Impact of post-applanation deformation curvature on the air-puff pressure temporal profile at the corneal apex

Abstract

Purpose : Air-puff load and corneal surface geometry have a synergistic relationship. We aim to investigate the effect of corneal concavity and deflection amplitude on the resultant impinging jet load.

Methods : Data on unobstructed CorVis ST air-puff measured by hot wire anemometry¹ was adaptively calibrated using Confinement Index (CI) to account for jet confinement (Fig. 1-a) with corneal concavity. CI was defined as inverse radius multiplied by half the peak to peak distance reported by CorVis ST dynamic corneal response parameters (DCR’s). DCR’s from a subset of 10 healthy eyes of 10 subjects were analyzed to compare the changes in magnitude and timing of maximum load between obstructed and unobstructed flow. Delayed corneal response to the applied air-puff was characterized by subtracting maximum pressure time from maximum deflection time. All data are presented as mean ± standard deviation.

Results : Accounting for post-applanation corneal curvature and deformation on unobstructed air-puff profile resulted in 35.9% ± 5.9% increment in maximum air-puff load on corneal apex (Fig. 1-b). Further, obstructed air-puff values impinging on the apex of the cornea reach a peak earlier than corneal deflection. This delay is measured to be 2.75 ± 0.32 ms in the study subjects (Fig. 1-c).

Conclusions : Precise load characterization is an essential step in assessing viscoelastic response of biomaterials. As explained by the Coanda effect², fluid jet tends to stay attached to the deforming surface, altering the angle of fluid outflow. The concave shape of the cornea after applanation results in backward air flow increasing the impact experienced on the apex. Results of this study indicate that the effect of post applanation deformation curvature cannot be neglected in characterizing the impinging jet load. Delayed corneal deflection in response to the air-puff shows that corneal viscoelasticity cannot be ignored in future studies. This model may be used to enhance the intraocular pressure estimation and may shed light into the effect of tissue properties in different disease states.
¹C. J. Roberts et al., J Refract Surg, 33, 2017.
²D. J. Tritton, Physical Fluid Dynamics, 2012

This is a 2020 ARVO Annual Meeting abstract.

 

(a) Confined impinging jet by deforming corneal concavity. (b) Hot wire measured unobstructed air-puff, vs. obstructed air-puff, and CorVis ST internal pressure. (c) Delayed corneal response to maximum obstructed air-puff load.