Biomechanical Health of Cornea in Keratoconus with Central and Paracentral Cone Location

Swanandi Gawde, B.Optom, PGDOVS, (M. Optom)

Optometry Faculty, ITM University – Navi Mumbai, Maharashtra, India

Corneal biomechanical changes were seen in ocular conditions like Glaucoma, keratoconus, and Fuchs dystrophy. ⁽¹⁾ Keratoconus is a progressive, non-inflammatory disorder resulting in thinning and protrusion of the cornea into a conical shape. The prevalence varies from 6.8 to 2,300 per 100,000. ⁽²⁾ Clinically early detection of keratoconus remains challenging. Corneal biomechanics help detect the keratoconus at a very early stage due to the abnormality in stromal collagen and extracellular matrix. ⁽³⁾

The corneal biomechanics can be measured with the Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) instrument. Corneal biomechanical parameters measurements are based on the analysis of the dynamic corneal deformation recorded by the ultra-fast Scheimpflug camera that captures 4,330 images per second. ⁽¹⁾ This instrument takes an 8.0 mm wide horizontal cross-sectional image of the cornea during deformation and the deformation profile is analysed in real-time to derive multiple parameters. In addition, this instrument has a light source an LED light of 455 nm wavelength and applies an air impulse with a maximum pressure of 25 Kilopascals.⁽³⁾

The biomechanical parameters such as Max Inverse Radius, deformation amplitude (DA) Ratio Max [2 mm], Pachy Slope, DA Ratio Max [1mm], Ambrosio relational thickness horizontal (ARTh), Integrated Radius, stiffness parameter at first applanation (SP-A1) and Corvis biomechanical index (CBI) can be analysed in central and paracentral cone location keratoconus with all the grades.

Due to the viscoelastic nature of the corneal tissue in keratoconus, biomechanical properties such as elasticity, creep, stress relaxation, and hysteresis are altered.  Moreover, the reduction of corneal elasticity within the cone area results in greater protrusion. The ratio of stress and strain changes according to the keratoconus progression.

The corneal biomechanical index (CBI) calculates based on the logistic regression formula from different Corvis ST parameters. A study reported that values below 0.25 indicate a low risk of developing ectasia, values between 0.25 and 0.5 indicate a moderate risk of developing ectasia, and values above 0.5 indicate a high risk of developing ectasia. ⁽⁴⁾ The Integrated Radius is the integrated area under the radius of the inversed curvature during the cornea in the concave phase. Due to the air puff, the central part of the cone showed higher convexity as compared to the periphery which results in the central keratoconus cone location getting the higher value of the integrated radius in all the grades of keratoconus as compared with the peripheral cone location keratoconus.

he Deformation amplitude (DA) ratio max [1mm] and [2 mm] increases more in the softer cornea than in the stiffer cornea moreover, it’s higher in the central cone keratoconus as compared to the peripheral cone keratoconus with all the grades. A previous study found that the other parameters of Corvis ST such as Max Inverse Radius, and Pachy slope also showed higher values in keratoconus eyes as compared to the normal eyes. ⁽⁴⁾

Keywords: Corneal biomechanics, Keratoconus cone location, Corvis ST

References:

1. Jędzierowska, Magdalena, and Robert Koprowski. “Novel dynamic corneal response parameters in a practice use: a critical review.” Biomedical engineering online 18.1 (2019): 1-18.
2. Rabinowitz, Yaron S. “Keratoconus.” Survey of ophthalmology 42.4 (1998): 297-319.
3. Vellara, Hans R., and Dipika V. Patel. “Biomechanical properties of the keratoconic cornea: a review.” Clinical and Experimental Optometry 98.1 (2015): 31-38.
4. Yang, K., Xu, L., Fan, Q. et al. Evaluation of new Corvis ST parameters in normal, Post-LASIK, Post-LASIK keratectasia, and keratoconus eyes. Sci Rep 10, 5676 (2020).

The blog is written solely for educational purposes, and it does not have any financial support or conflict of interest.