Mandibular and Facial Cortical Elastic Properties Differ in Cebus Monkeys

Objective: Cebus monkeys provide an excellent model for studying variation in craniofacial form during human evolution because of the broad range of skeletal adaptations to functional variation in this New World primate genus. The research objective was to determine material property variations in Cebus craniofacial cortical bone for input into finite element models for testing functional models of form variation.

Method: Fresh cortical bone was exposed on the craniofacial skeletons of five cebus monkey cadavers and 5mm trephine burs were used to remove 40 cortical specimens from the mandibles and facial skeletons of each. Bone densities were measured using Archimedes Principle and a microCT technique. Longitudinal and transverse ultrasonic velocities through the bone specimens at a variety of orientations were used to determine elastic properties in three dimensions.

Result: Results showed highly significant differences between mandibular (M) and facial (F) cortical bone in some features but not in others. Specifically, M had higher longitudinal velocities, indicating greater stiffness in the direction of maximum stiffness within the cortical plane (P<0.001) and in the normal orientation (cortical thickness) (P<0.02). No significant differences were found in the direction of minimum stiffness in the cortical plane, resulting in highly significant differences in anisotropy (M: 0.83, sd=0.07 vs. F: 0.90, sd=0.06, p<0.001). A highly significant difference (p<0.001) was also found for density which differed between M and F by an average of 4%.

Conclusion: Cebus mandibles have denser and stiffer cortical bone than facial skeletons. However, this difference is highly directional with no significant differences found in the direction of minimum stiffness in the cortical plane. These differences in structural and material properties will affect patterns of bone deformation as modeled with finite element techniques.

 (Supported by the NSF Physical Anthropology HOMINID program (NSF BCS 0725141 to PD) and the Baylor Oral Health Foundation).