Anatomy of temporal bone and it’s surgical importance

1. ANATOMY OF TEMPORAL BONE AND IT’S SURGICAL IMPORTANCE PRESENTER: DR. SRITAMA DE POST GRADUATE TRAINEE DEPARTMENT OF E.N.T GAUHATI MEDICAL COLLEGE AND HOSPITAL

2. EMBRYOLOGY

3. 0-8 WEEKS  The adult temporal bone is made up of five major components, namely the squamous part (squama), the petrous part (petrosa), the tympanic bone, the mastoid process, and the styloid process.  However, of these five components, the mastoid and styloid processes do not fully develop until after birth.  Both the squama and the tympanic bone are products of membranous bone development.  The petrous portion is represented by the cartilaginous otic capsule until 20 weeks of gestation during which ossification proceeds.  The styloid process also is preformed in cartilage.  It is not until the eight-week stage that one can first discern development of the squama of the temporal bone as commencing from an ossification center which extends into the zygomatic process.

4. 8-16 WEEKS  The tympanic part of the temporal bone begins its development at about 9 to 10 weeks of gestation.  In the ninth week, the squama and zygomatic process begin membrane bone formation.  By the end of the ninth week, the superior wall of the middle ear emerges as a projection of the otic capsule; known as the superior periotic process. It grows forward over the ossicles forming the lateral aspect of the tegmen tympani.  The medial part of the tegmen tympani consists of a fibrous tissue plate. A coronal section of the Skull of a Foetus, 4 months old

5. DEVELOPMENT AFTER 16 WEEKS  16 weeks-the postauditory process of the squama extends posterior to the tympanic ring forming the anterosuperior portion of the mastoid process.  20th to 24th week- a)the petrous bone, composed of the cartilaginous otic capsule, begins rapid ossification from multiple centers. b) At this point, the tympanic cavity and labyrinth have attained full size; however, the temporal bone, especially the mastoid process, continues to grow.  25 weeks-the floor of the middle ear develops, either as an independent bone located between the pyramid and the tympanic ring or as a bony lamellar projection of the petrous pyramid.  29th week- the tympanic process of the squama joins the antral segment of the periosteal otic capsule to form the lateral wall of the antrum.  At term an ossification center forms at the dorsal aspect of Reichert’s cartilage which fuses with the otic capsule to create the styloid eminence in the floor of the tympanic cavity and also part of the distal segment of the bony fallopian canal.

6. The external petrosquamous fissure demarcates the border between that part of the mastoid derived from the squama and the portion which arises from the petrosa. This fissure is visible in the newborn, but generally disappears by the second year of life. At birth the mastoid antrum is large with a thin shell of bone. The mastoid process develops as a prominence on the outer aspect of the petrous pyramid during the first year of life. As the mastoid grows, the antrum shrinks in relative size and assumes a more medial position, as does the facial nerve. The mastoid, although well developed by three years of age, does not achieve adult configuration for several more years. Postnatally, the styloid process forms as an ossification center in the upper portion of Reichert’s cartilage; concurrently, at its ventral aspect another ossification center appears which will become the lesser horn of the hyoid and the superior part of the body of the hyoid. The fusion of the separate components of the temporal bone then becomes the major process in its further development.

7.  In children the pinna needs to be pulled backwards, downwards and laterally to make the external auditory canal in line for examination as the developing temporal bone is horizontally placed which becomes vertically placed in adult.  The mastoid process in children is not fully developed, thus cannot be palpated easily. Hence the postauricular incision in children should be given more horizontally to prevent injury to the facial nerve. SURGICAL IMPORTANCE POST AURICULAR INCISION IN ADULT VS INFANT

8. ANATOMY

9. The temporal bone is a composite structure consisting of 1. The Tympanic Bone 2. The Mastoid Process 3. The Squama(Squamous portion of the temporal bone) 4. The Petrosa(Petrous portion of the temporal bone. ACCORDING TO SHAMBAUGH TEXTBOOK OF SURGERY OF THE EAR , ALTHOUGH THE STYLOID PROCESS IS CLOSELY RELATED TO THE TEMPORAL BONE, IT IS NOT CONSIDERED A PORTION OF IT.

10. TEMPORAL BONE (LATERAL SURFACE) The four parts visible here are: 1) squamous bone - flattened region that forms the lateral portion of the skull and is the origin for the temporalis muscle. The zygomatic process extends anteriorly from the squamous bone 2) tympanic bone - forms the floor, anterior and inferior wall of the bony portion of the external auditory canal 3) styloid bone - a slender process of variable length that extends in an anterior-inferior direction and serves as the attachment for the stylohyoid, styloglossus and stylopharyngeus muscles. 4) mastoid bone Immediately in front of the external auditory meatus is the glenoid (mandibular) fossa where the condyle of the mandible articulates. Also visible on the lateral side of the temporal bone are Macewen’s (suprameatal) triangle (i.e., a shallow depression posterior-superior to the EAM that marks the position of the mastoid antrum) and the spine of Henle which is a projection of variable prominence at the posteriosuperior aspect of the external auditory canal. LATERAL SURFACE OF RIGHT TEMPORAL BONE

11. TYMPANIC BONE  It forms the anterior, inferior and parts of the posterior wall of the external auditory canal.  It interfaces with 1) the squama at the tympanosquamous suture, 2) the mastoid at the tympanomastoid suture and 3) the petrosa at the petrotympanic fissure and constitutes the posterior wall of the glenoid fossa for the temporomandibular joint (TMJ).  The chorda tympani nerve, anterior process of the malleus, and anterior tympanic artery traverse the petrotympanic fissure.  Inferiorly, the vaginal process, a projection of tympanic bone, forms the sheath of the styloid bone.  Laterally, the tympanic bone borders the cartilaginous EAC, whereas medially it bears a circular groove, the annular sulcus. The annular sulcus houses the annulus of the tympanic membrane except superiorly, where it is deficient; at this point, known as the notch of Rivinus, the tympanic membrane attaches directly to the squama.  Anteriorly, the tympanic ring separates the external auditory canal from the glenoid fossa, which lies beneath the root of the zygoma. Temporomandibular joint dysfunction, as well as disease of the molar teeth, may manifest in referred otalgia, owing both to the proximity of the EAC and the shared innervation by the mandibular division of the trigeminal (fifth cranial) nerve.

12. SQUAMOUS PORTION OF TEMPORAL BONE • It serves as the lateral wall of the middle cranial fossa and interfaces with the parietal bone superiorly and with the zygomatic process and the sphenoid anteriorly. • Its medial surface is grooved by a sulcus for the middle meningeal artery, whereas the middle temporal artery runs in a groove on its lateral aspect. MASTOID PORTION OF THE TEMPORAL BONE • It is the inferiorly extending projection seen on the lateral surface of the temporal bone. It is composed of a squamous portion (laterally) and a petrous portion (medially) separated by Körner’s (petrosquamous) septum. • The fossa mastoidea (Macewen’s triangle) is defined by the linea temporalis (temporal line), the posterosuperior margin of the external auditory canal, and a tangent to the posterior margin of the external auditory canal. • The fossa mastoidea, a cribrose (cribriform) area, is identified by its numerous, perforating small blood vessels. • The mastoid foramen, located posteriorly on the mastoid process, is traversed by the mastoid emissary vein and one or two mastoid arteries. • Inferiorly, the sternocleidomastoid muscle attaches to the mastoid tip.

13. STYLOID PROCESS  Normal length-2.5cm  The styloid process is a slender pointed piece of bone just below the ear. It projects down and forward from the inferior surface of the temporal bone, and serves as an anchor point for several muscles associated with the tongue and larynx.  Its proximal part (tympanohyal) is ensheathed by the vaginal process of the tympanic portion.  Its distal part (stylohyal) gives attachment to the following:  stylohyoid ligament  stylomandibular ligament  styloglossus muscle (innervated by the hypoglossal nerve)  stylohyoid muscle (innervated by the facial nerve)  stylopharyngeus muscle (innervated by the glossopharyngeal nerve)

14. SURGICAL IMPORTANCE  The tympanosquamous and tympanomastoid sutures are landmarks for the “vascular strip” incisions used in tympanomastoid surgery. The elevation of EAC skin and periosteum at these two sutures often requires sharp dissection to divide the contained periosteum, particularly at the tympanosquamous suture.  Elevation of the tympanic membrane, as for a transcanal exploratory tympanotomy, typically commences just above the notch of Rivinus; the surgeon is thus able to identify the annulus and elevate the tympanomeatal flap in continuity with the tympanic membrane.  The tip of the mastoid process is easily palpated and is a landmark for the positioning of postauricular incisions.  The zygomatic process is also readily identifiable. The posterior root of zygomatic process forms the suprametal crest and the anterior root ends in a rounded eminence, the articular tubercle (eminentia articularis) which articulates with the mandible and gives attachment to the masseter muscle. On occasion, posterior bulging of the anterior canal wall may obscure full visualization of the tympanic membrane. Anterior canalplasty can improve surgical visualization but if overzealous may result in prolapse of the TMJ into the EAC with, for example, opening the mouth.

15. The tympanomastoid fissure is anterior to the tip of the mastoid and can be traced medially to the stylomastoid foramen, which is the exit point of the facial nerve. Thus, caution must be exercised when dissecting anterior to the mastoid tip during mastoid surgery, particularly in young children in whom the tip is not well developed. Vestibular schwannoma, Middle cranial fossa approach- It is utilized primarily for the purpose of hearing preservation in patients with small tumors, typically confined to the internal auditory canal. A small window of squamous part of temporal bone is removed to allow exposure of the tumor from the upper surface of the internal auditory canal, preserving the inner ear structures.  Styloid Process 1. The stylomandibular ligament is a specialized band of the cervical fascia, which extends from near the apex of the styloid process of the temporal bone to the angle and posterior border of the angle of the mandible. This ligament separates the parotid from the submandibular gland and medial pterygoid. 2. Eagle syndrome is characterized by recurrent pain in the oropharynx and face due to an elongated styloid process or calcified stylohyoid ligament. which interferes with adjacent anatomical structures giving rise to pain.

16. LINEA TEMPORALIS • A prominent ridge known as the temporal line (linea temporalis) runs posteriorly and slightly superiorly from the root of the zygoma and defines the inferior border of the temporalis muscle. • The linea temporalis is an avascular plane, a feature that makes it an ideal location for the superior limb of the “T” musculoperiosteal incision used in the postauricular approach to the tympanomastoid compartment. • The squamous portion of the temporal bone (the squama) extends above the temporal line, whereas inferiorly and anteriorly is the tympanic ring and posteriorly the mastoid. • The temporal line also approximates the position of the floor of the middle cranial fossa. TYMPANOMASTOID SUTURE •The posterior meatal skin is firmly adherent to the tympanmastoid suture, as such sharp and careful dissection should be carried out in this region to prevent tear of the tympanomeatal flap. •The facial nerve lies 8mm medial to the tympanomastoid line. •The tympanomastoid suture is traversed by Arnold’s nerve (auricular branch of vagus nerve).

17. FOSSA MASTOIDEA (MACEWEN’S TRIANGLE) • Macewen’s triangle (the fossa mastoidea), is delineated by the temporal line superiorly, a tangent to the posterior external auditory canal posteriorly, and the posterosuperior rim of the canal. • Macewen’s triangle is characterized by the presence of multiple small perforating vessels and hence is also known as the cribrose (cribriform) area. The mastoid antrum, medial to the fossa mastoidea (Macewen’s triangle), develops in the earliest stages of mastoid pneumatisation and is ordinarily present in even the least pneumatised temporal bones. • The fossa mastoidea is an important surgical landmark as it laterally overlies the mastoid antrum. Therefore, the fossa mastoidea is the site at which mastoid drilling ordinarily commences. • Henle’s spine which marks the anterior and inferior limit of dissection in a canal wall up mastoidectomy, is a content of the macewen’s triangle.

18. TEMPORAL BONE (MEDIAL SURFACE)  It features the porus of the internal auditory canal (IAC).  The foramen seen at the petrous apex is the internal carotid foramen, by which the internal carotid artery exits the temporal bone.  The sigmoid portion of the lateral venous sinus runs in the deep sulcus seen posteriorly, whereas the superior petrosal sinus runs in the sulcus located at the junction of the posterior and middle fossa faces of the temporal bone.  The petrous portion of the temporal bone houses part of the middle ear (e.g., ossicles) and inner ear (i.e., cochlear and vestibular end organs). Left adult temporal bone, medial aspect. 3 = squama; 11 = middle meningeal arterial sulcus; 12 = petrous bone; 13 = internal auditory canal; 14 = sigmoid sulcus; 15 = superior petrosal sulcus; 16 = inferior petrosal sulcus; 17 = petrous apex; 18 = arcuate eminence; 19 = internal carotid artery foramen.

19. The following landmarks are visible on the medial surface 1) cochlear aqueduct - a fine canal that connects scala tympani in the basal turn to the cerebrospinal fluid (CSF) space around the brain 2) vestibular aqueduct - a bony passage that runs from the vestibule to the subarachnoid space. It transmits the endolymphatic duct 3) internal auditory meatus (IAM) - a bony opening through which the 7th (i.e., facial) and 8th (i.e., vestibulocochlear) cranial nerves and internal auditory artery enter the petrous part of the temporal bone 4) mastoid process 5) sigmoid sinus sulcus - an ‘s’-shaped depression on the medial side of the temporal bone that is located posterior to the operculum and vestibular aqueduct. The sigmoid sinus is continuous with the transverse sinus and empties into the internal jugular vein 6) styloid process. 7) The jugular fossa marks the location of the jugular bulb. The temporal bone contains the air space of the middle ear and air cells in the mastoid, petrosa, perilabyrinthine and accessory areas

20. TEMPORAL BONE (POSTERIOR SURFACE)  The vertically oriented posterior face of the petrosa dominates the posterior view of the temporal bone as it delimits the anterolateral aspect of the posterior cranial fossa and lies between the superior and inferior petrosal sinuses.  The porus of the IAC, operculum, endolymphatic fossette cradling the endolymphatic sac, and subarcuate fossa are the key anatomic features on this surface. A view of the posterior surface of a left temporal bone. *Anterior lip, < Posterior lip, AE Arcuate eminence, AFL Anterior foramen lacerum component of the petrous apex, ELD Endolymphatic duct foramen, IAC Internal auditory canal, JB Jugular bulb, SP Styloid process, SS Sigmoid sinus groove

21. ACF Anterior condylar foramen, CL Clivus, ELD Endolymphatic duct foramen, FM Foramen magnum, FO Foramen ovale, FR Foramen rotundum, FS Foramen spinosum, IAC Internal auditory canal, JF Jugular foramen, JT Jugular tubercle, OB Occipital bone, OC Occipital condyle, PB Parietal bone, PCF Posterior condylar foramen, PR Petrous ridge, SB Squamous bone, SS Sigmoid sinus grooveA view of the posterior surface of an articulated temporal bone

22. SURGICAL IMPORTANCE  The posterior surface of the temporal bone forms the anterior border of the posterior cranial fossa.  The sigmoid sulcus is an indentation at the lateral aspect of the posterior surface and accommodates the sigmoid sinus.  Anterior to the sigmoid sulcus is the foveate fossa for the intradural portion of the endolymphatic sac. A ledge at the superior extent of the fossa, the operculum, covers the intraosseous portion of the endolymphatic sac. The vestibular aqueduct runs anteriorly, superiorly, and medially from the operculum to end at the medial wall of the vestibule.  The superior petrosal sulcus, located at the interface of the posterior and middle cranial fossa plates of the temporal bone, carries the superior petrosal sinus from the sigmoid sinus to the cavernous sinus anteriorly.

23. The internal auditory canal penetrates the posterior surface of the petrous ridge, runs anteromedially to posterolaterally, and contains the cochlear, vestibular, and facial nerves, along with their blood supply. The canal extends approximately 1 cm from the porus medially to the fundus laterally. At the fundus, the canal is divided into an upper and a lower portion by the transverse crest (crista falciformis). The inferior compartment contains the cochlear nerve anteriorly and the inferior vestibular nerve posteriorly. A vertical crest of bone, Bill’s bar, separates the superior portion of the canal into an anterior compartment, occupied by the facial nerve, and a posterior compartment containing the superior vestibular nerve. A branch of the inferior vestibular nerve, the posterior ampullary nerve or singular nerve , which innervates the ampulla of the posterior semicircular canal, exits the internal auditory canal through the singular canal. In rare cases of chronic persistent positional vertigo which do not respond to physiotherapy singular nerve neurectomy is a new surgical procedure for treatment.

24. The inferior surface of the temporal bone separates the upper neck from the skull base. Accordingly, many vital neurovascular structures traverse this surface.  Anteriorly and medially, the carotid foramen is the point at which the internal carotid artery enters the temporal bone.  Posteriorly, a ridge of bone, the jugulocarotid crest, separates the carotid canal from the jugular foramen.  Classically, the jugular foramen has been thought of as being divided into a posterolateral pars venosa, which is occupied by the jugular vein, and an anteromedial pars nervosa, which is traversed by the glossopharyngeal, vagus, and spinal accessory nerves.  The hypoglossal nerve exits the occipital bone by the hypoglossal canal, medial to the pars nervosa of the jugular foramen.  Lateral to the jugular foramen is the styloid process.  Immediately posteriomedial to the styloid process is the stylomastoid foramen, by which the facial nerve exits the temporal bone. TEMPORAL BONE (INFERIOR SURFACE)  The triangular opening of the cochlear aqueduct is located medial to the jugular foramen.  The inferior tympanic canaliculus runs in the jugulocarotid crest and carries the inferior tympanic artery (a branch of the ascending pharyngeal artery) and the tympanic branch of the glossopharyngeal nerve (Jacobson’s nerve) into the tympanic cavity.

25. An inferior view of an articulated temporal bone. AFL Anterior foramen lacerum, BO Basiocciput, CF Carotid foramen, DR Digastric ridge, FM Foramen magnum, GF Glenoid fossa, GWS Greater wing of the sphenoid, MT Mastoid tip, O Foramen ovale, OB Occipital bone, OC Occipital condyle, PCF Posterior condylar foramen, S Foramen spinosum, SMF Stylomastoid foramen, ZP Zygomatic process  The cochlear aqueduct runs from the medial aspect of the scala tympani of the basal cochlear turn to terminate anteromedial to the jugular bulb, it parallels, and lies inferior to, the IAC. Foramen Lacerum- Structures passing whole length: 1. Meningeal branch of Ascending pharyngeal artery 2. Emissary vein Other structures partially traversing: 3. Internal carotid artery 4. Greater petrosal nerve  Foramen Ovale- 1.Mandibular Nerve (CN V3) 2. Accessory meningeal nerve 3. Lesser petrosal nerve 4. Emissary vein (Cavernous sinus to pterygoid plexus) 5. Occasionally anterior trunk of middle meningeal artery  Foramen spinosum- 1. Middle meningeal artery and vein (posterior trunk) 2. Emissay vein 3. Nervus spinosus (Meningeal branch of mandibular nerve)

26. The jugular foramen is of particular importance in skull base surgery as it is traversed by the glossopharyngeal (ninth), vagus (tenth), and spinal accessory (eleventh) cranial nerves as they exit the skull .In the course of posterolateral skull base exposure, decortication and fibrous tissue dissection reveal the internal jugular vein, its bulb, and the internal carotid artery. Posterior retraction of the internal jugular vein and resection of the jugular bulb allow visualization of the lower cranial nerves exiting the skull (IX,X,XI). Glomus jugulare tumors are rare, slow- growing, hypervascular tumors that arise within the jugular foramen of the temporal bone. PHELP'S SIGN - loss of crest of bone as seen in CT-scan between carotid canal and jugular canal in glomus jugulare. SURGICAL IMPORTANCE A close-up view of the jugular foramen–carotid foramen area on the inferior aspect of a left temporal bone. * Jugulocarotid spine, < Cochlear aqueduct canal, > Jacobson’s nerve canal, AN Arnold’s nerve canal, D Dome of the jugular bulb, ICA Internal carotid artery canal, IPS Groove for the inferior petrosal sinus, SP Styloid process

27.  From the transmastoid perspective, the cochlear aqueduct is encountered when drilling medial to the jugular bulb; opening the aqueduct results in the flow of cerebrospinal fluid into the mastoid, a useful maneuver in translabyrinthine cerebellopontine angle tumor surgery as it decompresses cerebrospinal fluid pressure.  In addition, cranial nerve IX, the inferior petrosal sinus, and, in some cases, cranial nerves X and XI can be found immediately inferior to the lateral terminus of the cochlear aqueduct. Therefore, the cochlear aqueduct can be used as a guide to the lower limits of IAC dissection in, for example, the translabyrinthine approach as it allows full exposure of the IAC without risking the lower cranial nerves.  Medial to the mastoid tip is the digastric groove for the posterior belly of the digastric muscle. 1) This is an important landmark for the identification of facial nerve during parotid surgery. 2) This projects as the digastric ridge in the mastoid cavity which anteriorly traced leads to the stylomastoid foramen which delineates the vertical portion of the facial nerve.

28. TEMPORAL BONE (ANTERIOR SURFACE) The petrous apex is the wedge of bone that separates the greater wing of the sphenoid from the occipital bone. The most prominent feature of this surface is the internal carotid foramen, through which the carotid artery exits the temporal bone. The impression for the trigeminal ganglion is located on the lateral surface of the petrous apex. The semi canal for the tensor tympani is lateral to the carotid canal; the bony portion of the Eustachian tube runs inferior and parallel to the tensor tympani muscle. The thin medial wall of the eustachian tube forms the lateral wall of the carotid canal and is frequently dehiscent. Thus, the carotid canal is vulnerable to injury in the course of surgical manipulations in the anterior tympanic cavity and in the medial wall of the eustachian tube.

29. TEMPORAL BONE (SUPERIOR SURFACE)  The superior surface (tegmen) of the temporal bone is the floor of the middle cranial fossa, separating the tympanomastoid compartment from the temporal lobe.  The tegmen can be divided into 1) an anterior tegmen tympani (covering the tympanic cavity) and 2) a posterior tegmen mastoideum (covering the mastoid air cells).  The petrotympanic suture line forms the medial boundary of the tegmen.  Further medially, the dense petrous bone (petrosa) runs an oblique course from lateral to medial. The petrous portion of the temporal bone is marked by depressions and eminences corresponding to the convolutions of the brain and the internal structures of the temporal bone.  The greater petrosal nerve (GPN) separates from the geniculate ganglion and emerges through the facial hiatus to run in a groove that is slightly medial to the petrotympanic suture and that parallels the petrous ridge.  Lateral to and paralleling the greater petrosal nerve is the lesser petrosal nerve, which runs in the petrosquamous suture (superior tympanic canaliculus).  The tensor tympani muscle is inferior to the lesser petrosal nerve.

30.  The foramen lacerum, located at the junction of the base of the greater wing of the sphenoid, the petrous apex, and the basiocciput, is a false foramen that is filled with fibrous connective tissue and that forms the roof of the carotid canal.  The carotid canal also parallels the petrous ridge.  The gasserian (semilunar) ganglion lies in a depression at the lateral aspect of the petrous apex known as the Meckel’s cave.  Anteriorly, proceeding medially to laterally, are the foramen ovale (for the mandibular division of the trigeminal nerve) and the foramen spinosum (for the middle meningeal vessels and a recurrent branch of the mandibular nerve); these structures serve as surgical landmarks for the anterior limit of the temporal bone. * Meckel’s cave impression, AE Arcuate eminence, AFL Anterior foramen lacerum, FM Foramen magnum, FO Foramen ovale, FR Foramen rotundum, FS Foramen spinosum, GPN Groove for the greater petrosal nerve, PR Petrous ridge, SS Sigmoid sinus sulcus, ZP Zygomatic process A superior view of an articulated temporal bone.

31. THE PETROSA  It is evident on superior, medial, and posterior views of the temporal bone.  The term “petrous” (Greek for “rocklike”) stems from the extreme density of its bone, which guards the sensory organs of the inner ear.  Important landmarks seen on a superior view are the arcuate eminence (roughly corresponding to the superior semicircular canal), meatal plane (indicative of the internal auditory canal), foramen spinosum for the middle meningeal artery, and facial hiatus (marking the departure of the greater petrosal nerve from the anterior aspect of the geniculate ganglion). The lesser petrosal nerve, accompanied by the superior tympanic artery, occupies the superior tympanic canaliculus, lying lateral to and paralleling the path of the greater petrosal nerve to the petrous apex. The petrous apex points anteromedially and is marked by the transition of the intrapetrous to the intracranial internal carotid artery, orifice of the bony eustachian tube, and, anterolaterally, ganglion of the trigeminal nerve in Meckel’s cave.

32. SURGICAL IMPORTANCE ARCUATE EMINENCE  The arcuate eminence, present in about 85% of temporal bones, approximates the position of the superior semicircular canal (SSCC) and is a key landmark in middle cranial fossa surgery.  In this region, the dura is firmly adherent on the medial side however laterally it is loosely adherent. Thus, in case of brain abscess following chronic suppurative otitis media with complications the pus elevates the dura and tracts anteriorly thereby causing a swelling in the preauricular region known as POTT’S PUFFY TUMOUR.  In middle cranial fossa approach the arcuate eminence serves as an important landmark for identification of the internal auditory meatus. The bone anteromedial to the arcuate eminence and greater superficial petrosal nerve is termed the ‘meatal plane’ and lies above the inernal auditory canal. It is often marked by a shallow depression.  Superior canal dehiscence syndrome (SCDS) is a rare medical condition of the inner ear, leading to hearing and balance symptoms in those affected. The symptoms are caused by a thinning or complete absence of the arcuate eminence. MECKEL’S CAVE  For relief of pain in trigeminal neuralgia glycerol injection is given in the gasserian ganglion in this region.

33. Several large dural venous sinuses are intimately associated with the temporal bone and comprise the principal venous drainage of the brain and cranial vault. 1) The superior sagittal sinus and straight sinus merge at the internal occipital protuberance. 2) The right and left transverse sinuses extend beyond this junction. The right transverse sinus is primarily the continuation of the superior sagittal sinus and thus is generally larger in diameter than the left transverse sinus, which is primarily the continuation of the straight sinus. 3) The transverse sinuses lie just inferior to the tentorium and parallel its course. 4) Anteriorly, the superior petrosal sinus joins the transverse sinus, and this junction marks the beginning of the sigmoid sinus. VASCULAR ANATOMY

34. 1) The sigmoid sinus is the posterior boundary of the mastoid cavity. However, in particularly well- pneumatized bones, accessory air cells may extend posteriorly beyond the sigmoid sinus. The sigmoid sinus is most superficial (lateral) at its superior origin. The middle fossa dura approximates the superior portion of the sigmoid sinus at the sinodural angle of Citelli. From the sinodural angle, the sigmoid sinus runs inferiorly and medially, with a variable relationship to the bony labyrinth. At its inferior extent, the sigmoid sinus rises to the jugular bulb. The jugular bulb exhibits considerable variability in its height, location, and relationship to the labyrinth, internal auditory canal (IAC), and tympanic cavity. 2) The inferior petrosal sinus arises from the medial aspect of the jugular bulb and runs anteromedially to the cavernous sinus. 3) The jugular vein exits the skull through the jugular foramen, accompanied by the vagus, glossopharyngeal, and spinal accessory nerves.

35.  The internal carotid artery also travels through the temporal bone. Its entrance, the carotid foramen, is medial to the styloid process and anterior to the jugular foramen. The internal carotid artery travels superiorly until it encounters the dense bone of the cochlea, at which point it makes a 90-degree bend to run anteriorly and medially.  The carotid canal forms the medial wall of the eustachian tube; the internal carotid artery may be dehiscent and vulnerable to injury here.  Rarely, the internal carotid artery may encroach on the tympanic cavity proper

36. Drawing indicating approximate anatomic relationships of the internal carotid artery, superior petrosal sinus, facial nerve, bony labyrinth, and ossicular chain (right temporal bone).

37. Artist’s depiction of the posterior aspect of the right temporal bone, with neurovascular structures.

38. Emissary veins are drainage routes of the dural venous sinuses through the skull that communicate with the superficial veins of the scalp. A fairly constant emissary vein, the mastoid emissary vein, can be found at the junction of the temporal and occipital bones and usually communicates with the occipital or postauricular vein. Griesinger’s sign Tenderness and edema over the mastoid are pathognomonic for suppurative thrombophlebitis of the sigmoid sinus and reflect thrombosis of the mastoid emissary veins.

39. The majority of the cranial nerves are in close anatomic relationship to the temporal bone. Fifth Cranial (Trigeminal) Nerve  The trigeminal (gasserian, semilunar) ganglion lies on the lateral aspect of the anterior petrous apex and indents its surface.  This nerve supplies sensory and motor innervation to the face.  The first two divisions of the trigeminal nerve, the ophthalmic and the maxillary, are sensory only.  The motor branch (portia minor) lies medial to the sensory branch and joins the third division, the mandibular, to supply the muscles of mastication; a small branch supplies the tensor tympani muscle within the middle ear. CRANIAL NERVES IN RELATION TO TEMPORAL BONE

40. Sixth Cranial (Abducens) Nerve • The abducens nerve innervates the ipsilateral lateral rectus muscle. • It exits the brainstem from a groove between the superior medulla and inferior pons and then travels through Dorello’s canal, which is formed by the petroclinoid (Gruber’s) ligament and petrous apex. Inflammatory or neoplastic lesions in the petrous apex can present with lateral rectus palsy. GARDENIGO’S SYNDROME Also called Gradenigo-Lannois syndrome and petrous apicitis, is a complication of otitis media and mastoiditis involving the apex of the petrous temporal bone. It consists of a triad of symptoms consisting of 1)periorbital unilateral pain related to trigeminal nerve involvement, 2)diplopia due to sixth nerve palsy and 3)persistent otorrhea, associated with bacterial otitis media with apex involvement of the petrous part of the temporal bone (petrositis).

41. Seventh Cranial (Facial) Nerve  Preganglionic parasympathetic fibers destined for the pterygopalatine and submandibular ganglions and special sensory (taste) fibers comprise the nervus intermedius. This nerve joins the larger, motor root to form the facial nerve.  In the cerebellopontine angle, the nervus intermedius lies between the facial and cochlear nerves.  The facial nerve enters the temporal bone through the internal auditory canal, which it exits at the meatal foramen to travel anteriorly to the geniculate ganglion. This segment of the facial nerve, the labyrinthine segment, is the narrowest portion (0.61 to 0.68 mm) of the facial canal.  At the geniculate ganglion, the greater petrosal sinus travels anteriorly, carrying parasympathetic fibers to the pterygopalatine ganglion.

42.  The main trunk of the facial nerve turns posteriorly, inferiorly, and laterally to continue in its tympanic (horizontal) segment.  The nerve continues in this course until it turns inferiorly at the lateral semicircular canal (LSCC; the second genu), marking the terminus of the tympanic segment and the beginning of the mastoid segment.  The facial nerve continues to travel inferiorly, posteriorly, and laterally until it exits the temporal bone at the stylomastoid foramen.  Although the chorda tympani nerve usually separates from the mastoid segment of the facial nerve a few millimeters superior to the stylomastoid foramen, the exact location of this separation is quite variable. The chorda tympani nerve traverses the tympanic cavity to carry parasympathetic fibers to the submandibular ganglion and taste fibers to the anterior tongue.  The motor component of the facial nerve supplies the stapedius, posterior digastric, and stylohyoid muscles, as well as the muscles of facial expression.

43. COURSE OF FACIAL NERVE THROUGH TEMPORAL BONE SSC: SUPERIOR SEMICIRCULAR CANAL PSC: POSTERIOR SEMICIRCULAR CANAL LSC: LATERAL SEMICIRCULAR CANAL EAC: EXTERNAL AUDITORY CANAL VII N. : FACIAL NERVE

44. Eighth Cranial (Cochleovestibular) Nerve  The axons of the cochlear division of the eighth nerve arise from the bipolar cells of the spiral ganglion in the cochlea.  From this ganglion, the fibers pass through the modiolus and the foramina of the tractus spiralis foraminosus and into the anterior-inferior portion of the fundus of the internal auditory canal, at which point they fuse to form the cochlear nerve.  The vestibular portion of the eighth nerve divides into a superior and an inferior division in the internal auditory canal. The cell bodies for these nerves are in Scarpa’s ganglion, also located in the canal.  The superior vestibular nerve innervates the utricle, the superior semicircular canal and lateral semicircular canal, and the superior saccule. The inferior vestibular nerve innervates the posterior semicircular canal and the inferior saccule.

45. Ninth Cranial (Glossopharyngeal) Nerve  The glossopharyngeal nerve exits the upper lateral medulla and passes through the jugular foramen, accompanied by the vagus and spinal accessory nerves.  It carries preganglionic parasympathetic fibers to the otic ganglion and taste fibers from the posterior third of the tongue, general sensory afferents from the pharyngeal mucosa, and motor fibers to the stylopharyngeus muscle.  The tympanic branch (Jacobson’s nerve) re- enters the temporal bone through the inferior tympanic canaliculus and emerges onto the promontory to merge with sympathetic fibers at the tympanic plexus, forming the lesser petrosal nerve.  At the cochleariform process, the lesser petrosal nerve travels medial to the semicanal of the tensor tympani muscle to emerge on the floor of the middle cranial fossa.

46. Tenth Cranial (Vagus) Nerve  The vagus nerve is the longest of the cranial nerves.  It arises as 8 to 10 rootlets from the medulla oblongata; these roots unite into the vagus nerve, which passes beneath the flocculus to the jugular foramen and exits the skull within a dural sheath shared with the spinal accessory nerve. Eleventh Cranial (Spinal Accessory) Nerve  Cranial and spinal rootlets combine to form the eleventh nerve.  The spinal component extends to the level of C5 or C6.  These rootlets ascend through the foramen magnum into the cranial cavity, cross the occipital bone, and exit through the jugular foramen.  The spinal accessory nerve innervates the sternocleidomastoid and trapezius muscles. Twelfth Cranial (Hypoglossal) Nerve  The twelfth nerve arises from the medulla and exits the brainstem as a series of rootlets located between the pyramid and olive.  These rootlets fuse to form the hypoglossal nerve that exits the posterior cranial fossa through the hypoglossal canal of the occipital bone.

47. COMPARATIVE ANATOMY In many animals some of these parts stay separate through life: •Squamosal: the squama including the zygomatic process •Tympanic bone: the tympanic part: this is derived from the angular boneof the reptilian lower jaw •Periotic bone: the petrous and mastoid parts •Two parts of the hyoid arch: the styloid process. In the dog these small bones are called tympanohyal (upper) and stylohyal (lower). In evolutionary terms, the temporal bone is derived from the fusion of many bones that are often separate in non-human mammals: •The squamosal bone, which is homologous with the squama, and forms the side of the cranium in many bony fish and tetrapods. Primitively, it is a flattened plate-like bone, but in many animals it is narrower in form, for example, where it forms the boundary between the two temporal fenestrae of diapsid reptiles. •The petrous and mastoid parts of the temporal bone, which derive from the periotic bone, formed from the fusion of a number of bones surrounding the ear of reptiles. The delicate structure of the middle ear, unique to mammals, is generally not protected in marsupials, but in placentals, it is usually enclosed within a bony sheath called the auditory bulla. In many mammals this is a separate tympanic bone derived from the angular bone of the reptilian lower jaw, and, in some cases, it has an additional entotympanic bone. The auditory bulla is homologous with the tympanic part of the temporal bone. •Two parts of the hyoid arch: the styloid process. In the dog the styloid process is represented by a series of 4 articulating bones, from top down tympanohyal, stylohyal, epihyal, ceratohyal; the first two represent the styloid process, and the ceratohyal represents the anterior horns of the hyoid bone and articulates with the basihyal which represents the body of the hyoid bone.

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