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Mandibular Retromolar Foramen and Canal-A Systematic Review and Meta-Analysis

IntRoductIon The morphological changes that happen over the years in the mandible imply alterations in the positioning of the structures located in the mandibular rami, such as the mandibular foramen, condylar process, and coronoid process, as well as in the mandibular body, such as the path of the mandibular canal (MC) and the position of the mental foramen. Likewise, accessory foramina and canals will follow these changes. Therefore, it is important that we all should have this knowledge to carry out the necessary adjustments for the anesthetic and surgical procedures.[1]


MateRIals and Methods
An extensive review of published literature was done through use of general and meta search engines (Google scholar, PubMed, Science Direct) to harvest prominent medical database (Medline, Embase, Cochrane). The search strategies used were "Mesh" (key terms used were: Mandible, RMF, RMC, etc.,) "text word" searching, "reference list" harvesting and "related articles" feature. Strict inclusion and exclusion criteria were applied to select 100 articles ranging from the year 1987-2019, based on the context relevance.

Inclusion criteria
1. Dry skulls macroscopic investigations on RMF and RMC, investigations with plain radiography and computed tomography highlighting foramen gross morphology and morphometry, retrospective analysis of retromolar region. 2. Evaluative studies to detect the frequency, size, number, and position of RMF and RMC, reviews dealing with age and race-related variations.
Exclusion criteria 1. Studies on child skulls, animal mandibles, and dry mandible researches where race or sex were unidentified.
Key journals from library were hand searched. Endnote reference management system was used. Since most of the anatomical textbooks and dental literatures do not mention about this foramen or canal, we have studied the possible variations in the RMF and RMC.

Content of the retromolar canal
Gross examination and histological studies conducted so far have revealed that the canal has a neurovascular bundle which is found to contain predominantly thin myelinated nerve fibers and a little amount of adipose tissue. A recent study has mentioned that the diameter of nerve bundles varied between 40-60 µ, the larger ones ranged from 80 to 180 µ. The largest arteriole had a diameter of a maximum of 600 µ. [8] Along with nerve fibers and arterioles, there are numerous venules accompanying those arteries. [2] Importance of the retromolar foramen and the retromolar canal Neurovascular structures passes through RMC gives additional supply to mandibular molars, buccal area, posterior part of alveolar process of mandible, mucosa over retromolar fossa and also sometimes to temporalis muscle and buccinators muscle. [6,[8][9][10] Furthermore, the artery from the RMC anastomose with the branches of buccal artery and facial artery after arising from the MC. [11] It has been observed that the neurovascular bundle of foramen originated in the MC. High incidence of RMC is due to genetic crossbreeding of European individuals with Aboriginal Argentineans. Penetration of the neurovascular bundle into distal lamina dura of the distal root of the 3 rd molar has been noted. Clinician should be aware of this accessory innervation provided by RMC in the endodontic treatment. Postoperative hematomas caused by damage to the contents of canal and foramen during a surgical procedure or implantation should be kept in mind. The presence of RMF which was crossed by additional sensory fibers reported. The presence of this foramen can be the reason to the failed analgesia with the classical anesthetic techniques. Thus, detailed study of RMF is important to avoid failure in regional anesthetic techniques for blocking the inferior alveolar nerve and buccal nerve fibers. Singh, during surgery of a 3 rd molar, injured a nerve that crossed an unusual foramen located in the retromolar fossa. After the surgery, it was found that the patient presented paresthesia of the buccal mucosa from the retromolar region until the canine on the operated side. They have conducted many tests and found out that the nerve injured was a branch of the buccal nerve passing by foramen. Anderson et al. confirmed that the components in the RMF and canal are the nerves that provide innervation to the pulp of 3 rd molar, retromolar region and to the fibers of the temporalis and buccinator muscles. Damage to nerves of canal will have huge impact on the functions of temporalis and buccinator muscles.

Location of the retromolar foramen
The RMF is found in the retromolar fossa above the occlusal plane and below the coronoid process of the ramus. The retromolar area is bounded by the external oblique ridge, the attachment of the pterygomandibular raphe and the last molar in the mandible. The histological analysis determined that the retromolar nerve extends from the anterior border of the ramus and continues to the buccal gingival of up to two teeth anteriorly in the 1 st molar region. Potu et al. found that RMF is located mostly in the medial aspect of the retromolar fossa, proximal to the lingula. [3,4] According to Truong et al., some cases, particularly in RMCs with a large diameter (>1 mm), were positioned more anteriorly. Rarely, in cases with large diameters, the RMF was positioned in the anterior temporal crest of the coronoid process. [3] Age and the retromolar foramen Capote et al. found no significant difference in the presence of RMF based on age. [1] Higher occurrence rate in adolescent cohort has been found by Ossenberg and can be correlated with increased nutrient requirements related to the adolescent growth spurt and eruption of the wisdom tooth. [3,7,8,10] Furthermore, the preferential distribution of the nerve that runs through the RMC on the temporalis tendon may relate to the adolescent peak of RMF and increased masticatory strength. [3] Gender and the retromolar foramen Higher occurrence rate in male than female. [8] However, some studies have also shown higher occurrence rate in female than male. The bilateral occurrence of RMF is found to be also higher in females. [6] Overall, as such there is no gender predilection. [1,3,7,10,12] [3,13] Diameters of the RMF were 0.8 mm, 1.0 mm and, 1.1 mm respectively. Distance from distal edge of the 3 rd molar was measured as 4.0 m, 3.6 mm, and 0.5 mm respectively. [13] Side prevalence of the retromolar foramen

Number of the retromolar foramen
Many studies have different opinion. RMF in more common in the right side of the mandibles. [6] In general, there is no side predilection. [1,3] Unilateral RMF is more common than bilateral but the ratio of bilateral to unilateral increases with the population incidence. [7] Diameter of the retromolar foramen The diameter of RMF has been reported to range from 0.2 mm to 3.29 mm. Males have been reported to have larger diameters of RMF, which can be explained by the fact that male mandibles are usually larger than females. [3] Malik et al. had concluded that the mean diameter of the RMF is 1.01 mm. [11] Distance of the retromolar foramen from teeth The reported distances between RMF and the distal edge of the 3 rd molar were between 4.23 mm and 10.5 mm. The reported distances between the RMF and the distal edge of the second molar were between 11.91 mm and 16.8 mm. These values suggest that the locations of RMF are not constant. [3] Von Arx et al. by his study had shown that age can be a significant factor for the horizontal distance between the RMF and the 2 nd molar, with younger patients having a longer distance. It can be speculated that the presence of 3 rd molar in younger patients maintains a larger distance between the RMC and 2 nd molar. In older patients, whose 3 rd molars have been removed, the 2 nd molar might move slightly or tip distally, reducing the distance. [10] The position of RMF is nearer to mandibular 3 rd molar region in right side in comparison to the left side. [6] However, some author have proved that the RMF on the right side of  mandible. [3] So overall, it may be dependent on the age and development. [3,7,8,10]

Correlation with other anatomical variation
A statistical correlation was made between the occurrence of the RMF and accessary mandibular foramen, accessory mental foramen, mandibular 3 rd molar and three rooted mandibular 1 st molar. Only the accessary mandibular foramen showed a significant positive correlation with the RMF of the same side. That means when an accessary mandibular foramen is present, there is high chance of having horizontal bony canal leading to a foramen in retromolar fossa. [5,7,8,14] The bony canal could be the temporal crest canal that was first described by Ossenberg in 1986. [8,10] the mandible was overall found to be positioned further distally in the retromolar region than on the left side of the

Types of the retromolar canal
Ossenberg gave the first description of the types of retromolar canal based on its course [ Figure 2]. [7] On panoramic radiographic evaluation, RMC has been classified into five types according to course and morphology [ Figure 3]. [8,10] The most common appearance of the canal corresponded to type Al (vertical course), whereas type C (horizontal course) occurred least often. [3] On CT scan evaluation RMC can be classified into three types based on course and morphology [2] [ Figure 4].
The RMC is a type 1 bifidity of the MC. A bifid MC (BMC) is an anatomical variation wherein the MC divides into two parts. Each branch may carry its own neurovascular bundle. It is suggested that bifid and trifid MCs occur due to incomplete fusion of separate MC nerves from the incisors, primary molars, and permanent molars during embryonic development. The bifidity may be classified by its course. A type 1 bifidity is a unilateral or bilateral transverse bifidity. A type 2 bifidity is unilateral or bilateral and is limited to the ramus or body of the mandible. A type 3 bifidity is a combination of type 1 and type 2, thus it is a transverse and horizontal bifidity. [3] The most common variation of the RMC is a branch of the MC below the 3 rd molar. The nerve travels in a posterosuperior direction and opens in the retromolar fossa those posterior to the 3 rd molar. The second variation of the RMC opens in an anterior direction and the branches of the inferior alveolar nerve (IAN) as it enters the MC. The third and rarer variation of the RMC splits from a more proximal branch of the MC and enters the bone through a canal at the temporal crest, exiting anteriorly through the RMF. [3] Narayana et al. have observed that foramen opens upwards and backwards with a posterior smooth surface indicating the entry of the neurovascular bundle from the posterior aspect. [14] Length of the retromolar canal It has been shown that men have longer RMCs than women. This difference is not necessarily explained by the fact that men have an overall greater height of the mandible in the retromolar area, because the length of the canal was determined as the distance from the MC to the RMF. Hence, the length of the RMC is dependent on the location of the MC within the mandible. [8,10] Orhan et al. have reported 13.5 mm is the mean length of RMC. In addition, Naitoh et al. have reported mean length of the RMC 14.8 mm. [3] Another important aspect of these canals is that they vary considerably in their morphology, especially in their length, indicating that they could form the variable relation to the dental alveoli. [14] Frequency of retromolar foramen and canal The frequency of RMF as reported by the cone beam computed tomography (CBCT) studies ranges from 5.4% to 75.4%. The frequency of RMF reported by human dry mandible studies ranges from 3.2% to 72%. The frequency of RMF as reported by the panoramic studies ranges from 3.06% to 8.8%. This large range can be attributed to several factors, including ethnic differences, environmental and genetic factors, and variation in type and sample sizes across studies [3] [ Table 1]. However, studies have suggested that RMF and RMCs are normal anatomical variations of the IAN, rather than anomalies. [12] The RMF was found to occur more commonly in native populations of North America than in the other populations such as Africa, Europe, India, and Northeast India. [7] conclusIon RMC is often treated as a subtype of BMC. It is generally very narrow so it is difficult to detect it on panoramic radiographs. In addition, because the images in the retromolar region overlap the shadow of the opposite side of mandible and other soft tissues, the detailed status of RMC and RMF may be difficult to depict on panoramic radiograph. Hence, high resolution CBCT images are very usefull to know the prevalence of these anatomical variations. It remains unknown, how the RMC develops in the mandible, so there is need of further studies to understand its origin and evolutionary importance. RMC and RMF have great importance in the odontostomatological practice due to prevalence of the pathological processes and complications related to retromolar area involved in the practice. Hence, they are not rare anatomical structure and practitioner should not neglect it and have to take these into account in all anesthetic and surgical procedures involving the retromolar region to prevent possible complications such as failed anesthesia, paresthesia, excessive bleeding or traumatic neuroma. When there is any suspicious alteration in the MC, we suggest the indication of more accurate examination technique like CBCT.

Financial support and sponsorship
Nil.

Conflicts of interest
There are no conflicts of interest.