■ Part 2. Implications of Finite Element Analysis
Facial architecture has historically been perceived to be a buffer to injury of supra-adjacent structures. The cranio-facial buttresses act as an accordion in this proposed scenario, collapsing after impact to “cushion” the neuro-cranium and protect the cervical spine.3 , 53
The buttresses and flying buttresses “not only,” in the words of Cryer, “support direct forces acting externally, but also dissipate and diffuse shocks which would otherwise be transmitted to the cranium”9 ( Fig. 2.13 ).
This perception seems to correlate with the description of anatomic struts, descending from the cranial base to the palatal platform, by Testut, Cryer, Tandler and Sicher, and others.7 – 10 Support for this concept was endorsed by the authors of numerous subsequent clinical studies and was intuitively reinforced when plates and screws were introduced to reconstitute the craniofacial buttresses, restoring surface geometry and presumably reestablishing the distribution of load forces along these anatomic segments. Thin structures of the anterior, medial, lateral, and posterior maxilla are assumed by this historical perception to play a modest role in the restabilization of craniofacial structure (and the distribution of load forces) after repair.5
Lacquer impact studies and cinematography of the mid-20th century, though relatively ignored until recently, suggest a very different course of events, such that loads applied to the facial skeleton are freely and widely distributed to and from the cranium.1 , 2 Load forces, according to this hypothesis, have a pancraniofacial distribution, directly engaging the neurocranium and collateral areas.3 , 15 Extension of load forces to and from the cervical spine seems probable,54 – 57 but it is not yet proven.
This elegantly complex mechanism of load distribution throughout the craniofacial skeleton after impact is supported by selected studies (both in cohorts of survivors and notably non-survivors) over the past 60 years and by recent finite and strain-gauge analysis, as reported here and in pending publications15 , 59 , 60 ( Fig. 2.14 ).
All components of the pancraniomaxillofacial and pancraniobasilar skeleton, according to structural theory, participate in a balanced distribution of load forces. The contributions of the thin bone of the craniofacial skeleton are now less readily discounted as trivial.
A Current Finite Analysis Model
A computer model of the craniofacial skeleton captures the geometry of the entire skull and face by fixing the cranial base at the foramen magnum; the spine has been excluded. Anisotropic bone properties are included from multiple locations. The bite position in the computer model illustrated is in the area of the left canine, and the masseter musculature is simulated bilaterally for each bite condition. (“Compression” and “tension” values are related to the submental-vertex-axis. Red areas are “tension zones,” and blue “compression zones.” Regions in green depict the least stress.)
In this instance, the chosen left canine bite location triggers greater “tension” along the ipsilateral lingual cortex of the body and angle, “compression” along the contralateral lingual cephalad surface, and “compressive” stress in the midface along the four (medial and lateral) maxillary buttresses. The pattern in this case suggests concomitant “compression” of the central face, “compression” of the medial side of the lateral orbital rim, and “tension” at the ipsilateral lateral orbital rim (frontal process of the zygoma) ( Fig. 2.15A ).
In another illustration of load distribution, bilateral “tension” and “compression” can be provoked in many circumstances and measured using the computer model.
“Compression” is noted not only in the left body and symphysis of the mandible, ipsilateral to the bite load (pictured in blue) but is also captured in some regions of the contralateral mandible. Here also, the midsection of the lateral buttress of the midface is compressed, most notably on the same side as the bite load. The nasion is compressed below the frontal boss as noted, but in this case, the compression is in the midline. In addition, the ipsilateral infraorbital rim exhibits signs of “tension” along its superior surface, again supporting upward and cephalad finite displacement of the midface with the left-sided bite load ( Fig. 2.15B ).
Zones of “tension” and “compression” are also revealed in submental vertex planes. In the upper view, vertical “compression” is present along the posterior medial buttress during loading of bite forces. The sagittal plane demonstrates additional “compressive stress” (depicted in blue) along the right medial wall of the maxilla and vomer during loading ( Fig. 2.15C,D ).
During the same canine bite force, “compression” (in blue) is elicited simultaneously in both zygomatic arches and in bilateral regions of the skull base, including the temporal bones. The distribution of concomitant force to the subjacent cervical spine is not known ( Fig. 2.15E ).