A Novel Device for Clinical Measurement of Corneal Elasticity

Abstract

 

Purpose

 

The investigation of corneal biomechanics has been obfuscated by the lack of an instrument that can measure corneal elasticity. We have validated the principle of corneal indentation for measurement of corneal elastic modulus (E) in a silicone model of cornea, in porcine eyes (ex vivo testing) and rabbit eyes (in vivo testing). In this study, we designed and built a prototype for measurement of corneal E in glaucoma patients.

 

Methods

 

The prototype device is portable and designed to fit into a slit-lamp (Fig.1). It has 2 key components: (1) a corneal indenter with a subminiature load cell for measurement of the load, and (2) a miniature linear motorized actuator that propels the indenter and measures the displacement of corneal indentation. The load displacement data are transmitted to a notebook computer via Bluetooth to allow visual display of the load displacement curve. 30 eyes of 15 glaucoma/suspect patients were recruited for measurement of IOP (dynamic contour tonometry), CCT, corneal curvature (K) and corneal E (3 consecutive measurements) in the same visit. The test-retest variability of corneal E was evaluated and the association between IOP and corneal E was examined with linear mixed model.

 

Results

 

The mean IOP, CCT and K were 18.5±4.2mmHg, 560.5±35.2µm, and 43.8±1.6D, respectively. Corneal E was derived from the load displacement curve (Fig.2), taking individual K and CCT in the calculation. The corneal E ranged between 0.41 and 0.89 MPa (mean: 0.63±0.11 MPa). The ICC was 0.837 (95% CI: 0.727 - 0.913). There was a significant correlation in corneal E between fellow eyes (r=0.619, p=0.015) and the corneal E was positively associated with IOP (p=0.017) (with adjustment of correlation between fellow eyes).

 

Conclusions

 

This study provides an unprecedented documentation of corneal E in human eyes in vivo. Corneal E is often reported with little attention to the preload (the IOP). Our finding indicates that corneal elastic modulus should always be interpreted with IOP.

 

 

A prototype corneal indentation device for clinical measurement of corneal elastic modulus.

 

 

The loading portion of the load displacement curve (black line) is essentially linear and is used to calculate corneal elastic modulus (the tangent modulus). The unloading portion is non-linear (red line) and the area bounded by the loading and unloading curves represents corneal hysteresis.

 

Keywords: 568 intraocular pressure • 465 clinical (human) or epidemiologic studies: systems/equipment/techniques • 467 clinical laboratory testing