Description

Structures covered in this video:
• Fontanelles (Fonticuli cranii):
– Anterior fontanelle (Fonticulus anterior)
– Posterior fontanelle (Fonticulus posterior)
– Sphenoid (anterolateral) fontanelle (Fonticulus sphenoideus)
– Mastoid (posterolateral) fontanelle (Fonticulus mastoideus)

• Sutures (Suturae cranii):
– Coronal suture (Sutura coronalis)
– Sagittal suture (Sutura sagittalis)
– Lambdoid suture (Sutura lambdoidea)
– Squamous suture (Sutura squamosa)
– Parietomastoid suture (Sutura parietomastoidea)
– Sphenosquamous suture (Sutura sphenosquamosa)
– Frontal suture / Metopic suture (Sutura frontalis)
– Persistent frontal suture (Sutura frontalis persistens)

• Synarthroses (immovable joints):
– Sutures (Suturae cranii)
– Synchondroses (Synchondroses cranii):
• Sphenopetrosal synchondrosis (Synchondrosis sphenopetrosa)
• Petro-occipital synchondrosis (Synchondrosis petrooccipitalis)
• Spheno-occipital synchondrosis (Synchondrosis sphenooccipitalis)
• Sphenoethmoidal synchondrosis (Synchondrosis sphenoethmoidalis)
– Gomphoses (Gomphoses dentales):
• Root of tooth (Radix dentis)
• Alveolar socket (Alveolus dentalis)
• Periodontal ligament (Ligamentum periodontale)

• Temporomandibular Joint (Articulatio temporomandibularis):
– Condylar process of mandible (Processus condylaris mandibulae)
– Mandibular fossa of temporal bone (Fossa mandibularis ossis temporalis)
– Articular disc (Discus articularis)
– Superior synovial cavity (Cavitas synovialis superior)
– Inferior synovial cavity (Cavitas synovialis inferior)
– Articular capsule (Capsula articularis)
– Lateral temporomandibular ligament (Ligamentum temporomandibulare laterale)
– Sphenomandibular ligament (Ligamentum sphenomandibulare)
– Stylomandibular ligament (Ligamentum stylomandibulare)

• Accessory ligaments and fibrous structures:
– Stylohyoid ligament (Ligamentum stylohyoideum)
– Pterygospinous ligament (Ligamentum pterygospinosum)
– Pterygomandibular raphe (Raphe pterygomandibularis)

• Bones referenced for orientation:
– Frontal bone (Os frontale)
– Parietal bone (Os parietale)
– Occipital bone (Os occipitale)
– Temporal bone (Os temporale)
– Sphenoid bone (Os sphenoidale)
– Ethmoid bone (Os ethmoidale)
– Maxilla (Maxilla)
– Mandible (Mandibula)
– Zygomatic bone (Os zygomaticum)

Clinical anatomy and conditions discussed:
• Dehydration and sunken fontanelle
• Hydrocephalus and bulging fontanelle
• Meningitis and increased intracranial pressure
• Delayed closure of anterior fontanelle (congenital hypothyroidism, rickets, Down syndrome)
• Early closure of fontanelles and sutures (craniosynostosis)
• Persistent frontal suture (Sutura frontalis persistens)
• TMJ dysfunctions and ligament injuries
• Nerve compression from ossified pterygospinous ligament (Civinini’s foramen and mandibular nerve compression)

Transcript

0:00
You are born with gaps in your skull. It sounds strange, but there’s a very logical reason for it.
0:06
These gaps allow the skull to stay flexible during birth and give space for the brain to grow during
0:11
early development. We call them fontanelles, and this is what we call the developing skull.
0:17
As the child grows, those gaps slowly close, and the skull begins to form joints. Most
0:23
of these joints become completely stiff called sutures. Others, become a movable
0:28
joint that allows the jaw to move freely. In this video, we’ll first look at the
Content
0:33
developing skull, go through the fontanelles, and highlight the differences between a newborn and
0:37
an adult skull. Then I’ve made it very logical to understand the joints of the adult skull, and that
0:43
is by dividing them into two types. The immovable joints, or synarthroses, including sutures,
0:50
synchondroses, and gomphoses, and the one movable joint, the temporomandibular joint. We will cover
0:56
each of these and put in a few clinical notes along the way to make it interesting.
1:01
What’s up everyone, my name is Taim. I’m a medical doctor, and I make animated medical lectures to
1:05
make different topics in medicine visually easier to understand. If you’d like a PDF version or a
1:10
quiz of this presentation, you can find it on my website, along with organized video lectures
1:14
to help with your studies. Alright, let’s get started.
The Developing Skull
1:16
So let’s start with the developing skull. Let’s start by adding a newborn skull and an adult
1:22
skull. The newborn skull on the left represents about 0-1 months old, and the adult skull around
1:28
25 years old. What happens is, at birth, the bones of the skull are not fully fused. Instead,
1:35
they’re connected by soft connective tissue, forming what we call the fontanelles. And I’ll
1:40
touch upon that in more details in a minute, but these allow for flexibility during birth,
1:45
and also leave space for the brain to grow rapidly in the first months of life.
1:50
Over time, these close off and get replaced by solid bone. The timing varies slightly
1:56
between individuals, but most fontanelles begin to close between the first few months
2:01
and around 18 to 24 months of age. Now, look at the adult skull for a
2:06
moment. You can see that all the fontanelles have closed and been replaced by sutures,
2:10
which are immovable joints. These help stabilize the skull and fully protect the
2:16
brain as growth slows down and eventually stops. So somewhere along this timeline—from infancy
2:22
through adolescence—these gaps gradually close, and the bones of the skull begin to fuse. Some of
2:28
these fusions happen early, while others, like the closure of the spheno-occipital synchondrosis at
2:34
the skull base, don’t happen until late adolescence or even early adulthood.
2:39
Let’s highlight the most important differences between a newborn skull and an adult skull now.
2:44
First thing we can clearly see is the size of the viscerocranium, which is the facial part
2:49
of the skull. In newborns, the viscerocranium is much smaller in proportion to the neurocranium.
2:55
The face-to-cranium ratio in a newborn is about 1 to 8.
2:59
As the child grows, the facial bones develop—especially with the eruption
3:03
of teeth. By the time the skull reaches adult proportions, this ratio shifts to about 1 to 2.5.
3:10
So, the facial bones in newborns are small and make up a much smaller part of the skull. Now look
3:17
at this. What’s the first thing you notice? The mandible is small, and it’s positioned
3:22
in a way that allows forward movement during breastfeeding. The nasal cavity is also small,
3:28
and so are the airways. In fact, neonates have a very narrow inferior meatus, which limits
3:34
airflow through the lower nasal passage. Because of this, they have higher nasal
3:39
resistance, about 3.5 times higher than in adults. This makes them more prone to airway
3:45
obstruction and difficulty clearing nasal congestion. Another thing is the paranasal
3:50
sinuses. Paranasal sinuses are those holes in your skull that resonate the voice,
3:55
help filter the air you breath and so on. These are either underdeveloped or rudimentary at birth.
Newborn Skull VS Adult Skull
4:01
So overall, the facial bones in newborns are underdeveloped. That’s the first major
4:06
difference between the newborn and adult skull. First out of 3 main differences.
4:11
The second difference, proportion of the skull to the entire body. In a newborn, the head makes
4:17
up almost one-fourth of the total body length. That’s why babies appear to have such large heads.
4:23
But in adults, the head is only about one-seventh of body length, since the rest of the body
4:28
continues to grow more significantly after birth. So both the facial skeleton and the body grow to
4:35
catch up with the size of the cranium. This is the second major difference
4:39
between newborn and adult skulls. Now, the third and final one – open
4:43
sutures and fontanelles. What does that actually mean?
4:48
In the newborn skull, the bones are connected by wide sutures and fontanelles,
4:52
which you can see here. These are flexible, fibrous areas that allow movement and growth.
4:58
In the adult skull, these fontanelles eventually ossify, they turn into bone and close off. At
5:04
the same time, the sutures gradually fuse and lock the skull into a rigid structure.
Fontanelles
5:10
So… what are fontanelles? Well, when a baby is born,
5:14
their skull is not one solid piece of bone like in adults. It’s actually made up of several bones
5:20
that are separated by soft, flexible areas, and those soft spots are what we call fontanelles.
5:26
They’re basically little gaps between the bones of the skull, and instead of being filled with bone,
5:31
they’re covered by a tough membrane made of connective tissue. You can feel them if you
5:35
gently touch the top of a baby’s head. Fontanelles aren’t dangerous by themselves. but if something
5:41
goes wrong, they can tell you a lot. For example a sunken fontanelle is usually a red flag for
5:47
dehydration. A bulging fontanelle, especially if the baby isn’t crying, might suggest increased
5:53
intracranial pressure, which could be due to things like hydrocephalus, meningitis,
5:57
and hemorrhage. A delayed closure of the anterior fontanelle might point toward conditions like
6:02
congenital hypothyroidism, rickets, Down syndrome. On the flip side, if a fontanelle
6:08
closes too early, especially if it involves the sutures as well, we think about craniosynostosis,
6:14
which can restrict brain growth and might require surgical intervention. So, fontanelles are not
6:19
just developmental features, they’re important clinical windows into a baby’s health. They’re
6:24
flexible at birth, and they give the brain space to grow rapidly in the first years of life.
6:30
So, fontanelles are soft, membranous gaps where several bones meet. Sutures, however,
6:36
are the fibrous joints that connect two bones. They are still flexible in infants,
6:41
but they’re more narrow and more structured, and over time, they fuse and form solid bone
6:47
connections called synostoses. Fontanelles eventually close and become part of these sutures,
6:53
so they’re related, but not the same thing. For orientation we can see frontal bone, maxilla,
6:59
mandible, zygomatic bone, temporal, occipital, and parietal bone. There are four main fontanelles.
7:07
There’s the anterior fontanelle. It’s the one most people think of first, and is a diamond-shaped,
7:13
largest fontanelle that usually closes somewhere between 12 to 24 months of age. Then there’s the
7:19
posterior fontanelle, which is smaller and triangular, and closes much earlier,
7:23
typically by 6 to 8 weeks after birth. Next is the sphenoid fontanelle,
7:28
also called the anterolateral fontanelle, it’s found at the junction of the frontal,
7:33
parietal, temporal, and sphenoid bones, and usually closes around 2 to 3 months of age.
7:40
Then there is the mastoid fontanelle, or posterolateral fontanelle, siting between
7:45
the parietal, temporal, and occipital bones, and tends to close between 6 and 18 months.
7:51
So those are the four main fontanelles, and each of them follows a pretty predictable closure
7:56
timeline, which is useful both in development and in clinical exams. Apart from that,
Sutures
8:02
we can see the coronal suture, sphenosquamous suture, squamous suture, parietomastoid suture,
8:09
and the lambdoid suture. Let’s now turn the skull in this direction, and look at a superior view,
8:15
you’ll be able to see the top sagittal suture. And in the front, is the frontal suture. also known as
8:20
the metopic suture. It runs between the two halves of the frontal bone, as you can see here. In most
8:26
people, it fuses by the age of 2 years, but about 5 to 10% of the population have a persistent
8:34
frontal suture into adulthood. That’s called a sutura frontalis persistens, and it’s usually
8:40
just a normal variant, although in rare cases it can be mistaken for a skull fracture on imaging.
8:47
Now, let’s replace the skull with an adult skull. By this point, the sutures have completely
8:53
ossified, forming rigid joints between the bones, and the fontanelles have closed, they’ve been
8:58
replaced by these very same sutures and bones. From this view, we can see the coronal suture,
9:04
which separates the frontal bone from the parietal bones. If we turn the skull to the lateral view,
9:10
the coronal suture is even clearer, and just behind it, you’ll spot the squamous suture,
9:15
which connects the parietal bone to the temporal bone. And further back, there’s the lambdoid
9:20
suture, which runs between the parietal bones and the occipital bone, it almost looks like
9:24
an upside-down “V” from the posterior view. And superiorly, we can see the sagittal suture,
9:31
running right down the midline, connecting the two parietal bones. Notice how all these sutures have
9:37
that interlocking, saw-toothed appearance—that’s what gives them strength and stability.
Classification of the Skull Joints
9:43
Now, those are the sutures. They’re immovable joints in the adult skull,
9:48
what we call synarthroses. But don’t forget, these sutures were once flexible gaps filled with
9:54
fibrous connective tissue when we were newborns. So that brings us to a good question – what other
Immovable Joints (Synarthroses)
10:00
joints can we actually find in the adult skull? Besides sutures, we’ve got two more types of
10:05
synarthroses in the skull: synchondroses and gomphoses. And we also have one movable joint,
Movable Joints (Diarthroses)
10:12
the temporomandibular joint, or TMJ, which is a synovial joint. It’s the only freely movable
10:18
joint in the skull. Then, we’ve got ligaments, these aren’t joints themselves, but they’re
10:24
important supporting structures that reinforce and stabilize the joints around the skull.
10:29
So let’s start with synchondroses. Synchondroses are joints where bones
Synchondrosis
10:34
are joined by hyaline cartilage. Look here – One bone, here’s another bone. Between them,
10:40
a hyaline cartilage that is connecting them – this is a synchondrosis. They’re firm,
10:45
immovable, and typically found at the base of the skull, especially during development.
10:50
So, here we see the front side of the skull, let’s remove the skull cap, and look at the
10:55
skull from this perspective. We will see this. Can you tell the bones apart? The bone at the
11:01
back is called occipital bone, lateral to it are the temporal bones. We got the parietal bones,
11:07
Sphenoid bone, at the front, the frontal bone. And ethmoid chilling in the middle here.
11:13
————- Now, of the synchondroses we can
11:15
identify in the skull, one is the sphenopetrosal synchondrosis—this connects the sphenoid bone with
11:21
the petrous part of the temporal bone. Then there’s the petro-occipital synchondrosis,
11:26
which links the petrous temporal with the occipital bone. Next, we have the spheno-occipital
11:32
synchondrosis, which is a major one—it connects the body of the sphenoid with the basilar part of
11:38
the occipital bone. This one usually ossifies between the ages of 18 and 25. Then there is
11:44
sphenoethmoidal synchondrosis that lies between the sphenoid and ethmoid bones and typically fuses
11:51
a bit earlier, around the age of 10. Alright, next up is the gomphosis.
Gomphoses (Tooth Joints)
11:56
Gomphosis is a pretty unique joint because it’s only found in one place—the connection between
12:03
your teeth and your jawbones. So here, you see the tooth inserted into the socket of the
12:08
alveolar process, that’s a gomphosis joint. It consists of three parts: the root of the tooth,
12:14
the alveolar socket in either the mandible or maxilla, and the periodontal ligament which
12:19
acts as the connective tissue anchoring the two together. Even though this is a fibrous joint and
12:25
mostly immobile, it allows a tiny bit of movement to act like a shock absorber during chewing.
12:31
So those were the immovable joints of the skull—synarthroses— the sutures,
12:36
synchondroses, and gomphoses. Now let’s move on to the one movable joint in
Temporomandibular Joint (TMJ)
12:41
the skull—the temporomandibular joint, or TMJ. Let’s just rewind to the last video where we
12:47
covered the different types of synovial joints. Remember we said that synovial joints have a
12:52
joint cavity filled with synovial fluid, articular cartilage covering the surfaces,
12:57
have an inner synovial membrane and are surrounded by a capsule? We also said
13:01
that some synovial joints include accessory structures like ligaments, discs, and bursae.
13:06
Well, the TMJ is also a synovial joint that contains these structures. But it’s
13:13
a little bit more special than that. The temporomandibular joints are bicondylar,
13:18
hinge-type joint with gliding movements. What does that mean? Let’s break it down.
13:24
So, the temporomandibular joint is a connection between the condylar process of the mandible,
13:28
with the mandibular fossae of the temporal bone. The term “Bicondylar” means that it’s made up
13:35
of two condylar processes, meaning that the left and right temporomandibular joints work
13:41
together. They move simultaneously, they don’t function independently. That coordination is
13:48
what makes you able to move your jaw easily. Now, the TMJ is special. It has some special
13:55
features. It has an articular disc that divides the synovial cavity into two synovial
14:01
compartments, making it a complex joint. This is a really nice diagram from Physio-pedia. It
14:07
shows the content of the temporomandibular joint beautifully, you can see the articular disc that
14:13
divides the joint into a superior synovial cavity, and an inferior synovial cavity.
14:18
Each compartment allows different types of movement. The lower compartment allows
14:23
for hinge-like rotation, like opening and closing your mouth. The upper compartment
14:28
allows for sliding movements, like when you slide your jaw forward or from side to side.
14:33
Apart from this, there are several ligaments that support the movements of this joint,
14:38
and therefore they’re considered a part of the temporomandibular. So here we have the articular
14:43
capsule again. The main supporting ligament is the lateral temporomandibular ligament,
14:49
which reinforces the joint capsule and limits posterior displacement of the mandible. We
14:55
also have the sphenomandibular ligament, which connects the spine of the sphenoid bone to the
14:59
mandible and acts like a passive support, and the stylomandibular ligament, which
15:05
runs from the styloid process to the angle of the mandible and helps prevent excessive protrusion.
15:11
So that’s the TMJ, a unique, dynamic joint that’s responsible for stability, movement,
15:17
and coordination for the jaw. It’s the only synovial joint in the skull,
15:21
and it’s one you definitely want to know well in clinical practice.
Accessory Ligaments
15:26
Now let’s look at some of the accessory structures in the skull. These are ligaments that support the
15:31
joints and surrounding bones. They’re not joints themselves, but they help stabilize
15:36
and guide the movement of different parts of the head and neck. They’re highlighted here in blue.
15:41
The first one is the stylohyoid ligament. It stretches from the styloid process of the temporal
15:47
bone down to the lesser horn of the hyoid bone. This ligament helps suspend the hyoid in place
15:53
and plays a role in swallowing by anchoring muscles like the stylohyoid and digastric.
15:58
Then there’s the pterygospinous ligament. It runs between the spine of the sphenoid bone and the
16:04
lateral pterygoid plate. Now, this ligament isn’t always visible or even present in everyone. But in
16:10
some people, it can actually turn into bone. And when that happens, it can form a small bony bridge
16:16
that creates what’s called the pterygospinous foramen, or Civinini’s foramen. This region is
16:21
right next to branches of the mandibular nerve, and if the ligament becomes bone,
16:26
it can sometimes press on those nerves, and lead to symptoms like facial pain, numbness, or even
16:31
issues with taste, especially if the lingual nerve or other nearby branches are affected.
16:37
Next, we have the sphenomandibular ligament, which you already saw earlier when we covered the TMJ.
16:43
This ligament connects the spine of the sphenoid bone to the lingula of the mandible. It doesn’t
16:48
limit much motion, but it provides passive support to the mandible, especially during movements like
16:54
opening the jaw. It’s actually a remnant of Meckel’s cartilage from embryonic development.
17:00
Lastly, there’s the pterygomandibular raphe, which is a fibrous seam rather
17:04
than a traditional ligament. It goes from the pterygoid hamulus to the posterior part of
17:10
the mandible. It serves as an anchor point for two important muscles, the buccinator and the
17:16
superior pharyngeal constrictor, which makes it important for both chewing and swallowing.
17:21
So those are the main accessory ligaments associated with the skull. Pretty fascinating
17:26
how these tiny structures help stabilize the joints up here. But here’s the thing,
Outro and Next Steps
17:30
the skull doesn’t move on its own. Its ability to nod, rotate, and tilt comes
17:35
from how it’s connected to the bones of the spine, especially the cervical vertebrae. That’s where
17:40
things start to get even more interesting. If you want to understand how the head moves,
17:45
how posture is maintained, and how the bones of the spine generally look,
17:48
click on the next video, and I’ll see you there. If you want a handmade PDF version of this
17:52
lecture, take a quiz to test your knowledge, or access an organized list of all my videos,
17:57
you can find everything on my website. Thanks for watching! See you in the next one.