Tonometry is the primary risk assessment method for glaucoma, the leading cause of blindness in Hong Kong. The intraocular pressure (IOP) measured in tonometry is generally compared to a threshold IOP level (21 mmHg) as a reference for clinicians to determine if the subject is at risk. Both the measured IOP and the threshold IOP, however, ignore the individual variations of the ocular tissue biomechanical properties in human eyes. Some studies from the literature have reported increased ocular rigidity in glaucoma patients, and the error in IOP measurement caused by the individual variations in the corneal stiffness can be up to 15 mmHg. These make the existing tonometry fail to accurately distinguish the eyes that are at risk. The inclusion of individual ocular biomechanical properties...[ Read more ]

Tonometry is the primary risk assessment method for glaucoma, the leading cause of blindness in Hong Kong. The intraocular pressure (IOP) measured in tonometry is generally compared to a threshold IOP level (21 mmHg) as a reference for clinicians to determine if the subject is at risk. Both the measured IOP and the threshold IOP, however, ignore the individual variations of the ocular tissue biomechanical properties in human eyes. Some studies from the literature have reported increased ocular rigidity in glaucoma patients, and the error in IOP measurement caused by the individual variations in the corneal stiffness can be up to 15 mmHg. These make the existing tonometry fail to accurately distinguish the eyes that are at risk. The inclusion of individual ocular biomechanical properties is needed to improve the accuracy of the measured IOP and the clinical relevance of the threshold IOP, but a non-invasive method for characterizing ocular tissues in clinics is currently unavailable.

In clinics, Goldmann Applanation Tonometry (GAT) is generally regarded as the gold standard of IOP measurement, in which the subject’s cornea is applanated to a specific area (7.35 mm²) and the applied load is acquired to calculate the IOP. As this is a single-point load measurement, the individual corneal stiffness cannot be accounted for in GAT, causing a potential error of up to 15 mmHg. In this thesis, an Individual-Specific Tonometry (IST) is developed to measure the corneal stiffness and the IOP simultaneously. The cornea is indented by a flat-punch indenter in IST, and the load-displacement behavior during the indentation is acquired for the IOP measurement. The method and IOP analysis were verified on porcine eyes ex vivo, in which the IOP of the eyes were controlled by a manometer. The analysis showed that the IOP measurement errors can be more than 50% without individual corneal property correction, but were significantly reduced to less than 10% by incorporating the individual corneal properties into the IOP measurement.

The IOP measured by the tonometry is then compared to a threshold IOP reference to determine if a subject has high risk for glaucoma, while the individual ocular biomechanical properties of the subject are normally ignored during the comparison. 21 mmHg is generally used as the universal reference to distinguish low IOP and high IOP. Clinics has, however, found that eyes with IOP < 21 mmHg may still develop glaucoma, meaning that a universal threshold IOP is not sufficient. In this thesis, a finite element analysis (FEA) model of the eye is developed to determine if the ocular stiffness would affect the threshold IOP. The results showed that optic nerve damage and peripheral vision loss behavior in glaucoma can be phenomenologically modelled by a shear-based damage criterion. Inherently stiffer eyes or eyes with age-stiffened tissues were found to tolerate lower IOP level before the optic nerve being damaged. This means that the threshold IOP reference in tonometry may have to be adjusted individually according to the subject’s ocular tissue stiffness.

Although the measurement of ocular stiffness can be important in the risk assessment, non-invasive way to characterize in vivo ocular tissue properties is currently unavailable. An instrumented indentation technique is therefore developed for clinical measurement. The method was first verified and benchmarked with the standard 3-point bending method with a silicone eye model. It was then tested on ex vivo porcine eyes and human cadaver eyes, in which the IOP of the eyes were controlled by a manometer. The results showed that the stiffness and the tangent modulus of the eyes can be successfully measured by the indentation method. The method was then used to measure the ocular biomechanical properties of the glaucoma subjects and normal subjects in clinics. It was found that the results measured by the indentation method were comparable to the results measured by invasive methods reported in the literature. The indentation method may therefore act as a useful tool to study the effect of ocular stiffness on the threshold IOP in clinics.

With the individual-specific tonometry and the ocular stiffness characterization method, the accuracy of the IOP measurement and the clinical relevance of the threshold IOP in tonometry could be improved in the future. People who are more susceptible to glaucoma may be identified more accurately. This can enable earlier treatment and help arrest vision loss.