Dogs' and cats' teeth are perpetually at risk of being chipped, worn, or fractured.
Dogs' and cats' teeth are perpetually at risk of being chipped, worn, or fractured because of chewing activities or external trauma. Fractured teeth have been reported in 10% to 29% of patients in small-animal practice1-3 and in 25% of working military dogs.4
Many patients with dental fractures do not exhibit obvious clinical signs of pain or infection, and their appetites may remain unchanged. This lack of obvious harm has often led practitioners to take a wait-and-see approach to diagnostic work-up and timely therapeutic intervention. However, left untreated, fractured teeth can be a source of chronic pain and can lead to pulpitis, pulpal necrosis, osteitis, tooth root resorption, draining sinus tracts, facial swelling, and tooth loss.
An understanding of pertinent dental anatomy, fracture diagnosis, radiographic interpretation, and the pathologic consequences of fractured teeth should prompt you to aggressively assess and treat or refer dental fracture cases.
The teeth most susceptible to fracture vary according to the population studied. In millitary dogs trained in apprehension, the canine teeth appear to be most at risk, whereas in pet dogs, the maxillary fourth premolar teeth (carnassial teeth) appear to be at a greater risk of fracture than are other teeth.4,5
In a detailed study of dental pathology in feral cats, both the maxillary canine teeth and the carnassial teeth were most at risk for fracture.6 In addition to many other problems, most notably thoracic trauma, 17% of cats with high-rise syndrome had dental fractures.7
Dental fractures have two common causes—chewing hard objects and direct force trauma. Other causes are abrasion and attrition.
Dogs can generate tremendous biting forces with their carnassial teeth. If the object a dog is biting is as hard as or harder than the tooth, a fracture can occur. Materials that have caused dental fractures include animal bones (cooked or raw), nylon or hard plastic toys, cow hooves, and some of the harder compressed treats. Cage bars, metal collars or fences, and rocks are other obvious culprits.
Direct force trauma, such as from motor vehicle accidents, fighting (clicking teeth), bite work, kicks from farm animals, catching flying objects with teeth, falling, or sporting accidents (e.g. hit with a golf club or baseball bat), accounts for a marked number of dental fractures. Even without fractures, impact trauma may lead to pulpitis (see the boxed text "Pulpitis: Another outcome of trauma").
Pulpitis: Another outcome of trauma
Abrasion and attrition are two other causes of tooth structure loss that can predispose teeth to fracture. Abrasion is the mechanical wearing away of teeth caused by chewing on hard objects, while attrition is the wearing away of teeth caused by tooth-on-tooth contact (Figure 1).
1. A complicated crown fracture of the left maxillary canine tooth in a dog (A). Also note the abrasion on the right maxillary canine tooth (B) caused by chewing on a cage. This pattern of abrasion has been called cage chewers syndrome and can predispose the teeth to fracture, as has happened in this case. 2A. An uncomplicated crown fracture of the right maxillary fourth premolar tooth (carnassial tooth) in a dog. It was not known how long the tooth had been fractured. Note that there is no direct pulp exposure. The dark area (arrow) is tertiary or reparative dentin. 2B. A dental radiograph of the fractured tooth in Figure 2A. The periapical radiolucent areas indicate endodontic disease, illustrating the need for dental radiography even when the pulp is not exposed.
Preventing dental fractures in the first place is ideal. Proper husbandry practices such as avoiding certain hard chew objects (e.g. nylon bones, animal bones, cow hooves, rocks) and preventing cage chewing are essential for reducing the incidence of dental fractures.
When dental fractures do occur, early recognition, correct diagnosis, and adequate treatment are necessary to prevent pain and secondary disease and to save teeth when possible. Because early recognition is important, a conscious oral examination should be part of every physical examination. Additionally, comprehensive anesthetized oral examinations are recommended every 12 to 18 months or if any abnormalities are noted.
Enamel, the tooth structure that covers and protects the crown (the part of the tooth above the gum line), is the hardest substance in the body. Its composition is 96% inorganic calcium hydroxyapatite crystals and 4% water and collagen.8 The inorganic component is arranged in prisms surrounded by a collagen sheath. Enamel cannot regenerate after trauma.
Beneath the enamel is a layer of dentin that surrounds the hollow pulp chamber (root canal), which contains the pulp tissue. Dentin and pulp can be considered the same organ. Although morphologically different, the histologic boundary is not distinct. The pulp consists of connective, vascular, and nervous tissues along with reserve mesenchymal tissue, fibroblasts, and specialized cells called odontoblasts. Located at the periphery of the pulp, these cells continuously produce additional dentin, called secondary dentin, as long as the pulp is vital. Odontoblasts also send cytoplasmic processes into channels located in the dentin. These channels, called dentinal tubules, run at right angles to the complex of the dentin and pulp and the dentin and enamel.
In addition to the odontoblastic process, an ultrafiltrate of blood is in the dentinal tubule as free fluid. Because of blood pressure gradients, there is constant outward flow of this fluid.9 The dentinal tubules in dogs are 0.9 to 2.5 µm in diameter, with 20,000 to 90,000 dentinal tubules per square millimeter of exposed dentin. The width and number of dentin tubules depend on the distance from the pulp and the cusp of the crown.8,10
The pulp tissue in human teeth communicates with the body's vascular system through a single foramen, the apical foramen, located at the root apex. However, in dogs and cats, an apical delta exists where the pulp enters and exits the tooth root.11 As the pulp extends through the root apex, it branches several times, like a river delta. From there, it communicates with the periodontal ligament, periapical bone, and the vascular and lymphatic systems.
A dental fracture classification system has been adopted by the American Veterinary Dental College.12 The key diagnostic questions to ask are 1) What is the anatomical extent of the fracture? 2) Is the pulp exposed? 3) Is there radiographic evidence of pathology?
Uncomplicated fractures result in dentin exposure. With open dentin tubules exposed to the environment, A-delta nerve fibers in the pulp are stimulated. This stimulation, which is due to an increase in the flow rate of the free fluid in the dentinal tubule, is called the hydrodynamic mechanism of dentin sensitivity.9 Although the pulp is not directly exposed, oral bacteria are small enough to travel through an exposed dentinal tubule and invade the pulp.13 In uncomplicated dental fractures, the tooth may or may not become diseased
3. A complicated crown fracture of the right maxillary canine tooth in a cat. In domestic cats, the pulp chamber extends almost to the coronal tip. Always be suspicious of a complicated crown fracture with any amount of crown loss in a cat's canine teeth.
When the pulp is directly exposed to the oral environment (complicated fractures), pain and pulp necrosis are always the consequences. In the acute stage, these injuries are intensely painful because of activation of A-delta nociceptors. As the pulp dies, the A-delta nociceptors necrose, but the type C nociceptors are still intact, transmitting dull, aching pain. Additionally, as infection travels apically (toward the root tip), periapical disease can occur. The possible results include periapical osteitis, periapical granuloma, true abscess with or without draining sinus tracts, tooth resorption, and secondary periodontal disease.
Before therapy is selected, obtain dental radiographs to assess the alveolar bone, root, periodontal ligament space, pulp chamber space, and root canal space for radiographic evidence of endodontic disease and its extent. Even if extraction has been selected, radiographic assessment is necessary to plan the extraction in light of possible root ankylosis, fractured roots, supernumerary roots, and the health of the surrounding tissue.
Treatment options for fractured teeth include vital pulp therapy, root canal treatment, bonded composite restoration, crown restoration, and extraction.
Vital pulp therapy is an endodontic procedure that involves removing most of the coronal portion of the pulp, placing a pulp-capping agent to stimulate formation of tertiary dentin over the pulp, and then applying a bonded composite restoration (Figures 4A-4D). Recent research indicates that mineral trioxide aggregate is superior to traditional calcium hydroxide as a pulp-capping agent.14 The goal of vital pulp therapy is to maintain vital pulp tissue beneath the restoration and protect that vital tissue from noxious stimuli and bacterial invasion.
4Aâ4D. The chief steps in performing vital pulp therapy, in this case on the left mandibular canine tooth in a dog. In 4A, a tooth stump after amputation of the crown (an intentional complex crown fracture) is seen, with the red dot representing bleeding pulp tissue. In 4B, powdered mineral trioxide aggregate is being introduced into the pulp chamber with a sterile paper point. In 4C, a glass ionomer is being used as a cavity liner. And in 4D, a bonded composite restoration is being cured with ultraviolet light.; 5A & 5B. In the dental radiograph at top (5A), the left maxillary first incisor in a dog has a complex crown fracture (1) and periapical radiolucency (2), indicating endodontic disease. The dental radiograph at bottom (5B) shows the same tooth after root canal treatment. The root canal has been cleaned, shaped, and filled with a combination of rubber (gutta percha) and cement (AH-26ÃÂÃÂÃÂ-Dentsply International).
Root canal treatment involves completely removing pulp tissue and the diseased dentin and then completely filling the débrided canal with a sealant and core material (most often gutta percha.) It is a much less invasive treatment than tooth extraction, and the patient retains its teeth (Figures 5A & 5B).
Dental composite is a filled organic resin that closely mimics the color and strength of natural teeth. Dental composite can be bonded to the tooth surface (enamel or dentin) to restore defects, seal dentin tubules, and protect the underlying tooth structure. The bonding procedure involves treating the tooth with phosphoric acid to remove some of the hydroxyapatite crystals found in enamel and dentin and then placing a bonding agent that flows into the pits left by the crystal removal, which results in a micromechanical lock. The composite, made of a resin matrix and inorganic filler, is placed over the bonding agent, shaped, and polymerized into its solid form. Finally, the composite can be polished (Figures 6A-6D).
6Aâ6D. The chief steps in bonded composite restoration on a right mandibular canine tooth in a dog. In 6A, the uncomplicated enamel fracture is seen, and the white arrow points to the exposed dentin. Note that the pulp chamber is not exposed. In 6B, a dentin-bonding agent is being applied to an already-acid-etched surface. In 6C, a light-activated, low viscosity composite is being introduced to the prepared surface. In 6D, an ultraviolet light is curing the composite.
Crown restoration is the placement of a manufactured crown or cap over the injured and repaired tooth. Any endodontic treatment should be performed before crown placement. A margin is cut around the tooth for the crown to sit in, and the remaining tooth is shaped following the natural taper toward the cusp. Impressions of the tooth and the complete dentition, including a wax registration of the occlusion, are made and submitted to a dental laboratory. The laboratory creates the new crown through a process called the lost wax technique.
After the crown has been sent back to the clinician and evaluated, it is bonded to the tooth with resin cement. The crown may be made of metal, ceramic, or porcelain fused on metal. Of these materials, metal is the most durable and the most commonly used. In a study investigating the long-term results of crown therapy in dogs, 17 of 19 crowns were intact and functional at a mean follow-up time of 32 months.15
Although it is preferable to salvage teeth, it may not be possible because of extensive trauma, financial restrictions, or other circumstances. Successful extraction depends on your having the proper equipment and skills.
Enamel fractures do not expose dentin, and as enamel is essentially impermeable, the pulp should not become infected or sensitive. Obtain dental radiographs to look for other pathology. Additionally, if an enamel fracture is present, pulpitis may occur from the concussive trauma, so yearly radiographs are recommended. Because the enamel layer in dogs and cats is relatively thin, true enamel fractures in these species are rare. Treatment options for enamel fractures include either bonded composite restoration or no treatment with frequent (every six to 12 months) monitoring.
With uncomplicated crown fractures, exposed dentin can result in painful pulp stimulation and bacterial infection leading to pulp necrosis, especially immediately after the fracture when the dentinal tubules have been freshly exposed. Treatment should include composite restoration of the defect and possibly preparing for and placing a full- or partial-coverage crown.
Recent information has shown that canine dentin has less intertubular dentin (dentin between tubules) than human dentin does, possibly decreasing the bond strength of modern composites compared with those placed on human teeth.10 The decreased intertubular dentin could lead to weaker shearing forces and increased failure of composite restorations. Thus, placing a metal crown to protect the remaining tooth from further damage and bacterial ingress may be desired. However, the choice depends on the amount of remaining dental hard tissue, the thickness of the tooth axial walls surrounding the pulp, and client expectations.
Frequent (every six to 12 months) radiographic monitoring is necessary. If radiographic evidence of endodontic disease is observed, root canal therapy or, though less desirable, extraction is indicated.
Vital pulp therapy is a treatment option for acutely fractured teeth in patients less than 18 months old.16 Treatment success is related to the time between the injury that exposed the pulp and treatment. Marked bacterial contamination and histologic signs of inflammation in the coronal pulp occur just 48 hours after experimentally induced crown fracture.17 In a separate study, the success of vital pulp therapy decreased to 0% when it was performed more than seven days after the injury.18 If the therapy is unsuccessful or performed on a nonvital tooth, further pain and tissue loss from continued necrosis of the infected pulp may occur. Thus, vital pulp therapy is an excellent treatment option for intentional crown reduction procedures, but it should only be used for treating fractures that result from trauma in young (< 18 months) patients. Even then, the prognosis is guarded, and frequent radiographic follow-up is necessary.
In adult animals or animals with fractures that are more than 48 hours old, root canal therapy is a good treatment option. The reported failure rate for root canal therapy in dogs was only 5%.19 Root canal treatment allows a patient to maintain tooth function and is often less invasive than extraction. After root canal treatment, the tooth is restored with either a composite or metal crown, as mentioned above.
If salvaging the teeth is impossible, the only other recommendation for teeth with pulp necrosis or imminent pulp necrosis is extraction. However, teeth with severe endodontic disease may still have healthy periodontal attachment, and extracting these teeth can be challenging.
Matthew Lemmons, DVM, DAVDC
Circle City Veterinary Specialty and Emergency Hospital
9650 Mayflower Park Drive
Carmel, IN 46032
Daniel T. Carmichael, DVM, DAVDC
Veterinary Medical Center
75 Sunrise Highway
West Islip, NY 11795
Got a question about a difficult dental fracture case or other dental problem?
E-mail it to Dr. Carmichael by July 30, and look for his answer on vetmedpub.com in August!
Subject line: Dental Question
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