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Lingual nerve damage following lower third molar surgery
Trauma; PrepostheticSurgery
Lingual nerve damage following lower third molar surgery
D. A. Mason Department of Oral and Maxillofacial Surgery, St. Luke's Hospital, Bradford, UK
D. A. Mason: Lingual nerve damage following lower third molar surgery. Int. J. Oral Maxillofac. Surg. 1988; 17: 290-294. Abstract. A prospective study was carried out on the effects of surgical, operator, and anatomical variables on the incidence and duration of lingual dysaesthesia after the surgical removal of impacted lower third molars under general anaesthesia. The predictive value of the area of sensory loss in relation to recovery was also studied. 120 instances of dysaesthesia occurred following 1040 operations in 602 patients, an overall incidence of 11.5%. The effects of 22 possible causative factors, analysed initially by the Z2 test, are recorded. Since factors may not be independent, the results were then subjected to logit analysis, revealing highly significant effects from depth of impaction, removal of overhanging distal bone, lingual flap elevation, and operating time. Neither individual operators nor operator seniority had significant effects on the incidence. 6 cases of dysaesthesia failed to recover within 6 months. Where the area of sensory loss was incomplete, recovery occurred within 6 months in all cases.
Standard textbooks of oral surgery usually contain a good deal of information on the occurrence of damage to the inferior alveolar nerve during lower third molar surgery. Few, however, comment on the effects of surgery on the lingual nerve,other than to suggest that it may be at risk in some cases, and that damage, when it occurs, is likely to be due to the careless use of surgical instruments. Some textbooks do not mention this complication at all, which is surprising in view of the proximity of the lingual nerve to the operation site. Although the nerve usually lies on the inner surface of the mandible near the third molar roots, KIESSELBACH& CrI~aaBERLAIrq7 show that it may lie at the level of the alveolar crest or even higher in 17.6% of human cadavers. MACGREGORn notes that very little is known about the incidence of lingual anaesthesia, and suggests that the problems arising from lingual nerve involvement following lower third molar removal have been under-reported and minimized. SCHWARTZ 2° reports the results of a questionnaire sent to members of the American Association of Oral Surgeons. Of 1291 respondents, 860 reported a total of 2817 recorded instances of lingual nerve involvement, of which 418 cases were considered to be permanent. 53 cases resulted in litigation.
Elsewhere in the literature, there are few studies on this complication, and the reported incidence is variable. RUD 18 analyses the results of the split-bone technique in 718 operations for third molar removal. Impairment of lingual sensation occurred in seven patients (1.0%) of which only one had "permanent" dysaesthesia. Van Gool et aF z encountered only one incident of "paraesthesia" of the lingual nerve after 932 operations for third molar removal, 502 of which were described as "surgical removals". In a prospective study of complications associated with third molar surgery, BRUCE ET AL.l reported an incidence of lingual "paraesthesia" of 1.1% in 90 patients. In 1984, RUD 19 carried out a reassessment of the lingual split technique in which an incidence of impaired lingual sensation of 2.0% occurred where the lingual plate had been removed, compared with 3.0% where the plate had been split but retained. These studies involved operations carried out under local anaesthesia. ROOD16 describes a prospective study on 1400 operations carried out under general anaesthesia, in which the lingual split technique had been employed. An incidence of lingual sensory disturbance of 6.6°/'o was found, of which all but one case recovered within six months. Evidence of more severe damage to the lingual nerve is found in studies on
Key words: 3rd molar surgery; lingual nerve. Accepted for publication 18 May t988
microsurgical nerve repair 4, 13. 15 although the authors do not give any indication of the incidence of the complication. MOZSARY&. MIDDLETON14report 18 cases of lingual nerve injury repaired by microsurgical technique, ten of which occurred following lower third molar removal. The present study was a prospective investigation into the occurrence of lingual sensory disturbance following the removal of impacted lower third molars under general anaesthesia. The objects of the study were to investigate the following. (i) The incidence of lingual nerve damage manifest by disturbance in common sensation (dysaesthesia). (ii) The degree of such damage measured by plotting the duration of the dysaesthesia. (iii) The possible effect of different surgical techniques, operator variables and anatomical factors on the incidence and duration of the dysaesthesia. (iv) The possible predictive value of the area of sensory loss in relation to recovery of normal sensation. Material and methods
A pilot investigation on 100 operations was used to establish the range of surgical techniques and methods of assessment. Measurements of depth and angulation of the impac-
Lingual nerve damage tions were made from orthopantomographic radiographs. Although a slight degree of distortion may occur on these films, it has been shown that they are adequate for assessing third molar impactions l°,tT. The possibility of variation in the assessments of depth and angulation between senior operators was studied in the pilot investigation. The degree of variation was not statistically significant. An impacted lower third molar was judged to be at 'ocelusal level' if the mesial and distal cusps were on or within 2 mm. of a line extending back from the occlusal surfaces of the last 3 teeth in functional positions in the mandible, the so-called Curve of Spee. An estimation of depth was made by dropping a perpendicular line from the Curve of Spee to the mesial marginal ridge of the impacted tooth, and depth of impactions were classified as (i) occlusal level. (ii) under 1.0 cm, (iii) 1.0 to 1.5 cm., (iv) over 1.5 cm, Angulations of impaction were classified as vertical, mesio-angular, disto-angular, and horizontal, as described by Howe 5. A further category of "aberrant" impactions consisted of those not readily included in the four main groups, such as inverted teeth and those placed bucco-lingually across the mandible. The surgical techniques used were based on the Standard Operative Plan of KILLEY& KAY8 who describe the commonly used methods of incision, bone removal, elevation of the tooth from the socket, wound toilet and closure. The detection of lingual dysaesthesia was made on the 1st post-operative day, at 1 week, 1 month, 3 months and 6 months after surgery. The method of assessing sensory deficit took the form of a strict protocol of questionning the patient in a systematic way, supplemented by techniques of recording the modalities of common sensation based on Stn~DERLANt)21 and similar to those described by FERDOUSI • MACGREGOR2 Sensory impairment was detected by: (i) light touch, using a wisp of cotton wool, and graded thicknesses of nylon suture thread; (ii) tactile discrimination, using the sharp and blunt ends of a bi-angled probe; (iii) two-point discrimination, using blunt dividers; (iv) Pain awareness, using light pressure from a sharp probe. The method has been reported fully elsewhere 12. Assessments were carried out by the senior operators up to one month after operation, and subsequently by a single observer. This was not a double blind investigation comparing one technique with another, but was an open assessment primarily designed to highlight factors of importance. The possibility of bias thus has to be recognised. The data recorded on the study forms were tabulated for each factor (Table 1) and computer analysed for significant effects. The X2 test was applied to individual factors, but since they were not necessarily independent
Table 1. Data gained from study forms. Factors involving the tooth and related structures side of operation number of sides operated upon angulation of tooth depth of impaction overhanging ramus bone (requiring removal) state of eruption Factors involving surgical technique type of tongue retractor (hand-held or selfretaining) elevation of lingual flap visible damage to tissues on lingual surface of jaw nerve seen at operation method of bone removal (burs, chisel, or both) lingual plate-splitting technique operating time Factors involving personnel operator, and operator seniority assistant, and assistant seniority Patient data name and record number sex
age Lingual dysaesthesia the detection of lingual dysaesthesia the duration of dysaesthesia the area of sensory deficit
a supplementary technique of logit analysis 3 was used to distinguish influential factors. Results
There were 1040 operations for the surgical removal of impacted lower third molars in 602 patients. The age range was 14 to 58 years, with 63.8% of patients aged between 20 and 30 years. Females outnumbered males in a ratio of 1.6: 1. Analysis of the incidence of dysaesthesia recorded by senior operators in the first m o n t h showed no significant difference between operators. The detection of lingual dysaesthesia in the first few days proved difficult because o f problems of access due to swelling and discomfort. A t this stage, reliance had to be placed on the patients' responses to the protocol o f questionning. In subsequent examinations, objective testing was uncomplicated, and there was close correlation in all cases between the patient's description of the deficit and the results o f sensory testing. In m a n y instances loss of sensation was partial, but the area of sensory diminution could be mapped. The most effective objective tests proved to be tactile
291
discrimination and two-point awareness. Lingual dysaesthesia was found in some degree following 120 of the 1040 operations, an incidence of 11.5%. Both lower third molars were removed in 438 individuals, and a single impacted tooth was r e m o v e d in the remaining 164 cases. Dysaesthesia was unilateral in 94 patients, and bilateral in 13 patients. The incidence o f patient involvement was thus 17.8%. In the 164 patients from w h o m a single t o o t h had been removed, this was followed by some degree o f lingual dysaesthesia in 22 patients, an incidence of 13.4%. There was no instance of dysaesthesia occurring on the side from which the tooth had not been removed. X 2 tests applied to the tables of incidence for the factors showed no significant effects relating to the side of operation, the age or sex of the patient, the individual operator, operator seniority, the assistant or assistant seniority, the type of lingual retractor used, the m e t h o d of surgical division of the tooth, or the n u m b e r of sides operated upon. A n a t o m i c a l and surgical factors which had an effect on the incidence of linqual dysaesthesia are shown in Table 2. In general, factors which might be expected to lead to increased surgical difficulty were associated with an increased incidence of dysaesthesia. With regard to depth of impaction, the incidence varied from 4.2% when the tooth was at the occ'lusal level to 24.2% when the mesial marginal ridge was more than 1.5 cms. below the occlusal level. When the crown of the tooth was visible in the m o u t h the incidence was 7.9%, rising to 20.8% when the t o o t h was completely encased in bone. The presence of distal bone requiring removal to disimpact the tooth was associated with an incidence of 20.9%, compared with 9.0% when Table 2. Factors associated with an increased incidence of lingual dysaesthesia; these are very highly significant by the X2 test (p<0.001), apart from angulation of the tooth (t7 < 0.01) Anatomical factors depth of impaction overhanging bone distally state of eruption angulation of tooth Surgical J~tctors lingual flap elevation bone removal lingual plate-splitting operating time
292
Mason
no distal bone was removed. Vertical impactions were followed by lingual dysaesthesia in 4.4% of cases. The incidence was higher after disto-angular (10.2%), mesio-angular (13.7%), and horizontal impactions (15.2%). Aberrant impactions, i.e., those not conforming to the four most common categories, and including bucco-lingual impactions, were followed by an incidence of 15.0% Operators recorded instances where the elevation of a lingual flap proved difficult for any reason, such as dense scar tissue, tearing of the soft tissues during reflection, or unusual haemorrhage from this site. Dysaesthesia occurred in 20.2% of cases where these complications were recorded. Where no difficulties were encountered in elevating the lingual flap, the incidence of dysaesthesia was 16.6%. No bone removal was required in 354 operations, and these were associated with an incidence of lingual dysaesthesia in only 3.4% of cases. The incidence after removal with chisels alone was 14.6% which was not significantly different from that following the use of burs alone (17.1%). Chisels and burs together were generally employed in more difficult cases where one technique proved inadequate, and in these 70 cases the incidence of dysaesthesia was 20.0%. The lingual plate-splitting technique was associated with a higher incidence of lingual dysaesthesia than other techniques (Table 3). Retention of the lingual plate did not have a significant effect on the incidence of lingual dyseasthesia compared with removal of the plate, either before or after elevation of the tooth (Table 4). Operating time was not truly a surgical factor but was taken to represent the degree of difficulty of removal. Where the procedure was completed in less than 5 rain the incidence of lingual dysaesthesia was 4.5%. This increased to 14.3% for operations lasting 5 to 10 min, and 22.6% for operations of 11 to 15 min. There was an insignificant fall to 18.3% for operations lasting over 15 min.
Table 4. Effect of removal of the lingual plate after plate-splitting technique
Total operations
Number with lingual dysaesthesia
(%)
158 64 71
33 12 13
20.9 18.8 18.3
lingual plate retained plate removed before tooth elevated plate removed after tooth elevated Logit analysis for the incidence of lingual dysaesthesia showed that the only factors of significance were depth of impaction, presence of overhanging ramus bone, lingual flap elevation, and operating time. The overall reduction in deviance was 95 on 15 degrees of freedom, which is highly significant by any nominal ~ test.. The duration of dysaesthesia varied from 2 days to the end of the study period at 6 months, and the pattern of recovery is shown in Table 5. Similar tables were set up for each of the factors independently. Application o f z 2 tests in this instance was inappropriate because of the small size of some of the cells. Logit analysis of the figures dichotomised around the three-month follow up point showed only operator seniority to be of significance, there being a slightly higher incidence of prolonged dysaesthesia associated with senior operators (Table 6). 6 months after operation, only 6 instances of dysaesthesia (5.0% of the cases with this complication) had failed to recover completely, although two of these retained only residual areas of partial sensory deficit, amounting to almost complete recovery. The incidence of these more severe nerve injuries was 0.6% of the 1040 operations for third molar removal. These 6 incidents of dysaesthesia occurred in different patients, and the incidence of patient involvement for the most serve category of sensory deficit was 1.0% of the 602 patients in the study. Recovery of sensation according to the area of sensory deficit is shown in Fig. 1. If "tip only" and "lateral border only" are together taken to indicate incomplete nerve damage, and compared with the "composite area" representing involvement of the whole nerve, the results are highly significant (p < 0.01).
Table 3. The effects of the lingual plate-splitting technique on the incidence of lingual dysaesthesia, compared with all other techniques (X2 test, p < 0.001)
technique NOT used lingual plate-splitting technique
Total operations 747 293
Number with lingual dysaesthesia 62 58
(%) 8.3 19.8
Discussion
The incidence of lingual dysaesthesia in this study is higher than that reported in previous investigations. However, it should be noted that all degrees of sensory deficit were recorded here, including transient disturbances which subsided within one week of operation. ROOD16excluded instances of dysaesthesia which subsided within the first 10 days, and if these are excluded from the present investigation the difference in incidence between the two studies is not significant. Apart from ROOD 16, previous investigators have recorded lingual dysaesthesia occurring after operations under local anaesthesia. In the hospitals involved in the present study, local anaesthesia is used for most of the simpler impactions, and it is likely that there is a bias towards more difficult impactions when general anaesthesia is used. Length of operating time, which may be taken as a measure of the degree of operating difficulty, was shown to be a very highly significant factor in the incidence of lingual dysaesthesia. It has been shown 6,9 that instances of transient lingual dysaesthesia may infrequently occur following the administration of endotracheal anaesthesia for non-oral operations. In this study there was no instance of dysaesthesia occurring on the non-operated side where a single lower third molar had been removed. 22 (13.4%) of these 164 patients had dysaesthesia on the operated side. Non-operative factors are thus unlikely to have had a significant effect on the incidence. Although most of the cases of lingual dysaesthesia may be considered trivial in that complete recovery took place within a few weeks, the relatively high incidence of the complication illustrates the vulnerability of the nerve during lower third molar surgery, Whilst the incidence of more severe nerve damage resulting in duration of the dysaesthesia up to 6 months after operation was only 0.6% of operations, or 1% of patients, this figure should be seen in the context of the number of impacted third molars
Lingual nerve damage Table 5. Duration of lingual dysaesthesia
Acknowledgements
% recovering at Instances ofdysaesthesia (total no.) 120
less than 1 week
1 week to 1 month
1 month to 3 months
3 to 6 months
no. recovery by 6 months
25.0
28.3
32.5
9.2
5.0
Table 6. Effect of operator seniority on duration of lingual dysaesthesia
% recovered by % NOT recovered 3 months by 3 months senior junior
84.3 90.6
16.7 9.7
removed annually, and of the possible medico-legal consequences. No single factor could be identified as causative, but logit analysis showed that the most significant anatomical factor was the depth of the impactiou. Of the surgical factors, only the removal of overhanging distal bone and the elevation of a lingual flap were seen to be highly significant on logit analysis, together with the overall degree of surgical difficulty as measured by the length of the operating time. On the other hand, prediction of lingual dysaesthesia is complicated by the fact that there was no category of any factor in which dysaesthesia did not occur. For example, in studying the effect of bone removal it was found that there was an incidence of lingual dysaesthesia in 3.4% of cases where no bone had been removed. Full recovery took place within 3 m o n t h s in these instances, suggesting that the degree of trauma to the nerve was slight, such as might occur by stretching or compressing the nerve during the reflec-
°/o with Oysoesthesia 100- ~
Tip only
A
Loterol border o
80- i X riO1,020-
1 Week 1 Month
3 Months
5 Months
Fig. 1. Recovery of lingual sensation in re-
lation to area affected. The composite area involves both tip and lateral border, and is essentially the whole of the lingual nerve distribution.
293
tion of the lingual flap or "tenting" of the soft tissues during elevation of the tooth. Nevertheless, the vulnerability of the nerve to damage during surgery is well illustrated in these cases. It might have been expected that the incidence of lingual dysaesthesia for experienced operators would have been less than that for j u n i o r staff, but analysis showed no significant difference between the two groups. Similarly, there was no significant difference in the incidences recorded for individual operators, although the degree of experience of the surgeons was widely variable. There was no pre-selection of cases between operators, but where a senior and a j u n i o r operator were working together it is possible that the more experienced surgeon m a y have elected to undertake the more difficult operations. It should be noted, however, that no operator escaped this complication, the lowest incidence of dysaesthesia for an individual surgeon being 8.5% of operations. In the 120 instances in which lingual dysaesthesia did occur, complete recovery of sensation took place within 6 months in all cases in which sensory deficit affected less than the whole of the lingual distribution. In m a n y of these cases, the degree of nerve damage would be likely to have been neuropraxia, rather than severance or disruption of the nerve trunk. This would certainly apply where recovery took place within 1 month. A more severe degree of axonal damage may have been present in those cases in which recovery took up to three m o n t h s or longer. Where partial lingual sensory function remains it m a y be assumed that the nerve sheath and part of the nerve trunk remains undamaged, and that the prospects for complete recovery are therefore good. Where lingual sensation is lost over the whole of the nerve distribution, the degree of damage may still be slight, and in this study the majority of these cases also recovered within 3 months. In this group, however, early prediction of recovery is more difficult, although partial recovery of some function by one m o n t h was generally followed by complete restoration of sensation subsequently.
My thanks are due to Professor R. L. Turner and Mrs J. Stone for their invaluable help in this study, and to my colleagues in the Bradford and Airedale hospitals for their co-operation in the clinical work. I am also endebted to Drs. A. J. MacGregor and C. M. Hill for their assistance in the preparation of this paper.
References
1. Bruce, R. A., Fredrickson, G. C. & Small, G. S.: Age of patients and raorbidity associated with mandibular third molar surgery. JADA 1980: 101: 240-245. 2. Ferdousi, A. M. & Macgreor, A. J.: The response of the peripheral branches of the trigeminal nerve to trauma. Int. J. Oral Surg. 1985: 14: 41-46. 3. Fienberg, S. E.: The analysis of crossclassified data. MIT Press, 1977: 78107. 4. Hausamen, J. E.: Principles and clinical application of micronerve surgery and nerve transplantation in the maxillofacial area. Ann. Plast. Surg. 1981: 7:428 433. 5. Howe, G. L.: Minor oral surgery, 2nd edition John Wright & Sons Ltd., Bristol, 1971: 93-97. 313. 6. Jones, B. C.: Lingual nerve injury: a complication of intubation. Brit. J. Anaesth. 1971: 43: 730. 7. Kiesselbach, J. E. & Chamberlain, J. G.: Clinical and anatomic variations in the relationship of the lingual nerve to the mandibular third molar region, d. Oral Maxillofac. Surg. 1984: 42: 565-567. 8. Killey, H. C. & Kay, L. W.: The impacted wisdom tooth. Wright, Bristol 1980: 4~9. 9. Loughman, E.: Lingual nerve injury following tracheal intubation. Anaesthesia and Intensive Care 1983: 11: 171. 10. MacGregor, A. J.: The radiological assessment of ectopic lower third molars. Ann. Roy. Coll. Surg. Eng. 1979: 61: 107-113. I I. MacGregor, A. J.: The impacted lower wisdom tooth. Oxford University Press. 1985: 196-197. 12. Mason, D. A.: Lingual nerve damage following third molar removal. M. Phil. thesis, U. Bradford, 1986, 82-88. 13. Merrill, R. G.: Prevention, treatment and prognosis for nerve injury related to the difficult impaction. Dent. Clin. N. Amer. 1979: 23: 471~488. 14. Mozsary, P. G. & Middleton, R. A.: Microsurgical reconstruction of the lingual nerve. J. Oral. Maxillofac. Surg. 1984: 42: 415-420. 15. Mozsary, P. G., Middleton, R. A., Szabo, Z. & Machado, L.: Experimental evaluation of microsurgical repair of the lin-
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gual nerve. J. Oral. Maxillofac. Surg. 1982: 40: 329-331. 16. Rood, J. E: Lingual split technique. Brit. Dent. J. 1983: 154: 402-403. 17. Rowse, C. W.: Notes on the interpretation of the orthopantomogram. Brit. Dent. J. 1971: 130: 425-434. 18. Rud, J.: The split-bone technique for removal of impacted mandibular third molars. J. Oral. Surg. 1970: 28: 416-421. 19. Rud, J.: Re-evaluation of the lingual
split-bone technique for removal of impacted mandibular third molars. J, Oral. MaxilloJac. Surg. 1984: 42:114-117. 20. Schwartz, L. J.: Lingual anaesthesia following mandibular odontectomy. J. Oral. Surg. 1973: 31: 918-920. 21. Sunderland, S.: Nerves and nerve injuries, Churchill Livingstone, Edinburgh, 1978: 2nd edition: 351-355. 22. Van Gool, A. V., Ten Bosch, J. J. & Boering, G.: Clinical consequences of corn-
plaints and complications after removal of the mandibular third molars. Int. J. Oral. Surg. 1977: 6: 29-37. Address: D. A. Mason Department of Oral and Maxillofacial Surgery St. Luke's Hospital Bradford England
Lingual nerve damage following lower third molar surgery
D. A. Mason Department of Oral and Maxillofacial Surgery, St. Luke's Hospital, Bradford, UK
D. A. Mason: Lingual nerve damage following lower third molar surgery. Int. J. Oral Maxillofac. Surg. 1988; 17: 290-294. Abstract. A prospective study was carried out on the effects of surgical, operator, and anatomical variables on the incidence and duration of lingual dysaesthesia after the surgical removal of impacted lower third molars under general anaesthesia. The predictive value of the area of sensory loss in relation to recovery was also studied. 120 instances of dysaesthesia occurred following 1040 operations in 602 patients, an overall incidence of 11.5%. The effects of 22 possible causative factors, analysed initially by the Z2 test, are recorded. Since factors may not be independent, the results were then subjected to logit analysis, revealing highly significant effects from depth of impaction, removal of overhanging distal bone, lingual flap elevation, and operating time. Neither individual operators nor operator seniority had significant effects on the incidence. 6 cases of dysaesthesia failed to recover within 6 months. Where the area of sensory loss was incomplete, recovery occurred within 6 months in all cases.
Standard textbooks of oral surgery usually contain a good deal of information on the occurrence of damage to the inferior alveolar nerve during lower third molar surgery. Few, however, comment on the effects of surgery on the lingual nerve,other than to suggest that it may be at risk in some cases, and that damage, when it occurs, is likely to be due to the careless use of surgical instruments. Some textbooks do not mention this complication at all, which is surprising in view of the proximity of the lingual nerve to the operation site. Although the nerve usually lies on the inner surface of the mandible near the third molar roots, KIESSELBACH& CrI~aaBERLAIrq7 show that it may lie at the level of the alveolar crest or even higher in 17.6% of human cadavers. MACGREGORn notes that very little is known about the incidence of lingual anaesthesia, and suggests that the problems arising from lingual nerve involvement following lower third molar removal have been under-reported and minimized. SCHWARTZ 2° reports the results of a questionnaire sent to members of the American Association of Oral Surgeons. Of 1291 respondents, 860 reported a total of 2817 recorded instances of lingual nerve involvement, of which 418 cases were considered to be permanent. 53 cases resulted in litigation.
Elsewhere in the literature, there are few studies on this complication, and the reported incidence is variable. RUD 18 analyses the results of the split-bone technique in 718 operations for third molar removal. Impairment of lingual sensation occurred in seven patients (1.0%) of which only one had "permanent" dysaesthesia. Van Gool et aF z encountered only one incident of "paraesthesia" of the lingual nerve after 932 operations for third molar removal, 502 of which were described as "surgical removals". In a prospective study of complications associated with third molar surgery, BRUCE ET AL.l reported an incidence of lingual "paraesthesia" of 1.1% in 90 patients. In 1984, RUD 19 carried out a reassessment of the lingual split technique in which an incidence of impaired lingual sensation of 2.0% occurred where the lingual plate had been removed, compared with 3.0% where the plate had been split but retained. These studies involved operations carried out under local anaesthesia. ROOD16 describes a prospective study on 1400 operations carried out under general anaesthesia, in which the lingual split technique had been employed. An incidence of lingual sensory disturbance of 6.6°/'o was found, of which all but one case recovered within six months. Evidence of more severe damage to the lingual nerve is found in studies on
Key words: 3rd molar surgery; lingual nerve. Accepted for publication 18 May t988
microsurgical nerve repair 4, 13. 15 although the authors do not give any indication of the incidence of the complication. MOZSARY&. MIDDLETON14report 18 cases of lingual nerve injury repaired by microsurgical technique, ten of which occurred following lower third molar removal. The present study was a prospective investigation into the occurrence of lingual sensory disturbance following the removal of impacted lower third molars under general anaesthesia. The objects of the study were to investigate the following. (i) The incidence of lingual nerve damage manifest by disturbance in common sensation (dysaesthesia). (ii) The degree of such damage measured by plotting the duration of the dysaesthesia. (iii) The possible effect of different surgical techniques, operator variables and anatomical factors on the incidence and duration of the dysaesthesia. (iv) The possible predictive value of the area of sensory loss in relation to recovery of normal sensation. Material and methods
A pilot investigation on 100 operations was used to establish the range of surgical techniques and methods of assessment. Measurements of depth and angulation of the impac-
Lingual nerve damage tions were made from orthopantomographic radiographs. Although a slight degree of distortion may occur on these films, it has been shown that they are adequate for assessing third molar impactions l°,tT. The possibility of variation in the assessments of depth and angulation between senior operators was studied in the pilot investigation. The degree of variation was not statistically significant. An impacted lower third molar was judged to be at 'ocelusal level' if the mesial and distal cusps were on or within 2 mm. of a line extending back from the occlusal surfaces of the last 3 teeth in functional positions in the mandible, the so-called Curve of Spee. An estimation of depth was made by dropping a perpendicular line from the Curve of Spee to the mesial marginal ridge of the impacted tooth, and depth of impactions were classified as (i) occlusal level. (ii) under 1.0 cm, (iii) 1.0 to 1.5 cm., (iv) over 1.5 cm, Angulations of impaction were classified as vertical, mesio-angular, disto-angular, and horizontal, as described by Howe 5. A further category of "aberrant" impactions consisted of those not readily included in the four main groups, such as inverted teeth and those placed bucco-lingually across the mandible. The surgical techniques used were based on the Standard Operative Plan of KILLEY& KAY8 who describe the commonly used methods of incision, bone removal, elevation of the tooth from the socket, wound toilet and closure. The detection of lingual dysaesthesia was made on the 1st post-operative day, at 1 week, 1 month, 3 months and 6 months after surgery. The method of assessing sensory deficit took the form of a strict protocol of questionning the patient in a systematic way, supplemented by techniques of recording the modalities of common sensation based on Stn~DERLANt)21 and similar to those described by FERDOUSI • MACGREGOR2 Sensory impairment was detected by: (i) light touch, using a wisp of cotton wool, and graded thicknesses of nylon suture thread; (ii) tactile discrimination, using the sharp and blunt ends of a bi-angled probe; (iii) two-point discrimination, using blunt dividers; (iv) Pain awareness, using light pressure from a sharp probe. The method has been reported fully elsewhere 12. Assessments were carried out by the senior operators up to one month after operation, and subsequently by a single observer. This was not a double blind investigation comparing one technique with another, but was an open assessment primarily designed to highlight factors of importance. The possibility of bias thus has to be recognised. The data recorded on the study forms were tabulated for each factor (Table 1) and computer analysed for significant effects. The X2 test was applied to individual factors, but since they were not necessarily independent
Table 1. Data gained from study forms. Factors involving the tooth and related structures side of operation number of sides operated upon angulation of tooth depth of impaction overhanging ramus bone (requiring removal) state of eruption Factors involving surgical technique type of tongue retractor (hand-held or selfretaining) elevation of lingual flap visible damage to tissues on lingual surface of jaw nerve seen at operation method of bone removal (burs, chisel, or both) lingual plate-splitting technique operating time Factors involving personnel operator, and operator seniority assistant, and assistant seniority Patient data name and record number sex
age Lingual dysaesthesia the detection of lingual dysaesthesia the duration of dysaesthesia the area of sensory deficit
a supplementary technique of logit analysis 3 was used to distinguish influential factors. Results
There were 1040 operations for the surgical removal of impacted lower third molars in 602 patients. The age range was 14 to 58 years, with 63.8% of patients aged between 20 and 30 years. Females outnumbered males in a ratio of 1.6: 1. Analysis of the incidence of dysaesthesia recorded by senior operators in the first m o n t h showed no significant difference between operators. The detection of lingual dysaesthesia in the first few days proved difficult because o f problems of access due to swelling and discomfort. A t this stage, reliance had to be placed on the patients' responses to the protocol o f questionning. In subsequent examinations, objective testing was uncomplicated, and there was close correlation in all cases between the patient's description of the deficit and the results o f sensory testing. In m a n y instances loss of sensation was partial, but the area of sensory diminution could be mapped. The most effective objective tests proved to be tactile
291
discrimination and two-point awareness. Lingual dysaesthesia was found in some degree following 120 of the 1040 operations, an incidence of 11.5%. Both lower third molars were removed in 438 individuals, and a single impacted tooth was r e m o v e d in the remaining 164 cases. Dysaesthesia was unilateral in 94 patients, and bilateral in 13 patients. The incidence o f patient involvement was thus 17.8%. In the 164 patients from w h o m a single t o o t h had been removed, this was followed by some degree o f lingual dysaesthesia in 22 patients, an incidence of 13.4%. There was no instance of dysaesthesia occurring on the side from which the tooth had not been removed. X 2 tests applied to the tables of incidence for the factors showed no significant effects relating to the side of operation, the age or sex of the patient, the individual operator, operator seniority, the assistant or assistant seniority, the type of lingual retractor used, the m e t h o d of surgical division of the tooth, or the n u m b e r of sides operated upon. A n a t o m i c a l and surgical factors which had an effect on the incidence of linqual dysaesthesia are shown in Table 2. In general, factors which might be expected to lead to increased surgical difficulty were associated with an increased incidence of dysaesthesia. With regard to depth of impaction, the incidence varied from 4.2% when the tooth was at the occ'lusal level to 24.2% when the mesial marginal ridge was more than 1.5 cms. below the occlusal level. When the crown of the tooth was visible in the m o u t h the incidence was 7.9%, rising to 20.8% when the t o o t h was completely encased in bone. The presence of distal bone requiring removal to disimpact the tooth was associated with an incidence of 20.9%, compared with 9.0% when Table 2. Factors associated with an increased incidence of lingual dysaesthesia; these are very highly significant by the X2 test (p<0.001), apart from angulation of the tooth (t7 < 0.01) Anatomical factors depth of impaction overhanging bone distally state of eruption angulation of tooth Surgical J~tctors lingual flap elevation bone removal lingual plate-splitting operating time
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no distal bone was removed. Vertical impactions were followed by lingual dysaesthesia in 4.4% of cases. The incidence was higher after disto-angular (10.2%), mesio-angular (13.7%), and horizontal impactions (15.2%). Aberrant impactions, i.e., those not conforming to the four most common categories, and including bucco-lingual impactions, were followed by an incidence of 15.0% Operators recorded instances where the elevation of a lingual flap proved difficult for any reason, such as dense scar tissue, tearing of the soft tissues during reflection, or unusual haemorrhage from this site. Dysaesthesia occurred in 20.2% of cases where these complications were recorded. Where no difficulties were encountered in elevating the lingual flap, the incidence of dysaesthesia was 16.6%. No bone removal was required in 354 operations, and these were associated with an incidence of lingual dysaesthesia in only 3.4% of cases. The incidence after removal with chisels alone was 14.6% which was not significantly different from that following the use of burs alone (17.1%). Chisels and burs together were generally employed in more difficult cases where one technique proved inadequate, and in these 70 cases the incidence of dysaesthesia was 20.0%. The lingual plate-splitting technique was associated with a higher incidence of lingual dysaesthesia than other techniques (Table 3). Retention of the lingual plate did not have a significant effect on the incidence of lingual dyseasthesia compared with removal of the plate, either before or after elevation of the tooth (Table 4). Operating time was not truly a surgical factor but was taken to represent the degree of difficulty of removal. Where the procedure was completed in less than 5 rain the incidence of lingual dysaesthesia was 4.5%. This increased to 14.3% for operations lasting 5 to 10 min, and 22.6% for operations of 11 to 15 min. There was an insignificant fall to 18.3% for operations lasting over 15 min.
Table 4. Effect of removal of the lingual plate after plate-splitting technique
Total operations
Number with lingual dysaesthesia
(%)
158 64 71
33 12 13
20.9 18.8 18.3
lingual plate retained plate removed before tooth elevated plate removed after tooth elevated Logit analysis for the incidence of lingual dysaesthesia showed that the only factors of significance were depth of impaction, presence of overhanging ramus bone, lingual flap elevation, and operating time. The overall reduction in deviance was 95 on 15 degrees of freedom, which is highly significant by any nominal ~ test.. The duration of dysaesthesia varied from 2 days to the end of the study period at 6 months, and the pattern of recovery is shown in Table 5. Similar tables were set up for each of the factors independently. Application o f z 2 tests in this instance was inappropriate because of the small size of some of the cells. Logit analysis of the figures dichotomised around the three-month follow up point showed only operator seniority to be of significance, there being a slightly higher incidence of prolonged dysaesthesia associated with senior operators (Table 6). 6 months after operation, only 6 instances of dysaesthesia (5.0% of the cases with this complication) had failed to recover completely, although two of these retained only residual areas of partial sensory deficit, amounting to almost complete recovery. The incidence of these more severe nerve injuries was 0.6% of the 1040 operations for third molar removal. These 6 incidents of dysaesthesia occurred in different patients, and the incidence of patient involvement for the most serve category of sensory deficit was 1.0% of the 602 patients in the study. Recovery of sensation according to the area of sensory deficit is shown in Fig. 1. If "tip only" and "lateral border only" are together taken to indicate incomplete nerve damage, and compared with the "composite area" representing involvement of the whole nerve, the results are highly significant (p < 0.01).
Table 3. The effects of the lingual plate-splitting technique on the incidence of lingual dysaesthesia, compared with all other techniques (X2 test, p < 0.001)
technique NOT used lingual plate-splitting technique
Total operations 747 293
Number with lingual dysaesthesia 62 58
(%) 8.3 19.8
Discussion
The incidence of lingual dysaesthesia in this study is higher than that reported in previous investigations. However, it should be noted that all degrees of sensory deficit were recorded here, including transient disturbances which subsided within one week of operation. ROOD16excluded instances of dysaesthesia which subsided within the first 10 days, and if these are excluded from the present investigation the difference in incidence between the two studies is not significant. Apart from ROOD 16, previous investigators have recorded lingual dysaesthesia occurring after operations under local anaesthesia. In the hospitals involved in the present study, local anaesthesia is used for most of the simpler impactions, and it is likely that there is a bias towards more difficult impactions when general anaesthesia is used. Length of operating time, which may be taken as a measure of the degree of operating difficulty, was shown to be a very highly significant factor in the incidence of lingual dysaesthesia. It has been shown 6,9 that instances of transient lingual dysaesthesia may infrequently occur following the administration of endotracheal anaesthesia for non-oral operations. In this study there was no instance of dysaesthesia occurring on the non-operated side where a single lower third molar had been removed. 22 (13.4%) of these 164 patients had dysaesthesia on the operated side. Non-operative factors are thus unlikely to have had a significant effect on the incidence. Although most of the cases of lingual dysaesthesia may be considered trivial in that complete recovery took place within a few weeks, the relatively high incidence of the complication illustrates the vulnerability of the nerve during lower third molar surgery, Whilst the incidence of more severe nerve damage resulting in duration of the dysaesthesia up to 6 months after operation was only 0.6% of operations, or 1% of patients, this figure should be seen in the context of the number of impacted third molars
Lingual nerve damage Table 5. Duration of lingual dysaesthesia
Acknowledgements
% recovering at Instances ofdysaesthesia (total no.) 120
less than 1 week
1 week to 1 month
1 month to 3 months
3 to 6 months
no. recovery by 6 months
25.0
28.3
32.5
9.2
5.0
Table 6. Effect of operator seniority on duration of lingual dysaesthesia
% recovered by % NOT recovered 3 months by 3 months senior junior
84.3 90.6
16.7 9.7
removed annually, and of the possible medico-legal consequences. No single factor could be identified as causative, but logit analysis showed that the most significant anatomical factor was the depth of the impactiou. Of the surgical factors, only the removal of overhanging distal bone and the elevation of a lingual flap were seen to be highly significant on logit analysis, together with the overall degree of surgical difficulty as measured by the length of the operating time. On the other hand, prediction of lingual dysaesthesia is complicated by the fact that there was no category of any factor in which dysaesthesia did not occur. For example, in studying the effect of bone removal it was found that there was an incidence of lingual dysaesthesia in 3.4% of cases where no bone had been removed. Full recovery took place within 3 m o n t h s in these instances, suggesting that the degree of trauma to the nerve was slight, such as might occur by stretching or compressing the nerve during the reflec-
°/o with Oysoesthesia 100- ~
Tip only
A
Loterol border o
80- i X riO1,020-
1 Week 1 Month
3 Months
5 Months
Fig. 1. Recovery of lingual sensation in re-
lation to area affected. The composite area involves both tip and lateral border, and is essentially the whole of the lingual nerve distribution.
293
tion of the lingual flap or "tenting" of the soft tissues during elevation of the tooth. Nevertheless, the vulnerability of the nerve to damage during surgery is well illustrated in these cases. It might have been expected that the incidence of lingual dysaesthesia for experienced operators would have been less than that for j u n i o r staff, but analysis showed no significant difference between the two groups. Similarly, there was no significant difference in the incidences recorded for individual operators, although the degree of experience of the surgeons was widely variable. There was no pre-selection of cases between operators, but where a senior and a j u n i o r operator were working together it is possible that the more experienced surgeon m a y have elected to undertake the more difficult operations. It should be noted, however, that no operator escaped this complication, the lowest incidence of dysaesthesia for an individual surgeon being 8.5% of operations. In the 120 instances in which lingual dysaesthesia did occur, complete recovery of sensation took place within 6 months in all cases in which sensory deficit affected less than the whole of the lingual distribution. In m a n y of these cases, the degree of nerve damage would be likely to have been neuropraxia, rather than severance or disruption of the nerve trunk. This would certainly apply where recovery took place within 1 month. A more severe degree of axonal damage may have been present in those cases in which recovery took up to three m o n t h s or longer. Where partial lingual sensory function remains it m a y be assumed that the nerve sheath and part of the nerve trunk remains undamaged, and that the prospects for complete recovery are therefore good. Where lingual sensation is lost over the whole of the nerve distribution, the degree of damage may still be slight, and in this study the majority of these cases also recovered within 3 months. In this group, however, early prediction of recovery is more difficult, although partial recovery of some function by one m o n t h was generally followed by complete restoration of sensation subsequently.
My thanks are due to Professor R. L. Turner and Mrs J. Stone for their invaluable help in this study, and to my colleagues in the Bradford and Airedale hospitals for their co-operation in the clinical work. I am also endebted to Drs. A. J. MacGregor and C. M. Hill for their assistance in the preparation of this paper.
References
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plaints and complications after removal of the mandibular third molars. Int. J. Oral. Surg. 1977: 6: 29-37. Address: D. A. Mason Department of Oral and Maxillofacial Surgery St. Luke's Hospital Bradford England