Introduction

Mandibular fractures are a common traumatic injury in dogs and cats and are often associated with hit by car accidents as well as gunshot injuries, animal attack/bite wounds, and falls from a height.^1–6 Patients diagnosed with mandibular fractures should be carefully assessed for evidence of additional injuries, as these fractures often occur in association with more widespread maxillofacial trauma (such as maxillary or orbital fractures). Mandibular fractures are usually open fractures in communication with the oral cavity due to the relatively thin oral mucosal tissue layer that covers the mandible and the firm attachment of the oral mucosal tissue to the underlying periosteum of the mandible.^2,3 Untreated mandibular fractures typically do not heal or they heal as poorly functional malunions. Various options exist for treatment of mandibular fractures, including both non‐surgical (such as maxillomandibular splinting) and surgical stabilization options. Regardless of the method of fracture stabilization selected, one of the main considerations during mandibular fracture repair is restoration of normal dental occlusion.³ Failure to restore proper dental occlusion may lead to long‐term patient morbidity, including oral mucosal erosions, tooth damage, and/or persistent patient discomfort.³

The teeth occupy approximately 50% of the volume of the mandible, and consequently, fractures of the mandible often involve one or more teeth or tooth roots. Preservation of teeth involved in mandibular fractures should be attempted if possible. Stable teeth with an exposed root in a mandibular fracture site can typically be preserved.^5,7 A diseased or loose tooth in a mandibular fracture line or a tooth with a fractured root in association with a mandibular fracture may require extraction during mandibular fracture repair.⁸

Feeding tubes may be inserted intraoperatively at the time of mandibular fracture stabilization for use in the postoperative period at the discretion of the surgeon. Esophagostomy tubes are the feeding tube most commonly utilized after mandibular fracture repair, but gastrostomy tubes may also be utilized. The authors find that feeding tubes are rarely necessary for patient management after mandibular fracture repair in dogs, as most dogs will use their tongue to ingest a liquid diet soon after mandibular fracture stabilization. Cats are sometimes slow to eat in the postoperative period after mandibular fracture repair, particularly with maxillomandibular fracture stabilization techniques, so feeding tubes are more commonly placed in cats at the time of mandibular fracture repair.⁹ If a feeding tube is placed at the time of surgery, it can be removed once the patient starts to eat orally in the postoperative period.

Biomechanical Considerations

The primary forces acting on mandibular fractures after repair are bending forces, and these forces can be quite large due to the strong masticatory musculature that attaches to the mandibles.^3,10 Following a mandibular fracture, the bending forces exerted on the mandible primarily cause the rostral segment of the mandible to displace in a ventral and caudal direction. This means that the alveolar (upper) surface of the mandible is the tension surface of the bone and the aboral or lower surface of the mandible is the compression surface.^3,10 In order for mandibular fractures to heal appropriately, the fixation technique selected must appropriately counteract the forces acting on the mandible in order to achieve a fracture site environment in which bone healing can occur. Optimally, mandibular fracture fixation technique should result in rigid stabilization of the mandibular bone segments, as this provides the optimal environment for healing. Most implants used in mandibular fracture repair are strongest when subjected to tensile forces, and therefore, from a biomechanical perspective, should optimally be placed on or close to the tension (alveolar) surface of the mandible.^3,10 With the exception of interdental wire with an intraoral composite splint, it is unfortunately not feasible to place most orthopedic implants on the tension surface of the mandible without damaging the teeth. This means that most implants (bone plates and screws or external fixator pins) utilized to stabilize mandibular fractures must be applied in a biomechanically suboptimal location on the mandible. Due to the suboptimal location in which these implants are often applied and due to the significant bending forces acting on the mandible, implant constructs utilized for mandibular fracture repair need to be quite strong to minimize the risk of implant failure or delayed union/malunion.

Anatomy

A good working knowledge of regional mandibular anatomy is required to streamline mandibular fracture stabilization and minimize the risk of iatrogenic complications. The reader is encouraged to review the excellent overview of mandibular anatomy, including the supporting neurovascular anatomy, in Chapter 12 prior to attempting mandibular fracture repair. The main muscles commonly encountered during the surgical approach to the mandible include the digastricus, the masseter, the medial and lateral pterygoids, the mylohyoideus, and in some cases, the genioglossus. The digastricus, which functions to open the mouth, originates on the occipital bone and attaches to the caudoventral aspect of the mandibular body. The medial and lateral pterygoid muscles attach to the medial aspect of both the ramus of the mandible and the angular process. The masseter muscle attaches to the ventrolateral portion of the mandibular ramus and the angular process. The pterygoid muscles and the masseter all function to close the mouth. The mylohyoideus is a thin muscle sheet that runs transversely between the mandibles and attaches to the medial aspect of the mandibular body bilaterally. The mylohyoideus functions to support the tongue ventrally. The geniohyoideus, which is rarely encountered during mandibular fracture repair, attaches to the medial aspect of the mandibular body bilaterally at the caudal edge of the mandibular symphysis and is positioned dorsal to the mylohyoideus. An understanding of dental anatomy is also important to optimize implant positioning for mandibular fracture stabilization and minimize the risk of damage to tooth roots during implant application, which might ultimately result in tooth loss or persistent patient discomfort in the future. Mandibular dental anatomy of the dog and cat is summarized in Figure 42.1. Permanent mandibular dentition in the dog consists of 3 incisors, 1 canine, 4 premolars, and 3 molars for a total of 11 teeth per mandible and 22 total mandibular teeth.⁸ The incisors, canines, first premolar, and third molar of the dog all have one root while the second to fourth premolars and the first and second molars each have two roots.⁸ Permanent mandibular dentition in the cat consists of 3 incisors, 1 canine, 2 mandibular premolars, and 1 mandibular molar for a total of 7 teeth per mandible and 14 total mandibular teeth.⁸ The feline incisors and canine each have a single root while the premolars and molar each have two roots.⁸

Diagnostic Imaging

Mandibular fractures can usually be diagnosed based on physical examination due to the paucity of soft tissue structures around the mandibles and the significant mandibular instability that typically develops after even a simple unilateral mandibular fracture. Confirmation of diagnosis and determination of mandibular fracture configuration is achieved using diagnostic imaging. Traditionally, orthogonal skull radiographs, including oblique lateral projections, have been used to characterize mandibular fractures (Figure 42.2). Intraoral radiographs may also be utilized to gain more focused insight regarding mandibular fracture configuration. Due to the difficulty associated with interpretation of skull radiographs, particularly in the presence of multifocal maxillofacial injury, computed tomography (CT) scan is now commonly utilized to assess and characterize mandibular fractures. Mandibular fractures are more easily identified and more accurately characterized on CT images as compared to skull radiographs.¹² To simplify assessment of mandible fractures using CT scans and to guide surgical decision making, the CT images are often reconstructed into a virtual 3D image from which the surrounding soft tissue structures can be subtracted to facilitate assessment of the fractured bone (Figure 42.3).

Stabilization Options for Mandibular Fractures

Stabilization options for mandibular fractures may be divided into two major categories, those that are non‐surgical versus those that involve surgery. Some guidelines for selection of mandibular fracture stabilization technique based on fracture location and configuration are summarized in Table 42.1. Non‐surgical management of mandibular fractures is typically achieved via external maxillomandibular stabilization using a muzzle.^5,8 Muzzle stabilization of mandibular fractures does not result in rigid fracture site stabilization and is best utilized in young patients with simple, unilateral mandibular fractures that are only mildly unstable. Most mandibular fractures are best treated with surgical stabilization.

Surgical stabilization options for mandibular fractures may be subdivided into four major categories, including the following^{3,8,10,13–15}:

• Maxillomandibular stabilization
  
• Wiring techniques
  
• Bone plates and screws
  
• External skeletal fixation

Surgical maxillomandibular stabilization has been described using a bi‐gnathic encircling and retaining device (also known by the acronym BEARD).⁹ The BEARD consists of a loop of nylon leader line that is tunneled subcutaneously circumferentially around the maxilla and both mandibles just caudal to the canine teeth and is secured ventral to the mandibles with a metal crimp or via a knot in the suture (Figure 42.4). The BEARD significantly restricts mandibular movement thereby facilitating fracture healing. It should be secured tight enough to allow the dog or cat sufficient jaw movement to eat soft/wet food and no more. The BEARD also helps maintain dental occlusion but, similar to muzzle stabilization of mandible fractures, the BEARD does not result in rigid fracture site stabilization. It is best suited for stabilization of caudal mandibular fractures that are not amenable to other forms of fracture stabilization, but can also be utilized in younger patients with simple, relatively stable mandible fractures.⁹ The BEARD is commonly used in lieu of muzzle stabilization and is attractive in that it is mainly internal, thereby avoiding many of the challenges of muzzle stabilization such as muzzle dislodgement, moist dermatitis under the muzzle, and soiling of the tape muzzle. Although placing the BEARD is technically considered a surgery, it is very simple to perform, requires no specialized instrumentation, and does not need to be performed in a sterile OR setting.

Wiring techniques for mandibular fracture fixation may be subdivided into three groups^3,13,16:

• Circumferential wire technique
  
• Intraosseous wire techniques
  
• Interdental wiring with intraoral composite splint techniques

The circumferential wire technique consists of a single strand of orthopedic wire, which is tunneled subcutaneously around both mandibles at the symphysis region just caudal to the canine teeth.^8,16 The wire is tensioned and secured ventrally using a twist knot. The circumferential wire technique is used to stabilize mandibular symphysis separation fractures. Intraosseous wire techniques involve the use of multiple interfragmentary wires inserted through tunnels drilled in each bone segment and used to reduce and compress mandibular bone segments together at the fracture site. Intraosseous wire techniques have historically been commonly utilized to stabilize mandibular fractures but are associated with a relatively high complication rate, as interfragmentary wires commonly loosen in the postoperative period and are not the optimal implant type to counteract bending forces.^3,10 The use of intraosseous wire techniques has decreased with the development of intraoral splinting and bone plate fixation techniques. Interdental wiring with intraoral composite splinting consists of a combination of one or multiple strands of orthopedic wire interwoven and twisted around the base of the crowns of multiple teeth spanning a mandibular fracture site over which a layer of dental acrylic is applied to create an intraoral splint.^8,13 Interdental wiring combined with an intraoral splint is an excellent technique for treating more rostral simple to mildly comminuted mandibular fractures.

Bone plate and screw fixation techniques are commonly utilized for mandibular fracture fixation as bone plate stabilization typically results in very rigid fracture site stabilization that can effectively resist strong bending forces.^3,10 Small locking bone plates are preferred for mandibular fracture fixation over non‐locking plates, as locking plates provide angular stability and are very resistant to screw pullout even in the poor quality or thin bone that is often encountered in the mandible.¹⁷ Reconstruction‐style bone plates that allow plate‐contouring to be performed in multiple planes are often utilized due to the non‐linear shape of the majority of the mandible.^3,10 Bone plate stabilization is most applicable to fractures in the mid‐body or ramus regions of the mandible. In the rostral portion of the mandible, bone plate fixation is difficult to utilize, as it is typically challenging or impossible to insert screws in the rostral mandible without tooth root impingement.

External skeletal fixator (ESF) techniques utilize end‐threaded (recommended) or smooth (not recommended) pins percutaneously inserted into mandibular fracture segments and connected outside the body by an acrylic column or connecting bar to stabilize mandibular fractures. ESF techniques were commonly utilized for mandibular fracture stabilization prior to the development of small locking bone plating systems. Due to the limited and often relatively thin bone stock available for implant insertion in the mandible, ESF fixation typically does not result in rigid stabilization of mandibular fractures, and pin loosening or back‐out is a common occurrence in the postoperative period.³

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