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First published online November 17, 2005
Journal of Experimental Biology 208, 4509-4521 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01923

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Ontogeny of bone strain: the zygomatic arch in pigs

Susan W. Herring^*, Scott C. Pedersen and Xiaofeng Huang

Department of Orthodontics, University of Washington, Seattle, WA 98195, USA

View larger version (31K): [in a new window]   Fig. 1. The zygomatic arch, consisting of zygomatic (Z) and squamosal (Sq) bones, and the masseter muscle (Mass) in (A) a 3.5-week-old piglet and (B) a 3.5-month-old juvenile pig, both domestic farm animals. The masseter originates mostly aponeurotically from the zygomatic bone. In the piglet (A), the aponeurosis is so thin posteriorly that the underlying zygomaticomandibularis muscle (arrow) is clearly visible. In the juvenile (B) the aponeurosis is thicker and shows clearer margins (denoted by a dashed line), and only a small corner of the zygomaticomandibularis (arrow) is visible. The scale bar indicates 1 cm. The dotted lines in B show the reference axes for plotting the orientation of tensile strain. The 0–180° line is parallel to the occlusal plane. Modified from Herring and Wineski (1986).

View larger version (135K): [in a new window]   Fig. 2. Examples of skulls, strain gage locations, and measurements. (A) Skull of a piglet of the age used in the present study; condylobasal length approximately 100 mm. A strip gage, consisting of three parallel anterior-posteriorly oriented single-element strain gages, is illustrated on the zygomatic bone (Z), and a stacked rosette gage is shown on the squamosal bone (Sq). (B) Skull of a juvenile animal, a sibling of one used in the previous study (Herring et al., 1996); condylobasal length 200 mm. (C) Skull from a young adult (dentition complete except for third molars); condylobasal length approximately 270 mm. Note the progressive deepening and curving of the zygomatic arch with increasing age. (D) Ventral view of a disarticulated left squamosal bone, showing the articular eminence (AE) with the medial shelf (arrow) that sometimes continued to the level of the gage site, where squamosal thickness (SqTh) was measured. (E) Bone dimensions. Zygomatic body length (ZL) was measured from the most posterior part of the maxillary-zygomatic suture to the corner of the zygomatic-squamosal suture. Squamosal bone length (SqL) was measured from the zygomatic-squamosal suture corner to the posterior extent of the suture. Bone height (ZH and SqH) was measured vertically (relative to the occlusal plane) at the gage sites. Medial-lateral bone thickness (at the stars) was also measured at the gage sites. (F) Method for estimating ventral curvature. A straightedge was laid across the most ventral parts of the sutures (ZX and SqX). From these lines, the heights of the maximum perpendiculars (ZY and SqY) were measured. Curvature was estimated as ZY/ZX and SqY/SqX.

View larger version (146K): [in a new window]   Fig. 3. Piglet left zygomatic arch, showing magnitude and orientation of bone strain during feeding, based on rosette data from Tables 2, 7. Peak maximum principal strains (tension) are shown as arrows headed away from the gage sites, and peak minimum principal strains (compression) are shown as arrows headed towards the gage sites. (A) Comparison of food types in piglets. Mean peak tensile strains recorded during the ipsilateral mastication of hard and soft chow and during milk drinking are represented on the left zygomatic arch. Compressive strains were roughly equal to tensile strains and perpendicular to them. Strain magnitude tended to be higher for harder foods and to be higher for the squamosal (Sq) than for the zygomatic (Z) bone. (B) Comparison of piglet (solid arrows) and juvenile pigs (dotted arrows) for hard chow. The triangles superimposed on the tensile strain indicate one standard deviation in magnitude (height of triangle) and in orientation (angle at apex). Strain orientations tend to be more variable in piglets. In contrast to piglets, juvenile pigs have negligible compressive strain on the squamosal. Zygomatic strain orientations indicate an opposite direction of torsion in piglets vs juveniles.

View larger version (127K): [in a new window]   Fig. 4. Magnitude and orientation of zygomatic arch strain during tetanus of the ipsilateral masseter muscle, based on rosette strain gage data from Tables 4, 8. Conventions as in Fig. 3. (A) Piglet. (B) Juvenile. Except for squamosal tension, strain magnitudes are larger in piglets than in juveniles.

View larger version (22K): [in a new window]   Fig. 5. Changes in bone proportions and shape with increasing skull size. Scatter plots are based on 45 skulls of Hanford strain pigs. Reduced major axis parameters are presented in Table 6. The two boxed areas indicate the size ranges of the piglets used in the present study (92–106 mm condylobasal length) and the juveniles from the previous study (Herring et al., 1996; 196–222 mm condylobasal length). Arch dimensions measured are illustrated in Fig. 2D–F. (A) Bone height relative to bone length increased for both zygomatic and squamosal bones, but more so for the former. (B) Bone thickness relative to bone length decreased significantly for the squamosal but not for the zygomatic bone. (C) Curvature of the ventral border increased in both bones.

View larger version (61K): [in a new window]   Fig. 6. Disarticulated left squamosal bone, shown in a lateral and slightly dorsal view. The sutural contact areas for the zygomatic bone (open arrowheads), where masseteric loads would be transferred to the squamosal, are on the anterior section of the lateral surface and the ventral edge. Acting through the zygomatic bone at the suture, masseteric loading could give rise to the observed strain pattern (dotted lines; compare with Fig. 4A) by either twisting the lower edge of the squamosal medially (torsion) or pulling it ventrally (shear).

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