Clinical measurement of corneal biomechanical properties : tangent elastic modulus

Abstract

In vivo corneal biomechanical measurement is challenging to perform. Corneal Visualization Scheimpflug Technology (Corvis ST) is a recently launched non-contact tonometer, which was used to measure corneal deformation and its recovery during tonometry in the initial stage of the current study. Two of eight corneal biomechanical parameters (deformation amplitude and time to first applanation) showed favourable repeatability. The usefulness of the Corvis ST is limited because it is restricted to measuring parameters only at the central cornea. A novel corneal indentation device (CID) was developed which was able to precisely measure corneal stiffness, as the force required to indent the cornea to a unit depth. The tangent elastic modulus of the human cornea, or corneal tangent modulus in short, can be calculated from corneal stiffness, thickness, and radius of curvature measured at the central cornea. Using standard mechanical terminology, such as tangent modulus and stiffness, can better describe intrinsic properties of the cornea. Repeatability of the corneal stiffness and corneal tangent modulus measurements, as well as their diurnal variation was evaluated in human subjects before the CID was applied in other clinical studies. Corneal stiffness had an intraclass correlation coefficient (ICC) of 0.75 and a coefficient of variation (CV) of 7.32 %, whereas corneal tangent modulus had an ICC of 0.84 and a CV of 7.34 %. Neither parameter exhibited significant diurnal variation. The prevalence of myopia is high in Asian countries. It is not known whether axial elongation affects corneal tangent modulus in myopic eyes. The CID was used in two age- and central corneal thickness-matched low (n = 32) and high (n = 32) myopic subjects. Corneal stiffness (approximately 0.063 Nmm⁻¹) and corneal tangent modulus (approximately 0.48 MPa) were determined to be similar in the two groups. However, the intraocular pressure (IOP) was significantly higher in the high myopia group. Any difference in corneal tangent moduli could be masked by the difference in IOP between the two groups. In view of a linear dependency on IOP, the corneal tangent modulus in individual eyes was normalized to 15.5 mmHg, the mean IOP for normal eyes. The corneas of high myopes revealed a significantly lower tangent modulus (0.47 ± 0.087 MPa) than that of low myopes (0.57 ± 0.099 MPa). The corneas of the high myopes could be considered as less stiff. Due to the inherent continuity between cornea and sclera, corneal tangent modulus measurement may be useful as an index of the scleral coat of the eye. Orthokeratology has become one of the effective myopia intervention. A pilot study was conducted to monitor changes in corneal stiffness and tangent modulus in subjects receiving orthokeratology treatment for one month. Eighteen young myopes were recruited. Corneal stiffness was determined to be stable throughout the treatment period, and was approximately 0.063 to 0.065 Nmm⁻¹. The mean corneal tangent modulus measurement increased from a baseline of 0.47 MPa to 0.52 MPa at one month, returning to 0.48 MPa after cessation of lens wear for three months. The predictive role of pre-treatment corneal tangent modulus on orthokeratology response deserves further research. Regional mechanical alteration might be expected in diseased or postoperative corneas, but clinical devices cannot measure corneal biomechanics away from the central cornea. The feasibility of the CID to measure corneal stiffness at the peripheral cornea was examined in 25 young adults. Peripheral corneal measurements were performed twice at 3 mm from the temporal limbus, once with subjects looking straight ahead and then with subjects looking nasally. The mean central corneal stiffness was 0.070 Nmm⁻¹, while that of temporal corneal stiffness was 0.074 Nmm⁻¹ when the subjects were looking straight ahead, and 0.080 Nmm-1 when the subjects were looking nasally. The increased temporal corneal stiffness at nasal gaze may be attributable to corneal thickening and pulling of extraocular muscles. More work is needed to deduce the corneal tangent modulus at the peripheral cornea.