An understanding of the functional anatomy is the prerequisite for successful application of the fracture fixation devices
in the unfamiliar location of the mandible. These biomechanical principals must account not only for the very large forces
generated, but also the position of the teeth that can – and often do – interfere with implant application. Bending forces
are the primary distracting forces acting on the mandible that must be neutralized (Fig. 1). A continuum of tensile to compressive
stresses exists from one side of the bone to the other during bending stress. Maximal tensile stresses are present at the
oral (alveolar) surface, and maximal compressive stresses are present at the aboral surface (Fig. 1a); therefore, distraction
is created at the oral margin (Fig. 1b). These bending moments increase from caudal to cranial; furthermore, shear forces
are maximal at the ramus, while rotational forces are most prominent rostral to the canine teeth - and maximal at the mandibular
symphysis. Bending moments, however, remain as the most significant force that must be neutralized due to the anatomic configuration
of a long lever arm with absence of supplemental support.
Fig. 1 A) Line drawing of an the mandible demonstrating bending moments, i.e., the continuum of tensile to compressive stresses
from oral to aboral bone surface with closure of the jaw during normal (chewing, biting) function (medium arrows). Large
arrows indicate pull of the major muscles of mastication (T, temporalis m; M, masseter m; D, digastricus m; P, pterygoideus
m). B) With a fracture, distraction occurs at the oral (alveolar) margin; compression occurs only at the point of bone fragment
Application of the fixation must consider the tension and compression surfaces of the bone. All fixation devices are strongest
in tension (stresses parallel to the longitudinal axis of the implant); therefore, they should be placed along the lines of
tensile stress, or on the tension surface of the bone. In cases of mandibular fractures, this location is along the alveolar
border; however, the presence of the teeth will interfere with this most optimal biomechanical location.
The objectives of treatment are to provide early rigid skeletal fixation, with restoration of dental occlusion, thereby achieving
an early return to function (eating and drinking – without bypassing the oral cavity via alternate feeding methods, e.g.,
esophagostomy/gastrostomy tube placement). Techniques and devices most often described for this purpose include: intraosseous
wire, used alone or in combination with other skeletal fixation devices, and bone plates and screws.
Intraosseous wire fixation is a simple technique that is designed to be used on the basis of the tension-band principle.
Due to the wire's small size, a great deal of versatility is possible with the wire location. This permits the tooth roots
to be easily avoided when placing wires on the biomechanically advantageous alveolar surface of the bone. Intraosseous wire
fixation is used to provide direct support for distracting (due to bending) and shearing forces at the fracture site. A single
heavy wire suture (e.g., 1.5-mm) applied to thicker bones, as in the mandibular body or ridges of the vertical mandibular
ramus, sufficiently neutralizes bending forces by locating the wire nearer to the alveolar/oral margin rather than to the
ventral, or aboral, margin of the bone. This location effectively prevents bending and separation at the tension side of
the mandible during normal function (chewing, biting). If anatomic reduction can be maintained, lateral bending, shear and
rotational forces on the mandible are minimal. Single intraosseous wires, although most useful in neutralizing bending forces,
still may not effectively neutralize shear or rotational forces despite the latter's generally lower magnitude. The addition
of a second, or divergent, wire greatly enhances neutralization of these forces. When bony defects are present, either due
to trauma or tooth loss this form of fixation is not effective, as it cannot maintain distraction because it cannot function
as a buttress device. The limitation of orthopedic wire, therefore, is that it must be used with an understanding of the
functional anatomy (placed along the lines of tensile stress) and with an anatomic reconstruction of all bone fragments (used as a rigid suture to compress the fractured bone fragments together).