CRANIOFACIAL BONE - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 210

10.2 CRANIOFACIAL BONE STRUCTURE
10.2.1 CRANIOFACIAL MACROGEOMETRY
Both macrogeometry and microstructure are important to recapitulate in scaffold design. We illustrate
the advantages of 3DP approaches that can more readily capture the geometries on both length scales
relative to other approaches. On the macroscale, most of the 22 bones of the craniofacial skeleton
feature unique geometries that affect two categories of a patient's well-being: (1) facial structure and
(2) mechanical transduction.
10.2.1.1 Importance of Average and Symmetric Facial Structure on Psychosocial Health
The craniofacial skeleton provides each person's face with its shape, and consequently, craniofacial de-
fects necessarily result in outward facial deformities. The current gold standard for craniofacial repair,
the “ free flap,” ,” utilizes fragments of autologous bones (most commonly the fibula, but also ribs and
pelvis) to achieve craniofacial reconstruction ( Figure 10 . 1 ). There are obvious drawbacks—autologous
bone tissue is in limited supply and donor-site morbidity is unavoidable. It is even clearer that
autologous bone segments do not match well with the complex geometries found in the craniofacial
skeleton; as such, reconstructed bone using this method still result in a deformed outward facial
appearances.
Studies on the societal view of facial structure have established that society values facial geometries
that are similar to the population average ( Langlois and Roggman, 1990 ; Rhodes et al., 1999 ) as well
as faces with high degrees of symmetry ( Mealey et al., 1999 ), illustrating an established bias on accept-
able facial structures. This provides one motivation for reconstructive therapies that mimic the native
shape of craniofacial bone as closely as possible. In general, the autologous bone grafts do not result in
“normal” or symmetrical faces. Therefore, 3DP approaches can provide major advantages in providing
bone grafts that successfully match the facial structure and restore normal appearance following bone
resection and trauma.
10.2.1.2 Effect of Craniofacial Bone Shape on Mechanical Transduction of Forces
The major forces acting on the craniofacial skeleton result from mastication and are induced by the
action of four muscles—medial and lateral pterygoids, temporalis, and masseter—inserting in specific
locations on the posterior mandible. It was once postulated that the shapes of the bones within the cra-
niofacial skeleton evolved to distribute chewing forces evenly. This view was ultimately abandoned, as
the anterior root of the zygomatic arch experiences ∼ 10 times more strain than does the posterior root
during mastication ( Hylander and Johnson, 1997 ). However, it has been demonstrated that the forces
resulting from the four muscles of mastication are distributed in a very defined spatiotemporal pattern:
through the zygomatic arch, the lateral orbital wall, the supraorbital bone, the infraorbital bone, and
finally through the anterior root of the zygomatic arch in that order ( Ross et al., 2011 ). The shape of
these bones and the connections between them greatly affect the way they transduce and distribute me-
chanical stress. Therefore, for proper mechanical function of the craniofacial skeleton, it is crucial that
the scaffolds used as craniofacial grafts recapitulate the native geometries. Consequently, fibular flaps
do not appropriately transduce forces throughout the skull. Besides affecting mechanics of jaw move-
ment, altered mechanical environments affect cellular behavior, as the osteocytes that comprise the