Abstract
A prospective study of all lower third molar surgery performed in the outpatient extraction clinic of a teaching dental hospital was conducted from January 1998 through October 2005 to determine the incidence of subsequent neurosensory deficit due to inferior alveolar nerve (IAN) and lingual nerve (LN) injury, to examine possible contributing risk factors and to describe the pattern of recovery. 3595 patients were included (61% female, 39% male; age range, 14–82 years). Of the 4338 lower third molar extractions performed by various grades of operators, 0.35% developed IAN deficit and 0.69% developed LN deficit. Distoangular impaction was found to increase the risk of LN deficit significantly (p < 0.001). Depth of impaction was related to the risk of IAN deficit (p < 0.001). Undergraduates caused more LN deficits (p < 0.001). Sex, age, raising of a lingual flap, protection of LN with a retractor, removal of distolingual cortex, tooth sectioning and difficulty in tooth elevation were not significantly related to IAN or LN injury. Postoperative recovery from IAN and LN deficits was noted most significantly at 3 and 6 months, respectively. By the end of the follow-up period, 67% of IAN deficits and 72% of LN deficits had recovered completely.
Third molar tooth extraction is the most common surgical procedure in the oral cavity, and its major complications include postoperative neurosensory deficits. Studies from different countries have reported the incidence of various neurosensory deficits after lower third molar tooth surgery . For example, inferior alveolar nerve (IAN) injury, which usually presents with paresthesia or anesthesia of the lower lip, chin and buccal gingivae on the affected side, has a reported incidence of 0.26–8.4% . Lingual nerve (LN) deficit, which commonly presents with numbness of the ipsilateral anterior two-thirds of the tongue and taste disturbance, has a reported incidence of 0.1–22% . Neurosensory deficits can markedly affect the quality of life of affected patients . Continuous efforts have been made by researchers to investigate the risk factors associated with nerve injuries in lower third molar surgery. Factors such as the age of the patient, depth of tooth impaction, proximity of the roots to the IAN, surgical experience of the operator and surgical technique have been proposed as being associated with an increased risk of IAN injury . Perforation of the lingual plate and the lingual bone split technique have been found to be associated with LN damage. It remains unknown whether elevation of the lingual flap and an attempt to protect the LN by an instrument increase the risk of LN damage .
The aims of this prospective study were to determine the incidences of IAN and LN deficit after surgery for impacted lower third molars in a teaching dental hospital in Hong Kong; to evaluate the risk factors contributing to these postoperative neurosensory deficits; and to examine the pattern of recovery of these neurosensory deficits in affected patients.
Materials and methods
This was a prospective clinical study of all lower third molar surgeries performed in the outpatient extraction clinic of the Discipline of Oral and Maxillofacial Surgery at the Faculty of Dentistry, The University of Hong Kong, between January 1998 and October 2005. The study was approved by the Faculty’s Ethics Committee and patients gave written informed consent. Patients were excluded if they presented with conditions that were associated with the lower third molars, such as cysts and tumors, or with any preexisting neurosensory deficit related to the IAN and LN. The following data were recorded in a set of questionnaires. The preoperative data recorded were: sex, age, type of impaction (mesioangular, horizontal, distoangular or vertical), depth of impaction (by measuring the Winter’s lines from standardized orthopantomogram), operator’s experience and state of eruption of the lower third molars. The intraoperative data recorded were: raising of the lingual flap, use of a periosteal elevator to protect the LN, removal of distolingual cortex, tooth sectioning, root fragment elevation, difficulty with tooth elevation and any intraoperative complications.
20 orthopantomograms were selected randomly and the depth of impaction of the lower third molar was measured by an examiner. A reliability test was run between the original and the examiner’s measurement and was shown to have no statistical difference. The random error was also within acceptable limits.
All patients were reviewed 1 week after surgery, to assess wound healing status and the presence of any neurosensory deficits related to the lower third molar tooth surgery. Self-reported subjective sensory changes were recorded and objective assessments were performed with light touch test (with von Frey fibres), two-point discrimination threshold and pin-pick pain threshold in patients complaining of neurosensory disturbance. They were monitored regularly postoperatively to assess the pattern of recovery after 1 month, 3 months, 6 months, 1 year and 2 years and beyond, according to the standardized assessments.
Data were analyzed with the Statistical Package for Social Sciences (SPSS version 11.5; SPSS Inc, Chicago, IL, USA). The independent t test was used to examine whether cases of LN and IAN deficit differed from cases of uneventful healing depending on mean patient age and depth of impaction. The χ 2 test was used to examine whether the incidence of LN and IAN deficits varied according to the following possible risk factors: sex; type of impaction; experience of operator; whether the lingual flap was raised, a periosteal elevator was used as protection and distolingual cortical bone was removed; whether tooth sectioning was needed; and whether there was any difficulty with tooth elevation. The 5% probability level was taken as the cut-off for statistical significance and a 1% level was taken as the cut-off for highly statistical significance.
Results
Incidence of neurosensory deficits
A total of 3595 patients were enrolled in this study; 61% were female and 39% were male. Their ages ranged from 14 to 82 years (mean 27.2 years). Of the 4338 impacted mandibular third molars that were surgically extracted, 79% (3407) had partially erupted, 16% (673) were unerupted and the remainder (6%; 258) had erupted. The most common type of impaction was mesioangular (48%) and the mean depth of impaction of all types ranged from 3.0 to 6.5 mm ( Table 1 ). In terms of experience of the dental operator, 46% of the extractions were performed by undergraduate dental students; 40% by junior residents; 12% by oral and maxillofacial surgery (OMS) postgraduates; and 2% by OMS specialists.
| Pattern of impaction | Frequency (%) | Mean depth (SD) (mm) |
|---|---|---|
| Mesioangular | 2077 (47.9%) | 5.0 (2.8) |
| Horizontal | 1128 (26.0%) | 6.5 (3.1) |
| Distoangular | 465 (10.7%) | 4.5 (3.1) |
| Vertical | 668 (15.4%) | 3.0 (2.9) |
15 extractions (0.35%) resulted in IAN-related neurosensory deficits and 30 (0.69%) resulted in LN-related neurosensory deficits. The remaining 4293 (99%) extractions did not present with any neurosensory complications.
Risk factors of neurosensory deficits
Sex and age
The incidence of IAN deficit in males and females was 0.36% (6/1672) and 0.34% (9/2665), respectively. There was no statistical difference between the gender of IAN deficit (p = 0.99) The incidence of LN deficit in males and females was 0.84% (14/1672) and 0.60% (16/2665), respectively. There was no statistical difference between the gender of LN deficit (p = 0.66). The mean (SD) ages of patients who showed IAN and LN injury were 28.7 (8.6) years and 27.1 (6.6) years, respectively, and were not significantly different from the mean (SD) age (26.9 years, p = 0.46) of patients who showed uneventful healing (8.4).
Type and depth of impaction
Although the incidence of IAN deficit for each type of impaction ranged from 0.15% for vertical to 0.65% for distoangular, there was no statistical difference in the incidence of IAN deficit according to type of impaction (p = 0.48). The incidence of LN deficit by type of impaction ranged from 0.53% each for mesioangular and horizontal to 2% for distoangular, and the variation in incidence according to impaction type was highly statistically significant (p < 0.001; Table 2 ).
The mean depth of impaction was greater among lower third molar extraction cases leading to IAN deficit than among those that did not lead to IAN deficit (8.9 mm (S.D. 3.5 mm) versus 5.0 mm (S.D. 3.1 mm)); this difference was highly statistically different (p < 0.001). There was no statistical difference in the mean impaction depth among extractions leading to LN deficit and those that did not (5.7 mm (S.D. 3.4 mm) and 5.0 mm (S.D. 3.1 mm), p = 0.44, respectively; Fig. 1 ).
Experience of operators
Most of the LN deficits (77%; 23/30) occurred after surgeries performed by undergraduate students, whereas IAN deficits most commonly (53%; 8/15) occurred after surgeries performed by OMS postgraduates The patterns of incidence of IAN and LN injury differed by operator type with high statistical significance (p < 0.01 and p < 0.001, respectively; Table 3 ).
| Inferior alveolar nerve * | Lingual nerve* | Mean depth of impaction (S.D.) (mm) ** | |
|---|---|---|---|
| Undergraduate | 4 (26.7%) | 23 (76.7%) | 4.4 (2.8) |
| Junior resident | 3 (20.0%) | 3 (10.0%) | 5.0 (3.0) |
| OMS postgraduate | 8 (53.3%) | 3 (10.0%) | 7.4 (3.5) |
| Specialist | 0 | 1 (3.3%) | 4.8 (3.0) |
| Total | 15 (100%) | 30 (100%) |
The mean depth of impacted lower third molars ranged from 4.4 to 7.4 mm and was greatest after extractions that were performed by OMS postgraduates. The impaction depth varied with high significance among the different groups of operators (p < 0.001; Table 3 ). This finding suggests that the increase in incidence of IAN deficits could be related to the deeper impaction of lower third molars that are likely to be extracted by OMS postgraduates, even though they are surgically better trained than the undergraduates and junior residents.
Raising of lingual flap and lingual nerve protection
A lingual flap was raised during 33% (1427/4338) of surgeries, and an attempt was made to protect lingual tissue with a periosteal elevator during 80% (3447/4338) of surgeries. Of the operations involving a raised lingual flap, 0.91% (13/1427) led to postoperative LN deficits. 0.58% (17/2911) extraction cases of postoperative LN deficit were reported among the operations in which a lingual flap was not raised, but these two proportions were not significantly different (p = 0.58). Of the operations in which a periosteal elevator was placed lingually, 0.73% (25/3447) led to postoperative LN deficits, which was not statistically significant (p = 0.60) from the proportion of operations without periosteal elevator placement that resulted in postoperative LN deficits (0.56%; 5/891).
Removal of distolingual cortex
In about one-fifth (20%; 876/4338) of extractions, the distolingual cortex was removed by bone guttering. LN deficit occurred in 0.91% (8/876) of the surgeries with distolingual cortex removed, while LN deficit occurred in 0.64% (22/3462) without distolingual cortex removed. There was no significant difference in the incidences of LN deficit between the distolingual cortex removal and the preservation groups (p = 0.38).
Tooth sectioning
Of the 4338 operations, 2667 (62%) required tooth sectioning. Those cases most commonly involved mesioangular (55%; 1475/2667) and horizontal (39%; 1032/2667) impaction. The incidences of IAN deficit in groups with and without tooth sectioning were 0.30% (8/2667) and 0.42% (7/1671), respectively. The incidences of LN deficit in groups with and without tooth sectioning were 0.57% (15/2667) and 0.91% (15/1671), respectively. Proportions of extractions that did and did not use tooth sectioning were not significantly different for either the LN (p = 0.20) or IAN groups (p = 0.52).
Difficulty with tooth elevation
Difficult tooth elevations were reported by the operators in 726 (17% of 4338) procedures, of which 9 presented with LN deficit (1%) and 2 (0.28%) presented with IAN deficit postoperatively. Operations not encountering difficulty in tooth elevation had 21 LN deficits (0.58%) and 13 IAN deficits (0.36%) postoperatively. The incidences in LN and IAN deficit in surgeries encountering tooth elevation did not differ significantly when compared with surgeries without difficulty in tooth elevation (p = 0.051 and p = 0.72 respectively).
Recovery patterns
Inferior alveolar nerve
The 15 patients with unilateral IAN deficit after lower third molar surgery were reviewed postoperatively at 1 week and at 1, 3, 6, 12 and 24 months or beyond until resolution of the symptoms. No patients dropped out during the follow-up period, which lasted from 32 to 70 months (mean 49 months). By the end of follow-up, 10 of the 15 patients (67%) were deemed to have experienced total recovery ( Table 4 ). Almost half of the patients had recovered by 3 months, and most of those who showed complete recovery had done so by 1 year (60%; 9/15); only one more patient recovered between 1 and 2 years, and no more patients showed a full recovery after 2 years ( Fig. 2 ). The 5 patients who presented with incomplete recovery or persistent numbness beyond the 2 year review were considered to have permanent IAN damage. In this study, the rate of permanent neurosensory deficit of the IAN was 0.12%.
| No improvement | Some improvement | Total recovery (%) | |
|---|---|---|---|
| 1 month | 2/15 | 11/15 | 2/15 (13.0%) |
| 3 months | 1/15 | 7/15 | 7/15 (46.7%) |
| 6 months | 2/15 | 5/15 | 8/15 (53.3%) |
| 1 year | 3/15 | 3/15 | 9/15 (60.0%) |
| 2 years | 5/15 | 0 | 10/15 (66.7%) |
| >2 years | 5/15 | 0 | 10/15 (66.7%) |
Lingual nerve
Four of the 30 patients who had unilateral LN deficit after third molar surgery did not return for any follow-up after the first postoperative review; the drop-out rate was 13%. The follow-up duration ranged from 32 to 72 months (mean 56 months). One patient underwent LN exploration and re-anastomosis in the thirteenth postoperative month and was excluded from analysis after the 1-year follow-up. Total recovery from the LN deficit was achieved in most patients (58%; 15/26) within the first 6 postoperative months and in 18 patients at the end of the review period. Seven patients presented with incomplete recovery or persistent numbness at or beyond the 2-year review ( Table 5 ). The rate of permanent neurosensory deficit of the LN was 0.16%.
| No improvement | Some improvement | Total recovery (%) | |
|---|---|---|---|
| 1 month | 5/26 | 16/26 | 5/26 (19.2%) |
| 3 months | 5/26 | 13/26 | 8/26 (30.8%) |
| 6 months | 7/26 | 4/26 | 15/26 (57.7%) |
| 1 year | 8/26 | 1/26 | 17/26 (65.4%) |
| 2 years | 7/25 * | 0 | 18/25 (72.0%) |
| >2 years | 7/25 | 0 | 18/25 (72.0%) |
* 1 patient underwent re-anastomosis of the lingual nerve in the thirteenth postoperative month and was excluded from further analysis.
Results
Incidence of neurosensory deficits
A total of 3595 patients were enrolled in this study; 61% were female and 39% were male. Their ages ranged from 14 to 82 years (mean 27.2 years). Of the 4338 impacted mandibular third molars that were surgically extracted, 79% (3407) had partially erupted, 16% (673) were unerupted and the remainder (6%; 258) had erupted. The most common type of impaction was mesioangular (48%) and the mean depth of impaction of all types ranged from 3.0 to 6.5 mm ( Table 1 ). In terms of experience of the dental operator, 46% of the extractions were performed by undergraduate dental students; 40% by junior residents; 12% by oral and maxillofacial surgery (OMS) postgraduates; and 2% by OMS specialists.
| Pattern of impaction | Frequency (%) | Mean depth (SD) (mm) |
|---|---|---|
| Mesioangular | 2077 (47.9%) | 5.0 (2.8) |
| Horizontal | 1128 (26.0%) | 6.5 (3.1) |
| Distoangular | 465 (10.7%) | 4.5 (3.1) |
| Vertical | 668 (15.4%) | 3.0 (2.9) |
15 extractions (0.35%) resulted in IAN-related neurosensory deficits and 30 (0.69%) resulted in LN-related neurosensory deficits. The remaining 4293 (99%) extractions did not present with any neurosensory complications.
Risk factors of neurosensory deficits
Sex and age
The incidence of IAN deficit in males and females was 0.36% (6/1672) and 0.34% (9/2665), respectively. There was no statistical difference between the gender of IAN deficit (p = 0.99) The incidence of LN deficit in males and females was 0.84% (14/1672) and 0.60% (16/2665), respectively. There was no statistical difference between the gender of LN deficit (p = 0.66). The mean (SD) ages of patients who showed IAN and LN injury were 28.7 (8.6) years and 27.1 (6.6) years, respectively, and were not significantly different from the mean (SD) age (26.9 years, p = 0.46) of patients who showed uneventful healing (8.4).
Type and depth of impaction
Although the incidence of IAN deficit for each type of impaction ranged from 0.15% for vertical to 0.65% for distoangular, there was no statistical difference in the incidence of IAN deficit according to type of impaction (p = 0.48). The incidence of LN deficit by type of impaction ranged from 0.53% each for mesioangular and horizontal to 2% for distoangular, and the variation in incidence according to impaction type was highly statistically significant (p < 0.001; Table 2 ).
