Determination of Poisson Ratio of Bovine Extraocular Muscle by Computed X-Ray Tomography

Hansang Kim,Andrew Shin,Lawrence Yoo,Joseph L Demer

doi:10.1155/2013/197479

Abstract

The Poisson ratio (PR) is a fundamental mechanical parameter that approximates the ratio of relative change in cross sectional area to tensile elongation. However, the PR of extraocular muscle (EOM) is almost never measured because of experimental constraints. The problem was overcome by determining changes in EOM dimensions using computed X-ray tomography (CT) at microscopic resolution during tensile elongation to determine transverse strain indicated by the change in cross-section. Fresh bovine EOM specimens were prepared. Specimens were clamped in a tensile fixture within a CT scanner (SkyScan, Belgium) with temperature and humidity control and stretched up to 35% of initial length. Sets of 500–800 contiguous CT images were obtained at 10-micron resolution before and after tensile loading. Digital 3D models were then built and discretized into 6–8-micron-thick elements. Changes in longitudinal thickness of each microscopic element were determined to calculate strain. Green's theorem was used to calculate areal strain in transverse directions orthogonal to the stretching direction. The mean PR from discretized 3D models for every microscopic element in 14 EOM specimens averaged 0.457 ± 0.004 (SD). The measured PR of bovine EOM is thus near the limit of incompressibility.

Highlights

Since extraocular muscles (EOMs) are manipulated mechanically during strabismus surgery to correct binocular misalignment, the mechanical properties of the EOMs should be understood in order to optimize surgical results
The Poisson ratio (PR) of extraocular muscle (EOM) is almost never measured because of experimental constraints. e problem was overcome by determining changes in EOM dimensions using computed X-ray tomography (CT) at microscopic resolution during tensile elongation to determine transverse strain indicated by the change in cross-section
A variety of other biomechanical methods [12] have been employed to characterize more comprehensive constitutive models for EOMs that capture their timedependent relationships between stress and strain [1, 5, 8]

Summary

Introduction

Since extraocular muscles (EOMs) are manipulated mechanically during strabismus surgery to correct binocular misalignment, the mechanical properties of the EOMs should be understood in order to optimize surgical results. While conventional tensile elongation tests have been performed to investigate the uniaxial force and length relationship for EOMs [1,2,3,4,5], micro/nano indentation has permitted measurement of the compressive modulus of other orbital tissues [6, 7]. Despite such efforts [1, 5,6,7,8,9,10,11], many material parameters of orbital tissues have yet to be de ned. A variety of other biomechanical methods [12] have been employed to characterize more comprehensive constitutive models for EOMs that capture their timedependent relationships between stress and strain [1, 5, 8]

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Conclusion