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- Materials and methods
Damage to the branches of the trigeminal nerve can occur as a result of a variety of causes. The most common damage to all divisions of this nerve occurs as a result of facial trauma.
Unfortunately, iatrogenic damage to the inferior alveolar branch of the mandibular division of the trigeminal nerve is common because of its anatomical position within the mandible and its closeness to the teeth, particularly the third molar.
It has been reported there is an incidence of approximately 0.5% of permanent damage to the inferior alveolar nerve following third molar removal. Extraction of other teeth within the mandible carries a lower incidence of permanent damage. However, damage can still occur in the premolar area, where the nerve exits the mandible via the mental foramen.
Dental implants are a relatively new but increasing cause of damage to this nerve, particularly if the preoperative planning is inadequate. CT scanning is important for planning the placement of implants if this damage is to be reduced.
Primary repair of the damaged nerve will offer the best chance of recovery. However, if there is a gap, and the nerve ends cannot be approximated without tension, a graft is required. Traditionally, nerve grafts have been used for this purpose but other conduits have also been used, including vein grafts.
This article demonstrates the use of vein grafts in the reconstruction of the inferior dental branch of the mandibular division of the trigeminal nerve following injury, in this case due to difficulty in third molar removal, following sagittal split osteotomy and during the removal of a benign tumour from the mandible. In the five cases presented, this technique has demonstrated good success, with an acceptable return of function occurring in most patients.
- Top of page
- Materials and methods
Damage to the branches of the trigeminal nerve can occur as a result of a variety of causes. Trauma is the most common and damage to all divisions of this nerve can occur as a result of facial trauma. The third or mandibular division is commonly involved with fractures of the mandible but iatrogenic damage to the inferior-alveolar branch of this nerve during the removal of third molar teeth is the most common cause of damage.
The repair of the inferior alveolar nerve is difficult as it lies in a canal within the body of the mandible. Various methods of repair have been advocated including primary anastamosis of the nerve ends if they can be approximated with no tension, or a graft if not. Nerve grafts have been traditionally used for this purpose, either using the sural nerve from the leg or greater auricular nerve from the neck. Other grafts have been used for this purpose, including vein grafts1 which act as a conduit for the axons to grow down. In this paper, the posterior facial vein or the external jugular vein has been used for this purpose and has been found to be effective.2
It has been variously reported that there is an incidence of approximately 0.5% to 5% of damage to the inferior alveolar nerve following third molar surgery (Fig 1).3,4
Figure 1. Demonstrates the position of the inferior alveolar nerve within the mandible at the third molar site and the closeness of the teeth to this nerve.
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Extraction of other teeth within the mandible carries a lower incidence of permanent damage. However, damage can still occur, particularly in the premolar area where the inferior alveolar branch exits the mandible via the mental foramen.
Dental implants are a relatively new but increasing cause of damage to this branch, particularly if the position of the nerve is not determined preoperatively. Therefore, preoperative CT scanning is important for planning the placement of implants if this damage is to be reduced.
The third division of the trigeminal nerve, the mandibular division, enters the infratemporal fossa after leaving the base of the skull via the foramen ovale. The lingual nerve separates away from the mandibular division close to the base of the skull, eventually entering the mouth to supply the anterior two-thirds of the tongue. The motor branches of the trigeminal nerve similarly separate away from the mandibular division close to the base of the skull and supply the muscles of mastication.
The inferior alveolar nerve winds around the lower border of the lateral pterygoid muscle, turns sharply lateral and enters the inner aspect of the vertical ramus of the mandible by the mandibular foramen. Once inside the mandible, it traverses more laterally towards the outer cortical plate, below the mandibular teeth and exits the mandible via the mental foramen but before doing so it forms a loop more anteriorly and superiorly and then exits the foramen. The symphyseal branch continues on within the mandible to supply sensation to the anterior teeth and gum.5
The cell body of the trigeminal nerve is within the trigeminal ganglion with axons extending out into the sensory and motor receptors of the face. The central connections then extend from the trigeminal ganglion to the cortex and this pathway conducts sensory input to the brain for its interpretation.6
Histologically, the fundamental components of a nerve include: (1) a connective tissue outer sheath, the epineurium; and (2) within the epineurium is found the nerve fibrils containing the axons which are again contained within the perineurium.
Seddon7 classified nerve injury into three separate entities depending on the severity of damage: (1) neuro-praxia – traction injury; (2) axonotmesis – crush injury; and (3) neurotmesis – severance of the nerve.
Sunderland8 and Rood9 have presented alternative classifications for nerve injury. In this paper, Seddon’s classification will be used.7
Principles of inferior alveolar nerve repair
It is well accepted10 that perineural repair will offer the best chances of success. However, this concept needs to be looked at realistically as it is not always possible to perform a perineural repair in all cases and an epineural repair may have to suffice.11
The inferior alveolar nerve lies in a canal within the mandible for most of its course and it is in this position that damage can occur from the removal of teeth, a fractured jaw or the placement of dental implants. The inferior alveolar nerve consists of up to 18 fascicles12 of varying size in the area of the third molar tooth and as such it is impossible to carry out a perineural repair of all fascicles. Such a repair would cause considerable damage to the perineural tubes and invoke an inflammatory response from the surgery and sutures, leading to scar tissue formation and a non-functioning repair. It is also understood that the nerve ends should be placed together and repaired in a passive way under minimal or no tension. Miyamoto13 has determined that if there is a tension greater than 23 gms, the axons will not grow down the perineural tubes of the distal end as the gap between the two ends would be too great. It is felt that the inherent elasticity of the nerve produces this gap. Hausaman10 believes that if the nerve ends cannot be placed together in a passive way, a nerve graft should be used.
Various grafts have been used for this purpose, such as autologous nerve grafts taken from the same host, including the sural nerve in the leg or the greater auricular nerve in the neck. Alternatives have also been advocated and various different conduits have been used. Pogrel and Maghen1 has advocated the use of vein grafts for this purpose while other materials such as Gore-tex (WL Gore & Associates Inc, Flagstaff, AZ, USA) and Neurotube (Neurogen, LLC, Bel Air, MD, USA), made from resorbable poly glycolic acid14,15 have also been used for this purpose.
This paper presents five patients in whom vein grafts have been used to reconstruct the inferior alveolar branch of the trigeminal nerve following damage.
Materials and methods
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- Materials and methods
Five patients presented complaining of altered sensation to the inferior alveolar nerve, three patients suffered nerve damage following the surgical removal of third molar teeth, the fourth patient suffered nerve damage following the removal of a benign tumour from the mandible and the fifth patient as a result of a sagittal split osteotomy of the mandible.
All patients were consulted postoperatively when an explanation of the problem was discussed, including the need for close review. The third molar patients were referred early, as soon as it was noticed that damage had occurred. The fourth patient was noticed to have damage at the time of surgery for the removal of the tumour but because informed consent was a problem, it was decided to wait and see what recovery would occur before offering repair. The fifth patient was referred approximately 18 months following surgery for the correction of a mandibular deformity.
At the time of consultation, the area of numbness was mapped out and sensation was tested for two-point discrimination, light touch, brush directional stroke and sharp touch.2 These findings were compared with the other normal side. All findings were recorded in the notes and the patients followed up on a monthly basis. The sensation to the lip of the third patient was compared with the upper lip and anterior part of the face for comparison. Three +++ indicated normal or complete sensation and - - - indicated no sensation to the various parameters mentioned above. A simple example is shown in Fig 3 where the right side of the lip has full sensation and the left has no sensation.2
Once it had been determined that little or no recovery had occurred after three months of review, surgery was offered and informed consent obtained. The exception included the patient who had suffered damage as a result of the sagittal split osteotomy of the mandible because at the time of consultation it was already 18 months after injury.
All patients had fine-cut CT scans through the area of damage along with the normal OPG and PA of the mandible before surgery.
All patients were treated in the same way using the following procedure: under nasal endotacheal general anaesthesia, the mandible was approached and explored by an extraoral submandibular incision. The marginal mandibular branch of the facial nerve was exposed and protected in each case, and the facial artery and posterior facial vein exposed and tied off. The posterior facial vein or the external jugular vein was tagged for use as a graft once the amount of damage to the nerve had been determined. The mandible was decorticated in the area of expected damage, as determined by preoperative CT scan and OPG radiograph. The removed cortical plate was put aside and used as a graft after completion of the repair. Once the nerve was exposed and explored, the damaged section was resected and a suitable length of vein was harvested. The vein was turned inside-out in the manner described by Wang16 and sutured into position using 8/0 nylon sutures under magnification using an operating microscope. The cortical bone was then placed back into position and secured with a plate or with position screws, ensuring the graft was under no compression. It was felt that this would help protect the repair and reduce the chance of scar tissue formation around the nerve during the healing phase. The wound was then sutured in layers and 5/0 nylon to the skin.
All patients were discharged from hospital the next day and were reviewed five days later when the skin sutures were removed. At that time, sensation was tested using the above protocol and the patients were asked to return one month later. This was carried out on a monthly basis for three months followed by six months and then a yearly review.
All patients had axonotmesis of the inferior alveolar nerve with scar tissue and neuroma formation, and all were treated with grafts from the posterior facial vein.
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- Materials and methods
In the early stages following the initial injury, it is very difficult to know exactly what has happened to the nerve and what damage has been done, and therefore to classify the problem. It is prudent to wait for a period of time in order to determine if the nerve will recover.17 If, however, there are no signs of recovery at three months, exploration and repair should be considered, as it is at this time that active regeneration of the nerve is at its peak.18
It is accepted that the least one has to do to the nerve, the better and faster the recovery will be, so that an external or extra-fascicular neurolysis is better than primary repair, which is better than a graft. The timing is important and it is generally accepted that three months after injury is the optimal time for repair, unless it is known that the nerve has been severed at the time of operation, in which case repair at the time will offer the best chance of recovery. However, this is not often possible because of access and the need for informed consent.
Exploration and repair can be carried out from an intraoral approach using a sagittal split osteotomy19 (Fig 8) of the mandible to gain access to the nerve or by an extraoral approach and decortication of the mandible (Figs 4–7).20 If the sagittal split is used for access, it is necessary to decorticate the mandible from the vertical cut of the sagittal split to the mental foramen in order to gain adequate freedom of the nerve to successfully repair it. This then produces a number of problems in relation to recovery: (1) further iatrogenic damage to the nerve in carrying out the sagittal split and in decorticating the mandible to the mental foramen; (2) exposing the nerve over a long distance and therefore damaging the blood supply to the nerve; (3) replacement of the nerve after surgery and the need to externalize the nerve after repair, therefore exposing it to further damage at a later stage; (4) possible scarring around the nerve by the periosteum and subsequent lack of recovery; (5) the need to rigidly fix the fractured mandible with plates or screws and the effects of this on recovery.
If, however, the nerve is explored by the extraoral approach, one is committed to a nerve graft as it is not practical to mobilize the two ends of the nerve to carry out a tension-free repair. The external approach will leave a scar on the skin which may not be acceptable, particularly in the female patient and there is always the possibility of damage to the marginal mandibular branch of the facial nerve. A second donor site may also be necessary if a graft is taken from the leg.
The extraoral approach has been used with the current group of patients because of the ease of access and the need for obtaining a graft from the posterior facial or external jugular vein.
Methods of repair
Once a decision has been made to explore the nerve and an approach determined, a decision has to be made at the time the nerve is exposed as to what to do. If the nerve is intact and there is no neuroma formation associated with the nerve, then removal of scar or extra-fascicular neurolysis is all that is required. If the nerve has a neuroma associated with it, the neuroma will have to be resected and the nerve repaired. If, after preparation of the nerve ends, they cannot be approximated without tension, a graft will be required. This will invariably be the case if the extraoral approach is used as it is very difficult to mobilize the proximal and distal ends of the nerve in order to approximate the two ends in a tension-free manner.2
Types of graft
There are a number of different grafts which can be used for repair of the inferior alveolar nerve and these include various autografts or allografts: (1) nerve graft with the donor nerve being either the sural nerve in the leg or the greater auricular nerve in the neck. Other nerves can be used but these are the common sites for grafting purposes to the inferior alveolar and lingual nerves; (2) vein grafts will act as a conduit for the axons to grow down. It is also felt that if the vein is turned inside-out, the nerve growth factors in the external aspects of the vein will act as a stimulating factor for nerve regeneration;16 (3) denatured (freeze thawed) striated muscle grafts have been used and placed between the nerve ends;21 (4) various allografts including Gore-tex tubes and various different resorbable tubes have also been used with some success. These all act as a conduit for the axons to grow down.14,15
Some of these grafts are more effective than others but it is generally felt that the autologous vein graft is effective.15
The advantages and disadvantages of these materials for grafting requires some discussion. Autologous materials are generally the best for grafting purposes; they are from the same person and are therefore not subject to rejection. However, they are not without their problems. In each case there will be the need for a second operative site which will have its inherent problems, bleeding, pain, swelling and infection, and in the case of a nerve graft there will be a deficit of the donor graft distribution, and in the case of the sural nerve this will be a numbness on the lateral aspect of the foot which may not be acceptable. In the case of the greater auricular nerve, this will be of the ear and lateral face.
If the vein graft is taken from the long saphenous vein in the leg, a similar problem will occur but without the numbness to the foot. However, if the graft is taken from the neck (posterior facial vein or external jugular vein), there will be no need for a second donor site. The other problem in relation to the vein grafts is the possible collapse of the graft due to its lack of strength and also the chance of kinking.13 Theoretically, this could be a problem with the lingual nerve, in particular as it lies entirely within the soft tissues and therefore considerable movement could produce kinking. According to Pogrel and Maghen,1 this is the main reason for the lack of success of vein grafts with the lingual nerve. However, this is not the case with the inferior alveolar nerve which lies within the mandible and is therefore somewhat protected. This will not overcome the collapse of the vein but will prevent kinking which is an inherent problem with vein grafts.
Muscle grafts use the striated nature of the muscle to act as a conduit and have also been used for nerve grafting with limited success.
Weber21 reported favourably about the use of the resorbable conduits after he prospectively carried out 136 nerve repairs in the hand. However, very little has been reported on the use of these materials with the trigeminal nerve.15 The advantage of allografts relates to the lack of a second donor site and ease of use. They also have some inherent strength and will not collapse in the same way as a vein graft, and as such will act as a barrier membrane to hopefully reduce scar tissue formation and aid the progression of axons through the tube. This has to be balanced with the potential for foreign body reaction and the production of scar tissue.
It is apparent that many factors can affect the outcome of a nerve repair. This paper deals with some of these issues as it relates to the use of autologous vein grafts to repair the inferior alveolar nerve following damage.