Anatomy of Skullbase

Anatomy of Skullbase

1. ANATOMY OF SKULL BASE BY :- Dr.JINU IYPE 2 ND YEAR PG DEPARTMENT OF ENT

2. BIBILIOGRAPHY: Scott Brown’s Otorhinolaryngology head & neck surgery – 7 th edition Vol -3 Cummings Otolaryngology head and neck surgery - 6 th edition Glasscock Shambaugh surgery of the ear – 6 th edition Grey’s Anatomy

3. The skull base represents a central and complex bone structure of the skull that forms the floor of the cranial cavity on which the brain lies. It separates brain from facial structures and suprahyoid neck. Anatomical knowledge of this particular region is important for under standing several pathologic conditions as well as for planning surgical procedures. INTRODUCTION

4. Skull base boundaries :- Anterior ► Upper i ncisor teeth Posterior ► Sup. nuchal line of occipital bone Lateral ► Remaining upper teeth the zygomatic arch & its posterior root , the mastoid process

5. Three major areas of the skull base are described: 1. anterior 2. middle 3. posterior

6. Composed of five bones: Ethmoid Sphenoid 3) Occipital 4) Paired temporal 5) Paired frontal bones

7. OSTEOLOGY OF THE SKULL BASE The Frontal Bone

8. Orbital part- contain 2 openings- Anterior & Posterior ethmoidal foramen

9. The Ethmoid Bone consists of :- horizontal plate( cribriform plate) and a vertical plate in the midline, i.e. the perpendicular plate.

10. On closer examination, the cribriform plate Horizontal medial lamella Vertical lateral lamella This lateral lamella articulates with the frontal bone.

11. The skull base in this region— the ethmoid fovea —is formed medially by the lateral lamella of the cribriform plate , which is very thin bone and laterally by the frontal bone , which in contrast is a thicker bone.

12. The frontal bone forming the ethmoid fovea is 0.5 mm in thickness while the lateral lamella of the cribriform plate is 0.2 mm. The region where the anterior ethmoidal artery pierces the dura medially is the thinnest area in the skull base and is only 0.05 mm in thickness.

13. KEROS CLASSIFICATION The length of the lateral lamella and the depth of the olfactory fossa are classified into 3 types: • Type I — 1-3 mm • Type II — 4-7 mm • Type III — 8-17 mm

14. The Sphenoid Bone It separates the anterior and middle cranial fossa. looks like a bat with outstretched wings.

15. It consist of → a central body; two sets of wings– the greater and lesser, which course laterally ; & two pterygoid processes, directed inferiorly.

16. Posteriorly , the chiasmatic sulcus forms a slight depression & leads laterally to the optic canal. The tuberculum sellae , a bony elevation, just posterior to this sulcus

17. Followed by, posteriorly by sella turcica & dorsum sellae . The dorsum sellae terminates laterally into the posterior clinoid processes

18. The anterior surface of the body of sphenoid forms the roof & posterior wall of nasopharynx The body houses the sphenoid sinus . Lesser wings → Forms m edial portion of o rbital apex. Greater wings → Course upward & laterally from both sides of the sphenoid body-forms floor of M iddle C ranial F ossa .

19. The medial pterygoid plate projects back , where it articulates with the vertical plate of the palatine bone. Inferiorly, it ends in the pterygoid hamulus and superiorly in the pterygoid tubercle which projects back into the foramen lacerum . The lateral pterygoid plate extends back and laterally into the infratemporal fossa. The plate gives attachment to the pterygoid muscles.

20. Temporal bone Its made up of squamo us , mastoid, petrous & tympanic parts which ossify separately and later fuse creating squamotympanic & petrosquamo u s fissure 

21. The squamous temporal bone contains the hollow of the glenoid fossa joined laterally to the zygomatic process.

22. The petromastoid bone projects forwards and medially at 45°, wedged between the basiocciput and the greater wing of the sphenoid ; at the apex of the wedge the three bones do not quite meet, leaving a gap termed the 'foramen lacerum ’

23. Tympanic part has styloid process → b ehind its base, stylomastoid foramen & posterior the mastoid bone has the digastric notch, medial to which there is a groove for the occipital artery. Stylomastoid foramen transmits facial nerve.

24. ANTERIOR CRANIAL FOSSA The anterior cranial base can be defined as that portion of the skull base adjacent to the anterior cranial fossa.

25. ANTERIOR CRANIAL BASE Boundaries :- A n t e r i o r : P o s t e r i o r w a l l o f t h e f r o n t a l s i n u s Posterior : L esser wing of the sphenoid & anteior clinoid processes. Floor : roof of the nasal cavity & ethmoid sinuses medially. Lateral wall : O rbital plates of the frontal bone

26. An important visible landmark is the foramen cecum , which is the site of a communication between veins of the nasal cavity and the origin of the superior sagittal sinus. The next landmark, the crista galli , protrudes upward from the midline to provide attachment for the falx cerebri . On either side of the crista are the openings of the cribriform plate, through which olfactory nerves are transmitted

27. Just posterior to the last of these olfactory foramina is a smooth-surfaced area known as the planum sphenoidale , which forms the roof of the sphenoid sinus when the sinus is well pneumatized . 1 foramen cecum 2crista galli ; 3 cribriform plate; 4 planum sphenoidale ; 5 optic canal 6chiasmatic sulcus 7anterior clinoid process 8pituitary fossa 9, dorsum sellae ; 10, foramen ovale ; 11, foramen lacerum .

28. Frontal crest :- Midline bony ridge that projects upwards & provide attachment to the falx cerebri. Foramen caecum :- Transmits emissary vein from nose to superior sagittal sinus Crista galli :- Provides site for ant. most attachment of the falx cerebri . The contents & foramina's of ACF

29. Cribriform plate :- Sheet of bone contaning many small Olfactory foramina → Transmit olfactory nerve fibres into the nasal cavity.

30. The orbits contain several landmarks that can help surgical orientation during cranial base operations. The superior orbital fissure transmits the oculomotor , trochlear , ophthalmic, and abducens nerves and the ophthalmic vein , and it communicates with the middle cranial fossa The inferior orbital fissure contains the maxillary nerve (V2) and communicates primarily with the pterygopalatine fossa ; the lateral end of this fissure is an important landmark for the placement of lateral orbital osteotomies .

31. The optic canal transmits the optic nerve and the ophthalmic artery. Ant & post ethmoidal foramen :-Transmits Ant & post. ethmoidal artery, nerve,vein These ethmoid foramina mark the position of the frontoethmoid suture line, a valuable guide to the level of the ethmoid roof and the anterior fossa floor. The posterior ethmoid foramen is of additional significance because of its consistent relationship with the optic canal, found 4 to 7 mm posteriorly .

32. Optic foramen

33. MIDDLE CRANIAL BASE The middle cranial base forms the floor of the middle cranial fossa. Boundaries (from the intracranial perspective ) Anteriorly - the posterior edge of the lesser sphenoid wing Posteriorly - ends at the posterosuperior edge of the petrous part of the temporal bone.

34. Subdivisions of the lateral skull base Pharyngeal; Tubal; Neurovascular; Auditory; Articular ; Infratemporal fossa.

35. PHARYNGEAL AREA Situated centrally in the skull base, this area forms the roof of the nasopharynx .

36. Boundaries→ Formed by the line of attachment of the pharyngeal wall. The pharyngobasilar fascia is attached to the skull base and medial pterygoid plates → thickened posteriorly into a pharyngeal ligament that continues inferiorly as the pharyngeal raphe . The pharyngobasilar fascia is separated from the prevertebral muscles by the prevertebral fascia.

37. TUBAL AREA The tubal area lies just lateral to the pharyngeal area and occupied by the Eustachian tube. Anteriorly , the scaphoid fossa at the base of the medial pterygoid plate, between the petrous bone and the greater wing of the sphenoid

38. The pharyngobasilar fascia is attached to undersurface of the tube, & two 'paratubal' muscles arise one on each side of it. The levator palati arises medially (within the pharynx) & the tensor palati arises laterally (outside the pharynx). Both muscles are partly attached to the tube, and open it during swallowing

39. NEUROVASCULAR AREA Posterior to the tubal area lies the neurovascular area:- • Carotid sheath; • Styloid apparatus; • Facial nerve

40. Carotid sheath It is attached to the skull base around the carotid foramen and continues downwards as far as the aortic arch. Content : Internal carotid artery Vagus nerve. Internal jugular vein

41. In the neck, the carotid sheath + pretracheal fascia, is firmly attached anteriorly to the deep surface of the sternomastoid . Posteriorly , it is not attached to the prevertebral fascia, but is free to slide over it. This means that pus tracking laterally from a parapharyngeal abscess passes behind the sheath and behind the sternomastoid , to point in the posterior triangle.

42. The cervical sympathetic trunk lies behind the carotid sheath in front of the prevertebral fascia, just medial to the vagus nerve. It ends superiorly at the superior cervical ganglion. The DEEP PETROSAL NERVE arises from this ganglion and passes superiorly to join the greater superficial petrosal nerve to form the VIDIAN NERVE.

43. The jugular foramen is divided by two transverse septa(Jugular spine) of fibrous dura (which may ossify) into three compartments. The anterior compartment is occupied by the Glossopharyngeal nerve and the inferior petrosal sinus.

44. The middle compartment is shared by the vagus ( X ) and accessory ( XI ) nerves The posterior compartment is filled by the emerging internal jugular vein .

45. IX & XI TH CN lie more laterally than X TH CN in the foramen The right jugular foramen is larger than the left in 75% of the population. W h e n t h e r oo f o f t h e j u g u l a r b u l b i s s ee n a b o v e t h e l e v e l o f f l oo r o f I A C , i t i s called a high riding jugular bulb , which is more common on the righ t side . This is a dangerous variant & exposing during translabyrinthine surgery.

46. 3 muscles : Stylopharyngeus, Stylohyoid, Styloglossus . Stylopharyngeus :- Pass lateral to ICA . Origin - Deep aspect of base of styloid process. Insertion - Thyroid cartilage & side wall of pharynx. Nerve supply - Ninth nerve. Function:- Elevates larynx & pharynx . Styloid apparatus

47. Stylohyoid :- Pass lateral to ECA. Origin- Back of the base of styloid process Insertion- B ase of greater cornu of hyoid Nerve supply- Facial nerve. Function:- Elevates & retracts the hyoid.

48. Styloglossus :- L ateral to ICA Origin- front of the styloid process &upper part of stylohyoid ligament Insertion- Side of the tongue Nerve supply- Hypoglossal nerve Function:- Retract the tongue .

49. STYLOMASTOID FORAMEN It transmits the facial nerve and the stylomastoid artery . As soon as it emerges from the foramen, VII gives off the posterior auricular nerve (supplying the occipital belly of occipitofrontalis ) and a muscular branch (supplying the posterior belly of digastric and stylohyoid ). It then swings forward into the parotid gland, dividing zygomaticofacial & cerviciofacial division

50. AUDITORY AREA This small area anterolateral to the neurovascular area forming the floor and anterior wall of the external auditory canal and middle ear.

51. Petrotympanic fissure of Glaser – Transmits the chorda tympani and anterior tympanic branch of the maxillary artery, and the corresponding veins which drain into the pterygoid plexus.

52. Situated on each side of the body of sphenoid bone & extend from sup. orbital fissure anterior to petrous apex post erior . Receives :- Sup.& inf. ophthalmic vein , Sphenoparietal sinus. Drains into:- Petrosal sinus, Pterygoid plexus, Basilar plexus. Contents:- 1 ) C N III , IV, V1, V2 & VI 2) ICA The contents & foramina's of MCF Cavernous Sinus Only anatomic location in the body in which an artery travels completely through a venous structure

53. Triangular shaped fissure bounded med . → body of sphenoid, sup . → lesser wing , inf . → greater wing and is completed lat → frontal bone as greater & lesser wings converge. Optic strut separates optic canal from superior orbital fissure . Optic canal & superior orbital fissure together form the orbital apex. Contents are as in dia Superior orbital fissure

54. Extends from pterygopalatine fossa along orbital floor. Separates greater wings of the sphenoid from the maxilla. Content –1) Maxillary branch of trigeminal nerve 2)Infra orbital vessels. 3) Emissary veins connecting inf ophthalmic vein to pterygoid venous plexus. 4) Zygomatic nerve. Inferior orbital fissure

55. . Dural invagination at posterior aspect of cavernous sinus. Contains gasserian ganglion (trigeminal). Dural layers shows thin peripheral enhancement In MRI, 3 sensory divisions of t rigeminal nerve can be visualized leaving the gasserian ganglion MECKEL’S CAVE

56. Optic canal Formed by the lesser wing of sphenoid. The c ontents are - Optic nerve Ophthalmic Artery. Sympathetic fibers from carotid plexus

57. Foramen Rotundum Is actually a canal in the base of the greater sphenoid wing, is situated just inf & lateral to supe rior orbital fissure. It extends obliquely forward & slightly inferiorly, connecting the MCF to pterygopalatine fossa. Transmits the maxillary nerve ( V2), artery of the foramen Rotundum & emissary veins. Best visualized by coronal CT

58. Foramen O vale Endocranially , its situated post-lat to F. rotundum ; exocranially , it is found at the base of lateral pterygoid plate. Contents :- 1) Mandibular Nerve 2) Accessory meningeal nerve 3) Lesser petrosal nerve 4)Emissary vein 5)Occasionally ant. trunk of middle meningeal artery

59. Foramen spinosum Its an aperture in the greater wing of the sphenoid posterolateral to foramen ovale . Contents :- Middle meningeal artery & vein. 2)Emissary vein. 3)Nervous spinosus ( Meningeal branch of mandibular nerve)

60. Foramen Lacerum Its located at the base of medial pterygoid plate, ant to the petrous apex. Structures passing whole length : Meningeal branch of Ascending pharyngeal artery 2) Emissary vein Other structures partially traversing: Internal carotid artery Greater petrosal nerve .

61. Vidian Canal Also c/a pterygoid canal. Located in the floor of sphenoid sinus at the junction of the pterygoid process & the sphenoid body connecting the pterygopalatine fossa ant eriorly & the foramen lacerum posteriorly. Contents:- 1) Vidian Artery ( Br. Of Maxillary Artery). 2) Vidian Nerve (greater superficial petrosal nerve & deep petrosal nerve )

62. Infratemporal fossa It is the space between the skull base, lateral pharyngeal wall & the ramus of mandible.

63. BOUNDARIES :- 1) Lat . - Ramus and condylar process of the mandible. Med.- M edial pterygoid muscle . Ant. – Posterolateral wall of maxilla. 4) Post . – Carotid sheath & styloid apparatus 5) Sup . - Greater wing of the sphenoid bone. 6) Inf . – No anatomical.Continued down with neck

64. The contents of the fossa are :- Lateral and Medial pterygoid muscles The maxillary artery and its branches The pterygoid venous plexus and maxillary veins Branches of the mandibular nerve

65. Medial Pterygoid muscles Origin :- Superficial head → Maxillary tuberosity & pyramidal process of palatine bone Deep head → Medial surface of lateral pterygoid plate of the sphenoid bone Insertion into the angle of the mandible

66. Action: T o close the mouth and move the mandible towards the opposite side in chewing. Blood Supply:- Branch of Maxillary artery Nerve supply:- Medial pterygoid nerve branch of mandibular nerve

67. Origin :- Upper head → infra-temporal surface & crest of greater wing of sphenoid Lower head → Lateral surface of the lateral pterygoid plate I nsertion into the neck of the mandible Lateral Pterygoid muscles

68. Action: Depression & Protrusion of the mandible, Side to side movement Blood Supply:- Pterygoid vessels from Maxillary artery Nerve supply:- B ranches from the masseteric or buccal nerve, branch of the ant. trunk of the mandibular nerve

69. Its the larger of the 2 terminal branches of ECA →E nters the infratemporal fossa → Pass forward bwt ramus of mandible & sphenomandibular ligament → Then runs sup or deep to the lateral pterygoid muscle → Pterygomaxillary fissure → Pterygopalatine fossa . MAXILLARY ARTERY

70. BRANCHES OF MAXILLARY ARTERY

71. The pterygoid plexus & maxillary veins Plexus → lies within and on the lateral surface of the lateral pterygoid muscle, and receives branches of the maxillary artery. Drains into two short, large maxillary veins → superficial temporal vein and retromandibular vein .

72. The pterygoid plexus has three important communicating veins. The inferior ophthalmic vein → the facial vein. 2. A connecting vein passes vertically down →cavernous sinus 3. The deep facial vein → join the anterior facial vein.

73. The mandibular nerve It passes through the foramen ovale . The main trunk : 1.N ervus Spinosus (which reenters the middle fossa through the foramen spinosum ) 2 . Nerve to the Medial pterygoid → the tensor palati and tensor tympani.

74. The main trunk divides into anterior posterior divisions The anterior division:- Buccal nerve → buccinator to supply the mucous membrane of the cheek. Temporalis Masseter Lateral pterygoid .

75. The posterior division :- The Mylohyoid nerve. The Auriculotemporal nerve. The Inferior alveolar nerve The Lingual nerve is joined by the chorda tympani 2 cm below the base of the skull

76. The otic ganglion lies close to the mandibular nerve just below the foramen ovale . It relays secretomotor fibres for the parotid gland, which it receives by way of the lesser superficial petrosal nerve and transmits to the auriculotemporal nerve ..

77. A space between the pterygoid plates and the posterior wall of maxillary sinus. Shaped like an inverted pyramid . Pterygopalatine fossa

78. Boundaries :- Medially - Perpendicular plate palatine bone Laterally - Narrowing to pterygomaxillary fissure Anteriorly - Post wall of maxillary sinus Posteriorly - Med & Lat pterygoid plates Superiorly- Undersurface of body of sphenoid.

79. . The PPF is an important pathway for the spread of neoplastic and infectious processes : Med - with nasal cavity via sphenopalatine F. Lat - infratemporal fossa via the pterygomaxillary fissure. Ant - with orbit via the inferior orbital fissure Pterygopalatine fossa Communications

80. Post & sup - with Meckel c ave & cavernous sinus (of MCF) via the F. rotundum . Post & inf - with MCF via the vidian canal, which transmits the Vidian nerve. Inferiorly - with palate via the greater and lesser palatine canals

81. Contains:- 1) Pterygopalatine ganglion 2) Terminal third of the maxillary artery 3) Maxillary Nerve – CN V 4) Greater & deep petrosal nerve.

82. Posterior Cranial fossa Anterior margin :- The posterior surface of the clivus . Laterally:- superiorly the posterior surface of the petrous part of temporal bone Posteriorly :- mastoid portion of temporal bone & the squamous part of occipital bone.

83. The contents & foramina's of PCF FORAMEN MAGNUM The foramen magnum is entirely formed within the occipital bone. Contents :- 1. Medulla oblongata.2. Vertebral arteries and veins. Anterior & posterior spinal arteries. 3. spinal component of CN XI.

84. Also c/a ant condyloid canal . Located within occipital bone. Its paired bone passage that runs lateral & slightly forward from PCF to nasopharyngeal carotid space → Transmits hypoglossal nerve. I n t r a c a n a li c u l a r e n h a n c e m e n t i s a l w a y s p r e s e n t ( e m i ss a r y v e i n s ) , w i t h li n e a r filling defects ( nerve rootlets). HYPOGLOSSAL CANAL

85. INTERNAL ACOUSTIC CANAL Transmit VII &VIII from pontomedullary junction to inner ear Divided by a bony lamina ( falciform crest) into :- 1) Smaller superior part:Superior vestibular Nerve & Facial Nerve 2) Larger Inferior part:- Inferior vestibular Nerve & Cochlear nerve

86. Parapharyngeal , masticator, carotid & retropharyngeal spaces seen in close contact with the skull base along their cephalad aspect . Relation of skull base to the deep facial spaces

87. s . e Also c/a Lateral pharyngeal space, Pharyngomaxillary space . Shape like an inverted pyramid Parapharyngeal space

88. Boundaries:- Superior→Skull base, sphenoid & temporal bone Inferior → Greater cornu of hyoid bone Anterior → Pterygomandibular raphe . Medial → Sup. c onstrictor,b uccopharyngeal fascia Lat eral→ Ramus of mandible, deep lobe of parotid gland, medial pterygoid muscle Posterior lateral → Carotid sheath Posteriormedial → R etropharyngeal space

89. Has two compartments Prestyloid compartment Contains 2 muscles → Tensor palati & Levator palati muscles 2 artery → Ascending palatine & ascending pharyngeal artery & int. jugular vein. Retrostyloid compartment : It is neurovascular space, & contains the carotid sheath.

90. Internal carotid artery : Carotid foramen → curves upwards into F.lacerum in MCF → apex of petrous bone → enters the cavernous sinus It lies in front of cochlea & middle ear cavity, separated by thin plate of bone (may be dehiscent ) → gives off small intrapetrous branches, including carotico- tympanic artery → feeding vessels for a glomus tumour. Structures within the skull base

91. Jugular bulb It is the point at which sigmoid sinus feeds the upper end of IJV. Lies below posterior part of the floor of the middle ear. Inferior petrosal sinus joints jugular bulb at the skull base GSPN+ deep petrosal nerve= vidian N. Pterygoid canal → Pterygopalatine G. CN IX,X,IX, Jacobsons & Arnold nerve lies in & around the jugular foramen.

92. FRACTURES SUTURES Less than 2 mm in width Same width throughout the length Lighter on X-ray compared with fracture lines At specific anatomic sites Does not run in straight line Curvaceous Greater than 3 mm in width Widest at the centre & narrow at the ends Runs through both the outer & the inner lamina of bone, hence appears darker Usually over temporoparietal area Usually runs in a straight line Angular turns

93. There are 4 muscles as follow Masseter muscle Origin:- From zygomatic arch Insertion:- Lateral aspect o f mandible from the a ngle forwards along the lower border, & u pwards over the lower part of the ascending ramus. Nerve supply:- masseteric branch F r o f ant. division of the mandibular N. Action :-Elevation & p rotrusion of mandible Muscles superficial to the lateral skull base

94. Largest muscle of mastication & fan shape. Origin: From inf. temporal line , floor of temporal fossa & from overlying temporal fascia of the side of the skull. Insertion : To the coronoid process A ction : Elevation (anterior fibers ) & Retraction (posterior fibers) Nerve supply: Ant div. of mandibular N. Blood Supply:- middle temporal artery , branch of sup. temporal artery deep temporal arteries, branches of the maxillary artery Temporalis muscle

95. Origin:- from 2 heads: manubrium & clavicle. Inserted:- Curved line extending from tip of the mastoid process to superior nuchal line of the occiput. Nerve supply :- A ccessory nerve Action:- To protract the head (moving it forwards while keeping it vertical with a horizontal gaze). Sternocleidomastoid muscle

96. Two bellies united by tendon Origin – Anterior belly from diagastric fossa of mandible. Posterior belly from mastoid notch of temporal bone . Insertion – into the digastric fossa on the lower edge of the mandible. Nerve supply:- Post . b elly is supplied by Facial nerve (nerve to digastric) & the ant. belly by the mylohyoid nerve ( CN V3 ). Action:- To depress & retract the chin Digastric muscle