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Risk factors associated with inferior alveolar nerve injury after extraction of the mandibular third molar—a comparative study of preoperative images by panoramic radiography and computed tomography
Int. J. Oral Maxillofac. Surg. 2013; 42: 843–851 http://dx.doi.org/10.1016/j.ijom.2013.01.023, available online at http://www.sciencedirect.com
Clinical Paper Oral Surgery
Risk factors associated with inferior alveolar nerve injury after extraction of the mandibular third molar—a comparative study of preoperative images by panoramic radiography and computed tomography
T. Hasegawa1, S. Ri2, T. Shigeta1, M. Akashi1, Y. Imai1, Y. Kakei1, Y. Shibuya1, T. Komori1 1
Department of Oral and Maxillofacial Surgery, Kobe University Graduate School of Medicine, Kobe, Japan; 2Department of Oral and Maxillofacial Surgery, Kobe Steel Hospital, Kakogawa, Japan
T. Hasegawa, S. Ri, T. Shigeta, M. Akashi, Y. Imai, Y. Kakei, Y. Shibuya, T. Komori: Risk factors associated with inferior alveolar nerve injury after extraction of the mandibular third molar—a comparative study of preoperative images by panoramic radiography and computed tomography. Int. J. Oral Maxillofac. Surg. 2013; 42: 843–851. # 2013 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. In this study we investigated the relationships among the risk factors for inferior alveolar nerve injury (IANI), and the difference between preoperative imaging findings on panoramic radiographs and computed tomography (CT), by univariate and multivariate analyses. We determined the following to be significant variables by multivariate analysis: panoramic radiographic signs, such as the loss of the white line of the inferior alveolar canal or the diversion of the canal; excessive haemorrhage during extraction; and a close relationship of the roots to the IAN (type 1 cases) on CT examination. CT findings of type 1 were associated with a significantly higher risk (odds ratio 43.77) of IANI. In addition, many panoramic findings were not consistent with CT findings (275 of 440 teeth; 62.5%). These results suggest that CT findings may be able to predict the development of IANI more accurately than panoramic findings. Panoramic radiography alone did not provide sufficiently reliable images required for predicting IANI. Therefore, when the panoramic image is suggestive of a close relationship between the impacted tooth and the IAN, CT should be recommended as a means of conducting further investigations.
0901-5027/070843 + 09 $36.00/0
Key words: mandibular third molar; hypoesthesia; inferior alveolar nerve injury; computed tomography; multivariate analysis. Accepted for publication 21 January 2013 Available online 15 March 2013
# 2013 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
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The removal of the mandibular third molars to prevent future problems is widely acknowledged to be useful and is one of the most commonly performed surgical procedures in oral surgery. The surgical removal of these molars may damage the nerve and cause hypoesthesia of the lower lip as an unpleasant postoperative complication.1–22 Many investigators have reported that inferior alveolar nerve injury (IANI) is associated with mandibular third molar removal.1–22 However, there have been few quantitative analyses of the factors contributing to IANI. In addition, other than our recent study,23 only a few studies to date have evaluated the multivariate relationships among the various risk factors and IANI.11,16,17 In our recent study, we reported some radiographic signs (the loss of the white line of the inferior alveolar canal, the diversion of the canal, and a close relationship of the roots to the IAN on panoramic radiographic findings and not on CT findings) to be significantly associated with IANI following third molar extraction.23 Panoramic radiographs are the imaging modality most commonly used by oral and maxillofacial surgeons to view impacted third molars and to estimate the risk of IANI.13 In particular, Rood and Shehab reported that three of seven classic radiographic signs indicate a higher risk of IANI, implicating close proximity of the inferior alveolar nerve (IAN) to the lower third molar.7 However, some investigators have reported that panoramic radiography does not provide sufficiently reliable images to predict nerve lesions.13,24,25 In addition, some reports have indicated that it is only possible to determine the true relationship of the tooth root with the IAN with computed tomography (CT).13,24–26 In fact, axial, coronal, and sagittal CT images all provide surgeons with useful information, and such images are also beneficial for the preoperative planning of the surgical procedure because of the high-resolution quality of medical CT.25 In this study, we investigated the multivariate relationships among the various risk factors of IANI, and the differences between preoperative imaging findings on panoramic radiographs and medical CT images. Patients and methods
This was a non-randomized, retrospective (historic) cohort study of patients. Thus, this study was granted exemption from institutional review board approval. Between April 2006 and March 2010, 2528 surgical removals of mandibular third molars were performed by dentists
at the Department of Oral and Maxillofacial Surgery. A total of 376 (high risk of IANI; types 1, 2, and 3 as described in ‘Study design’ section) of the 2528 teeth were found to be in close association with the mandibular canal on panoramic radiograph, or were expected to be very difficult to extract (for example, in cases with hypertrophic roots and/or dilacerated teeth). Sixty-four (low risk of IANI; types 4 and 5) of the 2528 teeth were opposite to the other teeth that were in close proximity to the IAN. A total of 440 teeth from 295 patients (122 men and 173 women) were included in this study. The patients had a mean age of 36.2 12.2 years (range 16–71 years). Thus the data (total 440 teeth) for this study pertain to the impacted lower third molars considered to have a high level of risk of IANI during surgery. Before surgery, each patient was informed of the necessity to perform a CT examination and the possible complications, including the potential risk of IANI during the procedure. Surgical methods
Every intervention was carried out under local or general anaesthesia. The extraction of 147 teeth (33.4%) in 86 patients was carried out under general anaesthesia; for 293 teeth (66.6%) in 209 patients, the procedure was carried out under local anaesthesia. Sedation was not carried out. Envelope (sulcular) mucoperiosteal flaps were raised for superficial impactions, and triangular flaps were raised for deep impactions, followed by vestibular bone removal. If necessary, bone removal and crown and root sectioning were performed with tungsten fissure burs (Dentsply-Maillefer, Ballaigues, Switzerland). Sockets were irrigated with 20 ml sterile saline solution at room temperature, and exposure of the IAN was checked during and after precisely focused careful suction. A 3–0 silk suture was used to close the wound. An antibiotic and an anti-inflammatory drug were prescribed (usually oral cefcapene pivoxil hydrochloride hydrate 900 mg 3 times daily for 3 days and oral acetaminophen 1200 mg 3 times daily for 3 days). After 7 days, a surgeon removed the suture and explored the wound for closure and recorded any complications such as ecchymosis, trismus, swelling, postoperative bleeding, dry socket, infection, and IANI. Study design
Patients who underwent conventional panoramic radiography (Orthoceph
OC100CT; Yoshida, Tokyo, Japan) were included in the study. The radiation dose was 20–30 mSv. The medical CT images were taken using a High-Speed Advantage CT Scanner (GE Medical Systems-Japan, Tokyo, Japan). The exposure factors were set at 120 kV and 200 mA, and the duration of scanning was 6 s. The slice thickness of contiguous sections was 0.625 mm. Axial planes were set parallel to the occlusal plane, and continuous 0.625-mm slices were taken. Coronal and sagittal images were reconstructed from the raw data. The software program used was Advantage Workstation1 (GE Medical Systems-Japan). The radiation dose was 2.0–3.0 mSv. The primary predictive variable was the presence or absence of one or more panoramic radiographic or CT findings. The panoramic radiographic and CT findings were analyzed independently by the first and second authors. These two authors were blinded independently to the postoperative IANI status. The CT images were presented to the two observers in a randomized order to prevent referral to the panoramic findings. Whenever a disagreement occurred between the two observers’ assessments, a consensus was reached by discussion. At first, the panoramic findings were evaluated for the presence or absence of the following three classic radiographic signs, all of which have been reported as suggestive of a close proximity of the IAN to the lower third molar7: (1) darkening of the root where it crosses the inferior alveolar canal (as described below, this group was classified into types 1 and 2 on panoramic radiographic findings and not on CT findings); (2) interruption or obliteration of either of the radiopaque white (cortical) lines of the inferior alveolar canal; and (3) diversion or bending of the inferior alveolar canal in the region of the root apices. In addition, the relationship of the roots to the IAN in the panoramic radiographic findings was diagnosed radiographically in accordance with the method reported by Tanaka et al. (Fig. 1).27 In type 1 cases, the canal is superimposed over more than half of the root structure. In type 2 cases, the canal is superimposed over less than half of the root structure. In type 3 cases, the root structure impinges on the superior border of the canal. In type 4 cases, the distance between the root tip and the superior border of the canal is less than 2 mm, and in type 5 cases, the distance between the root tip and the superior border of the canal is more than 2 mm. The relationship of the roots to the IAN was diagnosed by CT with the same
Inferior alveolar nerve injury after extraction
Fig. 1. The classification of the relationship of the roots to the IAN.
method that was used for the panoramic radiographic findings (Fig. 2). In types 1 and 2, the disappearance of cortication of the canal is observed. In type 1, displacement of the nerve contents to curve around the root is observed. In the CT images, the relationship was also classified in terms of the buccolingual position into one of the four following categories: buccal, inferior, lingual, or inter-radicular (Fig. 3).
The following were analyzed and compared by the method reported in our previous study23: preoperative radiographic findings, such as the type of impaction (according to Pell and Gregory28), angulation (Winter classification29), three radiographic signs associated with IANI (listed above), and the classification (types 1–5) according to the method reported by Tanaka et al. (Fig. 1)27; procedure-related
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complications during surgery, such as the observation of the canal, excessive haemorrhage, and bone removal; and demographic factors, including patient age and gender. All factors are listed in Table 1. When the data were introduced into a multiple logistic regression model, the patients were divided according to the relationship of the roots to the IAN (type 1 vs. types 2–5; buccal and inferior vs. lingual and inter-radicular). Patients were similarly divided into two groups based on age (under 25 years and over 25 years), and into three groups based on the surgeon’s seniority (experience of 1–4 years, 5–9 years, and >10 years). In our previous study, we demonstrated the multivariate relationships among various risk factors on panoramic radiographic findings without CT findings and IANI. In this study, we investigated the multivariate relationships among various risk factors (including CT findings) and IANI. Only patients with preoperative sensory deficits were excluded. All patients were specifically questioned about their chin and lip sensitivity. IAN sensory impairment was detected by pin-prick and light touch test. In cases where there was diminution of sensation, a neurological examination assessed the degree of the deficit. The lesion was classified as either dysesthesia (painful sensation triggered by non-noxious stimuli), hypoesthesia (diminished sensation), or anaesthesia (absence of sensation), and was monitored after 10 and 20 days and 1, 3, and 6 months, until total recovery. Statistical analysis
Data collection and statistical analyses were carried out with SPSS 15.0 (SPSS, Chicago, IL, USA) and StatView J-4.5 software (Abacus Concepts, Berkeley, CA, USA). The association of each variable with the presence of IANI was tested by Mann– Whitney U non-parametric test for age and Fisher’s exact test for categorical variables. A P-value of <0.05 was considered statistically significant. All of the variables associated with IANI were introduced into a multiple logistic regression model. Forward stepwise algorithms were used, with the rejection of those variables that did not fit the model significantly. Multivariate odds ratios (ORs) and 95% confidence intervals (CIs) were also calculated for the significant signs. A P-value of <0.05 was considered statistically significant. Results Fig. 2. The classification of the relationship of the roots to the IAN on CT images.
The incidence of IANI at 1 month after surgery was 28 of 440 teeth (6.4%) in 26
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Hasegawa et al. 440 teeth (5.0%). The ratio of IANI in the extraction group with the loss of the white line of the inferior alveolar canal was significantly higher (P < 0.05) than in cases without the loss (Table 1). Similarly, panoramic radiographic evidence of the diversion of the canal was demonstrated in 28 of the 440 teeth (6.4%). The ratio of IANI in the extraction group with the diversion of the canal was significantly higher (P < 0.05) than in the patients without diversion (Table 1). The most common panoramic radiographic sign regarding the relationship of the roots to the IAN was type 1, which was diagnosed in 173 of the 440 teeth (39.3%). The least common panoramic radiographic sign was type 5, which was diagnosed in 26 of the 440 teeth (5.9%). The ratio of IANI in the extraction group with type 1 classification was significantly higher (P < 0.05) than in the patients with other types of classification (Table 1). However, there were no significant differences indicated by the multivariate analysis.
Fig. 3. The location of the IAN relative to the third molar roots on CT images. Radiographic factors: CT
patients. All of the patients with IANI had hypoesthesia. None of the patients had dysesthesia or anaesthesia. All of the patients with IANI received corticosteroids (hydrocortisone, 100 mg) 1 day after undergoing surgery. However, we did not perform a surgical intervention to the IAN canal in any of the cases because the patients with IANI did not wish to undergo such treatment. Demographic factors
The patients with IANI had a mean age of 40.5 12.4 years (range 22–69 years). The patients without IANI had a mean age of 35.4 11.9 years (range 16–71 years). The patients with IANI were significantly older than those without IANI by univariate analysis (P < 0.05) (Table 1). There were no significant differences between the sexes with regards to the presence or absence of IANI (Table 1). Procedure-related factors
IANI developed in three of 71 teeth (4.2%) in patients treated by the surgeons with 1– 4 years of experience, in 14 of 175 teeth (8.0%) in the group treated by surgeons with 5–9 years of experience, and 11 of 194 teeth (5.7%) in the group of patients treated by surgeons with >10 years of experience. The incidence of IANI after extraction by surgeons with 5–9 years of
experience was the highest in the three groups. However, there was no significant difference in the incidence based on surgeon seniority (Table 1). The IAN was exposed in 17 of all 440 teeth (3.9%). Postoperative IANI developed in five of these 17 cases of exposure (29.4%). The ratio of IANI in the extraction groups with nerve exposure was significantly higher (P < 0.05) than in patients without exposure (Table 1). Excessive haemorrhage occurred during the extraction of six of the 440 teeth (1.4%). Postoperative IANI developed in five (83.3%) of these six cases of excessive haemorrhage. The development of IANI in the extraction group with excessive haemorrhage was significantly higher (P < 0.05) than in the cases without excessive haemorrhage (Table 1). Radiographic factors: panoramic radiographs
The most common Winter classification was mesioangular (158 teeth, 35.9%), while the least common Winter classification was distoangular (22 teeth, 5.0%). The most frequent Pell and Gregory classification in the 440 teeth was IIB. In the multivariate analysis, no significant differences were indicated. Panoramic radiographic signs indicating the loss of the white line of the inferior alveolar canal were present in 22 of the
The most common CT radiographic sign regarding the relationship of the roots to the IAN was type 3, which was diagnosed in 127 of the 440 teeth (28.9%). The least common CT radiographic sign was type 2, which was diagnosed in 67 of the 440 teeth (15.2%). The ratio of the difference between the types of panoramic and CT radiographic signs was 275/440 (62.5%) (Table 2). Of the 173 teeth with type 1 panoramic findings, 24 (13.9%) were associated with IANI. Of the 86 teeth with type 1 CT findings, 26 (30.2%) were associated with IANI (Table 3). Of the 440 teeth, 146 (33.2%) IANs were in the buccal position, 195 (44.3%) were in the inferior position, 95 (21.6%) were lingual, and four (0.9%) were in the inter-radicular position (Table 1). The ratio of IANI in the extraction group with a lingual position between the roots was significantly higher (P < 0.05) than that in the group with other positions (Table 1). However, there were no significant differences indicated by a multivariate analysis. Applying the logistic regression model and forward stepwise algorithms, we determined the following to be significant variables: panoramic radiographic signs of the loss of the white line of the inferior alveolar canal or the diversion of the canal, excessive haemorrhage during extraction, and the close relationship of the roots to the IAN on CT examination (Table 4). Patient gender, surgeon seniority, and the Winter
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Inferior alveolar nerve injury after extraction Table 1. Characteristics and incidence rates of radiographic signs and their relationships to IANI after third molar extraction. Variables
IANI present, n (%)
IANI absent, n (%)
Demographic factors Sample size 28 (6.4) 412 (93.6) Gender Male 14 (53.8) 108 (40.1) Female 12 (46.2) 161 (59.9) Age, years Range 22–69 16–71 Mean SD 40.5 12.4 35.4 11.9 Panoramic findings Right vs. left Right 14 (50.0) 199 (48.3) Left 14 (50.0) 213 (51.7) Position Winter’s classification Vertical 4 (14.3) 73 (17.7) Horizontal 9 (32.1) 132 (32.0) Mesioangular 13 (46.4) 145 (35.2) Distoangular 1 (3.6) 21 (5.1) Transverse 1 (3.6) 41 (10.0) Pell and Gregory I 1 (3.6) 104 (25.2) II 16 (57.1) 226 (54.9) III 11 (39.3) 82 (19.9) A 3 (10.7) 120 (29.1) B 19 (67.9) 220 (53.4) C 6 (21.4) 72 (17.5) Extent of root tip–inferior alveolar canal overlap (the method of Tanaka et al.) (Fig. 1) Type 1 24 (85.7) 149 (36.2) Type 2 3 (10.7) 90 (21.8) Type 3 1 (3.6) 109 (26.5) Type 4 0 (0) 38 (9.2) Type 5 0 (0) 26 (6.3) Signs indicating close spatial relationship Loss of the white line Yes 9 (32.1) 13 (3.2) No 19 (67.9) 399 (96.8) Diversion of the canal Yes 11 (39.3) 17 (4.1) No 17 (60.7) 395 (95.9) CT findings Extent of root tip–inferior alveolar canal overlap (the method of Tanaka et al.) (Fig. 3) Type 1 26 (92.9) 60 (14.6) Type 2 2 (7.1) 65 (15.8) Type 3 0 (0) 127 (30.8) Type 4 0 (0) 81 (19.7) Type 5 0 (0) 79 (19.2) Signs indicating close spatial relationship Buccal position 3 (10.7) 143 (34.7) Inferior position 0 (0) 195 (47.3) Lingual position 21 (75.0) 74 (18.0) Inter-radicular position 4 (14.3) 0 (0) Procedure factors Canal observed Yes 5 (17.9) 12 (2.9) No 23 (82.1) 400 (97.1) Excessive haemorrhage Yes 5 (17.9) 1 (0.2) No 23 (82.1) 411 (99.8) Bone removal Yes 28 (100.0) 325 (78.9) No 0 (0) 87 (21.1) Surgeon experience 1–4 years 3 (10.7) 68 (16.5) 5–9 years 14 (50.0) 161 (39.1) >10 years 11 (39.3) 183 (44.4) IANI, inferior alveolar nerve injury; NS, not significant; SD, standard deviation. a Mann–Whitney U-test. b Fisher’s exact test.
P-value
NS <0.05a
NS
NS
<0.05b NS
<0.05b
<0.05b <0.05b
<0.05b
<0.05b
<0.05b <0.05b <0.05b
NS
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Table 2. Results of the comparison of the panoramic and CT findings regarding the extent of root tip–inferior alveolar canal overlap. Panoramic findings Type Type Type Type Type
1 2 3 4 5
CT findings
Ratio of discordance
Type 1
Type 2
Type 3
Type 4
Type 5
69 14 3 0 0
35 18 13 1 0
34 36 46 10 1
22 16 28 11 4
13 9 20 16 21
60.1% 80.6% 58.2% 71.1% 19.2%
(104/173) (75/93) (64/110) (27/38) (5/26)
62.5% (275/440)
Average
CT, computed tomography. The bold type demonstrated the number of the difference between the types of panoramic and CT radiographic signs. Table 3. Relationship between IANI and the extent of root tip–inferior alveolar canal overlap.a Panoramic findings
CT findings Type 1
Type Type Type Type Type
1 2 3 4 5
Total
Type 3
Type 4
Type 5
24/69 (34.8%) 2/14 (14.3%) 0/3 (0%) 0/0 (0%) 0/0 (0%)
0/35 (0%) 1/18 (5.6%) 1/13 (7.7%) 0/1 (0%) 0/0 (0%)
Type 2
0/34 (0%) 0/36 (0%) 0/46 (0%) 0/10 (0%) 0/1 (0%)
0/22 (0%) 0/16 (0%) 0/28 (0%) 0/11 (0%) 0/4 (0%)
0/13 (0%) 0/9 (0%) 0/20 (0%) 0/16 (0%) 0/21 (0%)
24/173 (13.9%) 3/93 (3.2%) 1/110 (0.9%) 0/38 (0%) 0/26 (0%)
26/86 (30.2%)
2/67 (3.0%)
0/127 (0%)
0/81 (0%)
0/79 (0%)
28/440 (6.4%)
CT, computed tomography; IANI, inferior alveolar nerve injury. a Complication present/absent (%).
classification were excluded from this model because these factors were not significant in the univariate analysis. The multivariate adjusted ORs and 95% CIs of the included factors were calculated. The discriminant hit ratio (96.1%) was considered to be excellent in this study. Discussion
Indications for the removal of third molars are pathologic findings and prevention. Whereas the removal of the third molars with pathologic findings is indisputably necessary, the benefit of preventive removal is controversial. In our department, the third molars are removed if their physiological eruption is not expected, in accordance with the general practice in Japan. In other countries such as the UK and the USA, preventive removal is partly rejected because of the immense health care resources that are required. The discussion deals with the question of whether or not preventive removal is more expen-
sive than therapy in the case of symptomatic pathologic findings. However, in general, no consensus guidelines presently exist. IANI is a characteristic complication following the removal of an impacted tooth.1–22 This complication often leaves the patient dissatisfied with the surgery. Predicting IANI before surgical intervention is thus a common desire for the surgeon and the patient. Therefore, a preoperative assessment should be carried out radiologically to identify the proximity of the impacted tooth to the inferior alveolar canal. Many investigators have reported that IANI is associated with mandibular third molar removal.1–5,7–15,18–20 However, few studies to date have evaluated the multivariate relationships among the various risk factors and IANI.11,16,17 We recently demonstrated the multivariate relationships among risk factors and IANI.23 We reported that some radiographic signs were significantly associated with IANI following third molar extrac-
tion.23 However, other reports have indicated that it is only possible to determine the true relationship of the tooth root with the IAN using CT.13,24–26 Therefore, in this study, we investigated the multivariate relationships among the various risk factors and IANI and the difference between preoperative imaging findings on panoramic radiographs and multi-planar reconstruction CT scans. Other studies have reported the incidence of postoperative IANI to be in the range of 0.4–8.4%.1–15 In the present study, the incidence of IANI was 6.4% (28/440 teeth). The ratio of permanent hypoesthesia (over 6 months) was 1.8% (8/440 teeth). The incidence in our current study was higher than that in many other studies, because these impacted lower third molars were considered to have a high risk of damage to the canal during surgery and were thought to need an examination by CT scanning. In our previous study, the ratio of temporary (at 1 month) hypoesthesia was 1.3% (34/2528
Table 4. Results of the multivariate logistic regression analysis of the risk factors of IANI. Variable
P-value
Odds ratio
Excessive haemorrhage The loss of the white line The diversion of the canal The close relationship of the roots to the IAN (type 1 of CT)
0.006 <0.001 0.001 <0.000
99.04 13.56 10.41 43.77
CT, computed tomography; IAN, inferior alveolar nerve; IANI, inferior alveolar nerve injury.
95% confidence interval Lower
Upper
3.83 3.34 2.75 9.27
2560.32 55.09 39.46 206.76
Inferior alveolar nerve injury after extraction
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Table 5. Rates of the different locations of the IAN of the third molars reported in other studies. Author
Cases
Ohman et al., 200631 Kaeppler, 200032 Tantanapornkul et al., 200733 Mahasantipiya et al., 200534 Ito et al., 199435 Tanaka et al., 200027 Hashizume et al., 200436 Maegawa et al., 200325 Present study
90 345 142 202 47 209 68 47 440
Locations (%) Buccal
Inferior
Lingual
Inter-radicular
31.1 53.6 25.4 15.3 55.3 39.2 23.5 51.1 33.2
25.6 6.0 45.1 42.6 36.2 47.4 33.8 19.1 44.3
33.3 13.0 26.1 30.2 2.1 10.0 39.7 25.5 21.6
10.0 26.8 3.5 12.4 6.4 3.3 2.9 4.3 0.9
IAN, inferior alveolar nerve.
teeth), and that of permanent hypoesthesia (over 6 months) was 0.6% (15/2528 teeth).23 In this study, no significant correlations were identified based on a multivariate analysis between age and sex of the patient, or surgeon seniority, and IANI. These results are similar to those of our previous study.23 It appears that IANI has a stronger relationship with procedurerelated and radiographic factors than demographic factors. The incidence of excessive haemorrhage during extraction was noted to be associated with an increase in IANI, both in our present report and in other reports.1,8,9,11,15,18 In this study, the presence of excessive haemorrhage during extraction was associated with a higher risk of IANI, similar to our previous study. In addition, the development of excessive haemorrhage (OR 99.04) was associated with a higher risk of IANI as indicated by a multivariate analysis. However, the identification of the IAN canal can be difficult due to inadequate access to visualize the canal or to haemorrhaging that obscures the canal. Therefore, it is unclear whether haemorrhaging is merely a consequence of a disruption of the IAN canal, or whether it originates from some other source and actually represents the cause of IANI, resulting from compression. For example, IANI may be related to indirect trauma from surgical oedema or to the formation of a haematoma. In the panoramic findings, the loss of the white line of the inferior alveolar canal (OR 13.56) and the diversion of the canal (OR 10.41) were significant risk factors for IANI, similar to other reports.1,6,7,11,16,18 Umar et al. also reported that loss of the radiopaque white lines of the inferior alveolar canal and diversion of the canal were both associated with loss of cortication of the canal on a cone beam CT scan.26 The authors also reported that the two signs were
crucial predictive signs for an increased risk of IANI during third molar extraction. These results are similar to those observed in our study. In almost all Japanese subjects, the buccolingual course of the IAN passes through one-third of the lingual side of the area between the mandibular foramen and the second molar. The course reaches a point about 6 mm away from the apex of the second molar and makes a turn to the buccal side of the mandibular bone.30 Some investigators have reported the rates of the different locations of the IAN of the third molars (Table 5).25,27,31–36 Several of these studies on the course of the mandibular canal reported a predominantly buccal course, although results vary (Table 5). Our results showed that it was buccal in 33.2%, inferior in 44.3%, lingual in 21.6%, and inter-radicular in 0.9% of cases; in this study, the inferior course was the most common, similar to the reports of Maegawa et al.,25 Tantanapornkul et al.,33 and Mahasantipiya et al.34 In fact, in type 5 cases, as determined by the panoramic findings (where the distance between the root tip and the superior border of the canal was more than 2 mm), the findings were similar to the CT findings. However, in type 1–4 cases, many panoramic findings were not consistent with the CT findings (275 teeth; 62.5%). Therefore, panoramic radiography alone did not provide images reliable enough for predicting nerve lesions. These results suggest that panoramic radiographic methods are marred by projection errors. In the panoramic radiographic methods, the vertical magnification factor can be regarded as reliable for practical clinical purposes if the object is properly positioned when performing the radiographs. However, minor differences in magnification factors may also exist when the object is at different distances from the film plane. In this study, of the 173 teeth with type 1 panoramic findings, 24 (13.9%)
were associated with IANI. Of the 86 teeth with type 1 CT findings, 26 (30.2%) teeth were associated with IANI. In fact, a close relationship of the roots with the IAN (type 1) by CT examination, but not by panoramic examination, was identified as a significant variable according to the multivariate analysis. The close relationship of the roots to the IAN, such as type 1 cases by CT findings, was associated with a significantly higher risk (OR 43.77) of IANI. These results indicate that the CT findings may predict the development of IANI more accurately than the panorama findings. Recently, Renton et al. described a radiological feature known as ‘the juxtaapical area’ and suggested that it was predictive of an increased risk of IANI.20 Therefore, we should consider this potential predictive feature in future studies. An enhanced understanding of the anatomical relationship may necessitate an alteration of the surgical approach for removal of the tooth, or allow the surgeon to plan an alternative risk-reducing surgical technique, for example, coronectomy.20,37 However, because of the different socioeconomic conditions in many countries, the high cost of CT scans, and the amount of radiation used for CT, panoramic radiography may be regarded as the acceptable method in preoperative radiologic evaluation. One obvious drawback of CT is the higher dose of radiation that the patient receives compared with conventional radiography. The routine use of CT scanning as a diagnostic method is currently constrained due to the relatively high radiation dose, as well as the time and labour required to complete the procedure, and also due to the resulting cost implications. Therefore, when the panoramic image is suggestive of an intimate relationship between the impacted tooth and the mandibular canal, CT is recommended for further investigation to clarify the three-dimensional relationship between the two structures.
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In conclusion, we successfully demonstrated the multivariate relationships among various risk factors and IANI, and the differences between the preoperative imaging findings on panoramic radiographs and CT images. Many panoramic findings were not consistent with the CT findings (275 teeth; 62.5%). Therefore, panoramic radiography alone does not provide sufficiently reliable images required for predicting nerve lesions. As a result, when the panoramic image is suggestive of an intimate relationship between the impacted tooth and the mandibular canal, CT is recommended for further investigation. Funding
None. Competing interests
None declared. Ethical approval
This was a non-randomized, retrospective (historic) cohort study of patients. Thus, this study was granted exemption from institutional review board approval by our institution. References 1. Kipp DP, Goldstein BH, Weiss Jr WW. Dysesthesia after mandibular third molar surgery: a retrospective study and analysis of 1,377 surgical procedures. J Am Dent Assoc 1980;100:185–92. 2. Rood JP. Lingual split technique. Damage to inferior alveolar and lingual nerves during removal of impacted mandibular third molars. Br Dent J 1983;154:402–3. 3. Sisk AL, Hammer WB, Shelton DW, Joy Jr ED. Complications following removal of impacted third molars: the role of the experience of the surgeon. J Oral Maxillofac Surg 1986;44:855–9. 4. de Boer MP, Raghoebar GM, Stegenga B, Schoen PJ, Boering G. Complications after mandibular third molar extraction. Quintessence Int 1995;26:779–84. 5. Carmichael FA, McGowan DA. Incidence of nerve damage following third molar removal: a West of Scotland Oral Surgery Research Group study. Br J Oral Maxillofac Surg 1992;30:78–82. 6. Miller CS, Nummikoski PV, Barnett DA, Langlais RP. Cross-sectional tomography. A diagnostic technique for determining the buccolingual relationship of impacted mandibular third molars and the inferior alveolar neurovascular bundle. Oral Surg Oral Med Oral Pathol 1990;70:791–7.
7. Rood JP, Shehab BA. The radiological prediction of inferior alveolar nerve injury during third molar surgery. Br J Oral Maxillofac Surg 1990;28:20–5. 8. Valmaseda-Castello´n E, Berini-Ayte´s L, Gay-Escoda C. Inferior alveolar nerve damage after lower third molar surgical extraction: a prospective study of 1117 surgical extractions. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2001;92:377–83. 9. Gu¨licher D, Gerlach KL. Sensory impairment of the lingual and inferior alveolar nerves following removal of impacted mandibular third molars. Int J Oral Maxillofac Surg 2001;30:306–12. 10. Bui CH, Seldin EB, Dodson TB. Types, frequencies, and risk factors for complications after third molar extraction. J Oral Maxillofac Surg 2003;61:1379–89. 11. Szalma J, Lempel E, Jeges S, Szabo´ G, Olasz L. The prognostic value of panoramic radiography of inferior alveolar nerve damage after mandibular third molar removal: retrospective study of 400 cases. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010;109:294–302. 12. Blaeser BF, August MA, Donoff RB, Kaban LB, Dodson TB. Panoramic radiographic risk factors for inferior alveolar nerve injury after third molar extraction. J Oral Maxillofac Surg 2003;61:417–21. 13. Sedaghatfar M, August MA, Dodson TB. Panoramic radiographic findings as predictors of inferior alveolar nerve exposure following third molar extraction. J Oral Maxillofac Surg 2005;63:3–7. 14. Jerjes W, Swinson B, Moles DR, El-Maaytah M, Banu B, Upile T, et al. Permanent sensory nerve impairment following third molar surgery: a prospective study. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006;102:e1–7. 15. Tay AB, Go WS. Effect of exposed inferior alveolar neurovascular bundle during surgical removal of impacted lower third molars. J Oral Maxillofac Surg 2004;62:592–600. 16. Nakagawa Y, Ishii H, Nomura Y, Watanabe NY, Hoshiba D, Kobayashi K, et al. Third molar position: reliability of panoramic radiography. J Oral Maxillofac Surg 2007;65:1303–8. 17. Chuang SK, Perrott DH, Susarla SM, Dodson TB. Age as a risk factor for third molar surgery complications. J Oral Maxillofac Surg 2007;65:1685–92. 18. Howe GL, Poyton HG. Prevention of damage to the inferior dental nerve during the extraction of mandibular third molars. Br Dent J 1960;109:355–63. 19. Rud J. Reevaluation of the lingual split-bone technique for removal of impacted mandibular third molars. J Oral Maxillofac Surg 1984;42:114–7. 20. Renton T, Hankins M, Sproate C, McGurk M. A randomised controlled clinical trial to compare the incidence of injury to the inferior alveolar nerve as a result of coronectomy
21.
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and removal of mandibular third molars. Br J Oral Maxillofac Surg 2005;43:7–12. Rud J. Third molar surgery: relationship of root to mandibular canal and injuries to inferior dental nerve. Tandlaegebladet 1983;87:619–31. Rud J. Third molar surgery: perforation of the inferior dental nerve through the root. Tandlaegebladet 1983;87:659–67. Hasegawa T, Ri S, Umeda M, Komori T. Multivariate relationships among risk factors and hypoesthesia of the lower lip after extraction of the mandibular third molar. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2011;111:e1–7. Monaco G, Montevecchi M, Bonetti GA, Gatto MR, Checchi L. Reliability of panoramic radiography in evaluating the topographic relationship between the mandibular canal and impacted third molars. J Am Dent Assoc 2004;135:312–8. Maegawa H, Sano K, Kitagawa Y, Ogasawara T, Miyauchi K, Sekine J, et al. Preoperative assessment of the relationship between the mandibular third molar and the mandibular canal by axial computed tomography with coronal and sagittal reconstruction. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2003;96:639–46. Umar G, Bryant C, Obisesan O, Rood JP. Correlation of the radiological predictive factors of inferior alveolar nerve injury with cone beam computed tomography findings. Oral Surg 2010;3:72–82. Tanaka T, Murakami K, Kishida T, Itou T, Morita Y, Noikura T. Relation between mandibular third molar and mandibular canal as assessed by three-dimensional computed tomographic reconstruction. J Jpn Oral Maxillofac Surg 2000;46:251–61. Pell GJ, Gregory BT. Impacted mandibular third molars: classification and modified techniques for removal. Dent Diag 1933;39:330–8. Winter GB. Principles of exodontia as applied to the impacted third molar. St. Louis, . p. 41–100. MO: American Medical Books; 1926 Kamijou Y. Oral anatomy. Tokyo: Anatomu; 1997. p. 217–24. Ohman A, Kivija¨rvi K, Blomba¨ck U, Flygare L. Pre-operative radiographic evaluation of lower third molars with computed tomography. Dentomaxillofac Radiol 2006;35:30–5. Kaeppler G. Conventional cross-sectional tomographic evaluation of mandibular third molars. Quintessence Int 2000;31:49–56. Tantanapornkul W, Okouchi K, Fujiwara Y, Yamashiro M, Maruoka Y, Ohbayashi N, et al. A comparative study of cone-beam computed tomography and conventional panoramic radiography in assessing the topographic relationship between the mandibular canal and impacted third molars. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007;103:253–9. Mahasantipiya PM, Savage NW, Monsour PA, Wilson RJ. Narrowing of the inferior
Inferior alveolar nerve injury after extraction dental canal in relation to the lower third molars. Dentomaxillofac Radiol 2005;34:154–63. 35. Ito M, Miyagishima T, Onizuka H, Takagi N. Relation between lower third molar and mandibular canal—preoperative evaluation using CT scan. J Jpn Oral Maxillofac Surg 1994;40:148–54. 36. Hashizume A, Nakagawa Y, Ishii H, Kobayashi K. Observation of positional relation between mandibular third molars and the mandibular canal on limited cone beam
computed tomography. J Jpn Oral Maxillofac Surg 2004;50:1–10. 37. Alessandri Bonetti G, Bendandi M, Laino L, Checchi V, Checchi L. Orthodontic extraction: riskless extraction of impacted lower third molars close to the mandibular canal. J Oral Maxillofac Surg 2007;65: 2580–6.
Address: Takumi Hasegawa Department of Oral and Maxillofacial
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Surgery Kobe University Graduate School of Medicine 7-5-1 Kusunoki-cho Chuo-ku Kobe 650-0017 Japan Tel: +81 78 382 6213; Fax: +81 78 351 6229 E-mail: [email protected]
Clinical Paper Oral Surgery
Risk factors associated with inferior alveolar nerve injury after extraction of the mandibular third molar—a comparative study of preoperative images by panoramic radiography and computed tomography
T. Hasegawa1, S. Ri2, T. Shigeta1, M. Akashi1, Y. Imai1, Y. Kakei1, Y. Shibuya1, T. Komori1 1
Department of Oral and Maxillofacial Surgery, Kobe University Graduate School of Medicine, Kobe, Japan; 2Department of Oral and Maxillofacial Surgery, Kobe Steel Hospital, Kakogawa, Japan
T. Hasegawa, S. Ri, T. Shigeta, M. Akashi, Y. Imai, Y. Kakei, Y. Shibuya, T. Komori: Risk factors associated with inferior alveolar nerve injury after extraction of the mandibular third molar—a comparative study of preoperative images by panoramic radiography and computed tomography. Int. J. Oral Maxillofac. Surg. 2013; 42: 843–851. # 2013 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. In this study we investigated the relationships among the risk factors for inferior alveolar nerve injury (IANI), and the difference between preoperative imaging findings on panoramic radiographs and computed tomography (CT), by univariate and multivariate analyses. We determined the following to be significant variables by multivariate analysis: panoramic radiographic signs, such as the loss of the white line of the inferior alveolar canal or the diversion of the canal; excessive haemorrhage during extraction; and a close relationship of the roots to the IAN (type 1 cases) on CT examination. CT findings of type 1 were associated with a significantly higher risk (odds ratio 43.77) of IANI. In addition, many panoramic findings were not consistent with CT findings (275 of 440 teeth; 62.5%). These results suggest that CT findings may be able to predict the development of IANI more accurately than panoramic findings. Panoramic radiography alone did not provide sufficiently reliable images required for predicting IANI. Therefore, when the panoramic image is suggestive of a close relationship between the impacted tooth and the IAN, CT should be recommended as a means of conducting further investigations.
0901-5027/070843 + 09 $36.00/0
Key words: mandibular third molar; hypoesthesia; inferior alveolar nerve injury; computed tomography; multivariate analysis. Accepted for publication 21 January 2013 Available online 15 March 2013
# 2013 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
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The removal of the mandibular third molars to prevent future problems is widely acknowledged to be useful and is one of the most commonly performed surgical procedures in oral surgery. The surgical removal of these molars may damage the nerve and cause hypoesthesia of the lower lip as an unpleasant postoperative complication.1–22 Many investigators have reported that inferior alveolar nerve injury (IANI) is associated with mandibular third molar removal.1–22 However, there have been few quantitative analyses of the factors contributing to IANI. In addition, other than our recent study,23 only a few studies to date have evaluated the multivariate relationships among the various risk factors and IANI.11,16,17 In our recent study, we reported some radiographic signs (the loss of the white line of the inferior alveolar canal, the diversion of the canal, and a close relationship of the roots to the IAN on panoramic radiographic findings and not on CT findings) to be significantly associated with IANI following third molar extraction.23 Panoramic radiographs are the imaging modality most commonly used by oral and maxillofacial surgeons to view impacted third molars and to estimate the risk of IANI.13 In particular, Rood and Shehab reported that three of seven classic radiographic signs indicate a higher risk of IANI, implicating close proximity of the inferior alveolar nerve (IAN) to the lower third molar.7 However, some investigators have reported that panoramic radiography does not provide sufficiently reliable images to predict nerve lesions.13,24,25 In addition, some reports have indicated that it is only possible to determine the true relationship of the tooth root with the IAN with computed tomography (CT).13,24–26 In fact, axial, coronal, and sagittal CT images all provide surgeons with useful information, and such images are also beneficial for the preoperative planning of the surgical procedure because of the high-resolution quality of medical CT.25 In this study, we investigated the multivariate relationships among the various risk factors of IANI, and the differences between preoperative imaging findings on panoramic radiographs and medical CT images. Patients and methods
This was a non-randomized, retrospective (historic) cohort study of patients. Thus, this study was granted exemption from institutional review board approval. Between April 2006 and March 2010, 2528 surgical removals of mandibular third molars were performed by dentists
at the Department of Oral and Maxillofacial Surgery. A total of 376 (high risk of IANI; types 1, 2, and 3 as described in ‘Study design’ section) of the 2528 teeth were found to be in close association with the mandibular canal on panoramic radiograph, or were expected to be very difficult to extract (for example, in cases with hypertrophic roots and/or dilacerated teeth). Sixty-four (low risk of IANI; types 4 and 5) of the 2528 teeth were opposite to the other teeth that were in close proximity to the IAN. A total of 440 teeth from 295 patients (122 men and 173 women) were included in this study. The patients had a mean age of 36.2 12.2 years (range 16–71 years). Thus the data (total 440 teeth) for this study pertain to the impacted lower third molars considered to have a high level of risk of IANI during surgery. Before surgery, each patient was informed of the necessity to perform a CT examination and the possible complications, including the potential risk of IANI during the procedure. Surgical methods
Every intervention was carried out under local or general anaesthesia. The extraction of 147 teeth (33.4%) in 86 patients was carried out under general anaesthesia; for 293 teeth (66.6%) in 209 patients, the procedure was carried out under local anaesthesia. Sedation was not carried out. Envelope (sulcular) mucoperiosteal flaps were raised for superficial impactions, and triangular flaps were raised for deep impactions, followed by vestibular bone removal. If necessary, bone removal and crown and root sectioning were performed with tungsten fissure burs (Dentsply-Maillefer, Ballaigues, Switzerland). Sockets were irrigated with 20 ml sterile saline solution at room temperature, and exposure of the IAN was checked during and after precisely focused careful suction. A 3–0 silk suture was used to close the wound. An antibiotic and an anti-inflammatory drug were prescribed (usually oral cefcapene pivoxil hydrochloride hydrate 900 mg 3 times daily for 3 days and oral acetaminophen 1200 mg 3 times daily for 3 days). After 7 days, a surgeon removed the suture and explored the wound for closure and recorded any complications such as ecchymosis, trismus, swelling, postoperative bleeding, dry socket, infection, and IANI. Study design
Patients who underwent conventional panoramic radiography (Orthoceph
OC100CT; Yoshida, Tokyo, Japan) were included in the study. The radiation dose was 20–30 mSv. The medical CT images were taken using a High-Speed Advantage CT Scanner (GE Medical Systems-Japan, Tokyo, Japan). The exposure factors were set at 120 kV and 200 mA, and the duration of scanning was 6 s. The slice thickness of contiguous sections was 0.625 mm. Axial planes were set parallel to the occlusal plane, and continuous 0.625-mm slices were taken. Coronal and sagittal images were reconstructed from the raw data. The software program used was Advantage Workstation1 (GE Medical Systems-Japan). The radiation dose was 2.0–3.0 mSv. The primary predictive variable was the presence or absence of one or more panoramic radiographic or CT findings. The panoramic radiographic and CT findings were analyzed independently by the first and second authors. These two authors were blinded independently to the postoperative IANI status. The CT images were presented to the two observers in a randomized order to prevent referral to the panoramic findings. Whenever a disagreement occurred between the two observers’ assessments, a consensus was reached by discussion. At first, the panoramic findings were evaluated for the presence or absence of the following three classic radiographic signs, all of which have been reported as suggestive of a close proximity of the IAN to the lower third molar7: (1) darkening of the root where it crosses the inferior alveolar canal (as described below, this group was classified into types 1 and 2 on panoramic radiographic findings and not on CT findings); (2) interruption or obliteration of either of the radiopaque white (cortical) lines of the inferior alveolar canal; and (3) diversion or bending of the inferior alveolar canal in the region of the root apices. In addition, the relationship of the roots to the IAN in the panoramic radiographic findings was diagnosed radiographically in accordance with the method reported by Tanaka et al. (Fig. 1).27 In type 1 cases, the canal is superimposed over more than half of the root structure. In type 2 cases, the canal is superimposed over less than half of the root structure. In type 3 cases, the root structure impinges on the superior border of the canal. In type 4 cases, the distance between the root tip and the superior border of the canal is less than 2 mm, and in type 5 cases, the distance between the root tip and the superior border of the canal is more than 2 mm. The relationship of the roots to the IAN was diagnosed by CT with the same
Inferior alveolar nerve injury after extraction
Fig. 1. The classification of the relationship of the roots to the IAN.
method that was used for the panoramic radiographic findings (Fig. 2). In types 1 and 2, the disappearance of cortication of the canal is observed. In type 1, displacement of the nerve contents to curve around the root is observed. In the CT images, the relationship was also classified in terms of the buccolingual position into one of the four following categories: buccal, inferior, lingual, or inter-radicular (Fig. 3).
The following were analyzed and compared by the method reported in our previous study23: preoperative radiographic findings, such as the type of impaction (according to Pell and Gregory28), angulation (Winter classification29), three radiographic signs associated with IANI (listed above), and the classification (types 1–5) according to the method reported by Tanaka et al. (Fig. 1)27; procedure-related
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complications during surgery, such as the observation of the canal, excessive haemorrhage, and bone removal; and demographic factors, including patient age and gender. All factors are listed in Table 1. When the data were introduced into a multiple logistic regression model, the patients were divided according to the relationship of the roots to the IAN (type 1 vs. types 2–5; buccal and inferior vs. lingual and inter-radicular). Patients were similarly divided into two groups based on age (under 25 years and over 25 years), and into three groups based on the surgeon’s seniority (experience of 1–4 years, 5–9 years, and >10 years). In our previous study, we demonstrated the multivariate relationships among various risk factors on panoramic radiographic findings without CT findings and IANI. In this study, we investigated the multivariate relationships among various risk factors (including CT findings) and IANI. Only patients with preoperative sensory deficits were excluded. All patients were specifically questioned about their chin and lip sensitivity. IAN sensory impairment was detected by pin-prick and light touch test. In cases where there was diminution of sensation, a neurological examination assessed the degree of the deficit. The lesion was classified as either dysesthesia (painful sensation triggered by non-noxious stimuli), hypoesthesia (diminished sensation), or anaesthesia (absence of sensation), and was monitored after 10 and 20 days and 1, 3, and 6 months, until total recovery. Statistical analysis
Data collection and statistical analyses were carried out with SPSS 15.0 (SPSS, Chicago, IL, USA) and StatView J-4.5 software (Abacus Concepts, Berkeley, CA, USA). The association of each variable with the presence of IANI was tested by Mann– Whitney U non-parametric test for age and Fisher’s exact test for categorical variables. A P-value of <0.05 was considered statistically significant. All of the variables associated with IANI were introduced into a multiple logistic regression model. Forward stepwise algorithms were used, with the rejection of those variables that did not fit the model significantly. Multivariate odds ratios (ORs) and 95% confidence intervals (CIs) were also calculated for the significant signs. A P-value of <0.05 was considered statistically significant. Results Fig. 2. The classification of the relationship of the roots to the IAN on CT images.
The incidence of IANI at 1 month after surgery was 28 of 440 teeth (6.4%) in 26
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Hasegawa et al. 440 teeth (5.0%). The ratio of IANI in the extraction group with the loss of the white line of the inferior alveolar canal was significantly higher (P < 0.05) than in cases without the loss (Table 1). Similarly, panoramic radiographic evidence of the diversion of the canal was demonstrated in 28 of the 440 teeth (6.4%). The ratio of IANI in the extraction group with the diversion of the canal was significantly higher (P < 0.05) than in the patients without diversion (Table 1). The most common panoramic radiographic sign regarding the relationship of the roots to the IAN was type 1, which was diagnosed in 173 of the 440 teeth (39.3%). The least common panoramic radiographic sign was type 5, which was diagnosed in 26 of the 440 teeth (5.9%). The ratio of IANI in the extraction group with type 1 classification was significantly higher (P < 0.05) than in the patients with other types of classification (Table 1). However, there were no significant differences indicated by the multivariate analysis.
Fig. 3. The location of the IAN relative to the third molar roots on CT images. Radiographic factors: CT
patients. All of the patients with IANI had hypoesthesia. None of the patients had dysesthesia or anaesthesia. All of the patients with IANI received corticosteroids (hydrocortisone, 100 mg) 1 day after undergoing surgery. However, we did not perform a surgical intervention to the IAN canal in any of the cases because the patients with IANI did not wish to undergo such treatment. Demographic factors
The patients with IANI had a mean age of 40.5 12.4 years (range 22–69 years). The patients without IANI had a mean age of 35.4 11.9 years (range 16–71 years). The patients with IANI were significantly older than those without IANI by univariate analysis (P < 0.05) (Table 1). There were no significant differences between the sexes with regards to the presence or absence of IANI (Table 1). Procedure-related factors
IANI developed in three of 71 teeth (4.2%) in patients treated by the surgeons with 1– 4 years of experience, in 14 of 175 teeth (8.0%) in the group treated by surgeons with 5–9 years of experience, and 11 of 194 teeth (5.7%) in the group of patients treated by surgeons with >10 years of experience. The incidence of IANI after extraction by surgeons with 5–9 years of
experience was the highest in the three groups. However, there was no significant difference in the incidence based on surgeon seniority (Table 1). The IAN was exposed in 17 of all 440 teeth (3.9%). Postoperative IANI developed in five of these 17 cases of exposure (29.4%). The ratio of IANI in the extraction groups with nerve exposure was significantly higher (P < 0.05) than in patients without exposure (Table 1). Excessive haemorrhage occurred during the extraction of six of the 440 teeth (1.4%). Postoperative IANI developed in five (83.3%) of these six cases of excessive haemorrhage. The development of IANI in the extraction group with excessive haemorrhage was significantly higher (P < 0.05) than in the cases without excessive haemorrhage (Table 1). Radiographic factors: panoramic radiographs
The most common Winter classification was mesioangular (158 teeth, 35.9%), while the least common Winter classification was distoangular (22 teeth, 5.0%). The most frequent Pell and Gregory classification in the 440 teeth was IIB. In the multivariate analysis, no significant differences were indicated. Panoramic radiographic signs indicating the loss of the white line of the inferior alveolar canal were present in 22 of the
The most common CT radiographic sign regarding the relationship of the roots to the IAN was type 3, which was diagnosed in 127 of the 440 teeth (28.9%). The least common CT radiographic sign was type 2, which was diagnosed in 67 of the 440 teeth (15.2%). The ratio of the difference between the types of panoramic and CT radiographic signs was 275/440 (62.5%) (Table 2). Of the 173 teeth with type 1 panoramic findings, 24 (13.9%) were associated with IANI. Of the 86 teeth with type 1 CT findings, 26 (30.2%) were associated with IANI (Table 3). Of the 440 teeth, 146 (33.2%) IANs were in the buccal position, 195 (44.3%) were in the inferior position, 95 (21.6%) were lingual, and four (0.9%) were in the inter-radicular position (Table 1). The ratio of IANI in the extraction group with a lingual position between the roots was significantly higher (P < 0.05) than that in the group with other positions (Table 1). However, there were no significant differences indicated by a multivariate analysis. Applying the logistic regression model and forward stepwise algorithms, we determined the following to be significant variables: panoramic radiographic signs of the loss of the white line of the inferior alveolar canal or the diversion of the canal, excessive haemorrhage during extraction, and the close relationship of the roots to the IAN on CT examination (Table 4). Patient gender, surgeon seniority, and the Winter
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Inferior alveolar nerve injury after extraction Table 1. Characteristics and incidence rates of radiographic signs and their relationships to IANI after third molar extraction. Variables
IANI present, n (%)
IANI absent, n (%)
Demographic factors Sample size 28 (6.4) 412 (93.6) Gender Male 14 (53.8) 108 (40.1) Female 12 (46.2) 161 (59.9) Age, years Range 22–69 16–71 Mean SD 40.5 12.4 35.4 11.9 Panoramic findings Right vs. left Right 14 (50.0) 199 (48.3) Left 14 (50.0) 213 (51.7) Position Winter’s classification Vertical 4 (14.3) 73 (17.7) Horizontal 9 (32.1) 132 (32.0) Mesioangular 13 (46.4) 145 (35.2) Distoangular 1 (3.6) 21 (5.1) Transverse 1 (3.6) 41 (10.0) Pell and Gregory I 1 (3.6) 104 (25.2) II 16 (57.1) 226 (54.9) III 11 (39.3) 82 (19.9) A 3 (10.7) 120 (29.1) B 19 (67.9) 220 (53.4) C 6 (21.4) 72 (17.5) Extent of root tip–inferior alveolar canal overlap (the method of Tanaka et al.) (Fig. 1) Type 1 24 (85.7) 149 (36.2) Type 2 3 (10.7) 90 (21.8) Type 3 1 (3.6) 109 (26.5) Type 4 0 (0) 38 (9.2) Type 5 0 (0) 26 (6.3) Signs indicating close spatial relationship Loss of the white line Yes 9 (32.1) 13 (3.2) No 19 (67.9) 399 (96.8) Diversion of the canal Yes 11 (39.3) 17 (4.1) No 17 (60.7) 395 (95.9) CT findings Extent of root tip–inferior alveolar canal overlap (the method of Tanaka et al.) (Fig. 3) Type 1 26 (92.9) 60 (14.6) Type 2 2 (7.1) 65 (15.8) Type 3 0 (0) 127 (30.8) Type 4 0 (0) 81 (19.7) Type 5 0 (0) 79 (19.2) Signs indicating close spatial relationship Buccal position 3 (10.7) 143 (34.7) Inferior position 0 (0) 195 (47.3) Lingual position 21 (75.0) 74 (18.0) Inter-radicular position 4 (14.3) 0 (0) Procedure factors Canal observed Yes 5 (17.9) 12 (2.9) No 23 (82.1) 400 (97.1) Excessive haemorrhage Yes 5 (17.9) 1 (0.2) No 23 (82.1) 411 (99.8) Bone removal Yes 28 (100.0) 325 (78.9) No 0 (0) 87 (21.1) Surgeon experience 1–4 years 3 (10.7) 68 (16.5) 5–9 years 14 (50.0) 161 (39.1) >10 years 11 (39.3) 183 (44.4) IANI, inferior alveolar nerve injury; NS, not significant; SD, standard deviation. a Mann–Whitney U-test. b Fisher’s exact test.
P-value
NS <0.05a
NS
NS
<0.05b NS
<0.05b
<0.05b <0.05b
<0.05b
<0.05b
<0.05b <0.05b <0.05b
NS
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Table 2. Results of the comparison of the panoramic and CT findings regarding the extent of root tip–inferior alveolar canal overlap. Panoramic findings Type Type Type Type Type
1 2 3 4 5
CT findings
Ratio of discordance
Type 1
Type 2
Type 3
Type 4
Type 5
69 14 3 0 0
35 18 13 1 0
34 36 46 10 1
22 16 28 11 4
13 9 20 16 21
60.1% 80.6% 58.2% 71.1% 19.2%
(104/173) (75/93) (64/110) (27/38) (5/26)
62.5% (275/440)
Average
CT, computed tomography. The bold type demonstrated the number of the difference between the types of panoramic and CT radiographic signs. Table 3. Relationship between IANI and the extent of root tip–inferior alveolar canal overlap.a Panoramic findings
CT findings Type 1
Type Type Type Type Type
1 2 3 4 5
Total
Type 3
Type 4
Type 5
24/69 (34.8%) 2/14 (14.3%) 0/3 (0%) 0/0 (0%) 0/0 (0%)
0/35 (0%) 1/18 (5.6%) 1/13 (7.7%) 0/1 (0%) 0/0 (0%)
Type 2
0/34 (0%) 0/36 (0%) 0/46 (0%) 0/10 (0%) 0/1 (0%)
0/22 (0%) 0/16 (0%) 0/28 (0%) 0/11 (0%) 0/4 (0%)
0/13 (0%) 0/9 (0%) 0/20 (0%) 0/16 (0%) 0/21 (0%)
24/173 (13.9%) 3/93 (3.2%) 1/110 (0.9%) 0/38 (0%) 0/26 (0%)
26/86 (30.2%)
2/67 (3.0%)
0/127 (0%)
0/81 (0%)
0/79 (0%)
28/440 (6.4%)
CT, computed tomography; IANI, inferior alveolar nerve injury. a Complication present/absent (%).
classification were excluded from this model because these factors were not significant in the univariate analysis. The multivariate adjusted ORs and 95% CIs of the included factors were calculated. The discriminant hit ratio (96.1%) was considered to be excellent in this study. Discussion
Indications for the removal of third molars are pathologic findings and prevention. Whereas the removal of the third molars with pathologic findings is indisputably necessary, the benefit of preventive removal is controversial. In our department, the third molars are removed if their physiological eruption is not expected, in accordance with the general practice in Japan. In other countries such as the UK and the USA, preventive removal is partly rejected because of the immense health care resources that are required. The discussion deals with the question of whether or not preventive removal is more expen-
sive than therapy in the case of symptomatic pathologic findings. However, in general, no consensus guidelines presently exist. IANI is a characteristic complication following the removal of an impacted tooth.1–22 This complication often leaves the patient dissatisfied with the surgery. Predicting IANI before surgical intervention is thus a common desire for the surgeon and the patient. Therefore, a preoperative assessment should be carried out radiologically to identify the proximity of the impacted tooth to the inferior alveolar canal. Many investigators have reported that IANI is associated with mandibular third molar removal.1–5,7–15,18–20 However, few studies to date have evaluated the multivariate relationships among the various risk factors and IANI.11,16,17 We recently demonstrated the multivariate relationships among risk factors and IANI.23 We reported that some radiographic signs were significantly associated with IANI following third molar extrac-
tion.23 However, other reports have indicated that it is only possible to determine the true relationship of the tooth root with the IAN using CT.13,24–26 Therefore, in this study, we investigated the multivariate relationships among the various risk factors and IANI and the difference between preoperative imaging findings on panoramic radiographs and multi-planar reconstruction CT scans. Other studies have reported the incidence of postoperative IANI to be in the range of 0.4–8.4%.1–15 In the present study, the incidence of IANI was 6.4% (28/440 teeth). The ratio of permanent hypoesthesia (over 6 months) was 1.8% (8/440 teeth). The incidence in our current study was higher than that in many other studies, because these impacted lower third molars were considered to have a high risk of damage to the canal during surgery and were thought to need an examination by CT scanning. In our previous study, the ratio of temporary (at 1 month) hypoesthesia was 1.3% (34/2528
Table 4. Results of the multivariate logistic regression analysis of the risk factors of IANI. Variable
P-value
Odds ratio
Excessive haemorrhage The loss of the white line The diversion of the canal The close relationship of the roots to the IAN (type 1 of CT)
0.006 <0.001 0.001 <0.000
99.04 13.56 10.41 43.77
CT, computed tomography; IAN, inferior alveolar nerve; IANI, inferior alveolar nerve injury.
95% confidence interval Lower
Upper
3.83 3.34 2.75 9.27
2560.32 55.09 39.46 206.76
Inferior alveolar nerve injury after extraction
849
Table 5. Rates of the different locations of the IAN of the third molars reported in other studies. Author
Cases
Ohman et al., 200631 Kaeppler, 200032 Tantanapornkul et al., 200733 Mahasantipiya et al., 200534 Ito et al., 199435 Tanaka et al., 200027 Hashizume et al., 200436 Maegawa et al., 200325 Present study
90 345 142 202 47 209 68 47 440
Locations (%) Buccal
Inferior
Lingual
Inter-radicular
31.1 53.6 25.4 15.3 55.3 39.2 23.5 51.1 33.2
25.6 6.0 45.1 42.6 36.2 47.4 33.8 19.1 44.3
33.3 13.0 26.1 30.2 2.1 10.0 39.7 25.5 21.6
10.0 26.8 3.5 12.4 6.4 3.3 2.9 4.3 0.9
IAN, inferior alveolar nerve.
teeth), and that of permanent hypoesthesia (over 6 months) was 0.6% (15/2528 teeth).23 In this study, no significant correlations were identified based on a multivariate analysis between age and sex of the patient, or surgeon seniority, and IANI. These results are similar to those of our previous study.23 It appears that IANI has a stronger relationship with procedurerelated and radiographic factors than demographic factors. The incidence of excessive haemorrhage during extraction was noted to be associated with an increase in IANI, both in our present report and in other reports.1,8,9,11,15,18 In this study, the presence of excessive haemorrhage during extraction was associated with a higher risk of IANI, similar to our previous study. In addition, the development of excessive haemorrhage (OR 99.04) was associated with a higher risk of IANI as indicated by a multivariate analysis. However, the identification of the IAN canal can be difficult due to inadequate access to visualize the canal or to haemorrhaging that obscures the canal. Therefore, it is unclear whether haemorrhaging is merely a consequence of a disruption of the IAN canal, or whether it originates from some other source and actually represents the cause of IANI, resulting from compression. For example, IANI may be related to indirect trauma from surgical oedema or to the formation of a haematoma. In the panoramic findings, the loss of the white line of the inferior alveolar canal (OR 13.56) and the diversion of the canal (OR 10.41) were significant risk factors for IANI, similar to other reports.1,6,7,11,16,18 Umar et al. also reported that loss of the radiopaque white lines of the inferior alveolar canal and diversion of the canal were both associated with loss of cortication of the canal on a cone beam CT scan.26 The authors also reported that the two signs were
crucial predictive signs for an increased risk of IANI during third molar extraction. These results are similar to those observed in our study. In almost all Japanese subjects, the buccolingual course of the IAN passes through one-third of the lingual side of the area between the mandibular foramen and the second molar. The course reaches a point about 6 mm away from the apex of the second molar and makes a turn to the buccal side of the mandibular bone.30 Some investigators have reported the rates of the different locations of the IAN of the third molars (Table 5).25,27,31–36 Several of these studies on the course of the mandibular canal reported a predominantly buccal course, although results vary (Table 5). Our results showed that it was buccal in 33.2%, inferior in 44.3%, lingual in 21.6%, and inter-radicular in 0.9% of cases; in this study, the inferior course was the most common, similar to the reports of Maegawa et al.,25 Tantanapornkul et al.,33 and Mahasantipiya et al.34 In fact, in type 5 cases, as determined by the panoramic findings (where the distance between the root tip and the superior border of the canal was more than 2 mm), the findings were similar to the CT findings. However, in type 1–4 cases, many panoramic findings were not consistent with the CT findings (275 teeth; 62.5%). Therefore, panoramic radiography alone did not provide images reliable enough for predicting nerve lesions. These results suggest that panoramic radiographic methods are marred by projection errors. In the panoramic radiographic methods, the vertical magnification factor can be regarded as reliable for practical clinical purposes if the object is properly positioned when performing the radiographs. However, minor differences in magnification factors may also exist when the object is at different distances from the film plane. In this study, of the 173 teeth with type 1 panoramic findings, 24 (13.9%)
were associated with IANI. Of the 86 teeth with type 1 CT findings, 26 (30.2%) teeth were associated with IANI. In fact, a close relationship of the roots with the IAN (type 1) by CT examination, but not by panoramic examination, was identified as a significant variable according to the multivariate analysis. The close relationship of the roots to the IAN, such as type 1 cases by CT findings, was associated with a significantly higher risk (OR 43.77) of IANI. These results indicate that the CT findings may predict the development of IANI more accurately than the panorama findings. Recently, Renton et al. described a radiological feature known as ‘the juxtaapical area’ and suggested that it was predictive of an increased risk of IANI.20 Therefore, we should consider this potential predictive feature in future studies. An enhanced understanding of the anatomical relationship may necessitate an alteration of the surgical approach for removal of the tooth, or allow the surgeon to plan an alternative risk-reducing surgical technique, for example, coronectomy.20,37 However, because of the different socioeconomic conditions in many countries, the high cost of CT scans, and the amount of radiation used for CT, panoramic radiography may be regarded as the acceptable method in preoperative radiologic evaluation. One obvious drawback of CT is the higher dose of radiation that the patient receives compared with conventional radiography. The routine use of CT scanning as a diagnostic method is currently constrained due to the relatively high radiation dose, as well as the time and labour required to complete the procedure, and also due to the resulting cost implications. Therefore, when the panoramic image is suggestive of an intimate relationship between the impacted tooth and the mandibular canal, CT is recommended for further investigation to clarify the three-dimensional relationship between the two structures.
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Hasegawa et al.
In conclusion, we successfully demonstrated the multivariate relationships among various risk factors and IANI, and the differences between the preoperative imaging findings on panoramic radiographs and CT images. Many panoramic findings were not consistent with the CT findings (275 teeth; 62.5%). Therefore, panoramic radiography alone does not provide sufficiently reliable images required for predicting nerve lesions. As a result, when the panoramic image is suggestive of an intimate relationship between the impacted tooth and the mandibular canal, CT is recommended for further investigation. Funding
None. Competing interests
None declared. Ethical approval
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Address: Takumi Hasegawa Department of Oral and Maxillofacial
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Surgery Kobe University Graduate School of Medicine 7-5-1 Kusunoki-cho Chuo-ku Kobe 650-0017 Japan Tel: +81 78 382 6213; Fax: +81 78 351 6229 E-mail: [email protected]