Assessment of the influence of viscoelasticity of cornea in animal ex vivo model using air-puff optical coherence tomography and corneal hysteresis

Abstract

Application of the air-puff swept source optical coherence tomography (SS-OCT) instrument to determine the influence of viscoelasticity on the relation between overall the air-puff force and corneal apex displacement of porcine corneas ex vivo is demonstrated. Simultaneous recording of time-evolution of the tissue displacement and air pulse stimulus allows obtaining valuable information related in part to the mechanical properties of the cornea. A novel approach based on quantitative analysis of the corneal hysteresis of OCT data is presented. The corneal response to the air pulse is assessed for different well-controlled intraocular pressure (IOP) levels and for the progression of cross-linking-induced stiffness of the cornea. Micrometer resolution, fast acquisition and noncontact character of the air-puff SS-OCT measurements have potential to improve the in vivo assessment of mechanical properties of the human corneas.

Keywords: corneal biomechanics; corneal hysteresis; optical coherence tomography; viscoelasticity.

Figure 1

Simplified schematic diagram of the air‐puff SS‐OCT instrument. (A) Swept‐source OCT set‐up combined with air puff and IOP control systems. (B) SS‐OCT M‐scan (composed of 1600 A‐scans) of the cornea during air‐puff stimulation. Temporal deformation of anterior corneal surface in marked with red curve. (C) A photography of the porcine eye placed in front of the air‐puff chamber. (D) Temporal profile of the force applied on the cornea during measurement. (E) Dynamic hysteresis curve for a single air‐puff stimulation

Figure 2

Parameters extracted from the measurements: (A) After segmentation of the corneal surfaces, the information about the maximum displacement of the apex (MAD) as well as the central corneal thickness before (CCT_bef), at maximum displacement (CCT_max) and after the puff (CCT_aft) application were extracted; (B) Secant slope (S _sec), high‐strain slope (S _high), and low‐strain slope (S _low) were determined based on the loading curve of hysteresis. The area enclosed by the HA is also calculated. Graphical description of S _low and S _high is conceptual (see detailed description in Section 2.6)

Figure 3

Impact of the IOP on the extracted parameters of hysteresis curve. Results for measurements on 35 porcine corneas ex vivo. (A) Evolution of the hysteresis shape under different IOP levels (for simplicity hysteresis curves only for ascending IOP phase of the cyclic inflation test are presented), (B) MAD, (C) central corneal thickness measured before the air puff (CCT_bef), (D) CCT‐corrected MAD (MAD/CCT_bef), (E) HA, (F) HR, (G) secant slope (S _sec), (H) low‐strain slope (S _low), (I) high‐strain slope (S _high) for ascending and descending IOP. Analysis results for ascending phase of hysteresis plot is presented with blue line and points, while descending phase is marked with red color. Errors are presented as shaded plots with width of ±SD. Asterisks indicate statistically significant differences between two analyzed groups. R is a Pearson's correlation coefficient

Figure 4

Impact of CXL treatment on extracted parameters of hysteresis curve. Results for measurements on 13 porcine corneas ex vivo. (A) Hysteresis curves at normal IOP of 15 mm Hg for different stages of collagen cross‐linking procedure. (B) MAD, (C) central corneal thickness measured before air puff (CCT_bef), (D) CCT‐corrected MAD (MAD/CCT_bef), (E) HA, (F) HR, (G) secant modulus (S _sec), (H) low‐strain slope (S _low), (I) high‐strain slope (S _high) measured at different stages of CXL procedure: after epithelium removal (EPI OFF), after RS saturation and after UVA light irradiation with RS application (RS + UV) under for two IOP levels. Results for IOP = 15 mm Hg were presented with green boxplots, while for 25 mm Hg with blue ones. Red crosses represent outliers