1.3  Midface Skeleton

Minimal extension of the ethmodial cells

Mean extension of the ethmoidal cells

Maximal extension of the anterior superior ethmoidal cells

Maximal extension of the medial superior ethmodial cells

Maximal extension of the posterior superior ethmoidal cells

Fig. 1.6  Variability of the sagittal and transversal extension of the ethmoidal cells in the floor of the anterior cranial fossa (mod. a. Lang 1983a, b, 1987, 1998)

1.2.3  Sphenoid

The sphenoid bone forms the posterior connection between the mid-facial skeleton and the cranial base, whereas the ethmoid bone, which has a delicate honeycombed structure, forms the anterior connection. Antero-laterally the sphenoid connects to the zygomatic bone and antero-inferiorly via the pterygoid process it connectes to the pyramidal process of the palatine bone.

The sphenoid sinuses border on the anterior, middle, and posterior cranial fossa, as well as on the sella turcica. The optic nerve passes through the lateral wall of the sphenoid sinus. It lies in close proximity to the internal carotid artery, the cavernous sinus and the cerebral nerves II-VI, as well as the sphenopalatine artery in the anterior sphenoid wall (Levine and May 1993; Messerklinger and Naumann 1995) (Fig. 1.7).

•A traumatic impact on the face can cause dislocated fractures in the frontobasal pneumatic cavities, which, in turn, may lead to disruptions of the inter-

Fig. 1.7  Parasagittal section of the lateral nasal wall with the subbasal ethmoidal-sphenoidal complex and its relationship to the frontal sinus and frontal base. The internal carotid artery and the optic nerve are prominent structures in the lateral wall of the sphenoid sinus. Note the relationship of the sphenopalatine artery to the inferior aspect of frontal wall of the sphenoid sinus (mod.a. Levine and May 1993) 1. Face of sphenoid 2. Internal carotid art. 3. Optic nerve 4. Posterior ethmoid art. 5. Air cell 6. Lamina papyracea viewed through ethmoid bulla 7. Anterior ethmoid art. 8. Agger nasi 9. Infundibulum 10. Lacrimal sac prominence 11. Uncinate process 12. Maxillary ostium 13. Sinus lateralis 14. Basal lamella 15. Lamella of superior turbinate 16. Sphenopalatine art.

nal mucous membranes, neighboring vital structures and dural injuries, so risking ascending intracranial infections (Boenninghaus 1971; 1983 Helms and Geyer; Theissing 1996).

1.3  Midface Skeleton

The central midfacial block, comprising the maxilla and the orbito-naso-ethmoidal region, constitutes the important osseous facial architecture. It incorporates the anterior skull base with the occlusal-mandibular complex and so predefines the vertical facial height. In the transverse plane, it combines both zygomaticorbital regions, so determining the facial width (Maisel 1984; Jackson et al. 1986; Manson et al. 1987).

The midface – conceptually designed as a biomechanical light-weight structure with thin walled cavities – is subject to specific construction principles.

•It is composed of osseous cavities and forceful trajectories,­ which in turn convey great, static, compressive­ forces to the stabile skull base. Force dispersion occurs via prominent vertical,

horizontal,­ and sagittal osseous trajectories (Rowe and Killey 1968; 1970; Haskell 1985; Ewers et al. 1995).

The three vertical midfacial trajectories are: the anterior naso-maxillary pillar, the mid-zygomatic-maxillary pillar, and the posterior pterygo-maxillary pillar.

•The naso-maxillary abutment: runs as naso-frontal pillar from the canine tooth region, adjacent to the anterior bony aperture of the nose, through the frontal process of the maxilla to the upper orbital border and naso-ethmoidal region as far as the glabella region of the frontal bone

•The zygomatico-maxillary abutment – the middle trajectory: protracts as the zygomatic-maxillary pillar­ vertically above the zygomatic bone to the fronto­ -zygomatic sutures, to the frontal bone and via the zygomatic bone and arches into the temporal region

•The pterygo-maxillary abutment: runs posteriorly along the dorsal maxilla and the pterygoid of the skull base to the sphenoid bone (Fig. 1.8)

The midface is stabilized horizontally by a lower horizontal pillar composing the alveolar process and an upper fronto-facial pillar formed by the fronto-cranial compartment as well as a middle infraorbital-zygoma- tico-temporal pillar (Rowe and Williams 1985).

One can observe that no sagittal columns exist between the palate and the upper frontal arch (Mc Mahon et al. 2003). The upper orbital-interorbital midface ­complex is stabilized by two horizontal and four vertical latticed pillars (Mathog et al. 1995) (Fig. 1.9).

•This anatomical construction is of relevance when considering injuries to the central and lateral midfacial region. As a result of its special construction, the comparatively thin-walled midfacial compartment can absorb intense kinetic energy, so reducing the injury to the neurocranium in craniofacial injuries.

Principally, the midface only exhibits strong resistance against vertically applied forces. Although there is a lesser resistance against antero-posteriorly applied forces, it is combined with a high structural absorption capacity.

There is a 45° angle between the stabile skull base and the palato-occlusal plane. In contrast to the midface, this inclination results in a high resistance against an antero-posterior compression.

Fig. 1.8  Diagram of the vertical maxillary buttresses of the midface. These buttresses (bone-trajectories) represent regions of thicker bone, which provide support for the maxilla in the vertical dimension (mod. a. Prein et al. 1998). 1 Anterior medial naso-maxillary buttress, 2 lateral zygomatico-maxillary buttress, 3 posterior pterygo-maxillary buttress

In the case of an anterior-posteriorly applied force, the midfacial complex is driven against the sphenoid body, in such a way that the midfacial complex is dislocated en bloc posteriorly and caudally. This results in comminuted midface fractures with a typical dish-face deformity.

This also applies to assaulting forces to the midface from an antero-superior or lateral direction, which may cause the entire midfacial complex to shear off transversally from the cranial base (Rowe and Williams 1985) and induce subbasal avulsion fractures in the midfacial region including:

•Greater wing of the sphenoid bone

•Alar processes

•Ethmoid complex

•Frontal sinus

Collateral skull base injuries can be seen within the fracture compartment.