In vivo surface strain and stereology of the frontal and maxillary bones of sheep: Implications for the structural design of the mammalian skull

Abstract

Does the skull of the sheep behave as a tube or as a complex of independent bones linked by sutures? Is the architecture within cranial bones optimized to local strain alignment? We attempted to answer these questions for the sheep by recording from rosette strain gauges on each frontal and maxillary bone and from single-axis gauges on each dentary of five sheep while they fed on hay. Bone structure was assessed at each rosette gauge site by stereological analysis of high-resolution radiographs. Structural and strain orientations were tested for statistical agreement. Ranges of strain magnitudes were Ϯ1200 ⑀ on the mandible, Ϯ650 ⑀ on the frontals, and Ϯ400 ⑀ on the maxillae. Each gauge site experienced one strain signal when on the working (chewing) side and a different one when on the balancing (nonchewing) side. The two signals differed in mode, magnitude, and orientation. For example, on the working side, maxillary gauges were under mean compressive strains of -132 ⑀ (S.D., 73.3 ⑀), oriented rostroventrally at 25°-70°to the long axis of the skull. On the balancing side, the same gauges were under mean tensile strains of ϩ319 ⑀ (S.D., 193.9 ⑀), at greater than 65°to the cranial axis. Strain patterns on the frontals are consistent with torsion and bending of the whole skull, indicating some degree of tube-like mechanical behavior. Frontal and maxillary strains also showed a degree of individual loading, resulting from modulation of strains across sutures and local effects of muscle activity. The sheep skull seems to behave as a tube made of a complex of independent bones. Structural orientation was in statistically significant agreement with the orientation of working-side compressive principal strain ⑀2, even though principal tensile strains may be as large or larger. Cranial bone architecture in sheep is not optimized to both strain signals it experiences. Anat Rec 264: [325][326][327][328][329][330][331][332][333][334][335][336][337][338] 2001.

Modulation of mandibular loading and bite force in mammals during mastication

“…Data sample The mandibular bone strain data analyzed for this study (Table·1) were recorded in three different laboratories in connection with other studies (Herring and Teng, 2000;Hylander et al, 1998;Liu et al, 2004;Liu and Herring, 2000a;Liu and Herring, 2000b;Rafferty, 2005;Ravosa et al, 2000;Ross and Hylander, 1996;Thomason et al, 2001;Williams, 2004). Data were available for 3084 chewing cycles from 31 individuals of 11 species, including seven species of primates (four anthropoid and three strepsirrhine species), as well as goats (Capra), horses (Equus caballus), alpacas (Lama pacos), and miniature pigs (Sus) (Table·1).…”

Masticatory muscles and the skull: A comparative perspective

“…For example, working and balancing mandibular bodies show opposite directions of bending because of the influence of the unilateral bite point. 18 Other groups, including many rodents, pigs, and higher primates, have some probability of occlusal contact occurring on the balancing side, and may even have a bilateral bolus and true bilateral chewing. Thus in pigs, mandibular and maxillary strains are identical on the working and balancing sides.…”

Comparison of beam theory and finite‐element analysis with in vivo bone strain data from the alligator cranium

“…Numerous experimental studies have shown that sutures typically exhibit strain magnitudes that are an order of magnitude higher than those in the bones that they connect, and strains can be reduced or reoriented across sutures (Jaslow, 1990;Jaslow and Biewener, 1995;Rafferty and Herring, 1999;Herring and Teng, 2000;Metzger and Ross, 2001;Rafferty et al, 2003;Lieberman et al, 2004). The principal strain values recorded from the alligators in vivo are on average greater than those recorded from any other vertebrate cranial bones that have been extensively sampled (Hylander, 1979;Hylander and Johnson, 1992;Herring et al, 1996;Ross and Hylander, 1996;Hylander and Johnson, 1997;Herring and Teng, 2000;Ravosa et al, 2000;Ross, 2001;Thomason et al, 2001;Lieberman et al, 2004;Ross and Metzger, 2004). If sutural strain increases as a function of bone strain, the very high bone strain magnitudes recorded in this study predict extremely high sutural strains, suggesting that sutural morphology might be of great importance in the functioning of the Alligator skull, as it appears to have been in dinosaurs (Rayfield, 2005, this issue) and at least some mammals (Herring and Teng, 2000).…”