Endodontic treatment of lower molar in a patient with paraesthesia of inferior alveolar nerve – A Case Report

SUMMARY Root canal (endodontic) treatment is demanding and complex procedure. A variety of difficulties can occur in different phases of endodontic procedure. Complex anatomorphological tooth structure, curved canals, close proximity of lower molars and premolars to inferior alveolar nerve make endodontic treatment even more challenging. During endodontic treatment, an inferior alveolar nerve may become traumatized and symptoms may vary from mild neuro-sensory dysfunction to a complete loss of sensation in the innervation area of damaged nerve. The aim of this paper is to present a clinical case of endodontic treatment of lower second molar with C-shaped root canal in a patient with paraesthesia of inferior alveolar nerve due to endodontic origin.


INTRODUCTION
Successful endodontic treatment implies complete removal of microorganisms and their products as well as organic and inorganic contents from root canal space. Removal of pathologically changed pulp and contaminated dentin, instrumentation and irrigation of root canal and finally, adequate tridimensional obturation of endodontic space are basic principles of endodontic treatment. Modern concepts of biological endodontic treatment include instrumentation, irrigation, medication and obturation only within the root tooth canal space, without any contact with periapical and other surrounding tissues [1]. Inadequate application of anesthetic solution, irrigation and activation of hand and mechanical instruments as well as anatomical proximity of these structures may cause perforation of mandibular canal. This can lead to extrusion of irrigation solution, obturation sealers, numbness of mandibular nerve or its branches and contamination of mandibular canal with microorganisms from infected root canal [2]. Neurological symptoms and disorders may appear as intense pain, hyperesthesia, hypoesthesia, anesthesia, dysesthesia and paraesthesia. Symptoms may differ from mild neurological dysfunction to a complete loss of sensation in the innervation area of damaged nerve [2,3]. Paraesthesia may occur as a consequence of local endodontic or even systemic factors. Local endodontic factors may be chemical (local anaesthetics, irrigation solutions, intersession medicaments), mechanical (over instrumentation) thermal (heated gutta-percha) and pressure factors on certain structures [2,3]. Other local factors may be trauma (jaw fractions, contusions etc.), local infections (osteomyelitis, peri-implant infections), compressive lesions (benign and malignant neoplasms and cysts), tooth impactions, iatrogenic lesions upon tooth extraction and implantation techniques (in most cases, there is a swelling which causes compression and leads to the loss of sensitivity) [4,5]. Systemic factors that cause paraesthesia might be multiple sclerosis, sarcoid, metastatic changes, viral and bacterial infections, leukemia, lymphoma, diabetes mellitus [5,6].
Consequences of peripheral nerve injury and prognosis depend on proper and accurate diagnosis. Neuropraxia, or irritation is a first degree injury and it represents only physiological block of conduction, without interruption of axon continuity. The cause of conduction interruption is probably of biochemical origin on myelin sheath level. Axonotmesis, second degree injury, represents an injury with the loss of axon continuity and myelin sheath. Third degree injury is characterized by damaged endoneurium with scarring that supports axon regeneration. In the case of fourth degree injury, nerve continuity is preserved even though it is maintained by scarring tissue. Neurotmesis, fifth degree, is an injury with a complete disruption of nerve continuity [7]. It is important to mention that early stages of neurotmesis and axonotmesis are difficult to differ. In such case, only clinical examination may show which of the two injuries will spontaneously heal and which injury requires surgical intervention. Complications of endodontic treatment greatly depend on the complexity of canal configuration. In certain situations, lower second molar may have merged roots with C-shaped canal which usually implies deeper localization of pulp space and atypical anatomical openings [8].
The aim of this paper was to present the clinical case of endodontic treatment of lower second molar with Cshaped root canal in a patient with paraesthesia of inferior alveolar nerve due to endodontic origin.
A 38-year old female patient was referred to the Department of Restorative Dentistry and Endodontics, School of Dentistry, University of Belgrade in February 2012. The patient suffered from intense pain in the right lower lip and mental region. During the tooth drilling (#47), the pulp was exposed and upon the application of anaesthesia (Ubistesin tm forte 4%, 3M Espe, Germany), her dentist initiated endodontic treatment. After initial exploring of the canal, symptoms of sensitivity of inferior alveolar nerve in the right chin area and right half of the lip appeared. The patient was then referred to the Department of Restorative Dentistry and Endodontics, School of Dentistry, University of Belgrade.
Upon the patient's arrival at the clinic, dental examination showed the presence of temporary crown on the tooth 47 and excellent oral hygiene. At that moment, the patient suffered from severe pain in the right part of her lower lip and mandible from the tooth 47 to incisal region. The pain was intensified in response to thermal stimuli (especially to cold) and percussion test. The patient was taking 3x1 tablet of 400 mg Ibubrufen per os in order to ease the pain. Periapical radiography showed possible close contact between neurovascular bundle of the tooth 47 and inferior alveolar nerve, and possible irritation of this nerve caused by endodontic treatment. Temporary crown, filling and intersession medicament were removed. The working length was determined using apex locator followed by carefully performed endodontic treatment of C-shaped canal using manual endodontic instruments. Preparation of the apical third was performed to the size of ISO # 25. It was intermittently irrigated with 1% solution of NaOCl, heated up to the body temperature. During the procedure, the patient suffered from pain and discomfort. When the treatment was finished a medicament dressing in the form of pad soaked in chlor-phenol camphor solution was administered to the patient. Painkillers were also prescribed, if necessary. After a week, intense pain was gone and the patient suffered only from the symptoms of right inferior alveolar nerve paraesthesia (tingling sensation, mild numbness and insensitivity of lower right lip and mental region). Canal obturation was performed with Guttaflow paste and adequate gutta-percha points with improved retention characteristics (Coltene, Whaledent AG, Switzerland). Upon obturation, the patient was referred to 3D orthopantomography of appropriate region that showed good canal obturation in all dimensions. The patient kept doing check-ups in the following period. Symptoms slowly eased and relieved and finally disappeared 8 months after their first occurrence.

DISCUSSION
Treatment of the tooth that caused neurosensory dysfunction depends on the type and severity of injury. The most common injuries of inferior alveolar nerve occur during surgical interventions, especially extraction of lower wisdom teeth and poorly planned implant placement [9,10,11]. Many authors reported occurrence of paraesthesia caused by endodontic treatment of root canal [5,[12][13][14][15]. Almost all of the materials used in endodontic treatment are neurotoxic at some level and can cause various inflammatory reactions that can lead to cell damage, ulceration, hemolysis and necrosis in contact with periapical tissues [4,5,6,16]. Irrigation solutions (NaOCl and EDTA) can   reach mandibular canal and cause chemical nerve damage [17,18]. NaOCl is an endodontic irrigant of choice since it has excellent antimicrobial effect and ability to dissolve tissue. It is cytotoxic even in low concentrations and causes protein denaturation, releases chlorine gas and draws fluids osmotically into periapical space [12]. Ethanol, irrigant as well, may cause oversensitivity of apical tissue since it has very strong dehydration potential [12]. Hulsmann and Hahn studied the application of different concentration of NaOCl (3% and 5,25%) for irrigation during endodontic treatment of mandibular premolars and molars and found in some cases paraesthesia combined with sensitivity dysfunction in lower lip [19]. They determined that toxicity of this irrigant caused chemical damage to neurovascular bundle [19]. An adequate irrigation technique with the application of small pressure and use of special needles with lateral perforations will decrease the possibility for periapical irritation. Damage of inferior alveolar nerve may also be caused by inadequate instrumentation of root canals of lower molars and premolars, mechanical irritation of apex or even separated instrument in mandibular canal [11,16,20]. Ca(OH) 2 as an intersessional medicament and a strong base that may cause nerve damage even though such cases are rarely described in literature. [2] The literature most often describes compression syndrome caused by extrusion of endodontic sealers beyond apex [12,14]. Experimental studies confirmed the role of eugenol and paraformaldehyde in neurotoxic reactions [1,4,12]. Kozam and Trowbridge reported in 1977 that eugenol has neurotoxic effect that can cause paraesthesia of inferior alveolar nerve. Eugenol causes chemical destruction of axon by protein coagulation [21,22]. Canal sealers, AH 26 and AH 26 plus, also have cytotoxic potential [23]. AH 26, a synthetic resin, together with formaldehyde causes tissue necrosis and inflammation [12]. Ehrmann was the first to report paraesthesia case caused by overfilling with N2 paste [11]. Gutta-percha is the material of choice for root canal obturation. It is inert material but it may cause paraesthesia if mechanically irritates the nerve [24]. Vertical condensation technique and other obturation techniques that require heated gutta-percha may also cause nerve damage [24]. Block anesthesia of inferior alveolar nerve may as well cause paraesthesia. Injury is mostly provoked by nerve damage with injection needle, compression effect or even combination of the two [14].
It is very important to understand variations of anatomorpholocigal characteristics of certain teeth groups since deviations from the average morphological characteristics are most common reasons for failure of endodontic treatment. One of the most interesting anatomical variations is C-shaped root and canal system. The shape and number of roots are defined by Hertwig's epithelial sheath that bends in horizontal dimension below cementoemanel junction and fuses in the center leaving the openings of the canals. C-shaped root may be formed due to constant deposition of cement over time [8]. Studies on lower second molar have shown high incidence of C-shaped roots and canals (10-31,5%) in Japanese, Chinese, Hong Kong Chinese, Lebanese and Thai populations [8]. Clinical recognition of C-shaped canals is based on definition of observable criteria (anatomy of the floor of pulp chamber and persistent haemorrhagia or pain when separate canal orifices are found). Pulp chamber in teeth with C-shaped canals may be large in occlusoapical dimension with low bifurcation. Sometimes, the canal can be calcified thus masking its C-shape configuration.
Nerve recovery after its damage depends on the severity of damage and rapidity of cause removal. Often, after the removal of cause, symptoms of paraesthesia continue to exist since the injury was not just mechanical but chemical as well. Endodontic material can spread to periapex in four different ways (through the nerve bundle, by drainage through lymphatic vessels, periapical capillary system and diffusion between the bones and mucosal membrane toward soft tissues) [2,25]. The anatomy of lower jaw favors diffusion of endodontic material, especially in the posterior area of lower jaw due to trabecular properties of cancellous bone that facilitates diffusion of different materials into the surrounding tissues. Special attention should be paid to the distance between anatomical openings of mandibular molars and mandibular canal. According to one study, this distance varies from 1 and 4 mm in the case of first lower molar, while it is less than 1 mm with second and third molar [25]. Cone Beam computed tomography-modern diagnostic method could help with therapy planning and prevention of paraesthesia occurrence as a complication of endodontic therapy [1].
Adequate endodontic treatment of the tooth that caused neurosensory dysfunction is important with additional application of cold packs, analgesics, antibiotic therapy, nonsteroidal anti-inflammatory drugs, synthetic corticosteroids (dexamethasone), proteolytic enzymes (which disintegrate coagulum), vitamin B complex, vitamin C (it has antioxidative action which reduces ischemic effects), and adenosine thriphosphate that regenerates tissues for restoring nerve function [15,16]. It is important to diagnose paraesthesia as soon as possible and remove potential causes of this dysfunction, preferably within 48 hours [12,27]. Surgical treatment includes extraction of causal tooth, apicoectomy and surgical removal of foreign body [27].
Based on the patient history and clinical findings, nerve damage in our study was classified as second-degree damage by Seddon: recognized axonotmesis manifested by paralysis of motor and sensitive nerve function. Due to the close connection between anatomical opening and mandibular canal, endodontic hand instrument most probably injured the axon and myelin sheath while NaOCl caused chemical irritation. Healing occurred spontaneously after adequate endodontic treatment and disinfection of root canal that was of utmost importance. Healing conditions were improved by low concentration of NaOCl used as an irrigant and the use of Guttaflow paste for final obturation.
The key to successful endodontic treatment of complex canal configurations is to know dental anatomy and apply adequate instrumentation and obturation techniques. This case report shows, apart from properly conducted endodontic treatment, positive features of guttaflow paste that is the sealer of choice in cases of close relation between the tooth apex and mandibular canal.