MANAGEMENT OF MIDFACE INJURIES
The midface is defined as the area between a superior plane drawn through the zygomatiocofrontal sutures tangential to the base of skull and an inferior plane at the level of the maxillary dental occlusal surfaces. These planes do not parallel each other but coverage posterior at a level approximating that of the foramen magnum. The mid face can therefore be considered a triangular region with its widest dimension facing anteriorly. This arrangement helps protect vital posterior structures such as the proximal spinal cord as well as cranial nerves and vessels entering and exiting the cranium.
Different functional and anatomic units joined by direct sutural connections form the midface. it is a composite arrangment with contributions from the orbits. naso-or-bital-ethmoid (NOE) complex, zygomatic complex, and maxilla. Consequently, injuries to this region may involve these structures and their soft tissue contents to varying extents. The comprehensive management of midface injuries therefore involves a mandatory evaluation of these structural neighbors as well as corrective measures that take into account the separate complexes and their respective functions.
In this section, several issues that relate directly to the pattern and subsequent treatment of midface injuries are discussed. The bony morphology and important soft tissue structures are also described.
Areas of Weakness As previously stated, the midface is composed of the maxilla, orbits, NOE complex, and paired zygomatic complexes. Developmental sutures between these structures represent potential areas of weakness and are often the sites of fracture. Common sutural fracture sites are the frontozygomatic suture, zygomaticomaxillary suture, zygomaticosphenoid suture, nasofrontal suture, maxillofrontal suture, nasomaxillary suture, and midpalatal suture.
Fractures are not confined to these junctions but also occur in areas of relative weakness. B ones are less resistant to external forces when they surround an anatomic space of neurovascular bundle. Areas weakened in this manner include the mid-maxilla containing the maxillary sinuses bilaterally, the posterior maxilla where the pterygomaxillary junction separates the maxilla from the pterygoid plates, the midfraorbital rims advancement to the infraorbital foramen, the medial orbital walls overlying air cells of ethmodial sinus, and the lateral orbital walls, which are thinned out to accommodate the globe and adnexae medially and the temporalis muscle laterally.
Areas of Strength Fractures also result from the diversion of forces from areas of relative strength of weaker adjacent sites. Strength is imparted to regions where bone is thickened, contains a higher ratio o cortical to cancellous bone, or surrounds hard tissue structures that help absorb forces without disruption. An analogy has been drawn to architectural concept of support, and this has led to the characterization of these areas as vertical and horizontal pillars of the face. The horizontal pillars are composed of the supraorbital rims joined by the nasal process of the frontal bone, the infraorbital rims, and the alveolar process of the maxilla. Vertical support is derived from the zygomatic butteresses, pyfiform apertures (continuing superiorly as the frontal processes of the maxilla), and pterygomaxillary junctions. These areas provide structural reinforcement to the midface and help maintain its integrity unless excessive force is applied.
Soft Tissue Attachments
Lining Tissues the Midface skeleton is characterized by the presence of several air-filled cavities. Where present, soft tissue invests the bones on two sides. The outer or facial surfaces are covered by a firmly attached periosteal layer. This primitive structure contains a rich nonaxial network of vascular and neural elements that supplies the underlying bone. The periosteum is also a repository of undifferentiated mesenchymal cells that, under certain conditions, undergo transformation to osteogenic cells. The inner surfaces of the midface (i.e., surfaces adjacent to anatomic spaces) and; covered by specialized forms of epithelium that support the particular function of the space. The maxillary sinus and nasal cavity, for instance, are lined with ciliated epithelium containing secretary cells. This membrane also contains a rich vascular network, providing the bones of the midface with multiple nutrient sources. Thickened areas of the midface that do not contain spaces (e.g., the zygomatic buttress) derive an additional endosteal vascular supply from the cancellous marrow. This combination of periosteal, endosteal, and lining vascularity supports the survival of separate fracture segments even when gross communution is present.
Muscular Attachments Unlike the mandible, the midface is a static structure. Consequently, muscular forces applied to this region are reduced in magnitude. Closed reduction techniques or the fixation of fractures with wires and small bone plates are therefore more likely to be successful. Three muscles of mastication derive origins from different portion of the midface: the masseter (zygomatic arch) and the medial and lateral pterygoids (pterygoid plates). Fractures extending into these sites rarely undergo significant displacement. This reflects the relatively broad area of muscles attachments that tend to splint the fracture interfaces. other muscles connected to the midface include the various facial mimetic muscles (zygomaticus major and minor, lavatory superioris, levator anguli oris, levator superioris alaeque nasi, and risorius), the extraocular muscles, and the buccinator. Aside from the extraocular muscles, which provide movement to the globe, and the buccinator, which functions as a diaphragm, the remaining muscles insert into the overlying skin and subcutaneous tissues. Lack of firm anchorage reduces the ability of mimetic muscles to displace fractured segments. However, their orientation and tone are responsible for facial form and symmetry, and failure to reestablish their bony attachments may create an alteration of the soft tissue drape.
Classic descriptions of midface injuries invoke the patterns described by French surgeon Rene LeFort, who observed three levels of fracture determined by the magnitude and direction of an externally applied blunt force. Fractures occurred along lines of relative weakness and avoided areas of strength, as described previously. The Lefort I fracture is a horizontal fracture through the maxilla above the level of the alveolar process. It extends through the midportion of the pyriform rims and nasomaxillary suture anteriorly and continues posteriorly below the zygomticbuttresses before culminating as a horizontal fracture of the pterygoid plates. Such fractures permit separate mobility of the maxilla relative to the rest of the midface. A LeFort 1 fracture is also known as a Guerin fracture. LeFort II fracturesinvolve seperation of the nasofrontal suture anteriorly. These continue posteriorly through the thin medial orbital walls and weakened infraorbital floor and rim adjacent to the infraorbital canal and foramen and extend inferiorly through the anterior maxilla aand backward below the zygomatic buttresses and pterygoid plates. This fracture, also known as a pyramidal fracture, allows the central portion of the midface and maxilla to be mobilized independently from the cranial base, but the lateral orbital walls remain intact. The leFord III fracture is also known as a craniofacial dysjunction, because the entire midface is separated from the skull base. This is a result of fractures through the nasofrontal suture, medial orbital walls, orbital floors, lateral orbital walls, zygomaticofrontal sutures, zygomatic arches, maxilla (below the buttresses), and ptergoid plates.
Fracture patterns in the midface are the result of multivariate interactions between the forces applied to the skeletal components and the resistance offered by these structures. The amount of force obviously affects the type and pattern of fracture. A less known variable is the angle of impact. It has been suggested that forces applied obliquely to the horizontal pillars of the midface tend to provide leForT III fracture.