Skull and Mandible

Introduction

Fractures of the mandible and maxilla are fairly common in dogs and cats. They are usually of traumatic origin, occurring with vehicular trauma, gunshot wounds, horse kicks, bite wounds and “high-rise syndrome” in cats. Non-traumatic causes include severe periodontal disease, oral neoplasia, radiation therapy induced osteonecrosis and metabolic derangements. Recently, the increased use of recombinant bone morphogenic-2 (rhBMP-2) protein has allowed surgeons to repair challenging fractures or non-unions, fix critical defects, and perform aggressive oncologic maxillofacial surgery that would have otherwise failed with conventional therapy. When repairing mandibular and maxillary fractures the primary goal is to permanently restore proper dental occlusion. The secondary goal of obtaining anatomical reduction of the fracture is achieved when possible.

Maxillary fractures do not require surgical intervention as often as mandibular fractures. If displacement of a maxillary fracture is not severe, occlusion is appropriate, oronasal communication and facial deformity are minimal, and no obstruction of airflow through the nasal cavity is appreciated, then conservative management is an option. Although most fractures of the oral cavity are open and may have substantial soft tissue trauma, they generally heal quickly due to abundant vascularization.

Anatomy

The mandible is made up of two halves which are firmly, but not rigidly united at the mandibular symphysis. Each half is then composed of a horizontal body and a vertical ramus (Figure 46-1). The alveoli of the lower teeth lie within the mandibular body, with the tooth roots occupying approximately two-thirds of it. The ventral third of the mandible contains the mandibular canal, through which the alveolar nerve, artery and vein course. Several mental foramina lie within the rostral aspect of the mandible. These foramina are the exit points for the mental branches of the inferior alveolar nerve and vessels, providing sensory innervation to the teeth. Typically one foramen is larger than the others.
 

The ramus of the mandible consists of three distinct processes, the most dorsal being the coronoid process. It projects into the temporal fossa and provides an area for attachment of the temporalis muscle. The condylar (or condyloid) process is just ventral and slightly caudal to the coronoid process. This process is a point of articulation between the maxilla (retroarticular process) and mandible. Disruption of this region will result in temporomandibular joint luxation. The most ventral process is the angular process. This structure provides an area for attachment of the digastricus muscle. The masseter muscle inserts laterally on the mandible, while the pterygoid muscle inserts medially. The mandibular foramen is also located medially on the caudal aspect of the mandible.

The masseter, temporalis, and pterygoid muscles are responsible for closing the jaw and will contribute to dorsal displacement of caudal mandibular fractures. An appreciation of mandibular muscle biomechanics is important as this may influence treatment options. For example, an oblique caudo-ventral fracture of the mandibular body will be particularly unstable compared to an oblique caudo-dorsal fracture or a transverse fracture (Figure 46-2).

The maxilla is a slightly more challenging structure to understand from an anatomic standpoint (Figure 46-3). For a complete review the author refers you to the suggested reading list. Although not entirely correct, most veterinarians consider the incisors as part of the maxilla. However, these are actually housed by the incisive bone. Adding confusion, fractures of the frontal, palatine, zygomatic and nasal bones are often considered part of a maxillary fracture. In reality, the maxillary bone itself sits between all these structures. Fractures of the maxillary region may require additional evaluation due to the potential for penetration into the nasal cavity, the close proximity of the orbit, and potential damage to cranial nerves.
 

Clinical Presentation

As mentioned above, most dogs and cats will present subsequent to trauma. It is critical to perform a complete evaluation of the patient prior to focusing on non life-threatening oral trauma. On presentation, stabilization of the patient is the top priority. Assess the airway for patency. Evaluate the patient for thoracic trauma via auscultation and perform thoracic chest radiographs if indicated. As with all trauma patients, it is important to rule out concurrent traumatic conditions such as pneumothorax, diaphragmatic hernia, pulmonary contusions, cardiac arrhythmias and hemothorax. Ensure that there is no abdominal trauma requiring immediate surgical intervention (hemoabdomen, uroabdomen, perforated intestine etc.). Finally, do not overlook the patient’s neurological and mental status. Did head trauma result in brain injury? Stabilize the patient adequately prior to managing the fracture repair.

Diagnosis

Examination of the oral cavity with the patient under sedation or short term anesthesia will provide a great deal of information and cannot be underestimated. Evaluate for gingival lacerations and trauma as this may indicate the location of the fracture.

Each individual tooth should be palpated for instability. In dogs, fractures of the mandible will most commonly occur in the premolar, molar and symphyseal regions. In cats, over 70% of fractures occur at the level of the mandibular symphysis. Assess the patient’s occlusion as part of your examination. Many fractures will be open and easy to diagnose. Additionally, one can palpate for crepitus and instability of the mouth. A mandibular symphyseal fracture will allow movement of one hemimandible in relation to the other.

Traditionally, radiographs have been the first line of diagnostics for viewing mandibular and maxillary fractures. A thorough knowledge of skull radiography is needed to properly interpret the more complex oblique and intraoral views that are sometimes necessary. Radiographs are ideally performed under general anesthesia. Radiographs are excellent for evaluation of multiple tooth roots. Evaluating radiographs of the head can be challenging due to the plethora of overlying bony structures. As computed tomography (CT) has become more widely available and affordable, many surgeons opt to augment conventional radiography with this imaging modality in situations where fracture evaluation is more difficult. Fractures of the ramus, caudal mandibular body and mandibular condyle are key areas that may be difficult to interpret radiographically. CT allows certain advantages for surgical planning such as greater detail, rapid acquisition of images and the ability to create three dimensional reconstructions of the fracture site. CT is particularly desirable when dealing with pathologic fractures due to neoplasia in which resection rather than fracture repair is the treatment of choice.

Conservative Management

The mainstay of medical management of maxillary and mandibular fractures is the tape muzzle. This option is acceptable in situations where the fracture is minimally displaced, there is proper dental occlusion, and the patient has good healing potential. Tape muzzles usually remain in place for approximately 6 to 8 weeks or until there is clinical union. There are a few important considerations when placing a tape muzzle. The muzzle must allow enough slack for the patient to be able to drink water and eat soft foods or a gruel. Additionally, the patient should be kept cool and restricted as heat dissipation via panting is compromised. Placement of a tape muzzle is seldom a viable option for brachycephalic dogs and cats.

To place a tape muzzle simply cut a length of the appropriate size tape to go around the muzzle of the patient (Figure 46-4A). The “sticky” side of the tape should be up. A spacer (a pencil or a pen) should be placed between the upper and lower incisor teeth to ensure that a small gap is present after the tape muzzle is applied. This gap is not large enough to jeopardize proper alignment, but is sufficient to enable the patient to lap fluid or gruel. The next piece of tape is placed behind the neck and along both sides of the muzzle (Figure 46-4B). This piece of tape is also placed with the “sticky” side up. The ends of the tape on both sides should extend past the nose for an equal distance to that from the nose to the middle of the patient’s back. The third piece of tape is placed “sticky” side down around the muzzle. Then the long ends of the second piece can be folded back onto itself. A caudo-ventral mandibular support strap may also be incorporated if desired (Figure 46-4C). It is strongly recommended to apply a tape muzzle under heavy sedation or anesthesia. An alternative to a tape muzzle is a pre-fabricated restraint muzzle. This can be conveniently swapped out with another muzzle and washed periodically. The same considerations mentioned above will apply to sizing of pre-fabricated muzzles. One of the drawbacks to muzzles is their propensity to cause a moist dermatitis. This typically resolves without complications once the muzzle is removed.

Bonding of the mandibular and maxillary canine teeth (Figure 46-5) is a conservative repair technique that may result in less morbidity than stabilizing the fracture with a tape muzzle. In order for this technique to be successful, it is necessary for all canine teeth to be present and healthy. These teeth must first be cleaned, pumiced, and acid etched. The acid etching allows the bonding agent (acrylic) to adhere more reliably to the teeth. The teeth are then aligned with the mouth open enough to allow for drinking and eating as described above and the acrylic is then applied. If the patient is unlikely to eat due to the severity of trauma or an inability to lap food, a temporary esophagostomy tube should be placed. After application, any rough or sharp edges of the acrylic are smoothed with a dental burr. The fixation is removed with a dental burr once the fracture has healed. Although application is easy, this conservative option will require two anesthetic events and the risk of anesthesia should be weighed against the benefits of intraoral bonding in each individual patient.
 

Anesthetic Considerations

Understanding the potential ramifications of inducing anesthesia on a patient with possible head trauma is important and the anesthetist should plan accordingly. When repairing a mandibular or maxillary fracture that has compromised occlusion, it is challenging to assess proper occlusion if the patient is intubated in the customary manner. It is recommended to translocate the endotracheal tube to a pharyngostomy incision in these situations. Once the patient is anesthetized and intubated in the usual fashion, locate the lateral pharyngeal region of the patient just cranial to the hyoid apparatus, with a curved Carmalt forceps inserted through the mouth. Incise over this region through the skin, subcutaneous tissues, and mucous membrane. Make the incision large enough to reroute the endotracheal tube. Grasp the endotracheal tube through the incision and feed it retrograde through the incision. Once the patient is extubated the incision is left open to heal by second intention.

Another anesthetic consideration that is commonly overlooked is the application of local nerve blocks in the oral cavity. Mepivicaine, lidocaine, and bupivicaine are all commonly used agents in small animal dentistry. Of these, bupivicaine will have the longest duration of action (approximately 6 hours). Nerve blocks are relatively simple to perform, may help prevent “wind-up” of pain receptors and decrease the amount of inhalant anesthesia required.

There are several nerve blocks that are commonly utilized in oral procedures and these should be considered when repairing fractures of the maxilla or mandible. The mental nerve block is performed apical to the mesial root of the second mandibular premolar (Figure 46-6). This block will result in anesthesia of all ipsilateral incisors and canines of the mandible. A caudal mandibular nerve block can be performed when regional anesthesia of all the ipsilateral teeth of the mandible is necessary. The injection is made near the mandibular foramen on the lingual aspect of the mandible (Figure 46-7).
 

If regional anesthesia of the maxillary incisors, canines and premolars is desired then a palatine nerve block should be performed (Figure 46-8). This block may only be partially effective as some of the innervation of the region comes from the infraorbital nerve. For this reason, it is not uncommon to perform a palatine nerve block in conjunction with an infraorbital nerve block in dogs. To perform a palatine nerve block an injection is made at the midpoint between the mesial aspect of the maxillary carnassial tooth and midline of the palate. The cranial infraorbital block will result in anesthesia of the ipsilateral incisors and canine teeth of the maxilla. To perform this block, an injection is made apical to the distal root of the maxillary third premolar. This corresponds to the opening of the infraorbital foramen (Figure 46-9). Once the foramen is localized by palpating through the oral mucosa the syringe is advanced approximately 1 mm into the foramen prior to injection. In cats it is not recommended to advance the needle because the infraorbital canal is short and orbital trauma may result. For anesthesia of all the ipsilateral teeth of the maxilla, a caudal infraorbital block may be performed by advancing the needle 2 to 3 mm into the infraorbital canal. Again, this is not recommended in the cat.
 

Surgical Considerations

There are multiple structures that must be considered when performing surgery of the mandible and maxilla. These structures are frequently compromised prior to surgical intervention, making it that much more important to limit additional iatrogenic trauma. Use of an intranasal Folley catheter may assist with reduction and controlling nasal bleeding. The maxillary nerve passes through the alar canal and should be avoided during maxillary fracture repair.

Tooth roots must be avoided when drilling into the mandible or maxilla. Damage to the nerve root may necessitate additional intervention at a later time. The teeth should be thoroughly evaluated for damage and viability. If a fractured tooth root is present with weak periodontal ligament attachment, the tooth should be removed. If the viability of the tooth is questionable then adequate follow-up is necessary to minimize future complications. When approaching the caudo-lateral aspect of the mandible, the parotid duct and gland must be avoided. Positioning will depend on the region that is fractured. Typically, mandibular body fractures are repaired with the patient in dorsal recumbency, mandibular ramus fractures in lateral recumbency, and maxillary fractures in ventral recumbency.

External Fixation

External methods of mandibular and maxillary fracture stabilization include external skeletal fixation, interdental wiring and interdental fixation. These techniques offer the potential advantage of being less invasive than internal fixation methods. Decreased morbidity, avoidance of iatrogenic trauma to important structures of the mandible and maxilla, and preserving blood supply are all potential benefits of external fixation.

Interdental Wiring

Wires that are placed around teeth adjacent to a fracture are called interdental wires. Placement relies on a solid tooth-bone interface and any loose teeth incorporated into the wire may result in instability and subsequent failure of the repair. For placement, drill a hole on the superficial aspect of the mandible or maxilla between the two teeth closest to the rostral fracture fragment and then do the same on the caudal fracture fragment (Figure 46-10). An alternative method is to use a hypodermic needle or Kirschner wire passed through the gingival line at the level of the neck of the tooth. The cerclage wire is then fed through the holes and contoured around the teeth in a figure eight fashion. Twist and tighten the wire evenly. Bend the wire ends into the mucosa to avoid damaging surrounding surfaces. Twenty to 24 gauge wire is appropriate for most dogs and cats. Interdental wiring is commonly bolstered with interdental fixation.

Interdental Fixation

Similar to interdental wiring, healthy, intact teeth are required rostral and caudal to the fracture line when applying an interdental fixation. As mentioned above, interdental wires are commonly applied prior to interdental fixation in dogs. Doing so results in a stronger, more stable repair. Interdental fixation involves placement of an acrylic layer over prepared teeth to act as an intraoral splint (Figure 46-11). The teeth must first be cleaned, polished and acid etched. The acrylic is then applied, usually spanning at least two teeth rostral and caudal to the fracture. The acrylic splint is left in place for approximately 6 weeks or until healing has been confirmed. The splint may then be removed by sectioning it with a dental burr.
 

External Skeletal Fixation

The advent of positive profile pins for use in external skeletal fixation has allowed greater versatility and success. Highly comminuted mandibular fractures (i.e. gunshot wounds) are ideal candidates for repair with external skeletal fixation. Type 1-a fixators are used for mandibular fractures (Figure 46-12). Typically, all fixation pins are half-pins, but a centrally-threaded full-pin can be applied across the mandibular symphysis. The fixator is applied percutaneously by making release incisions through the skin, pre-drilling the bone, and placing positive-profile end-threaded pins. The pins are placed in the ventro-lateral aspect of the mandible to avoid tooth roots. Ideally, three pins are placed on either side of the fracture, but there may only be enough room for two pins on either side. This varies depending on patient size and fracture extent. The fixation frame can be built with clamps and rods, or acrylic.

Premature pin loosening is one of the major limiting factors of the external skeletal fixation system. Thermal osteonecrosis is thought to contribute to premature pin loosening. To avoid thermal osteonecrosis and subsequent premature loosening of fixations pins, pin sites should be pre-drilled prior to insertion of a fixation pin with a low speed (< 200 RPM) power drill. Acrylic fixators are discussed later in this chapter and add yet another level of versatility to external skeletal fixation. This method of fixation may be a superior alternative in certain comminuted fractures of the mandible in which pins need to be placed on the rostral aspect of the mandible. An acrylic column can be molded to any shape desired around the mandible (Figure 46-13). Alternatively, epoxy putty found at hardware stores may be substituted for acrylic to form the fixation frame.
 

Internal Fixation

Internal fixation of mandibular and maxillary fractures may include interfragmentary wiring and bone plating. An advantage of internal fixation is the ability to achieve excellent reduction and stabilization. It is important to remember, however, that proper dental occlusion takes precedence over apparent “anatomic reduction” of the internally fixated oral fracture. Additionally, post-operative morbidity may be reduced with internal fixation compared to external skeletal fixation. This is attributable to the absence of percutaneous implants.

Interfragmentary Wires

Interfragmentary wires involve using cerclage wire to reduce relatively simple fractures (Figures 46-14 and 46-15). They should only be used in situations where anatomical reconstruction can be achieved. Placement of two interfragmentary wires is recommended as this will help counter shear and rotational forces. Always drill and pre-place all wires prior to tightening. Wire size will depend on the animal; sizes between 16 to 22 gauge are typically used. Holes are pre-drilled into the bone for application of the wire. The holes are drilled with Kirschner wires perpendicular to, and 5 to 10 mm away from the fracture line. As with all other drilling of the mandible or maxilla, care must be taken to avoid drilling through tooth roots. Slight angling of the holes toward the fracture line will allow for easier tightening of the wire once it is applied.
 

After the wires are passed through the drill holes they are tightened with wire twisters. Pull evenly on both wires and twist; with unequal tension, one wire will twist around the other and the fixation will fail due to knot slippage. It is important to ensure that the wire is tight to avoid any unnecessary instability. The ends of the wire are bent toward the bone and away from the gingival margin to avoid damage or irritation of nearby structures. This is done by twisting and bending the wire at the same time to prevent any loosening of the wire while it is being bent. Care must be taken to ensure that the wire is not over- or under-twisted. As the wire is over-twisted and becomes tighter, its color will become dull instead of shiny. Many surgeons twist the wire until the point at which it starts to dull. Once the wire is appropriately bent it is then cut with wire cutters. Three twists are typically left behind to ensure knot security of the tightened wire. Interfragmentary wires are used frequently with maxillary fractures since other fixation methods are either difficult to apply in the presence of the nasal passages or cost prohibitive. Sometimes a Kirschner wire is incorporated into a figure eight wire fixation to help prevent collapse of a maxillary fracture into the nasal passages.

Interfragmentary wires have also been used successfully for the treatment of midline palatal fractures (Figure 46-16).
 

Symphyseal Fracture of the Mandible

Symphyseal fractures are the most common type of mandibular fracture encountered in cats and they typically occur as a component of “high rise syndrome”. Diagnosis can usually be done on initial physical examination by palpating instability at the level of the symphysis (Figure 46-17). The clinician should be able to appreciate one side of the mandible moving in relation to the other. However, once the patient’s overall status is evaluated and confirmed stable, a sedated exam is likely to be much less stressful for the patient and the clinician. This will also help to avoid missing other oral cavity injuries.
 

Proper reduction of the symphyseal fracture is accomplished through placement of a wire around the rostral mandible just caudal to the canine teeth (Figure 46-18). To do this, make a small incision ventral to the symphysis that is large enough to insert two 16 or 18 gauge hypodermic needles. Insert one needle just caudal to the canine tooth along the lateral aspect of the mandible, as close to the mandible as possible to avoid iatrogenic damage to soft tissues. Next, thread an appropriately sized cerclage wire through the needle (usually 18 or 20 gauge). Place another hypodermic needle in the same fashion as described above on the contralateral side. Thread the cerclage into this hypodermic needle and out the ventral aspect of the mandible. The result is encircling of the symphyseal fracture with cerclage. The cerclage can now be tightened with wire twisters, being sure to achieve proper reduction while tightening. The excess wire can then be cut while leaving at least three twists with the patient. The wire ends are bent down and the small incision is closed or left to heal by second intention.

After the fracture has healed, the wire is usually removed by cutting it with wire cutters. This is best done by cutting the wire intraorally and then either pulling the wire through ventrally or cutting the wire twists to create two separate wires. This latter approach may allow easier passage of the wires. Once the wires are removed, palpate the symphysis to verify that it is stable. Heavy sedation of the patient is recommended as this greatly facilitates wire removal.
 

Bone Plates

With recent advancement in plating technologies there are several different options depending on the repair goals. Bone plates have a distinct advantage of allowing the surgeon to apply them in compression, neutralization, or buttress. For easily reconstructable fractures a limited contact plate (LC), dynamic compression plate (DCP), or even better, LC-DCP is recommended. Small locking plates like a 2.0 mm string of pearls plate (SOP) can also be very versatile as the plate behaves as an “internal” external fixator and allows the surgeon the ability to contour the plate in almost any direction (Figure 46-19). Miniature maxillofacial reconstruction plates are also available and are very easy to work with from a contouring standpoint. Their main disadvantage is weakness in comparison to other plating options. In human maxillofacial surgery it is common practice to use resorbable plates, however, the high cost of these plates generally precludes their use in veterinary medicine.

When bone plates are applied on the mandibular body they are placed on the ventro-lateral aspect of the mandible. The tooth roots should be avoided when drilling and placing screws. This may be a disadvantage as a result of the “tension band” effect on the alveolar surface. Due to the pull of the muscles of mastication there is a bending force at the alveolar surface causing separation of the fracture at the tooth surface and compression at the ventral aspect of the fracture. To counter this “tension band” effect an interdental wire may be applied or a miniature plate placed more dorsally on the mandible that will allow screw placement between the tooth roots. Lastly, when applying bone plates care must be taken during contouring of the plate to avoid iatrogenic malocclusion.
 

Postoperative Care

In the immediate postoperative period, the patient should be monitored carefully for any airway obstruction secondary to the fracture repair. Adequate intravenous analgesia should also be provided. Patients should be transitioned to oral analgesics and maintained on these for 5 to 10 days. If substantial discomfort is expected when trying to administer oral medications then a feeding tube (e.g. esophagostomy tube) should be used. These are relatively easy to place and require minimal care. When placing an esophagostomy tube, always check proper placement into the distal third of the esophagus with a lateral radiograph. Initiate an appropriate feeding regimen according to the patient’s needs. If oral intake is possible, easily swallowed soft foods are recommended. The ideal food item is nutritionally complete, requires minimal chewing and does not adhere to the repair site. Recheck examination will depend on the type of fracture, repair method used, and patient age. In general most fractures of the maxilla and mandible will heal by 6 to 8 weeks. Once clinical union is achieved implants may be removed if necessary. Potential complications of fracture repairs include sequestra, osteomyelitis, implant failure, malocclusion, nonunion or malunion, tooth root injury and periodontitis.

Suggested Readings

Bennet JW, Kapatkin AS, Marretta SM. Dental composite for the fixation of mandibular fractures and luxations in 11 cats and 6 dogs. VetSurg 23:190, 1994.

Bos RR, Rozema FR, Boering G, et al. Bio-absorbable plates and screws for internal fixation of mandibular fractures. A study of six dogs. Int J Oral Maxillo Surg 18:365, 1989.

Boudrieau RJ: Fractures of the mandible In Johnson AL, Houlton JEF, Vannini R, ed.: AO principles of fracture management in the dog and cat. Thiemie: AO publishing, 2005, p98.

Boudrieau RJ: Fractures of the maxilla In Johnson AL, Houlton JEF, Vannini R, ed.: AO principles of fracture management in the dog and cat. Thiemie: AO publishing, 2005, p116.

Evans HE: The skeleton In Miller’s Anatomy of the Dog. Philadelphia: WB Saunders, 1993, p128.

Johnson AL: Management of Specific Fractures In Fossum TW, ed.: Small Animal Surgery. Saint Louis: Mosby, 2007, p1015.

Kern DA, Smith MM, Stevenson S, et al. Evaluation of three fixation techniques for repair of mandibular fractures in dogs. J Am Vet Med Assoc 206:1883, 1995.

Legendre L. Intraoral acrylic splints for maxillofacial fracture repair. J Vet Dent 20:70, 2003.

Lopes FM, Gioso MA, Ferro DG, et al. Oral fractures in dogs of Brazil-a retrospective study. J Vet Dent 22:86, 2005.

Umphlet RC, Johnson AL. Mandibular fractures in the dog. A retrospective study of 157 cases. Vet Surg 19:272, 1990.

Umphlet RC, Johnson AL. Mandibular fractures in the cat. A retrospective study. Vet Surg 17:333, 1988.

Verstraete FJ, Maxillofacial fractures In Slatter D, ed.: Textbook of Small Animal Surgery. Philadelphia: Saunders, 2003, p2190.

Verstraete, FJ ed.: Oral and maxillofacial surgery in dogs and cats. Elsevier, 2012, p233.
 

Acrylic pin splints are external skeletal fixators that use acrylic as both the connector rod and linkage. This fixation method can be accomplished in numerous ways, using either homemade materials or commercial kits (Acrylic Pin External Fixation System, Innovative Animal Products, Rochester, MN). Use of an acrylic pin splint has several advantages over standard metal external skeletal fixators for the mandible. The acrylic pin splint is lightweight, radiolucent, and versatile. The acrylic pin splint enables the surgeon to position pins to avoid tooth roots and vital structures easily and to combine pins of various sizes in a singular frame (Figure 46-20). The acrylic pin splint is easy to contour to the shape of the mandible (Figure 46-20). The advantage of using a homemade acrylic pin splint is that the surgeon can purchase specifically needed components from different sources. The commercial kit provides convenience of application because it contains all materials in a single package.
 

The homemade splint consists of methylmethacry-late, which can be obtained as either hoof repair (Tech-novit Hoof Acrylic, Jorgensen Laboratories, Loveland, CO) or dental molding acrylic (Orthodontic Resin, L.B. Caulk Co., Milford, DE). The acrylic column can be free-formed or injected into a tube to serve as a mold. When free-formed, the acrylic is molded by hand to the required shape. The free-form method is easiest with most applications to the mandible, especially for smaller dogs and cats. The tube method may be best for larger dogs. The commercial kit uses a tube method. Research has shown that a 3/4-inch acrylic column diameter provides fixation strength comparable with or greater than that of the medium Kirschner 3/16-inch connector rod. Given this guideline, the surgeon can extrapolate the needed width of the acrylic column to various sizes of animals.

Two considerations are important to predictable and consistent success when using acrylic pin splints for mandibular fractures. First, the surgeon needs to establish normal occlusion and mastication for the patient. Failure to accomplish this goal predisposes the patient to abnormalities of the temporomandibular joint and pain, with the possibility of negative consequences on nutritional balance. Normal occlusion in the dog is seen when the mandibular canine teeth are positioned between the maxillary incisors and canine teeth and the mandibular fourth premolar is situated between the maxillary third and fourth premolars. Achieving normal occlusion is always a higher priority than accomplishing accurate reduction at the fracture site (See Figure 46-18). When performing surgical correction of mandibular fractures, placement of the endotracheal tube through a pharyngostomy enables the surgeon to assess occlusion during the operative procedure. When the endotracheal tube is positioned routinely, it interferes with normal closure of the mouth and prevents the surgeon from assessing accurate occlusion.

The second important consideration is the need to use screws or positive profile end-threaded pins (fasteners) for attachment of the acrylic column to the mandible. The mandible is a relatively flat bone without two nicely separated dense cortices. This configuration predisposes nonthreaded pins to premature loosening, which leads to discomfort and, possibly, to delayed healing with the fixation of mandibular fractures. Because of this situation, screws or threaded pins, which provide a screwed-in anchorage, are advantageous when used for treating fractures of the mandible with external skeletal fixation. Bone screws work well for this purpose because they can be obtained in varied sizes corresponding to patient size. The head of the screw and exposed thread provide a secure linkage to the acrylic column. For use in particularly small animals is a small-diameter (0.9, 1.1, 1.6, 2.0, and 2.4 mm) positive-profile end-threaded pin (Miniature Interface Fixation Half Pins. IMEX Veterinary, Inc., Longview, TX) that is an excellent fastener designed to be used with acrylic. One end is intended to provide screwed-in fixation with the bone, and the opposite end is a roughened thread to allow for strong linkage with the acrylic column. The pin is remarkably stiff, given its diminutive size, a positive mechanical property not found in most small-diameter pins. Fully threaded Steinmann pins or negative-profile threaded pins should not be used with the acrylic pin splint because they are mechanically weak and are predisposed to loosening or breakage. The threaded pins and screws should be inserted by first drilling a hole with a sharp drill bit sized to approximate the core diameter of the fastener. This gives maximum stability to the fastener-bone interface. The tip of the threaded pin must exit the transcortex completely to engage thread throughout the bone.

Often, a combination of threaded and smooth fasteners is used together. When this method is used, at least one threaded fastener needs to be positioned in each bone segment on either side of the fracture. By combining smooth and threaded fasteners, the surgeon gains both stability and ease of application, especially when using a biphase technique (see later). At least two fasteners need to be positioned in each bone segment on either side of the fracture. Frequently, more fasteners are placed in each individual bone segment, a maneuver that enhances the strength of the construct. It is possible, and advantageous, with the acrylic pin splint to stabilize fractures involving both hemimandibles with a singular acrylic column (See Figure 46-20). The vertical ramus of the mandible is a poor location for securing fasteners because this soft, flat bone does not hold a fastener well. Because of this limitation, caudal mandibular fractures do not lend themselves well to fixation with the acrylic pin splint.

Free-Form Acrylic Pin Splint

The patient is administered a perioperative antibiotic regimen. The appropriate number of fasteners is placed into each bone segment. Aseptic technique is always used when applying the fasteners and during fracture manipulation and closure of the soft tissues, when using an open reduction technique. Aseptic technique is not necessary for application of the acrylic connector when this procedure is done after closure of the wound. Fasteners can be wedged between tooth roots, but they should not be drilled through these roots, and mandibular vessels and nerves need to be avoided. If smooth pins are used in the configuration, they need to be bent to lie parallel to but elevated from the skin, to allow secure adherence to the column and room to accommodate the acrylic mass (Figure 46-21). Fasteners should be positioned so, after the acrylic column is in place, distance of 1 to 2 cm will be present between the acrylic column and the skin. This distance is necessary to avoid thermal damage to the soft tissues and bone while the acrylic sets. Moistened gauze sponges can be placed to protect the skin and to cool the pins, thereby impeding conduction of excessive heat to the bone. All methylmethacrylate products use two components, a liquid (monomer) and a powder (polymer). For the Caulk orthodontic dental resin, three parts powder are mixed with one part liquid. For Technovit Hoof Acrylic, two parts powder are mixed with one part liquid. A disposable cup and wooden tongue depressor can be used to mix the acrylic. These two portions are mixed until they become doughy (3 to 4 minutes). The acrylic is hand molded to form a column long enough to incorporate all the preplaced fasteners and wide enough to provide adequate strength for the particular size of the animal. Approximate occlusional alignment and fracture reduction are achieved. The acrylic column is placed on the fasteners and is conformed to the appropriate shape; then final occlusional alignment and fracture reduction are completed and held. The acrylic is adequately hardened 8 to 12 minutes after mixing, to enable the surgeon to abandon temporary holding of occlusion and reduction.
 

A biphase technique can be used to facilitate application of the acrylic pin splint. With this technique, the surgeon applies a temporary adjustable reduction device such as Kirschner clamps and connector rods (phase 1) separate and external to the acrylic column, to hold occlusional alignment and fracture reduction temporarily (Figure 46-22). The acrylic is mixed and molded to all fasteners and is allowed to set (phase 2); then the external device is removed. The long pins are cut short once the acrylic has hardened; alternatively, the long pins can be bent over to lie flush with the acrylic column, and more acrylic can be mixed and added to the column to incorporate the bent pins (Figure 46-23). This technique enhances the stability of the smooth pin acrylic linkage. The surgeon must bend the pins over using one pair of pliers as a lever positioned at the point of bend and another pair of pliers or hand chuck to exert bending of the pin. This prevents the formation of high stresses at the fastener-bone interface when bending over the pin.
 

Tube Acrylic Pin Splint

For this splint, either a commercial device or a homemade tube can be used. A homemade method is described here. The patient is administered a perioperative antibiotic regimen. The appropriate number of fasteners is placed into each bone segment using sterile technique, and surgical wounds are closed. Plastic tubing is pressed over the ends of the fasteners and is positioned 1 to 2 cm from the skin. Corrugated plastic anesthetic tubing (anesthesia breathing circuit, 1/2-inch for small frames and 3/4-inch for large frames, King Systems Corp., Nobelsville, IN) or Silastic tubing works well, serving as an injection mold for the acrylic. The most dependent end of each tube is plugged with cotton, and modeling clay is used at each junction of the tube and fastener to prevent excessive leakage of acrylic at these sites. Approximate occlusional alignment and fracture reduction are achieved. The powder and liquid components of the acrylic are mixed and are poured into the top end of the plastic tube. This maneuver can be facilitated by using a large-dose syringe to inject the acrylic into the plastic tube. Accurate occlusional alignment and fracture reduction are maintained until the acrylic sets. The acrylic must fill the tube completely, and no air bubbles can be present to weaken the acrylic column. If large air bubbles are noted, holes can be made in the plastic tube and more acrylic can be injected into the area before or after the acrylic sets. The biphase technique can easily be adapted to the tube acrylic pin splint, with considerations and technique similar to that described for the free-form method. A frame alignment kit (Innovative Animal Products, Rochester, MN) is available and is advantageous because it allows phase 1 reduction equipment to be placed either above or below the plastic tubes.

The fracture reduction or the splint can be adjusted after the acrylic has set by removing a short segment of the acrylic column with a hacksaw blade, obstetric or Gigli wire, or a cast cutter. A portion of the tubing is peeled back, and several channels are drilled into a portion of acrylic on either end of the cut column to provide an anchor for the new acrylic patch. A small amount of acrylic is mixed and hand molded to fill the gap and to overlap a portion of the exposed acrylic containing the channels. Occlusional alignment or fracture reduction is then manipulated, while the acrylic is still soft, and is held until the acrylic hardens. New fasteners can be placed to add additional strength to the configuration or to replace fasteners that are loose. Fasteners are placed adjacent to the existing acrylic column using aseptic technique. The fasteners are then incorporated into the column with the addition of a new patch of acrylic.

Suggested Readings

Egger EL. Management of mandibular fractures with external fixation. In: Proceedings of the 5th annual Complete Course in External Skeletal Fixation. Athens, GA:, 1996:113-115.

Toombs JP. Nomenclature and Instrumentation of external skeletal fixation systems. In: Proceedings of the 5th annual Complete Course in External Skeletal Fixation. Athens, GA:, 1996:2-9.