Variations in Cortical Material Properties Throughout the Human Dentate Mandible C.L. Schwartz-Dabney 1 and P.C. Dechow 2 * 1 Division of Oral and Maxillofacial Surgery, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9109 2 Department of Biomedical Sciences, Baylor College of Dentistry, Texas A&M University System Health Science Center, Dallas, Texas 75246 KEY WORDS ultrasound; bone; biomechanics; development; function ABSTRACT Material properties and their variations in individual bone organs are important for understand- ing bone adaptation and quality at a tissue level, and are essential for accurate mechanical models. Yet material property variations have received little systematic study. Like all other material property studies in individual bone organs, studies of the human mandible are limited by a low number of both specimens and sampled regions. The aims of this study were to determine: 1) regional variabil- ity in mandibular material properties, 2) the effect of this variability on the modeling of mandibular function, and 3) the relationship of this variability to mandibular struc- ture and function. We removed 31 samples on both facial and lingual cortices of 10 fresh adult dentate mandibles, measured cortical thickness and density, determined the directions of maximum stiffness with a pulse transmission ultrasonic technique, and calculated elastic properties from measured ultrasonic velocities. Results showed that each of these elastic properties in the dentate human mandible demonstrates unique regional variation. The direction of maximum stiffness was near parallel to the occlusal plane within the corpus. On the facial ramus, the direction of maximum stiffness was more vertically ori- ented. Several sites in the mandible did not show a con- sistent direction of maximum stiffness among specimens, although all specimens exhibited significant orthotropy. Mandibular cortical thickness varied significantly (P  0.001) between sites, and decreased from 3.7 mm (SD = 0.9) anteriorly to 1.4 mm posteriorly (SD = 0.1). The cortical plate was also significantly thicker (P  0.003) on the facial side than on the lingual side. Bone was 50 –100% stiffer in the longitudinal direction (E 3 , 20 –30 GPa) than in the circumferential or tangential directions (E 2 or E 1 ; P  0.001). The results suggest that material properties and directional variations have an important impact on mandibular mechanics. The accuracy of stresses calcu- lated from strains and average material properties varies regionally, depending on variations in the direction of maximum stiffness and anisotropy. Stresses in some parts of the mandible can be more accurately calculated than in other regions. Limited evidence suggests that the orienta- tions and anisotropies of cortical elastic properties corre- spond with features of cortical bone microstructure, al- though the relationship with functional stresses and strains is not clear. Am J Phys Anthropol 120:252–277, 2003. © 2003 Wiley-Liss, Inc. The deficit of information on cortical material properties and on their directional and regional variations in the human mandible creates problems in interpreting the results of experimental and mod- eling approaches to mandibular function. While lim- ited information exists on material property vari- ability in the cortices of human long bones (Cowin, 1989; Katz and Meunier, 1987; Weiner et al., 1999; Wirtz et al., 2000), less information is available for the craniofacial skeleton. Studies of mandibular cor- tical material properties (Arendts and Sigolotto, 1990; Ashman and van Buskirk, 1987; Carter, 1989; Dechow et al., 1992, 1993) have small numbers of specimens from limited anatomical regions. Inadequate information on material properties complicates the interpretation of mandibular finite element models (Kabel et al., 1999; Korioth et al., 1992). Some mandibular finite element models (Kaewsuriyathumrong and Soma, 1993; Meijer et al., 1993) assume that cortical and cancellous bone is isotropic (material properties do not vary by di- rection), homogeneous, and linearly elastic (linear relationship between stress and strain is invariant with changes in time and frequency of loading). Other models (Hart et al., 1992; Korioth et al., 1992) incorporate material properties, even though the as- sumptions about those properties are speculative, including: 1) direction of maximum stiffness, 2) lack Grant sponsor: VA Dental Research Fellowship; Grant sponsor: NIH; Grant numbers: DE05691, DE07256. *Correspondence to: Paul C. Dechow, Ph.D., Department of Bio- medical Sciences, Baylor College of Dentistry, 3302 Gaston Ave., Dallas, TX 75246. E-mail: pdechow@tambcd.edu Received 28 June 2001; accepted 29 March 2002. DOI 10.1002/ajpa.10121 Published online in Wiley InterScience (www.interscience.wiley. com). AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 120:252–277 (2003) © 2003 WILEY-LISS, INC.