Non-Metric Variations of the Mandible
Non-metric variations of the mandible are differences in normal anatomy that should be understood in order to be differentiated from features of traumatic origin when carrying out forensic assessment.
Stafne’s Defect
Also termed Stafne’s Bone Cavity (SBC), this rare defect is idiopathic and asymptomatic in nature and is usually an incidental finding on a dental panoramic radiograph. First described by Edward Stafne in 1942, SBCs are usually found unilaterally and posteriorly, often in the lingual cortex between the inferior alveolar canal and the inferior border of the mandible.
Posterior SBCs appear radiographically as unilocular, well-demarcated, ovoid or round lesions. The most common contents of a posterior cavity is salivary gland tissue. Although much less common, there have been reports of SBCs in the anterior mandible, where the contents include lymphoid or lipid tissue. Although the cause of SBCs is unknown, it is theorised that an ectopic portion of the submandibular salivary gland causes the bone of the lingual cortical plate to remodel.
Forensically, the defect will appear as a shadow in the bone if in its early stages of development. In the late stages, the defect will have compromised the outer cortex of bone and will appear as an open cavity on the lingual aspect of the mandible.
Bifurcated Mandibular Condyle
Bifurcation of the head of the mandibular condyle is a very rare anatomical deviation which affects 0.3-1.8% of the population. The causes of this defect are usually congential (present at birth), or from a history of trauma to the condylar region of the mandible. The duplication is most often observed in the mediolateral plane, rather than the anteroposterior or superior-inferior planes.
The head of the mandibular condyle is a component of the tempromandibular joint (TMJ), which during normal function allows opening and closing of the lower jaw by hinge and slide movements. Some symptoms experienced by a person with a bifurcated mandibular condyle include hypomobility of the TMJ, arthralgia (pain in the joint), noisy articulation of the joint and ankylosis, or fusion of the TMJ.
Shallow Fossa on the Mandibular Condyle with Perforations
In normal anatomy, the mandibular condyle is a rounded protuberance of the jaw which articulates with the mandibular process of the temporal bone. One variant observed in the condylar process is the presence of a shallow depression, or fossa, in the head of the eminence. Also observed within this fossa are small perforations. Both of these findings are rare and their cause is poorly understood, but it is theorised they are due to disease of the joint – for example in individuals with osteoarthritis.
Also termed Stafne’s Bone Cavity (SBC), this rare defect is idiopathic and asymptomatic in nature and is usually an incidental finding on a dental panoramic radiograph. First described by Edward Stafne in 1942, SBCs are usually found unilaterally and posteriorly, often in the lingual cortex between the inferior alveolar canal and the inferior border of the mandible.
Posterior SBCs appear radiographically as unilocular, well-demarcated, ovoid or round lesions. The most common contents of a posterior cavity is salivary gland tissue. Although much less common, there have been reports of SBCs in the anterior mandible, where the contents include lymphoid or lipid tissue. Although the cause of SBCs is unknown, it is theorised that an ectopic portion of the submandibular salivary gland causes the bone of the lingual cortical plate to remodel.
Forensically, the defect will appear as a shadow in the bone if in its early stages of development. In the late stages, the defect will have compromised the outer cortex of bone and will appear as an open cavity on the lingual aspect of the mandible.
Bifurcated Mandibular Condyle
Bifurcation of the head of the mandibular condyle is a very rare anatomical deviation which affects 0.3-1.8% of the population. The causes of this defect are usually congential (present at birth), or from a history of trauma to the condylar region of the mandible. The duplication is most often observed in the mediolateral plane, rather than the anteroposterior or superior-inferior planes.
The head of the mandibular condyle is a component of the tempromandibular joint (TMJ), which during normal function allows opening and closing of the lower jaw by hinge and slide movements. Some symptoms experienced by a person with a bifurcated mandibular condyle include hypomobility of the TMJ, arthralgia (pain in the joint), noisy articulation of the joint and ankylosis, or fusion of the TMJ.
Shallow Fossa on the Mandibular Condyle with Perforations
In normal anatomy, the mandibular condyle is a rounded protuberance of the jaw which articulates with the mandibular process of the temporal bone. One variant observed in the condylar process is the presence of a shallow depression, or fossa, in the head of the eminence. Also observed within this fossa are small perforations. Both of these findings are rare and their cause is poorly understood, but it is theorised they are due to disease of the joint – for example in individuals with osteoarthritis.
Long Genial Tubercles
Genial tubercles are bony projections which extend posteriorly from the lingual aspect of the midline of the mandibular symphysis. There are four projections in total – superior and inferior pairs. They serve as attachment sites for the bilateral genioglossus and geniohyoid muscles, respectively.
The genial tubercles are usually small eminences, but they can become elongated through excessive force of the attached muscles. The genial tubercles may also become hypertrophic by consequence of alveolar ridge resorption in the edentulous mandible. In most cases, lengthening of the genial tubercles is completely asymptomatic.
Cavitation Defect in Retromolar Fossa
The retromolar fossa is the part of the mandible immediately posterolateral to the lower third molar tooth. It is bound laterally by the anterior border of the ramus and medially by the temporal crest of the coronoid process, to which the temporalis muscle attaches.
In the majority of individuals, the bone composition of the retromolar fossa is solid and intact. However, in some cases there may be a retromolar foramen present within the retromolar fossa. This would form the opening to a retromolar canal - an accessory canal with an accessory neurovascular bundle which branches from the inferior alveolar canal and bundle, supplying accessory innervation to the third molars. This foramen may be of varying size, and may even cavitate to form a reasonable sized hollow in the retromolar fossa of the mandible.
This cavitation defect in the retromolar fossa may have an impact on achieving anaesthesia for removal of mandibular third molars. The incidence of this defect is largely neglected in research literature, but one study showed a prevalence of 8%. This cavitation otherwise causes no known symptoms or side effects to the individual.
Perforation of the Mylohyoid Canal
The mylohyoid canal/groove is a bilateral feature found on the medial surface the mandible, which starts at the angle and extends along the body of the jaw at the approximate level of the dental root apices. It is the point of origin of the mylohyoid muscle, which then inserts at the hyoid bone.
One variation in anatomy of the mylohyoid groove is the presence of a small perforation in the centre of the canal, most commonly at the angle of the mandible.
Genial tubercles are bony projections which extend posteriorly from the lingual aspect of the midline of the mandibular symphysis. There are four projections in total – superior and inferior pairs. They serve as attachment sites for the bilateral genioglossus and geniohyoid muscles, respectively.
The genial tubercles are usually small eminences, but they can become elongated through excessive force of the attached muscles. The genial tubercles may also become hypertrophic by consequence of alveolar ridge resorption in the edentulous mandible. In most cases, lengthening of the genial tubercles is completely asymptomatic.
Cavitation Defect in Retromolar Fossa
The retromolar fossa is the part of the mandible immediately posterolateral to the lower third molar tooth. It is bound laterally by the anterior border of the ramus and medially by the temporal crest of the coronoid process, to which the temporalis muscle attaches.
In the majority of individuals, the bone composition of the retromolar fossa is solid and intact. However, in some cases there may be a retromolar foramen present within the retromolar fossa. This would form the opening to a retromolar canal - an accessory canal with an accessory neurovascular bundle which branches from the inferior alveolar canal and bundle, supplying accessory innervation to the third molars. This foramen may be of varying size, and may even cavitate to form a reasonable sized hollow in the retromolar fossa of the mandible.
This cavitation defect in the retromolar fossa may have an impact on achieving anaesthesia for removal of mandibular third molars. The incidence of this defect is largely neglected in research literature, but one study showed a prevalence of 8%. This cavitation otherwise causes no known symptoms or side effects to the individual.
Perforation of the Mylohyoid Canal
The mylohyoid canal/groove is a bilateral feature found on the medial surface the mandible, which starts at the angle and extends along the body of the jaw at the approximate level of the dental root apices. It is the point of origin of the mylohyoid muscle, which then inserts at the hyoid bone.
One variation in anatomy of the mylohyoid groove is the presence of a small perforation in the centre of the canal, most commonly at the angle of the mandible.
Accessory Mandibular Foramen
The mandibular foramen is found on the medial surface of the ramus of the mandible. It forms the opening to the inferior alveolar canal, through which runs the neurovascular bundle that supplies the lower molar teeth and gingival tissue.
Occasionally, individuals may have additional foramina which can be found around the main mandibular foramen. In these cases, the inferior alveolar nerve enters the mandible at several sites instead of a through a singular site.
4 Accessory Mandibular Foramina
A rare and minor variant of mandibular anatomy is the presence of four accessory foramina on the buccal surface, close to the mental foramen. These small features are likely to be openings of the mandibular canal which transport small branches of the inferior alveolar neurovascular bundle. These numerous foramina could be indicative of a split mandibular canal.
Large Mental Foramen with a Small Foramen
The mental foramen is found bilaterally on the buccal aspect of the body of the mandible, inferior to the lower premolar teeth. The terminal branches of the inferior alveolar neurovascular bundle exit the mandible here as the mental nerve which supplies the anterior teeth, gingiva, and lower lip on the corresponding side.
This foramen is most often a singular small opening in the lower jaw, but a normal variation of anatomy of the mental foramen includes a larger main foramen with a smaller foramen adjacent to it. In these rare cases, the mental nerve exits the mandible through both foramina. The prevalence of an accessory mental foramen ranges from 2.0-14.3%.
Bilateral Buccal Foramina Superior to the Gonial Angle
The presence of accessory buccal foramina (ABF) is a rare variation in anatomy of the mandible. ABFs are the umbrella term for any accessory foramen found on the buccal surface of the lower jaw. Here, a small hole is seen bilaterally on the lateral surface of the mandible, superior to the gonial angle. It is thought to be a small continuation of the mandibular canal.
Small accessory foramina
A small accessory foramina can be observed here on the buccal surface of the mandible and posterior to the last standing molar tooth. This feature would be categorised under the umbrella term of ABF, as previously described.
The mandibular foramen is found on the medial surface of the ramus of the mandible. It forms the opening to the inferior alveolar canal, through which runs the neurovascular bundle that supplies the lower molar teeth and gingival tissue.
Occasionally, individuals may have additional foramina which can be found around the main mandibular foramen. In these cases, the inferior alveolar nerve enters the mandible at several sites instead of a through a singular site.
4 Accessory Mandibular Foramina
A rare and minor variant of mandibular anatomy is the presence of four accessory foramina on the buccal surface, close to the mental foramen. These small features are likely to be openings of the mandibular canal which transport small branches of the inferior alveolar neurovascular bundle. These numerous foramina could be indicative of a split mandibular canal.
Large Mental Foramen with a Small Foramen
The mental foramen is found bilaterally on the buccal aspect of the body of the mandible, inferior to the lower premolar teeth. The terminal branches of the inferior alveolar neurovascular bundle exit the mandible here as the mental nerve which supplies the anterior teeth, gingiva, and lower lip on the corresponding side.
This foramen is most often a singular small opening in the lower jaw, but a normal variation of anatomy of the mental foramen includes a larger main foramen with a smaller foramen adjacent to it. In these rare cases, the mental nerve exits the mandible through both foramina. The prevalence of an accessory mental foramen ranges from 2.0-14.3%.
Bilateral Buccal Foramina Superior to the Gonial Angle
The presence of accessory buccal foramina (ABF) is a rare variation in anatomy of the mandible. ABFs are the umbrella term for any accessory foramen found on the buccal surface of the lower jaw. Here, a small hole is seen bilaterally on the lateral surface of the mandible, superior to the gonial angle. It is thought to be a small continuation of the mandibular canal.
Small accessory foramina
A small accessory foramina can be observed here on the buccal surface of the mandible and posterior to the last standing molar tooth. This feature would be categorised under the umbrella term of ABF, as previously described.
