Very high-frequency ultrasound corneal analysis identifies anatomic correlates of optical complications of lamellar refractive surgery

Abstract

OBJECTIVE: To examine the utility of very high-frequency (VHF) ultrasound scanning in determining the anatomic changes and correlates of optical complications in lamellar refractive surgery. STUDY DESIGN: Case series. PARTICIPANTS: Cases analyzed included marked asymmetric astigmatism postautomated lamellar keratoplasty (ALK), image ghosting despite normal videokeratography post-ALK, uncomplicated myopic laser in situ keratomileusis (LASIK), and hyperopic LASIK with regression. METHODS: A prototype VHF ultrasound scanner (50 MHz) was used to obtain sequences of parallel B-scans of the cornea. Digital signal processing techniques were used to measure epithelial, stromal, and flap thickness values in a grid encompassing the central 4 to 5 mm of the cornea, enabling pachymetric mapping of each layer with 2-micron precision. MAIN OUTCOME MEASURE: The appearance of the corneas in VHF ultrasound images and thickness values of individual corneal layers determined from VHF ultrasound data. RESULTS: VHF ultrasound resolved the epithelial, stromal cap, or flap and residual stromal layers 1 year after lamellar surgery. Asymmetric stromal tissue removal was differentiated from stromal cap irregularity. Epithelium acted to compensate for asymmetry of the stromal surface about the visual axis and for localized surface irregularities. Irregularities in the epithelial-stromal interface accounted for image ghosting present despite apparently normal videokeratography. Epithelial thickening was shown after uncomplicated myopic LASIK. Hyperopic LASIK demonstrated relative epithelial thickening localized to the region of ablation accounting for refractive regression. CONCLUSIONS: VHF ultrasound shows promise as a sensitive method of determining the anatomic correlates of optical complications in lamellar refractive surgery.