Microsurgical repair of nerves injured during third molar surgery* Robert H. B. Jones, MDS, FRACDS, FRACDS (OMS)?
Key words: Inferior dental nerve, lingual nerve, function, injury review, prevention of injury, surgical repair. Abstract The inferior dental and lingual nerves are in close proximity to the mandibular third molar. In approximately 5 per cent of cases of the surgical removal of this tooth one or other of these nerves may be injured. In most cases the injury is reversible but in 1 per cent a distressing dysaesthesia may occur. The diagnosis of the sensory deficit is made by repeated sensory testing. If there is no improvement by six to nine months then complete spontaneous recovery will not occur. These irreversible nerve injuries can be successfully treated microsurgically. (Received for publicationApril 199 1. Accepted July 1991.)
Introduction It has been consistently reported that in at least 5 per cent of all surgical extractions of mandibular third molars there is an injury to either the inferior dental or lingual nerves. In most cases this is temporary and normal sensation returns within a few weeks or months. However in about 1 per cent of cases the patient is left with a permanent and frequently distressing sensory loss.’-6 Management of these post-surgical nerve injuries in the past has been passive, with reassurance to the patient that the sensory loss will spontaneously return. This passive management is usually ‘Based on a paper presented at the Inaugural Meeting, Section of Oral and Maxillofacial Surgery, Royal Australasian College of Dental Surgeons, February 1990. ?Senior Lecturer (half-time), Oral and Maxillofacial Surgery Unit, The University of Adelaide. Specialist oral and maxillofacial surgeon, Adelaide. Australian Dental Journal 1992;37(4):253-61.
continued for a long time as nerves recover slowly for up to two years. As the majority of injuries spontaneously heal, this approach is satisfactory in most cases. It is, however, devastating for the patient with dysaesthesia to be eventually told that nothing can be done and that they have a permanent problem. Not surprisingly, and maybe justifiably, the patient will consider litigation, both for the initial injury and for the subsequent inactivity.’ This paper reviews the current state of knowledge of nerve injuries and the application of microsurgical techniques to their repair. In particular, it proposes that management of nerve injury following third molar surgery should be early and intensive and that active microsurgical intervention should be considered.
Review of nerve injury Anatomy The surgical anatomy of the inferior dental and lingual nerves is demonstrated in Fig. 1 and 2. The inferior dental nerve is usually in close relationship to the roots of the mandibular third molar and this can be assessed radiographically. The key features of an intimate relationship are an apparent narrowing of the canal, loss of cortical outline and increased radiolucency over the root8 (Fig. 3). These features are not inclusive however and nerve injury may occur when there is apparently little relationship between the roots and the mandibular canal. The inferior dental nerve can also be injured by needle penetration during block anaesthesia, molar endodontic instrumentation or filling past the apex, or by surgery in the premolar region. The result of inferior dental nerve injury is loss of sensation to the mandibular teeth, gingiva and the lower lip. The lingual nerve is in close proximity to the lingual plate of the mandible covering the third 253
Fig. 1. -Anatomical dissection showing inferior dental nerve (I); lingual nerve (L).
Fig. 2.-Anatomical dissection showing inferior dental nerve (I); lingual nerve (L); facial artery (F). 254
Australian Dental Journal 1992;37:4.
Fig. 3.-Periapical radiograph of mandibular third molar with inferior dental nerve in close proximity.
Fig. 4. -Transverse section of mandibular nerve showing axons (A); perineurium (P); epineurium (E); blood vessel (B). H&E. x 20.
molar. Its position is quite variable vertically, being at any point from the level of the gingival crevice to that of the mylohyoid m ~ s c l e Thus . ~ the nerve is vulnerable to any surgery involving the lingual periosteum in the posterior mandible, whether it be third molar removal or mylohyoid reduction in the edentulous patient. It may also be damaged in lingual block anaesthesia. Australian Dental Journal 1992;37:4.
The result of injury to this nerve is loss of sensation and taste to the anterior twethirds of the tongue. The lingual nerve usually also supplies the lingual gingivae but this may be supplied by the mylohyoid nerve.'O The general structure of a peripheral nerve in transverse section is shown in Fig. 4. The epineurium is the outer connective tissue sheath and is impor255
Table 1. Classification of nerve injury ~
Neuropraxia
Grade 1
Axonotrnesis
Grade 2 Grade 1 Grade 2
Neurotmesis
Grade 1 Grade 2
Functional conduction block (ischaemia and oederna) Dernyelination (no axon loss) Axon degeneration (Wallerian degeneration) Axon degeneration and endoneural disruption Perineural disruption (a closed nerve injury) Nerve division (epineurium disrupted)
tant for support and protection. The perineurium is a thinner connective tissue sheath which surrounds each individual fascicle or group of nerve fibres. The perineurium provides elasticity to the nerve and is important in maintaining the integrity of the nerve when it is stretched. The number of fascicles within a nerve varies, with a range of six to eighteen in the mandibular nerve" and one to seven in the lingual nerve. Each nerve fibre is surrounded by the endoneurium which has both protective and electrical insulation functions. The nerve fibres vary from large myelinated to small myelinated and unmyelinated fibres. An essential and frequently overlooked aspect of intraneural anatomy is the blood supply. Within the epineurium there is an arteriole and venous plexus with branches penetrating both the perineurium and endoneurium to form an intrafascicular network. The relationship of these vessels to the nerve is the same as the blood-brain barrier and thus is selectively impermeable. Interference with the blood supply rapidly results in areas of anoxia and is an essential component of nerve injury and its capacity to repair.I2 N e m e injury The classification of nerve injury is presented in Table l . l 3 - I 5 It is essential to understand that this classification is based primarily on histological findings and clinically may only be applied retrospectively after the nerve response to injury is known. At the time of injury it is at best, using the operative microscope, possible only to diagnose a neurotmesis grade 2. In all other instances the nerve appears intact from the outside. Neuropraxia and axonotmesis injuries rapidly spontaneously recover; with neurotmesis, spontaneous recovery may occur but this will be slow, take up to two years and may be incomplete. Factors which influence the extent of repairs are: 256
1. The amount of damage to the proximal end of the nerve When a nerve fibre is transected there is retrograde degeneration for a short distance at the site of injury but more importantly nerve cell death within the trigeminal ganglion. Thus when a nerve is transected there is considerable loss of ganglion 2. The amount of damage at the distal end of the nerve When a nerve fibre is transected there is degeneration from the site of injury to the sensory receptor. The endoneural tubes shrink to about 70 per cent of their initial diameter within a few months of injury. This shrinkage is reversible however if new axons re-enter the tube.'* New axons may also produce new endoneural tubes or grow down arterioles, venules or even implanted silastic tubes.19
3. The site of injury The nature and extent of injury is probably the most important factor in recovery. A crush injury will result in more extensive damage than a sharply transected nerve. Increased fibrosis, infection, foreign bodies and a large discontinuity will all adversely affect the chances of recovery. The closer the site of injury to the periphery the better the chance of recovery. Thus the transected mental nerve is more likely to recover than an injured inferior dental nerve at the third molar site. The inferior dental nerve is more likely to recover due to its containment within the mandibular canal, while the lingual nerve is loosely tied in the floor of the mouth and is subject to muscle movement.
4. The nature of the nerve Mixed nerves of motor, sensory and sympathetic fibres are less likely to recover than when all fibres are of similar type. Thus mandibular nerves, which are almost entirely sensory, are more likely to recover than the lingual nerve which consists of sensory fibres and sympathetic fibres from the chorda tympani. Nerve function Sensory nerve testing can be divided into:" 1. Mechanoceptive This relates to the ability to determine touchpressure sensations which are largely mediated by larger myelinated fibres. Methods of mechanoreceptor testing are two-point discrimination, static light touch and directional brush stroke. Australian Dental Journal 1992;37:4.
2. Nociceptive This is the ability to discriminate between different types of stimuli which are largely mediated by small myelinated and unmyelinated fibres. This is tested by pin prick and thermal discrimination.
Table 2. Indications for surgery 1. Known transection of the nerve
Ifthe nerve is seen to be transected or there was profuse operative haemorrhage from the mandibular canal with profound anaesthesia post-operatively, then early exploration and repair is indicated. 2. Hyperaesthesia for some time without improvement
3. Proprioceptive This tests the perception of position. If fascicular cross-over has occurred in recovery then the patient will interpret touch in one position in the nerve distribution as occurring elsewhere in the nerve distribution. This assumes greater clinical importance in completely transected nerves particularly in long-standing injury and in older patients. Somatosensory-evoked potential recording using electrophysiological equipment is an objective and sensitive method of assessing sensory nerve function.’l It does, however, require extensive equipment and expertise as compared with clinical testing. Clinical aspects
Prevention Prevention of nerve injury is clearly an essential aspect of pre-operative and operative technique. Adequate radiographs (Fig. 3) must be carefully evaluated for the relation of the roots to the mandibular canal. Essential points of operative technique are tooth sectioning and elevation of roots away from the nerve. It is not possible to pre-operatively evaluate the position of the lingual nerve although usually these are bilaterally symmetrical. Operatively one can either keep the approach entirely to the buccal without disturbing the lingual tissues at all or carefblly elevate the lingual mucoperiosteum and place a retractor to protect the nerve from all cutting and elevating instruments.” On completion of tooth removal the operative site should be carefully examined to see if the inferior dental or lingual nerves have been exposed. If so they should be carefully examined, preferably with magnification, for evidence of disruption. If a nerve is severed then either seek immediate specialist advice or at least align the two ends. Any bone fragments compressing the inferior dental nerve should be gently removed from around the nerve. Post-operative evaluation At the first post-operative visit patients should be specifically asked if they have any areas of numbness. If so the area of sensory loss must be defined using the methods outlined earlier. The area of Australian Dental Journal 1992;37:4.
Hyperaesthesia is an unpleasant feeling of burning, stinging, or frank pain. It is made worse by stimulation. 3. Anaesthesia for six months without improvement Anaesthesia is complete lack of sensation. Pin-prick testing to the point of bleeding will be felt as blunt pressure.
4. Hypoaesthesia for at least six months without improvement and troublesome to the patient Hypoaesthesia is a relative lack of sensation with reduced discrimination to all tests as compared with the uninjured side.
sensory loss must be recorded in the notes using either an accurate anatomical description or, preferably, by diagram. The nature of the possible injury and probable spontaneous recovery should be explained to the patient. At this stage reassurance and passive observation is appropriate. A further appointment should be made for not more than one month. At one month the clinical evaluation should be repeated and recorded. If there is no sign of recovery or a partial dysaesthesia then specialist referral should be considered and foreshadowed to the patient. The patient should then be reviewed monthly and if there are no signs of recovery by six months the patient must be referred for specialist evaluation. There is evidence that a nerve injury which has not shown clear signs of recovery by six to nine months is unlikely to recover and surgical intervention is indicated. Surgical repair
Indications for surgery The indications for surgery are presented in Table 2. The timing of surgery is partly dependent on the symptoms, the degree of distress to the patient, and the surgeon’s assessment of the injury. Generally surgery is best performed at six to nine months following injury as this allows time for assessment of recovery. Transection and troublesome hypoaesthesia can be operated earlier than this and hypoaesthesia which does not greatly trouble the patient can be left longer. 257
Fig. 5.-Dissection of the inferior dental nerve (I) exposed by a sagittal split osteotomy (0)ofthe mandible. Sucker tip (S) retracting the nerve.
Fig. 6.-Dissection ofthe lingual nerve (L) through operating microscope, x 10 power. Alveolus mandible (A); tongue (T).
The realization and effects of central degeneration following peripheral injury and the ramifications of the problems which stem from it would indicate early rather than late repair. However, there are now reliable new diagnostic tests which will allow the determination of the extent of the problem and act as guidelines for intervention. 258
Surgical access dental neyOe
The inferior dental nerve is best approached intraorally Using a modified sagittal split osteotomy. (Fig. 5). The osteotomy cuts are in the conventional position for a mandibular advancement procedure Australian Dental Journal 1992;37:4.
Fig. 7. -Hand instrumentation required for microsurgery.
with the exception that the anterior vertical cut is level with the first molar. Decortication to the mental foramen will allow further exposure of the nerve if required. The osteotomy is carehlly opened ensuring that the nerve is on the distal segment and is not hrther damaged by the osteotomy. The proximal segment is rotated laterally with no muscle stripping. Meticulous haemostasis is obtained prior to commencing the microsurgical procedure. On completion of the microsurgical phase, bone is removed from the inner aspect of the proximal fragment so that the nerve is not compressed when the osteotomy is reduced. Fixation is gained using three rigid futation screws and intermaxillary fixation is not required. The inferior dental nerve may also be exposed via an extra-oral submandibular approach. This certainly gives wide access and has been advocated by some surgeons without orthognathic expertise. It does, however, leave a large unaesthetic scar in the neck and bleeding from the exposed marrow can be troublesome. There is also the disastrous risk of damaging the marginal mandibular branch of the facial nerve and thus ending up with a paralysed as well as numb lip.
Lingual nerve The lingual nerve is approached intraorally by one of two exposures. The direct approach is by an incision along the anterior edge of the vertical ramus along the lingual gingival crevice to the first molar. The lingual mucoperiosteum is widely elevated and held superiorly with stay sutures. The nerve is visualized through the periosteum, the periosteum incised parallel to the nerve and the nerve exteriorized by blunt dissection. Australian Dental Journal 1992;37:4.
Alternatively the lingual nerve is exposed via an anterior approach through the floor of the mouth. The nerve is identified as it crosses under the submandibular duct and followed posteriorly by blunt dissection (Fig. 6). This approach is essential if an interpositional nerve graft is required. Bleeding from the many veins in the floor of the mouth can be troublesome, with access difficulty, particularly if the microscope is to be.used. Microsurgical procedure The minimum equipment required is shown in Fig. 7. The first step is to thoroughly examine the exposed nerve using the operating microscope. The nerve is manipulated with fine microsurgical forceps and a magnification of 10 x is preferred. The usual options are an intact nerve with surrounding adhesions, a transected nerve or a neuromatous enlargement. Any adhesions to the surrounding bone, periosteum or scar tissue are severed, using microsurgical scissors, until the nerve is completely free. The epineurium is then lifted and incised parallel to the long axis of the nerve. The perineurium of each individual fascicle is exposed and all the nerve bundles separated from each other. Each bundle is examined for evidence of adhesion, transection or neuroma. If all fascicles appear normal then the nerve has been satisfactorily microsurgically decompressed. If the nerve either in total or individual fascicles has been transe ed then the epineurium is removed for at least 1 m T from the injury. The perineural tubes at the proximal end are then examined and 259
resected until axons are seen to ooze from the cut ends. The oozing axons are removed until no further oozing occurs. The distal end of the nerve is then treated in a similar fashion except that axons do not ooze from the distal end. The perineurium of the two ends is then gently approximated and sutured with a 10/0 nylon suture. Four sutures, one in each quadrant, are placed. If the perineural ends do not approximate with minimal tension, an interposition graft is indicated. If examination shows an enlargement ofthe nerve tissue the condition is most likely a neuroma. These may be either lateral, in which case they can be excised, leaving the nerve fascicles intact, or central. Central neuromas require resection and the resulting gap usually requires an interpositional graft. The donor site for interposition grafts is either the sural nerve in the leg if a long graft is required, or the great auricular nerve in the neck. Nerve preparation and suturing is similar to that for direct approximation. The results of microsurgical repair of the inferior dental and lingual nerves has been reported in a number of studie~.’~-’~ Similar results have been obtained by the author.
Discussion This paper reviews the essential aspects of the basic science of nerve injury following third molar removal and presents an approach for active evaluation and microsurgical repair of irreversibly injured nerves. It can be argued that as the incidence of nerve injury is low, about 5 per cent, and that as most recover anyway, a passive approach is satisfactory. However, removal of impacted third molars is a common procedure so that there is an unacceptably large number of patients with irreversibly injured nerves. For example, there are currently 500 000 people aged between 17 to 19 years in Australia at present; if only 10 per cent have their third molars removed there will be a possible 500 nerve injuries which do not spontaneously resolve. There is also increasing medico-legal activity following nerve injury.6 Certainly, if patients have been given full information so that they can give informed consent, and if the procedure is performed using good pre-operative evaluation and careful technique, then any medico-legal challenges are defensible. However, if the nerves are injured, with less than rigorous follow-up evaluation, particularly in cases where early microsurgery could be of value, then there is increasing risk of successful medicolegal action. The author is aware of a number of such cases where patients have obtained out-of-court settlement in the order of $25 000 for their 260
distressing and now treatable complaint. It is in the interests of both the patient and practitioner that these injuries are adequately assessed and treated with early referral to an appropriate specialist oral and maxillofacial surgeon with further training in microsurgery. There have been a number of training courses conducted in Australia, either for surgical specialists of all types or specifically for oral and maxillofacial surgeons. Following the introductory course the surgeon must practise on a regular basis on laboratory animals to maintain these skills and then use them on a reasonable number of patients. Besides the training and practice, the essential requirements for microsurgery are time, patience and extreme sensitivity with the tissues. There are oral and maxillofacial surgeons with such skills in most capital centres.
Acknowledgements The author wishes to thank Grant Townsend, Associate Professor in Oral Anatomy, for Fig. 1 and 2; and Professor Alastair GOSS,Head, Oral and Maxillofacial Surgery Unit, for help with the manuscript.
References 1. Goldberg MH, Nemarick AN, Marco WP. Complications after mandibular third molar surgery. A statistical analysis of 500 consecutive procedures in private practice. J Am Dent ASSOC1985;111:277-9. 2. Osborn TP, Frederickson G Jr, Small IA, Rogerson TS. A prospective study of complications related to the mandibular third molar. J Oral Maxillofac Surg 1985;43:767-9. 3. Van Goo1 AV, Fosch JJ, Boering G. Clinical consequences of complaints and complications after removal of the mandibular third molar. Int J Oral Surg 1977;6:29-37. 4. Alling CC. Dysaesthesia ofthe lingual and inferior alveolar nerves following third molar surgery. J Oral Maxillofac Surg 1986;44:454-7. 5. Kipp DP. Dysaesthesia after mandibular third molar surgery. A retrospectivestudy and analysis of 1377 surgical procedures. J Am Dent Assoc 1980;10:185-92. 6. Blackburn CW, Bramley PA. Lingual nerve damage associated with the removal of lower third molars. Br Dent J 1989;167: 103-7. 7. Haskell R. Medicolegal consequencesof extracting lower third molar teeth. Medical Protection Society Annual Reports and Accounts 1986;94:51-2. 8. Rood JP, Shehab BA. The radiological prediction of inferior alveolar injury during third molar surgery. Br J Oral Maxillofac Surg 1990;28:20-5. 9. Kiesselbach JE, Chamberlain JG. Clinical and anatomical observations on the relationship of the lingual nerve to the mandibular third molar region. J Oral Maxillofac Surg 1989;42:565-7. 10. Tier GA. Sensory nerve supply to the tongue. A clinical reappraisal. Br Dent J 1984;157:354-6. Australian Dental Journal 1992;37:4.
11. Svane TJ, Wolford LM, Milam SB, et al. Fascicular characteristics of the human inferior alveolar nerve. J Oral Maxillofac Surg 1986;44:431-4. 12. Lundborg G. Structureand function ofthe intra-neural vessels as related to trauma oedema formation and nerve function. J Bone Joint Surg 1975;57A:938-48. 13. Sunderland S. Nerve and nerve injuries. 2nd edn. London: Churchill Livingstone, 1978. 14. Seddon HJ. Three types ofnerve injury. Brain 1943;66:237. 15. Rood JP. Degree of injury to the inferior alveolar nerve sustained during the removal of impacted third molars by means of the lingual split technique. Br J Oral Surg 1983;21: 103-16. 16. Aldskogius H, Ardmudsson J. Nerve cell degeneration and death in the trigeminal ganglion of the adult rat following peripheral nerve transection. J Neurocytol 1978;7:229-50. 17. D’Rozario RH. The central effects ofperipheral nerve injury on the trigeminal nerve ofthe rat. Adelaide, Australia: The University of Adelaide, 1985. MDS thesis. 18. McObrien B, Morrison W.Reconstructive microsurgery. London: Churchill Livingstone, 1987: 137-40. 19. Lundborg G, Hansson H. Nerve regeneration through preformed pseudosynovial tubes. J Hand Surg 1980;5:35-8. 20. Ghalli GE, Epker B. Clinical neurosensory testing. Practical applications. J Oral Maxillofac Surg 1989;47: 1074-8. 2 1. Ghalli GE, Jones DL, Wolford LM. Somatosensory evoked potential assessment of the inferior alveolar nerve following third molar extraction. Int J Oral Maxillofac Surg 1990;19: 18-21.
Australian Dental Journal 1992;37:4.
22. Stacy G. Lingual exposure during mandibular third molar surgery. Int J Oral Surg 1977;6:334-9. 23. Mozsary PG, Syers C. Microsurgical correction ofthe injured inferior alveolar nerve. J Oral Maxillofac Surg 1985;43:353-8. 24. Mozsary PG, Middleton RA. Microsurgical reconstruction ofthe lingual nerve. J Oral Maxillofac Surg 1984;42:415-20. 25. Hausaman JE, Schmidseder D. Repair of the mandibular nerve by means of autologous nerve grafting after resection of the lower jaw. J Maxillofac Surg 1975;1:74-8. 26. Wessberg GA, Epker BN. Transoral inferior alveolar neurorrhaphy via a sagittally split mandible. J Maxillofac Surg 1982;IO: 173-6. 27. Wessberg GA, Wolford LM, Epker BN. Experiences with microsurgical reconstmction of the inferior alveolar nerve. J Oral Maxillofac Surg 1982;40:651-5. 28. Hiroyasi N, Takashi K, Genydu Y,Kenichi S. Repair of the mandibular nerve by autologous grafting after partial resection ofthe mandible. J Oral Maxillofac Surg 1986;44:31-6. 29. Donoff RB, Guralnick W. The application of microsurgery to oral neurological problems. J Oral Maxillofac Surg 1982;40: 156-9.
Address for correspondence/reprints: Oral and Maxillofacial Surgery Unit, Department of Dentistry, The University of Adelaide, GPO Box 498, Adelaide, South Australia, 5000.
26 1
Key words: Inferior dental nerve, lingual nerve, function, injury review, prevention of injury, surgical repair. Abstract The inferior dental and lingual nerves are in close proximity to the mandibular third molar. In approximately 5 per cent of cases of the surgical removal of this tooth one or other of these nerves may be injured. In most cases the injury is reversible but in 1 per cent a distressing dysaesthesia may occur. The diagnosis of the sensory deficit is made by repeated sensory testing. If there is no improvement by six to nine months then complete spontaneous recovery will not occur. These irreversible nerve injuries can be successfully treated microsurgically. (Received for publicationApril 199 1. Accepted July 1991.)
Introduction It has been consistently reported that in at least 5 per cent of all surgical extractions of mandibular third molars there is an injury to either the inferior dental or lingual nerves. In most cases this is temporary and normal sensation returns within a few weeks or months. However in about 1 per cent of cases the patient is left with a permanent and frequently distressing sensory loss.’-6 Management of these post-surgical nerve injuries in the past has been passive, with reassurance to the patient that the sensory loss will spontaneously return. This passive management is usually ‘Based on a paper presented at the Inaugural Meeting, Section of Oral and Maxillofacial Surgery, Royal Australasian College of Dental Surgeons, February 1990. ?Senior Lecturer (half-time), Oral and Maxillofacial Surgery Unit, The University of Adelaide. Specialist oral and maxillofacial surgeon, Adelaide. Australian Dental Journal 1992;37(4):253-61.
continued for a long time as nerves recover slowly for up to two years. As the majority of injuries spontaneously heal, this approach is satisfactory in most cases. It is, however, devastating for the patient with dysaesthesia to be eventually told that nothing can be done and that they have a permanent problem. Not surprisingly, and maybe justifiably, the patient will consider litigation, both for the initial injury and for the subsequent inactivity.’ This paper reviews the current state of knowledge of nerve injuries and the application of microsurgical techniques to their repair. In particular, it proposes that management of nerve injury following third molar surgery should be early and intensive and that active microsurgical intervention should be considered.
Review of nerve injury Anatomy The surgical anatomy of the inferior dental and lingual nerves is demonstrated in Fig. 1 and 2. The inferior dental nerve is usually in close relationship to the roots of the mandibular third molar and this can be assessed radiographically. The key features of an intimate relationship are an apparent narrowing of the canal, loss of cortical outline and increased radiolucency over the root8 (Fig. 3). These features are not inclusive however and nerve injury may occur when there is apparently little relationship between the roots and the mandibular canal. The inferior dental nerve can also be injured by needle penetration during block anaesthesia, molar endodontic instrumentation or filling past the apex, or by surgery in the premolar region. The result of inferior dental nerve injury is loss of sensation to the mandibular teeth, gingiva and the lower lip. The lingual nerve is in close proximity to the lingual plate of the mandible covering the third 253
Fig. 1. -Anatomical dissection showing inferior dental nerve (I); lingual nerve (L).
Fig. 2.-Anatomical dissection showing inferior dental nerve (I); lingual nerve (L); facial artery (F). 254
Australian Dental Journal 1992;37:4.
Fig. 3.-Periapical radiograph of mandibular third molar with inferior dental nerve in close proximity.
Fig. 4. -Transverse section of mandibular nerve showing axons (A); perineurium (P); epineurium (E); blood vessel (B). H&E. x 20.
molar. Its position is quite variable vertically, being at any point from the level of the gingival crevice to that of the mylohyoid m ~ s c l e Thus . ~ the nerve is vulnerable to any surgery involving the lingual periosteum in the posterior mandible, whether it be third molar removal or mylohyoid reduction in the edentulous patient. It may also be damaged in lingual block anaesthesia. Australian Dental Journal 1992;37:4.
The result of injury to this nerve is loss of sensation and taste to the anterior twethirds of the tongue. The lingual nerve usually also supplies the lingual gingivae but this may be supplied by the mylohyoid nerve.'O The general structure of a peripheral nerve in transverse section is shown in Fig. 4. The epineurium is the outer connective tissue sheath and is impor255
Table 1. Classification of nerve injury ~
Neuropraxia
Grade 1
Axonotrnesis
Grade 2 Grade 1 Grade 2
Neurotmesis
Grade 1 Grade 2
Functional conduction block (ischaemia and oederna) Dernyelination (no axon loss) Axon degeneration (Wallerian degeneration) Axon degeneration and endoneural disruption Perineural disruption (a closed nerve injury) Nerve division (epineurium disrupted)
tant for support and protection. The perineurium is a thinner connective tissue sheath which surrounds each individual fascicle or group of nerve fibres. The perineurium provides elasticity to the nerve and is important in maintaining the integrity of the nerve when it is stretched. The number of fascicles within a nerve varies, with a range of six to eighteen in the mandibular nerve" and one to seven in the lingual nerve. Each nerve fibre is surrounded by the endoneurium which has both protective and electrical insulation functions. The nerve fibres vary from large myelinated to small myelinated and unmyelinated fibres. An essential and frequently overlooked aspect of intraneural anatomy is the blood supply. Within the epineurium there is an arteriole and venous plexus with branches penetrating both the perineurium and endoneurium to form an intrafascicular network. The relationship of these vessels to the nerve is the same as the blood-brain barrier and thus is selectively impermeable. Interference with the blood supply rapidly results in areas of anoxia and is an essential component of nerve injury and its capacity to repair.I2 N e m e injury The classification of nerve injury is presented in Table l . l 3 - I 5 It is essential to understand that this classification is based primarily on histological findings and clinically may only be applied retrospectively after the nerve response to injury is known. At the time of injury it is at best, using the operative microscope, possible only to diagnose a neurotmesis grade 2. In all other instances the nerve appears intact from the outside. Neuropraxia and axonotmesis injuries rapidly spontaneously recover; with neurotmesis, spontaneous recovery may occur but this will be slow, take up to two years and may be incomplete. Factors which influence the extent of repairs are: 256
1. The amount of damage to the proximal end of the nerve When a nerve fibre is transected there is retrograde degeneration for a short distance at the site of injury but more importantly nerve cell death within the trigeminal ganglion. Thus when a nerve is transected there is considerable loss of ganglion 2. The amount of damage at the distal end of the nerve When a nerve fibre is transected there is degeneration from the site of injury to the sensory receptor. The endoneural tubes shrink to about 70 per cent of their initial diameter within a few months of injury. This shrinkage is reversible however if new axons re-enter the tube.'* New axons may also produce new endoneural tubes or grow down arterioles, venules or even implanted silastic tubes.19
3. The site of injury The nature and extent of injury is probably the most important factor in recovery. A crush injury will result in more extensive damage than a sharply transected nerve. Increased fibrosis, infection, foreign bodies and a large discontinuity will all adversely affect the chances of recovery. The closer the site of injury to the periphery the better the chance of recovery. Thus the transected mental nerve is more likely to recover than an injured inferior dental nerve at the third molar site. The inferior dental nerve is more likely to recover due to its containment within the mandibular canal, while the lingual nerve is loosely tied in the floor of the mouth and is subject to muscle movement.
4. The nature of the nerve Mixed nerves of motor, sensory and sympathetic fibres are less likely to recover than when all fibres are of similar type. Thus mandibular nerves, which are almost entirely sensory, are more likely to recover than the lingual nerve which consists of sensory fibres and sympathetic fibres from the chorda tympani. Nerve function Sensory nerve testing can be divided into:" 1. Mechanoceptive This relates to the ability to determine touchpressure sensations which are largely mediated by larger myelinated fibres. Methods of mechanoreceptor testing are two-point discrimination, static light touch and directional brush stroke. Australian Dental Journal 1992;37:4.
2. Nociceptive This is the ability to discriminate between different types of stimuli which are largely mediated by small myelinated and unmyelinated fibres. This is tested by pin prick and thermal discrimination.
Table 2. Indications for surgery 1. Known transection of the nerve
Ifthe nerve is seen to be transected or there was profuse operative haemorrhage from the mandibular canal with profound anaesthesia post-operatively, then early exploration and repair is indicated. 2. Hyperaesthesia for some time without improvement
3. Proprioceptive This tests the perception of position. If fascicular cross-over has occurred in recovery then the patient will interpret touch in one position in the nerve distribution as occurring elsewhere in the nerve distribution. This assumes greater clinical importance in completely transected nerves particularly in long-standing injury and in older patients. Somatosensory-evoked potential recording using electrophysiological equipment is an objective and sensitive method of assessing sensory nerve function.’l It does, however, require extensive equipment and expertise as compared with clinical testing. Clinical aspects
Prevention Prevention of nerve injury is clearly an essential aspect of pre-operative and operative technique. Adequate radiographs (Fig. 3) must be carefully evaluated for the relation of the roots to the mandibular canal. Essential points of operative technique are tooth sectioning and elevation of roots away from the nerve. It is not possible to pre-operatively evaluate the position of the lingual nerve although usually these are bilaterally symmetrical. Operatively one can either keep the approach entirely to the buccal without disturbing the lingual tissues at all or carefblly elevate the lingual mucoperiosteum and place a retractor to protect the nerve from all cutting and elevating instruments.” On completion of tooth removal the operative site should be carefully examined to see if the inferior dental or lingual nerves have been exposed. If so they should be carefully examined, preferably with magnification, for evidence of disruption. If a nerve is severed then either seek immediate specialist advice or at least align the two ends. Any bone fragments compressing the inferior dental nerve should be gently removed from around the nerve. Post-operative evaluation At the first post-operative visit patients should be specifically asked if they have any areas of numbness. If so the area of sensory loss must be defined using the methods outlined earlier. The area of Australian Dental Journal 1992;37:4.
Hyperaesthesia is an unpleasant feeling of burning, stinging, or frank pain. It is made worse by stimulation. 3. Anaesthesia for six months without improvement Anaesthesia is complete lack of sensation. Pin-prick testing to the point of bleeding will be felt as blunt pressure.
4. Hypoaesthesia for at least six months without improvement and troublesome to the patient Hypoaesthesia is a relative lack of sensation with reduced discrimination to all tests as compared with the uninjured side.
sensory loss must be recorded in the notes using either an accurate anatomical description or, preferably, by diagram. The nature of the possible injury and probable spontaneous recovery should be explained to the patient. At this stage reassurance and passive observation is appropriate. A further appointment should be made for not more than one month. At one month the clinical evaluation should be repeated and recorded. If there is no sign of recovery or a partial dysaesthesia then specialist referral should be considered and foreshadowed to the patient. The patient should then be reviewed monthly and if there are no signs of recovery by six months the patient must be referred for specialist evaluation. There is evidence that a nerve injury which has not shown clear signs of recovery by six to nine months is unlikely to recover and surgical intervention is indicated. Surgical repair
Indications for surgery The indications for surgery are presented in Table 2. The timing of surgery is partly dependent on the symptoms, the degree of distress to the patient, and the surgeon’s assessment of the injury. Generally surgery is best performed at six to nine months following injury as this allows time for assessment of recovery. Transection and troublesome hypoaesthesia can be operated earlier than this and hypoaesthesia which does not greatly trouble the patient can be left longer. 257
Fig. 5.-Dissection of the inferior dental nerve (I) exposed by a sagittal split osteotomy (0)ofthe mandible. Sucker tip (S) retracting the nerve.
Fig. 6.-Dissection ofthe lingual nerve (L) through operating microscope, x 10 power. Alveolus mandible (A); tongue (T).
The realization and effects of central degeneration following peripheral injury and the ramifications of the problems which stem from it would indicate early rather than late repair. However, there are now reliable new diagnostic tests which will allow the determination of the extent of the problem and act as guidelines for intervention. 258
Surgical access dental neyOe
The inferior dental nerve is best approached intraorally Using a modified sagittal split osteotomy. (Fig. 5). The osteotomy cuts are in the conventional position for a mandibular advancement procedure Australian Dental Journal 1992;37:4.
Fig. 7. -Hand instrumentation required for microsurgery.
with the exception that the anterior vertical cut is level with the first molar. Decortication to the mental foramen will allow further exposure of the nerve if required. The osteotomy is carehlly opened ensuring that the nerve is on the distal segment and is not hrther damaged by the osteotomy. The proximal segment is rotated laterally with no muscle stripping. Meticulous haemostasis is obtained prior to commencing the microsurgical procedure. On completion of the microsurgical phase, bone is removed from the inner aspect of the proximal fragment so that the nerve is not compressed when the osteotomy is reduced. Fixation is gained using three rigid futation screws and intermaxillary fixation is not required. The inferior dental nerve may also be exposed via an extra-oral submandibular approach. This certainly gives wide access and has been advocated by some surgeons without orthognathic expertise. It does, however, leave a large unaesthetic scar in the neck and bleeding from the exposed marrow can be troublesome. There is also the disastrous risk of damaging the marginal mandibular branch of the facial nerve and thus ending up with a paralysed as well as numb lip.
Lingual nerve The lingual nerve is approached intraorally by one of two exposures. The direct approach is by an incision along the anterior edge of the vertical ramus along the lingual gingival crevice to the first molar. The lingual mucoperiosteum is widely elevated and held superiorly with stay sutures. The nerve is visualized through the periosteum, the periosteum incised parallel to the nerve and the nerve exteriorized by blunt dissection. Australian Dental Journal 1992;37:4.
Alternatively the lingual nerve is exposed via an anterior approach through the floor of the mouth. The nerve is identified as it crosses under the submandibular duct and followed posteriorly by blunt dissection (Fig. 6). This approach is essential if an interpositional nerve graft is required. Bleeding from the many veins in the floor of the mouth can be troublesome, with access difficulty, particularly if the microscope is to be.used. Microsurgical procedure The minimum equipment required is shown in Fig. 7. The first step is to thoroughly examine the exposed nerve using the operating microscope. The nerve is manipulated with fine microsurgical forceps and a magnification of 10 x is preferred. The usual options are an intact nerve with surrounding adhesions, a transected nerve or a neuromatous enlargement. Any adhesions to the surrounding bone, periosteum or scar tissue are severed, using microsurgical scissors, until the nerve is completely free. The epineurium is then lifted and incised parallel to the long axis of the nerve. The perineurium of each individual fascicle is exposed and all the nerve bundles separated from each other. Each bundle is examined for evidence of adhesion, transection or neuroma. If all fascicles appear normal then the nerve has been satisfactorily microsurgically decompressed. If the nerve either in total or individual fascicles has been transe ed then the epineurium is removed for at least 1 m T from the injury. The perineural tubes at the proximal end are then examined and 259
resected until axons are seen to ooze from the cut ends. The oozing axons are removed until no further oozing occurs. The distal end of the nerve is then treated in a similar fashion except that axons do not ooze from the distal end. The perineurium of the two ends is then gently approximated and sutured with a 10/0 nylon suture. Four sutures, one in each quadrant, are placed. If the perineural ends do not approximate with minimal tension, an interposition graft is indicated. If examination shows an enlargement ofthe nerve tissue the condition is most likely a neuroma. These may be either lateral, in which case they can be excised, leaving the nerve fascicles intact, or central. Central neuromas require resection and the resulting gap usually requires an interpositional graft. The donor site for interposition grafts is either the sural nerve in the leg if a long graft is required, or the great auricular nerve in the neck. Nerve preparation and suturing is similar to that for direct approximation. The results of microsurgical repair of the inferior dental and lingual nerves has been reported in a number of studie~.’~-’~ Similar results have been obtained by the author.
Discussion This paper reviews the essential aspects of the basic science of nerve injury following third molar removal and presents an approach for active evaluation and microsurgical repair of irreversibly injured nerves. It can be argued that as the incidence of nerve injury is low, about 5 per cent, and that as most recover anyway, a passive approach is satisfactory. However, removal of impacted third molars is a common procedure so that there is an unacceptably large number of patients with irreversibly injured nerves. For example, there are currently 500 000 people aged between 17 to 19 years in Australia at present; if only 10 per cent have their third molars removed there will be a possible 500 nerve injuries which do not spontaneously resolve. There is also increasing medico-legal activity following nerve injury.6 Certainly, if patients have been given full information so that they can give informed consent, and if the procedure is performed using good pre-operative evaluation and careful technique, then any medico-legal challenges are defensible. However, if the nerves are injured, with less than rigorous follow-up evaluation, particularly in cases where early microsurgery could be of value, then there is increasing risk of successful medicolegal action. The author is aware of a number of such cases where patients have obtained out-of-court settlement in the order of $25 000 for their 260
distressing and now treatable complaint. It is in the interests of both the patient and practitioner that these injuries are adequately assessed and treated with early referral to an appropriate specialist oral and maxillofacial surgeon with further training in microsurgery. There have been a number of training courses conducted in Australia, either for surgical specialists of all types or specifically for oral and maxillofacial surgeons. Following the introductory course the surgeon must practise on a regular basis on laboratory animals to maintain these skills and then use them on a reasonable number of patients. Besides the training and practice, the essential requirements for microsurgery are time, patience and extreme sensitivity with the tissues. There are oral and maxillofacial surgeons with such skills in most capital centres.
Acknowledgements The author wishes to thank Grant Townsend, Associate Professor in Oral Anatomy, for Fig. 1 and 2; and Professor Alastair GOSS,Head, Oral and Maxillofacial Surgery Unit, for help with the manuscript.
References 1. Goldberg MH, Nemarick AN, Marco WP. Complications after mandibular third molar surgery. A statistical analysis of 500 consecutive procedures in private practice. J Am Dent ASSOC1985;111:277-9. 2. Osborn TP, Frederickson G Jr, Small IA, Rogerson TS. A prospective study of complications related to the mandibular third molar. J Oral Maxillofac Surg 1985;43:767-9. 3. Van Goo1 AV, Fosch JJ, Boering G. Clinical consequences of complaints and complications after removal of the mandibular third molar. Int J Oral Surg 1977;6:29-37. 4. Alling CC. Dysaesthesia ofthe lingual and inferior alveolar nerves following third molar surgery. J Oral Maxillofac Surg 1986;44:454-7. 5. Kipp DP. Dysaesthesia after mandibular third molar surgery. A retrospectivestudy and analysis of 1377 surgical procedures. J Am Dent Assoc 1980;10:185-92. 6. Blackburn CW, Bramley PA. Lingual nerve damage associated with the removal of lower third molars. Br Dent J 1989;167: 103-7. 7. Haskell R. Medicolegal consequencesof extracting lower third molar teeth. Medical Protection Society Annual Reports and Accounts 1986;94:51-2. 8. Rood JP, Shehab BA. The radiological prediction of inferior alveolar injury during third molar surgery. Br J Oral Maxillofac Surg 1990;28:20-5. 9. Kiesselbach JE, Chamberlain JG. Clinical and anatomical observations on the relationship of the lingual nerve to the mandibular third molar region. J Oral Maxillofac Surg 1989;42:565-7. 10. Tier GA. Sensory nerve supply to the tongue. A clinical reappraisal. Br Dent J 1984;157:354-6. Australian Dental Journal 1992;37:4.
11. Svane TJ, Wolford LM, Milam SB, et al. Fascicular characteristics of the human inferior alveolar nerve. J Oral Maxillofac Surg 1986;44:431-4. 12. Lundborg G. Structureand function ofthe intra-neural vessels as related to trauma oedema formation and nerve function. J Bone Joint Surg 1975;57A:938-48. 13. Sunderland S. Nerve and nerve injuries. 2nd edn. London: Churchill Livingstone, 1978. 14. Seddon HJ. Three types ofnerve injury. Brain 1943;66:237. 15. Rood JP. Degree of injury to the inferior alveolar nerve sustained during the removal of impacted third molars by means of the lingual split technique. Br J Oral Surg 1983;21: 103-16. 16. Aldskogius H, Ardmudsson J. Nerve cell degeneration and death in the trigeminal ganglion of the adult rat following peripheral nerve transection. J Neurocytol 1978;7:229-50. 17. D’Rozario RH. The central effects ofperipheral nerve injury on the trigeminal nerve ofthe rat. Adelaide, Australia: The University of Adelaide, 1985. MDS thesis. 18. McObrien B, Morrison W.Reconstructive microsurgery. London: Churchill Livingstone, 1987: 137-40. 19. Lundborg G, Hansson H. Nerve regeneration through preformed pseudosynovial tubes. J Hand Surg 1980;5:35-8. 20. Ghalli GE, Epker B. Clinical neurosensory testing. Practical applications. J Oral Maxillofac Surg 1989;47: 1074-8. 2 1. Ghalli GE, Jones DL, Wolford LM. Somatosensory evoked potential assessment of the inferior alveolar nerve following third molar extraction. Int J Oral Maxillofac Surg 1990;19: 18-21.
Australian Dental Journal 1992;37:4.
22. Stacy G. Lingual exposure during mandibular third molar surgery. Int J Oral Surg 1977;6:334-9. 23. Mozsary PG, Syers C. Microsurgical correction ofthe injured inferior alveolar nerve. J Oral Maxillofac Surg 1985;43:353-8. 24. Mozsary PG, Middleton RA. Microsurgical reconstruction ofthe lingual nerve. J Oral Maxillofac Surg 1984;42:415-20. 25. Hausaman JE, Schmidseder D. Repair of the mandibular nerve by means of autologous nerve grafting after resection of the lower jaw. J Maxillofac Surg 1975;1:74-8. 26. Wessberg GA, Epker BN. Transoral inferior alveolar neurorrhaphy via a sagittally split mandible. J Maxillofac Surg 1982;IO: 173-6. 27. Wessberg GA, Wolford LM, Epker BN. Experiences with microsurgical reconstmction of the inferior alveolar nerve. J Oral Maxillofac Surg 1982;40:651-5. 28. Hiroyasi N, Takashi K, Genydu Y,Kenichi S. Repair of the mandibular nerve by autologous grafting after partial resection ofthe mandible. J Oral Maxillofac Surg 1986;44:31-6. 29. Donoff RB, Guralnick W. The application of microsurgery to oral neurological problems. J Oral Maxillofac Surg 1982;40: 156-9.
Address for correspondence/reprints: Oral and Maxillofacial Surgery Unit, Department of Dentistry, The University of Adelaide, GPO Box 498, Adelaide, South Australia, 5000.
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