The Etiology of Altered Sensation in the Inferior Alveolar,
Lingual, and Mental Nerves as a Result of Dental Treatment

In a review of 163 consecutive patients referred with trigeminal nerve (inferior alveolar or lingual nerve) involvement following dental treatment, the most common etiology was third-molar removal (87 patients). The second most common cause was an inferior alveolar nerve block injection (34 patients), with a smaller number of endodontic and periodontal complications. Female patients outnumbered male 3.3 to 1. Twenty-seven patients were offered surgical exploration and possible nerve repair surgery; of them, 14 underwent surgery. Forty percent of the patients admitted to being involved in litigation during the time they were undergoing treatment.

There is a perception that nerve involvement as a result of dental treatment is becoming more frequent.¹ Differences in gender and etiology of the condition in patients requesting treatment have also been observed.¹ There appears to be a predominance of female patients requesting reassurance or treatment of nerve involvement; and patients often seem most disabled when they suffer nerve involvement after relatively minor dental treatment, whereas patients with the same nerve involvement as a result of maxillofacial trauma, tumor resection, or major orthognathic surgery rarely appear to present or request treatment.¹ In an effort to clarify and elucidate the factors involved in patients presenting for evaluation and possible management of altered nerve function following dental treatment, the following study was undertaken.

Materials and Methods

This is a prospective study of 163 consecutive patients referred to the Department of Oral and Maxillofacial Surgery at the University of California, San Francisco. These patients were referred for evaluation and management of altered sensation of one or more areas served by the third division of the trigeminal nerve following dental treatment. All patients were seen and evaluated by one surgeon (MAP). The age and gender of the patients were recorded, as was the type of dental treatment being performed, the specialty status of the dentist performing the treatment, and the actual nerve involved. Testing was by pinprick to outline the affected area with a skin marker. The area was then photographed and the degree of involvement tested as previously described² with Von Frey's hairs,³ two-point discrimination, and Minnesota thermal discs⁴ for temperature sensation. This provided a baseline evaluation against which to measure natural progression and identify those patients who might be helped by micro-neurosurgery.

Results

Of the 163 consecutive patients included in the study, 125 were female, and 38 were male. Ages ranged from 21 to 72 years for males (mean 28 years) and from 16 to 83 years for females (mean 28 years). Table 1 shows the specialty of the dentist carrying out the dental treatment that was presumed to be the causative factor and also the type of treatment being performed. General dentists and oral and maxillofacial surgeons made up the majority of the dentists. Tooth removal was the most common etiological factor, but local anesthesia injections alone were the second most common factor. The patients in the "other" group were three patients having surgical pathology removed, which were mainly cystic lesions. Table 2 shows the specialty of the dentist involved and the distribution of the teeth involved when dental extraction was the presumed causative agent. Third molars were the most common teeth involved. Table 3 shows the actual nerve involved (inferior alveolar nerve, lingual nerve, or mental nerve only), the side involved, and the specialty of the dentist involved. There were no cases of long buccal nerve involvement in this particular series of patients. Table 4 shows the nerve involved and the specialty of the dentist for each of the lower third molars (Nos. 17, 32). Tables 5 and 6 show a breakdown of those 34 cases where the nerve involvement is assumed to have arisen from an inferior alveolar nerve block of local anestheticalone. They indicate the nerve involved and the type of local anesthetic used. These are cases where there would appear to be no other possible etiological factor in the causation of nerve involvement apart from a local anesthetic nerve block. In particular, no surgical procedure was carried out that could possibly have caused the nerve involvement. All cases had only simplerestorative treatment carried out.

Table 1 Associated Dental Treatment and Specialty of Dentist in 163 Consecutive Cases of Involvement of the Third Division of the Trigeminal Nerve
Specialty of Dentist	Tooth Removal	Orthognathic Surgery	Endodontic Therapy	Implant	Periodontic Surgery	Other	Local Anesthesia
OMFS	48	8		2		3
General dentist	45		7	3	2		30
Periodontist	2		1	1	4		2
Endodontist			2		1		2

Table 2 Teeth Involved When Dental Extraction Was Related to Nerve Inolvement
	Tooth No. 32	Tooth No. 17	Teeth Nos. 18, 19, or 31
OMFS	24	22	2
General dentist	21	18	6
Periodontist	2

Table 3 The Nerve Involved and the Specialty of the Dentist in 163* Consecutive Cases
	Right			Left
Specialty of Dentist	IAN**	Lingual	Mental	IAN**	Lingual	Mental
OMFS	17	21	0	20	13	0
General dentist	15	30	1	25	25	0
Periodontist	1	2	1	2	3	1
Endodontist	3	0	0	1	0	1
* The total is more than 163 because 19 patients had more than one nerve involved. **Inferior alveolar nerve.

Table 4 analysis of the Type of Dentist Involved, The Nerve Involved, and the tooth Involved When Nerve Involvement is Related to Tooth Extraction*
	Tooth Removed
	Tooth No. 17		Tooth No. 32		Teeth Nos. 18, 19, 31
Specialty of Dentist	IAN*	Lingual	IAN*	Lingual	IAN*	Lingual
OMFS	11	11	10	19	3	1
General dentist	13	11	8	18	3	1
Periodontist	0	1	0	1	0	0
* The total exceeds the number of patients because 16 patients had more than one nerve involved after tooth extraction. ** Inferior alveolar nerve

Table 5 Cases Caused by an Inferior Alveolar Nerve Block
Type of Anesthetic	Number of Cases	Usage by LA by dentists in the U.S.**
Lidocaine 2% with epinephrine 1:100,000	16	62%
Prilocaine 4%	1	13%
Prilocaine 4% with 1/2000,000 epinephrine	11
Mepivicaine 3%	3	23%
Unknown*	3
* In all cases because the dentist had not documented the type of local anesthetic and at least two types were in routine use in the office. ** Personal communication, Astra Corporation, 1998

Table 6 Cases Caused by an Inferior Alveolar Nerve Block
Nerve Involved	Number of Cases
Inferior alveolar	6
Lingual	23
Both	5

Twenty-seven patients were believed to fulfill the current criteria of the Department of Oral and Maxillofacial Surgery to be offered nerve exploration and possible repair. Of these, 14 patients underwent surgery; and, for a variety of reasons, the other 13 did not. Of the 14 who underwent surgery, two obtained good improvement in sensation, seven obtained some improvement, and five obtained no improvement, as assessed by Von Frey's hair, two-point discrimination, and Minnesota thermal discs. Of interest is the fact that at least 40 percent (65 patients) of these 163 patients were involved in some form of litigation. This information was discovered through patients volunteering it, as a result of a subpoena for records, or through depositions.

Discussion

The rate of nerve involvement following some dental procedures has been documented. The incidence of inferior alveolar nerve involvement following third-molar removal has been estimated at from 0.5 percent to 5 percent.^1,5-9 The incidence of lingual nerve involvement following third-molar removal has been quoted as from 0.6 percent to 2 percent.^10-13 The incidence of temporary nerve involvement following bilateral sagittal split osteotomies has been put as high as 80 percent to 100 percent with an incidence of permanent nerve involvement of about 11 percent.^14-16 The vast majority of the patients in this study were not referred nor seen until several months after the causative incident because any transient neuropraxia would have been expected to resolve by that time. In the case of the relatively small group of patients who were seen in the first few weeks following injury, they were all followed up until any neuropraxia would have been expected to resolve so that all the cases in the study may be considered permanent. Estimates have stated that up to 97 percent of inferior alveolar nerve injuries and 83 percent of lingual nerve injuries recover spontaneously.¹ The inferior alveolar nerve may recover more consistently since it is contained within a bony canal, which helps guides regeneration.

The patients seen in this study are likely to be an atypical group of patients since they have been referred to a tertiary care center. They may well not represent the true incidence and spectrum of the problem. In particular, the number of cases resulting from local anesthetic injections alone may be atypical. This was felt to be a very unusual condition, and it was only following a previous publication¹⁷ that the possibly true incidence of this condition has become more apparent. Following the publication of the aforementioned article,¹⁷ the senior author (MAP) has received in excess of 400 telephone calls from dentists around the United States (and some from abroad) describing patients with long-term nerve involvement where the etiological factor could only have been an inferior alveolar nerve block. Those patients were not included in this study, and the only patients included were those who were personally seen and examined by the senior author. Even so, the incidence of this problem would appear to be higher than has previously been supposed. Utilizing the current figures and the approximate numbers applied in the previous article¹⁴ where the number of dentists in the San Francisco Bay Area is known and the approximate usage of local anesthetic and the distribution of inferior alveolar nerve blocks is known, it is estimated that long-term nerve involvement may result from approximately one in every 175,000 inferior alveolar nerve blocks. This is four to five times more frequent than previous numbers have suggested^17-19 and still almost certainly underrepresents the condition. As an example, there are no cases of nerve involvement due to an injection when the dentist was an oral and maxillofacial surgeon since all these patients had surgery that was presumed to be the etiological factor. It is possible that some of the cases where the nerve involvement has been attributed to a surgical procedure may in fact have the local anesthetic injection as the etiological factor; but this could never be determined, and the nerve involvement has been ascribed to the surgical procedure. When the proportion of each local anesthetic used nationally in the United States is considered (Table 5), it does appear that prilocaine may be overrepresented and mepivicaine underrepresented; but the numbers are small and local usage in the San Francisco Bay Area may not mirror national figures.

The difference in the referral rates in females as opposed to males with nerve involvement is hard to explain when it is assumed that the incidence of the condition would not have a gender bias. It is possible that some of the same etiological factors are involved as in temporomandibular joint dysfunction, where the incidence may be equal between the sexes, but females present more frequently to a referral center.²⁰

One interesting figure is the 40 percent increase in lingual nerve involvement in relation to the removal of tooth No. 32 compared with No. 17. This may be a statistical anomaly due to the relative small numbers in the study, but it could also be of clinical importance. Since most dentists are right-handed and, in the United States, both tooth No. 32 and No. 17 are normally taken out from the right side, there are some fundamental differences in removal technique. In removing bone for tooth No. 17, one is basically working forehand, whereas removal of No. 32 involves working backhand for a right-handed surgeon. This may create problems in some cases. Additionally, one should have relatively better visibility for tooth No. 32 as opposed to No. 17 if one is sitting on the same side, and this may produce overconfidence and perhaps a greater tendency to curette the socket aggressively or remove follicular remnants on the lingual side, where the lingual nerve may become involved.

The criteria for surgical exploration and possible nerve repair are evolving. During the time span of this study, the authors' criteria stated that nerve surgery was offered to anyone with less than 30 percent of residual feeling (tested with Von Frey's hairs, two-point discrimination, and Minnesota thermal discs) and to those patients with dysesthesia that was materially affecting their quality of life. With total anesthesia, patients were offered surgery at two to three months, while paresthesia patients were offered surgery at four to six months if there had been no improvement for two months. Dysesthesia patients were offered surgery at two to three months if the condition was not improving. It was believed that 27 patients fulfilled these criteria; but, of these, only 14 underwent surgical exploration and possible repair. In some cases, patients declined because they did not wish to undergo further surgery or did not wish to accept the possible risks and complications. In some cases, there were financial implications that caused patients to decline surgery.

The fact that at least 40 percent of these patients were involved in litigation is of obvious concern. The incidence may have been higher since this figure represents only those patients who spontaneously offered this information or from whom a communication had been received from an attorney. Patients were not actively questioned on this point.

Cases caused by root canal treatment or implant insertion were all inferior alveolar nerve cases where the treatment was performed posterior to the mental foramen. It seems obvious that implant insertion and endodontic treatment carried out posterior to the mental foramen must have adequate preoperative radiographic identification of the inferior alveolar nerve and suitable precautions to avoid its involvement, which in some cases would mean not carrying out the treatment. The eight cases associated with orthognathic surgery were all sagittal split osteotomies. Six were inferior alveolar nerves, and two were lingual nerves. Of the seven cases related to periodontal surgery, five were lingual nerve involvement related to the distal wedge procedure, and two were a mental nerve related to a crown lengthening procedure (one carried out by a periodontist and one by an endodontist). It does appear that some distal wedge techniques in the retromolar area may place the lingual nerve at risk when one considers that in from 15 percent to 20 percent of cases, the lingual nerve may be placed anatomically superiorly^18-20 and thus may not be protected by the lingual plate of bone.

Conclusion

It would appear that permanent nerve involvement to the inferior alveolar and lingual branches of the trigeminal nerve as a result of dental treatment remains a problem. The most common single cause would appear to be associated with third-molar removal; but, among other causes, iatrogenic involvement from an inferior alveolar nerve block appears to be a significantly underrecognized problem. Many cases are not preventable, which raises the issue of informed consent. Although surgical correction is available in some cases, the results are suboptimal.

Authors/

M.A. Pogrel, DDS, MD, is a professor and the chairman of the Department of Oral and Maxillofacial Surgery at the University of California San Francisco School of Dentistry.

Sri Thamby, BDS, MDSc, is chief resident in the Department of Oral and Maxillofacial Surgery at UCSF School of Dentistry.

References/

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To request a printed copy of this article, please contact: M.A. Pogrel DDS, MD,

Department of Oral and Maxillofacial Surgery, UCSF School of Dentistry, 521 Parnassus Avenue, C-522, San Francisco, CA 94143-0440.