Dental injuries are among the most numerous claims against anesthesia providers (1, 2); however, only 20% of such cases are described as “difficult intubations” (3). This questions the assumption that the application of large forces with the laryngoscope is the usual cause of injury. In this case report, a fractographic analysis of a retrieved tooth implicated postoperative chewing on an endotracheal tube as the probable cause of failure. To prevent this kind of dental trauma, protective tooth guards may be more effective if they are used postoperatively rather than only during the intubation procedure itself (4).
Several days after an atraumatic laryngoscopy, a 65-yr-old male (ASA physical status IV) discovered that his maxillary right canine tooth (#6) was broken in the root, just below the gingival margin. At the time of surgery, the patient’s oral condition was generally good, with some partially edentulous areas and serviceable restorations. Although many tooth fractures are assumed to initiate during intubation procedures (5,6), confirmatory analyses are rarely, if ever, done. The patient in this case, however, saved the broken tooth fragment, enabling a fractographic failure analysis.
Fractography is broadly defined as “means and methods for characterizing a fractured specimen or component” (7). This often involves examination of fracture surfaces to identify relevant features, such as fracture plane angles, which can determine the directions of the break forces. Fractography has been used for design and failure analyses in the aircraft (8–10) and automotive (11,12) industries, and more recently in biomedical applications (12–15). Fractographic analyses are generally most informative when applied to brittle fracture surfaces, where there is little or no deformation to obscure the original paths of the growing cracks. Teeth and some bone meet this criterion.
Figure 1a shows a view of the fracture surface of the broken canine tooth from the front, or labial direction. In Fig. 1b, the tooth fragment is rotated and angled to show the flattened incisal edge. Viewer orientation may be aided by noting that the discolored region on the right side of Fig. 1a is visible near the center of Fig. 1b. Some of the discoloration in this tooth was the result of restorative work that was not expected to influence the fracture path or strength (15).
Fractographic analysis indicated that the tooth fracture initiated from a combination of large bending and shear stresses at the root surface. Bending stresses can arise from the configuration shown in Fig. 2, where a remote force is applied to the crown while the embedded root resists the force. The highest bending stresses are at the approximate location indicated as the fracture initiation site in Fig. 2. These stresses tend to pull the specimen apart in tensile mode fracture at the root surface. The other side of the root is in compression. Because the root is firmly embedded, there is a shear component as well. Shear stresses result from tangential or sliding forces. In this case, a force as indicated in Figure 2 would not only cause the specimen to bend, but would also tend to “shear off” the specimen near the gingival margin.
The curvature as the crack proceeded from the root on the lingual side toward the occlusal and then toward the labial surface is expected in this type of mixed mode fracture. There is a small, thin ridge where the fracture changed direction at the labial root surface just above the enamel crown junction. This ridge is referred to as “compression curl” in fractography and is characteristic of bending failures (16).
To confirm the fractographic analysis and determine an approximate break load, an intact tooth with a root of similar size and shape was embedded in a configuration to simulate the natural tooth environment. The specimen was then loaded in a commercial testing machine (Instron Model 5500R, Canton, MA) until failure at 480 N (106 lb) resulted in fracture surfaces matching the original tooth. The break load of 480 N is typical for natural canine teeth (15). A more detailed fractographic analysis and description of the subsequent testing procedure is described in an engineering journal (17).
The break force from the lingual direction could only be applied by the laryngoscope if it were being withdrawn or levered from directly behind the right canine tooth more to the side of the mouth than the center. The approximate force magnitude of 480 N is high, more than half the weight of a normal person. This combination of force and direction is unlikely during a routine intubation procedure.
Also noteworthy is that the tooth fracture began at the root, in a location protected by the surrounding alveolar process. Thus, contact forces could not have directly initiated the critical crack, which started a distance away from the location of direct force contact (Fig. 2). Padding or otherwise softening the point of force application would not have averted fracture in this case.
A more reasonable explanation for the tooth fracture that is consistent with the fractographic findings is involuntary biting (bruxism) on the endotracheal tube. The flattened occlusal surface of the canine tooth, seen in Fig. 1b, is characteristic of a history of bruxism, confirmed from the patient’s dental records. Anxiety, such as may be associated with surgery, has also been linked with bruxism (18). Involuntary tooth grinding in the presence of an oral tube can concentrate biting forces to only a few teeth. Nocturnal total bite forces exceeding 800 N have been measured for bruxers (19) and this is far above the tolerance limit for individual anterior teeth (20).
The use of mouthguards to avoid dental trauma associated with anesthesia has been reported to range from helpful (21) to totally ineffective (22). Because most such studies focus on dental protection only during the intubation procedure itself, a significant incidence of tooth fracture during the recovery period might explain the mixed results. Also, it should be mentioned that dental injuries from Guedel oral airways are a recognized problem (23) that have been attributed to the concentration of biting forces on a few front teeth (24).
In conclusion, this study suggests that some tooth fractures attributed to the intubation procedure might actually be the result of biting on the endotracheal tube during the recovery period. Because bruxers comprise an estimated population fraction of 4.4% (25) and some drugs list bruxism as a side effect (26), the use of prudent precautions such as postoperative mouthguards or shields may reduce the potential for bruxism-related dental injury.
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