Corneal Biomechanics in Ectatic Diseases: Refractive Surgery Implications

Abstract

Background: Ectasia development occurs due to a chronic corneal biomechanical decompensation or weakness, resulting in stromal thinning and corneal protrusion. This leads to corneal steepening, increase in astigmatism, and irregularity. In corneal refractive surgery, the detection of mild forms of ectasia pre-operatively is essential to avoid post-operative progressive ectasia, which also depends on the impact of the procedure on the cornea.

Method: The advent of 3D tomography is proven as a significant advancement to further characterize corneal shape beyond front surface topography, which is still relevant. While screening tests for ectasia had been limited to corneal shape (geometry) assessment, clinical biomechanical assessment has been possible since the introduction of the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, USA) in 2005 and the Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) in 2010. Direct clinical biomechanical evaluation is recognized as paramount, especially in detection of mild ectatic cases and characterization of the susceptibility for ectasia progression for any cornea.

Conclusions: The purpose of this review is to describe the current state of clinical evaluation of corneal biomechanics, focusing on the most recent advances of commercially available instruments and also on future developments, such as Brillouin microscopy.

Fig. (1)

ORA Signal.

Fig. (2)

Patient with very asymmetric keratoconus. The topometric indices were within the normality in OD (A). The Belin/Ambrósio Enhanced Ectasia Display (B) demonstrated relatively normal elevation maps, but a thin cornea with 1.22 mm displacement of the thinnest point towards the inferior-temporal quadrant, an ART-Max of 322µm and a final deviation value of 1.30. The ORA KMI and KMP also allowed the detection of ectasia in both eyes (C and D).

Fig. (3)

Corvis ST Overview display.

Fig. (4)

Scheimpflug images representing the ingoing applanation (above), highest concavity (middle), and outgoing applanation (below) moments. Numbers 1 and 4 are the applanation lengths at the ingoing and outgoing phases, respectively. Number 2 represents the radius of curvature at highest concavity or inverse concave radius. Number 3 represents the deformation amplitude at the highest concavity moment.

Fig. (5)

Deformation amplitude, deflection amplitude and whole eye movement parameters graphic representation (plotted versus the time). The deformation amplitude is the sum of deflection amplitude and whole eye movement.

Fig. (6)

Radius of curvature at highest concavity or inverse concave radius algorithm (above). The parameters has also a graphic representation that is plotted versus the time (below).

Fig. (7)

Demonstrative scheme of the peak distance parameter. It describes the distance between the two highest points of the cornea at the highest concavity moment (1).

Fig. (8)

Demonstrative scheme of the deformation amplitude ratio between the apex and at 2 mm from the apex (DA Ratio 2 mm). It describes the ratio between the deformation amplitude at the apex (red arrow) and the average deformation amplitude at a 2 mm nasal and temporal zone (greens arrows).

Fig. (9)

Demonstrative scheme of the delta arc length. It represents the change of the arc length during the highest concavity moment in a defined 7 mm zone.

Fig. (10)

Vinciguerra Report Before (A) and After (B) Crosslinking.

Fig. (11)

Ambrósio, Roberts & Vinciguerra (ARV) Biomechanics and Tomographic Assessments with TBI of the Right Eye with clinical ectasia from a patient with highly asymmetric ectasia, with left eye presented in Fig. 12.

Fig. (12)

Ambrósio, Roberts & Vinciguerra (ARV) Biomechanics and Tomographic Assessments with TBI of the Left Eye with normal topography.