Biaxial mechanical testing of posterior sclera using high-resolution ultrasound speckle tracking for strain measurements

Abstract

This study aimed to characterize the mechanical responses of the sclera, the white outer coat of the eye, under equal-biaxial loading with unrestricted shear. An ultrasound speckle tracking technique was used to measure tissue deformation through sample thickness, expanding the capabilities of surface strain techniques. Eight porcine scleral samples were tested within 72 hours postmortem. High resolution ultrasound scans of scleral cross-sections along the two loading axes were acquired at 25 consecutive biaxial load levels. An additional repeat of the biaxial loading cycle was performed to measure a third normal strain emulating a strain gauge rosette for calculating the in-plane shear. The repeatability of the strain measurements during identical biaxial ramps was evaluated. A correlation-based ultrasound speckle tracking algorithm was used to compute the displacement field and determine the distributive strains in the sample cross-sections. A Fung type constitutive model including a shear term was used to determine the material constants of each individual specimen by fitting the model parameters to the experimental stress-strain data. A non-linear stress-strain response was observed in all samples. The meridian direction had significantly larger strains than the circumferential direction during equal-biaxial loadings (P’s<0.05). The stiffness along the two directions were also significantly different (P=0.02) but highly correlated (R2=0.8). These results showed that the mechanical properties of the porcine sclera were nonlinear and anisotropic under biaxial loading. This work has also demonstrated the feasibility of using ultrasound speckle tracking for strain measurements during mechanical testing.

Mapping 3D Strains with Ultrasound Speckle Tracking: Method Validation and Initial Results in Porcine Scleral Inflation

“…Previous surface strain measurements showed minimal shear in the posterior sclera, 3 possibly a characteristic of the scleral surface but not representative of the internal stroma. Our previous biaxial testing in the more peripheral region of the sclera also did not detect significant shear during equal biaxial loading in the porcine posterior sclera; 4 however the biaxial loading does not replicate the structural bending of the peripapillary region during inflation caused by structure and material property differences between the sclera and the ONH. By definition, the maximum shear lies in the plane of the maximum and minimum principals and is oriented in the direction of the principal vectors rotated by 45°.…”

“…This modulus is of the same order of magnitude as what we previously measured in porcine posterior sclera using biaxial testing. 4 …”

“…5,7,21,26 Biaxial mechanical testing (loading in two orthogonal in-plane directions) has shown a significantly different response between the circumferential and meridional directions in the porcine posterior sclera, 4 but the contrast was less obvious in the human sclera. 6 Uniaxial and biaxial mechanical testing provide measurements of mechanical constants but have several known drawbacks including the disruption of tissue integrity during specimen excision and the introduction of artificial pre-stresses on sample surfaces that are naturally curved.…”

Corneoscleral stiffening increases IOP spike magnitudes during rapid microvolumetric change in the eye

“…It would therefore be interesting to evaluate the level of changes in dynamic IOP that can be expected with localized stiffening of the ocular shell. Moreover, we evaluated how an increased steady-state IOP affects IOP spikes, because an increase in IOP effectively increases the “apparent” stiffness of the entire corneoscleral shell due to the nonlinear mechanical properties of the ocular shell (Cruz Perez et al, 2014; Eliaghi et al, 2010; Girard et al, 2009). This allows us to gain insight into the relationship between the clinical mean IOP and its fluctuations from the perspective of corneoscleral biomechanical properties.…”

Shear Behavior of Bovine Scleral Tissue

“…[58]. For eye tissue, shear strains have been determined from biaxial tests through the measurements of three normal strains and then strain transformation [43,59]. Recent partial globe studies have imaged shear strain on the surface of the posterior globe [32,34].…”