Abstract
Purpose: :
Porohyperelastic (PHE) finite element models (FEMs) are used to study normal and pathological ocular mechanics. Ocular soft tissues have been analyzed using models based on elastic [1,2], viscoelastic [3], and PHE [4] material laws. Recent PHE FEMs [4] of ocular structures have demonstrated the role of mobile tissue fluid in response to normal and elevated intraocular pressure (IOP). Unconfined compression (UCC) and confined compression (CC) experimental methods and preliminary results are described that will provide essential PHE material properties in these FEMs.
Methods: :
A UCC/CC experimental apparatus was developed (extending earlier designs [5]) for testing excised porcine ocular tissues. A 5 mm diameter corneal trephine was used to obtain scleral tissue specimens that were kept in BSS at 37 0C. The retina and choroid layers were removed leaving scleral tissue specimens. These cylindrical specimens were placed between porous platens and compressed after removal from the trephine (UCC) or within the trephine (CC). Displacements were measured using a LVDT and force using a 1500ASK–50 load cell.
Results: :
UCC and CC testing produced characteristic creep–consolidation (constant force, measured displacement) and/or relaxation (constant displacement, measured force) response comparable to other soft tissues; e.g. for a constant load of 6.44 N (200 gm); creep strain reached 20% in 100 sec. Material properties were determined using ABAQUS FEMs and generalized least squares data reduction procedures for UCC and CC tests. Typical preliminary values for the material parameters are (linear poroelastic model) drained elastic modulus = 580 kPa and Poisson ratio = 0.45; or (neo Hookean, ABAQUS model) C01 = 100 kPa and D1= 0.000001. A nonlinear permeability was determined with a value of 8x10–13 m/sec for porosity of 0.5.
Conclusions: :
The UCC/CC methods are to be extended to allow measurement of transport of mobile species in ocular tissues. Permeability, diffusivity, osmotic and convection coefficients will be determined for newly developed FEMs that will couple the ABAQUS PHE (deformation–convection) to ABAQUS mass transport (diffusion–convection) FEMs. These FEMs will allow quantitative description of fluid and species transport that is important in the design of local drug delivery strategies. The methods reported here will initiate comprehensive PHE–transport testing of single and multilayered specimens obtained from various regions of the porcine eye.
References: :
[1] Sigal et al, 2005; [2] Burgoyne et al, 2005; [3] Downs et al, 2005; [4] Simon et al, 2004; [5] Mow et al, 1980.
Keywords: computational modeling • sclera • retina