To investigate the spatiotemporal variation of pressure on the corneal surface under an air puff load among normal, keratoconic, and highly myopic corneas.

From January 2022 to December 2022, two healthy subjects (two eyes), two patients with keratoconus (two eyes), and two patients with high myopia (two eyes) were selected for the study at the Beijing Tongren Ophthalmic Center affiliated to Capital Medical University. Among them, the healthy subjects comprised one male (one eye) and one female (one eye) with an average age of 31 (ranging from 30 to 32 years). The keratoconus patients comprised one male (one eye) and one female (one eye) with an average age of 26 (ranging from 22 to 30 years). The high myopia patients comprised one male (one eye) and one female (one eye) with an average age of 25 years (ranging from 24 to 26 years). The geometric parameters of the anterior and posterior surfaces of the cornea from three groups of subjects: normal cornea, keratoconus, and high myopi using computational fluid dynamics coupled with three dimensional Scheimpflug imaging were performed. A personalized three-dimensional corneal model was constructed. After setting up the computational domain and boundary conditions, simulate and visualize the effects of the air puff from the ocular response analyzer on the cornea, recording the flow field structure and surface pressure distribution.

When the flow field was 5 ms, a relatively stable shear layer was not formed, and the airflow near the axis was obstructed, decelerated, and pressurized, forming an axial reverse pressure gradient. The airflow away from the axis rolled up and reached the wall at about 7 ms. When the flow field was 10 ms, the shear layer was still unstable, and the pressure waveform was formed and only existed for 7 ms~13 ms during the jet acceleration process. At a flow field of 15 ms, a relatively stable mixing layer was formed after 13 ms, and the pressure fluctuations within the mixing layer weaken and disappear. The jet′s injection effect on the surrounding flow was enhanced, and the pressure decreases. At a flow field of 20 ms, the jet dynamic pressure decreased, the stagnation pressure decreased, the absolute negative pressure outside the shear layer decreases, and the pressure gradient tended to flatten. Unstable shear layer pressure waves appeared in the 25 ms flow field, with pressure hysteresis near the corneal stagnation point. When the velocity of the healthy corneal jet reaches its peak, the radial flow of surface pressure exhibited typical subsonic shock jet flow characteristics. With the corneal center as the stationary point, all dynamic pressure was converted into static pressure, and the velocity stagnates to 0. When the pressure reacheed its peak, the fluid outside the stationary point then turned and flowed downstream along the surface of the detection piece. The maximum stationary pressure on the corneal surface occured between 15 ms and 16 ms. This characteristic was described by Navier-Stokes system of equations. The radial distribution curve of pressure on the surface of keratoconus was lower than that of healthy cornea near the stagnation point, and the pressure distribution curve decreased more slowly than that of healthy cornea. The absolute value of the slope was smaller. When more significant with the degree of disease, and the radial position where the surface pressure exceeded that of healthy cornea was further away. The radial distribution curve of corneal surface pressure in myopic eyes was higher than that of healthy cornea in a considerable ranged except for the corneal edge, and the decrease was slower before the radial dimensionless distance of 0.4.

The air puff reaches the apex of the cornea in approximately 7 ms. The changes in pressure over time and space for normal corneas, keratoconus corneas, and corneas with high myopia exhibit consistent patterns, differing only in numerical distribution. The pressure changes symmetrically around the apex, with a decrease in flow velocity and an increase in pressure near the stagnation point. There was a hysteresis phenomenon during the acceleration and deceleration processes of the jet, and the maximum pressure at the stagnation point closely coincides with the maximum jet velocity, with both time curves exhibiting similar shapes.