Management of symphyseal and parasymphyseal mandibular fractures

Arch bars and maxillomandibular fixation (MMF) are almost always necessary to establish the premorbid relationship of the mandibular and maxillary teeth. Although there are many ways to establish occlusion, standard Ernst arch bars and wire loop maxillomandibular fixation are commonly used in this technique. Simple fractures may also be managed with MMF via Ivey loops, maxillomandibular fixation screws, or Ernst ligatures. However, care must be taken to avoid overtightening the MMF, which can cause flaring of the mandibular angles. If using the arch bar as the tension band, care should be taken to ensure a solid bar which can last for the period of fracture healing.

The most common approach to the symphysis and parasymphysis is the transoral gingivolabial and gingivobuccal incision. The key to the design of the incision is to stay approximately a centimeter away from the attached gingival to allow enough tissue for a watertight closure at the end of the procedure (Figure 1). Care must be taken to avoid injuring the mental nerves because they exit the mandible and supply sensation for the lower third of the middle portion of the face. After incising through the mucosa, the mentalis muscle is divided and the periosteum is elevated off of the mandible, exposing all of the fractures and enough of the adjacent mandible for placement of the hardware. If necessary to provide sufficient access, the mental nerves may be exposed. Care is taken when dissecting around the mental foramen, to not cut or stretch the nerves. The periosteum is circumferentially elevated away from the foramen, allowing the soft tissue to be retracted away from the foramen for improved exposure. On occasion, the periosteum may need to be incised radially in the direction of the nerve to further increase exposure. With larger, comminuted fractures, an external approach may be necessary to accurately and rigidly fixate the mandible. In this approach a standard cervical incision may be made approximately 2 fingerbreadths below the mandible to avoid injury to the marginal mandibular nerve. After elevating the submandibular gland fascia to protect the nerve, the inferior border of the mandible may be approached. The periosteum is incised and elevated in the same fashion as the transoral approach to expose the fracture and allow for the placement of the appropriate hardware.

Once the occlusion has been established with the arch bars and MMF and the fracture has been exposed, attention is turned to reduction and fixation of the fracture. Reducing the fracture is usually straightforward in this technique. However, with severely comminuted fractures, care must be taken to ensure the viability of the segments with careful periosteal dissection and fragment manipulation. Once the fracture has been reduced to the anatomic position, the fixation is applied. The classic approach to fractures has been to place an upper border, monocortical tension band and a large lower border bicortical (often compression) plate. When properly performed, this AO-ASIF (i.e., Association for the Study of Internal Fixation) technique has resulted in very successful fracture management. However, over the years, many other excellent alternative approaches have been shown to be equally effective with similar results. Currently, smaller plates (miniplates) or lag screw fixation are most commonly used for repair of symphyseal and parasymphyseal fractures.

As with all fractures, the key to successful repair is accurate fracture reduction. This is especially true of symphyseal and parasymphyseal fractures, where there is a strong tendency for the mandible to flair at the angles. Lag screw fixation is a very useful technique in the symphysis and parasymphysis. In this description, references to screw size and instruments are to the AO/ASIF 2.4-mm mandibular trauma system although most mandibular plating systems will have the necessary equipment. It is imperative to have a wide variety of screw lengths to make sure the appropriate length is available for every fracture. Monocortical drill holes are placed on each side of the fracture and a bone reduction forcep is applied to hold the fracture in reduction (Figure 2). When the lag screws are applied, it is imperative to reduce the lingual border of the fracture and reestablish the appropriate intragonial distance by squeezing the mandibular angles together (Figure 3). While holding the reduction, the lag screws may be applied. This is accomplished by drilling a gliding hole with the 2.4-mm (larger drill bit) through the proximal bone to the level of the fracture line. Then, by sliding a drill guide into the gliding hole, the trajectory of the distal hole may be maintained. The drill bit is then changed to the 1.8 mm (smaller drill bit), and the distal bone is drilled until the drill exits the cortex (Figure 4A and B). Care is taken to insure that the trajectory of the drill bit is away from the mental nerves and below the level of the tooth roots (Figure 4C). A countersink is then used to allow for flush placement of the screw, and the screw is inserted into the hole (Figure 4D). For optimal strength a second lag screw is placed in a similar fashion. The bone reduction forceps are removed and the occlusion is checked.

Once the fracture is reduced and the angles have been squeezed together, plates may be applied for fixation of the fracture. Historically, this involved a smaller monocortical tension band along the upper mandible and a larger and sometimes even a compression bicortical plate along the lower border. Thanks to the work of Champy, the lines of optimal osteosynthesis have been established (Figure 5). Several authors have shown that miniplate fixation along these lines is a very effective way to fixate these fractures.^, 

The fracture is reduced with the bone reduction forceps and manual compression of the mandibular angles to avoid flaring. If the arch bars are going to be removed, a monocortical tension band is then applied to the upper Champy line using a 4 to 5 hole, 2.0-mm plate, and 4- to 5-mm screws. These short screws are used to avoid injury to the underlying dental roots. The lower border bicortical plate is than applied. Several authors have advocated overbending the plate as yet another way to get accurate reduction of the lingual cortex and overcome the tendency of the fracture to remain splayed open (Figure 6A). The drill holes are made adjacent to the fracture one at a time and the drill guide is utilized to measure the thickness of the bone. The measurement allows the correct choice of screw length and the screw is inserted. Once the holes on either side of the fracture are filled, the remaining holes of the plate are filled (Figure 6B). A minimum of 2 holes are needed on either side of the fracture for stable fixation.

With severe comminution or an edentulous mandible, larger, load-bearing hardware is required. In these circumstances, the larger locking, reconstruction plates are the optimal choice to maximize the chances of stable fixation and bone healing. With these fractures, 3 holes are desired on each side of the affected segment for stable fixation.

Once the hardware has been placed, the occlusion is checked and attention is turned to closure. After copious irrigation, the intraoral incision is closed with care taken to reattach the mentalis muscle. A watertight closure of the mucosa is then performed with absorbable sutures (Figure 7). A decision is then made to either leave the arch bars in place or to remove them. In most cases, with accurate reduction and appropriate fixation the arch bars can be removed and the patient can start to function immediately after surgery. Before leaving the operating room, the skin is washed and a splint of foam tape may be used to reapproximate the mental soft tissue over the bony mentum. Patients are instructed on oral care with frequent mouth rinses utilizing peroxide or chlorhexidine rinses. A soft diet is recommended for the 4 to 6 weeks of fracture healing. The patients are followed postoperatively for signs of malocclusion, wound breakdown, and infection.

With any mandibular fracture repair, malocclusion, malunion, nonunion, infection, dental injury, would breakdown, and nerve injury are possible. By using the aforementioned techniques, one can minimize these complications. Postoperatively, the patient's wounds are followed closely for signs of breakdown, hardware exposure, inflammation that might suggest nonunion or loose hardware, and frank infection. A postoperative panorex or computed tomography scan is frequently obtained to confirm and document accurate reduction of the fractures. Patients should be followed for at least 6 weeks to insure accurate reduction and occlusion during the fracture healing.