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Joum.al of Oral and Maxillofacial Surgery (1990) 28,204
1990 The British Association of Oral and h$xiIlofacial
Surgeons
The radiological prediction of inferior alveolar nerve injury during third molar surgery J. P. Rood, B. A. A. Nooraldeen
Shehab
Department of Oral and Maxillofacial Surgery, Turner Dental School, Manchester
The surgical removal of an impacted mandibular third molar may result in damage to the SUMMARY. inferior alveolar nerve and may cause disabling anaesthesia of the lip; anaesthesia of the lower gingivae and anterior teeth may also result. Assessing the likelihood of injury depends to a great extent on preoperative radiographic examination. Seven radiological diagnostic signs have been mentioned in the literature; the reliability of these signs as predictors of likely nerve injury have been evaluated through retrospective and prospective surveys. Three signs were found to be significantly related to nerve injury and a further two were probably important clinically.
lining of the canal between the source of X-rays and the film (MacGregor, 1976).
INTRODUCTION The inferior alveolar nerve runs in a canal within the mandible usually near the apices of the third molar and, if the molar is impacted, a close relationship of the roots to the nerve is likely. Sometimes, during the surgical removal of a mandibular third molar, the inferior alveolar nerve is damaged leading to impairment of sensation in the lower lip; which is one of the most unpleasant postoperative complications. Pre-operative assessment must be carried out radiologically in an attempt to identify the proximity of the impacted tooth to the inferior alveolar canal. This evaluation is the first stage in assessing the possible postoperative occurrence of labial sensory impairment and thus its prevention. A review of the literature revealed that seven radiological signs had been suggested as indicative of a close relationship between the mandibular third molar tooth and the inferior alveolar canal. Four of these signs are seen on the root of the tooth and the other three are changes in the appearance of the inferior alveolar canal. This investigation was designed to identify the most important signs.
Deflected roots Deflected roots or roots hooked around the canal are seen as an abrupt deviation of the root, when it reaches the inferior alveolar canal (Fig. 2). The root may be deflected to the buccal or lingual side or to both sides so that it may completely surround the canal (Stockdale, 1959); or it may be deflected to the mesial or distal aspect (Waggener, 1959). Narrowing of the root Seward (1963) stated ‘If there is narrowing of root where the canal crosses it, it implies that greatest diameter of the root has been involved by canal, or that there is deep grooving or perforation the root’. (Fig. 3).
the the the of
Dark and bifid root This sign appears when the inferior alveolar canal crosses the apex (Fig. 4) and is identified by the double periodontal membrane shadow of the bifid apex (Seward, 1963).
Darkening of the root Usually the density of the root is the same throughout its length and this is not disturbed when the images of the tooth and inferior alveolar canal overlap. When there is impingement of the canal on the tooth root, there is loss of density of the root (Fig. 1); the root appears darker (Main, 1938; Miles & West, 1954; Durbeck, 1957; Seward, 1963; Killey & Kay, 1975; Kipp et al., 1980; Howe, 1985). Howe and Poyton (1960) reported that 93.1% of the teeth in true relationship to the canal showed this sign. Darkening of the root is attributed to the decreased amount of tooth substance or loss of the cortical
Interruption of the white line(s) The white lines are the two radio-opaque lines that constitute the ‘roof’ and ‘floor’ of the inferior alveolar canal. These lines appear on a radiograph due to the rather dense structure of the canal walls (Durbeck, 1957). The white line is considered to be interrupted if it disappears immediately before it reaches the tooth structure (Fig. 5); either one or both lines may be involved (Howe & Poyton, 1960; Killey & Kay, 1975; MacGregor, 1976; Kipp et al., 1980; Rud, 20
Radiological prediction of inferior alveolar nerve injury
1983b). The interruption of the white line(s) is considered to indicate deep grooving of theroot if it appears alone or perforation of the root if it appears with the narrowing of the inferior alveoiar canal (Seward, 1963; Howe, 1985). The interruption is considered by some to be a ‘danger sign’ of a true relationship betwe&n tooth root and canal (Summers, 1975). Diversion of the inferior alveolar canal The canal is considered to be diverted if, when it crosses the mandibular third molar, it changes its direction (Fig. 6), (Miles & West, 1954; MacGregor, 1976; Kipp et al., 1980; Rud, 1983a). Seward (1963) attributed an upward displacement of the inferior alveolar canal to the contents of the canal passing through the root and hence, during eruption of the third molar, the contents are dragged upwards with it. Rud (1983a) reported a 1% incidence of an upward deflection of the canal where it overlapped the root and 4% when the root was grooved. Narrowing of the inferior alveolar canal The inferior alveolar canal is considered to be narrowed if, when it crosses the root of the mandibular third molar, there is a reduction of its diameter (Fig. 7) (Poyton, 1982). This narrowing could be due to the downward displacement of the upper border of the canal (Kipp et al., 1980; Rud; 1983a) or the displacement of the upper and lower borders toward each other with the hourglass appearance (Cogswell, 1942; Rud, 1983a). The hourglass form indicates a partial encirclement of the canal (Seward, 1963; MacGregor, 1976) or a complete encirclement (Waggener, 1959; Killey & Kay, 1975; Stimmers, 1975; Howe, 1985); or it may mean either of these alternatives (Cogswell, 1942; Austin, 1947; Miles & West, 1954; Uotila & Kilpinen, 1968). Howe and Poyton (1960) reported 33.7% of teeth in a true relationship with the canal to have this sign.
METHODS Retrospective and prospective surveys were carried out involving 1560 impacted mandibular third molars requiring surgical removal. In the retrospective survey, cases of lip sensation impairment were identified from the records. The cases were randomly selected and the teeth were removed. -between 1980 and’ 1984 in Manchester Dental Hospital and Manchester Royal Infirmary by many operators of varying ‘experience. The radiological signs of nerve proximity to the mandibular third molar were noted for each c,ase and the apparent relationship was recorded. ‘The radiographs, periapical and orthopantomogram, were mounted on an Xray viewer for examination and a magnifying lens was used. In the prospective survey, the periapical and orthopantomogram views were examined prior to operation, The signs of impingement between the tooth
and the inferior alveolar canal were recorded prediction of likelihood of nerve damage was for each case preoperatively. All preoperative ographs were examined for the presence of the radiological signs mentioned earlier.
21
and a made radiseven
Post-operative findings
The main symptoms of injury to the inferior alveolar nerve were anaesthesia or altered sensation of the lower lip and chin (Simpson, 1958). The degree of disturbance varied from mild paraesthesia, only noticed when the skin was touched, to dense anaesthesia (Rood, 1983). Extra-orally the affected area was bounded medially by the midline and laterally by a line extending downward and slightly backward from the corner of the mouth to the inferior border of the mandible which forms the inferior boundary (Simpson, 1958). The extent of sensory loss may vary from a small area, frequently the vermilion border, to involvement of all of the skin innervated by the mental nerve (Rood, 1983). Intra-orally the affected areas were the inner surface of the lip, the adjacent labial mucosa of the alveolar process and the mandibular teeth of the affected side (Simpson, 1958). Each patient was examined on the first postoperative day and again at a follow-up appointment 7 to 10 days later. Sensation was assessed using cotton wool, blunt probe and pin prick (Rood, 1983). Cases with isolated areas of submental anaesthesia, without involvenient of the lower lip or gingivae, were excluded as they were considered likely to have suffered injury to the mylohyoid nerve (Roberts & Harris, 1973). Statistical analysis
The findings of the relationship of seven radiological signs to the impairment of labial sensation were analysed by applying the chi square (x2) test (Von Fraunhofer & Murray, 1976). For the present study the following levels of significance were used:If ~~~3.841, then the result was significant at 5% level (PcO.05). If ,x2>6.635, then the result was significant at 1% level (PcO.01). If x2>10.827, then the result was (very highly) significant at 0.1% level (P
Joum.al of Oral and Maxillofacial Surgery (1990) 28,204
1990 The British Association of Oral and h$xiIlofacial
Surgeons
The radiological prediction of inferior alveolar nerve injury during third molar surgery J. P. Rood, B. A. A. Nooraldeen
Shehab
Department of Oral and Maxillofacial Surgery, Turner Dental School, Manchester
The surgical removal of an impacted mandibular third molar may result in damage to the SUMMARY. inferior alveolar nerve and may cause disabling anaesthesia of the lip; anaesthesia of the lower gingivae and anterior teeth may also result. Assessing the likelihood of injury depends to a great extent on preoperative radiographic examination. Seven radiological diagnostic signs have been mentioned in the literature; the reliability of these signs as predictors of likely nerve injury have been evaluated through retrospective and prospective surveys. Three signs were found to be significantly related to nerve injury and a further two were probably important clinically.
lining of the canal between the source of X-rays and the film (MacGregor, 1976).
INTRODUCTION The inferior alveolar nerve runs in a canal within the mandible usually near the apices of the third molar and, if the molar is impacted, a close relationship of the roots to the nerve is likely. Sometimes, during the surgical removal of a mandibular third molar, the inferior alveolar nerve is damaged leading to impairment of sensation in the lower lip; which is one of the most unpleasant postoperative complications. Pre-operative assessment must be carried out radiologically in an attempt to identify the proximity of the impacted tooth to the inferior alveolar canal. This evaluation is the first stage in assessing the possible postoperative occurrence of labial sensory impairment and thus its prevention. A review of the literature revealed that seven radiological signs had been suggested as indicative of a close relationship between the mandibular third molar tooth and the inferior alveolar canal. Four of these signs are seen on the root of the tooth and the other three are changes in the appearance of the inferior alveolar canal. This investigation was designed to identify the most important signs.
Deflected roots Deflected roots or roots hooked around the canal are seen as an abrupt deviation of the root, when it reaches the inferior alveolar canal (Fig. 2). The root may be deflected to the buccal or lingual side or to both sides so that it may completely surround the canal (Stockdale, 1959); or it may be deflected to the mesial or distal aspect (Waggener, 1959). Narrowing of the root Seward (1963) stated ‘If there is narrowing of root where the canal crosses it, it implies that greatest diameter of the root has been involved by canal, or that there is deep grooving or perforation the root’. (Fig. 3).
the the the of
Dark and bifid root This sign appears when the inferior alveolar canal crosses the apex (Fig. 4) and is identified by the double periodontal membrane shadow of the bifid apex (Seward, 1963).
Darkening of the root Usually the density of the root is the same throughout its length and this is not disturbed when the images of the tooth and inferior alveolar canal overlap. When there is impingement of the canal on the tooth root, there is loss of density of the root (Fig. 1); the root appears darker (Main, 1938; Miles & West, 1954; Durbeck, 1957; Seward, 1963; Killey & Kay, 1975; Kipp et al., 1980; Howe, 1985). Howe and Poyton (1960) reported that 93.1% of the teeth in true relationship to the canal showed this sign. Darkening of the root is attributed to the decreased amount of tooth substance or loss of the cortical
Interruption of the white line(s) The white lines are the two radio-opaque lines that constitute the ‘roof’ and ‘floor’ of the inferior alveolar canal. These lines appear on a radiograph due to the rather dense structure of the canal walls (Durbeck, 1957). The white line is considered to be interrupted if it disappears immediately before it reaches the tooth structure (Fig. 5); either one or both lines may be involved (Howe & Poyton, 1960; Killey & Kay, 1975; MacGregor, 1976; Kipp et al., 1980; Rud, 20
Radiological prediction of inferior alveolar nerve injury
1983b). The interruption of the white line(s) is considered to indicate deep grooving of theroot if it appears alone or perforation of the root if it appears with the narrowing of the inferior alveoiar canal (Seward, 1963; Howe, 1985). The interruption is considered by some to be a ‘danger sign’ of a true relationship betwe&n tooth root and canal (Summers, 1975). Diversion of the inferior alveolar canal The canal is considered to be diverted if, when it crosses the mandibular third molar, it changes its direction (Fig. 6), (Miles & West, 1954; MacGregor, 1976; Kipp et al., 1980; Rud, 1983a). Seward (1963) attributed an upward displacement of the inferior alveolar canal to the contents of the canal passing through the root and hence, during eruption of the third molar, the contents are dragged upwards with it. Rud (1983a) reported a 1% incidence of an upward deflection of the canal where it overlapped the root and 4% when the root was grooved. Narrowing of the inferior alveolar canal The inferior alveolar canal is considered to be narrowed if, when it crosses the root of the mandibular third molar, there is a reduction of its diameter (Fig. 7) (Poyton, 1982). This narrowing could be due to the downward displacement of the upper border of the canal (Kipp et al., 1980; Rud; 1983a) or the displacement of the upper and lower borders toward each other with the hourglass appearance (Cogswell, 1942; Rud, 1983a). The hourglass form indicates a partial encirclement of the canal (Seward, 1963; MacGregor, 1976) or a complete encirclement (Waggener, 1959; Killey & Kay, 1975; Stimmers, 1975; Howe, 1985); or it may mean either of these alternatives (Cogswell, 1942; Austin, 1947; Miles & West, 1954; Uotila & Kilpinen, 1968). Howe and Poyton (1960) reported 33.7% of teeth in a true relationship with the canal to have this sign.
METHODS Retrospective and prospective surveys were carried out involving 1560 impacted mandibular third molars requiring surgical removal. In the retrospective survey, cases of lip sensation impairment were identified from the records. The cases were randomly selected and the teeth were removed. -between 1980 and’ 1984 in Manchester Dental Hospital and Manchester Royal Infirmary by many operators of varying ‘experience. The radiological signs of nerve proximity to the mandibular third molar were noted for each c,ase and the apparent relationship was recorded. ‘The radiographs, periapical and orthopantomogram, were mounted on an Xray viewer for examination and a magnifying lens was used. In the prospective survey, the periapical and orthopantomogram views were examined prior to operation, The signs of impingement between the tooth
and the inferior alveolar canal were recorded prediction of likelihood of nerve damage was for each case preoperatively. All preoperative ographs were examined for the presence of the radiological signs mentioned earlier.
21
and a made radiseven
Post-operative findings
The main symptoms of injury to the inferior alveolar nerve were anaesthesia or altered sensation of the lower lip and chin (Simpson, 1958). The degree of disturbance varied from mild paraesthesia, only noticed when the skin was touched, to dense anaesthesia (Rood, 1983). Extra-orally the affected area was bounded medially by the midline and laterally by a line extending downward and slightly backward from the corner of the mouth to the inferior border of the mandible which forms the inferior boundary (Simpson, 1958). The extent of sensory loss may vary from a small area, frequently the vermilion border, to involvement of all of the skin innervated by the mental nerve (Rood, 1983). Intra-orally the affected areas were the inner surface of the lip, the adjacent labial mucosa of the alveolar process and the mandibular teeth of the affected side (Simpson, 1958). Each patient was examined on the first postoperative day and again at a follow-up appointment 7 to 10 days later. Sensation was assessed using cotton wool, blunt probe and pin prick (Rood, 1983). Cases with isolated areas of submental anaesthesia, without involvenient of the lower lip or gingivae, were excluded as they were considered likely to have suffered injury to the mylohyoid nerve (Roberts & Harris, 1973). Statistical analysis
The findings of the relationship of seven radiological signs to the impairment of labial sensation were analysed by applying the chi square (x2) test (Von Fraunhofer & Murray, 1976). For the present study the following levels of significance were used:If ~~~3.841, then the result was significant at 5% level (PcO.05). If ,x2>6.635, then the result was significant at 1% level (PcO.01). If x2>10.827, then the result was (very highly) significant at 0.1% level (P
