$388 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

LASIK; the other three had not. Corneal topography (Orbscan II) and wavefront analysis (Zywave) were performed on each eye before and after corneal edema induction. Changes in central and peripheral corneal thickness, as well as axial and tangential curvature, were evaluated. Zernike second and fourth order term changes were also calculated. Changes were compared between post-LASIK eyes and normal eyes. Results: Analysis of corneal thickness change revealed an increase in corneal thickness in all subjects following nitrogen exposure. The post-lasik group had a larger (p=0.0039) mean increase in corneal thickness nitrogen re- sponse. (Mean central change=44.53±9.48~t for post-lasik; Mean central change=29.34±3.73~t for control). Mean increase in corneal thickness was significantly (p <0.0001) larger in the central cornea than peripheral cornea when both groups were considered together (post-lasik and control). The inter- action term was not significant. Anterior tangential curvature change revealed a pattern in the post-lasik group of greater peripheral steepening than central steepening following nitrogen exposure and the interaction term between the two groups was significant (p=0.0364); control group did not exhibit this pattern. The posterior corneal surface underwent central steepening in the post-lasik group. The control group experienced central posterior curvature flattening; the group effect was significant (p=0.0212). Zywave analysis did not reveal significant change in either the second or fourth order terms. Conclusions: Lasik eyes demonstrate an exaggerated response to induced corneal edema, manifested by larger increases in corneal thickness than normal subjects with intact anterior lamellae. The post-lasik cornea responds differently to induced edema in terms of peripheral anterior curvature and central posterior curvature. These findings indicate the biomechanical and swelling forces acting within and upon the cornea are altered after LASIK.

7153 Mo, 14:45-15:00 (P11) Three-dimensional stress as a s ignal ing tool for develop ing a tissue engineered cornea

A. Shah 1 , A. Voorhees 2, E. Orwin 1,2. Departments of 1Engineering and 2Biology, Harvey Mudd College, Claremont, California, USA

Corneal transplants have been very successful for restoring vision to patients with corneal defects. However, the global demand for corneal transplants exceeds the number of available donor corneas. A tissue engineered (TE) cornea could ease the demand for donor corneas. The specialized function of the cornea makes it particularly challenging to engineer. The transparency of the cornea is due to the parallel alignment of collagen fibers and the expression of certain proteins which regulate the refractive properties of the keratocytes that populate the tissue. Our current TE culture model includes human corneal fibroblasts seeded in a collagen sponge matrix. We have designed a biore- actor system to study the effects of intraocular pressure (lOP) on corneal replacement tissue growth. The human cornea normally experiences an lOP of 15-20 mmHg, creating hoop stress within the cornea. The bioreactor allows for sterile culture and live optical coherence microscopy (OCM) while holding the tissue in a three dimensional stress state; we believe this stress state is critical to our end goal, as it will provide mechanosignaling similar to that seen in the in vivo environment. The literature, and particularly work focused on TE articular cartilage development, suggests that such signaling, in addition to other growth factors and chemical signals, is necessary to ensure proper phenotypic expression. When corneal keratocytes are seeded in unstressed cell culture models, they express high levels of ~J,-Smooth Muscle Actin (~J, SMA) and low levels of aldehyde dehydrogenase 3A1; this is contrary to expression patterns seen in vivo. Initial studies of keratocytes grown in collagen sponges have determined that the quantity of ~J, SMA is inversely related to the cell seeding density. However, preliminary results from tissue constructs grown in the bioreactor system show that ~J, SMA density in this model may be independent of seeding density. This data, along with OCM data, demonstrates the contraction of these tissues and suggests a high level of wound healing activity in constructs grown under 3D stress.

5432 Mo, 15:00-15:15 (P11) Modeling the human cornea - Mapping the corneal collagen f ibri l architecture based on X-ray diffraction measurements

P.M. Pinsky 1 , D. van der Heide 1 , D. Chernyak 2, K.M. Meek 3, C. Boote 3. 1Stanford University, Stanford, CA, USA, 2AMO Inc., Santa Clara, CA, USA, 3 Cardiff University, Cardiff, UK

X-ray diffraction has been successfully used to map the architecture of corneal tissue. Recently Meek et al. have refined this methodology and succeeded in characterizing the collagen density and spatial organization of fibrils in the human cornea and surrounding sclera and resulting in a complete two dimen- sional atlas of fibril directions. Since the mechanical strength of the cornea depends entirely on the collagen network within the corneal lamellae, this data is extremely valuable for constructing mathematical constitutive models of corneal tissue. As a step toward this goal, a mathematical model of the

X-ray scattering data is presented in a form which can be employed in finite element models of the tissue behavior. X-ray scattering data from experiments on human corneas indicates that at each point on the corneal/scleral surface the collagen present through the thickness may be divided into two groups of fibrils; one group has a uniform angular orientation (spatially isotropic) and the second group displays a normal distribution about a preferred orientation (spatially anisotropic). For example, Meek et al. discovered that the fibril organization of the human eye is orthogonal in the inferior-superior and nasal-temporal directions in the central part of the cornea whereas it is primarily tangential at the limbus. A mathematical model of the experimental data has been created using a composition of normal distribution functions. At each point on the corneal and scleral surfaces, the model provides a probability distribution for collagen over all orientations. It reproduces the experimental data with good accuracy and is in a form which can be used for tissue modeling. The experimental data has also been used to test hypotheses about the overall architecture of collagen in the cornea. One such hypothesis posits that the lamellae run from limbus to limbus over great circles on the corneal dome and it is shown that under reasonable conditions this model is consistent with the X-ray diffraction data.

6559 Mo, 15:15-15:30 ( P l l ) Creep properties of Descemet's membrane and lens capsule fo l lowing enzymatic digestion of glycosaminoglycans C.C. Danielsen. Dept. of Connective Tissue Biology, Institute of Anatomy, University of Aarhus, Denmark

Descemet's membrane (DM) has been found to possess higher stiffness than lens capsule (LC) during initial deformation in both volume-strain and creep testing [1]. The aim of this study was to explore the possible role of proteoglycans in relation to the different mechanical properties of the ocular membranes. Bovine DM and anterior LC were subjected to unconfined creep test before and after sequential incubation with five different glycanases that removed 80-85% of the glycosaminoglycans (GAG). The specimen was subjected to a constant load (0.2 MPa) corresponding to an adjusted weight of the plunger of a high-resolution electronic length gauge. To obtain a uniform and quick load application, the plunger was lowered and raised at a constant velocity by a stepmotor via a balanced lever, and the load start ( t=0) and the initial thickness of specimen were established by means of a load cell inserted between the stepmotor and the lever arm. The initial thickness of the membranes was not influenced by the enzyme treatment. Confirming previous results, the creep response of intact LC was higher than that of DM due to a more rapid increase in strain immediately (from -1 s) after load application. Control incubated specimens showed minor changes in creep properties. GAG-depletion increased creep response in both membranes, initially to a very similar extent, but later on more pronounced in DM. Thus, during the initial load, GAG-depleted LC and DM showed the same difference in creep response as that of intact membranes, but, later on, the strain values of the two membranes approached. These results indicate that proteoglycans contribute to the creep response in the membranes, but the difference in the immediate creep response, inverse related to Young's modulus [2], seems not to be explained by the proteoglycans.

References [1] Danielsen C.C. Exp. Eye Res. 2004; 79: 343-350. [2] Armstrong C.G., et al. J. Biomech. Eng. 1984; 106: 165-173.

20.2. Ears

6035 Mo, 16:00-16:15 (P13) Biomechanical simulation of middle ear using hyperelastic models E Gentil 1, R.M. Natal Jorge 2, A.J.M. Ferreira 3, M.EL. Parente 2, EA.L.S. Martins 2, E. Almeida 1 . 1Escola Superior de Tecnologia da Sa~de do Porto, Clinica ORL, Widex, Portugal, 21DMEC-Polo FEUR Faculdade de Engenharia, 31NEGI, Faculdade de Engenharia, Universidade do Porto, Portugal

The ear is made up of two systems: the auditory system which is involved with hearing, and the vestibular system which is related with body equilibrium, orientation, and balance. Anatomically, it can be divided into three parts: the outer, middle, and inner ear [1]. The outer ear has the function of collect sound waves and conducts them to the eardrum. The eardrum separates the outer ear from the middle ear. The middle ear contains three small bones, the ossicles (malleus, incus and stapes) that transfer sound vibrations from the eardrum to the inner ear. These ossicles are connected to exterior by small ligaments, and are attached to the eardrum by the handle of the malleus and the footplate of stapes connects to the inner ear, by the oval window. However, the ossicles can suffer from several damages, for example, the Oto- sclerosis, being a need the application of mechanical prosthesis, by chirurgic intervention, keeping the right travel of sound wave.