Anatomy of the temporomandibular joint and muscles of mastication

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Introduction0:00–0:14

Anatomy of the temporomandibular joint and muscles of masticationGetting into medical school must have been a jaw-dropping moment, but have you ever wondered how it is that your jaw didn’t fall off?
You can thank the temporomandibular joint for that!Now, the temporomandibular joint is a modified synovial hinge joint. So first, just like a hinge joint, it allows for flexion and extension, which translate to elevation…….and depression of the jaw, respectively.

Temporomandibular joint0:14–2:12

Additionally, the TMJ also allows for gliding and rotation. The articular components of this joint include the mandibular fossa and the articular tubercle of the temporal bone as the superior surface, and the head of the mandible as the inferior surface.
Unlike your average hinge joint, the articular surfaces don’t make direct contact here. They are separated by an articular disc, which is attached to the inner surface of the joint's fibrous capsule.
The articular disc divides the joint into two parts: the superior articular cavity, which allows the mandible to glide causing protrusion…….and retrusion, and the inferior articular cavity, which allows the hinge movements, elevation or closing of the jaw and depression or opening of the jaw, as well as rotational movements.
With all of these movements at the temporomandibular joint, there needs to be strong surrounding support so it doesn’t become dislocated.
This is where ligaments come in handy. The temporomandibular joint is enveloped by the joint capsule and strengthened by one intrinsic and two extrinsic ligaments.
The intrinsic ligament, called the lateral ligament, is a thickening of the joint capsule and extends from the articular tubercle to the neck of the mandible.
It strengthens the joint laterally and helps prevent posterior dislocation. The two extrinsic ligaments are the sphenomandibular ligament and the stylomandibular ligament.
The sphenomandibular ligament extends from the spine of the sphenoid bone to the lingula of the mandible. It supports the joint and serves as a prop during hinge movements.
The stylomandibular ligament extends from the styloid process of the temporal bone to the angle of the mandible. It strengthens the joint, but not as much as other ligaments.Now, the joint can’t move on its own - it needs muscles, specifically the muscles of mastication, which are all innervated by branches of the mandibular division of the trigeminal nerve, also known as the mandibular nerve.

Temporalis2:12–2:49

First up, there’s the temporalis muscle. It originates from the temporal fossa and the deep surface of the temporal fascia.
The temporalis inserts on the coronoid process of the mandible and the anterior border of the ramus of the mandible. It is innervated by deep temporal nerves.
The temporalis elevates the mandible and its posterior fibers, which are more horizontal, retrude the mandible. The next muscle is called the masseter, and it’s a strong muscle that originates on the zygomatic arch and the maxillary process of the zygomatic bone.

Masseter2:49–3:11

It inserts on the angle and the lateral surface of the ramus of the mandible. The masseter mainly elevates the mandible, but superficial fibers contribute to protrusion and it’s innervated by the masseteric nerve.
The third muscle of mastication, called the medial pterygoid muscle, is made of two heads: the deep head and the superficial head.

Medial pterygoid muscle3:11–3:53

The deep head originates from the medial surface of the lateral pterygoid plate and the pyramidal process of the palatine bone.
The superficial head originates from the tuberosity of the maxilla. Both heads insert on the medial surface of the ramus and angle of the mandible.
The medial pterygoid muscle is innervated by the medial pterygoid nerve. Bilaterally, it acts very similarly to masseter, producing elevation and protrusion of the mandible.
When alternate unilateral contraction occurs it also produces small side-to-side grinding movements of the jaw. The last muscle of mastication is the lateral pterygoid muscle, which is also made of two heads.

Lateral pterygoid muscle3:53–4:34

The superior head originates from the infratemporal surface and crest of the greater wing of the sphenoid bone and inserts directly on the joint capsule and the articular disc.
The inferior head originates from the lateral surface of the lateral pterygoid plate and inserts on the pterygoid fovea on the neck of the mandible.
This muscle is innervated by the lateral pterygoid nerve. Bilaterally, the lateral pterygoid muscle is involved in protrusion and depression of the mandible, while unilaterally it helps with lateral or side-to-side chewing movements.Let’s take a quick break before we continue.

Quiz4:34–4:49

Can you identify all muscles of mastication on this image?Now let’s take a comprehensive look at how these muscles work together to produce the various movements that occur at the TMJ.

Movements4:49–6:13

Hinge movements, which include elevation and depression, occur between the head of the mandible and the articular disc in the inferior articular cavity.
Elevation involves the temporalis, the masseter, and the medial pterygoid muscle. The main force acting during depression is gravity, but it also involves the lateral pterygoid muscle.
Now, to open the mouth completely, like when you’re at the dentist, the mandible and the articular disc have to glide forward until the head of the mandible is under the articular tubercle.
So, this movement involves both depression and protrusion. Gliding movements like protrusion and retrusion occur in the superior articular cavity as the head of the mandible and the articular disc move simultaneously against the mandibular fossa and the articular tubercle of the temporal bone.
Protrusion involves the lateral pterygoid, medial pterygoid, and masseter muscles, while retrusion involves temporalis.Lastly, the mandible can also move laterally during chewing and grinding movements.
During lateral movement, one side protrudes, while the other side rotates slightly. This movement is carried out by the temporalis and the masseter on the side the mandible is moving toward, and the lateral and the medial pterygoid muscles on the opposite side.
Lastly, let’s talk about the innervation of the temporomandibular joint. Hilton law says that a joint is innervated by articular branches of nerves that innervate the muscles acting on the joint.

Innervation6:13–6:37

So, naturally, the temporomandibular joint is innervated by articular branches of the mandibular nerve, specifically branches of the auriculotemporal and masseteric nerves.All right, as a quick recap… The temporomandibular joint is a modified synovial hinge joint, between the mandibular fossa and the articular tubercle of the temporal bone superiorly and the head of the mandible inferiorly.

Review6:37–7:46

Articular components are separated by the articular disc, which divides the joint into the superior and inferior articular cavities.
The temporomandibular joint is strengthened by one intrinsic ligament, called the lateral ligament, and two extrinsic ligaments: the stylomandibular ligament and the sphenomandibular ligament.
The muscles of mastication are the temporalis, the masseter, the medial pterygoid, and the lateral pterygoid muscle. They are all innervated by branches of the mandibular nerve: deep temporal nerves, the masseteric nerve, the medial pterygoid nerve, and the lateral pterygoid nerve, respectively.
The temporomandibular joint is innervated by articular branches of the mandibular nerve. The temporomandibular joint allows for hinge movements, like elevation and depression, as well as gliding movements, like protrusion and retrusion, and lateral or side-to-side movements used