Abstract
Purpose::
To investigate the viscoelastic properties of porcine and human eyewall layers.
Methods::
Dynamic mechanical analysis was performed on the retinal, choroidal and scleral layers of twenty-five porcine (average age 6 months) and five human eyes (average age 76 years). Porcine eyes were collected and stored at 4°C for 6 hours, whilst the human eyes were collected and stored at 4°C for up to 36 hours prior to testing. Specimens were prepared as 2mm x 8mm sections by means of a custom razor tool. Each section was measured microscopically by the difference in focus between nanospherules placed on the inner and outer surfaces. All sections were held between two specialized grips positioned 2mm apart and stretched at a constant rate to 100% strain. The tests were run with both load and displacement control, using a high-resolution control configuration for retina and choroid.
Results::
The load-strain and stress-strain relationships of all tissue sections were nonlinear, and therefore elastic stiffness and elastic modulus varied as a function of strain. The average elastic modulus was calculated for the initial loading and elastic deformation regions of the stress-strain curve. The resultant moduli were 3kPa and 3.2kPa for porcine retina, 1.6kPa and 10kPa for human retina, 25.7kPa and 353.6kPa for porcine choroid, 35.3kPa and 57.5kPa for human choroid, 1251kPa and 6551kPa for porcine sclera and 511.8 and 4181kPa for human sclera, respectively.
Conclusions::
Mechanical strength of the porcine and human eyewall layers are similar in many respects. The biomechanical behavior of the eyewall is characterized by a short elastic phase and a broad plastic phase with irreversible deformation which could serve as a protective mechanism. This data allows researchers to use the viscoelastic properties of the porcine eyewall to develop a finite element model of the eye and refine parameters for human ophthalmic procedures.
Keywords: retina • choroid • sclera