Patterns of bone strain in the zygomatic arch.

Abstract

The transmission of force through the skull is complicated by the irregular form of the bones, the interposed sutures, and the multiplicity of loads from the teeth, muscles, and environment. The in vivo relationship between bone strain and muscle function in the mammalian skull is best investigated empirically. We studied the zygomatic arch of pigs (Sus scrofa) by simultaneous strain gauge recording and electromyography. Seventeen juvenile animals were used, employing multiple strain gauges arranged either in rosettes or strips. Strain was recorded during mastication and muscle stimulations. Bony architecture was examined on sectioned specimens. Strain patterns were complex even in this beamlike structure. During masseteric contraction, the more anterior zygomatic bone showed in-plane bending such that its lower border became more convex, and the major principal strain axis (tension) was parallel to the masseter muscle. The posterior squamosal bone was slightly bent in the opposite direction, and the major principal strain was rotated 45-60 degrees from the masseteric line of action. Strain magnitudes in the squamosal were larger than those in the zygomatic. Woven bone composing the surface of the arch appeared denser in the zygomatic bone, where its predominant orientation corresponded with compressive strain. In the squamosal bone trabeculae were more regularly arranged, but their orientation did not correspond with strain axes. The magnitude differences are probably related to the different architecture of the zygomatic and squamosal bones, whereas the different strain patterns primarily reflect the influence of the sutures in selectively damping or transmitting loads. In particular, the zygomatic bone may be loaded by three-point, distributed-load bending, whereas the squamosal, loaded at only two points, may be sheared. We conclude that each cranial bone functions in a unique strain environment, with the sutures serving to redirect loading.