You are watching a preview. Go Premium to access the full video: Bony structures of the mandible.
Related study unit
Miss Kenhub versus Miss Mandible! And here we go! A few jabs here and there and ooh! A sucker punch to the door for Miss Mandible. Didn't seem to bother her, though. Oh my! Another one, and oh! Ano... Read more
Miss Kenhub versus Miss Mandible! And here we go! A few jabs here and there and ooh! A sucker punch to the door for Miss Mandible. Didn't seem to bother her, though. Oh my! Another one, and oh! Another one. You know what they say – the mandible is the strongest bone of the skull and Miss Mandible seems to be as solid as a rock!
Still though, that's got to rattle a few teeth. Okay, this is getting brutal. Why don't we take a break here and find out more about the anatomy of the unbelievably strong mandible!
So today we're going to learn all about the anatomy of the mandible. We will begin with taking a look at the main divisions of the mandible which include the body of the mandible and the two rami. We'll explore the bony landmarks and foramina within each division and also take a look at its adjacent articulations and main functions. Finally, we'll finish off today's tutorial with a quick look at some clinical notes on the mandible to help consolidate our understanding.
As you can see here, the mandible is our lower jaw bone. It is the largest, strongest, and lowest bone of the human skull. It is the only mobile bone of the skull, apart from the auditory ossicles.
The mandible is often considered to be a part of the viscerocranium; however, in some literature, it is not included as part of the facial skeleton. The mandible, as we have already mentioned, consists of a body anteriorly and two rami posteriorly.
Let's begin with taking a look at the body of the mandible.
The body of the mandible is somewhat U-shaped extending posteriorly on either side. The body of the mandible contains two borders – an upper alveolar part of the mandible, sometimes referred to as the superior border, and a lower base of the mandible, also known as the inferior border. It also has two surfaces – an outer or external surface and an inner or internal surface.
The alveolar part of the mandible anchors all 16 of your lower teeth; that is, if you don't choose boxing as a career, right? Alveolar originates from the Latin word alveus which means little cavity or hollow, pertaining to the little cavity which each tooth sits in.
The other 16 upper teeth can be found anchored in the paired maxillae. The maxilla, also known as the upper jaw, is a bone of the viscerocranium which lies just superior to the mandible. The body of the mandible articulates with the maxilla via the teeth when you close your mouth. Separating each adjacent dental alveoli is a thin plate of bone known as the interalveolar septa of the mandible. These septa ensure that the teeth have enough space to grow.
Let's move on to the inferior border of the body of the mandible, which is known as the base of the mandible. This structure is a free edge that extends posterolaterally from the midline. Now, if we flip the skull and look at the mandible from the back, you can see that near the midline of the base of the mandible, there are two shallow depressions – one on each side – which are known as the left and right digastric fossae of the mandible. These are the attachment sites of the anterior bellies of the left and right digastric muscles.
Now that we've explored the borders of the body of the mandible, let's take a look at the landmarks of the external surface of the body of the mandible. But before we do that, we need to go a bit back in time – all the way to fetal development.
During fetal development, the mandible actually consists of two cartilaginous structures – a right half and a left half. These ossified structures begin to fuse together forming the bony mandible at around two years after birth. After the bones have fused, an inconstant faint ridge of bone at the midline of the mandible remains on both the internal and external surface. This ridge of bone is known as the mandibular symphysis. It is not a true symphysis as there is no cartilage between the two sides of the mandible. We can actually feel the structure if we place our finger between the tip of our chin and lower lip and move our finger back and forth. Have you ever noticed how some people have a little dimple or cleft on their chin? This occurs as a result of a failure of the two sides of the mandible to fuse completely.
Inferiorly, the mandibular symphysis divides to enclose a triangular bony elevation inferior to the mandibular symphysis known as the mental protuberance. The term mental originates from the Latin word mentum which simply means chin. So this structure, as the name suggests, contributes to the prominence of the chin.
On either side of the mental protuberance are the left and right mental tubercles of the mandible. If we look closely at the triangular mental protuberance, we can see that the base of this triangle is depressed at the center but raised on either side to form the mental tubercles. These two small protrusions on the external surfaces of the body of the mandible also contribute to the prominence of the chin.
Next, we meet the two small little holes that we can see here on the external surface of the body of the mandible. These are the mental foramina, which are located halfway between the alveolar part and base of the mandible and superolateral to the mental protuberance. The left and right foramen lie just beneath the inferior second premolar teeth on either side. This little hole forms the external opening of the mandibular canal which transports the inferior alveolar nerve and vessels. When these structures exit the mandibular canal, they become the mental nerve and vessels.
Now let's isolate the mandible so we can look at the final external landmark on the external surface of the body of the mandible which are the oblique lines. The oblique lines are two bony ridges which extend from the anterior border of the ramus in the anteroinferior direction towards the mental tubercle. The distal aspect of these left and right ridges provide the attachment site for the depressor anguli oris muscles of the face.
Now onto the landmarks of the internal surface of the body of the mandible.
The first landmarks that we meet on the internal surface are the mental spines, also known as the genial tubercles. The mental spines are two small midline processes found on the posterior aspect of the mandibular symphysis. They can be divided into a superior and inferior mental spine.
The superior mental spine provides the attachment site for one of the extrinsic muscles of the tongue, the genioglossus muscle, while the inferior mental spine provides the attachment site for the right and left geniohyoid muscles.
The internal surface of the mandible is divided by oblique paired mylohyoid lines. The mylohyoid lines are bony ridges which extend from just below the third molar tooth in an anteroinferior direction towards the inferior border of the mandibular symphysis. As its name suggests, the left and right mylohyoid lines provide the attachment sites for the right and left mylohyoid muscles which support the floor of the mouth.
Above the anterior one-third of the mylohyoid lines is the left and right sublingual fossae. The sublingual fossa is an osseous indentation on either side of the posterior aspect of the mandibular symphysis. This little shallow depression that we can see highlighted here provides a little nook for the sublingual salivary gland to sit in.
Just inferior to the posterior two-thirds of the mylohyoid line is another osseous indentation known as the submandibular fossa. This is also a paired fossa meaning that there is a left fossa here and a right fossa here. These paired fossae house another pair of salivary glands known as the submandibular glands.
Now that we have explored the surfaces and borders of the body of the mandible, let's move posteriorly to the rami of the mandible.
The two broad rami ascend posteriorly on each side from the body of the mandible. Each ramus is quadrilateral and consists of four borders – an inferior border, a posterior border, an anterior border, and a superior border. You can also find two surfaces – a medial surface and a lateral surface.
We'll begin with the borders.
The inferior border of the ramus is a thick straight bony structure which is continuous with the mandibular base. The posterior border of the ramus is slightly rounded and is usually completely covered by the parotid gland as we can see here. The inferior border of the ramus of the mandible extends posteriorly to meet the posterior border at the angle of the mandible.
Divisions of the mandible are highly variable across texts and, therefore, some sources identify the angle of the mandible as the angle where the body and the ramus meet. However, for the purpose of this tutorial, we will classify the angle of the mandible as being formed by the intersection between the inferior and posterior borders of the ramus.
In this lateral view of the skull, we can clearly see the slightly horizontal inferior border intersecting with the vertical posterior border of the ramus to form the angle of the mandible. This angle, also known as the gonial angle, can range from 110 to 130 degrees and can vary between age, sex, and ethnicity. It is typically everted or turned outwards in males but frequently inverted or turned inwards in females.
The anterior border of the ramus is thinner than its posterior counterpart and is continuous with the oblique line of the body of the mandible. The superior border is the most complex border of the mandibular ramus and so we're going to take a look at a few more relevant structures here.
The superior border consists of two bony processes separated by a bony notch. The coronoid process is the triangular-shaped bony extension which projects in an anterosuperior direction. The anterior border of the coronoid process is continuous with the anterior border of the ramus. This process is an important attachment site for the temporalis and masseter muscles of mastication.
Just posterior to the coronoid process is the mandibular notch. The mandibular notch is the bony indentation on the superior aspect of the ramus stretching between the two processes of the superior border. These paired notches create a passageway for the masseteric nerves and vessels to pass through.
The condylar processes extend posterosuperiorly from the junction of the superior and posterior borders of each rami. They can be found posterior to the mandibular notches as we can see here. Each condylar process is made up of the head and neck of the mandible.
As I briefly mentioned, the superior aspect of the condylar process is known as the head of the mandible. This is a rounded bony surface which articulates with the squamous part of the temporal bone forming the temporomandibular joint.
The temporomandibular joint connects the ramus of the mandible to the neurocranium when the mouth is open and closed. In between the two articulating surfaces of the temporomandibular joint lies a fibrocartilaginous articular disc. This disc separates the joint into two compartments. The upper part of the joint allows for protraction and retraction of the mandible while the lower part of the joint permits movements of elevation and depression.
The head of the mandible is supported and continues inferiorly with the neck of the mandible. The neck of the mandible forms the inferior aspect of the condylar process and is continuous inferiorly with the ramus of the mandible. On the anterior aspect of the neck of the mandible is a small bony indentation known as the pterygoid fovea. This small fossa provides a distal attachment site for the lateral pterygoid muscle.
Now that we've gone through the borders of the ramus of the mandible, let's take a look at each of its surfaces.
As we already know, the ramus of the mandible has both a lateral or external and a medial or internal surface. The lateral surface is a flat sheet of bone found on the outer aspects of both the left and right rami. The majority of the lateral surface is covered by the strongest muscle in the human body – the masseter muscle – which is one of the muscles of mastication.
The lateral surface of the angle of the mandible contains a rough area known as the masseteric tuberosity. This is an important attachment point for the masseter muscle. The medial surface can be found at the inner or internal aspect of each ramus of the mandible. There are some bony landmarks on this surface and we're going to take a closer look at them.
I know…more landmarks…But hey! This is what happens when you learn bone anatomy.
At the central portion of the medial surface of the ramus is a small foramen known as the inferior alveolar foramen. The inferior alveolar foramen forms the internal opening of the mandibular canal and allows the passage of the inferior alveolar nerve and vessels.
Just anterior to the inferior alveolar foramen is a small bony projection known as the lingula of the mandible. This bony projection provides an attachment site for the sphenomandibular ligament which aids in supporting the mandible and muscles of mastication.
Next, we come to the mylohyoid groove of the mandible. The paired mylohyoid grooves extend in an anteroinferior direction from the inferior aspect of each inferior alveolar foramen. Each mylohyoid groove presents an oblique depression which allows for the passage of the mylohyoid nerve and artery. The nerve and artery travel through this groove to supply the mylohyoid muscles and the anterior belly of the digastric muscle.
The final bony landmark that we'll meet today is located on the medial surface of the angle of the mandible. Here we can find a bony roughening known as the pterygoid tuberosity. This is an important attachment site for the medial pterygoid muscle.
So now that we've covered the anatomy of the mandible, let's take a quick look at its main function.
As you may have already guessed, the bony mandible is fundamental in the process of mastication. As we have seen, the muscles of mastication attach to the surfaces of the mandible eliciting movements of this bone at the temporomandibular joint. The movements include elevation and depression as well as protraction and retraction of the mandible. These movements allow us to open and close our mouths when we chew our food and talk – hopefully not at the same time.
And that's it for the anatomy of the mandible. Don't go anywhere just yet because we have a clinical note on the mandible just for you.
Now and again, we might have to get a teeth pulled or maybe we might lose a tooth in a fight as we saw earlier. When we lose an adult tooth of the lower jaw, resorption of the alveolar bone may occur. Over time, when a tooth is absent, the alveolar bone begins to resorb. Osteoclasts break down the bone releasing the minerals back into the bloodstream. This is a natural process; however, it may be accelerated by dietary deficiencies such as a vitamin D deficiency or as a result of bone density diseases such as osteopenia, osteoporosis, and rickets.
If a large portion of teeth are missing, gradually over time, inferior structures of the mandible may be found closer to the superior border due to extensive resorption and breakdown. In some cases, bone resorption may reach the level of the mental foramen leading to its complete erosion. Neurovascular structures traveling through this foramen are left exposed and at risk of injury. Treatment includes supplements and drugs which decrease vitamin deficiencies. In more progressive bone resorption, bone grafting or gum grafting may be carried out.
And we're just about done. Before we finish off, let's do a quick summary to remind us of what we learned today.
We began this tutorial with the body of the mandible. Here we explored the alveolar part and its associated interalveolar septa. Moving to the inferior aspect of the body of the mandible, we meet the base of the mandible. The digastric fossa can be found here, near the midline.
Next, we explored the external surface of the body of the mandible and identified in the center, the mandibular symphysis. Just inferior to the mandibular symphysis, we found the triangular-shaped eminence known as the mental protuberance which was bordered on either side by the bony mental tubercles. Superior and lateral to these structures, we met the mental foramina, which allow for the passage of the mental nerve and vessels.
The final structures of the external body that we looked at were the oblique lines which extend from the anterior border of the ramus to the mental tubercle. On the internal surface of the body of the mandible, we meet the mental spines which are found at the posterior aspect of the mandibular symphysis. The internal surface of the mandible is divided by oblique paired mylohyoid lines. These lines form the attachment site for the mylohyoid muscles.
Just superior to the mylohyoid line, we identified the sublingual fossa which is a home for the sublingual gland. Inferior to the mylohyoid line, we found the last bony landmark of the internal surface – the submandibular fossa – which houses another salivary gland, the submandibular gland.
Next we met the paired left and right rami of the mandible. Here we defined its four borders and two surfaces. We began with the posterior border which is usually covered in its entirety by the parotid gland followed by the inferior border which is continuous with the base of the mandible. At this point, we identified the angle of the mandible which is formed by the intersection of the posterior and inferior borders of the ramus. Next we came to the anterior border which is continuous with the oblique line of the body of the mandible.
The final border of the ramus is the superior border which contains three bony landmarks – the coronoid process, the mandibular notch, and the condylar process. The tip of the condylar process is known as the head of the mandible. The head of the mandible articulates with the temporal bone at the temporomandibular joint. Supporting the head of the mandible is the neck of the mandible. The neck of the mandible is the inferior portion of the condylar process. On the anterior aspect of the neck of the mandible is a small indentation known as the pterygoid fovea which provides the distal attachment of the lateral pterygoid muscle.
Next we explore the surfaces of the ramus beginning with the lateral surface. The lateral surface is largely covered in its entirety by the masseter muscle. On the lateral surface of the angle of the mandible, we identified the masseteric tuberosity which is the attachment site of the masseter muscle.
Moving on to the medial surface, we identified the inferior alveolar foramen which forms the internal opening of the mandibular canal. This is where the inferior alveolar nerve and vessels pass through. Just anterior to the inferior alveolar foramen is the lingula of the mandible which acts as an attachment site for the sphenomandibular ligament. Next we met the mylohyoid groove which extends in an anteroinferior direction from the inferior aspect of the inferior alveolar foramen.
The final bony landmark of the ramus is the pterygoid tuberosity, located on the medial aspect of the angle of the mandible.
After dealing with the anatomy of the mandible, we wrapped up with the clinical note. We learnt about resorption of the alveolar bone which may occur as a result of mandibular tooth loss.
That's all we have for today. Thanks for watching and happy studying!