PATTERNS OF BONE STRAIN IN THE ZYGOMATIC ARCH

Background: The transmission of force through the skull is complicated by the irregular form of the bones, the interposed sutures, and the m ultiplicity of loads from the teeth, muscles, and environment. The in vivo relationship between bone strain and muscle function in the mamma lian skull is best investigated empirically. Methods: We studied the z ygomatic arch of pigs (Sua scrofa) by simultaneous strain gauge record ing and electromyography. Seventeen juvenile animals were used, employ ing multiple strain gauges arranged either in rosettes or strips. Stra in was recorded during mastication and muscle stimulations. Bony archi tecture was examined on sectioned specimens. Results: Strain patterns were complex even in this beamlike structure. During masseteric contra ction, the more anterior zygomatic bone showed in-plane bending such t hat its lower border became more convert, and the major principal stra in axis (tension) was parallel to the masseter muscle. The posterior s quamosal bone was slightly bent in the opposite direction, and the maj or 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 appeare d denser in the zygomatic bone, where its predominant orientation corr esponded with compressive strain. In the squamosal bone trabeculae wer e more regularly arranged, but their orientation did not correspond wi th strain axes, Conclusions: The magnitude differences are probably re lated to the different architecture of the zygomatic and squamosal bon es, whereas the different strain patterns primarily reflect the influe nce of the sutures in selectively damping or transmitting loads. In pa rticular, the zygomatic bone may be loaded by three-point, distributed -load bending, whereas the squamosal, loaded at only two points, may b e sheared, We conclude that each cranial bone functions in a unique st rain environment, with the sutures serving to redirect loading. (C) 19 96 Wiley-Liss, Inc.