Abstract
Purpose :
Despite the importance of corneal biomechanics on the refractive response to surgery, the impact of LASIK surgery on the cornea mechanical properties is not well known. We used air-coupled ultrasonic optical coherence elastography (ACUS-OCE) to measure full spatial-dependent elasticity of corneas pre- and post-LASIK surgery. We hypothesize that post-LASIK corneal rigidity decreases due to the partial removal of the anterior stroma containing high lamellae interweaving.
Methods :
A 500-kHz ultrasonic air-coupled transducer co-focused with a swept source phase-sensitive Optical Coherence Tomography system was used for non-contact excitation of the corneal apex. Lamb wave propagation speed and average corneal thickness were measured within 16 corneal semi-meridians (360 degrees, 1-s acquisition time) and used to calculate the Shear Modulus (G). Measurements were conducted on 30 healthy control subjects (n=60 corneas; age:20-50 yo; corneal astigmatism<2 D) and 8 patients (n= 16 corneas; age: 23-36 yo) pre- and 1-month post-LASIK. Changes of the speed-thickness index (STI) and the spatial anisotropy of wave speed (SAWS) with LASIK were compared against the baseline metrics of healthy corneas (95% confidence level), when projected into the speed-thickness plot (STP).
Results :
Wave speed decreased with thickness at a rate of 0.014±0.002 m/s/um in healthy corneas (p < 0.001), and at significantly (p<0.001) higher rate of 0.027±0.011 m/s/um post-LASIK. Moreover, G decreased from 52±4 kPa to 43±5 kPa for pre-and post-LASIK corneas, respectively, compared to the baseline 47±6 kPa of healthy corneas. In 12 out 16 corneas post-LASIK, wave speed in all semi-meridians fell within healthy corneal values in the STP and STI; but, in average, 4 semi-meridians in 4 post-LASIK corneas were outside the baseline range. Finally, SAWS did not change significantly post-LASIK.
Conclusions :
LASIK decreased corneal stiffness, which is consistent with the loss of anterior stroma containing high lamellae interweaving. While in the majority of patients, LASIK did not decrease corneal stiffness outside the baseline range, the higher corneal thickness-stiffness slope suggests LASIK may result in abnormal mechanics in some cases. The ACUS-OCE technique may identify preoperatively patients at risk of developing iatrogenic ectasia and allows biomechanical follow-up post-LASIK.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.