Biomechanical diagnostics of the cornea

Abstract

Corneal biomechanics has been a hot topic for research in contemporary ophthalmology due to its prospective applications in diagnosis, management, and treatment of several clinical conditions, including glaucoma, elective keratorefractive surgery, and different corneal diseases. The clinical biomechanical investigation has become of great importance in the setting of refractive surgery to identify patients at higher risk of developing iatrogenic ectasia after laser vision correction. This review discusses the latest developments in the detection of corneal ectatic diseases. These developments should be considered in conjunction with multimodal corneal and refractive imaging, including Placido-disk based corneal topography, Scheimpflug corneal tomography, anterior segment tomography, spectral-domain optical coherence tomography (SD-OCT), very-high-frequency ultrasound (VHF-US), ocular biometry, and ocular wavefront measurements. The ocular response analyzer (ORA) and the Corvis ST are non-contact tonometry systems that provide a clinical corneal biomechanical assessment. More recently, Brillouin optical microscopy has been demonstrated to provide in vivo biomechanical measurements. The integration of tomographic and biomechanical data into artificial intelligence techniques has demonstrated the ability to increase the accuracy to detect ectatic disease and characterize the inherent susceptibility for biomechanical failure and ectasia progression, which is a severe complication after laser vision correction.

Keywords: Corneal biomechanics; Corneal ectasia; Corneal imaging.

Fig. 1

Ocular response analyzer (ORA) measurements showing the air pulse deforming the cornea (ingoing phase) and registering corneal signal (Y axis) through time (X axis) in milliseconds, in which P1 is the first applanation moment. The Gaussian configuration is from when the air pulse signal is shut off, then with the continuing increase in magnitude of the air pulse due to inertia in the piston, the cornea assumes a concave configuration. In the outgoing phase (air pressure decreases), the cornea passes through a second applanation, when the pressure of the air pulse (P2) is again registered. The pressure-derived parameters generated are corneal hysteresis (CH) and corneal resistance factor (CRF). This figure is a composite made by the authors of classic pictures available in public domain

Fig. 2

The impact of the chamber pressure on the deformation of two different contact lenses. The toughest lens (525 μm thick with 62% hydroxyethyl methacrylate) in its natural state (a) is compared to the most pliable lens (258 μm thick with 42% methyl methacrylate) in its natural state (b). Note that each lens deforms more at higher chamber pressures and that the toughest lens deforms less when compared to the most pliable lens under the same pressure levels of 5 mmHg (c and d), 25 mmHg (e and f), and 45 mmHg (g and h). However, note the toughest lens deforms more under low pressure (c) than the most pliable lens under high pressure (h) [55]. Personal archive

Fig. 3

Standard Corvis ST parameters. The figure shows the deformation amplitude (DA), applanation lengths (AL), corneal velocities (CVel) recorded during ingoing and outgoing phases and the radius of curvature at the highest concavity (Curvature radius HC), and thereby calculating and registering corneal thickness and IOP. Personal archive

Fig. 4

The Vinciguerra Screening Report. This display provides correlations of normality values and a biomechanically adjusted intraocular pressure. It uses a calibration factor to calculate the IOP value based on the pressure at the time of the first applanation. It empowers the calculation of the Ambrósio Relational Thickness over the horizontal meridian (ARTh) and the Corvis Biomechanical Index (CBI). Personal archive

Fig. 5

The ARV (Ambrósio, Roberts & Vinciguerra) Biomechanical and Tomographic Display showing the Corvis Biomechanical Index (CBI), tomographic biomechanical index (TBI) from the VAE-NT case with uncorrected distance visual acuity of 20/20. Personal archive

Fig. 6

The Ambrósio, Roberts & Vinciguerra (ARV) Display from the VAE-E (fellow eye of the eye on Fig. 5). Personal archive

Fig. 7

Comparative Corvis ST display before (A in red) and after CXL (B in blue), including the overlap image at higher deformation, the SSI (Stress-Stain Index), and the stress-strain curves, along with comparative DA ratio, integrated radius, and the Stiffness Parameter at first Applanation (SPA1) indicating stiffer behavior after the procedure. Personal archive