Anatomy, head and neck, jaw (2023)

introduction

The lower jaw is the largest bone in the human skull. It holds the lower teeth in place, aids in chewing, and shapes the lower jaw line. The lower jaw is made up of the body and ramus and is situated below the upper jaw. The body is a horizontally curved part that forms the line of the lower jaw. The branches are two vertical processes located on both sides of the body; they connect the body at the angle of the lower jaw. On the superior side of each ramus, the coronoid and condylar processes articulate with the temporal bone to form the temporomandibular joint, which allows mobility. Other than the ossicles, the lower jaw is the only bone in the skull that is mobile, allowing the bone to contribute to chewing.

During development, the first pharyngeal arch forms two processes that eventually fuse at the mandibular symphysis to form the mandible. At birth, the mandibular symphysis consists of fibrocartilage. Within a year of life, the symphysis is fused and a subtle midline ridge remains on the front surface of the body.[1]

structure and function

The lower jaw consists of the following parts: the body and two rami.

Body

The body is the front part of the lower jaw and is bounded by two surfaces and two margins. The body ends and the rami begin on each side at the angle of the mandible, also known as the gonial angle.

External surface:The outer surface contains the midline symphysis of the mandible, recognized as a subtle ridge in the adult. The lower part of the ridge is divided and encloses a midline depression called the mental ridge. The edges of the mental pons are elevated, forming the mental tubercle. Next to the crest and below the incisors is a depression known as the incisal fossa. Below the second premolar is the mental foramen, through which the mental nerve and vessels exit. The oblique line runs posteriorly from the mental tubercle to the anterior border of the ramus.
inner surface:The inner surface contains the median crest in the midline and mental spines located just to the side of the crest. The mylohyoid line begins in the midline and runs up and back to the alveolar rim.
Alveolar:The alveolar rim, the upper edge, contains the sockets that house the sixteen lower teeth.
Bottom edge:The lower border forms the line of the lower jaw and contains a small groove through which the facial artery passes.

rama

The ramus contributes to the lateral part of the mandible on both sides. The coronoid and condyloid processes lie on the superior aspect of the ramus. The coronoid process is anterior and the condyloid process is posterior; the two are separated by the mandibular notch. Bounded by two surfaces and four margins, the branch contains two processes.

cara lateral:The lateral surface contains part of the oblique line that began on the outer surface of the body. This surface also forms the origin of the masseter muscle.
median surface:The medial surface contains the mandibular foramen, through which the inferior alveolar nerve and inferior alveolar artery enter, and the posterior course of the mandibular canal. On the anterosuperior side of the mandibular foramen is a sharp process called the lingula mandibularis. On the posteroinferior side of the mandibular foramen is the mylohyoid groove, against which the mylohyoid vessels course.
Upper limit:The upper border leading to the coronoid and condyloid processes.
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Bottom edge:The lower border is continuous with the lower border of the body of the lower jaw and contributes to the jaw line.
rear edge:The posterior border is continuous with the lower border of the ramus and extends deep to the parotid gland. This limit is used in conjunction with the lower limit of the body of the mandible to determine the gonial angle.
Limite frontal:The anterior border is continuous with the oblique line of the outer surface of the body.

CoronoidesProzess

The coronoid process is located on the superior aspect of the ramus. Its anterior border merges with that of the ramus and its posterior border forms the anterior border of the mandibular notch. The temporal muscle and the masseter insert on its lateral surface.

condyloid process

The condyloid process is also found on the superior face of the ramus and is divided into two parts, the neck and the condyle. The neck is the thinnest part of the condyloid process that protrudes from the ramus. The condyles are the upper part and contribute to the transition from the TMJ to articulation with the articular disc.

Embryology

The mandible appears in the sixth week of intrauterine development and is the second bone after the clavicle to ossify.[1]Meckel's cartilage develops from the first pharyngeal arch, the arch of the lower jaw. This cartilage serves as a template for the development of the lower jaw. A fibrous membrane covers the left and right Meckel's cartilages at their ventral ends, each giving rise to a single ossification center. These two halves are eventually fused together by fibrocartilage at the mandibular symphysis. At birth, the lower jaw consists of two separate bones. Ossification and fusion of the mandibular symphysis occurs during the first year of life, resulting in a single bone. The remainder of the mandibular symphysis is a subtle ridge in the midline of the mandible.[1]

The lower jaw is constantly changing throughout a person's life. At birth, the gonial angle is about 160 degrees. By the age of four, teeth have formed, causing the jaw to lengthen and widen; These changes in mandibular dimensions cause the gonial angle to decrease to approximately 140 degrees. In adulthood, the gonial angle is reduced to about 120 degrees.[1]

blood supply and lymphatic vessels

The blood supply to the jaw is carried out through small periosteal and endosteal vessels. The periosteal vessels arise mainly from the inferior alveolar artery and supply the ramus of the mandible. The endosteal vessels arise from the perimandibular branches of the maxillary, facial, external carotid, and superficial temporal arteries; these feed the body of the lower jaw.[2]The mandibular teeth are supplied by dental branches of the inferior alveolar artery.

Lymphatic drainage from the mandible and mandibular teeth occurs primarily via the submandibular lymph nodes; However, the symphysis mandibular region drains into the submental lymph node, which subsequently drains into the submandibular lymph nodes.

Annoy

The main nerve associated with the mandible is the inferior alveolar nerve, which is a branch of the mandibular division of the trigeminal nerve. The inferior alveolar nerve enters the mandibular foramen and travels anteriorly in the mandibular canal, where it sends branches to the lower teeth and provides sensation. In the mental foramen, the inferior alveolar nerve branches into the incisive nerve and the mental nerve. The mental nerve emerges from the mental foramen and ascends to give sensation to the lower lip. The incisal nerve runs in the incisal canal and supplies the mandibular premolars, canines, and lateral and central incisors.[3]

muscles

Muscles extending from the lower jaw.

mentalist- comes from the cutting pit
Lata Spherical- comes from the cutting pit
lower lip depressor- comes from the oblique line
depressing the corner of the mouth- comes from the oblique line
Buccinador- originates from the alveolar process
digastric anterior abdomenIt originates from the digastric fossa.
my idea- comes from the mylohyoid lineage
Genius idea- comes from the lower part of the mental spine
genioglosso- comes from the top of the mental spine
superior pharyngeal constrictor- derives in part from the pterygomandibular raphe, which derives from the mylohyoid lineage

Muscles that attach to the lower jaw

platysma- Inserts in the lower part of the lower jaw
superficial masseter- Inserts on the lateral surface of the ramus and angle of the mandible
deep masetero- Inserts on the lateral surface of the ramus and angle of the mandible
medial pterygoid- Inserts on the medial surface of the mandibular angle and mandibular ramus
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Inferior head of the lateral pterygoid- Insertions in the condyloid process
temporal-Insertions in the coronoid process

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Physiological variants

Males generally have more angular and prominent jaws than females. This is due to the larger size of the mental lobe in males and the reduced gonial angle. The gonial angle is 90 degrees in men and 110 degrees in women.

In rare cases, there may be a bifid or trifid inferior alveolar canal. This can be seen on the radiograph as a second or third mandibular canal. Branches of the inferior alveolar nerve often pass through these additional foramina and may present a risk of inadequate anesthesia during mandibular surgical procedures.[4]

A cleft chin can result from insufficient or absent fusion of the mandibular symphysis during embryonic development. This often results in deepening of the overlying soft tissue in the midline of the mandible. This is a genetic disorder that is inherited in an autosomal dominant fashion and is more common in the male population.[5]

surgical considerations

Orthognathic surgery, which includes mandibular osteotomies and split sagittal osteotomies, is corrective surgery of the jaw performed to improve bite misalignment, sleep apnea, TMJ disorders, and structural problems such as cleft palate and micrognathia.

Mandibular osteotomy is performed on patients with micrognathia, a condition in which the jaw is too small. Micrognathia can lead to pain and difficulty chewing: correction is often required. This procedure is performed by dividing the mandible bilaterally between the first and second molars; The lower jaw is extended to its new position and stabilized with hardware.

The sagittal split osteotomy is performed in patients with prognathism, a condition in which the lower jaw is oversized and results in an underbite. This procedure is performed by splitting the mandible bilaterally, moving it to a more posterior position, and stabilizing it with hardware.

Complications include postoperative facial numbness due to nerve damage. Recovery from nerve damage usually occurs within 3 months of the procedure.

clinical significance

Jaw fractures are most often caused by trauma and usually occur in two places. The parasymphysis is particularly prone to fracture due to the incisive fossa and mental foramen. A direct blow to the lower jaw can cause a condylar neck fracture because the articular disc of the temporomandibular joint prevents it from moving backwards.[6]

For patients with traumatic jaw injuries, a standard four-view X-ray film series can be created. In many cases, mandibular series do not provide enough detail to accurately diagnose condyle fractures due to overlapping anatomy. This problem can be solved by using a reverse Towne view to obtain images. Newer modalities, such as CT scans, have been shown to be more sensitive than X-rays and are commonly used.[7]

The most common dislocation of the mandible is posteriorly, but anterior and inferior dislocations may also be seen. The patient may present with an inability to close the mouth or an asymmetrical jaw. A previous dislocation is the biggest risk factor. Manual reduction is often used to correct the injury. Barton's bandages are used after reduction to hold the jaw in place and stabilize it.

review questions

Cipher

Labeled lower jaw. Image courtesy of S Bhimji MD

references

1.

Lipski M, Tomaszewska IM, Lipska W, Lis GJ, Tomaszewski KA. The mandible and its foramen: anatomy, anthropology, embryology, and resulting clinical implications.Lámina de morfol (Warsaw).November of 2013;72(4):285-92.[PubMed: 24402748]

2.

Saka B, Wree A, Henkel KO, Anders L, Gundlach KK. Blood supply to the mandibular cortex: an experimental study in Göttingen minipigs with special consideration of the condyle.J Craneomaxilofac Surg.February 2002;30(1):41-5.[PubMed: 12064882]

3.

Lee MH, Kim HJ, Kim DK, Yu SK. Histological characteristics and fascicular arrangement of the inferior alveolar nerve.Arco Oral Biol.December 2015;60(12):1736-41.[PubMed: 26433190]

4.

Wadhwani P, Mathur RM, Kohli M, Sahu R. Mandibular canal variant: a case report.J Oral Pathol Med.February 2008;37(2): 122-4.[PubMed: 18197857]

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5.

Ladani P, Sailer HF, Sabnis R. Tessier 30 Mandibular symphyseal cleft: simultaneous early correction of soft and hard tissues: a case report.J Craneomaxilofac Surg.December 2013;41(8):735-9.[PubMed: 23454264]

6.

Morrow BT, Samson TD, Schubert W, Mackay DR. Evidence-based medicine: mandibular fractures.Plast Reconstr. SurgeryDecember 2014;134(6):1381-1390.[PubMed: 25415101]

7.

Gupta M, Iyer N, Das D, Nagaraj J. Analysis of different treatment protocols for fractures of the condylar process of the mandible.J Oral and maxillofacial surgery.January 2012;70(1):83-91.[PubMed: 21549492]