Finite-element analysis (FEA) is the method of choice for theoretical analysis of the mechanical behavior of complex shapes in biology. The approach of FEA approximates real geometry using a large number of smaller simple geometric elements (e.g., triangles, bricks, tetrahedrons). Since complex shapes defy simple mathematical solution (i.e., in terms of engineering formulas), FEA simplifies a problem by analyzing multiple simple elements of known shapes with established mathematical solutions. These multiple solutions are in the end combined together to depict states of stress and strain through the entire structure.

The last half of the 20th century saw increasing interest in testing and analysis of bone biomechanics (Cowin, 2001), including increased use of finite-element (FE) methods. Many theoretical and experimental studies have sought to quantify the distribution of stresses and strain in the mandible (Knoell, 1977; Hylander, 1984; Bouvier and Hylander 1996; Daegling and Hylander, 1997, 1998; Dechow and Hylander, 2000), and several finite-element analyses of human mandibles have been developed. One of the first mandibular finite-element models was developed over 30 years ago (Gupta et al., 1973). This half-mandible model was symmetrical about the symphysis. The authors attempted to study the stress distribution and the deformation that occur during biting. A model designed 4 years later offered improved resolution through the inclusion of the full mandibular dentition (Knoell, 1977). The use of computed tomography (CT) scans to develop a three-dimensional geometric model was pioneered by Hart and Thongpreda (1988). Hart et al. (1992) offered further refinements of better geometry, mesh density, and realistic material properties assignment. The work of Korioth et al. (1992) represents the first attempt to include the periodontal ligament and dental structures in combination with cortical and cancellous bone in a whole mandible model. More recently, model designs have incorporated a voxel-based approach using CT data (Vollmer et al., 2000).

All these studies attempted to portray the stress-strain behavior of the mandibular bone, but the degree to which they succeeded in doing so remains unknown. The lack of information about mandibular material properties, the uncertainty of correct load distribution or assigning the proper boundary conditions all compromised these finite-element studies to some extent (Korioth and Versluis, 1997). Recent studies on the primate skull have emphasized the limited utility of unvalidated models (Ross and Patel, 2003; Strait et al., 2003).

In this article, we explore some of the important issues involved in model validation. We proceed from the assumption that issues of geometric reconstruction are largely solved and focus instead on the impact of decisions made concerning assignment of material properties and specification of boundary conditions. We constructed a finite-element model of a *Macaca fascicularis* mandible after recording in vitro strain data from this specimen under controlled loading conditions. In effect, our validation of the finite-element model is an attempt to reconstruct the conditions of this in vitro experiment.

The design of a finite-element model proceeds by first obtaining a geometric model and then converting that precise geometry into a finite-element model. The geometric model can be obtained through direct or indirect methods, i.e., by reconstruction of a 3D model from a stack of CT scan images or from a cloud of coordinate points or by using the dimensions of the bone to build an approximate model with a computer-aided design system (Gupta et al., 1973; Knoell, 1977; Meijer et al., 1993). The model obtained in this latter (indirect) way is in fact an idealized model, an approximation of the real object. Reconstruction from CT scans usually generates an improved virtual model because simplifying assumptions of geometry are avoided (Fütterling et al., 1998; Hart and Thongpreda, 1988; Keyak et al., 1990; Hart et al., 1992; Korioth et al., 1992; Hollister et al., 1994; van Rietbergen et al., 1995; Lengsfeld et al., 1998; Vollmer et al., 2000). Obtaining geometry by CT is the preferred method since it offers more accuracy than reconstructions based on planar radiographs. The advantage of CT scanning is that it gathers multiple images of the object from different angles and then combines them together to obtain a series of cross-sections.

Comparisons between the experimental and finite-element analyses presented here represent our continuing attempt to validate the model originally described by Daegling et al. (2003). In the current study, we aim to improve existing modeling efforts by introducing new parameters: designing a less stiff edentulous model, imposing more realistic boundary conditions, and incorporating heterogeneity and transverse isotropy into the models.