK with a single element focused transducer (50 MHz arc scanning ultrasound). Biometric techniques were used to evaluate the treatment spot depth, corresponding corneal thickness, and spot profile between patients, eyes (left/right), and by location on the cornea. The identical technique was used in a rabbit immediately after LTK to compare ultrasound versus histologic findings.
Results: The mean treatment spot penetration depth ranged between 0.373 and 0.533 mm, representing 64% to 78% of the corneal thickness compared to previous reports of 80% to 90%. Treatment spot depth, the corresponding corneal thickness, and percentage of overall cornea penetrated differed significantly across patients. Treatment spot depth was not significantly related to the level of applied laser energy (230 to 258 mJ) (0.082 Pearson sign). Spot profiles were not uniformly cone-shaped; W- and wedge-shaped were also identified. Ultrasound findings in the rabbit were similar to histology results and confirmed evidence of epithelial remodeling.
Conclusions: Very high-frequency ultrasound of 128 treatment spots after non-contact LTK demonstrates epithelial remodeling and inconsistencies in penetration depth and profile.