Surface wave elastometry of the cornea in porcine and human donor eyes

Abstract

Purpose: To introduce a nondestructive technique for characterization of corneal stiffness, determine measurement precision, and investigate comparative stiffness values along central, radial, and circumferential vectors in porcine corneas. The effects of epithelial debridement, relaxing incisions, and crosslink-mediated stiffening on surface wave velocity are also studied.

Methods: A handheld prototype system was used to measure ultrasound surface wave propagation time between two fixed-distance transducers along a ten-position map. Repeatability was assessed with replicate measurements in 6 porcine corneas. In 12 porcine globes with controlled intraocular pressure (IOP), serial measurements were performed before and after epithelial removal, then after 250- and 750-microm-deep relaxing incisions. In human globes with constant intravitreal pressure, central wave velocity and transcorneal IOP measurements were compared before and after collagen cross-linking.

Results: Measurement repeatability across all regions was between 2.2% and 8.1%. Epithelial removal resulted in increases in measured stiffness in 67% of eyes, but statistical power was insufficient to detect a systematic change. Wave velocity across a central incision decreased significantly after 250-microm keratotomy (P < .001), but did not undergo a significant further decrease with deeper keratotomy. Meridional stiffness changes consistent with coupling effects were detected after keratotomy. Surface wave velocity and transcorneal IOP measurements increased markedly after collagen cross-linking despite maintenance of a constant IOP.

Conclusions: Handheld corneal elastometry provides a repeatable measure of regional stiffness changes after relaxing incisions and collagen cross-linking in in vitro experiments. Surface wave elastometry allows focal assessment of corneal biomechanical properties that are relevant in refractive surgery, ectatic disease, and glaucoma.

Figure 1

The elastometer probe incorporates surface-contact transducers and measures the time of flight of a sonic wave at the corneal surface.

Figure 2

Measurement template incorporating 10 regional and directional measurements of surface wave velocity (central [1, 2], radial peripheral [3−6], and circumferential peripheral [7−10]). Black bars demarcate the span of the transducer tips at each location (4.5 mm), and numbers indicate the order in which each measurement was obtained.

Figure 3

Sequential results of central stiffness measurements at position A) 1 and B) 2 in 12 porcine corneas maintained at IOP 15 mmHg. Each cornea was measured before debridement, after debridement, after a 3-mm long, 250-μm deep central keratotomy was performed along position 2 (see Figure 2), and after extension of the incision to 750 μm.