Mechanical properties of mandibular cortical bone in the American Alligator

Objectives: The American Alligator (Alligator mississippiensis) has become an important model for palatogenesis and other research in craniofacial development. The 3D skull shape of alligators is well-structured to resist feeding stresses. This study examines another parameter of alligator craniofacial structure, namely cortical material properties. We hypothesize that material properties and their regional variations will be similar to those found in mammals, and that they will reflect the important relationships among shape, bone quality, and feeding function in alligators.

Methods: The elastic modulus, the shear modulus, and Poisson's ratio, were measured on cylindrical Alligator bone specimens obtained from the labial and buccal surfaces of 3 fresh Alligator mandibles. Values were obtained by using an ultrasonic wave technique in three orthogonal directions relative to the material axes of the specimens, previously determined using longitudinal ultrasound.

Results: The elastic modulus in cortical bone of the alligator mandibles varied by orientation as in mammalian mandibles. Maximum stiffness averaged 20.3GPa in 25 specimens (SD=2.8). Minimum stiffness in the plane of the cortical plate (2 direction) averaged 10.5GPa (SD=1.8) and 8.7GPa (SD=0.9) normal (1 direction) to this plane. These averages are similar to those found in mammals, with the exception that the bone is less orthotropic; in alligators, E in the 1 and 2 directions are more similar than in mammals. Relative values of E in these directions are similar to those found in the cortices of postcranial long bones. Regional variation is apparent, although its consistency is unclear from this limited number of mandibles.

Conclusion: The elastic modulus shows transverse isotropy in the alligator mandibles, which is different from the orthotropy of mammalian mandibles, but similar to that of mammalian postcranial long bones.

Supported by COLCIENCIAS (Colombia) and LASPAU.