NSRCT mandibular premolars Flashcards
Describe the morphological characteristics and treatment considerations of the root canal system of mandibular premolars.
Consistently high levels of success in endodontic treatment require an understanding of root canal anatomy and morphology. Mandibular premolars may present fairly simple root and root canal con- figuration, that is, single root/single canal without severe curvature. Nevertheless, mandibular premolars, particularly 1st premolars, may present with multiple roots and/or canals with considerable variations, making them challenging for endodontic treatment.
The frequency of single‐canal mandibular premolars has been reported as 74–80.6% (1st premolars) and 88.4–97.5% (2nd premolars) (Zillich & Dowson 1973; Vertucci 1984; Calişkan et al. 1995). The pulp chamber of the single‐canal mandibular premolar is usually oval and directed buccolingually.This shape may become round in the apical portion of the canal. However, it has been reported that the frequency of long‐oval canals, where the long canal diameter is at least twice the short canal diameter, in single‐canal mandibular premolars was 13% and 27% at 1–3 mm and 5 mm, respectively, from the apex (Wu et al. 2000). Instrumentation of the entire wall of long‐oval canals can be difficult to achieve and a considerable portion of the canal wall can be uninstrumented (Versiani et al. 2013). Such recesses may harbor pulp remnants or bacterial biofilms, and thus may serve as potential sites of persistent intracanal infection.To disinfect the complexities associated with long‐oval canals, meticulous irriga- tion using sonic/ultrasonic irrigation systems and apical negative pressure systems is recommended. One study has demonstrated that passive ultrasonic irrigation with sodium hypochlorite and a final rinse with chlorhexidine facilitates disinfection of E. faecalis‐infected oval canals (Alves et al. 2011).
Mandibular premolars with multiple roots and root canals may present considerable morphological variations. In a study by Vertucci (1984), mandibular 1st premolars were classified as: one canal (70%); two canals joining at a common foramen (4%); two independent canals (1.5%); one canal bifurcating at the apex (24%); and two separate canals in two independent roots (0.5%). Mandibular 2nd premolars are less variable; a single canal presents in 97.5%, and a canal that bifurcates at the apex in 2.5% (Vertucci 1984). Mandibular premolars with three canals (Figure 11.7A, B, C, D, E) or a C‐shaped canal can also be found as rare variants.
Careful interpretation of preoperative radiographs is essential for providing insight into the number of existing root canals. In mandibular premolars with multiple roots and canals, the roots and canals may look unusual but the canals may not be evident in radiographs. Sudden narrowing or disappearance of the root canal space may indicate the presence of one or more extra canal(s). Radiographic appear- ance of the corresponding contralateral tooth may help in detecting additional canal(s), since bilateral teeth may present a similar morphology.Three‐ dimensional data obtained with cone beam‐com- puted tomography (CBCT) is extremely useful in identifying the multiple‐canal variants (Figure 11.7B).
The lingual canal of two‐canal mandibular premo- lars may be difficult to locate, although direct access to the buccal canal may be readily possible.This is because the lingual canal often diverges from the buccal (main) canal at an acute angle.Thus, to facilitate the location of the lingual canal, the lingual wall of the access cavity should be extended lingually.
What are some advantages of the crown‐down preparation technique?
Crown‐down preparation is classified as a coro- nal‐to‐apical preparation technique, where the preparation proceeds from coronal flaring to work- ing length determination and apical preparation. This technique was originally advocated for handfile preparation as the “crown‐down pressureless technique” (Morgan & Montgomery 1984) and has now been incorporated into various NiTi file systems.
With the crown‐down technique, the coronal portion of the root canal system is first prepared mechanically using Gates–Glidden drills.The apical portion of the canal is then gradually approached sequentially with instruments of larger‐to‐smaller sizes until the apical constriction is reached. During this process, a fully cleaned and tapered canal space is left behind the preparation.The true working length is determined when the instrumentation reaches within 2–3 mm from the apical constriction (Morgan & Montgomery 1984).
The crown‐down technique has several advan- tages over traditional apical‐to‐coronal preparation techniques such as standardized preparation and step‐back technique. Early coronal flaring provides an “escape way” that reduces intracanal hydrostatic pressure generated in an apical direction. Early coronal flaring also facilitates penetration of irri- gants into the root canal system and helps to create a fully cleaned coronal portion. For these reasons, the crown‐down technique may provide less risk of apical extrusion of intracanal contents, i.e., bacteria, debris, dentin mud, and irrigant solution, which can cause postoperative flare‐ups and delayed healing (Siqueira 2003). In the case presented, the primary reason for applying the crown‐down preparation was to avoid postoperative flare‐ups and the result- ing exacerbation of paresthetic symptoms. Another advantage of the crown‐down technique is that it provides less likelihood of working length shorten- ing, which can occur during preparation of curved canals.This is because, in the crown‐down tech- nique, working length is determined after the achievement of straight‐line access.
The crown‐down technique is usually recom- mended in protocols for the use of NiTi rotary file systems in order to reduce the risk of intracanal instrument separation. Due to the presence of a space coronal to the site of preparation, the crown‐ down technique limits the binding of NiTi instru- ments to the root canal dentin, except in the apical flutes, and reduces torsional loads to the instru- ments during root canal preparation (Roland et al. 2002). In particular, this technique helps in reducing the risk of large torsional stress generation due to “taper lock,” where a file is engaged into dentin over the length of its cutting blades and thus is at a great risk of fracture.
Describe the anatomical relationship of the inferior alveolar nerve and mental foramen with mandibular premolars.
Knowledge of the spatial relationship between the inferior alveolar nerve and root apices is impor- tant in avoiding inadvertent nerve damage during endodontic procedures.This is because the inferior alveolar nerve is sensory, and thus its damage can cause disorders of sensory functions, such as numbness and neuropathic pain, which are uncom- mon, but serious, treatment complications.
After entering the mandibular canal through the mandibular foramen, the inferior alveolar nerve runs through the mandible body to the mental foramen, which is usually located in the premolar region. Within the mandible, the inferior alveolar nerve is located beneath the tooth roots and sometimes very close to the tooth apices. Although there are varia- tions in the position of the nerve bundle in patients, mandibular 2nd premolars often show close proxim- ity to the inferior alveolar nerve. In one study where human dried mandibles were used to measure the distance between the tooth apex and the mandibu- lar canal, 2nd premolars and 2nd molars had the smallest distances, with a mean value of 4.7 mm and 3.7 mm, respectively (Denio,Torabinejad & Bakland 1992). Another study described how the inferior alveolar nerve rises to allow the mental branch to exit the mental foramen in the 2nd premo- lar area, which is associated with the proximity of the apex of this tooth type to the nerve (Knowles, Jergenson & Howard 2003). However, a recent study, where the distance was evaluated in CBCT images, described the apices of 2nd molars as being significantly closer to those of 2nd premolars and 1st molars (Kovisto, Ahmad & Bowles 2011).
The location of the mental foramen also needs to be considered when performing non‐surgical as well as surgical endodontic therapy in mandibular premolars to avoid inadvertent neural damage.The mental foramen is an opening of the mandible and transmits the mental nerve, which is a branch of the posterior trunk of the inferior alveolar nerve and transmits the sensation from the buccal gingiva of the mandibular incisors, canine, and premolars, as well as the anterior aspects of the chin and lower lip. The foramen is usually located apical to the 2nd mandibular premolar or between the apices of the 1st and 2nd premolars, although it can be seen apical or mesial to the 1st premolar or distal to the 2nd premolar. A recent CBCT analysis showed that the root apex of the mandibular 2nd premolar (70%) was the closest to the mental foramen, followed by the 1st premolar (18%), and then the 1st molar (12%) (Chong et al. 2017).This study also described only 4% of root apices as being located within 3 mm from the mental foramen, with the position of the mental foramen being superior to the apices of the adjacent premolars in only 18% of cases (Chong
et al. 2017).These findings may be associated with the fact that the incidence of paresthesia following endodontic treatment of mandibular premolars is low (0.96%) (Knowles et al. 2003). In addition, more than one mental foramen may be present; two mental foramina were noted in 1.8% (N = 110) of Asian skulls (Agthong, Huanmanop & Chentanez 2005).The additional foramina may be difficult to locate with panoramic and periapical films, but may be detected with CBCT scans.
Radiographic assessment of the mandibular canal and mental foramen is important for identification of the actual clinical location of the inferior alveolar nerve. However, radiographs must be interpreted cautiously, since these structures may not be clearly visible for several reasons, as will be discussed below.The mandibular canal is usually detected as a narrow radiolucent ribbon bordered by radio‐ opaque lines, although it may not always be a distinct bony‐walled channel. In the anterior region, the canal wall is thinner, and thus less detectable on radiographs.
The advantages of panoramic radiography over periapical radiography in detecting the mandibular canal and mental foramen include the ability to view the entire body of the mandible. One study has shown that the detection rate of the mental foramen in panoramic radiographs was 94% (N = 545), although only 49% showed clear visibility (Jacobs
et al. 2004). With respect to periapical radiographs, the detection rates of the mental foramen are smaller and have been reported to be 46.8% (N = 1000) in one study (Fishel et al. 1976), and 75%
(N = 75) in another study (Phillips, Weller & Kulild 1990). In periapical films, the mental foramen sometimes mimics an inflammatory periapical lesion, particularly when the radiolucency is overlap- ping the apex of a premolar (Figure 11.8). In such a case, however, the mental foramen can be differenti- ated from pathologic conditions by its radiographic appearance, that is, better‐maintained integrity of the lamina dura and periodontal ligament space. Exposures at different angulations are useful in the differentiation, since the radiolucency representing the mental foramen moves from the apex by chang- ing the angulation.
There are several reasons why the mandibular canal and the mental foramen are not always detectable in radiographs; these include difficulty in differentiating these structures from the trabecular pattern, and low radiographic contrast due to the thin mandibular bone or thick lingual cortical plate of the bone. In periapical films, these structures can be missed because of the narrower coverage, that is, when they are located out of the film edge.
The use of CBCT provides 3‐D evaluation of the mandible, and its measurement accuracy is superior to panoramic and periapical radiographs.Thus, CBCT is currently the best available imaging tech- nique to determine the accurate location of the mandibular canal and mental foramen (Aminoshariae, Su & Kulild 2014).
What are the causes of mental nerve paresthesia related to non‐surgical endodontic treatment?
The overall incidence of mental nerve paresthesia is not clear, although one study reported that the incidence of paresthesia associated with non‐surgical endodontic treatment of mandibular premolars was 0.96% (Knowles et al. 2003), indicating that such complication is fortunately uncommon.
Mental nerve paresthesia due to diseases of endodontic origin, that is, intracanal and periapical infection, is caused by several factors. One factor is mechanical pressure to the inferior alveolar nerve or mental nerve, which is associated with inflammatory reaction; inflammatory edema formation and accumulation of purulent exudates may result in an increase in local pressure to a level sufficient to induce paresthesia. Nerve ischemia due to inflam- mation may also be a factor associated with pares- thesia. A second factor is local production of bacterial metabolic products that are toxic to nerves.
Paresthesia is also a complication that is associ- ated with endodontic treatment, and can be attribut- able to various causes (Ahonen & Tjäderhane 2011). Overinstrumentation and/or extrusion of endodontic materials into the vicinity of the inferior alveolar nerve or mental nerve are the major causes of mental nerve paresthesia. During chemomechanical root canal instrumentation, inadvertent extrusion of sodium hypochlorite can result in tissue necrosis due to the strong cytotoxicity and high tissue‐dissolving activity of this solution, leading to pain, swelling, and possibly anesthesia of the mental nerve. Bacterial irritation can also occur during root canal instrumentation due to the extrusion of infected debris, which may also induce paresthesia by mech- anisms similar to those induced by endodontic infection. Extruded calcium hydroxide intracanal medicament can also induce inferior alveolar nerve paresthesia (Ahlgren, Johannessen & Hellem 2003), possibly due to the causative potential of calcium hydroxide to induce inflammation and/or inhibition of nerve transmission by excessive calcium and hydroxide ions. Postoperative flare‐ups following root canal instrumentation can be accompanied by mental nerve paresthesia, likely due to polietiologi- cal mechanisms, including bacterial, mechanical, and chemical irritation to the mental nerve (Morse 1997).
Overextruded root canal filling materials can induce paresthesis. Although gutta‐percha is consid- ered to be an inert root‐filling material, overfilling of thermoplastic gutta‐percha within the mandibular canal can generate paresthesia, likely due to thermal irritation and nerve compression. Another potential cause of paresthesia is root canal sealer, which can cause chemical irritation. In particular, zinc oxide and eugenol‐based sealers show neurotoxic effects due to the action of eugenol, especially in the freshly mixed state. Paraformaldehyde‐containing pastes are known to induce strong neurotoxic effects, and are not recommended for endodontic obturation.
Explain clinical management of mental nerve paresthesia associated with endodontic infection and iatrogenic events related to endodontic treatment.
The recovery potential of the nerve may be dependent on the extent of the damage and duration of the irritation, which show consider- able variation among cases.Thus, paresthesia following transient nerve irritation, such as that induced by overinstrumentation, may resolve spontaneously within days or weeks. However, nerves suffering from prolonged damage due to chemical and mechanical irritation, such as that caused by gross overextension of neurotoxic materials, may not recover to the same degree (Ahonen &Tjäderhane 2011). In general, the first choice should be a more conservative treatment, including prescription of vitamin B12, which has the action of promoting peripheral nerve regen- eration, and antibiotics to control infection. However, if surgical removal of foreign materials is deemed necessary, such as in the case of overextended neurotoxic material migrating along the mandibular nerve bundle, immediate surgical intervention, preferably within 48 hours, is recommended (Pogrel 2007).
Mental nerve paresthesia related to endodontic infection usually resolves after appropriate endodontic therapy in combination with drug therapy using antibiotics, corticosteroids, and/or vitamin B12 (Morse 1997). In the case presented, incision to establish drainage was effective for the resolution of symptoms related to acute inflammation, although great care should be taken not to damage the mental nerve during the incision. The decompression may have contributed, to a certain degree, to the recovery of sensation, whereas root canal therapy was necessary for the definitive resolu- tion of the paresthetic symptoms. During the root canal therapy, great care should be taken to avoid iatrogenic events that could lead to the aggravation of paresthetic symptoms. In particular, postoperative acute exacerbation (flare‐ups) following root canal instrumentation can be accompanied by mental nerve paresthe- sia (Morse 1997). Although acute exacerbation is caused by polietiological mechanisms, including bacterial, mechanical, and chemical irritation, and is often unpredictable, some procedures may have the potential to reduce the incidence of acute exacerbation. In this regard, instrumentation techniques with lesser amounts of apically extruded debris should be considered, such as the crown‐down prepara- tion technique, as discussed earlier. Copious and frequent irrigation is recommended since it can enhance the removal of canal contents such as infected dentin chips and microbial cells. Other possible measures include: com- pletion of chemomechanical root canal prepa- ration in one visit to remove maximum amount of irritants; correct measurement of the working length; and use of intracanal medicaments to facilitate microbial elimination
(Siqueira 2003).
