Osteology of the Skull and Cranial Fossa PDF

The Gross Anatomy of the Head: The skull 1  THE HEAD Osteology of the skull Introduction: superior part of the body that is attached to the trunk by the neck Composition: the brain and its protective coverings, the ears...

The Gross Anatomy of the Head: The skull 1  THE HEAD Osteology of the skull Introduction: superior part of the body that is attached to the trunk by the neck Composition: the brain and its protective coverings, the ears, and the face The Cranium Introduction: The cranium (skull) is the skeleton of the head The many bones of the head collectively form the skull Most of these bones are interconnected by sutures which are immovable fibrous joints Parts: has two parts:  the neurocranium /cranial vault  the viscerocranium/ facial skeleton 2 3 4  Neurocranium (Cranial vault) Introduction: is the bony covering (case) of the brain and its membranous coverings, the cranial meninges It also contains proximal parts of the cranial nerves and the vasculature of the brain Parts: The neurocranium has; a dome-like roof called the calvaria (skullcap) and a floor or cranial base (basicranium) 5 Basicranium 6 Composition: The neurocranium in adults is formed by a series of eight bones:  Four are singular bones  and the other two sets of bones occur as bilateral pairs  The four singular bones are:  frontal  ethmoidal  sphenoidal  occipital 7  While two sets of bones occurring as bilateral pairs are:  Temporal  parietal note  The ethmoid bone is an irregular bone that makes a relatively minor midline contribution to the neurocranium but is primarily part of the viscerocranium Viscerocranium (facial skeleton) comprises the facial bones forms the anterior part of the cranium consists of the bones surrounding the mouth (upper and lower jaws), nose/nasal cavity, and most of the orbits (eye sockets or orbital cavities) 8 Composition: consists of 15 irregular bones: Three are singular bones While 6 bones occur as bilateral pairs  The three singuar bones are:  Mandible  Ethmoid  Vomer  The 6 bilateral paired bones are:  Maxillae  inferior nasal conchae  zygomatic  palatine  nasal  lacrimal bones 9  Note: The maxillae and mandible house the teeths They provide the sockets and supporting bone for the maxillary and mandibular teeth. The Maxillae:  contribute the greatest part of the upper facial skeleton,  form the skeleton of the upper jaw  It is fixed to the cranial base The Mandible :  forms the skeleton of the lower jaw,  It is movable because it articulates with the cranial base at the temporomandibular joints (TMJs) 10  Anterior or facial(frontal) aspect of the Cranium Features include:  the frontal bone  zygomatic bones  orbits  nasal region  maxillae  mandible 11 12 13 14  The frontal bone it is a squamous (flat) part forms the skeleton of the forehead articulates inferiorly with the nasal and zygomatic bones. The supraorbital margin of the frontal bone has a supraorbital foramen (notch) for the passage of the supraorbital vessels and nerves Just superior to the supraorbital margin is a ridge called the superciliary arch Between these arches is a smooth, slightly depressed area called the GLABELLA The Glabella is the anterior most projecting part of the forehead Just above the glabella is a point on the midline of the forehead called the Ophyron On the midline of the mandible is the the most anterior point on the mandible which is the most prominent part of the chin. This point is called the pogonion Gonion. a landmark located at the most inferior, posterior, and lateral point on the external angle of the mandible, being the apex of the maximum curvature of the mandible 15 16  The intersection of the frontal and nasal bones is called the NASION, a depressed area (bridge of the nose) The frontal bone also articulates with the lacrimal, ethmoid, and sphenoids bones The nasal region made up of a pair of nasal bones which are joined together at midline by the nasal septum Inferior to these nasal bones is a pear shaped piriform aperture the bony nasal septum can be observed through this aperture dividing the nasal cavity into the right and left parts On the lateral wall of each nasal cavity are curved bony plates called the nasal conchae 17 18 19 Maxillae form the skeleton of the upper jaw Occur in pairs The two maxillae are united at the intermaxillary suture in the median plane their alveolar processes include the tooth sockets (alveoli) and house the maxillary teeth They articulate with the frontal bone anteriorly and zygomatic bones laterally On the body of the maxilla is the infraorbital foramen for passage of the infraorbital nerve and vessels 20 Mandible is a U-shaped bone forms the skeleton of the lower jaw Its alveolar process supports the mandibular teeth.  consists of:  of a horizontal part (the body)  and a vertical part (the ramus ) Inferior to the second premolar teeth are the mental foramina for the passage of the mental nerves and vessels The base of the mandible has a midline swelling called the mental protuberance Lateral to the mental protuberance on either side is a slightly more pronounced bumps called the mental tubercles 21 22 23 Zygomatic Bone Also known as cheek bone, malar bones On the lateral aspect of this bone is a foramen called the zygomaticofacial foramen this foramen allows for the passage of the zygomaticofacial nerve This bone articulates with the frontal, sphenoidal, maxillae and temporal bones 24  Clinical anatomy  Injury to the superciliary arches The superciliary arches are relatively sharp bony ridges; consequently, a blow to them (e.g., during boxing) may lacerate the skin and cause bleeding. Bruising of the skin surrounding the orbit causes tissue f lu id and blood to accumulate in the surrounding connective tissue, which gravitates into the superior (upper) eyelid and around the eye  Malar Flush The zygomatic bone was once called the malar bone. Clinically the term malar f lush refers to redness of the skin covering the zygomatic prominence (malar eminence), which is associated with a rise in temperature in various fevers occurring with certain diseases, such as tuberculosis 25  Superior aspect of the skull Also called the calvaria or skull cap Bones forming it include  Frontal bone (unpaired)  Paired parietal bones  Occipital bone (unpaired) The outer surface of the skull possess a number of landmarks  The frontal bone articulates with the paired parietal bones at the coronal suture  The two parietal bones articulate with each other at the sagittal suture  The paired parietal bones articulate with the occipital bones at the lambdoid suture 26 27 28  The junction(intersection) between the coronal and sagittal sutures is called the bregma  The junction (intersection) between the sagittal suture and lambdoid sutures is called the lambda  The vertex is the most superior point of the skull near the midpoint of the sagittal suture  The parietal foramen on the parietal bone is variable, and it allows for the passage of emissary veins  Obelion: The region of the skull between the two parietal foramina where the closure of the sagittal suture usually begins Clinical Anatomy Fracture of the Calvaria  Depressed fractures:  this can results from hard blows in thin areas of the calvaria in which a bone fragment is depressed inward, compressing and/or injuring the brain 29 30  Linear calvarial fractures :  the most frequent type  usually occur at the point of impact; but fracture lines often radiate away from it in two or more directions  Comminuted fractures:  the bone is broken into several pieces  If the area of the calvaria is thick at the site of impact, the bone may bend inward without fracturing; however, a fracture may occur some distance from the site of direct trauma where the calvaria is thinner  Contrecoup (counterblow) fracture:  no fracture occurs at the point of impact, but one occurs on the opposite side of the cranium Obliteration of the Cranial Sutures The obliteration of sutures between the bones of the calvaria usually begins between the ages of 30 and 40 years on the internal surface and approximately 10 years later on the external surface Obliteration of sutures usually begins at the bregma and continues sequentially in the sagittal, coronal, and lambdoid sutures 31 32  Skull of a newborn baby The halves of the frontal bone in the newborn are separated by the frontal suture the frontal and parietal bones are separated by the coronal suture and the maxillae are separated by the intermaxillary suture mandible consists of two halves united in the median plane by the mandibular symphysis, this union begins during the 1st year and the halves are fused by the end of the 2nd year No mastoid process at birth No styloid process The mastoid processes form gradually during the 1st year The bones of the calvaria of a neonate are separated by membranous intervals called frontanelles 33 34  They include:  anterior fontanelle  posterior fontanelle  paired sphenoidal fontanelle  Paired mastoid fontanelles Anterior fontanelle intro: the largest frontanelle shape: diamond or star shaped boundary: bounded by the halves of the frontal bone anteriorly and the parietal bones posteriorly. Location: at the junction of the sagittal, coronal, and frontal sutures, the future site of the bregma closure: By the 18 months of age, the surrounding bones have fused and the anterior fontanelle is no longer clinically palpable Frontal bone of a newborn Union of the halves of the frontal bone begins in the 2nd year In most cases, the frontal suture is obliterated by the 8th year in approximately 8% of people, a remnant of the frontal suture persist called the metopic suture 35 36  Posterior fontanelle Shape: is triangular boundary: bounded by the parietal bones anteriorly and the occipital bone posteriorly location: at the junction of the lambdoid and sagittal sutures, the future site of lambda Closure: begins to close during the f irst few months after birth and by the end of the 1st year, it is small and no longer clinically palpable Sphenoidal and mastoid fontanelles overlain by the temporal muscle, fuse during infancy and are less important clinically than the midline fontanelles  note The halves of the mandible fuse early in the 2nd year The two maxillae and nasal bones usually do not fuse 37  Molding of the calvaria The softness of the cranial bones in infants and their loose connections at the sutures and fontanelles enable the shape of the calvaria to change (mold) during birth During passage of the fetus through the birth canal, the halves of the frontal bone become f lat, the occipital bone is drawn out, and one parietal bone slightly overrides the other Within a few days after birth, the shape of the calvaria returns to normal The resilience of the cranial bones of infants allows them to resist forces that would produce fractures in adults The fibrous sutures of the calvaria also permit the cranium to enlarge during infancy and childhood The increase in the size of the calvaria is greatest during the first 2 years, the period of most rapid brain development The calvaria normally increases in capacity for 15-16 years After this, the calvaria usually increases slightly in size for 3 -4 years as a result of bone thickening 38 39  Craniosynostosis and Cranial Malformations P r e m a t u r e c l o s u r e o f t h e c ra n i a l s u t u r e s i s c a l l e d craniosynostosis results in several cranial malformations Premature closure of the sagittal suture, in which the anterior fontanelle is small or absent, results in a long, narrow, wedge- shaped cranium, a condition called scaphocephaly 40  When premature closure of the coronal or the lambdoid suture occurs on one side only, the cranium is a condition known as plagiocephaly Premature closure of the coronal suture results in a high, tower-like cranium, called oxycephaly or turricephaly oxycephaly or turricephaly is more common in females  Note: Premature closure of sutures usually does not affect brain development 41  Lateral aspect of the skull  This includes :  lateral part of the neurocranium  Lateral part of the viscerocranium (facial skeleton) Composition: bones forming the lateral portion of the neurocranium include: the frontal, parietal, occipital, sphenoid, and temporal bones bones forming the visible part of the facial skeleton include the nasal, maxilla, and zygomatic bones and the mandible The main features of the neurocranial part are the temporal fossa, the external acoustic opening, and the mastoid process of the temporal bone T h e m a i n fe a t u re s o f t h e v i s c e ro c ra n i a l p a r t a re t h e infratemporal fossa, zygomatic arch, and lateral aspects of the maxilla and mandible  The junction where the frontal, parietal, sphenoid(greater wing) and temporal bone meet is called the pterion It is usually indicated by an H-shaped formation of sutures 42 43 44 45 Clinical anatomy The pterion is an important area because it overlies the middle meningeal artery Fracture to the pterion can rupture the anterior branch of this artery resulting in hematoma which exerts pressure on the underlying cerebral cortex An untreated meningeal artery can cause hemorrhage which can lead to death in a few hours 46  In the anatomical position, the cranium is oriented in such a way that the inferior margin of the orbit and the superior margin of the external acoustic meatus opening of both sides lie in the same horizontal plane This standard craniometric reference/ plane is called the orbitomeatal plane (Frankfort horizontal plane) The temporal bone of the lateral aspect of the skull It has:  a squamous part which articulates with the greater wing of sphenoid at the sphenosquamous suture  a zygomatic process which articulates with the zygomatic bone to form the zygomatic arch  a tympanic part which has the external acoustic meatus  a petromastoid part which is usually separated into the petrous and mastoid part 47 48  A large bony prominence projects from the inferior border of the mastoid part of the temporal bone called the mastoid process Medial to the mastoid process is the styloid process which projects from the lower border of the temporal bone The point where the superior temporal line cuts the coronal suture is called the stephanion While the located at junction of three sutures: parietomastoid, occipitomastoid, and lambdoid is called the asterion 49 Posterior aspect of the skull 50 51 Bones include: Occipital bone Parts of the parietal bones Temporal bones (mastoid part) Occipital bone  The occipital bone is separated from the paired parietal bone by the lambdoid suture  Along the lambdoid suture are small bones called sutural or wormian bones  Wormian bones or sutural bones are additional small bones sometimes found between the cranial sutures, most commonly in relation to the lambdoid suture. They are abnormal ossicles that develop from extra ossification centers within the cranium  there is a midline projection called the external occipital protuberance  Curved lines extend laterally from this projection called the superior nuchal lines  The most prominent part of the external occipital protuberance is the inion 52  About 1 inch (2.5cm) below the nuchal lines are additional 2 lines called the inferior nuchal lines which curves laterally  Extending downwards from the external occipital protuberance is the external occipital crest  In between the mastoid and the occipital in is the occipitomastoid suture Note  Obliteration of Cranial Sutures The obliteration of sutures between the bones of the calvaria usually begins between the ages of 30 and 40 years on the internal surface and approximately 10 years later on the external surface Obliteration of sutures usually begins at bregma and continues sequentially in the sagittal, coronal, and lambdoid sutures  The region of the skull between the 2 parietal foramina where the closure of the sagittal suture begins is called the OBELION 53 Craniometric Points of the cranium 54 55 The Internal surface of the cranial base (cranial cavity/fossae) has three large depressions that lie at 3 different levels:  anterior crania fossa  middle cranial fossa  posterior cranial fossa These form the bowl-shaped floor of the cranial cavity The anterior cranial fossa is at the highest level(shallowest) w h i l e t h e p o s t e r i o r c ra n i a l f o s s a i s a t t h e l o w e s t level(deepest)  Sitting on the f loor of the anterior cranial fossa is the frontal lobes of the cerebral hemisheres  Sitting on the f loor of the lateral portion of the middle cranial fossa is the temporal lobe, and in the midline of the middle crania fossa is the pituitary gland 56 57 58 59 60 61 62  Sitting on the anterior portion of the f lo or of the posterior cranial fossa is the brainstem (midbrain, pons and medulla oblongata) and occupying the rest of the posterior cranial fossa are the cerebellar hemispheres  Note: the occipital lobe do not sit on the f loor of the posterior cranial fossa, it is the cerebellum that sit of the f lo or of the posterior cranial fossa Anterior cranial fossa Introduction: Shallowest of the 3 cranial fossae Lodges the frontal lobe of the brain Bony formation: Formed by 3 bones, namely:  Frontal bone in the anterior and lateral directions  Ethmoid bone in the middle portion  2 parts of the sphenoid bone in the posterior portion 63  The part of the frontal bone in the anterior cranial fossa is the orbital part/orbital surface The ethmoid bone of the anterior cranial fossa is made up o f:  A ridge which projects superiorly called the crista galli  And a sieve- like cribriform plate of ethmoid which lies on each sides of this ridge The 2 parts of the sphenoid bone in the anterior cranial fossa are:  The jugum  The lesser wing of sphenoid Foramen  The cribriform plate of ethmoid has numerous tiny foramina that transmit olfactory nerves (CN 1) from the olfactory area of the nasal cavity to the olfactory bulb of the brain 64 65  The frontal bone also has a bony median extension called the frontal crest, at the base of the frontal crest is the foramen cecum This foramen varies in size in different individuals, and is frequently impervious; when open, it transmits the emissary vein from the nose to the superior sagittal sinus This has clinical importance in that infections of the nose and nearby areas can be transmitted to the meninges and brain Each lesser wing widens, curves posteriorly, and ends as a rounded anterior clinoid process Just anterior to each anterior clinoid process is a circular opening in the lesser wing of the sphenoid called the optic canal The optic canal allows for the passage of  ophthalmic artery and  optic nerve [CN II] to pass through as they exit the cranial cavity to enter the orbit 66  Middle crania fossa It is butterfly shaped  separating the anterior and middle cranial fossae are the sharp sphenoidal crests laterally and the sphenoid limbus centrally  The sphenoid limbus forms the posterior boundary of the jugum and the anterior boundary of the prechiasmatic sulcus  (note:the prechiasmatic sulcus is bounded posteriorly by the tuberculum sellae)  The prechiasmatic sulcus is a smooth groove stretching between the optic canals across the body of the sphenoid Bony formation Formed mainly by the: i. Parts of the sphenoid bone ii. Parts of the temporal bone  The parts of sphenoid involved are: i. Sella turcica which is part of the body of sphenoid ii. greater wings of sphenoid laterally 67 68 69 Sella turcica Has 3 parts: i. an anterior wall called the turberculum sella ii. a posterior wall called the dorsum sella iii. a deep central part lying in between the turberculum sella and dorsum sella  This central part is called the hypophyseal fossa, which houses the pituitary gland On each side of the body of the sphenoid are four foramina perforating the roots of the greater wings of the sphenoid, these include : i. Superior orbital f issure: a diagonal gap that separates the greater wing from the lesser wing of sphenoid 70  This fissure allows for the passage of the:  Cranial nerves III, IV and VI (occulomotor, trochlear and abducens nerves)  Ophthalmic division of trigeminal nerve (CN V1)  ophthalmic vein ii foramen rotundum: allows for the passage of the maxillary division of the trigeminal nerve (CN V2) iii foramen ovale: allows for the passage of the mandibular division of the trigeminal nerve (CN V3) iv foramen spinosum: allows for the passage of the middle meningeal artery and the meningeal branch of mandibular nerve NOTE: Memory device: LR 6 SO 4 AO 3 ; lateral rectus, CN VI; superior oblique, CN IV; all others, CN III 71  Parts of the temporal bone include:  squamous part of the temporal bones laterally  and the petrous part of the temporal bones posteriorly On the petrous part are grooves for both the greater and lesser petrosal nerves Posterolateral to the hypophyseal fossa is the foramen lacerum (lacerated or torn foramen) is not part of the crescent of foramina and is an artifact of a dried cranium In life, it is closed by a cartilage plate Only some meningeal arterial branches and small veins are transmitted vertically through the cartilage completely traversing this foramen The lateral parts of the middle cranial fossa support the temporal lobes of the brain The boundary between the middle and the posterior cranial fossae is the:  superior border of the petrous part of the temporal bone laterally 72  Posterior cranial fossa Largest and deepest of the 3 cranial fossa Lodges the brainstem (medulla oblongata, pons and midbrain) and the cerebellum Formed mainly by the occipital bone and the temporal bone There is a slope that extend down from the dorsum sellae leading to the foramen magnum called the clivus posterior to the foramen magnum are cerebellar fossae which accommodate the cerebellar hemispheres The cerebellar fossae are partly divided by the internal occipital crest into bilateral concave impression The internal occipital crest ends superiorly in a bony prominence called the internal occipital protuberance At the base of the petrous ridge of the temporal bone is the jugular foramen Anterosuperior to the jugular foramen is the internal acoustic meatus 73 Grooves and foramen in the posterior cranial fossa internal acoustic meatus: passage of facial nerve (CN VII) and vestibulochear nerve (CN VIII) and the labyrinthine artery Jugular foramen: allows the passage of:  3 cranial nerves pass through it, which are cranial nerves IX, X and XI {glossopharyngeal,vagus and accessory nerve}  2 important venous structures, the dura venous sinuses, coming in from the posterior side is the sigmoid sinus and from the anterior side is the inferior petrosal sinus  Note: these two sinuses enter the jugular foramen and when they emerge from the other side, they fuse together and once fused together, they are called the internal jugular vein hypoglossal canal: located superior to the anterolateral margin of the foramen magnum and allows for the passage of the hypoglossal nerve {CN XII} 74  Foramen magnum: allows for the passage of;  The spinal cord which is continuous with the medullar obongata  The two vertebral arteries  The spinal contribution of accessory nerve, the spinal accessory nerve 75 76  External aspect of cranial base The base of the skull is seen in the inferior view and e xte nd s anteriorly fro m the middle incisor teeth, posteriorly to the superior nuchal lines and laterally to the mastoid processes and zygomatic arches  For descriptive purposes the base of the skull is often divided into: 1. an anterior part, which includes the teeth and the hard palate; 2. a middle part, which extends from behind the hard palate to the anterior margin of the foramen magnum; 3. a posterior part, which extends from the anterior edge of the foramen magnum to the superior nuchal lines 77 78 79 80  Anterior part The main features are:  The teeth project from the alveolar arches of the two maxillae These arches are arranged in a U-shaped conf iguration that borders the hard palate on three sides  The hard palate is composed of the palatine processes of each maxilla anteriorly and the horizontal plates of each palatine bone posteriorly The paired palatine processes of each maxilla meet in the midline at the intermaxillary suture and the paired horizontal plates of each palatine bone meet in the midline at the interpalatine suture the paired maxilla and the paired palatine bones meet at the palatomaxillary suture 81 Several additional features are also visible when the hard palate is examined: the incisive fossa for the passage of nasoplatine and sphenopalatine vessels the greater palatine foramina for the passage of greater palatine vessels and nerve just posterior to the greater palatine foramina is the lesser palatine foramina for the passage of lesser palatine vessels and nerve a midline pointed projection in the free posterior border of the hard palate called the posterior nasal spine 82  Middle part The middle part of the base of the skull is complex:  forming the anterior half are the vomer and sphenoid bones;  forming the posterior half are the occipital and paired temporal bones. Anterior half Vomer Anteriorly, the small vomer is in the midline, resting on the sphenoid bone It contributes to the formation of the bony nasal septum separating the two choanae Sphenoid Only 3 parts of the spheniod is seen in the inferior view They are: body Greater wings 2 pterygoid processes on each side 83  The pterygoid process consist of a medial plate and a lateral plate separated by the pterygoid fossa The body: It ar ticulates anteriorly with the vomer, ethmoid, and posterolaterally with the temporal bones, and posteriorly with the occipital bone The greater wing: Articulates laterally and posteriorly with the temporal bone Important features on the greater wing are:  Foramen ovale: passage of mandibular nerve (V3)  Foramen spinosum: passage of middle meningeal artery 85 Posterior half Made up of Part of Occipital bone Parts of paired temporal bones  Occipital bone This include the basilar part of the occipital bone, which is posterior to the body of sphenoid posterior to this part is the foramen magnum and it is bounded laterally by the temporal bone Prominent structure on the basilar part is the pharygeal tubercle  Temporal bone  Has a rough petrous part Features here include: 86  Carotid canal for the passage of internal carotid artery Mandibular fossa for the head of the mandible (condylar process) External acoustic meatus Styloid process Posterior part Extend from the anterior edge of foramen magnum to the superior nuchal line posteriorly Consist of the occipital bone in the central and posterior part and the temporal bone laterally Features on the occipital bone 1. Jugular notch: for the passage of  Cranial nerve IX to XI  Internal jugular vein 87 2. Foramen magnum located in the occipital bone for the passage of:  The spinal cord which is continuous with the medullar oblongata  The two vertebral arteries  The spinal contribution of accessory nerve, the spinal accessory nerve 3. Occipital condyles: located on each side on the anterolateral margin of the foramen magnum for articulation with the atlas (the 1st cervical verterbra) Features on the temporal bone: Mastoid process  between the base of the styloid process and the mastoid process is the stylomastoid foramen This foramen allows for the passage of the facial nerve (CN VII) 88