Statistical effects on FEA-simulated stress and strain in feeding rodents when teeth are modeled and implanted as separate structures

Background: Finite element analysis (FEA) is used in paleontology to visualize the stress and strain distributions in extinct animals during biting and to make ecological inferences. Most previous work models teeth as physically and materially continuous with the cranium and mandible. When teeth are modeled separately the focus is usually on individual teeth or the mandible. We test the impacts of modeling the teeth as separate structures on the cranium, mandible, and teeth, with the first statistical comparison for mammals of stress and strain results in separate-tooth versus simplified continuous models.

Methods: We used CT scans of Rattus norvegicus (ID: 55306) and Cavia porcellus (ID: 55304) from Morphosource. We produced three models: (1) a continuous bone-material model of the cranium and mandible with the teeth fused, and models with teeth as separate objects with; (2) enamel or; (3) dentine properties applied. We imported models into Strand7 for FEA. Forces from published literature were applied for the muscles of mastication: the superficial masseter, anterior and posterior deep masseter, temporalis, anterior, posterior, and infraorbital zygomaticomandibularis, and the internal and external pterygoid muscles. We applied elastic moduli of 17 GPa for bone, 16.9 GPa for dentine, and 83 GPa for enamel. By constraining respective teeth, we simulated a bilateral incisor bite to simulate gnawing, and the start of a unilateral left sided chewing motion. Simulated direct attachments connected the teeth to the cranium and mandible.

Results: von Mises overall stress, and von Mises, tensile and compressive (first and third principal) strains, were sampled from twenty-two consistent mandible and tooth locations, and compared stress and strain between models using Kruskal-Wallis and Tukey-Kramer tests. Modeling the teeth as separate has a varying effect on the resulting stress in the two taxa. The rat showed more differences between the three models. The guinea pig model showed no significant difference in stress when modeling teeth as separate structures. The dentine separate teeth analysis resulted in both the guinea pig bites having overall higher strain compared to the bone and enamel models. The rat molar bite with teeth modeled as enamel showed high von Mises and tensile strain anterior to the left maxillary incisor, and the dorsal surface of the left mandibular ramus. With incisor bites, the highest strain occurred with teeth as dentine and lowest strain on the all-bone model.

Conclusions: This is the first study in rodents that examines the statistical differences in FEA results when teeth are modeled as separate structures. We demonstrate that such modeling promises a more realistic distribution of stress and strain during both molar and incisor biting. Detaching the teeth from the jaw reveals where stress and strain transfer from the teeth to the skull. The difference in modeling teeth as enamel or dentine is expected, as enamel has lower strain values with its greater stiffness. The difference in results between the rat and guinea pig are potentially related to differences in diet, chewing motion, or phylogeny. This will be resolved in future research by including more rodents.