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Editors Selection IGR 21-4

Anatomical Structures: Corneal Biomechanics

Comment by Cynthia Roberts on:

72979 Comparison of Corneal Biomechanical Properties between Indian and Chinese Adults, Chua J; Nongpiur ME; Zhao W et al., Ophthalmology, 2017; 124: 1271-1279

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It is important to investigate differences in corneal biomechanical properties between races in the context of multiple diseases, and the authors are commended for this detailed study. The authors describe corneal hysteresis (CH) as an indicator of the viscous properties of the cornea, and corneal resistance factor (CRF) as an indicator of the elastic properties of the cornea. However, both CH and CRF are calculated as linear functions of the first applanation pressure (P1) in the loading phase, and second applanation pressure (P2) in the unloading phase of the Ocular Response Analyzer (ORA).¹ Therefore, both are viscoelastic parameters and highly correlated. The distinction is that CRF was designed to have maximum correlation with central corneal thickness (CCT).² This likely contributes to the result that CH showed significant differences between Indian and Chinese populations, while CRF did not, when ocular parameters including CCT were included as co-variates.

Both CH and CRF are viscoelastic parameters and highly correlated

Additional insight might be gained by analyzing the pressure and infrared (IR) signals produced by the ORA with each exam. For example, the magnitudes of the IR signal peaks have been shown to be associated with stiffness, such that the greater the magnitude of the peaks, the greater the corneal stiffness.¹ This might allow the conclusions to be extended beyond CH, to determining which group has the stiffer cornea.

Interestingly, the authors report that Body Mass Index (BMI) is significantly greater in the Indian population than the Chinese. It has been reported that BMI is positively correlated with intracranial pressure (ICP).³ Thus, the lower BMI in the Chinese population might also indicate a lower ICP, which is an additional risk factor for glaucoma,⁴ and might also contribute to its prevalence in the Chinese despite lower IOP and higher CCT, compared to Indian persons.

It would valuable to extend our knowledge base with similar studies on other racial groups, as well as incorporate newer technology to clinically evaluate corneal biomechanics.

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