What is the Jawbone
The jawbone is the largest, sturdiest, and the only skull bone capable of movement. It forms the lower jaw, and thus is also known as the lower jaw bone. It helps with the process of chewing along with the maxilla or upper jaw bone.
The bone has derived its name from the Latin word ‘mandibula’, which is derived from ‘mandere’, meaning ‘to chew’, and ‘bula’, an instrumental suffix.
Where is the Jawbone Located
As stated, the jawbone is located in the lower jaw, just inferior to the maxilla or upper jaw. You can easily feel the bone by touching your lower jawline.
Quick Facts
| Type | Irregular bone |
| Length | In males: Approximately 17-19 cm In females: Approximately: 16-18 cm |
| How many are there in the human body | 1 |
| Articulates with | Temporal bone |
Functions
- Forms the lower jawline, shaping the face and chin. The teeth in the lower jaw are also rooted to this bone.
- Helps in mastication or chewing of food.
- Being the only mobile skull bone, it aids in the movement of the mouth, helping with eating, and talking.
- Forms and protects the organs and vessels in the oral cavity.
Parts and Anatomy of the Jawbone
It is a single, horseshoe-shaped bone consisting of a horizontal body on the anterior side and two vertical rami on the posterior side.
Body
The body is curved, having two borders and two surfaces.
Borders
The superior border is called the alveolar border, whereas the inferior one is the base.
Alveolar Border (Superior): An essential bony landmark, the alveolar process extends superiorly from this border. It comprises two bony plates: a thick buccal and a thin lingual. This is the most vital region of the jawbone, as it holds 16 sockets or cavities for the lower set of teeth. Each side of the jawbone holds 5 primary teeth and 7-8 permanent teeth.
Base (Inferior): It is the lower border of the bone, where the digastric muscle attaches medially. It also provides a small groove through which the facial artery passes.
Surfaces
The part of the bone facing the oral cavity is the inner surface, whereas the one facing outwards is the outer surface.
Inner Surface: On the medial side of the inner surface, there are some important structures, including the superior and inferior genial tubercles or mental spine, digastric fossa, and mylohyoid line. The mylohyoid line divides the jawbone into submandibular and sublingual fossae, where the two salivary glands, submandibular and sublingual, rest. The pterygomandibular raphe attaches to the posterior border of the mylohyoid line.
The internal surface contains the median ridge at the midline and mental spines, which are just lateral to the ridge. The mylohyoid line begins at the midline and courses superiorly and posteriorly to the alveolar border.
Outer Surface: On the lateral sides of the outer surface, external oblique line, mandibular symphysis, and mental foramen can be seen.
Jawbone symphysis is a bony landmark present in the midline of the jawbone. It is a small bone ridge representing the fusion of the two halves during development. Laterally to the ridge and below the incisive teeth, there is a depression known as the incisive fossa. The symphysis encloses a triangular eminence called the mental protuberance, which forms the characteristic shape of the chin. The edges of the mental protuberance are elevated, forming the mental tubercle.
Ramus
As mentioned, the bone features two mandibular rami (singular: ramus) that project perpendicularly upwards from the angle of the jawbone. The body and the rami meet on each side at the angle of jawbone or gonial angle. This angle can range from 110-130 degrees and vary depending on age, sex and ethnicity.
At the superior point, the ramus divides into two processes, the coronoid process on the anterior side and the condylar process on the posterior side. A jawbone notch separates these two processes.
Head: It is situated posteriorly and articulates with the temporal bone, forming the temporomandibular joint.
Neck: It supports the head of the ramus and also acts as a site of attachment for the lateral pterygoid muscle.
Coronoid Process: It is located on the superior side of the ramus. The temporalis and masseter muscle that helps in chewing attaches to its lateral surface. The anterior border of this process is continuous with that of the ramus, and its posterior border makes the anterior boundary of the jawbone notch.
Condylar Process: It is also located at the ramus’s superior side and divided into neck and condyle. The neck is a slender stalk-like structure projecting from the ramus. There is a protuberance on the top of the neck, the condyle that forms the lower bony component of the temporomandibular joint, along with the temporal bone. The anteromedial surface of the condylar process has a small depression called the pterygoid fovea or pterygoid fossa, where the lateral pterygoid muscle attaches.
Foramina
The jawbone is marked by two foramina:
1. Mandibular Foramen
The inner medial side of the ramus contains the mandibular foramen, through which the inferior alveolar nerve and inferior alveolar artery pass. These neurovascular structures travel through this foramen into the jawbone canal and exit at the mental foramen. At the anterosuperior side of the mandibular foramen, there is a sharp process called the lingula of the jawbone. Similarly, at the posteroinferior aspect of the foramen is the mylohyoid groove, through which the mylohyoid vessels run.
2. Mental Foramen
The outer surface of the jawbone body contains another foramen, called mental foramen. It is located lateral to the mental protuberance, below the second premolar, and houses the inferior alveolar nerve and artery that exit through the jawbone canal. When the inferior alveolar nerve passes through the mental foramen, it becomes the mental nerve that innervates the skin of the lower lip and the front part of the chin.
Muscle and Ligament Attachments
Muscles originating from the jawbone
- Mentalis: From the incisive fosanterior portion: From the digastric fossa
- Depressor of the corner of the mouth: From the oblique line
- Cheek muscles: From the alveolar process
- Muscles of the floor of the mouth: From the mylohyoid line
- Chin muscle: From the lower part of the genial tubercle
- Tongue muscle: From the upper part of the genial tubercle
- Upper throat constrictor: From the pterygomandibular raphe
Muscles attached to the jawbone
- Platysma: On the lower edge of the mandible
- Deep masseter: On the side surface of the ramus and angle of the mandible
- Superficial masseter: On the side surface of the ramus and angle of the mandible
- Inferior head of the lateral pterygoid: On the condyloid process
- Medial pterygoid: On the inner surface of the mandibular angle and ramus of the mandible
- Temporalis: On the coronoid process
An important ligament connection of the bone is the sphenomandibular ligament.
Connections
The mandible is the only bone that does not connect with its nearby skull bones by using sutures. It makes only one connection:
Joint of the Temporomandibular: The condyle part of the condylar process connects with the temporal bone, creating the temporomandibular joint with the help of the articular disk.
It is linked to the maxilla through muscle and joint connections, although they do not articulate with each other. The only time they are directly connected is when the upper and lower teeth meet when the mouth is shut.
Formation and Growth
Formation of the mandible begins during the sixth week of fetal development. It is the second bone to form.
The first set of the pharyngeal arch, or the mandibular arch, gives rise to the left and right Meckel cartilage, which serves as a pattern for the development of the bone. A fibrous membrane covers this cartilage at their ventral ends. From each of the left and right cartilage arises an ossification center. These two halves eventually fuse via fibrocartilage at the mandibular symphysis. As a result, the mandible is still composed of two separate bones at birth.
Ossification and fusion of the mandibular symphysis happen during the first year of life, resulting in a single bone. The remainder of the mandibular symphysis remains as a slight ridge at the midline of the mandible.
The mandible undergoes constant changes throughout an individual’s life. At birth, the gonial angle is approximately 160 degrees. By age four, teeth begin to form, causing the jaw to lengthen and broaden. These changes in the dimensions of the mandible cause the gonial angle to decrease to approximately 140 degrees. By adulthood, the gonial angle decreases to approximately 120 degrees.
FAQs
Q.1. What muscles move down and pull back the mandible?
Ans. The lateral pterygoid muscles move the mandible down, while the rear fibers of the temporalis muscle pull it back.
Q.2. What are the variances between a male and a female mandible?
Ans. Usually, in males, the angle of the mandible is curved outwards, resulting in a sharper angle at the jaws. Females often lack this outward curve and have a more rounded jaw. In some females, the angle may even invert towards the midline.
Sources
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- The Mandible – Teachmeanatomy.info
- Anatomy, Head and Neck, Mandible – Ncbi.nlm.nih.gov
- Mandible – Radiopaedia.org
- Mandible – Kenhub.com
