■ Part 2. Operative Technique and Exemplary Repair
Occlusion is a dynamic concept, and, according to Sicher, occlusal positions are “all those in which contact between some or all upper and lower teeth occurs.”7 Median occlusion represents the interdigitation of mandibular teeth against maxillary teeth, and, as noted by Dingman and Natvig, maintenance of median occlusion is important to ongoing healing osteosynthesis of the fracture site.24 Current rigid fixation devices have freed patients from the untoward sequelae and inconvenience of prolonged, intermaxillary fixation (IMF) after surgery.
The maxillary and mandibular first molars are guides for estimating normal occlusal position25: the mesiobuccal cusps of the maxillary first molars should align with the mesiobuccal grooves of the mandibular first molars. A slight overbite and an overjet of the anterior teeth are allowed. The engagement assumes what is colloquially called the [Davenport-Angle] “occlusal plane” (see annotated references 5 and 18 in the Additional Bibliography of Chapter 2) ( Fig. 5.12 ).
Establishing maxillomandibular (so-called intermaxillary) fixation is a significant measure in the repair of all lower facial fractures, whether with Erich arch bars, Ernst ligatures, or intermaxillary posts and wire loops ( Fig. 5.13A–C ).
“Seating” of the condyles in their appropriate position (craniomandibular articulation) is key to reestablishment of the dimensions of the lower third of the face.
Potential Bias of Intermaxillary Fixation
Appliances used to achieve IMF are easily applied and often utilized as an initial step in operative repair. In some instances, however, IMF appliances create bias that must be overcome (by rigid stabilization), because the IMF appliances complement forces at the fracture site, tending to rotate the fragments in a lingual direction.
Lingual (dental) version is seen most with fractures of the body of the mandible that are mesial to the molar teeth. For example, fractures occurring within the body are subject to the downward and inward pull of the mylohyoid muscle, such that the inferior margins of the fragments tend to splay outwardly. Elastics applied to the labial surface of the upper and lower teeth complement the mylohyoid “pull” and add a second force (in the same direction), favoring fragment version instead of reduction ( Fig. 5.14 ).
Because of this disadvantage, interdental wires and intermaxillary elastics have limited postoperative use, except with fractures of the body distal to the molar teeth, condylar fractures, and so-called simple maxillary fractures.26
Because pretraumatic malocclusion is common, it is important to assess dentition and occlusive patterns preoperatively. Abnormal wear facets, crowding, and other features are carefully documented before surgery is initiated, as they may significantly bias repair and could lead to false claims of induced malocclusion after surgery. Normal (“neutral”) occlusion and two classes of malocclusion (distocclusion and mesiocclusion) have been thoroughly described as Classes, I, II, and III, respectively.7 , 24
The relation of the mesiobuccal cusp of the maxillary first molar to the mandibular teeth is the key upon which Angle’s classification of occlusion is based ( Fig. 5.15A–C ).
Muscle Attachments and Their Influence
A detailed discussion of the mandible and its attachments can be found in standard books or atlases of anatomy, but the surgeon should be aware that basic movements of the mandible depend upon two groups of muscles.
These muscle groups may influence the position of the fractured segments,7 , 24 , 27 , 28 such that the fracture is considered “functionally stable” (thus “favorable”) or “functionally unstable” (thus “unfavorable”). The muscles of facial expression attached to the mandible have no significant effect on fractured segments.
The geniohyoid and digastric muscles act in concert as a depressor-retractor group; they are the weaker of the two systems. The masseter, temporalis, and medial pterygoid are comparatively powerful and serve to elevate the lower jaw, as the stronger system of muscles.
The lateral pterygoid is a major force in the protrusion of the mandible, and the mylohyoid has modest influence, because most of its fibers join in the midline at the mylohyoid raphe. The mylohyoid, nevertheless, may cause inward (lingual) rotation and splaying after fracture because of its high plane of attachment to the anterior segment.
The bias on fractures created by these muscle attachments has been thoroughly described by other texts.7 , 24 They, the presence of dentition, and the pathomechanics of impact profoundly affect the clinical presentation and radiographic findings of the injured patient ( Fig. 5.16A–C ).
Regions of Mandibular Fractures
Mandibular fractures may be characterized by the type of fracture: greenstick, simple, complex, or comminuted. However, a classification according to anatomical location is more germane to most clinical settings, as offered by Dingman and Natvig, with modification24 ( Fig. 5.17 ):
Region of the symphysis and parasymphysis. This is the region bounded by vertical lines mesial to the first premolar.
Region of the body. This region extends from the premolar line to a line that coincides with the anterior border of the masseter muscle.
Region of the angle. The region of the angle is triangulated and bears the attachment of the masseter.
Region of the ramus. This region is bounded inferiorly by the region of the angle and superiorly by two equal lines dropped at 90 degrees from the sigmoid notch.
Region of the condylar process. This region comprises the condylar process above the ramus. It includes the neck and the mandibular condyle.
Region of the coronoid process. This region includes the coronoid process above the ramus.
Region of the alveolar process. This region is restricted to the alveolus.
Proclivity to Fracture
The sites of predilection for fracture27 , 29 – 32 were identified in general by cinematography at many frames per second in the 1960s,33 – 38 relatively confirmed by cross-sectional studies in the 1980s and 1990s that depict the mandible as a tubular bone,4 and refined recently by finite analysis.39 , 40 The finite studies describe the mandibular trajectories as load-bearing pathways, as reviewed in Chapter 2.
The higher incidence of fractures of the mandible in the area of the symphysis-parasymphysis and in the body cannot be explained by cross-sectional area. The proclivity to fracture in these regions of the anterior segment appears rather to depend on the presence of dental roots, the location of the mental foramen and mandibular canal, tooth abutments, and the sites of insertion of the muscles of mastication,4 to name a few.
The incidence of fractures of the angle and particularly the region of the posterior segment near the condyle(s) is, by comparison, in part explained by cross-sectional area. The fracture line often passes obliquely downward and backward, from the sigmoid notch to the posterior border of the upper ramus. The fracture is extracapsular in this region and is colloquially labeled a subcondylar fracture.41
Condylar fractures may reach more proximally, within the anatomical neck, or may reach the joint cavity, with comminution.
Dislocation of the condyles into the middle cranial fossa may occur. As noted by Fonseca,42 however, when the mentum is struck with the mouth open (as in surprise), the medial and lateral poles of the condyles are impacted against the more thickened (medial and lateral) margins of the glenoid fossa. The prepositioning prevents the condyles from penetrating the more vulnerable center of the roof of the glenoid fossa. The thin roof of the glenoid (mandibular fossa) just anterior to the petrous pyramid is made apparent when viewed from above under appropriate lighting ( Fig. 5.18 ).
Fracture of the subcondylar region reduces the likelihood of penetration of the glenoid fossa and entry of an intact condyle into the cranial vault by decompressing the load force ( Fig. 5.19 ).
With fractures of the glenoid fossa, the condyle(s) may on occasion be displaced outside the fossa. High-resolution computed tomography ferrets out this unusual variant.
Fractures of the ramus exhibit relatively little displacement, because, to a large extent, the fractured segments are splinted by the masseter and medial pterygoid muscles. Similarly, fracture of the coronoid is splinted by the tendinous insertion of the temporalis muscle.2
After the subcondylar region, the angle has a large incidence of fracture. The angle fracture line begins at the inner cortex, passes to the outer (buccal) cortex, and may extend to involve the posterior body or adjacent alveolus. The fault commonly traverses the anterior root socket of the third molar, then obliquely reaches the distal root socket of the third molar, to exit through the outer (buccal) cortex ( Fig. 5.20 ).2 The inward and backward obliquity of this angular fracture may bias segment manipulation and realignment at surgery. On occasion, the molar at the site of fracture must be extracted to complete reduction of the fragments, but more often than not, the molar is used to maintain the operative realignment.
Most fractures of the body are buttressed by the dentition, particularly when impact is delivered from a lateral source. The powerful muscles attached to the ramus tend to displace the proximal body fragment, particularly when edentulous, upward, and inward relative to the maxillary teeth ( Fig. 5.21 ).
Fracture through the thick, inferior border of the symphysis is unusual but tends to be oblique when it occurs. The fragment from which the genial muscles arise tends to be displaced lingually, and both fragments consistently rotate medially, because of the medial pull of the mylohyoid muscles.2 The medial rotation is apparent on clinical presentation because the incisors adjacent to the fracture line consistently overlap and the anterior segment tends to splay at the angles ( Fig. 5.22 ).
The obliquity of the fracture makes realignment of the fragments difficult. Once reduced with forceps, however, the fracture can be stabilized with two or more lag screws, aided by inward pressure at each angle (by a surgical assistant). Alternately, the fracture can be stabilized with a strong, bicortical locking plate at the lower margin and a monocortical tension band more superiorly. When the fracture occurs near the deep root of the canine or involves the mesial roots of the first premolar, it is referred to as a parasymphyseal fracture.
Preoperative Assessment and Indications for Repair
Low-velocity fractures cause less dislocation and simplify clinical examination. Soft-tissue swelling may be minimal, and airway and vascular compromise are less often witnessed.
With improved restraint systems, rapid retrieval, and triage of the severely injured, a greater number of patients survive high-velocity fractures. Airway compromise is often reversed, hemorrhage is frequently contained, and limited fluid resuscitation is commonly initiated (see Chapter 3) before arrival at tertiary centers.
Discomfort is often present with motion and palpation over the site of injury. The least movement of the jaw may trigger excruciating pain, and thus chewing, speaking, or swallowing are minimized. Clinical examination may be compromised. Swelling, ecchymosis, hematoma, or crepitus in the vicinity of fracture “flags” the site of injury.
Damage to teeth may occur in conjunction with the fractures of the anterior segment and are most common in the symphysis and parasymphysis, where the teeth are relatively exposed. The teeth of the body may be sheared off at the time of impact. Teeth in the line of fracture are often a benefit to prealignment and stability of the fragments, even after rigid appliances have been applied. Nevertheless, if a single tooth or a proximal fragment interferes with reduction, the tooth is removed.
Numbness in the distribution of the mental nerve to the chin and lower lip is usually transient but frequently reported. Approximately 15% of patients have persistent areas of hypesthesia or anesthesia 2 years following fracture in uncontrolled study.
Displacement of the mandibular fragments may be suggested by the presence of asymmetry and deformity of the lower face. Bilateral subcondylar fractures, with upward and backward telescoping of the rami, typically trigger an open bite and the appearance of elongation of the face.7 , 24
Displacement sufficient to cause malocclusion increases the odds of finding an open fracture ( Fig. 5.23 ).
With displacement of the condylar neck, the mandible tends to shift toward the fractured side when any attempt is made to open the mouth. And, on protrusive motion, the jaw also shifts to the side of injury because of the relative inability of the lateral pterygoid muscle to function on the side of injury.24
Multiple fractures of the mandible are common. More than one fracture of the mandible is present in one of five or six cases. The incidence of distortion and malocclusion is greater in these more complex circumstances.
The Mandible as an “Osseous Ring”
Some clinicians describe the architecture of the mandible as though it was a “ring” and draw an analogy to the pelvis, where more than one fracture is common. The analogy to the “pelvic ring” would require, however, that the temporosphenoid aggregate (cranial base) and each glenoid fossa be an integral part of the mandibular architecture and, clearly, this is not the case.