Biomechanical Measurement of Rabbit Cornea by a Modified Scheimpflug Device

Optimization of a Rabbit Dry Eye Model Induced by Topical Instillation of Benzalkonium Chloride

Purpose

To optimize a rabbit dry eye model induced by topical instillation of benzalkonium chloride (BAC), reduce the days of instillation of the original model by increasing the concentration of BAC from 0.1% to 0.2%.

Materials and Methods

An experimental, prospective, and randomized study was performed on 10 male New Zealand white rabbits, divided into two groups, considering both eyes: 5 rabbits as control (n = 10) and 5 rabbits with 0.2% BAC treatment (n = 10). Saline solution (control) and 0.2% BAC were instilled for 5 consecutive days, twice daily. Tear secretion with and without anesthesia, tear breakup time, tear osmolarity, corneal staining, conjunctival hyperemia, density of goblet cells, height of mucin cloud, and transcript levels of IL-6 were measured before and after the treatment.

Results

After the instillation of 0.2% BAC for 5 consecutive days, there was a significant increase in tear secretion without anesthesia (P < 0.001), corneal staining (P < 0.001), conjunctival hyperemia (P < 0.001), and levels of IL-6 mRNA (P=0.005) compared to the control group. Conversely, there was a decrease in tear secretion with anesthesia (P < 0.001), tear breakup time (P=0.007), tear osmolarity (P < 0.001), density of goblet cells (P < 0.001), and height of mucin cloud (P < 0.001).

Conclusions

The topical instillation of 0.2% BAC for 5 consecutive days, twice daily, was a proper procedure to induce a rabbit dry eye model, reducing the number of days of instillation compared to the original model (14 days).

The Mechanical Interpretation of Ocular Response Analyzer Parameters

Purpose

Ocular Response Analyzer (ORA) is one of the most widely used devices in clinic, while the mechanical interpretations of parameters obtained from ORA have not been understood completely. The aim of this research is to explore the mechanical interpretation of ORA parameters.

Methods

Rabbits aged 3-24 months were measured with ORA in vivo and corneal strips uniaxial tensile tests to get ORA parameters and corneal biomechanical parameters (corneal elastic modulus, relaxation time, and relaxation limit). The mechanical interpretation of ORA parameters was cognized preliminarily by analyzing the correlation between ORA parameters and corneal biomechanical parameters. On the other hand, finite element method was applied to simulate ORA measurements with different corneal biomechanical parameters to obtain quantitative relationship between ORA parameters and corneal biomechanical parameters further.

Results

Biomechanical experimental results showed that Corneal Resistance Factor (CRF) was correlated with corneal elastic modulus and relaxation limit significantly, while the significant correlations between Corneal Hysteresis (CH) and corneal biomechanical parameters were not observed. Results of finite element analysis showed that both CH and CRF were correlated with corneal elastic modulus, relaxation limit, and relaxation time significantly. Besides, corneal elastic modulus was positively correlated with upslop1 and upslop2 and negatively correlated with w2.

Conclusions

For all ORA parameters, CH, CRF, the upslope, and the width of the peaks are parameters which may reflect corneal elastic properties. It is viable to cognize mechanical interpretation of ORA parameters by the comparisons of the data from ORA and biomechanical tests of rabbits with different ages and the simulations of ORA based on finite element methods. Further studies are needed to confirm the mechanical interpretation.