ATLAS OF ANATOMY OF CRANIAL FOR DENTISTRY

KEY CONCEPTS AND ILLUSTRATIVE TABLES ATLAS OF ANATOMY OF FOR DENTISTRY

KEY CONCEPTS AND ILLUSTRATIVE TABLES

Introduction VII.!Facial

There has been an interest in anatomy since ancient VIII.! times. The skull and its precious content have always IX.!Glossopharyngeal Nerve been some of the most fascinating and complex ana- X.! tomic elements. The head and the include hi- ghly specialized parts of the body. The structures con- XI.! tained in them are closely related, being packed in XII.! an extremely small area. Plus, the head and neck area have an innervation that concerns the work of The cranial nerves have seven specific functional the dentist. Dentists deal with some nerves regularly; components that can be passed within them. No cra- it is, therefore, important for the dentist to know the nial nerve has all the function within it. Each cranial anatomical aspect linked with the head and neck. nerve has specific patterns responsible for receiving Anesthesia allows us to avoid any painful stimulus sensory input through the receptors or producing the during the treatment. It was the basis for this Ana- motor function’s outputs. An additional component is tomy Compendium of the Nerves, focused on the that of proprioception, which can be traced back to a ones of the oral cavity. sensory input that presents the muscles that are in- nervated by cranial nerves [1].

Cranial Nerves Motor Functions – Output Somatic Efferent: the motor innervation of the skele- There are twelve pairs of cranial nerves. Originating tal muscles developed by somitis. from the brain, they pass through some foramina of the skull and distribute to the head-neck area. The General Visceral Efferent: the motor fibers that in- cranial vagus nerve continues in the and abdo- nervate smooth muscles, muscles, and glands. men, innervating some entrails. The cranial nerves Only four cranial nerves transmit parasympathetic are named and numbered in sequence with Roman fibers: the oculomotor, facial, glossopharyngeal and Numbers, proceeding in the craniocaudal direction. vagus nerves. Sensory Functions – Input I.! General Somatic Afferent: the general sensation II.! (touch, pressure, temperature, pain) of the around the front and side of the face. An example of III.! this function is given by the when it IV.! covers a wide part of the skin and mucous membra- nes of the face, while the facial, glossopharyngeal V.!Trigeminal and vagus nerves cover the area of the . VI.!

1 General visceral afferent: composed of the fibers ganglion, located in a subdural dimple (called the that carry the general sensation of the viscera, ge- Meckel) near the apex of the petrosal part of the tem- nerally perceived as pressure and pain. poral bone. From an embryological viewpoint, the tri- geminal forms the nerve of the first , Special somatic afferent: carries the sensations co- the same origin of all the tributary muscles mentio- ming from the and ear. ned above. Special visceral afferent: these fibers are associa- The nerve leaves the endocranial space through the ted with the special of smell, carried in the oval foramen located in the olfactory nerve and , transmitted in the facial, () and emerges in the infratemporal glossopharyngeal and vagus nerves. fossa. Still close to the base of the skull, the main Cranial nerves and oral cavity functions trunk releases the sensory meningeal branch and so- Let's now review the main features of the nerves in me of the motor fibers, and then divides into a smal- the oral cavity: trigeminal, facial, glossopharyngeal, ler anterior branch and a larger posterior branch. vagus, accessory, hypoglossal. Ophthalmic branch TRIGEMINAL NERVE The provides the bulb and palpe- The fifth pair of cranial nerves (V), the trigeminal ner- bral , the tear gland, the skin of the head ve, carries this name because it is divided into three and nose and the mucous membranes of the parana- main branches: ophthalmic (V1), maxillary (V2) sal sinuses with sensory innervation. Before entering and mandibular (V3). The latter represents the main the upper orbital fissure, it releases three terminal branch because, unlike the other two carrying only branches, the tear nerve, the , and the afferent fibers, it also carries efferent fibers: it is, the- . refore, due to the alone that the tri- The tear branch is of great interest concerning the geminal can be called a mixed nerve. The V is also ophthalmic branch when it comes to the oral cavity. the largest of all the cranial nerves, being responsib- The tear nerve is the smallest branch of the ophthal- le for serving—with an eminently sensory role—a lar- mic division; it runs along the lateral muscle ge part of the face, the dental arches and the support- distributing itself to the tear gland and the adjacent ing structures, a large part of the oral cavity and, in conjunctiva. While in the eye socket, it communica- general, of the mucous lining structures of the head. tes with the zygomatic-temporal branch of the The apparent origin of the nerve is located in the of the maxillary division of the , exactly on the anterior face of the bridge trigeminal nerve, which carries postganglionic pa- near the middle cerebellar peduncles. The large, flat- rasympathetic fibers from the pterygopalatine gan- tened sensory root is located laterally to the thin mo- glion. These parasympathetic fibers are then transmit- tor root, which is responsible for innervating the ma- ted to the tear gland through the tear nerve, thus pro- sticatory musculature, the anterior belly of the diga- viding secretomotor innervation. stric muscle, the tensor of the tympanum and that of the soft , as well as transporting fibers (VII pair of cranial nerves) through the . Maxillary Branch

The neurons that make up the sensitive fibers have The second trigeminal nerve branch, the maxillary a trophic center located at the level of the Trigeminal nerve, is only sensory and serves the skin of the side

2 of the nose, , , central part of the face, nerve divides while it is in the canal to nasopharynx, tonsils, palate, , gum, form a minor palatine nerve, which co- teeth and structures associated with the upper jaw. mes out to the palate through two or The nerve emerges from the cranial vault through the three homogeneous foramins serving the round foramen after passing through the back of the , the tonsil and the uvula. . The nerve flows out of the round o Upper posterior nasal branches: they foramen through the to enter enter the from the foramen the floor of the at the bottom of it. Here, the ner- sfenopalatino. The nasopalatino nerve ve becomes the and enters the in- emerges. fraorbital canal of the same and comes out of the in- fraorbital hole. This nerve can be anaesthetized if an • Foramina of Scarpa: the nerve nasopalatine upper arch procedure is required, including the sinus or foramina of Scarpa is a collateral branch of lift. A publication [2] has precisely evaluated the the pterygopalatine nerve; the internal branch anesthetic effectiveness of The Wand in this procedu- of division of the maxillary branch of the trigemi- re, finding excellent results. nal nerve (V pair of cranial nerves). It runs along the floor of the nasal cavity, attached to Let's see which branches it releases during its cour- the periosteum of the septum, moving from the se. back of the septum forward. It then enters the • Medium meningeal nerve: detaches before foramina of Scarpa to access the oral cavity the engages in the round hole; through the hole (or anterior palatine). follows the middle meningeal artery in its ramifi- Here we observe a confluence between the cations in the . two nasopalatine nerves, which, in turn, can • Zygomatic nerves: releases the zygomatic-fa- anastomize with the major palatine branch. The cial branch that provides the sensitivity of the serves the anterior third of cheek and the zygomatic-temporal branch that the hard palate with its mucosa. It may involve innervates the side of the . The fore- the gum and support structures of the inter- head side releases a branch directly to the tear chain region. The nerve block of this structure, nerve before leaving the orbit. This carries indicated in interesting mucous and periosteal postganglion parasympathetic fibers derived interventions and also in periodontal treat- from cranial nerve VII from the pterygopalatine ments at the same area. Dental pulp is not in- ganglion. volved and anesthesia is not recommended when treatment is focused on one or two ele- • : they emerge near ments. [3] the homonymous ganglion. Consequences of the block of the foramina o Orbital nerve: the orbital branches enter of Scarpa the orbit to supply the periorbite and the The method is beneficial because it allows the posterior ethmoidal and sphenoidal si- anesthesia of a large portion of the palate with nus. a minimum amount of local anesthetic. At the o : comes out from same time, it can be problematic, as the injec- the homonymous hole located in the pala- tion procedure is often painful in this area. The te about 1 cm apically to the upper mo- main anatomical finding is represented by the lars; it goes forward to the midline to ser- interincisive papilla, under whose palatal conti- ve the entire palate. The major palatine nuation lies the foramen itself. Malamed recom-

3 mends the use of a 27 or 25-gauge needle, pre- led the upper middle alveolar, to the lateral wall ferably short for greater stability and, to appro- of the maxillary sinus, innervating it. The bran- ach the area in the least invasive way possible, ches of this nerve also innervate the mesial suggests to consider two different methods of buccal root of the first molar and all the roots of operation. the upper premolars. The infraorbital nerve is a branch of the maxillary nerve, the second The first is the single injection technique to be branch of the V pair of cranial nerves. It emer- performed with the patient lying open-mouthed ges from a hole of the same name on the mala- and with the neck stretched out. With a cotton rial surface and conducts the sensitivity of the swab, ideally soaked in a topical anesthetic so- skin and mucous membranes of the middle lution, the area adjacent to the papilla is pres- third of the face. Once out, it is divided into four sed to blanch the tissue. On the other hand, distal branches, dedicated to the specific skin the needle is made to enter: during the deepe- areas, and three proximal ones: the latter corre- ning, small quantities of solution are released. spond to the upper, anterior, middle and poste- As soon as the bone is reached (at a 5 mm rior alveolar nerves, which together form the depth), the needle is recoiled by about one milli- plexus that serves the entire upper dental arch. meter and, after aspiration (the positive rate is An anatomical peculiarity of this nerve is repre- almost zero), another anesthetic is given. With sented by the so-called canalis sinuosus. It a quarter of a vial tube, on average, the tissue was first described in 1939 by Jones as a ner- appears stiffened as well as further blanched. ve file that emerges from the posterior portion • Pharyngeal branch: leaves the rear aspect of of the infraorbital hole, and then runs into a tor- the pterygopalt ganglion to enter the pharyn- tuous bone channel, about 2 mm in diameter, geal canal. It serves the next to the nasal cavity. The canal originates and the rhinopharynx up to the acoustic meato. from the terminal tract of the infraorbital canal (i.e., before it flows into the respective hole), • Upper posterior alveolar nerves: derived initially running forward towards the nasal ca- from the main trunk of the maxillary nerve, whi- vity, and then moving downwards after passing le still in the pterygopalatine fossa. They pass through an "S". The term "canalis sinuosus" [4] over the tuberosity of the upper jaw providing refers to this characteristic segment. The canal, branches to the mucous membrane of the after an intraosseous path of more than 5 cm, cheek and the adjacent gum. The posterior al- finally emerges from a secondary hole at the veolar nerves then enter the foramen of the sa- palatal level: the latter tract is subject to impor- me name to provide the innervation of the ma- tant anatomical variability. The structure hou- xillary sinus and the molar teeth where they ses the course of the anterior superior alveolar form the dental plexus, with the exception of nerve (with the respective comitant vessels) the buccal mesial root of the first molar. Senso- and is, therefore, not underestimated, especial- ry innervation of this root is provided by the up- per middle from the infraorbital. ly in the area's implant surgery. Fortunately, the final tract of the nerve, which, as mentioned, • Infraorbital branch: this is the terminal branch participates in the plexus dedicated to the up- of the maxillary nerve. After passing through per arch, presents a series of anastomoses ab- the pterygopalatine fossa, the maxillary nerve le to vicariaire any damage: in some cases, enters the floor of the orbit, thus becoming the however, it is possible to find neurological disor- infraorbital nerve. Upon entering the floor of the ders related to traumatic events. Moreover, the- orbit, the infraorbital nerve sends a branch, cal-

4 se symptoms are often difficult to interpret from Sensory component Motor component a clinical viewpoint , as they are not better eva- Sensory fibers serve the skin The motor component provides luated previously. Ferlin's group has conducted around the lower third of the all the muscles developed within face, the cheek, the lower , the the first pharyngeal arch: the a systematic review of the subject, obtaining ear, the external acoustic meatus, mastication muscles, including from the Scopus, Medline and Web Of Science the temporomandibular joint and the temporal, masseter, medial databases a total of seventy records, eleven of the temporal region. It also and lateral pterygoid muscles, as which have been evaluated qualitatively. Most innervates the mucous well as the timpani and palatine membrane of the cheek, the veils tensors and the anterior of the studies consist of CT cone beams to eva- mucous membrane of the belly of the digastric and luate accessory channels. However, 90% of anterior two-thirds of the , milooideian muscles. the evidence relating to these documents was the mandibular teeth, the gingiva, the mastoid and the evaluated as being of moderate or high quality. body of the . The study concluded by observing the possible variability of the canalis sinuosus in terms of From the latter emerge the palatine tensor and pala- position, diameter, route and presence of ac- tine veil tensor branches. cessory channels. The canalis sinuosus is pre- After leaving the oval hole, it divides into its two sent between 88 and 100% and its accessories terminal branches: between half and 70% of the time. As far as cli- • Anterolateral: almost exclusively motor, nical management in surgical cases is concer- composed of temporo-buccinator nerves, ned, the authors underline the importance of medium deep temporal, temporo-masseteri- not underestimating the problem, especially du- ne and external pterygoid, buccal nerve. ring the surgical programming phase: the most recommended diagnostic measure, from this • Posteromedial: almost exclusively sensory. viewpoint, is CT cone beam. It breaks down into auriculo-temporal, lin- gual, lower alveolar, internal pterygoid ner- ves, the tensor muscle of the palatine veil Mandibular branch and the tensor muscle of the tympanum. It's the only branch that contains both a sensory and Of the anterolateral component we see in detail the a motor component. In addition to the mandibular ner- buccal nerve. The buccal nerve, also called long ve, the otical, submandibular and sublingual ganglion buccal nerve in the English literature, represents are attached. one of the two terminal branches of the inner trunk of The mandibular nerve leaves the skull through the mandibular branch of the trigeminal nerve (V3). the oval hole in the large sphenoid wing. Before This structure collects and conducts the sensitivity of the division from the main trunk occurs, the recurrent the mucosa of the posterior area of the cheek; it also meningeal nerve and the medial pterygoid nerve serves the vestibular side of the gingiva of the molar are detached. region. In some cases, it may contribute to the inner- vation of the skin of the corresponding extraoral re- gion.

The origin is rather high at the level of the infratempo- ral fossa and the course is in good caudal and lateral substance. After the origin, the nerve has an initially horizontal trajectory, running between the two ends of the . It then effectively mo- ves downwards against the medial face of the mandi-

5 bular branch. Finally, it crosses the retromolar trigon, The three terminal branches of the mandibular moving towards the cheek. It innervates the exter- branch are: the auriculo-temporal nerve, the lin- nal pterygoid muscle, mucous membrane of the gual nerve and the lower alveolar nerve. cheek, gum of the external surface of the lower After having emerged at the level of the neck of the alveolar process. mandible, the nerve rotates above with the superfi- The buccal nerve is an important structure from the cial temporal artery and penetrates within the parotid anesthesiologic viewpoint: by adopting the common gland structure. It continues to rise, exiting the gland technique of lower alveolar blockage, in fact, it is not to pass over the zygomatic arch to distribute sensory possible to reach the area of the gingiva served by fibers as a superficial temporal nerve to the skin of the nerve. the temporal region. In its course, the auriculo-tempo- ral nerve sends articular branches to the temporo- For this reason, the procedure is integrated with an mandibular joint, anterior auricular branches to the administration of anesthetic distally to the lower mo- anterior portion of the , to the outer acoustic lar region. On the contrary, the Gow Gates techni- meatus and with some branches to the parotid gland. que involves upstream the mandibular nerve and, These are made up of postganglion parasympathetic consequently, its main branches, including the lower fibres from the . and buccal alveolars themselves. Sicher and Du- brul (1991) [5] state that the nerve crosses the upper part of the retromolar fossa and it is in this area that the nerve can be exposed to the anesthetic solution. The lingual nerve is one of the two terminal bran- Malamed defines the long vestibular injection techni- ches of the inner trunk of the mandibular branch of que and ensures an almost absolute success rate. the trigeminal nerve [7]. It conducts the somatic sen- From a surgical viewpoint, it is now established that sitivity of the anterior 2/3 of the tongue—to which is the buccal nerve does not present any particular criti- added the special gustatory sensitivity conducted via cality. It may actually be affected during third molar the tympanum string—of the sublingual mucosa, the extraction operations included if the release incision oral floor and part of the lingual aspect of the gingiva. is conducted too deeply. For this reason, the ma- The tympanum cord also carries preganglial para- nuals recommend that the flap be set up in full sympathetic fibers to the submandibular and su- thickness only in correspondence of the retromolar blingual ganglia. It innervates the mucous membra- trigon, and then continue to half thickness along the ne of the lower and upper surface of the lingual body. upright mandibular branch. According to Hendy This nerve structure is of great importance in oral (1996), an incision more than 12 mm below the ma- and maxillo-facial surgery. Temporary or permanent jor concavity of the oblique line puts the buccal nerve damage is an infrequent but nevertheless contem- at risk [6]. The not complete reliability of this finding, plable eventuality in a series of operations at the le- especially in edentulous subjects , means that today vel of the regions of the third mandibular molar and the main reference is the anterior margin of the of the oral floor. The extraction of the lower third branch of the mandible. The nerve usually crosses molar is the most routine of the surgical acts invol- the upper half of this line. ving this anatomical region. In the case of implant sur- Let us now focus on some of the main structures of gery, on the other hand, direct involvement of the the posteromedial component. The only element re- structure is less frequent, which may be involved if it sponsible for motor innervation is the miloioid nerve is necessary to extend the surgical access to the site that leads to the muscle of the same name. by setting up release incisions. For this reason, proto- cols always recommend conducting such incisions with a clear vestibular pattern.

6 The lingual nerve detaches from the lower alveolar tions: it may communicate, for example, with the mi- nerve before it enters the . Three loideo nerve (dependence on the mandibular anatomical patterns have been identified depending branch) or even with the hypoglossal nerve (XII pair on the height of the bifurcation, to which a fourth one of cranial nerves). with a plexiform appearance has been added. Star- ting from the origin, the lingual nerve runs between Lower alveolar nerve the external and internal pterygoid muscles in an in- The lower alveolar nerve is the middle branch of fratemporal fossa, and then continues forward and the mandibular branch and emerges above the spine down along the medial surface of the internal pte- of the Spix. It is carried behind and to the side of the rygoid itself, the relationship with which it is variable. lingual nerve between the two pterygoid muscles and It is also close to the internal surface of the upright penetrates the mandible body through the mandibu- branch, and then moves not far (postero-inferiorly) lar body. Before penetrating the canal it releases the from the junction with the body of the mandible. It en- miloioid nerve. ters the submandibular region after having moved forward and medially, close to the lower margin of The lower alveolar nerve runs through the canal un- the upper constrictor muscle of the . From til it reaches the foramen mentoniero near the roots here it will be in close relationship with the region of of the premolars; from here it is divided into three the lower third molar. It declines adhered to the perio- branches that innervate the skin of the chin, the mu- steum on the medial surface of the mandible, pro- cosa of the lip and the lower alveolar surface. The tected only by the mucoperiosteal tissues. It is in con- lower dental plexus innervates the and dental tact with the lingual cortical in 20-62% of cases and elements. A significant surgical risk is represented is located at or above the alveolar crest in 4.6-21% of by the iatrogenic trauma—and the possible neuro- cases. If it is not found at the level of the lingual corti- pathic consequences: paresthesia-distesthesia, cal, 0.57-7.10 mm median to this and below etc.—and by the subsequent neuropathic outcomes. (2.28-16.8 mm) the alveolar crest runs. The retromo- The trauma is caused by the , lar trigon proper is actually an infrequent site or better, by its terminal tract, in the case of a loop (0.15-1.5% of cases). The average distance from the inside the canal, just before it emerges from the men- retromolar region is 7.2 mm. The nerve can emit he- tal foramen. The resulting chinner nerve—directed re a collateral, also called "gingival branch," which to the lower lip, skin of the chin guard region and part extends up to the fifth-sixth dental element area. of the vestibular gum—is generally considered the main terminal branch of the lower alveolar nerve, The course of the main nerve moves towards the ton- which usually continues with a small incisor branch gue decisively, running in an antero-medial direction directed precisely to the frontal elements of the lower on the upper surface of the milioid muscle. Then, it arch. It is possible that the main branch, before emer- crosses the Wharton's , running laterally first, ging as a chinner nerve, passes through the hole, de- then below, and then resorting antero-medially. Next, scribing an anterior convex loop, then returning to- it turns towards the surface of the halibut muscle. Stu- wards the hole and, finally, coming out. In the texts dies on corpses report this ratio in 62.5% of cases. we commonly speak of "anterior loop" of the nerve. However, it is inverted; i.e., the nerve passes above The study by Velasco-Torres (2017) proposed, the duct in a considerable number of cases, equal to among other evaluations, to estimate on CT cone the remaining 37.5%. Current evidence suggests that beam the prevalence and extension of the anterior the point of intersection is inconstant. loop, in a sample of 348 patients. The analysis Before resolving in the terminal branches along the showed a negative correlation (lower prevalence and lingual belly, the nerve may present further varia- extension) with age growth. This is potentially an indi-

7 cation of great clinical interest: making an example of so nerve (originating from the internal carotid plexus, treatment at risk, Todorovic, in his very recent work, mainly sympathetic vasomotor), thus giving rise to refers to full-arch rehabilitation with four to five interfo- the pterygoid canal nerve (or vidian nerve). This ner- raminal implants. Approaching the foramen, the co- ve thus arrives at the pterygo-palatine ganglion (or verage area is extended in an antero-posterior direc- sfenopalatine), from which these fibers, which have tion and the cantilever is reduced. The study in que- become postganglionic, will join the zygomatic nerve stion aims to quantify prevalence and size through (branch of the maxillary nerve, second branch of the reformatted multislice CT investigations (retrospecti- trigeminal nerve) leading first to the zygomatictempo- ve study on examinations required for other clinical ral nerve, terminal branch of the zygomatic, and reasons). Unlike the previously mentioned study, then, due to an anastomosis, to the tear nerve, finally which focused on Caucasian subjects, the latter deli- reaching the tear gland. From the pterygo-palatine berately did not consider possible racial differences ganglion will also originate post-ganglion parasimpat- (it is also a sample of 188 South African patients). In hetic fibers directed to the glands of the nasal muco- this case, a statistically significant difference in sex sa and the palatine glands. was reported (higher prevalence in males). It can al- - nerve for the stapedium muscle: provides for ready be said that the anterior loop is an anatomical the motor innervation of the stapedium muscle. event to be considered in the context of implant preo- perative, much more than other much rarer variables, - tympani chord: the tympani chord slowly moves such as the doubling of the hole. towards a separate channel. Leaves the tympanic cavity to enter the petroympanic fissure, then exits the skull at the level of the posterior spine of the FACIAL NERVE sphenoid bone. The nerve receives communication from the otic ganglion and joins the lingual branch of The components of the facial nerve and their func- the mandibular division of the trigeminal nerve. It con- tions are: special visceral motor, general sensory, tains special sensory fibers intended for the taste visceral sensory, special sensory, parasympathe- buds on the anterior two-thirds of the tongue and pre- tic [8]. gangular parasympathetic fibers intended for the The nerve has two roots, a large motor root and a . The smaller root, called the , contai- receives the preganglial parasympathetic fibers of ning the special sensory fibers for taste, parasympat- the tympani chord nerve through the parasympathe- hetic fibers and sensory fibers. The two roots emerge tic root. The postganglion parasympathetic fibers from the brain between the bridge and the lower cere- from the submandibular gland pass to the submandi- bellar peduncle. bular gland or return to the lingual nerve to be distri- The course through the skull ends when the facial buted to the and the minor salivary emerges from the stilomastoid foramen. The gan- glands in the floor of the , providing them with glion that accompanies the nerve is called the geni- secretomotor innervation. culate. - When the facial nerve leaves the stilomastoid fo- Numerous branches rise from the nerve as it flows ramen, the posterior auricular nerve that passes through the temporal bone, including: between the auricle and the mastoid process begins, dividing into occipital and auricular branches. - major petroso nerve: it carries motor and parasym- pathetic fibers (respectively from the facial nerve nu- - They originate from the same area where someti- cleus and from the muco-lacrimatory nucleus); along mes with a single emergency the nerves for the mo- its route it contracts anastomosis with the deep petro- tor component of the posterior belly of the diga-

8 stric muscle and styloid for the homonymous portion of the tongue and to the circumvallated buds. muscle. Other branches communicate with the lower palatine nerve of the and lead to the tonsils. - Parotid plexus: after entering the parotid gland, the facial nerve divides into temporo-facial and cervi- cal-facial divisions that join into the parotid plexus. From here emerge the branches that provide motor VAGUS NERVE innervation to the muscles of facial expression, cal- The vagus nerve is important and has a longer cour- led branches: se than other cranial nerves. The vagus nerve also -! Temporal enters the chest to serve the heart and and -! Zygomatic continues in the . Despite its many func- tions, compared to the oral cavity, it carries special -! Buccal visceral afferent fibers for the base of the tongue. -! Mandibular It also has fibers with special visceral efferent func- -! Cervical tion, general somatic afferent, visceral somatic affe- rent, and general visceral efferent.

GLOSSOPHARINGEAL HYPOGLOSSAL NERVE The glossopharyngeal nerve has fibers that carry si- gnals of type: The most caudal of the cranial nerves is the hypo- glossal nerve, the motor nerve of the tongue mu- - Special visceral differentiator. scles. It enters the muscle structure of the tongue, - Efferent visceral parasympathetic: for the parotid which it provides, proceeding towards the ventral gland, other minor salivary glands of the posterior part of the tongue. area of the tongue and the adjacent area of the pha- rynx. NERVE INJURIES RELATED TO DENTAL PROCEDU- - Special visceral afferents: for the papillae of the RES posterior third of the tongue and the circumvallate ones. Lower alveolar nerve - General visceral afferents: provides the papillae Neuropathic damage is undoubtedly a form of iatroge- of the posterior third of the tongue and the palatine nic damage that is not common but, at the same tonsil. time, frightening, precisely because of its rarity as well as the difficulty of its diagnostic and therapeutic - Somatic afferent generals. approach. Most of the time, these are transitory con- Dwelling on the branches of greatest interest in the ditions that tend to resolve autonomously within a cer- dental field, it is possible to observe that the main tain period of time. In some cases, however, the da- trunk of the glossopharyngeal nerve ends as several mage may require invasive treatments of a neurologi- pharyngeal branches to enter the posterior wall of cal and neurosurgical nature. The variability of the the pharynx; some of these branches then lead to type of damage is reflected in the clinical picture, the tongue as lingual branches providing a general which ranges from hypo-anesthesia, to different de- sensation to the posterior third of the tongue and brin- grees of paresthesia, up to allodynia-hyperalgesia. In ging special sensory fibers to the taste buds on that 70% of cases, a painful symptom is observed.

9 In the case of the lower alveolar nerve, the possible As regards localization, the length of the roots of the causes of iatrogenic involvement in dentistry can be first premolar, first and second molars should be eva- traced back to four main categories: luated. A second aspect to be evaluated in the growing patient is the degree of patency of the - damage caused by local injective anesthesia techni- apexes. que. Damage to the lower alveolar nerve due to - damage caused by endodontic therapy. anesthesia - damage on a surgical basis by implanting and avul- Anesthetic techniques at the level of the lower al- sion of the third molar. veolar nerve may represent a significant clinical pro- According to the 2011 Renton and Yilmaz study, the blem, especially considering the rate of failure that latter is the most common cause (alone covering affects the most common technique, namely the 60% of cases), followed by local anesthesia techni- lower alveolar block. However, cases are descri- ques (19%), on a par with implant surgery. Only 8% bed, certainly rarer, in which the technique is associa- of the episodes would represent the complication of ted with real complications, some of which are seve- an endodontic treatment. re and not entirely imponderable [9]. Starting from these data and considering these same This is the case of neuropathic damage secondary broad categories, Martinez and colleagues (2014) to anesthetic procedure: as already mentioned in have conducted a review of the literature referring to the first part of the text, it would be the cause of al- the period 2008-2013. The evidence collected, the most two out of ten episodes in this field. Regardless main points of which are reported below, refer to a of the anesthesiologic technique used—lower alveo- corpus of forty-five scientific articles, selected from lar nerve block, Gow-Gates technique or Vazirani-Aki- an initial pool of almost four hundred. nosi technique—the repetition of anesthesia can be useless in a context of refractoriness to pharmacolo- Damage to the lower alveolar nerve in endodon- gical action, as can be the acute pulpitis. More useful tics in these cases is the use of different additional techni- As anticipated, neuropathic damage of the lower ques, primarily intraligamentous anesthesia. alveolar nerve does not represent a common compli- The higher the number of injecting procedures, cation of canal treatment. On the contrary, the ruptu- the higher the risk that one of them will favor the re of an instrument is a feared but contemplated pos- onset of a complication, even a remote one. sibility for the endodontist, especially when it comes When using a computerized anesthesia device to rotating instruments and especially in the presen- such as The Wand, the position for the injection ce of thin channels and important curvatures. Howe- is not changed for the aspiration test. Simply ask ver, it is difficult for the instrument to go beyond the the machine to aspirate, and check that the aspi- apex until it engages the mandibular canal where the ration test is negative and inject. This avoids a nerve runs, since the fracture follows precisely on second injection, which will inevitably be a few contact with a sudden obstacle. millimeters displaced from the first, thus making It is more likely that the complication follows the extru- the clinician really sure they are not inside a sion from the apex of canal cement, hot gutta-percha blood vessel and reducing the risk of "hitting" or other endodontic materials. It is, therefore, impor- the lower alveolar nerve and lingual. tant to know the potential neurotoxicity of some of [Movement control during aspiration with diffe- these products. rent injection systems via video monitoring-an in

10 vitro model. Kämmerer PW1, Schneider D2, Pacyna anatomical variations: the most significant, in this sen- AA3, Daubländer M4. Clin Oral Investig. 2017 se, is the possible bifid course of the nervous trunk. Jan;21(1):105-110.] Neuropathic damage of the lower alveolar nerve Although rare, cases of permanent damage to the during surgery lower alveolar nerve have been reported. This struc- The last cause treated is represented by the compart- ture would be at lower risk than the lingual nerve, ment of surgical damage which, in reality, represents which runs much more superficially and, therefore, the most numerically relevant possibility, able to co- would be more often affected; the painful symptom, ver almost eight cases out of ten of damage to the however, would be systematically more severe in the lower alveolar nerve. case of the lower alveolar nerve. The traumatic outcome of implant surgery is, in many Fortunately, the anesthetic solutions currently used cases, the expression of deficient planning and, in ensure high standards of safety. According to the six- particular, of an imperfect phase of radiological dia- year study conducted by Progrel and colleagues, gnosis. If, on the contrary, the planning is correct, the however, any molecule would be able to lead to pa- nerve damage is a rare complication and, if it occurs, resthesia: the classic lidocaine would be linked to not serious, because it is secondary to the inflamma- 25% of cases of paresthesia, articain 33% and the tory state or to the bone remodeling following the sur- now outdated prilocaine 34%. These differences gery. Such involvement tends to resolve sponta- would not be significant and, therefore, not be useful neously, at the latest within a few months. to define the causal relationship with the individual species of anesthetic. Hillerup, on the contrary, indi- From an anatomical viewpoint, it is difficult to think cates in the articaine the drug with the highest neuro- that a trauma of this type can manifest in a jaw, eden- toxicity. tulous aside, that is healthy. More likely to be an ac- quired aspect, as typically happens in the presence The complication referred to follows a procedure, at of complete undefeated edentulousness, associated least in appearance, conducted regularly. Apart from with severe bone base resorption. A process of this possible secondary lesions to events such as needle type leads to a relative approach of the mandibu- rupture during injection, this possibility occurs more lar canal to the alveolar crest. Even during frequently in children and anxious subjects, being sy- anesthesia it is advisable to evaluate the thinning stematically secondary to sudden and unexpected of the tissues to avoid getting too close to the patient movements. nerve component even with a simple plexus injec- In addition, accuracy during the needle insertion pha- tion at the site of the lower premolars where the- se is a factor to be considered. When trying to per- re is the emergence of the nerve from the mental form conventional anesthesia, in many cases the de- foramen. flection of the needle does not allow to reach the posi- In cases like this, the use of short implants may be tion that was set. Using a bi-directional technique du- a viable option for the prevention of alveolar nerve ring insertion and advancement allows greater preci- complications. It should be remembered, however, sion to be achieved; this type of technique is allowed that short implants are arousing clinical interest in wi- by the use of a device with a shape similar to that of der fields of application than the one under examina- a pen such as The Wand Handpiece [10, 11] . tion: once again, therefore, the preventive measure In the final analysis, it should be considered that an is not justified solely on the basis of complication, anesthetic technique can lead to complications, inclu- which remains in all cases a rare event. ding neuropathic damage, even in the presence of

11 In the surgical field and also in absolute terms, howe- mic corticosteroids. One of the existing protocols ver, the main cause of involvement of the lower alveo- uses prednisone to the extent of 1mg/kg/day. lar nerve (as, in fact, also of the previously mentio- ned lingual nerve) is represented by the surgery of the third molar. DENTAL ANAESTHESIA AND NERVE ANATOMY Smith observes that some of the most common pro- Most dental procedures are carried out after proper cedures of aggression to the malpositioned third mo- administration of local anesthetic. Let's briefly see lar, ostectomy, dentitomy or lingual separation of the the nerves involved during conventional anesthesia element are able to induce excessive bleeding, techniques: which, in turn, can cause transient deficits in neuro- sensory conduction. Jerjes has defined the anatomi- - Upper arch vestibular plexic anesthesia: this is the cal risk factors underlying lower alveolar nerve pa- technique most frequently used to obtain anesthesia resthesia: horizontal position of the element and of the dental element, the buccal periosteum, the con- proximity to the alveolar canal. To this the author nective tissue and the mucosa. The large terminal adds the little experience on the part of the clinician. branches of the dental plexus are anaesthetized. As far as the anatomical aspect is concerned, Park - Infraorbital nerve anesthesia: used when more than indicates as a critical factor the loss of cortical bone two maxillary teeth and their respective vestibular tis- near the mandibular canal. Aspects concerning root sues need to be treated. Anesthetized nerves are the morphology can also be added. anterior, middle and infraorbital upper alveolars (invol- These indications also seem to underline the impor- ving the areas below the , laterally to the nose tance of the case study, especially from the radio- and above the lip). graphic viewpoint: in the face of a concrete suspicion - Palatine major nerve anesthesia: Used for procedu- of proximity between element and canal, deepening res at the level of the areas distal to the canine, invol- through the execution of a TC cone beam can cor- ving soft and hard tissues. It is required for conserva- rectly direct the surgical approach to the case. tive procedures involving more than two dental ele- Bell's palsy ments and for surgical and periodontal procedures at the palatal level. Atypical orofacial pains constitute a broad syndro- mic compendium of considerable clinical importance - Nasal-palatine nerve anesthesia: indicated for resto- and commitment. The lesion to the lower facial motor rative treatments involving more than two dental ele- neuron is commonly known as Bell's palsy. It mani- ments and for surgical and periodontal therapies fests "in the fridge"; i.e., after exposure to the cold, from canine to canine. typically during the mid-season. It can also occur as - Lower arch buccal infiltration anesthesia: similar to a result of or as a result of surgery. what happens in the case of the upper infiltration, The deficit may also be associated with its analgesia with the only difference that the terminal branches of during dental therapies that may occur when working the dental plexus of the lower alveolar are anaestheti- in the parotid area. zed. Paralysis of the causes ptosis of the - Lower alveolar nerve block anesthesia: The nerves eye, depression of the corner of the mouth with spit involved are the lower alveolar, the incisor, menta- out of saliva, and speech disorder. land the lingual, thus anaesthetizing the teeth, the body of the mandible, the vestibular mucosa anterior The treatment normally implemented in the most se- to the the two-thirds anterior of the vere cases, involves the administration of syste-

12 tongue, the oral floor, the periosteum and the lingual mucosa. - Gow Gates truncular anesthesia: affects the entire third branch of the trigeminal nerve, including the lower alveolar, lingual, miloioid, mental, incisor, auri- cular-temporal and buccal. - Long Buccal nerve anesthesia: used to anaestheti- ze mucous membrane and vestibular periosteum di- stally from the mandibular molars [12].

13 AA3, Daubländer M4. Clin Oral Investig. 2017 Jan;21(1):105-110.]

10.Hochman MN, Friedman MJ. In vitro study of need- le deflection: a linear insertion technique versus a bidirectional rotation insertion technique. Quintes- sence Int. 2000 Jan;31(1):33-9.

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9. Movement control during aspiration with different injection systems via video monitoring-an in vitro model. Kämmerer PW1, Schneider D2, Pacyna

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