2. INFERIOR ALVEOLAR NERVE PARESTHESIA
Paresthesia is defined as altered sensation exhibited as numbness, burning or tingling of patient skin . The etiology of inferior alveolar nerve paresthesia is somewhat unknown, yet may occur following various dental procedures ranging from simple anesthetic injections [6, 9], surgical , orthodontic procedures . Third molar extractions  and oral pathologies  can also cause inferior alveolar paresthesia as well. Endodontic treatment has been associated with paresthesia , Froes et al. , reported paresthesia as a result of endodontic sealer extrusion that leaked into the mandibular canal.
Inferior alveolar or lingual nerve paresthesia [6, 16] is a complication of inferior alveolar nerve blocks . In some cases, paresthesia can be interpreted as injury to the inferior alveolar or lingual nerve bundle. The majority of cases involving lingual nerve neuropathies (89%) were more frequent following mandibular nerve blocks .
There is a wide variety of anesthetic agents used in dental procedures. Articaine is the anesthetic of choice used in many dental practices . Its use is thought to be optimal; as it is proven to be efficient and it is easily diffusible through bone and tissue. The formula most commonly used in US and Canada is Articaine hydrochloride 4% with epinephrine 1:100,000 [17, 18]. Certain anesthetic formulations such as Articaine 4% and Prilcaine 3-4% have been suggested to have a neurotoxic effect causing sensory loss  simply because of the higher concentration of the anesthetic . Articaine-related paresthesia ranged as high as 71%  and as low as 33% in other studies . This wide range of difference in Articaine-related paresthesia may depend on the method, search queries and the database used in the study . Articaine paresthesia was significantly higher than marketshare paresthesia in majority of studies . However, cell culture experiments and animal studies did not find a higher toxicity of Articaine compared to other anesthetics.
Gaffen and Haas (2009)  reported that Articaine had the lion’s share of paresthesia cases (59%) compared to other classes of anesthetic used between 1999 and 2008. This data was supported by a previous study by Haas and Lennon (1995)  that indicated that Articaine was used in the majority of non-surgical paresthesia cases in Ontario between 1973 and 1993. Prilocaine came in the second place following Articaine. Tongue paresthesia, then lip paresthesia was the most frequent side effects, with combined tongue and lip paresthesia in a few cases. Taste loss, speech impairment and drooling are symptoms that may accompany oral paresthesia. It is argued that anesthesia could have a neurotoxic effect to the inferior alveolar nerve fibers especially if the needle delivering anesthesia penetrates the neuronal sheath. However, almost half of the cases of inferior alveolar nerve damage exhibit no signs of nerve trauma caused by needle injection .
There are numerous studies that present an array of clinical cases of prolonged lingual and mandibular anesthesia . Loss of sensation may resolve in a matter of days, weeks or few months [7, 9]. In a few rare cases there was an unresolved loss of sensation that lasted for more than six months . Brann et al (1999)  found that patients who underwent general anesthesia before third molar extraction had a five times higher incidence of lingual or inferior alveolar nerve damage.
Third molar extractions are very common procedures in most dental practices. Many complications might result from third molar extractions including bleeding, pain, swelling, dry socket, trismus and paresthesia [12, 21]. The chief postoperative complaint of was numbness . The proximity of the third molar roots to the inferior alveolar nerve might play a role in developing inferior alveolar or lingual nerve damage postoperatively . Many radiological studies focus on the location of the inferior alveolar nerve and its relation with the third molar, subsequently finding that there has been narrowing of the mandibular canal in that region. Thickness of the alveolar bone surrounding the mandibular canal is a crucial in preventing dental paresthesia to avoid perforation into the inferior alveolar nerve especially when performing implant placement  or treating periodontal pocket accompanied with alveolar bone loss . However, there are new promising treatments to restore bone loss of the alveolar bone using biodegradable chitosan derivatives [24, 25].
It was reported in 31 out of 134 cases of paresthesia, patients had felt an “electric shock” sensation at the time of anesthetic administration  which might be an indication of nerve injury by trauma at the site of injection. Seddon (1943)  has attempted to classify nerve injuries based on the severity of the injury. Neurapraxia, the mildest form of nerve injury , may lead to mild paresthesia that resolves within a few hours or days. Axonotmesis, the second type of nerve injury which is more severe, is exhibited as severe paresthesia that may take several months to resolve. At this level of nerve injury the nerve bundle is generally intact but there is a small degree of nerve degeneration. Neurotmesis is most severe type of nerve injury where the nerve is completely disrupted. A complete loss of sensory innervation is observed in this case. Sunderland (1978)  used a similar classification of neuronal injury as Seddon. Sunderland’s nerve injury scale ranges from grade I to V escalating in terms of severity.
It has been suggested that dental injection complications might be due to neurotoxicity and concentration of anesthetic agent used  or simply caused mechanical injury to the nerve by barbed needle . In general local anesthetics including Articaine are considered safe if the dental injection is performed properly. The procedure difficulty, needle gauge and age of the dentist  are all factors that should be taken into consideration to avoid inferior alveolar nerve damage.
To understand the nature of dental paresthesia and their relationship with the site of inferior alveolar or mandibular nerve blocks, dentists need to appreciate the anatomy of pterygomandibular fossa which will be described in this next section.
3. ANATOMY OF PTERYGOMANDIBULAR FOSSA
Understanding the anatomy of pterygomandibular fossa is crucial for dentists since it is the target space for local anesthetic administration prior dental treatment. Key structures that are present in pterygomandibular fossa include lingual nerve, inferior alveolar nerve and nerve to the mylohyoid. The sphenomandibular ligament and the interpterygoid fascia are also integral structures that define this area [28, 29]. The borders of pterygomandibular fossa are bounded posteriorly by parotid glandular tissue and anteriorly by pterygomandibular raphe made by the union of buccinator and superior constrictor muscles. The lateral border is defined by the mandibular ramus and the medial border is formed by medial and lateral pterygoid muscles. The inferior alveolar nerve is one of the most important branches of the mandibular branch of the trigeminal nerve. Before its entry to the mandibular foramen near the lingula of the mandible; it gives off their well-known branch the mylohyoid nerve supplying both the mylohyoid and anterior belly of digastric muscles. Toward the end of the mandibular foramen; the inferior alveolar nerve continues as the mental nerve emerging from the mental foramen and the incisive nerve continuing in course anteriorly. Both of these nerve branches are sensory, the mental nerve supplies the skin of chin and oral mucosa while the incisive nerve is responsible for providing sensory innervation for premolar, canines and incisors. The inferior alveolar nerve is also accompanied with inferior alveolar artery and vein. Inferior alveolar artery is a branch of the mandibular artery, although it has been reported that it can branch off the external carotid artery . Another study of 56 hemisected cadavers  head analyzed crucial structures in pterygomandibular fossa indicating an average of two inferior alveolar veins per specimen. In addition to that the inferior alveolar nerve was found to be anterior to inferior alveolar vasculature in most specimens.
There is a considerable amount of literature regarding anatomical variations in pterygomandibular fossa especially the inferior alveolar nerve . Variant of the inferior alveolar nerve has been a classified into extra-osseous and intra-osseous multiple branches of the nerve. The presence of extra-ossesous branches has been concurrent with the presence of multiple accessory foramina in the mandible.
Lingual nerve injury has been reported in some cases of inferior alveolar nerve blocks [6, 16]. In study by Morris et al., 2010  simulated inferior alveolar injections were performed in cadavers head to estimate probability of lingual nerve trauma associated with inferior alveolar nerve blocks. The location of the lingual nerve was highly variable; although about 96% of the injections were made lateral to the lingual nerve. A small fraction 4.5% of these simulated injections has actually penetrated the lingual nerve.
The location of lingual nerve in cadavers’ study done by Kiesselbach and Chamberlain, 1984  was found to be in direct contact with lingual crest in majority of cadavers (62%) and superior to the lingual crest in only 17.6% of cadavers dissected in the study.
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or otherwise.