Strain in the braincase and its sutures during function

Abstract

The skull is distinguished from other parts of the skeleton by its composite construction. The sutures between bony elements provide for interstitial growth of the cranium, but at the same time they alter the transmission of stress and strain through the skull. Strain gages were bonded to the frontal and parietal bones of miniature pigs and across the interfrontal, interparietal and coronal sutures. Strains were recorded 1) during natural mastication in conjunction with electromyographic activity from the jaw muscles and 2) during stimulation of various cranial muscles in anesthetized animals. Vault sutures exhibited vastly higher strains than did the adjoining bones. Further, bone strain primarily reflected torsion of the braincase set up by asymmetrical muscle contraction; the tensile axis alternated between +45 degrees and -45 degrees depending on which diagonal masseter/temporalis pair was most active. However, suture strains were not related to overall torsion but instead were responses to local muscle actions. Only the coronal suture showed significant strain (tension) during jaw opening; this was caused by the contraction of neck muscles. All sutures showed strain during jaw closing, but polarity depended on the pattern of muscle usage. For example, masseter contraction tensed the coronal suture and the anterior part of the interfrontal suture, whereas the temporalis caused compression in these locations. Peak tensile strains were larger than peak compressive strains. Histology suggested that the skull is bent at the sutures, with the ectocranial surface tensed and the endocranial surface predominantly compressed. Collectively, these results indicate that skulls with patent sutures should be analyzed as complexes of independent parts rather than solid structures.

Material properties of the inner and outer cortical tables of the human parietal bone

“…The greater average mineralization of the inner table may reflect a slightly lower remodeling rate. If, as suggested by Herring and Teng (2000), the ectocranial surface of the cranial vault (outer table) is under tension during biting and the endocranial surface (inner table) is under compression, than the difference in mineralization correlates with that found in artiodactyl and perissodactyl calcanei by Skedros et al (1997). In these bones, remodeling in the compression cortex occurs at a slower rate than remodeling in the tension cortex.…”

“…Tensile strains across the sagittal suture in macaques during isotonic temporalis contraction (Behrents et al, 1978) suggest that the outer table is loaded during biting. However, at least during growth prior to sutural fusion, tension at the sagittal suture would have a dampening effect on parietal bone strain above the temporalis attachment, as strains in the sagittal suture of pigs are much larger than those in the adjacent bone (Herring and Teng, 2000).…”

Bone strain gradients and optimization in vertebrate skulls

“…The data for Otolemur come from experiments on one adult O. garnetti, two adult male (Ravosa et al 2000 a) and one adult female O. crassicaudatus (Ross 2001). Data are also available from numerous individuals of min- (Herring and Teng 2000;Herring et al 1996Herring et al , 2001Rafferty et al 2000) and two Alligator mississipiensis (Ross and Metzger, unpublished data).…”

“…When peak c-max values were not reported (Dechow and Hylander 2000), the maximum values of e 1 and e 2 re- corded during a single experiment were used to estimate peak c-max. In the case of the data from the pigs, neither c-max nor maximum e 1 and e 2 data were available, so peak c-max data are not presented (Herring and Teng 2000;Herring et al 1996Herring et al , 2001Rafferty et al 2000). The primate and pig mean c-max values were calculated using only working-side mastication data, pooling data from all food types.…”

Finite element analysis in functional morphology

“…This makes it difficult to assign empirical elastic properties into a global reference frame if the bone shape is highly curved, as it is in most part of the craniofacial skeleton. Researchers also face the challenge of modeling differences in elastic properties in different tissue types, such as skull sutures or cancellous bone regions (Herring and Teng, 2000). More research is needed to assess the impact of various simplifications of the complex variation found in the elastic properties of some skeletal elements on the reliability of associated FE models.…”