ANATOMY
The mandible is composed of two halves with the horizontal portion (body) supporting teeth and the vertical portion (ramus) providing surfaces for muscle attachment and articulation with the skull. Each half of the mandible joins at the rostral midline to form the mandibular symphysis. Major neurovascular structures—including the mandibular artery and vein and the inferior alveolar nerve—enter the mandible on the caudomedial aspect through the mandibular foramen. These structures provide the major neurovascular input to the mandible and sole neurovascular supply to the mandibular teeth. The mandibular canal opens mesially to form 2–3 mental foramina. The mandibular contribution to the temporomandibular joint (TMJ) consists of a transversely elongated condyle. The temporal bone contribution to the joint is tubular and closely surrounds the condyle. The TMJ also has a disc or meniscus providing an interface between bony components.
BIOMECHANICS
Mandibular fracture biomechanics indicates that the tension side of the mandible is the alveolar border, or tooth side of the mandible. Techniques that engage teeth secure the tension surface, allowing natural compression of the ventral surface. Intraoral splints, interdental acrylic, interdental wire, and combinations of interdental wire and acrylic can be applied to the tension band surface. Oblique mandibular body fractures can be classified as advantageous or disadvantageous. Fracture lines that are oriented from dorsocaudal to ventromesial are considered advantageous since muscular attachments to the mandible have a primarily ventral insertion causing compression of the fracture line. Fracture lines that are oriented from dorsomesial to ventrocaudal are considered disadvantageous since similar muscle forces lead to distraction of the mesial fragment. Knowledge of the inherent biomechanical nature of the fracture may affect decision-making concerning the technique for fracture fixation.
FIXATION TECHNIQUES
Mandibular fractures in the dog occur secondary to vehicular trauma, falls, kicks, gunshots, and fights with other animals. Mandibular fractures represent 3-6% of all fractures seen in the dog while 15% of all fractures in the cat affect primarily the mandibular symphysis. The most common location for fracture in dogs is between the PM 1 and M 2. Pathologic mandibular fracture may occur secondary to periodontal disease, neoplasia, and metabolic diseases. The primary objective for repair of mandibular fractures in small animals is return to normal function. Caudal malalignment of 2–3 mm may prevent closure of the mesial portion of the mouth by a full centimeter. Therefore, it is necessary to maintain occlusive alignment while providing adequate stability for bony union. Basic principles of mandibular fracture repair include anatomic reduction and restoration of occlusion, application of a stable fixation to neutralize negative forces on the fracture line, gentle handling of soft tissues, avoidance of iatrogenic dental trauma, extraction of diseased teeth within the fracture line, minimizing excessive soft tissue elevation, and application of techniques which restore a rapid return to function.
Fractures of the mandible in dogs present several unique challenges to the veterinarian since it withstands different forces compared with weight–bearing bones. Mandibular fractures will heal in the presence of fracture gaps and some mobility, as long as vascularity is protected, revascularization encouraged, and infection prevented. The fixation method should allow immediate restoration of function; be light and not cumbersome, economical, and readily available; and require only a reasonable amount of time, expertise, and ancillary equipment for application. Trauma to tooth roots and neurovascular structures may not result in clinical signs; however, endodontic and periodontal complications including alveolar bone resorption, tooth root involvement, pulpitis, and tooth loss may occur. The inferior alveolar artery and its branches provide the sole blood supply to alveolar bone, periodontal ligament, and teeth. Its importance in the healing of mandibular fractures and tooth structures after injury and any subsequent clinical effects is unknown. Likewise, the clinical occurrences of painful neuroma after damage to the inferior alveolar nerve during fracture have not been documented in the dog.
Temporary muzzle coaptation may be applied during the preoperative period to support mandibular fracture. If used, the patient should be monitored to insure the muzzle does not interfere with breathing status or cause unnecessary, potentially detrimental excitement. Muzzle coaptation is also the most common definitive stabilization technique for mandibular fracture in dogs. Its common use indicates that it is successful in providing bony union most of the time. Post-treatment occlusion may not be optimal, however patients tend to do well clinically. Complications and problems associated with muzzle application include malocclusion, aspiration of food contents secondary to vomiting, hyperthermia from decreased ventilatory function (negative effect on panting), and moist dermatitis. This fixation method is inexpensive and does not negatively affect fracture fragment vascular supply or tooth roots and neurovascular structures of the mandibular canal. Although often successful in providing fracture fragment stability sufficient to promote secondary bony healing, mandibular healing because of muzzle application may be associated with permanent malocclusion. Other potential complications, which may occur during the treatment period, include patient noncompliance, and delayed return to function related to restriction of normal mastication. External fixation methods using intrafragmentary pins and acrylic side bars may provide adequate mandibular fracture stabilization; however, iatrogenic trauma of structures of the mandibular canal is possible based on recommendations for location of pin placement and mandibular anatomy. Loosening and infection are the two most frequent problems associated with the use of larger external skeletal fixation pins and are primarily a result of thermal necrosis of bone and soft tissue.
Other potential complications associated with the use of external fixation methods are pin–tract infections, patient intolerance of the appliance, and disruption of the fixator bar on household furnishings. Internal fixation methods such as intramedullary pinning and plates and screws may also be associated with iatrogenic trauma of tooth roots and neurovascular structures. Disruption of fracture fragment vascular supply during implant application may complicate healing. Drawbacks to plating are the expense of the equipment, substantial time investment required to learn technical principles, and the penetration through or interference with the blood supply to the roots of the mandibular teeth resulting in endodontic disease. Methods using interdental fixation (IF) are an important component of temporary or primary mandibular fracture stabilization in humans. Clinical reports have recommended IF for stabilization of mandibular fractures in dogs. Advantages of interdental fixation for stabilization of mandibular fractures include avoidance of iatrogenic trauma to tooth roots and neurovascular structures of the mandibular canal, minimal disruption of fracture fragment vascular supply, restoration of occlusion, and early return to function. In summary, several fixation methods may be used for mandibular fracture repair that are quick to perform and provide sufficient stabilization for healing. Techniques that have these attributes may be used for emergency management of mandibular fractures.
INTERDENTAL FIXATION
Interdental fixation methods for human mandibular fracture stabilization include Ivy loop, Stout loop, modified Stout loop, acrylic splints, and Erich arch bar. The ability of IF methods to provide mandibular fracture stabilization while avoiding iatrogenic complications inherent with other more conventional fixation methods makes these techniques particularly desirable. The low cost of materials, relative ease of application, and frequency of mandibular fracture in dogs contribute to their potential uses in veterinary medicine.
Although acrylic does not adhere well to metal, it conforms to crown shape and interdigitates with gross metal architecture (metal cleats of the arch bar) and deformation (wire twists). Enamel adherence properties are propagated by formation of microporosities within prism cores or around rod peripheries of enamel using phosphoric acid gel etching of the enamel surface. Microporosity depth has been reported to range from 20 to 50 microns. Dental acrylic materials have been shown to penetrate these microporosities-forming, finger–like projections, resulting in a strong bond between the acrylic material and enamel.
A technique utilizing 24 gauge stainless steel orthopedic wire and poly(methyl)methacrylate has been developed. The wire is place around the teeth in Stout loop fashion, the teeth are prepared by acid etching, and dental acrylic is bonded to the teeth to create the interdental fixation.
The dog is generally positioned in ventral recumbency and the mouth opened with a speculum or other device. This technique is best applied to fractures in the premolar to molar area.
If the fracture is open, debridement and mucosal wound care is performed.
Twenty-four gauge orthopedic wire is cut to a length that will incorporate at least two teeth on either side of the fracture. The wire is applied in an intertwining fashion and tightened to the teeth using a twisting fashion as one would with cerclage wiring.
Once the wire has been applied, the teeth are cleaned with an ultrasonic scaler, followed by acid etching with 40% phosphoric acid gel, then rinsed and dried. The etching is performed on the buccal and lingual surfaces of the first through third premolars and lingual surface only of the fourth premolar and molars, taking into account the scissor bite of the maxillary and mandibular arcades.
The dental acrylic is mixed in a 2:1 ratio. Three cubic centimeters (by volume) of monomer powder is placed in a mixing container. One-and-one–half cubic centimeter of polymer liquid is added to the powder. The mixture is stirred for a brief period and then transferred to a 3 cc syringe with a needle attached. The plunger is inserted partially, the syringe is inverted, and the needle is removed once the air bubble has reached the top. The plunger is then inserted fully into the syringe, evacuating the air from the syringe. The acrylic is allowed to “cure” until it reaches the doughy stage of polymerization. This can be ascertained by testing the acrylic on a piece of paper or between your fingers. The acrylic is then applied to the buccal and lingual surfaces of the teeth that have been etched.
Further reduction of the fracture may be performed while the acrylic hardens (the acrylic can still be molded). Irrigation with cool tap water or saline can be used at this stage to decrease the heat generated by the exothermic polymerization of the acrylic.
Once the acrylic is cured, its shape can be modified with dental burs and files. Additional acrylic can be added right onto the original acrylic once it is cleansed of debris and dried. If the appliance breaks prior to fracture healing, additional acrylic can be applied directly to that already bonded to the teeth.
Recent studies compared the strength of various types of interdental fixation including wire, wire and arch bar, acrylic, acrylic and wire, and acrylic with wire and arch bar. Results indicated that acrylic interdental fixation reinforced with metal is the strongest fixation when tested in bending. The strongest interdental device of those tested was acrylic with wire and arch bar. Similar to the technique described, wire and arch bar may be applied using 24-gauge wire and orthodontic arch bar. The arch bar is applied to the lingual aspect of the mandibular arcade by using individual loops of 24-gauge wire around the neck of each tooth and under the cleat of the arch bar adjacent to the tooth. The wire is twisted on the lingual aspect of the mandibular arcade, in effect cerclage wiring the arch bar to individual teeth. Acrylic is applied in the same manner as described previously. In summary, the order of lesser to greater strength was wire < wire and arch bar < acrylic < acrylic and wire < acrylic with wire and arch bar.