The radiological prediction of inferior alveolar nerve injury during third molar surgery

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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.

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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
RESULTS Retrospective survey results The hospital records of, 553 pitients were examined. The case notes recdrded ,details :of treatment involving 800 impacted third molar teeth. Impairment of labial sensation wa? found related to 19 impacted mandibular third mqlars (2.4%) which were removed from 17 patients under local or general anaesthesia. The preoperative radiological signs appeared 73 times in 800 cases (9.1%). Table 1 reveals the rate of appearance of each of the seven radiological signs in the preoperative X-ray views. The signs were

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British Journal of Oral and Maxillofacial Surgery Figures prepared as tracings from radiographs.

Fig. 1 - Darkening of root.

Fig. 2 - Deflection of root.

Fig. 3 - Narrowing of root.

Fig. 4 - Dark and bilid apex of root.

Fig. 5 - Interruption of white line of canal.

Fig. 6 - Diversion of canal.

Fig. 7 - Narrowing of canal.

Radiological urediction of inferior alveolar nerve iniurv Table 1 - Incidence survey)

of seven radiological

Radiographic signs 1. Darkening of the root 2. Deflected root 3. Interruption of the white line 4. Narrowing of the inferior alveolar canal 5. Diversion of the inferior alveolar canal 6. Dark and bifid root 7. Narrowing of the root Total sign appearance ** P
signs (retrospective

Table

3 -

Incidence

of seven radiological

23

signs (prospective

survey)

Sign appearance

Percent of total population (800 cases)

26 13

3.25*** 1.625***

11

1.375**

9

1.125

7 6 1 73

0.875 0.75 0.125 9.125

*** P-CO.001

Radiographic signs 1. Darkening of the root 2. Deflected root ‘, 3. Interruption of‘ the white line 4. Diversion of- the inferior alveolar canal 5. Narrowing of the inferior alveolar canal 6, Dark and bifid root 7. Narrowing of the root Total sign appearance * P-CO.05

Sign appearance

Percent of total population (800 cases)

46 42

6.05*** 5.53***

35

4.61*

18

2.37

11 3 2 157

1.45 0.39 0.26 20.66

*** P
Table 2 - Incidence of seven radiological signs and impaired lip sensation (retrospective survey)

Table 4 - Relation of seven radiological signs and impaired lip sensation (prospective survey)

Sign appearance Lip impairment Non-lip cases impairment cases

Sign appearance Lip impairment Non-lip cases impairment cases

Radiographic signs 1. Interruption of the white line 2. Darkening of the root 3. Deflected root 4. Diversion of the inferior alveolar canal 5. Narrowing of the root 6. Dark and bifid root 7. Narrowing of the inferior alveolar canal Total sign appearance ** P-CO.01

7*** lo*** 3***

0 16 10

2***

5 0 5

1** 1 1 25

8 48

*** P
tabulated according to their predominance. related to nerve injury. The relationship between the radiological signs and the impairment of labial sensation is shown in Table 2. A significant relationship was found with the first five signs shown in the table. Signs unrelated to nerve injury. The appearance of dark and bifid root and the narrowing of the inferior alveolar canal did not appear to be significant when related to lip sensation impairment. These two signs therefore may be less important clinically.

Signs

Prospective

survey results

This part of the investigation was designed to examine an equivalent number of third molars. The number of patients studied was 552, requiring surgery to remove 760 impacted mandibular third molars. Preoperative radiological signs suggestive of nerve involvement were associated with 125 teeth (16.45%). 17 patients (3.08%) experienced impairment of labial sensation (four bilaterally and 13 unilaterally). Radiological signs appears 157 times in the 760 cases (20.66%). This was twice that found in the retrospective survey. More patients had both periapical and orthopantomogram views, with more

Radiographic signs 1. Diversion of the inferior alveolar canal 2. Darkening of the root 3. Interruption of the white line 4. Narrowing of the root 5. Dark and bifid root 6. Narrowing of the inferior alveolar canal 7. Deflected root TotaP sign appearance

6*** g***

12 38

5*** 0 0

30 2 3

0 2 21

11 40 136

*** P
adequate orthopantomograms in the prospective survey leading to a higher identification of the signs. Table 3 shows the seven radiological signs listed according to their frequency of appearance. The increase of X-ray signs in this series was mainly due to the increase of the first four signs in Table 3. Positive identifications of the deflected root and interrupted white line were considerably increased. Signs related to nerve injuly. The most significant sign which related to nerve injury was diversion of the inferior alveolar canal (P
DISCUSSION

By preoperative examination of the intra-oral periapical view, Howe and Poyton (1960) claimed to

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British Journal of Oral and Maxillofacial Surgery

forecast accurately the presence of grooving, notching or perforation of third molar roots by the inferior alveolar canal. Azaz et al. (1976) also accurately diagnosed the intimate relationship of the third molar with the inferior alveolar canal in preoperative periapical views. The findings of these two groups of workers supported the use of the periapical X-ray view for preoperative assessment. Seven radiological signs were mentioned in the literature as indicative of a close relationship of the impacted mandibular third molar to the inferior alveolar canal. All seven signs were found on the radiographs during these retrospective and prospective studies. Only three were found to be significantly related to inferior alveolar nerve injury in both studies. These were diversion of the canal, darkening of the root and interruption of the white line. Rud (1983a) found that there was a significant relation between the diversion of the canal and grooved roots. In this study a significant relation was found between the diversion of the canal and nerve injury (P
operative nerve injury. Kipp et al. (1980) and Rud (1983a) also found that this sign was not significantly related to the incidence of post-operative labial sensation impairment. In some cases, more than one of the X-ray signs were present-the maximum combination of the signs found in any single case was four. Although it may be expected that the presence of several signs would increase the likelihood of nerve injury, this could not be shown due to the small numbers involved in this study. One hundred and four patients with ‘positive’ Xray appearances did not suffer nerve injury. The presence of these X-ray signs does not indicate that nerve injury will definitely occur. Five patients experienced labial sensation impairment without ‘positive’ preoperative radiological appearances. The absence of pre-operative X-ray signs does not ensure that injury to the inferior alveolar nerve will not occur during surgery. References Austin, L. T. (1947). Perforation of roots of impacted lower third molars by contents of mandibular canal: Report of a case. American Journal of Orthodontics and Oral Surgery, 33,623.

Azaz, B., Shteyer, A. & Piamenta, M. (1976). Radiographic and clinical manifestations of the impacted mandibular third molar. International Journal of Oral Surgery, 5,153. Cogswell, W. W. (1942). Surgical problems involving the mandibular nerve. Journal of the American Dental Association, 29,964.

Durbeck, W. E. (1957). The Impacted Lower Third Molar, 2nd Ed.. .*. DD. 24: 109-110. London: Henrv Kimoton. Howe, G. L. (1985). Minor Oral Surgery: 3rd Ed., pp. 126,129. Bristol: Wright. Howe, G. L. & Poyton, H. G. (1960). Prevention of damage to the inferior dental nerve during the extraction of mandibular third molars. British Dental Journal, 109,3X3-363. Killey, H. C. & Kay, L. W. (1975). The Impacted Wisdom Tooth. 2nd Ed., pp. 13; 24-25; 28. Edinburgh: Churchill Livingstone. Kipp, D. P., Goldstein, B. H. & Weiss Jr, W. W. (1980). Dysesthesia after mandibular third molar surgery: a retrospective study and analysis of 1377 surgical procedures. Journal of the American Dental Association, 100,185.

MacGregor, A. J. (1976). The radiological assessment of ectopic lower third molars. DDSc Thesis, University of Leeds. Main, L. R. (1938). Further roentgenographic study of mandibular third molars. Journal of the American Dental Association, 25,1993.

Miles, A. E. W. & West, W. H. (1954). The relationship of the mandibular third molar to the mandibular canal. Dental Practitioner, 4,370.

Poyton, H. G. (1982). Oral Radiology, 1st Ed., pp 35; 166168. Baltimore: Williams and Wilkins Co. Roberts, G. D. D. & Harris, M. (1973). Neurapraxia of the mylohyoid nerve and submental analgesia. British Journal of Oral Surgery, 11,110.

Rood, J. P. (1983). Degrees of injury to the inferior alveolar nerve sustained during the removal of impacted mandibular third molars by the lingual split technique. British Journal of Oral Surgery, 21,103.

Rud, J. (1983a) Third molar surgery: relationship of root to mandibular canal and injuries to inferior dental nerve. TandZaegebZadet, 87,619.

Rud, J. (1983b) Third molar surgery: perforation of the inferior dental nerve through the root. Tandlaegebladet, 87,659. Seward, G. R. (1963). Radiology in general dental practice: VIII-Assessment of lower third molars. British Dental Journal, 115,145.

Radiolosical orediction of inferior Simpson, H. E. (1958). Injuries to the inferior dental and mental nerves. Journal of Oral Surgery, 16,300. Stockdale, C. R. (1959). The relationship of the roots of mandibular third molars to the inferior dental canal. Oral Surgery, Oral Medicine, Oral Pathology, 12,1061.

Summers, L. (1975). Assessment of lower third molars. Australian Dental Journal, 20,368.

Uotila, E. and Kilpinen, E. (1968). Relationship of the roots of an impacted third molar and the mandibular canal determined by stereo-roentgenography. Odontologisk Tidskrifi, 76, 55.

Von Fraunhofer, J. A. & Murray, J. J. (1976). Statistics in Medical, Dental and Biological Studies, pp 62-68; 108. London: Tri-Med Books Ltd. Waggener, D. T. (1959). Relationships of third molar roots to the mandibular canal. Oral Surgery, Oral Medicine, Oral Pathology, 12,853.

dVeOhI’

nerve iniurv

The Authors Professor J. P. Rood MDS, MSc, MBBS, FDSRCS, FRCS Head of Department Mr B. A. A. Noorakleen Shehab MSc, BDS Postgraduate Student Department of Oral and Maxillofacial Surgery Turner Dental School University Dental Hospital of Manchester Higher Cambridge Street Manchester Ml5 6FH Correspondence and requests for offprints to Professor J. P. Rood. Paper received 1 September 1989 Accepted 21 September 1989

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