Strain in the braincase and its sutures during function

The skull is distinguished from other parts of the skeleton by its composit e 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 fronta l and parietal bones of miniature pigs and across the interfrontal, interpa rietal and coronal sutures. Strains were recorded 1) during natural mastica tion in conjunction with electromyographic activity from the jaw muscles an d 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 pa ir was most active. However, suture strains were not related to overall tor sion but instead were responses to local muscle actions. Only the coronal s uture showed significant strain (tension) during jaw opening; this was caus ed 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 t he 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 ectocra nial surface tensed and the endocranial surface predominantly compressed. C ollectively, these results indicate that skulls with patent sutures should be analyzed as complexes of independent parts rather than solid structures. (C) 2000 Wiley-Liss, Inc.