Root canal systems resemble river systems in many ways. Both follow highly variable and unpredictable pathways. Mark Twain once said, "The Mississippi river will always have its own way; no engineering skill can persuade it to do otherwise."55 In attempting to control bacteria, clinicians face a similarly formidable task. The canal morphology often is complex and unpredictable, severely limiting the practitioner's capabilities and posing challenges to modern medicines, materials, and instruments. The clinician who is knowledgeable about canal morphology and the science of dental equipment and materials will be able to practice safer, more efficient, and more effective techniques.
The past decade has seen many changes in the practice of endodontics, such as in materials, techniques, equipment, instrument design, and the types of metals used to manufacture endodontic instruments. However, the goals of endodontics, as stated so clearly by Schilder,246 have not changed: "Root canal systems must be cleaned and shaped: cleaned of their organic remnants and shaped to receive a three dimensional hermetic (fluid-tight seal) filling of the entire root canal space." The goal for cleaning and shaping of the root canal system is to obtain a continuously tapering funnel from the coronal access (widest diameter) to the apex (narrowest diameter) that flows with the shape of the original canal.246 Preparation of the canal, especially the apical segment, without weakening the remaining dentin or perforating the root is essential to proper infection control and obturation and to long-term success.114,199 An important finding in a recent clinical study is that these goals were better achieved and the success rate was higher when nickel-titanium instruments were used rather than instruments made of stainless steel.219 The ultimate goal (infection control) of all clinicians is to rid the canal and periapical tissues of bacteria.
Radiography is an essential part of endodontic diagnosis. Modern technology is rapidly shifting toward digital filmless imaging and other new image-enhancing methods (see Chapter 26).* Therefore the clinician must be well versed in this diagnostic field.
*References 29, 127, 168, 187, 206, and 211.
Pulp Testing Materials
Materials for Thermometric Evaluation
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Figure 8-1 Endo Ice. (Courtesy Coltene/Whaledent, Inc., Cuyahoga Falls, OH.)
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Figure 8-2 CO2 snow is an alternate method of thermal testing. (From Johnson WT: Color atlas of endodontics, St. Louis, 2002, Saunders.)
Pulp stimulation with cold or heat is the oldest method of evaluating the pulp's health and its ability to respond to external stimulation. However, evaluation of pulpal response must not be confused with vitality testing, which requires assessment of pulpal circulation. A cold test commonly is done by applying ice, a liquid refrigerant, or dry ice. Ice (32° F [0° C]) has only a limited usefulness because it normally is effective only on intact teeth in the anterior part of the mouth. Ethyl chloride is not normally sufficient for stimulating a tooth with extensive restorations or full crown coverage. Liquid refrigerants, such as 1,1,1,2-tetrafluoroethane (Endo Ice; Hygienic Corp., Akron, OH) (-21° F [-30° C]), are a good means of lowering the tooth temperature57 (Fig. 8-1). Dry ice (-108° F [-78° C]) is also an excellent stimulant (Figs. 8-2 and 8-3). Endo Ice and carbon dioxide (CO2) snow are equally reliable when used in adults.94
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Figure 8-3 CO2 dry stick with holder applied to tooth #9. (From Johnson WT: Color atlas of endodontics, St. Louis, 2002, Saunders.)
Seeking a means to improve the sensitivity and specificity of thermal tests, Leffingwell et al171 conducted a study involving a prototype apparatus capable of precisely controlling the temperature delivered to the tooth. These researchers found that the device may show promise for the testing of longitudinal pulp dynamics. They also concluded that tetrafluoroethane seemed to perform best in distinguishing teeth with increased pulpal irritation.
A cold water bath is the most effective testing method, because this technique is the only one in which the tooth is immersed in chilled water, often producing a true reproduction of the patient's chief complaint. Therefore, regardless of the restoration, the ice cold water test is effective. (See Chapter 1 for more details.)
The heat test is performed by applying heated gutta-percha to the tooth (this stimulus may reach 168.8° F [76° C] before burning). Special care must be taken not to damage the pulp with excessive heat. (Other methods of performing a heat test, such as a warm water bath or the use of electrical devices, are described in Chapter 1.)
Although the levels of cold (-l08° F [-78° C]) and heat (168.8° F [76° C]) are extreme, the health of the pulp is not jeopardized if testing is done with care.228 An understanding of pain responses to thermometric pulp testing is based on the hydrodynamic theory of the sensitivity of dentin, because the pulp has no thermosensing nerve endings. Therefore a sensation of pain requires the existence of some intact pulp tissue, including odontoblasts, for the hydrodynamic mechanisms to function. In other words, the cold- or heat-induced sensation depends on the presence of remnants of a morphologically intact pulp.
Electrical pulp testing is often overused and poorly understood. Mumford and Björn202 described some of the requirements for successful use of this testing method: "The basic requirements are an adequate stimulus, an adequate technique of applying this to the teeth, and a careful interpretation of the result." Unfortunately, these standards often are not met.
Responding nerve endings can be evaluated with an electrical pulp tester (Figs. 8-4 and 8-5). However, an electrical pulp tester is only as good as the person interpreting the patient's response. The sensation the patient may feel when an electrical current is passed through the tooth is the result of direct nerve stimulation. However, no reasonable assurance exists that these nerves are located in an intact pulp. Necrotic and disintegrating pulp tissue often leaves an excellent electrolyte in the pulp space. This electrolyte can easily conduct the electrical current to nerves farther down into the pulp space, simulating a normal pulp response. The situation becomes even more complicated with a multirooted tooth, in which the health status of the pulp may vary from root to root. A positive response to electrometric recordings alone should not be used for differential diagnosis of pulpal disease.164 Provided the examination was conducted properly, a lack of response suggests the lack of responding nerve endings. In most cases this means pulp necrosis. If the nerves to the pulp were transected during surgery, the pulp may still be vital.
Pulp tissue is much more sensitive to electrical stimulation than gingival or periapical tissue.34 Most modern pulp testers cannot put out sufficient current to stimulate periradicular tissues.
Several types of pulp testers are commercially available. Some units have both electrical pulp testing and electronic apex locator capabilities. Studies have shown that a pulsating direct current with a duration of 5 to 15 ms provides the best nerve stimulation.34,202 Optimal stimulation is achieved when the cathode is used to provide the stimulus. The faster the current rises, the more effective the stimulation, and the less compensation takes place in the nerves.
Somewhat simplified, Ohm's law (E = R × I) (E = electromotive force, R = resistance and I = current flowing through resistance) applies to electrical pulp testing, although the response most likely is a combination of impedance and resistance. Pulp testers operate at a relatively high-potential difference (i.e., several hundred volts) but at a very low current (mA). Enamel and dentin constitute very high resistance in the electrical circuit through the tooth. Of the two, enamel has the highest resistance. In dentin the lowest resistance occurs parallel with the tubules. The product of E and I (E × I) results in the neural response. This energy can be "consumed" in the hard-tissue part of the tooth, leaving too low a level of stimulation for the pulpal nerves. Therefore the tooth electrode must be applied at the same location and with the same conduction for each recording if the recordings are to be compared accurately. Clinical conditions make this practically impossible.
Electrometric recordings are often used to monitor traumatic injuries to the teeth. Under these conditions, the clinician must keep in mind that the need to increase stimulation from one observation time to another very often suggests increased hard-tissue formation in the pulp space rather than real changes in the pulp's ability to respond.
The tooth must be kept very dry when electrometric recording is performed. Because of the high electrical potential used, the current tends to creep along any wet external tooth surface to the gingiva, creating a short circuit and leaving too little energy for the pulp. For the same reason, critical recordings must be done after the teeth have been isolated from the saliva with a rubber dam and insulated from each other by the insertion of Mylar strips through the contact points.
A recent study found that the chance that a nonresponsive pulp was necrotic was 89% with cold testing, 48% with heat testing, and 88% with electrical testing.218 This study also found that the chance that a positive response represented a vital pulp was 90% with cold testing, 83% with heat testing, and 84% with electrical testing. Therefore the electrometric pulp tester should not be the instrument of choice for assessing pulpal health. A positive result on cold testing is a more accurate response that is easier to interpret. A positive result on either cold testing or electrical testing does not guarantee that the pulp is vital or healthy.