Common Anatomical Variations of Neurovascular Canals and Foramina Relevant to Oral Surgeons: A Review

: (1) Background: The anatomical variations of neurovascular canals that are encountered in oral surgery are often overlooked by anatomy textbooks or provided with insufﬁcient information. The aim of this study is to analyze the most common variations, describing their morphology, prevalence and clinical implications. (2) Methods: A review of published literature from the 20th century onwards was performed using the PubMed electronic database as well as anatomical textbooks. The variations being investigated were: retromolar canal (RMC) and foramen (RMF), accessory mental foramen (AMF), midline (MLF) and lateral (MLF) lingual foramina and canalis sinuosus (CS). (3) Results: Anatomical variants of neurovascular canals and foramina have a signiﬁcant incidence and important clinical implications in the most common oral surgery procedures such as third molar extraction, bone harvesting and implantology. (4) Conclusions: Knowledge of these variables is highly important both for students to have a more accurate anatomical awareness and for professional surgeons to be able to provide better diagnoses and prevent complications during oral surgery techniques.


Introduction
Anatomy represents the fundament of surgery and forms the basis for diagnosis, treatment and care. Anatomical knowledge is crucial for meeting diagnostic challenges but also for developing surgical procedures. Therefore, anatomy is a major requirement for all surgical specialities, including oral and maxillofacial surgery.
The concept of normality in anatomy is called relative: this is inferred from a series of repeated observations. Organisms vary within the same population but also the same species. Anatomical variations may contribute to unusual manifestations of clinical symptoms, influence the course of clinical examination or the interpretation of imaging and prove of primary importance during surgical procedures [1].
Insufficient anatomical awareness can lead to serious complications and poor postoperative results both aesthetically and functionally. Therefore, to perform safe and effective surgeries, it is essential to have a thorough understanding of clinical anatomy and potential anatomical differences that may be found between individuals.
Morphological changes caused by atrophy of alveolar processes following tooth loss may sometimes prevent the surgeon from locating anatomical landmarks. This condition is complicated by anatomical variations, representing potential risk factors in oral and maxillofacial surgery. Variations in nerves and blood vessels can lead to serious complications such as uncontrollable haemorrhage with potentially fatal outcomes and permanent nerve trunk injuries which negatively impact patients' quality of life. To

Retromolar Canal and Foramen
The mandibular canal (MC) is now known as the main trunk from which multiple minor branches originate, running approximately parallel to it [2]. These accessory canals result from failure of primitive canal fusion during the prenatal period and are usually detected as incidental radiographic findings since they have no clinical landmarks [3]. However, a better understanding of the openings and location of mandibular accessory canals is important to ensure the safety of surgical procedures in the posterior part of the mandible.

Retromolar Canal
The retromolar canal (RMC) is a bifid variant of the MC that involves branches of the inferior alveolar nerve (IAN). These branches divide from the top of the canal and travel antero-superiorly within the bone to emerge from the retromolar foramen (RMF) into the retromolar fossa or retromolar triangle [4].

Classification of RMC
The retromolar canal was first classified by Ossenberg (1987) [5] and later by Von   [6] into three main types. Later Patil et al. (2013) [7] introduced an additional subclassification in two subtypes.
The resulting classification is as follows: (Figure 1) • Type A1: RMC with a vertical course that branches off from the MC and courses postero-superiorly to open into the retromolar fossa. • Type A2: RMC with a vertical course that forms an additional horizontal anterior branch before opening posterosuperiorly into the retromolar fossa • Type B1: RMC with a posteriorly curved course • Type B2: RMC with posteriorly curved course forming an additional anterior horizontal branch before opening into the retromolar fossa • Type C: RMC with a horizontal or transverse course with a posterior opening behind the temporal crest and an anterior opening in front of the temporal crest in the retromolar fossa.
• Type A1: RMC with a vertical course that branches off from the MC and co postero-superiorly to open into the retromolar fossa. • Type A2: RMC with a vertical course that forms an additional horizontal an branch before opening posterosuperiorly into the retromolar fossa • Type B1: RMC with a posteriorly curved course • Type B2: RMC with posteriorly curved course forming an additional an horizontal branch before opening into the retromolar fossa • Type C: RMC with a horizontal or transverse course with a posterior opening b the temporal crest and an anterior opening in front of the temporal crest i retromolar fossa. Zhang et al. [8] also described this classification system into three subtypes: • Subtype 1, which runs directly over the surface of the bone; • Subtype 2, which reaches the retromolar region with a single curve, givin impression of a "V"; • Subtype 3, with three segments and two main curves before ending in the retro region, resembling a "U". In spite of the numerous classification systems proposed to describe RMC various locations and configurations, most studies conducted over the years have fo on the most common type of RMC (type 1) proposed by Ossenberg (1987) [10,11].

Prevalence of RMC
According to the literature, RMC has an extremely variable prevalence ranging 0% to 75.4%, depending on the method applied. Studies based on orthopantomog reported a low prevalence of RMC [5,[12][13][14][15] while studies in which OPT and CBCT associated found significantly higher results. These outcomes indicate the importa three-dimensional investigations and their great advantage in detecting these anato structures [5,13,14,16]. The prevalence of RMC reported by studies carried out on ranges from 24.65% to 75.4% [7,14]. Zhang et al. [8] also described this classification system into three subtypes: • Subtype 1, which runs directly over the surface of the bone; • Subtype 2, which reaches the retromolar region with a single curve, giving the impression of a "V"; • Subtype 3, with three segments and two main curves before ending in the retromolar region, resembling a "U".
Another classification was proposed by Luangchana et al. (2018) [9], which distinguished five types of RMC by further modifying the classification of Patil  In spite of the numerous classification systems proposed to describe RMC in its various locations and configurations, most studies conducted over the years have focused on the most common type of RMC (type 1) proposed by Ossenberg (1987) [10,11].

Prevalence of RMC
According to the literature, RMC has an extremely variable prevalence ranging from 0% to 75.4%, depending on the method applied. Studies based on orthopantomography reported a low prevalence of RMC [5,[12][13][14][15] while studies in which OPT and CBCT were associated found significantly higher results. These outcomes indicate the importance of three-dimensional investigations and their great advantage in detecting these anatomical structures [5,13,14,16]. The prevalence of RMC reported by studies carried out on CBCT ranges from 24.65% to 75.4% [7,14].

Content of the RMC
Based on radiologic studies, cadaver studies, and biopsies, it has been reported that the retromolar canal contains a neurovascular bundle that is predominantly endowed by thin myelinated nerve fibres, venules and arterioles covered by collagen fibres, and adipose tissue in small amounts.
A recent study has mentioned that the diameter of nerve bundles varied between 40 to 60 microns, and the larger ones ranged from 80 to 180 microns. The largest arteriole had a diameter of a maximum of 600 microns [12].

Morphometric Measurement of RMC
The distance from the mandibular foramen to the origin of the RMC is 21.5 (11.2) mm [11]. The diameter of the RMC has been reported to range from a mean of 0.75 mm to 2.28 mm [8,18]. The narrowest RMC reported was 0.27 mm, while the widest was 3.29 mm in diameter [19]. The reported mean length of the RMC ranges from 6.9 mm to 16.2 mm [19,20]. The longest RMC ever reported was 33.2 [19]. The mean height of the canal has been reported to range from 6.66 (2.18) mm to 15.3 (4.6) mm from the MC [5,19].

Retromolar Foramen
RMF is the accessory foramen that allows RMC outflow and is often located on the medial [21,22] or lingual [23] surface of the retromolar trigone.
It has been suggested that the foramen on the medial aspect of the retromolar trigone may carry fibres of the mylohyoid nerve; this may be responsible in part for the dolorific sensibility of the third molar. The RMF often occurs as a single unilateral or bilateral foramen, although up to three foramina may be present at a single site [24].

Prevalence of RMF
The prevalence of the RMF as reported in studies of dry mandibles ranges from 3.2% to 92.7% [25]. Studies using orthopantomography reported a prevalence of only 5.3% [26], while CBCT studies reported a range between 7.33% and 12.4% [25,26]. This wide range has been attributed to ethnic differences, environmental and genetic factors, techniques of study, and differences in the number of samples studied [25].
Morphometric Measurement of RMF RMF was first analysed in its size and location by Lofgren in 1957 [27]. The average diameter of the RMF ranged from 1.2 mm to 2.97 mm [28,29]. The smallest reported diameter was 0.1 mm [28]. Studies by Haas et al. (2016) [30] reported that RMF on the right side of the mandible was located more distally than on the contralateral side in the retromolar area. RMF is approximately 4.23 mm to 9.71 mm distant from the posterior edge of the third molar's socket [29,30]. However, it can be located as close as 1 mm from the tooth [29]. It lies 8.02 mm from the anterior aspect of the branch [4,31]. It is closer to the buccal cortex than to the lingual plate [4].

Clinical Significance
Currently, in clinical practice, a large number of procedures are performed in the retromolar area, and this is evidence of how RMC is a structure of considerable importance that deserves more attention in daily surgical practices.
The main complications that can be caused by accessory canals relate to failure to achieve complete local anaesthesia and intraoperative injury to neurovascular bundles if not detected before surgery [32].
The presence of an RMC is often associated with the so-called "escape pain phenomenon", which is the persistence of painful symptomatology in procedures such as third molar extraction despite proper local anaesthesia.
This escape pain is related to the presence of nerve fibres in the RMC responsible for innervation of the retromolar triangle mucosa, buccal mucosa and gingiva of the mandibular posterior region [33].

Local Anesthetic Failure
Accessory channels are often blamed for the failure to achieve a complete local anaesthetic block. Some studies, however, have suggested that this can be achieved provided the clinician has awareness of the anatomy of the region.
Suazo et al. (2008) [34] proposed a technique accounting for RMC, which allows the achievement of 72.5% anaesthesia with a latency of ten minutes. They argued that the technique allows for easy tracking of the puncture site, which can be observed during the entire deposition of the anaesthetic agent, without bleeding. Owing to the limited vascularization and strongly adherent periosteum of this area, this technique was proposed by the authors as the first-choice approach for anaesthesia in patients with blood dyscrasia because of its low risk of vascular injury.

Haemorrhage
The presence of a retromolar canal, given its neurovascular content, is one of the causes of excessive bleeding during surgical procedures in the retromolar trigone. There may be an initial haemorrhage from blood vessels that emerge from the retromolar canal which are torn when the flap is raised, or spontaneous haemorrhage may occur later during healing and lead to a hematoma.
These complications can be managed locally by crushing the bone in the area occupied by the canal or filling the opening with bone wax [35].

Neurosensory Disturbance
According to numerous case reports, such as that reported by Sigh in 1981 [36], the retromolar canal may contain an aberrant branch of the buccal nerve.
This anatomical variation is of considerable clinical significance since surgical procedures in the retromolar area could result in complications such as unilateral paresthesia or hypoesthesia subsequent to its damage.
Normally, the buccal nerve emerges from the mandibular nerve in the infratemporal fossa before heading low to the inner portion of the branch and crossing the external oblique line up to 3 mm from the deepest concavity of the anterior aspect of the branch. In the event that an aberrant long buccal nerve emerges from the retromolar foramen, it would be at a closer distance than expected from the surgical site. Thus, an injury to the RMC nerve bundle potentially results in neurosensory disturbances such as temporary or permanent sensory impairment or traumatic neurinoma formation, negatively impacting the patient's quality of life.

Accessory Mental Foramina
The mental foramen (MF) is the opening through which the mental nerve emerges from the mandible in its anterolateral aspect. It is usually located either between the roots of the first and second mandibular premolars or apical to the second premolar. The mental nerve represents one of the terminal branches of the mandibular nerve and divides into three branches supplying the lower lip, cheeks, chin, and the vestibular gingival of mandibular incisors [37]. Although the anatomy of the mandibular nerve is well established, some anatomical variations have been reported that must be taken into consideration to avoid clinical complications. One of these is the accessory mental nerve, which runs through small foramina in the area surrounding the MF, known as accessory mental foramina (AMF) [38].
The accessory mental nerve is a relevant anatomic structure in dental practice with special relevance to local anaesthesia and surgical procedures involving this region.
AMF are smaller than mental foramina and outflow from the mandibular canal. Naitoh et al. (2009) distinguished AMFs from nutrient foramina, which may also border the MF but are not connected to MC [39].

Prevalence and Morphometric Measurement of AMF
The AMF has been first reported by Toh et al.'s study in 1992 [38]. The study was on a cadaver case report of three samples only. Once CBCT became a common practice in the dental field, many researchers reported a higher prevalence of AMF [40][41][42].
AMF has been reported in multiple configurations for instant, separated, diffused or continuous in relation to MF (Figure 2). Muinelo-Lorenzo et al. (2014) observed that the presence of AMF influences the size of MF, finding that on the sides where AMF occurred, MF was significantly smaller [47]. Naitoh et al. (2009) also found MF size to be smaller on sides with AMF but did not find statistical significance [39]. The presence of the anterior loop of mandibular nerve (ALM) is an added complexity to the mental foraminal area. Naitoh et al. (2009) distinguished AMFs from nutrient foramina, which may also border the MF but are not connected to MC [39].

Prevalence and Morphometric Measurement of AMF
The AMF has been first reported by Toh et al.'s study in 1992 [38]. The study was on a cadaver case report of three samples only. Once CBCT became a common practice in the dental field, many researchers reported a higher prevalence of AMF [40][41][42].
Studies show a prevalence of AMF that range between 2% and 13% [43][44][45], with an average reported prevalence of 8.3%, according to the cumulative results [46]. AMF has been reported in multiple configurations for instant, separated, diffused or continuous in relation to MF (Figure 2). Muinelo-Lorenzo et al. (2014) observed that the presence of AMF influences the size of MF, finding that on the sides where AMF occurred, MF was significantly smaller [47]. Naitoh et al. (2009) also found MF size to be smaller on sides with AMF but did not find statistical significance [39]. The presence of the anterior loop of mandibular nerve (ALM) is an added complexity to the mental foraminal area.
The average size of the reported measurement of the AMF is 1.55 mm from all the studies that calculated the AMF opening size.  detailed the bundle that exits from AMF, he and his colleagues in a dissection anatomical investigation showed nerve and blood vessels are passing through this foramen [42].

Clinical Significance
The position of the Mental Foramen (MF) and the Accessory Mental Foramen (AMF) is of utmost importance to modern dentistry and oral surgeons in general. The location and prevalence of any particular variation of the inferior alveolar nerve (IAN) and MN are vital for dental practitioners. Injuries to the mandibular nerve bundle can cause permanent lifelong disability and psychological damage. These include numbness of the lower lip and chin, tingling sensation of the same area and possibly dysesthesia. Many The average size of the reported measurement of the AMF is 1.55 mm from all the studies that calculated the AMF opening size.  detailed the bundle that exits from AMF, he and his colleagues in a dissection anatomical investigation showed nerve and blood vessels are passing through this foramen [42].

Clinical Significance
The position of the Mental Foramen (MF) and the Accessory Mental Foramen (AMF) is of utmost importance to modern dentistry and oral surgeons in general. The location and prevalence of any particular variation of the inferior alveolar nerve (IAN) and MN are vital for dental practitioners. Injuries to the mandibular nerve bundle can cause permanent lifelong disability and psychological damage. These include numbness of the lower lip and chin, tingling sensation of the same area and possibly dysesthesia. Many reported injuries in the dental field to the MN due to endodontic therapy [48], implant placement [49] and dental extraction [50].
Modern developments in radiology facilitate the detection of such landmarks by CBCT and panoramic imaging compared to periapical radiograph [46]. Panoramic imaging has a great advantage of ease of use and availability in many dental practices. Using panoramic imaging is beneficial to localize the inferior alveolar never and the MF location in relation to dentitions. However, this modality has been reported to be inefficient to show the location of AMF due to AMF's relatively small size [47]. In addition, the panoramic image provides only a two-dimensional image that masks other important anatomical structures, such as the ALM [51].
On the other hand, the CBCT exams present several important features related to its feasibility in dentistry, such as low dose of radiations, high level of accuracy (an average of 95.5%), high resolution, and three-dimensional orientation [46]. Importantly, AMF is visible on CBCT and has been showing a relatively high incidence: Lam et al. (2019) reported a 6.4% presence of AMF in a 4000 CBCT study [52]. Interestingly, the AMF was known to the dental community since 1992 [38] through cadaver studies. However, the prevalence of AMF in radiology exams increased after the CBCT was introduced to dentistry. Other important anatomical structures in the mandible such as the anterior loop and the bifid mandibular canal [53] were also not delineated before the CBCT introduction to dentistry.

Mandibular Lingual Foramina
Lingual foramina are accessory foramina on the lingual surface of the mandible. They are typically located in the interforaminal area [54] but can also be commonly observed in the area of the second premolar [54][55][56]. Vascular and nerve anastomoses arising from branches of the submental artery, lingual artery, and mylohyoid nerve are found in these structures.
The lingual foramen is also known as medial lingual canal, lingual vascular canal, lateral lingual canal, and genial spinal foramen, which is itself subdivided into superior, when it is at or above the mental or genial spines, or inferior, when it is below the mental or genial spines. Other names by which it is described include supraspinous foramen (when it is located above the mental or genial spines); interspinous foramen (when it is located at the level of the mental or genial spines); infraspinous foramen (when it is located below the mental or genial spines); mental spinal foramen, lingual accessory foramen, mandibular accessory foramen, and mandibular lingual foramen [54,55].
The interforaminal zone, delimited by first premolars, is a mandibular area that is traditionally considered safe from a surgical point of view [57]. This area, on its vestibular side, is routinely used as a donor site for bone grafts of the mental block and implant placements [58]. However, these practices are often not supported by clinical protocols that take into account possible interference with lingual foramina. In fact, these structures are not described by current dental anatomy textbooks [59], and only radiographic anatomy textbooks have occasionally reported their presence [59,60]

Midline Lingual Foramina
MLF are the mandibular lingual foramina that are most often encountered and documented, with a prevalence ranging from 96% to 100% [62].
MLFs can often be divided into superior, middle, and inferior lingual foramina depending on their vertical position in relation to genial tubercles (or bilateral mental spines). As a result, these foramina are called supraspinous, interspinous, and infraspinous foramina. [62] Generally, one to three foramina are found, although some authors have described up to four midline lingual foramina [56]. Most studies describe two MLFs as the most common occurrence [62].
Studies have reported that if only one MLF is present, it is usually located above the mental spine. In mandibles with two foramina, the largest was located above the genial tubercles. Unlike, most mandibles with three MLF showed two of the three foramina located below the genial tubercles [59].

Lateral Lingual Foramina
LLF has been less commonly evaluated than MLF and has been observed in a lower number of cadavers and patient mandibles. The frequency of LLF generally reported in the literature ranges from 6 to 80% [61]. According to a study published by Katakami et al. (2009) [56], LLF is encountered with a relatively high frequency in the area corresponding to the first premolar (36%) but also in central incisor and canine areas (each 19.4%).
Von Arx et al. (2011) described frequencies of 22.6% and 20.3% for LLF in the first and second premolar areas, respectively [62]. The high occurrence of LLF under the first premolar was also confirmed in a cadaver study where 45.7% of the sides of dry cadaveric skulls had a notable lingual foramen in that region [63].
Concerning bilateral recurrence of LLF, in a CT study of 70 patients, bilateral lingual canals were observed in 29.7% of the cases [64]. Another CT study carried out on 200 patients found a 44% frequency of bilateral LLF [54]. However, no information on the symmetry of LLFs was provided in both studies.

Morphometric Measurement of Lingual Foramina
The average size of MLF reported in the literature ranges from 0.6 to 1.17 mm. The smallest size was 0.1 mm in diameter and the largest was 2.29 mm. Higher mean values were found for the superior MLF (0.75-1.12 mm) than for the inferior MLF (0.58-0.9 mm) [61]. CBCT studies found a greater height than the width in the MLF, while an inverted ratio in the LLF [62]. Regarding ratios from the MLF or LLF to adjacent anatomical

Midline Lingual Foramina
MLF are the mandibular lingual foramina that are most often encountered and documented, with a prevalence ranging from 96% to 100% [62].
MLFs can often be divided into superior, middle, and inferior lingual foramina depending on their vertical position in relation to genial tubercles (or bilateral mental spines). As a result, these foramina are called supraspinous, interspinous, and infraspinous foramina [62]. Generally, one to three foramina are found, although some authors have described up to four midline lingual foramina [56]. Most studies describe two MLFs as the most common occurrence [62].
Studies have reported that if only one MLF is present, it is usually located above the mental spine. In mandibles with two foramina, the largest was located above the genial tubercles. Unlike, most mandibles with three MLF showed two of the three foramina located below the genial tubercles [59].

Lateral Lingual Foramina
LLF has been less commonly evaluated than MLF and has been observed in a lower number of cadavers and patient mandibles. The frequency of LLF generally reported in the literature ranges from 6 to 80% [61]. According to a study published by Katakami et al. (2009) [56], LLF is encountered with a relatively high frequency in the area corresponding to the first premolar (36%) but also in central incisor and canine areas (each 19.4%).
Von Arx et al. (2011) described frequencies of 22.6% and 20.3% for LLF in the first and second premolar areas, respectively [62]. The high occurrence of LLF under the first premolar was also confirmed in a cadaver study where 45.7% of the sides of dry cadaveric skulls had a notable lingual foramen in that region [63].
Concerning bilateral recurrence of LLF, in a CT study of 70 patients, bilateral lingual canals were observed in 29.7% of the cases [64]. Another CT study carried out on 200 patients found a 44% frequency of bilateral LLF [54]. However, no information on the symmetry of LLFs was provided in both studies.

Morphometric Measurement of Lingual Foramina
The average size of MLF reported in the literature ranges from 0.6 to 1.17 mm. The smallest size was 0.1 mm in diameter and the largest was 2.29 mm. Higher mean values were found for the superior MLF (0.75-1.12 mm) than for the inferior MLF (0.58-0.9 mm) [61]. CBCT studies found a greater height than the width in the MLF, while an inverted ratio in the LLF [62]. Regarding ratios from the MLF or LLF to adjacent anatomical structures, distances from the lower edge of the mandible have been observed by several studies.
The average distance to the lower edge of the mandible ranged from 12 to 18.4 mm for the superior MLF and from 2.2 to 7 mm for the inferior MLF [61]. Only two studies [65,66] evaluated the distance from the MLF to the alveolar crest because measurements and comparisons are hampered by periodontal diseases or atrophic bone. The average distances from the superior MLF to the alveolar crest were 14.2 mm [65] and 14.4 mm [66]. For LLF, the average distances from the lower edge of the mandible ranged from 5.3 to 11.5 mm. The reported distance of LLF of the incisor/canine area from the inferior edge of the mandible was 11.5 mm, while the distance of LLF of the premolar/molar area was 7.1 mm.

Canals of Lingual Foramina
MLF is always associated with a canal, often described in radiology textbooks as a radiopaque area surrounding the lingual foramen.
The medial lingual canal may be located superiorly, at the same level, or inferior to the genial tubercles. The MLC is almost always (99%) perpendicular to the mandibular lingual borders, from an axial view [62]. From the sagittal view, the superior MLC usually runs in an anteroinferior direction, whereas the inferior MLC runs in an anterosuperior direction [54,62]. In addition, the s-MLC can rarely run horizontally or upward (3% and 1%), while the i-MLC sometimes runs horizontally or downward (3% and 21%) [66]. As reported in some studies, the MLC may join the lingual and labial plaques [60].
It was reported that a lingual canal associated with LLF occurs in 95% of individuals. From the axial view, LLCs usually course anterior to the foramen (43%), even if 21% also course perpendicular to the lingual bone surface of the mandible [61,62].
Despite the fact that the most common type of lingual vascular canal is the single canal (77%), studies observed that 20% have bifurcations and 3% triple canals [67].

Content of Lingual Foramina and Canals
The LLF contains a branch of the submental artery, which may form an anastomosis with the lingual branch of the inferior alveolar artery or be independent and give branches supplying the incisional area and the lower lip.
In cases where anastomosis occurs, the inferior alveolar artery also supplies the incisive area [68,69].
LLC arising from the premolar region anastomoses with the incisor canal about 87% of the time and with the mandibular canal, 38% of the time [62]. Anastomoses with the mandibular canal can form in the anterior loop, in the area adjacent to the mental foramen, or posterior to the mental foramen [56]. In the incisor-canine region, all communication occurs with the incisor canal [62], thus connecting the CLL with the area concerning the mental nerve. For that reason, the occurrence of the lateral lingual foramen is a strong predictor of communication with the incisor canal [61].
The superior MLC contains branches of the lingual artery and vein. The superior MLC and anterior MLC are crossed by an anastomosing branch of the sublingual artery [59,60]. The inferior MLC includes submental or sublingual branches [60]. The MLCs do not connect in 92% of cases [62]. When they anastomose, the artery in the MLC may anastomose with the incisive artery in 8-40% of cases but less frequently than in MLC anastomoses with the incisive canal. That is because this canal is not typically present in the middle of the symphysis [56,62,67].
The superior MLC contains a branch of the lingual nerve, while the inferior MLC contains a branch of the mylohyoid nerve [60].

Clinical Significance
In light of the significant neurovascular content of both MLC and LLC and given the involvement of the interforaminal region in many surgical procedures, a thorough awareness of the location, size, and content of these structures is of utmost importance in daily clinical practice.

Dental Implantation
Implant rehabilitation represents one of the most common rehabilitative procedures that are performed in the interforaminal area in edentulous patients with bone atrophy. Implant placement allows recovery of function and aesthetics by taking advantage of an anatomical area that is considered surgically safe.
Nevertheless, after tooth loss, horizontal bone resorption occurs, drastically reducing the distance between the lingual foramen and the bone crest: this results in an increased risk of intra-and post-operative complications. Thus, the presence of at least one lingual foramen in almost all patients has prompted increased interest in identifying the exact position of MLF and LLF and the possible management of vessel injury therein.

Haemorrhage
Hemorrhages occurring in association with surgery can be late or early. The latter is most often caused by perforation of the lingual cortical. Intraoperative injury to the interforaminal lingual cortical may result in damage to the sublingual artery, submental artery as well as the inferior alveolar artery branch, which leads to an oral floor haematoma [70].
Since the interforaminal lingual cortical area is characterized by a rich blood supply, if the haemorrhage persists, it could cause an upward dislocation of the oral floor and tongue, leading to a potentially fatal upper airway obstruction [59,66].
In addition, in some individuals with mylohyoid muscle defects, haemorrhage from the sublingual space may extend to the submandibular space, causing airway obstruction as well [68]. Airway patency can be regained by intubation and tracheostomy. A better assessment of mandibular anatomy is therefore essential to avoid complications related to lingual cortical perforation [70].
Knowledge of the distance between the lingual canal and the alveolar ridge is particularly important in surgical procedures such as implant placement, as immediate implants placed deeper than the natural alveolus and implant platform preparation can damage the lingual canals [67]. Long dental implants should not be placed in canine regions and atrophic jaws. In all cases in which implant placement caused life-threatening haemorrhage, a drilling depth greater than 15 mm was observed to cause perforation of the lingual cortical [54,71]. Edema is the result of surgical bleeding and, if observed, should be immediately controlled. Additional prescription of antibiotics and steroids might be helpful when the bleeding resolves [72].

Canalis Sinuosus
The Canalis Sinuosus (CS) is a tortuous intraosseous canal that originates from the infraorbital nerve. It emerges posterior to the infraorbital foramen and runs in an anterolateral direction up to the anterior wall of the nasal cavity below the orbital margin, at this point, it turns sharply downwards along the pyriform opening describing an S-shaped course and then heading towards the low and emerge into the palatine mucosa through an accessory foramen (Figure 4). The term Canalis Sinuosus (CS) describes the double curvature of the latter, which runs for about 55 mm along the maxilla and is characterized by a thin overlying bone that makes it more susceptible to injury in case of trauma. For this reason, it is considered an important branch of the orbital nerve (ION). The anterosuperior alveolar nerve (ASAN) and corresponding veins and arteries run within it. The CS commonly presents anatomic variations anterior to the incisive canals in the anterior palate, called accessory channels (ACs). The neurovascular branches in the CS innervate incisor and canine region and adjacent soft tissues through the dental plexus [73].
Most of the studies found no significant statistical difference in frequency [74,75,80]; only one study showed the left side as more commonly affected [81].
The average diameter of the CS was found to be approximately 1 mm.

Clinical Significance
There are many surgical procedures involving the anterior maxilla from all branches of dentistry: endodontics, periodontology, implantology, oral surgery and orthognathic [76].  2017), the frequency of CS in the patients of these studies was 88% and 100% respectively [74,75].

Morphometric Measurement of CS
Most studies have focused on the terminal portion of the CS, showing that the final portions of the CS and the ACs variations are more frequent in the incisor and canine region near the palate [75,80].
Most of the studies found no significant statistical difference in frequency [74,75,80]; only one study showed the left side as more commonly affected [81].
The average diameter of the CS was found to be approximately 1 mm.

Clinical Significance
There are many surgical procedures involving the anterior maxilla from all branches of dentistry: endodontics, periodontology, implantology, oral surgery and orthognathic [76].
Because of its anatomic course, the CS should be always considered in the treatment planning and outcome of dental surgery.
In implantology, invasion of the neurovascular bundle of the CS and Acs can compromise osseointegration and cause temporary or permanent paresthesia as well as blood loss and neurovascular disturbance in the region [82,83].
CS injuries may result in a clinical condition known as post-traumatic midface pain, characterized by paroxysmal neuropathic pain localized to the central and lateral incisors, canines, and maxilla. These symptoms are related to ASAN involvement. Injuries to ASAN can occur after midface fractures but can also be iatrogenic, following Lefort I-type osteotomies or dental surgery [84].
For all the reasons just listed, it is essential to know the trajectory and calibre of CS during the surgical programming phase. At present, some authors report essential the individuation of the CS in order to avoid the risk of haemorrhage and paraesthesia [83,85]. Jacobs et al. (2000) analysed the anatomical structures of the maxillary bones and any associated risks. The authors claim that there is great individual variability regarding the presence and the course of the CS and Acs. The identification of these anatomical structures in any surgical intervention in the maxillary region is essential. To avoid complications, it is very important surgical programming through radiographic examinations of the second level [86].

Discussion
Anatomically, the structures of significance in oral surgery contain neurovascular bundles and related foramina that, both in their classical anatomy and in their anatomical variations, are of crucial importance in the vascularization and innervation of dental structures and their surrounding tissues. Their knowledge allows professionals to select an appropriate approach to the patient's requirements.
This study shows that the reported prevalence of RMC is 24.6-75.4%, RMF is 7.33-12.4%, AMF is 2-13%, and MLF is 96-100%, while that of CS is 88-100%. Thus, these are anatomical variants that can be termed anything but rare and may be encountered with relative frequency in surgical practice.
The diagnosis of these variations has been changing as technology has evolved. In the first reported studies, the findings were presented on the basis of cadaveric mandibles and in the best of cases PANs [87], where it was often difficult to find them due to the ghost images that are generated in relation to the superposition of adjacent structures which is expected within a 2D image.
Currently, the range of tools available to diagnose anatomical variations of any neurovascular channel in the oro-maxillofacial territory has made the CBCT technique the first preference or the one with the best clinical acceptance. This is thanks to the high quality of 3D images that allow better investigation forms the presence of these variations within the mandibular and maxillary structure.
This review describes the clinical implications of encountering each of the listed variables. The main complications that can be found arise from the neurovascular content of these structures and are represented by marked bleeding, temporary or permanent sensitivity changes of the supplied areas, as well as local anaesthesia failure. Further, these accessory foramina may present an entry point and the related canals may act as a route of infection or, most rarely, tumour metastases [32,88].
The clinician's awareness of such eventualities should always be kept in mind during surgical procedures in order to be able to manage them properly.

Conclusions
The anatomical variations of the oromaxillofacial district occur with a relatively high incidence. Knowledge of them is of crucial importance both for professors, who provide morphological training, and for professional surgeons, who work daily in the oromaxillofacial territory. In addition, it is very important to have new lines of research that focus their efforts on describing and diagnosing these anatomical variations and being able to associate in which ethnic groups or genders it occurs in greater frequency.
In conclusion, all health professionals must continually challenge themselves in knowing the different anatomical variations that the human body presents and how these may affect clinical practice in order to perform a surgery that is as safe and minimally invasive as possible.
Funding: This research received no external funding.