■ Part 1. Basic Craniofacial Biomechanics
The structure of the craniofacial skeleton1 – 3 is relatively delicate and vulnerable to the impact of new physiologic loads, whereas in adjacent areas, such as the skull, the bone is thick and comparatively robust. All areas participate in the absorption of force loads, but logic would suggest that reinforced areas (buttresses) transfer and then release greater force loads during normal function and are less vulnerable to injury. In the mid-20th century, Gurdijan and Webster studied deformation of bone by first applying lacquer (so-called Stresscoat®) to the surface of bone and by then employing various, controlled forces.1 , 2 Gurdijan and Webster did not have the initial goal of studying craniofacial fractures but demonstrated that loads applied in a given area are routinely and widely distributed throughout the craniofacial skeleton,3 even with low-velocity impact.
High-speed computer (finite element) analysis reveals the functional behavior of craniomaxillofacial structure at more diminutive levels than those possible in the mid-20th century. Upon the application of measurable load forces, creation and flow of facial force equilibrium circuits of stress to and from the components of the craniofacial region (collagen matrix, dentition, cartilage, muscle, tendon, fascia, and notably bone) can now be demonstrated using finite computer models.4 – 6 The distribution of circuits of stress to and from the cervical spine during force applied to the craniofacial skeleton can be speculated but is yet to be demonstrated.
The admix of thick and thin, first noted by Le Fort, Testut, and Cryer,7 – 9 makes the craniofacial skeleton subject to torsion, rotation, translation, and other complex geometrical events1 – 3 , 10 – 15 after low-velocity and particularly after high-velocity impact. These seven buttresses of the mid-face, by example, are often sheared just after their ascent from the palatal platform, creating what Le Fort called a Level I fracture ( Fig. 2.1 ). With higher velocities of impact, more obscure patterns of injury are witnessed.
Terms Essential to Understanding the Biomechanics of the Craniofacial Skeleton
Only biomechanical perspectives germane to craniofacial anatomy and repair will be presented in this chapter.16 , 17 Terms common to engineering are reduced to a pragmatic level, with some admitted, but acceptable, risk of oversimplification. That said, all structures in nature undergo a physical, geometrical change in space when exposed to “force.” The craniofacial skeleton is no exception. Alteration of length is described as “tension” or “compression” and is defined by the relative change in position of the atoms of a given material. “Tension,” or “tensile force,” results in atoms moving further apart, in effect acting to microlengthen or microstretch a material. With “compression,” or “compressive force,” the atoms move more closely together, acting to diminutively shorten linear dimensions of a material.5 , 12 , 13 “Tension” patterns in craniofacial bone have been arbitrarily shown by various means, and they are reproducible. The patterns of “tensile force” in the accompanying mandible follow the application of force in the region of the anterior body near the right mental foramen, with the condyles fixed and immovable. The inward bending of the right body leads to lines of “tension” at the right inner cortex, the inner cortex of the ipsilateral condyle, and the outer cortex below the opposite condylar neck. The tension lines, in this arbitrary system, that may be inconsistent with a functional, living system, have been darkened to dramatize the imposed patterns12 ( Fig. 2.2 ).
“Torsion” is rotation caused by tensile and compressive forces that act as a unit to create displacement. A “displacement force” (previously referred to as a “shearing force” by Huelke and Harger12 ), thus, can be pictured as causing one part of an object to slide over an adjacent part ( Fig. 2.3 ).
“Torsion” is suspected as a key component, by example, when comminution of the buttresses of the midface occurs during high-velocity impact and displacement of the intact palate and alveolus beneath the periorbit and cranial base occurs. Torsion of the face upon a stable cranium interrupts the vertical and horizontal buttresses, creating a “twisted skull” ( Fig. 2.4 ).
The terms “stress” and “strain” are often used synonymously, but the terms are not in any way interchangeable. “Strain” refers to the elongation or the shortening of the linear dimensions of an object when it is under tension or compression; “strain” is expressed in a dimensionless ratio, as the amount of linear deformation per unit of length. By contrast, “stress” is a measure of the intensity of applied force, and it is expressed in units of force per unit area, most often as pounds per square inch.12 , 13 , 18 , 19
When there is “strain,” there is by definition a measurable deformation. “Tensile-strain” is therefore the elongation of an object by tensile forces, and “compressive strain” is shortening of an object under compressive force, as measured by increasingly sophisticated methodology that is able to define the “stress-strain relationship” of various materials, both commercial and biologic. But “stress” merely refers to the existence of intermolecular forces that are generated by the action of two objects on one another, exerting forces of equal magnitude along the same axis but in a different direction.5
“Strength” refers to the capacity of materials to resist failure.20 Thus, “tensile strength” refers to the capacity of a material to resist the action of tension (“tensile force”). “Compressive strength” depicts the capacity of a given material to resist compression (“compressive force”). “Stiffness” is the capacity to resist motion from compressive and tension forces. And, “elasticity” refers to the ability of a material to return to the shape that was present prior to the application of force. The material able to resist permanent change uses the energy stored during loading to return to the preloaded shape.4 , 5 , 12