Anatomic Considerations of Mandible

The important anatomic structures/areas which influence implant surgery in mandible are discussed as below.

Anatomic landmarks on mandible

1) Neurovascular bundles
Inferior alveolar nerve and artery: A branch of mandibular nerve, it enters the mandibular foramen on the medial aspect of the ramus above the lingula and exits on the lateral aspect of the mandibular body at the mental foramen which may be located between the premolars or at the apex of the 2 nd premolar. In the molar region, the inferior alveolar nerve divides into mental and incisal branches. The mental nerve and blood vessels exit through the distal half of the mental foramen dividing into three branches, anterior, middle and posterior unless it has an anterior loop which goes anterior to mental foramen, then loops back to exit from mesial aspect of mental foramen. The incisal branch runs anteriorly in the incisive canal and can be felt at mesial aspect of mental foramen.

The entire course of mandibular canal may be clearly discerned through the
caudal lamella of compact bone(arrows). 1. Mandibular foramen; 2. Mental foramen

Surgical significance

In an excessively resorbed ridge, the mental foramen with its contents of mental nerve and vessels, can be found on the crest of the ridge. An incision or reflection of the mucosa in this area must avoid injury to these vital structures. The knowledge of position of the inferior alveolar canal in vertical as well as buccolingual dimension is of paramount importance during site preparation for implants. Intrusion into the mandibular canal during surgery results in increased risk of haemorrhage, visibility impairment, and increased potential of fibrous tissue formation at the surface of the implant. Encroachment of the mandibular canal, mental foramen during osteotomy can result in injury to these nerves and accompanying blood vessels causing paraesthesia, hyperesthesia, hypoesthesia, dysthesia or anaesthesia of the teeth, lower lip or surrounding skin and mucosa. Also may cause venous/arterial bleeding.Inferior alveolar nerve repositioning to facilitate the placement of endosseous implants posterior to mental foramen is associated with very high incidence of temporary inferior alveolar nerve damage.The inferior dental nerve in some cases may divide into two or three rami that occupy separate canals as the nerve travels in the mandible to supply the bone. These variations should be determined radiographically and the surgical approach accordingly modified to avoid nerve injury.

Mental foramen and nerve
Mental foramen is a strategically important landmark during osteotomy procedures.Its location and the possibility that an anterior loop of mental nerve may be present on the mesial of the mental foramen, needs consideration before placing implant medial to foramen to avoid mental nerve injury. An OPG is used to see the possibilityfor an anterior loop which is found to extend about 1-3 mm anterior to the mental foramen in about 12 per cent of cases. If the mandibular canal on an OPG approaches the mental foramen at the same height, the nerve always enters the distal portion of the foramen. However, a loop occurs when the mandibular nerve approaches from below (the mental foramen), proceeds anteriorly to about 1-3 mm and curves upward and distally to enter the mesial aspect of mental foramen The anterior loop should be verified either by surgical exposure of the mental foramen or with a CT scan. A blunt curved probe is used to check the patency at the distal aspect from where the nerve exits. If not patent, it means that the nerve extends to the mesial side and that either it can be an anterior loop or the incisal branch of inferior alveolar nerve.

In order to avoid nerve injury, leaving a 2 mm safety zone (Misch) above the nerve is desired when placing the implant. The anterior loop can extend anterior to the mental foramen by 1-3 mm. Therefore a safety margin of 2mm added to 3mm anterior to the foramen provides a safe zone. Explanation of these will be made in the unit explaining stage one surgery.

Anterior extension of inferior alveolar canal beyond mental foramen (arrow)

Lingual nerve and artery: is the branch of mandibular nerve which enters the oral cavity above the posterior edge of the mylohyoid muscle close to the 3 rd molar region proceeding on the surface of hypoglossal muscle to enter floor of the mouth and tongue. Lingual artery (br. of inferior alveolar artery) descends with the lingual nerve and must be taken care off during surgery.

Significance: Because the nerve lies medial to the retromolar pad, incisions in this region should remain lateral to the pad and mucosal reflection should be done with the periosteal elevator in constant contact with bone to avoid injury to the nerve. As the branches of the lingual nerve carry sensory information from the lingual mucosa, mucosa of the floor of the mouth and anterior two-third of the tongue, improper reflection of lingual mucoperiosteal flap may injure the lingual nerve and produce ipsilateral paraesthesia or anaesthesia of the innervated mucosa, loss of taste and reduction of salivary secretion.

Nerve to Mylohyoid: motor branch of inferior dental nerve which descends in a groove on the medial surface of the mandibular ramus. Surgical intervention in this area may lead to injury of the nerve.Long buccal nerve: The long buccal nerve is a branch of the mandibular division of the trigeminal nerve which provides sensory innervation to the buccal gingiva and mucosa of the cheek and emerges deep to the mandibular ramus. In an anatomical variation, known as Turner’s variation, the long buccal nerve branches off the inferior alveolar nerve within the mandible and arises from a foramen in the retromolar fossa. Surgical trauma of the Turner’s variation may lead to paresthesia in the retromolar- canine buccal gingiva.

2 )Important Muscles associated with Mandible
Mylohyoid muscle: Surgical manipulation at the crest of a severely resorbed ridge may injure the mylohyoid muscle. Manipulation of the tissues of the floor of the mouth for placement of subperiosteal implant may lead to edematous swelling of sublingual space or submandibular space. Extensive bilateral cellulitis of sublingual space may push the tongue backward or compress the pharynx resulting in airway obstruction.

Genioglossus: originates from superior genial tubercles which are located near the crest of the alveolar ridge in atrophic mandible. Therefore, one should be aware of it during the elevation of lingual mucosa for making impression for subperiosteal implant to prevent its injury. The muscle should not be completely detached from the tubercle as it may cause retrusion of the tongue and possible airway obstruction.Medial pterygoid: muscle bounds the pterygomandibular space medially which is entered when an inferior alveolar nerve block is administered. Infection of this space is dangerous due to its proximity to the parapharyngeal space and potential spread of the infection to mediastinum.

Muscle attachements on medial aspect of mandible

Mentalis: The mental tubercles on either side of mental protruberance(in midline) gives origin to the mentalis muscle. Above the mentalis origin, the incisivus muscle takes origin from small fossa called the incisivus fossa. Complete reflection of mentalis muscle for the purpose of extension of a subperiosteal implant or symphyseal intraoral graft should be avoided as it may result in “witch’s chin” probably caused by the failure of muscle reattachment.

Temporalis: Surgical exposure of the mandibular ramus medially involves the temporalis tendon-fascial complex with its content of muscle fibers, nerves, and vessels leading to tendinitis and postoperative pain. Incisions placed on the ascending ramus for subperiosteal implants or harvesting bone from external oblique ridge and ramus should be inferior to the insertion of the two tendons of the temporalis muscle.

3) Surgical Considerations in anterior mandibular region
During the harvesting of a monocortical symphyseal block of bone from the mandibular symphysis region (autogenous block bone graft), incisive branch of the inferior alveolar artery is often severed as it continues medially and anastomose with the artery of the opposite side if proper care is not taken. The apical limit of bone harvesting is 5mm coronal to the inferior border of the chin, coronal limit is 5mm apical of the apex of the anterior teeth and lateral limit is 5mm mesial to the mental foramen.Life threatening haemorrhage has been reported when a drill perforates the sublingual region of the mandible and injures the sublingual artery (branch of lingual artery).Upper airway obstruction secondary to severe bleeding in the floor of the mouth has been occasionally reported as a rare but potentially fatal complication of implant surgery. Excessive bleeding and formation of massive lingual, sublingual, and submandibular hematomas as a result of arterial trauma that occurred during the osteotomy preparation have the tendency of displacing the tongue and floor of the mouth to obstruct the airway.The arterial bleeding in the floor of the mouth may be due to injury to lingual artery( which stops when tongue is pulled out) or submental artery( branch of facial artery) which is stopped by placing pressure over the facial artery notch anterior to the mandibular angle.

4) Mandibular Ramus
Serves as an intraoral autogenous bone donor site. Cortical bone upto 4mm thickness may be harvested from the ramus. Harvest of bone from this area requires knowledge of the mandibular canal anatomy to prevent nerve injury and also injury to the buccal artery when incision is placed along the external oblique ridge.Prominent external oblique ridge is also an ideal site for harvesting autogenous bone graft. However, caution must be exercised to avoid injury to the teeth or to the inferior alveolar nerve at the inferior extent of the graft harvest, and the lingual nerve medially.