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Sphenoid bone

The sphenoid bone is a single, unpaired, irregular bone of the human cranium, characterized by its complex, wedge- or butterfly-shaped structure that spans the midline of the skull base and articulates with nearly every other cranial and several facial bones, earning it the designation as the "keystone" of the skull.^[1] Located centrally at the skull's base, it lies posterior to the frontal bone, anterior to the occipital bone, and inferior to the parietal bones, while contributing to the posterior walls of the orbits, the lateral portions of the nasal cavity, and the floors of the anterior, middle, and posterior cranial fossae.^[2] This bone's intricate anatomy includes a central cuboidal body containing the sella turcica (a saddle-shaped depression that houses the pituitary gland) and bilateral sphenoid sinuses, paired greater wings that extend laterally to form parts of the temporal and orbital walls, paired lesser wings that project anteriorly to roof the orbits, and downward-projecting pterygoid processes with medial and lateral plates for muscle attachments.^[1] The sphenoid bone's structure facilitates critical functions in supporting the brain's weight, transmitting forces from the face to the cranium, and providing passageways for vital neurovascular structures through multiple foramina, including the optic canal for the optic nerve, the superior orbital fissure for cranial nerves III, IV, V1, and VI, the foramen rotundum for the maxillary nerve (V2), the foramen ovale for the mandibular nerve (V3), and the foramen spinosum for the middle meningeal artery.^[2] The sphenoid bone articulates with the frontal, ethmoid, occipital, parietal, temporal, palatine, vomer, and zygomatic bones, creating a rigid framework that integrates the neurocranium with the viscerocranium, while the pterygoid plates serve as origins for the medial and lateral pterygoid muscles, and the greater wings provide attachment sites for the temporalis muscle, key muscles of mastication.^[1] The paired sphenoid sinuses, the most posterior paranasal sinuses, occupy the body and help reduce the skull's weight while contributing to vocal resonance and air humidification.^[2] Embryologically, the sphenoid bone develops from presphenoid, postsphenoid, and orbitosphenoid centers derived from cephalic mesoderm and neural crest cells, with ossification initiating around the eighth week of fetal life and completing postnatally through multiple synchondroses that fuse by early adulthood.^[1] Clinically, its central position makes it susceptible to fractures from trauma, involvement in sphenoid sinusitis, and a key structure in surgical approaches like transsphenoidal hypophysectomy for pituitary adenomas, underscoring its role in protecting and facilitating access to endocrine and neurological elements.^[1]

Overview

Location and orientation

The sphenoid bone occupies a central position at the base of the skull, serving as a keystone that connects the neurocranium to the facial skeleton and forms critical components of the cranial floor.^[1] It is situated in the midline of the cranial cavity, posterior to the frontal bone and anterior to the occipital bone, contributing substantially to the floor of the middle cranial fossa through its greater wings and the anterior walls of this fossa via its lesser wings.^[2] Additionally, the lesser wings help delineate the boundary between the anterior and middle cranial fossae, while the central body contributes to the roof of the posterior nasal cavity.^[3] Characterized by its distinctive butterfly-like shape, the sphenoid bone features a cuboidal central body from which paired lesser and greater wings extend laterally, and pterygoid processes project inferiorly.^[4] The body is oriented forward and downward, positioning its anterior surface toward the nasal region and its superior aspect toward the brain, which houses the sphenoidal sinuses and the sella turcica.^[3] The lesser wings extend horizontally superolaterally from the body, contributing to the roof of the orbits, whereas the greater wings project obliquely in a lateral, superior, and posterior direction, forming parts of the lateral skull walls.^[4] In terms of spatial relationships, the sphenoid bone lies anterior to the temporal bones, with its greater wings articulating along the sphenosquamosal suture, and posterior to the ethmoid bone via the sphenoethmoidal suture on the body.^[1] It is positioned superior to the palatine bones, as the pterygoid processes extend downward to form lateral walls of the nasal cavity and support structures near the hard palate.^[2] These relations underscore its role in bridging multiple cranial and facial elements, though detailed articulations are addressed elsewhere.^[3]

Articulations and connections

The sphenoid bone serves as the keystone of the skull base, articulating with twelve other bones to provide structural integrity and interconnect the neurocranium with the viscerocranium.^[5]^[3] These articulations include the unpaired frontal, ethmoid, vomer, and occipital bones, as well as the paired parietal, temporal, zygomatic, and palatine bones.^[3]^[6] This extensive connectivity underscores its central role in stabilizing the cranium and facilitating the passage of neurovascular structures. Specific joints and sutures define these connections. The sphenoid joins the frontal bone via the sphenofrontal suture, the ethmoid bone via the sphenoethmoidal suture, the occipital bone via the sphenobasilar synchondrosis (a cartilaginous joint that ossifies by early adulthood), and the temporal bone via the sphenosquamosal suture.^[4]^[7]^[1] Additional articulations occur with the palatine bone through the sphenopalatine suture, the vomer via the sphenovomerine suture, and the zygomatic bone via the sphenozygomatic suture.^[3]^[4] Intrinsic to the sphenoid are ligamentous and cartilaginous elements that reinforce its structure. The interclinoid ligaments connect the anterior and posterior clinoid processes of the sphenoid body, while synchondroses form between its multiple ossification centers during development, including remnants like the sphenoethmoidal and sphenopetrosal synchondroses that fuse progressively in postnatal life.^[1]^[8] These articulations contribute to key foramina that transmit critical neurovascular elements. The optic canal, formed between the lesser wing and the sphenoid body, carries the optic nerve (CN II) and ophthalmic artery.^[3] The superior orbital fissure, situated between the lesser and greater wings, transmits cranial nerves III (oculomotor), IV (trochlear), V1 (ophthalmic division of trigeminal), and VI (abducens), along with the superior ophthalmic vein.^[1] Within the greater wings, the foramen rotundum conveys the maxillary nerve (CN V2); the foramen ovale transmits the mandibular nerve (CN V3), accessory meningeal artery, and lesser petrosal nerve; and the foramen spinosum allows passage of the middle meningeal artery and vein.^[3]^[1]

Body of the sphenoid

Superior surface

The superior surface of the body of the sphenoid bone forms part of the floor of the anterior and middle cranial fossae. Anteriorly, it presents the jugum sphenoidale, a smooth, quadrilateral plate bounded by the ethmoidal spines laterally and articulating with the cribriform plate of the ethmoid bone. Posterior to the jugum lies the chiasmatic groove (sulcus chiasmatis), which lodges the optic chiasm and is bounded laterally by the optic canals. The tuberculum sellae marks the posterior end of the chiasmatic groove.^[3] Centrally, the surface features the sella turcica, a saddle-shaped depression divided into the anterior tuberculum sellae, the central hypophyseal fossa that accommodates the pituitary gland, and the posterior dorsum sellae. The dorsum sellae is flanked by the posterior clinoid processes, which serve as attachments for the tentorium cerebelli and petroclinoid ligaments. Laterally, the superior surface transitions to the greater wings, contributing to the middle cranial fossa floor. This surface bears impressions from the frontal and temporal lobes of the brain.^[1]

Inferior surface

The inferior surface of the body of the sphenoid bone is directed toward the nasopharynx and features the sphenoidal rostrum, a downward-projecting, triangular process in the midline that articulates with the posterior aspect of the vomer bone, contributing to the nasal septum. On either side of the rostrum, the vaginal processes project medially and form the vomerovaginal canals with the alae of the vomer.^[9] The majority of this surface forms the roof of the nasopharynx and is perforated by the ostia of the sphenoid sinuses, which open into the sphenoethmoidal recess of the nasal cavity. The sinuses themselves occupy the body, separated by a median septum, and their walls are thin and variable in pneumatization. Laterally, the inferior surface gives attachment to the medial pterygoid plates of the pterygoid processes. This surface helps lighten the skull and resonates during speech.^[1]

Anterior surface

The anterior surface of the sphenoid bone body presents a sloping, quadrilateral expanse directed forward and upward toward the nasal cavity. This surface primarily articulates with the perpendicular plate and labyrinth of the ethmoid bone, providing structural support at the posterior aspect of the nasal region.^[3] In the midline, the surface features a prominent vertical ridge known as the sphenoidal crest (crista sphenoidalis), which articulates directly with the perpendicular plate of the ethmoid bone to form the posterior portion of the nasal septum. At its superior extremity, the crest terminates in a small, forward-projecting ethmoidal spine (also referred to as a small alar spine), serving as an attachment point for ethmoidal structures. Inferiorly, the crest extends into the sphenoidal rostrum, a narrow, triangular projection that descends to form the foundational base of the nasal septum by articulating with the vomer bone.^[10]^[11] Laterally, the anterior surface is bounded by the medial margins of the lesser wings of the sphenoid, creating a smooth, roughened area for broad articulation with the ethmoid bone's lateral masses. On each side of the sphenoidal crest, subtle ethmoidal crests provide attachment sites for the posterior aspects of the superior nasal turbinates (conchae), facilitating the organization of nasal airflow and mucosal drainage. These lateral regions contribute to the overall stability of the ethmoidal complex without dedicated vascular foramina specific to the ethmoidal arteries, which instead traverse nearby orbital structures. The medial borders of the lesser wings, adjacent to this surface, participate in forming the optic canal superolaterally.^[3]^[12]^[1]

Posterior surface

The posterior surface of the body of the sphenoid bone is a rough, quadrilateral area directed posteroinferiorly, forming the anterior portion of the clivus at the base of the skull.^[9] This surface articulates directly with the basilar part of the occipital bone via the spheno-occipital synchondrosis, a cartilaginous joint that typically begins to ossify around age 12–15 and fuses completely by ages 17–25, creating a stable midline junction essential for skull base integrity.^[13] The resulting clivus is a smooth, sloping bony platform that extends inferiorly toward the anterior margin of the foramen magnum, providing structural support between the cranial cavity and the upper cervical spine.^[3] At its superolateral margins, the posterior surface is bounded by the paired posterior clinoid processes, which extend laterally from the dorsum sellae as small, triangular projections.^[14] These processes lie in close proximity to the foramen magnum and serve as key attachment points for dural structures, including the tentorium cerebelli.^[4] The dorsum sellae itself forms the posterior wall of the adjacent sella turcica on the superior surface, enclosing the pituitary fossa.^[3] A shallow groove runs along the posterolateral aspect of this surface for the abducent nerve (cranial nerve VI), which ascends the clivus lateral to the midline before piercing the dura to enter Dorello's canal near the petrous apex.^[15] The posterior clinoid processes also provide attachments for the petroclinoid ligaments, specifically the posterior (Gruber's) ligaments, which span from the clinoid processes to the anterior margin of the petrous part of the temporal bone, forming the superior boundary of Dorello's canal and stabilizing dural reflections.^[16] In the midline, the posterior surface exhibits vascular impressions from the basilar artery, which lies in a longitudinal groove along the clivus, indenting the bone as it courses superiorly from the vertebrobasilar junction.^[10] These impressions highlight the intimate relationship between the sphenoid's posterior aspect and major posterior circulation, influencing surgical approaches to the skull base.^[1]

Sphenoidal sinuses

Structure and location

The sphenoidal sinuses are paired paranasal air-filled cavities housed within the body of the sphenoid bone, a central unpaired bone at the base of the cranium. They occupy a posterior position relative to the nasal cavity and lie inferior to the pituitary fossa, also known as the sella turcica, which forms their superior boundary. These sinuses are separated by a midline septum and are enclosed by the anterior, posterior, superior, and inferior surfaces of the sphenoid body.^[17]^[18]^[1] Each sphenoidal sinus drains through a narrow ostium located on its anterior wall, opening into the sphenoethmoidal recess—a narrow passage in the superior aspect of the nasal cavity that leads to the superior meatus. The mucosal lining of the sinuses consists of pseudostratified ciliated columnar epithelium, which is continuous with the nasal mucosa, facilitating mucus drainage and humidification. In adults, the average volume of each sinus typically ranges from 0.5 to 8 mL, though this can vary based on individual pneumatization extent.^[17]^[19]^[17] Pneumatization patterns of the sphenoidal sinuses are classified into four main types depending on their extension relative to the sella turcica: conchal, where pneumatization is minimal or absent anterior to the sella; presellar, where the sinus extends up to but not beyond the anterior wall of the sella; sellar, where the sinus pneumatizes beneath the sella between its anterior and posterior walls; and postsellar, where the sinus extends posterior to the sellar floor. These patterns influence the overall size and shape of the sinuses but do not alter their fundamental drainage or lining characteristics.^[19]^[20] Anatomically, the medial walls of the paired sphenoidal sinuses contribute to forming portions of the nasal septum, while their lateral walls are in close proximity to the cavernous sinuses, which contain critical neurovascular structures. This positioning underscores the sinuses' central role in the posterior cranial base, adjacent to the optic nerves superiorly and the internal carotid arteries laterally.^[17]^[19]

Variations and clinical notes

The sphenoid sinus exhibits several patterns of pneumatization, classified based on the extent of aeration relative to the sella turcica: the conchal type features minimal or no pneumatization, with the sinus limited to the presellar region; the presellar type extends anteriorly to the anterior wall of the sella; the sellar type reaches beneath the pituitary fossa; and the postsellar type extends posterior to the dorsum sellae.^[21] These variations influence surgical planning, as the degree of pneumatization affects the proximity to critical neurovascular structures. Prevalence differs across populations; for instance, in one study of 114 pneumatized sinuses, conchal type occurred in 5.2%, presellar in 26.3%, and sellar in 68.4%, while another analysis of 113 patients reported conchal at 1.8%, presellar at 7.3%, sellar at 47.7%, and postsellar at 43.3%.^[21]^[22] Sellar and postsellar types are generally the most common, comprising over 80% in many cohorts.^[20] Asymmetry in sphenoid sinus size and shape is frequent, often due to a deviated intersinus septum that divides the sinus into unequal cavities, leading to an irregular sella turcica outline on imaging.^[23] Septation variations, including multiple or accessory septa, further contribute to this asymmetry and can complicate endoscopic access during transsphenoidal procedures by altering the sinus cavity's configuration and potentially obstructing the surgical corridor.^[24] Such septa may attach to the internal carotid artery (ICA) or optic nerve canal, increasing the risk of inadvertent injury if not identified preoperatively via CT imaging.^[25] Clinically, the sphenoid sinus's relations to the optic nerve and ICA are critical, with dehiscence or protrusion of these structures into the sinus occurring at varying rates; for instance, ICA protrusion is observed in 26.3% of cases, optic nerve protrusion in 13%, and optic nerve dehiscence in 1.5%, heightening risks of visual or vascular complications during sinus surgery or pituitary interventions.^[26] The average ostium diameter measures 2-3 mm, which can limit drainage and ventilation, predisposing to pathology or challenging balloon dilation techniques.^[27] In children, pneumatization is often incomplete, beginning around 2 years of age and reaching full extent by 10-12 years, with only 90% showing pneumatization by age 4 and 100% by age 10; this developmental delay necessitates age-adjusted imaging interpretations to avoid misdiagnosis of hypoplasia.^[28]

Lesser wings

Superior surface

The superior surface of each lesser wing of the sphenoid bone is flat and forms the posterior part of the floor of the anterior cranial fossa, providing support for the frontal lobe of the brain.^[1] This surface is continuous medially with the jugum sphenoidale and articulates anteriorly with the frontal bone along the sphenoidal ridges.^[29]

Inferior surface

The inferior surface of each lesser wing of the sphenoid bone is a smooth, triangular area that forms the posterior portion of the roof of the orbit, providing support for orbital contents including fat and associated structures.^[29] This surface is adapted to accommodate the levator palpebrae superioris muscle, which originates from its inferior aspect superior to the annulus of Zinn, facilitating eyelid elevation.^[30] Additionally, the region near the orbital apex supports attachments related to the extraocular rectus muscles via the common tendinous ring, which encircles the optic canal at the base of the lesser wing.^[31] The posterior margin of the inferior surface contributes to the superior orbital fissure, a key passageway located lateral to the optic canal, through which structures such as the oculomotor, trochlear, abducens, and ophthalmic nerves, along with the superior ophthalmic vein, transmit into the orbit.^[4] The optic canal itself is formed medially by the space between the two roots of the lesser wing and the adjacent body of the sphenoid bone, allowing passage of the optic nerve and ophthalmic artery.^[32] On this surface, subtle vascular grooves may accommodate the course of the ophthalmic artery after its entry via the optic canal, aiding its distribution within the orbital cavity.^[4] The lesser wing measures approximately 1 to 2 mm in thickness, rendering it particularly fragile and prone to fractures in cases of orbital or cranial trauma, which can lead to complications such as optic nerve compression or cerebrospinal fluid leakage.^[1]

Greater wings

Superior surface

The greater wings of the sphenoid bone are broad, quadrilateral extensions that project laterally from the sides of the body, forming the majority of the floor of the middle cranial fossa.^[1] This superior surface of each greater wing is concave and irregular, accommodating the basal aspects of the frontal and temporal lobes of the cerebrum.^[1] The surface bears subtle impressions corresponding to the gyri and sulci of these cerebral lobes, providing a molded interface for the brain's inferior surface.^[33] Prominent on this surface are shallow sulci that accommodate the branches of the middle meningeal artery, which supplies the dura mater and calvaria after entering through the foramen spinosum.^[1] Three key foramina perforate the greater wings near their medial margins, facilitating the passage of neurovascular structures into the cranial cavity: the foramen rotundum transmits the maxillary nerve (CN V2); the foramen ovale conveys the mandibular nerve (CN V3), accessory meningeal artery, and lesser petrosal nerve; and the foramen spinosum allows entry of the middle meningeal artery and vein.^[1] The infratemporal crest is a transverse ridge on the greater wing that demarcates the superior (cerebral or temporal) surface from the inferior (infratemporal) surface, serving as an attachment site for the upper fibers of the lateral pterygoid muscle.^[1]

Temporal surface

The temporal surface of the greater wing of the sphenoid bone constitutes the superior portion of its lateral aspect, separated inferiorly by the infratemporal crest, and faces the temporal fossa.^[10] This surface is rough and concave anteroposteriorly, providing attachment for the superficial fibers of the temporalis muscle, which originates along its extent to contribute to mastication.^[1] The infratemporal crest itself serves as an additional origin for the superior fibers of the lateral pterygoid muscle, though this attachment pertains more directly to the adjacent infratemporal surface.^[1] Anteriorly, the temporal surface articulates with the zygomatic bone and the greater wing of the sphenoid meets the frontal bone near the pterion, while posteriorly it forms the sphenosquamosal suture with the squamous portion of the temporal bone.^[10] At the posteroinferior angle of the greater wing lies the sphenoidal spine, a downward-projecting process that anchors the superior end of the sphenomandibular ligament, which extends to the lingula of the mandible and helps stabilize the temporomandibular joint.^[34] The spine's medial surface occasionally presents a shallow groove traversed by the chorda tympani nerve as it emerges from the petrotympanic fissure and descends toward the infratemporal fossa.^[35] This surface maintains close anatomical relations with branches of the mandibular division of the trigeminal nerve (CN V3), including the lingual and inferior alveolar nerves, due to their emergence via the nearby foramen ovale located along the posterior aspect of the greater wing.^[1] These neurovascular structures underscore the temporal surface's role in the infratemporal region's complex wiring, though the surface itself does not directly transmit them.

Orbital and infratemporal surfaces

The orbital surface of the greater wing of the sphenoid bone constitutes the posterior portion of the lateral orbital wall, providing a smooth, concave expanse that accommodates the periorbita, the periosteal lining of the orbit. This surface articulates anteriorly with the orbital process of the zygomatic bone, contributing to the structural integrity of the orbital cavity. The superior orbital fissure, a narrow cleft situated between the lesser and greater wings of the sphenoid, demarcates the boundary of this surface superiorly and serves as a conduit for the oculomotor nerve (CN III), trochlear nerve (CN IV), abducens nerve (CN VI), ophthalmic division of the trigeminal nerve (CN V1), and superior ophthalmic vein. Small vascular foramina on the orbital surface transmit branches of the ophthalmic artery and vein, supporting orbital circulation. The infratemporal surface lies on the medial aspect of the greater wing, inferior to the infratemporal crest, and forms the superior boundary of the infratemporal fossa, a key space for masticatory structures. This roughened surface provides the primary origin for the superior head of the lateral pterygoid muscle, facilitating mandibular movements such as protrusion and lateral deviation. The foramen rotundum, located anteriorly within the greater wing near this surface, transmits the maxillary nerve (CN V2) from the middle cranial fossa into the adjacent pterygopalatine fossa.^[1]

Pterygoid processes

Lateral pterygoid plate

The lateral pterygoid plate is a thin, curved bony lamina that constitutes the lateral division of the pterygoid process, projecting inferiorly from the junction of the sphenoid bone's body and greater wing. This structure arises as part of the paired pterygoid processes, which extend posteroinferiorly and bifurcate into medial and lateral plates separated by a V-shaped pterygoid fossa posteriorly. The plate is broader superiorly and tapers toward its inferior extremity, forming a horseshoe-like configuration when viewed with its medial counterpart.^[36]^[37] The lateral surface of the plate faces the infratemporal fossa, serving as its medial wall and providing structural support for the contents of this space, including branches of the maxillary artery and divisions of the mandibular nerve. The infratemporal surface, particularly its upper portion, offers the primary origin site for the inferior head of the lateral pterygoid muscle. In contrast, the medial surface of the plate contributes to the pterygoid fossa and gives attachment to the deep head of the medial pterygoid muscle, facilitating the integration of these masticatory elements. At its root, the plate forms part of the posterior boundary of the pterygopalatine fossa, with the anterior opening of the pterygoid canal located nearby for the passage of the nerve of the pterygoid canal.^[1]^[37]^[36] Although the lateral pterygoid plate does not form direct synovial articulations, it relates indirectly to the maxilla through the shared boundaries of the infratemporal and pterygopalatine fossae, influencing the spatial arrangement of adjacent bony elements. Its position underscores the sphenoid bone's role in bridging cranial and facial structures, with the plate's curvature adapting to the lateral expansion of the skull base.^[36]

Medial pterygoid plate

The medial pterygoid plate constitutes the medial lamina of each pterygoid process, extending inferiorly as a thin, quadrangular bony projection from the posteroinferior aspect of the sphenoid body, near its junction with the greater wings. This plate orients nearly parallel to the midline, forming the lateral boundary of the nasopharynx and contributing to the medial wall of the pterygopalatine fossa. Its posterior border remains free, lacking direct bony articulation, while the anterior border articulates with adjacent structures such as the perpendicular plate of the palatine bone.^[4]^[38]^[39] In its upper portion, the medial pterygoid plate features a subtle perpendicular orientation, with the posterosuperior region exhibiting a bifurcated posterior border that encloses the small, triangular scaphoid fossa, a shallow depression serving as the origin for the tensor veli palatini muscle. This fossa lies immediately superior to the pterygoid fossa and facilitates the muscle's passage toward the soft palate. The upper anterior margin includes a curved vomerine process, a short projection that articulates with the inferior border of the vomer bone, thereby supporting the posterior nasal septum. The posterior border in this region also anchors the pharyngobasilar fascia, a key component of the pharyngeal wall.^[38]^[39]^[4] The lower portion of the medial pterygoid plate tapers inferiorly, with its anterior margin articulating directly with the perpendicular plate of the palatine bone to form part of the lateral pterygoid wall. Terminating at the plate's inferior extremity is the pterygoid hamulus, a slender, hook-shaped bony process that curves laterally and serves as a pulley for the tendon of the tensor veli palatini muscle while providing attachment for the palatopharyngeal ligament and select fibers of the superior pharyngeal constrictor muscle. This configuration aids in elevating and tensing the soft palate during swallowing and phonation. Additionally, the superior aspect of the plate's base houses the medial opening of the palatovaginal canal (also termed the pharyngeal canal), a short bony passage that transmits the pharyngeal branch of the maxillary nerve (from the pterygopalatine ganglion) along with pharyngeal arteries and veins to innervate and vascularize the nasopharyngeal mucosa.^[36]^[4]^[1]

Development and ossification

Presphenoid ossification

The presphenoid bone originates from the orbito-sphenoidal component of the chondrocranium, a cartilaginous template that forms during the early fetal period as bilateral mesenchymal condensations anterior to the basisphenoid. By Carnegie stage 18 (approximately 6 weeks gestation), these condensations become evident as bilateral presphenoid elements, fusing in the midline by stage 20 to connect with the mesethmoid cartilage. This cartilaginous precursor provides the structural foundation for the anterior aspects of the sphenoid, including the rostrum and the anterior portion of the body, while the lesser wings derive from the adjacent orbitosphenoid cartilage that integrates with the presphenoid during development. The lesser wings ossify endochondrally from the orbitosphenoid centers starting around 12-16 weeks gestation, integrating with the presphenoid.^[1] Ossification of the presphenoid initiates from a primary center in the third fetal month, around 12-17 weeks gestation with variations reported in literature, typically involving bilateral centers medial to the optic struts that contribute to the anterior body.^[40]^[41] Additional accessory centers may appear sequentially, including anterior and posterior pairs, leading to a layered ossification pattern that progresses from the chiasmatic sulcus region laterally and inferiorly. By 17–18 weeks gestation, the main presphenoid centers are visible on imaging, with growth occurring proportionally in sagittal diameter, projection surface area, and volume during the late second to third trimester, though transverse diameter grows logarithmically.^[41]^[42] The presphenoid fuses with the postsphenoid via the intersphenoidal synchondrosis, which begins to close postnatally around 4 months of age, though complete ossification of the sphenoid body may extend into early childhood.^[43] This fusion process facilitates the onset of sphenoid sinus pneumatization, where epithelial invaginations into the presphenoid cartilage during the fourth gestational month evolve into air cells that expand starting around age 3 years, initially in posterior and inferior directions.^[44]^[45] Developmental anomalies such as presphenoid hypoplasia can result from disrupted ossification centers, leading to underdevelopment of the anterior cranial base and associated orbital dystopia, where the orbits exhibit vertical misalignment due to inadequate support from the hypoplastic bone. Such defects are often linked to genetic or syndromic conditions affecting chondrocranial growth, emphasizing the presphenoid's role in maintaining orbital symmetry.^[46]^[47]

Postsphenoid ossification

The postsphenoid, also known as the basisphenoid, represents the posterior portion of the sphenoid bone body and undergoes endochondral ossification from multiple centers during early fetal development. It typically develops from two medial and two lateral ossification centers, with the medial centers appearing first and often fusing into a single unit to form the sella turcica, enclosing the pituitary gland; these centers emerge around 8–10 weeks of gestation (approximately the 3rd fetal month), with visible ossification progressing by 11–13 weeks.^[48]^[49]^[50] The postsphenoid contributes to the posterior body of the sphenoid, including the dorsum sellae and clivus, while the medial pterygoid plates arise from its cartilaginous precursor; the greater wings, however, ossify primarily from the adjacent alisphenoid cartilage via intramembranous ossification starting at 8 weeks, though they integrate with postsphenoid elements later.^[1]^[49]^[51] As part of the basibregmatic region of the chondrocranium, the postsphenoid fuses with the presphenoid anteriorly via the intrasphenoidal synchondrosis, which typically ossifies shortly before or around birth, and with the basioccipital posteriorly via the spheno-occipital synchondrosis, which remains patent until puberty and completes fusion by ages 15–25 years in most individuals.^[50]^[49]^[52] This progressive fusion contributes to the complete ossification of the skull base by early adulthood, around 20–25 years. The pterygoid plates ossify from separate centers, with the medial plate developing endochondrally from postsphenoid cartilage around 12–16 weeks and the lateral plate via intramembranous ossification starting around 8-9 weeks gestation.^[1]^[39] Variations in postsphenoid ossification include delayed fusion of the medial centers or persistence of the intrasphenoidal or spheno-occipital synchondroses beyond typical timelines, potentially resulting in incomplete bony enclosure of the pituitary or prolonged cartilage remnants into adolescence; such delays occur in a minority of cases and may influence skull base morphology.^[48]^[53]^[52]

Functions

Structural support

The sphenoid bone functions as the keystone of the cranium, articulating with twelve other bones to maintain the overall integrity and stability of the skull.^[3] These articulations include the unpaired frontal, ethmoid, vomer, and occipital bones, as well as the paired parietal, temporal, zygomatic, and palatine bones, enabling the bone to anchor the neurocranium to the facial skeleton and distribute mechanical forces from activities such as mastication and external impacts across the cranial structure.^[3]^[1] This central positioning allows the sphenoid to act as a primary buttress, bridging the cranial and facial regions while transmitting loads to prevent localized stress concentrations.^[3] Within the sphenoid body lies the sella turcica, a saddle-shaped depression that cradles and protects the pituitary gland, ensuring its safeguarding amid the skull's foundational architecture.^[1] The lesser wings of the sphenoid contribute to this supportive framework by forming the posterior boundary of the anterior cranial fossa and the anterior margin of the middle cranial fossa, thereby separating these compartments and aiding in the even dispersal of intracranial pressures.^[3] In contrast, the greater wings extend laterally to reinforce the temporal and orbital regions, composing part of the skull's lateral walls and the floor of the middle cranial fossa to bolster resistance against lateral forces.^[1]^[4] Load-bearing capacity is further enhanced by the sphenoid body and its clinoid processes, which serve as key attachment points for dural folds like the tentorium cerebelli, distributing the weight of the brain and stabilizing the skull base against vertical and horizontal stresses.^[3] The anterior clinoid processes project from the lesser wings, while the posterior ones arise from the dorsum sellae, collectively reinforcing the central architecture to uphold the cranium's biomechanical equilibrium.^[1] This integrated design underscores the sphenoid's pivotal role in sustaining the skull's form and resilience.^[4]

Muscle and ligament attachments

The sphenoid bone serves as a critical attachment site for numerous muscles and ligaments, facilitating key functions such as eye movement, mastication, and deglutition.^[1]

Muscles

Several extraocular muscles originate from the sphenoid bone, particularly at the annulus of Zinn located at the orbital apex, enabling precise control of eye movements. The superior rectus muscle arises from the superior portion of the annulus of Zinn on the sphenoid, contributing to elevation, adduction, and intorsion of the eyeball.^[54] Similarly, the inferior rectus originates from the inferior part of the annulus, facilitating depression, adduction, and extorsion.^[54] The medial rectus attaches to the medial aspect of the annulus, promoting adduction, while the lateral rectus originates from the lateral portion, aiding abduction.^[54] The superior oblique muscle originates from the periosteum of the sphenoid body superomedial to the optic foramen, passing through the trochlea to support depression, abduction, and intorsion.^[54] On the greater wing, the temporalis muscle attaches to the temporal surface, assisting in mastication by elevating the mandible.^[1] The pterygoid processes provide origins for the pterygoid muscles; the lateral pterygoid arises from the infratemporal crest and lateral surface of the lateral pterygoid plate, promoting protrusion and lateral deviation of the mandible during chewing.^[1] The medial pterygoid originates from the medial surface of the lateral pterygoid plate and the pterygoid fossa, contributing to elevation and medial movement of the jaw.^[1] Additionally, the tensor veli palatini muscle originates from the scaphoid fossa at the base of the medial pterygoid plate and the spine of the sphenoid bone, with its tendon hooking around the pterygoid hamulus to insert on the palatine aponeurosis, aiding in tensing the soft palate during swallowing and opening the Eustachian tube.^[55]

Ligaments

The sphenomandibular ligament attaches superiorly to the spine of the sphenoid bone and inferiorly to the lingula of the mandible, providing passive stabilization to the temporomandibular joint.^[56] The pterygospinous ligament spans from the upper part of the posterior border of the lateral pterygoid plate to the spine of the sphenoid, potentially ossifying and serving as a boundary for infratemporal structures.^[1]^[57] Caroticoclinoid ligaments connect the anterior clinoid process to the middle clinoid process or the petrous apex, occasionally ossifying and relating to the cavernous sinus, through which nerves such as the oculomotor, trochlear, and abducens pass en route to the orbit.^[1] These attachments on the pterygoid plates and other surfaces integrate the sphenoid into coordinated soft tissue dynamics.^[1]

Clinical significance

Fractures and trauma

Fractures of the sphenoid bone are relatively uncommon but carry significant morbidity due to the bone's central location at the skull base and its proximity to critical neurovascular structures. These injuries typically result from high-energy trauma, such as motor vehicle accidents or falls, and are often associated with other facial or cranial fractures.^[58] Sphenoid fractures can be categorized by their anatomical location. Central body fractures often arise from direct facial trauma, involving the sphenoid sinus and potentially extending to the sella turcica. Greater and lesser wing fractures are characteristic of basal skull injuries, where the wings may fracture transversely or obliquely, leading to orbital or middle cranial fossa involvement. Pterygoid process fractures frequently occur as part of Le Fort midfacial fracture patterns, with all Le Fort types (I, II, and III) involving separation at the pterygoid plates.^[59]^[60] The incidence of sphenoid fractures among patients with facial trauma varies across studies, ranging from approximately 3% to 15%, with higher rates in high-energy mechanisms. These fractures are frequently accompanied by cerebrospinal fluid (CSF) leaks, occurring in up to 30% of basal skull cases involving the sphenoid, and vision loss due to optic nerve compression or orbital apex involvement.^[58]^[61] Classification of sphenoid fractures distinguishes between direct and indirect mechanisms. Direct fractures result from blunt force applied to the midface or skull base, causing immediate disruption. Indirect fractures occur via transmission of force, such as from mandibular impacts propagating upward through the pterygoid processes. The pattern of fracture lines may be influenced by the bone's developmental ossification centers and their fusion, which can create preferential planes of weakness.^[59]^[1] Complications of sphenoid fractures are severe and multifactorial. Vascular injuries, including internal carotid artery dissection or cavernous sinus fistula, arise particularly with fractures involving the carotid canal, carrying a risk of up to 35% for significant vascular compromise. Cranial nerve palsies affecting nerves III (oculomotor), IV (trochlear), and VI (abducens) are common, resulting from compression at the superior orbital fissure or cavernous sinus, leading to ophthalmoplegia or ptosis. Diagnosis relies on high-resolution computed tomography (CT) imaging with thin-section multiplanar reconstructions to delineate fracture extent and associated injuries.^[59]^[62] The sphenoid sinuses, paired air-filled cavities within the body of the sphenoid bone, are susceptible to inflammatory conditions such as sinusitis, which can manifest as acute or chronic forms. Acute sphenoid sinusitis typically arises from bacterial or viral infections and presents with severe headache, often retro-orbital or occipital, alongside symptoms like fever and nasal congestion, while chronic sphenoid sinusitis persists beyond 12 weeks and may involve persistent mucopurulent discharge and facial pressure.^[63]^[64] Vision changes, including diplopia or blurred vision, can occur due to proximity to the optic nerve and cavernous sinus.^[65] A critical complication is the spread of infection to the cavernous sinus, leading to thrombophlebitis or thrombosis, which may result in cranial nerve palsies, permanent visual loss, or life-threatening conditions like meningitis.^[66]^[67]^[68] Tumors involving the sphenoid sinuses often erode the surrounding bone, leading to structural compromise and neurological deficits. Sphenoid mucoceles, cystic expansions of the sinus lined by respiratory epithelium, can cause bony resorption of the sphenoid body or clivus, resulting in symptoms such as headache and diplopia from compression of adjacent neurovascular structures.^[69]^[70] Pituitary adenomas, particularly ectopic variants, may originate or extend into the sphenoid sinus or clivus, eroding the sellar floor and leading to hormonal hypersecretion or hypopituitarism.^[71]^[72] Chordomas at the clivus, arising from notochordal remnants, frequently invade the sphenoid bone, presenting with cranial neuropathies and requiring multidisciplinary management due to their locally aggressive nature.^[73]^[74] Transsphenoidal surgery represents the primary approach for accessing and resecting pathologies within the sphenoid sinus and sella turcica, particularly for pituitary adenomas, involving an endoscopic route through the nasal cavity to the sphenoid sinus.^[75] This minimally invasive technique allows precise tumor removal but carries risks including cerebrospinal fluid (CSF) rhinorrhea, occurring in 0.5-4% of cases due to dural tears or incomplete sellar reconstruction.^[76]^[77] Hormonal imbalances, such as diabetes insipidus from posterior pituitary injury or hyponatremia, affect up to 20-30% of patients postoperatively, necessitating vigilant endocrine monitoring.^[78]^[79]^[80] Isolated sphenoid sinusitis accounts for 1-3% of all paranasal sinus infections, often underdiagnosed due to nonspecific symptoms, with anatomical variations such as dehiscent carotid canals or optic nerves increasing the risk of vascular or orbital complications during disease progression or intervention.^[81]^[82]^[83] These variations, present in up to 40% of individuals, heighten the potential for intracranial spread in inflammatory or neoplastic processes.^[26]

Comparative anatomy

In mammals

The sphenoid bone in mammals exhibits a homologous structure consisting of a central body, greater and lesser wings, and pterygoid processes, which are consistently present across species and contribute to the cranial base, orbit formation, and muscle attachments.^[84] These elements provide structural continuity for the neurocranium, though significant variations occur in their proportions and pneumatization, particularly in sinus size; for instance, the sphenoid sinus is notably large in equids like horses, where it extends extensively and connects with other paranasal sinuses.^[85] In contrast, the sphenoid sinus is absent or minimal in some carnivores, such as dogs, highlighting adaptive differences in cranial weight and sensory specialization.^[84] In rodents, the sphenoid features reduced greater wings adapted to the compact skull, with prominent pterygoid processes that anchor masticatory muscles in a laterally oriented temporalis fossa.^[86] Primates, including both quadrupedal and bipedal forms, display an expanded sphenoid body and wings that enhance cranial base flexion and balance, particularly in hominoids where the angled basicranium supports upright posture by redistributing skull mass over the vertebral column.^[87] Carnivores possess a more robust sphenoid overall, with thickened pterygoid plates and body to withstand high bite forces transmitted via the masseter and pterygoid muscles during predation.^[88] Herbivores, such as ruminants, often show extensive pneumatization of the sphenoid body and wings, forming large sinuses that reduce cranial weight while maintaining structural integrity for grazing adaptations.^[89] Fusion patterns of the sphenoid components—presphenoid, basisphenoid, and alisphenoid—typically complete in adult mammals, resulting in a single ossified unit that strengthens the skull base, unlike the more segmented, cartilaginous configuration retained in reptiles. This mammalian fusion occurs progressively postnatally, with variations in timing; for example, it is more rapid in humans compared to most other mammals.^[90]

Evolutionary aspects

The sphenoid bone in mammals traces its phylogenetic origins to distinct reptilian elements, primarily the parasphenoid and pleurosphenoid, which contributed to the braincase structure. In reptiles, the parasphenoid serves as an unpaired dermal bone forming the floor of the braincase, extending anteriorly from the basisphenoid and providing attachment for jaw adductor muscles.^[91] The pleurosphenoid, a cartilaginous or ossified sidewall element derived from the pila antotica, reinforces the lateral braincase and supports orbital structures.^[92] During the transition to synapsids in the late Carboniferous to Permian periods, these elements began integrating within the expanding braincase, with the parasphenoid fusing into the basisphenoid and the pleurosphenoid evolving toward the alisphenoid component, marking early steps toward the composite mammalian sphenoid. Fossil evidence from early therapsids, such as Permian cynodonts like Thrinaxodon, reveals these elements as largely separate, with underdeveloped orbitosphenoids (presphenoid precursors) and epipterygoids (alisphenoid precursors) leaving much of the braincase cartilaginous and open.^[93] Ossification and fusion progressed in advanced cynodonts during the Triassic, including expansion of the orbitosphenoid and alisphenoid to enclose the braincase more fully, as seen in probainognathians like Brasilodon. By the Early Jurassic, true mammals and mammaliaforms, such as those from the Yanliao Biota, exhibit complete integration of these elements into a single sphenoid bone, correlating with increased encephalization and a rigid cranial base.^[93] Key adaptations in mammalian evolution include the expansion of the middle cranial fossa, facilitated by the greater wings of the sphenoid, which accommodated rapid brain growth, particularly in primates where temporal lobe enlargement drove anterolateral projection of the sphenoid's anterior pole.^[94] This shift enhanced cognitive capacity by providing structural support for expanded cerebral hemispheres. Additionally, sphenoid sinus development emerged in higher mammals, pneumatizing the bone's body through outgrowths from the nasal cavity, potentially aiding vocal resonance by modifying sound transmission, though primary functions likely involve structural lightening and force dissipation during mastication.^[95] Functionally, the sphenoid transitioned from a reptilian role in jaw support—where the parasphenoid and pleurosphenoid buttressed adductor muscles and the quadrate-articular joint—to a central mammalian cranial base element anchoring the pituitary fossa, optic chiasm, and major neurovascular foramina, thereby stabilizing the expanded brain and facilitating endothermy-related metabolic demands.^[96] This reconfiguration underscores the sphenoid's pivotal role in the synapsid-mammal transition, enabling the shift from reptilian ectothermy to mammalian homeostasis.

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The base of the cranium (i.e. the basioccipital, the sphenoid and the temporal bones) undergoes significant morphological change within the hominin lineage, and ...
[89]
Craniometric Characteristics of Selected Carnivora Species Kept in ...
May 1, 2024 · Lateral and medial pterygoid muscles run from the sphenoid bone, pterygoid hamulus, and pterygoid bone, inserting into the medial surface of the ...
[90]
Sphenoid sinus in the Holstein cow: A sectional and 3D ...
Nov 4, 2023 · The median part, which pneumatized the body of the sphenoid bone, was observed bilaterally in seven animals, while the alar part, which ...
[91]
Presphenoid bone - vet-Anatomy - IMAIOS
The two parts that fuse early on in Men in order to form the sphenoid bone, stay separated much longer in the other Mammals ... Comparative anatomy in humans.
[92]
Fetal and perinatal morphogenesis of the sphenoid bone in primates
Aug 16, 2021 · These data are highly suggestive of synchondroseal fusion, and indicate that the ISS may begin to fuse immediately before birth or during early ...
[93]
The evolutionary and morphological history of the parasphenoid ...
Jun 23, 2015 · Here we investigate the parasphenoid bone across the major vertebrate lineages (amphibians, reptiles, mammals and teleosts) including both ...
[94]
Palaeos Vertebrates: Bones: Braincase: Sphenoid: Pleurosphenoid
This important article leads us back to the middle of the braincase and a re-evaluation of a number of structures, including the pleurosphenoid.
[95]
Craniodental anatomy in Permian–Jurassic Cynodontia and ... - NIH
May 15, 2023 · Mammalia can be identified in the fossil record by the occurrence of over 37 osteological characters, including a squamosal-dentary jaw ...
[96]
[PDF] Middle Cranial Fossa Anatomy and the Origin of Modern Humans
Recent developmental evidence in primates points to a bidirectional relationship of physical brain expansion, basicranial development, and growth under genetic ...
[97]
The Paranasal Sinuses: The Last Frontier in Craniofacial Biology
Oct 24, 2008 · This special issue of the Anatomical Record explores the presence and diversity of paranasal sinuses in distinct vertebrate groups.
[98]
Evolution of the mammalian middle ear and jaw - PubMed Central
Jun 11, 2012 · Mammals have three middle ear ossicles, unlike one in reptiles/birds. The malleus and incus are homologous to jaw bones, and the new jaw joint ...

Sphenoid bone

Overview

Location and orientation

Articulations and connections

Body of the sphenoid

Superior surface

Inferior surface

Anterior surface

Posterior surface

Sphenoidal sinuses

Structure and location

Variations and clinical notes

Lesser wings

Superior surface

Inferior surface

Greater wings

Superior surface

Temporal surface

Orbital and infratemporal surfaces

Pterygoid processes

Lateral pterygoid plate

Medial pterygoid plate

Development and ossification

Presphenoid ossification

Postsphenoid ossification

Functions

Structural support

Muscle and ligament attachments

Muscles

Ligaments

Clinical significance

Fractures and trauma

Sinus-related pathologies

Comparative anatomy

In mammals

Evolutionary aspects

References