In the development of biomedical components engineered for restoring joint mobility, a thorough knowledge of mechanical properties of the adjacent hard tissues (e.g. bone, cartilage, etc.) is often needed. Similarly, the success of restorative dentistry is largely dependent on properties of the restorative materials and their interaction with dentin and enamel, the hard tissues of teeth. The physical and mechanical properties of biological tissues are generally more difficult to evaluate than those of conventional engineering materials due to size limitations, the need for maintaining hydration, and difficulties encountered in fabrication of specimens. In addition, material anisotropy and inhomogeneity can further limit the applicability of common experimental techniques that are enrolled for examining the mechanics of materials. In these situations, optical techniques such as moiré interferometry and electronic speckle pattern interferometry can serve as valuable alternatives, which permit a detailed analysis. We utilize a variety of optical techniques in our research to examine the properties of biological tissues and changes that occur with specific mechanical, physical, and chemical conditions.

We are currently evaluating the influence of storage conditions on the elastic modulus of cortical bone using electronic speckle pattern interferometry (ESPI). In this study the elastic modulus of bone from mature bovine femurs is determined using a simple inverse analysis. Displacements resulting from 3-point bending of bovine beams are determined using ESPI and used as the solution for the analytical expression for beam deflection defined by classical mechanics for either isotropic or orthotropic materials. The speckle fringe fields are analyzed using specialized methods of fringe processing for determination of the transverse displacements. Speckle fringe patterns are obtained from a "superposition" of the speckle patterns before and after application of the dead weight loads according to the subtraction method of fringe processing. A typical speckle fringe pattern obtained on the surface of a bovine specimen is shown below.

Fringe pattern processed using electronic speckle pattern interferometry on the surface of a 3-point bend specimen obtained from a bovine femur

We are also using ESPI to examine the influence of amalgam and composite fillings on the mechanical behavior of restored molars and their contribution to restored tooth failures. Extracted restored molars are obtained from the University of Maryland Dental School and from local dental practices in the Baltimore area. The molars are sectioned, placed in appropriate fixtures to accommodate the experiments and subjected to occlusal contact loads that simulate mastication (biting). The in-plane displacement field resulting from loading is determined using ESPI and compared with results from a finite element model for the restored tooth. ESPI is less sensitive to environmental disturbances and does not require detailed surface preparation, both of which make it valuable for our analysis. The surface displacements are revealed through fringe patterns, which may be displayed and analyzed in real time or further processed to extract more detailed information. An example of a fringe field and corresponding unwrapped phase map on the surface of a restored molar subjected to simulated occlusal loads is shown below.

Fringe pattern on the surface of a sectioned molar restored with Class II amalgam restorations. The fringe field corresponds to vertical displacement (v) resulting from a contact load of 14 kg applied on the occlusal (top) surface.	The unwrapped phase map of the fringe field (left). Displacement is indicated by the grayscale where "0" (white) indicates maximum displacement and "255" (black) represents the minimum displacement. Note the pulp cavity at the center of the tooth.

We are also using moiré interferometry to examine the mechanics of fracture during stable crack growth in the dentin of human and bovine teeth. Dentin comprises the majority of the tooth by volume and is approximately 30% organic, 50% inorganic, and 20% fluid. An examination of teeth in our laboratory has shown that cracks are often found in the dentin of restored molars and that the orientation of crack growth is generally perpendicular to the dentin tubule orientation. Thus, we are interested in understanding how the structure of dentin contributes to the mechanics of fracture. An example of a moiré fringe pattern obtained during stable crack growth within a double cantilever beam (DCB) dentin fracture specimen that results from wedge loading is shown in below. A magnified view of the near-tip displacement field is also presented. The fringe field in these figures clearly elucidates a deformation zone of non-linear behavior that develops adjacent to the crack tip. According to Eqn. (1) the nonlinear deformation zone extends approximately 250 µm laterally on each side of the propagating crack. Also evident is the appearance of crack branching, which implies that a microcracking zone develops in front the dominant crack tip. Therefore, the fracture of mineralized dentin appears to be an elastic-plastic process and is influenced by the presence of a fracture process zone. We are continuing our work in this area and investigating the contribution of these mechanisms to energy dissipation in the fracture of dentin.

A fringe pattern on the surface of a dentin DCB specimen resulting from wedge loading. The fringe field corresponds to opening mode displacement (u) in the x-direction.	Each fringe corresponds to a region of equivalent displacement. Note the fringe curving on the crack boundary and "forked" crack tip