Head & Neck Anatomy | Bones of the Skull | INBDE

hey everyone ryan here and welcome back to our head and neck anatomy series of videos this video will be about bones of the skull so the skull is a bony structure that forms a protective cavity for the brain and it supports the face sutures are fibrous joints unique to the skull between the skull bones that allow for brain growth during childhood and adolescence and finally fused together in adulthood once brain growth is complete neural crest forms the anterior part of the skull here in blue whereas paraxial mesoderm that eventually contributes to semitomeres and somites forms

the posterior part of the skull here in red those are terms that we talked about in our last video on embryology the bones of the skull can also be divided into those of the cranium here in yellow and the face here in green so the cranium is the part of the skull that's protecting the brain and the face of the skull is going to be the part that supports the face so this is a functional division the previous slide was an embryologic division so let's talk more about the cranium also called the neurocranium and this

can be further subdivided anatomically into a roof and a base so the roof over here protects the brain from above and the cranial base supports the brain from below so the calvarium is comprised of frontal parietal and occipital bones the cranial base is comprised of a few more those being the frontal ethmoid sphenoid temporal parietal and occipital then we also have the face which is also called the visceral cranium so the skeletal face supports soft tissues of the face and subsequently determines much of our facial appearance and this is comprised of 14 individual bones that

fuse to house the orbits the nasal cavity and the oral cavity as well as all the sinuses and this is comprised of zygomatic maxilla nasal lacrimal palatine bones the ethmoid bone which also includes superior and middle nasal conchae the inferior nasal conchae the vomer and the mandible so a lot of bones going into supporting our face so the fontanelles are these incompletely fused joints that manifest as these wide membranous gaps between the skull bones as an infant and they fuse within 18 months of birth so there are two fontanelles to remember there's the frontal fontanelle

which is out in front it's located at the junction of the coronal suture that crosses over horizontally and the sagittal suture which is right in the center and the occipital fontanelle is in the back and that's located at the junction of the same sagittal suture and the lambdoid suture at the back of the skull and so as the skull matures over time these fontanelles disappear and we have sutures that are left and these are immovable fibrous joints unique to the skull that we mentioned before and so these sutures close completely by around age 20. the

one exception though is the frontal or metabolic suture which is located at the midline of the frontal bone this one closes much earlier even earlier than the fontanelles within three to nine months after birth the coronal suture is the same part we just saw in the last image that's right out in front here across horizontally and it fuses the frontal bone with the two parietal bones the sagittal suture right down the center fuses both parietal bones together the squamous suture is right down here along the side of the skull and that one fuses the parietal

bone with the temporal bone the lambdoid suture across the back fuses the occipital bone with the two parietal bones and then we have two intersection points which are vestigial from the fontanelles the bregma is out in front that's where the frontal fontanelle used to be that's where the coronal and sagittal sutures intersect and then the lambda in the back which matches with lambdoid is where the sagittal and lambdoid sutures intersect and that's where the occipital fontanelle used to be now i love drawing things for anatomy to kind of simplify things as much as possible and

i like to use different colors to help it stick so i'll be showing you a lot of my drawings throughout this video series on anatomy and i encourage you to make your own drawings while you study to help you remember all these names and how they're positioned so this is a very simplified way of reproducing the sutures come exam time if you wanted to have a quick drawing to remember alright so craniosynostosis involves premature closure of the fontanelles and or the sutures and so this is where cranial skeletal growth lags behind brain growth and basically

the brain continues to expand despite the sutures being closed and so there are some specific manifestations of craniosynostosis scaphocephaly is caused by early closure of the sagittal suture and that's pictured here so this sagittal suture down the center closes too early and the brain continues to grow and develop and so this is going to result in a skull that becomes elongated in an ap anteroposterior direction brachycephaly is caused by early closure of the coronal and lambdoid sutures so these two horizontal sutures are are in this brachycephaly are prematurely closed and so that means that the

skull is going to become elongated superior inferiorly it's going to grow vertically too tall and then plagiocephaly is caused by early closure of the coronal and landoid sutures on just one side of the skull and so that's going to result in the skull becoming distorted asymmetrically so just some examples of specific types of craniosynostosis all right how about the sinuses so sinuses develop as diverticula or basically pouches of the lateral nasal wall and they extend into the frontal ethmoid sphenoid and maxilla bones and eventually creating sinuses of those same four names so the primary function

of these sinuses these air spaces essentially is to reduce the weight of the front of the skull the secondary functions are they affect resonance of the voice they can insulate sensitive structures like tooth roots and eyes from rapid temperature fluctuations within the nasal cavity and oral cavity they can regulate intranasal gas pressure and they can provide some immunological defense so these sinus pouches will start out really small and expand and expand over time the ethmoid air cells or ethmoid sinuses are pneumatized or air filled at birth while the frontal sinus and the maxillary sinuses begin

later and reach near maximum size by about age 20. so both the frontal and maxillary sinuses are expanding mostly with vertical growth to the developing midface region and sinuses will honestly continue to remodel in response to forces throughout life occurring in the skull mostly masticatory forces so the mandible is an interesting bone and the structure of the mandible is greatly influenced by its alveolar process the part of the bone that supports the teeth and because the angle of the mandible adapts to the changes in the alveolar process the actual angle will change with age based

on what the dentition is doing so this is really interesting at birth the mandible is without teeth and the alveolar portion this part right here that supports the teeth has not formed yet so this angle is very obtuse at about 150 degrees in children the mandible bears those small deciduous teeth and hence the alveolar portion is still poorly developed but a little bit more so so that angle becomes a little bit less obtuse at 140 degrees in adults the mandible bears the permanent teeth and hence the alveolar portion of the bone is fully developed and

so that angle decreases to about 120 230 degrees on average and then edentulous mandibles are characterized by resorption of that alveolar process and also a change or remodeling and position of the mental foramen down here and this whole bone kind of reverts to a more infant like mandible and that angle can increase back to 140 to 150 ish so again these exact degrees aren't something you have to memorize because it varies from individual to individual but that's the general trend and it's based on how the angle adapts to changes in the alveolar process so pretty

cool so there's some lines that are understood within the skull these imaginary lines and their principle lines of force and principle lines of fracture so the lines of force are the strong areas of the skull here in blue and they develop along the lines of force irresponsible to local mechanical stress primarily masticatory forces so the skull adapts to the forces that it receives and is strong in those regions however in the horizontal direction there's really no masticatory force directed in that direction so principal lines of fracture called the fort fracture lines are these weak areas

in the skull and they're the most common sights of fracture so it's really interesting because these lefort fracture lines are also used as guides by oral surgeons to make cuts during orthognathic surgery mostly the fort one is the most common type of cut that we're using in surgery another notoriously weak area of the skull is called the terion that's p p-t-e-r-i-o-n and we can take a look at this slide right here it gives you a pretty good look at what the terion looks like right here and so the terion is represents a region in the

lateral skull where the frontal parietal sphenoid and temporal bones articulate and it's actually the weakest area of the calvaria and it's coursing just beneath that terion runs the middle meningeal artery which can be lacerated following trauma to that area and that results in an epidural hematoma potentially very dangerous situation all right so let's finish up this video with some really great high yield facts and let's start with what bones make up the orbit and test test examiners love asking about this what bones make up the orbit and i have a great mnemonic for this just

imagine an astronaut who is orbiting in space and really wants to listen to the radio and he says fm please so fm please stands for frontal maxillary palatine lacrimal ethmoid sphenoid and zygomatic so frontal is pictured here in yellow the maxillary and purple that little lateral that little piece of the palatine bone sticking out in teal we have the lacrimal bone in green the ethmoid bone sticking up there in brown that's the lateral plate of the ethmoid then we have the sphenoid and finally the zygomatic out here in blue the nasal bone in dark green

is not part of the orbit is not included in the orbit that's usually a distractor answer choice that's included if they ask what bones are included in the orbit all right so let's unpack the ethmoid bone a little bit more because it's kind of hidden within the skull this is an aerial view of the ethmoid kind of looks like an aerial view of a dragon or something with its wings spread out and some of the noteworthy parts of the ethmoid are the krista gauli that's this part right here it stands for crest of the rooster

that's what this means and basically it's this crest that sticks up and the fox cerebri of the dura mater attaches at that point the unsonic process not pictured here is a sickle shaped extension that directs sinus drainage to the middle meatus we'll talk more about the meatuses in just a little bit the cribriform plate or horizontal plate is where the cranial nerve one passes through that's the olfactory nerve passes through there that makes sense this is up here by the nose and the superior and middle nasal conchae or turbinates are also part of the ethmoid

bone we mentioned that before and there are these curled shelves of bone that direct air flow in the nose the sphenoid bone uh also from an aerial view here and honestly it kind of looks like a dragon as well here we have the face the two eyes and maybe the wings extending out like that and this bone has a ton of holes or foramina that we'll talk about in the next video so the sphenoid bone has the sella turcica this is that saddle-shaped depression that houses the pituitary gland the clinoid processes there's these four little

extensions that are in the center of the sphenoid bone they surround the sella turcica and they also attach to the dura mater then we have the lesser bone up here is part of the anterior cranial fossa the greater wing the large part of the wing of the dragon is part of the middle cranial fossa and it also houses the majority of those foramina the nasal septum is the vertical wall separating the left and right nasal airways it's this thin wall right down the center of the nose and if someone has a deviated septum it means

that this wall is displaced to one side or the other it's composed of three parts the septal cartilage out in front the perpendicular plate of the ethmoid bone and the vomer bone another really great high-yield fact could easily come up on exam day and lastly we have the nasal meatuses these are a distinct air passages of the lateral nasal cavity so on the right and the left located inferior to each nasal conchae of the same name so in other words the superior meatus is just underneath the superior concha the middle medius is right underneath the

middle concha and so on so the superior meatus houses the openings to the sphenoid sinus back here the posterior ethmoid sinus as well as the sphenopalatine foramen the middle meatus houses openings to the anterior and middle ethmoid air cells the maxillary sinus and the frontal sinus all via this opening called the semilunar hiatus it's a really good thing to remember for the board exam how i remember this is that i took a hiatus in the middle of the week i took a hiatus in the middle of the week that's how i remember that where that

is is the middle meatus and then the inferior meatus houses the opening of the nasolacrimal duct down here all right so that's it for this video thank you so much for watching everyone please like this video if you enjoyed it and consider subscribing to this channel for much more on dentistry if you're interested in supporting this channel and what i do here please check out my patreon page thank you to all of my patrons here for all of your loyal support you can unlock extras like access to these video slides if you want to take

notes on them and practice questions that i've designed for the board exams so go check that all out the link is in the description thanks again for watching everyone and i'll see you in the next video