The root of the problem

The elimination of infected pulp and dentine, and adequate root canal preparation and sealing constitute the basic principles of endodontic treatment. Ideally, the filling material should be limited to the root canal without extending to periapical tissues or other neighbouring structures.

When root canal therapy is performed on mandibular molar and premolar teeth, damage to the inferior nerve is possible[1]. Serious problems such as paraesthesia and similar neural complications may occur during endodontic treatment. Paraesthesia is a condition that involves perverted sensations of pain, touch or temperature. It is described by patients as warmth, cold, burning, aching, prickling, or pins and needles. With inferior alveolar or mental nerve paraesthesia, patients may also experience dryness of the affected mucosa and damage to the lip or tongue when chewing. There are several terms related to paraesthesia. Dysaesthesia is the impairment of any of the senses, but most importantly touch. Hypoaesthesia means diminished sensation. Paraesthesia can be considered as a form of dysaesthesia.

Such dangers are becoming more common in the treatment of teeth whose apices are close to the inferior alveolar canal[2]. More cases have been reported in connection with the lower second molars, but cases related to the lower first molars and the premolars have also been reported. Three possible mechanisms have been postulated:

  • mechanical trauma from over-instrumentation into the inferior alveolar canal;
  • a pressure phenomenon from the presence of the endodontic point or sealant within the inferior alveolar canal;
  • a neurotoxic effect from the medicaments used to clean the canal or that are in the sealant.

Over-instrumentation of the root canal with hand or mechanically driven files can perforate the mandibular canal, allowing the extrusion of sealers, dressing agents and irrigation solutions into the canal during endodontic treatment.

A review of the endodontic paraesthesia literature reveals that most cases have resulted from overfill of a paraformaldehyde paste into the vicinity of the mental or inferior alveolar nerves. Totally biocompatible materials are not available. Consequently, their spread beyond the apical foramen can give rise to clinical manifestations in relation to the toxicity of the product, though minor material extrusions are generally well tolerated by the peri-radicular tissues[3]. Whilst studies have shown that all root canal sealants are toxic to some degree, especially before they have set[3], the most neurotoxic appear to be those containing paraformaldehyde or one of its analogs, including Sargenti paste (N2) or Endomethasone. Other sealants contain analogs of formaldehyde, particularly before they have set (AH26). Even root canal sealants that are believed to be more benign, such as zinc oxide/eugenol and calcium hydroxide have been shown to be neurotoxic in vitro and are almost certainly neurotoxic in vivo.[2]

Morse[2] reports that, from a study of relevant case reports, it appears that the least damage occurs from a brief period of nerve irritation. Therefore cases of paraesthesia caused by absorbent point penetration and over instrumentation should resolve within days. Although sodium hypochlorite, especially at lower concentrations (one per cent) is relatively innocuous when it is kept within the confines of the root canal., it can be extremely damaging when it is expressed periapically[4][5]. When contact is made with the inferior alveolar or mental nerves with obturating materials containing paraformaldehyde, long-term or even permanent paraesthesia results[2]. In all of these cases, it has been suggested that periapical surgery is required to remove the overextended material as soon as possible to prevent permanent paraesthesia.

Case report

A 40 year old woman had conventional endodontic therapy to tooth 45, a bridge abutment. At the time of presentation prior to the commencement of endodontic treatment, the patient reported paraesthesia in the area of the right mental dermatome. The practitioner thought that this was due to the infection present. However, after the completion of the treatment, the patient symptoms had diminished and largely disappeared. However, there was a persistent tingling in the lower lip for more than one week, so the dentist referred the patient to an Oral Maxillofacial Surgeon.

The surgeon noted:

“…she had a significant swelling in the right buccal sinus which was quite painful and this swelling immediately overlies the mental foramen. The right mandibular second premolar tooth is not sensitive to percussion. Objective neurosensory testing of the right mental dermatome revealed that the mucosal lip is quite numb with almost complete lack of directional sensibility and no nocioception.”

The surgeon ordered a CT scan, as the completion radiograph revealed endodontic filling material outside the root apex of the tooth. The scan revealed root filling material within the alveolar canal just proximal to the mental foramen. He suggested that the patient undergo a reparative operation to debride the canal and the local area. The surgeon was confident of a good outcome, with a high probability of return to near normal sensation.

17 days after the completion of the endodontic procedure, the surgeon carried out the surgical debridement. 

“…I apicectomised the premolar tooth and de-roofed the inferior alveolar canal. A small quantity of root filling material was removed from the periapex and the inferior alveolar canal and following this the inferior alveolar and mental nerve was found to be relatively clean and intact. The surgical wound was closed in a single layer, catgut to mucosa…”

At a post operative review some two weeks later, the patient reported more feeling and return to near normal sensation both immediately post operatively and at the time of review.

At a further review one month later subsequently the surgeon noted:

“…I found that objectively she had perfect mechano and nocioceptive sensibility in the right mental dermatome. There has been almost complete return of sensation in the right mental dermatome.”

The dentist had contacted DDAS at the time of his noting of the dysaesthesia. The matter was logged with his insurer (Guild Insurance) and a file opened. It was suggested to the practitioner that he assist the patient by having the surgeon send the invoice(s) directly to the practitioner, not the patient. Subsequently, the insurer sent a cheque to the practitioner for the total cost of the surgeon’s surgical revision of the area, some $3,000. The case was closed following the excellent result which was obtained through the skill of the Oral Maxillofacial Surgeon and the early referral from the practitioner.

Comment

Early referral in this case proved to be critical. There is ample evidence in the literature that when there is demonstrable root filling material in the mandibular canal combined with symptoms of dysaesthesia, early surgical exploration of the area is indicated. The chemical properties of the sealer and the location of the sealer are important factors affecting the recovery with surgical treatment. A longer wait-and-see period might increase the risk of irreversible damage to the nerve when the more toxic sealers (e.g. Endomethasone) are extruded into the canal.[6]

In some patients there is contact between the apices of the molar teeth and the mandibular canal, and the inadvertent extrusion of the root canal sealer is more likely to occur in such case. An initial pretreatment radiograph will reveal the proximity of the canal to the apices. During endodontic treatment, working radiographs will help to prevent perforation of the canal and possible subsequent damage to the inferior alveolar nerve. Although the mandibular canal is surrounded by dense hypermineralised bone, it can be perforated by a rotating file on a handpiece or sometimes even with hand files. Also the irrigation solutions, such as sodium hypochlorite and EDTA may leak into the canal and damage the nerve chemically. The use of lentulo spirals increases the risk of sealer extrusion into the canal.

Gross overfilling of mandibular premolars and molars is to be avoided at all costs due to the possibility of interference with the function of the inferior alveolar and mental nerves. Special care should be taken when performing endodontic treatment in this area.

A literature review supports the contention that paraformaldehyde containing pastes not be used to root fill mandibular molars and premolars. Sealers that contain both eugenol and paraformaldehyde, such as Endomethasone and N2, were found to be the most toxic.[6]

Non-drug methods of paraesthesia prevention

  • Careful reading of the radiograph to identify at-risk mandibular premolars and molars.
  • When irrigating, use the lowest concentration of sodium hypochlorite (0.5 per cent) and avoid wedging the irrigation needle into the root canal. Express only a small amount of liquid at a time and avoid excess force.
  • When drying the canal, do not force the point beyond the apex and have it fitting relatively loosely in the canal.
  • Avoid excess canal medicaments, especially if formocresol on a cotton pellet is to be used as the intracanal medicament.
  • Take as many working radiographs as necessary to ensure that overfilling of the canal will not occur
  • Avoid using Endomethasone or other paraformaldehyde root filling pastes in mandibular premolar and molar teeth.
  • If paraesthesia occurs after root filling, refer EARLY to an experienced Oral Maxillofacial Surgeon for ongoing care.

Even with these precautions, paraesthesia can still occur. With the less injurious procedures which result in nerve damage (such as local anaesthetic, over-instrumentation, irrigation penetration, absorbent point penetration, and intra-canal medication), the paraesthesia is generally short-lived provided that the irritant is removed or stopped as soon as possible. In all of these cases, the patient should be informed that the paraesthesia is probably temporary. If in doubt, the practitioner should of course consult with an experienced Endodontist or Oral Maxillofacial Surgeon familiar with paraesthesia cases. Whilst administration of the corticosteroid Dexamethasone can be useful in these cases, early referral is the best action.

By Dr Roger Dennett – ADA (NSW)

 

Bibliography

[1] Pogrel MA. Damage to the inferior alveolar nerve as the result of root canal therapy. J Am Dent Assoc. 2007 Jan;138(1):65-9.

[2] Morse DR. Endodontic-related inferior alveolar nerve and mental foramen paresthesia. Compend Contin Educ Dent. 1997 Oct;18(10):963-8, 970-3, 976-8

[3] Poveda R, Bagán JV, Fernández JM, Sanchis JM. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2006 Nov;102(5):e46-9. Epub 2006 Sep 12.

[4] Reeh ES, Messer HH: Long term paraesthesia following inadvertent forcing of sodium hypochlorite through perforation in maxillary incisor Endod Dent Traumatol 5:200-203, 1989

[5] Joffe E: Complications during root canal therapy following accidental extrusion of sodium hypochlorite through the apical foramen Gen Dent 39:460-461, 1991

[6] Koseoglu B Tanrikulu S Subay R Sencer S Anesthesia following overfilling of a root canal sealer into the mandibular canal” A case report Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006;101:803-6


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