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Zygomatic bone
Zygomatic bone
Zygomatic bone
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Zygomatic bone
Position of the zygomatic bone
Animation of the zygomatic bone
Details
Part ofSkull
ArticulationsMaxilla, temporal bone, sphenoid bone and frontal bone
Identifiers
Latinos zygomaticum, zygoma
TA98A02.1.14.001
TA2818
FMA52747
Anatomical terms of bone

In the human skull, the zygomatic bone (from Ancient Greek: ζῠγόν, romanized: zugón, lit. 'yoke'), also called cheekbone or malar bone, is a paired irregular bone, situated at the upper and lateral part of the face and forming part of the lateral wall and floor of the orbit, of the temporal fossa and the infratemporal fossa. It presents a malar and a temporal surface; four processes (the frontosphenoidal, orbital, maxillary, and temporal), and four borders.

Etymology

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The term zygomatic derives from the Ancient Greek Ζυγόμα, zygoma, meaning "yoke". The zygomatic bone is occasionally referred to as the zygoma, but this term may also refer to the zygomatic arch.

Structure

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Surfaces

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The malar surface is convex and perforated near its center by a small aperture, the zygomaticofacial foramen, for the passage of the zygomaticofacial nerve and vessels; below this foramen is a slight elevation, which gives origin to the zygomaticus muscle.

The temporal surface, directed posteriorly and medially, is concave, presenting medially a rough, triangular area, for articulation with the maxilla (articular surface), and laterally a smooth, concave surface, the upper part of which forms the anterior boundary of the temporal fossa, the lower a part of the infratemporal fossa. Near the center of this surface is the zygomaticotemporal foramen for the transmission of the zygomaticotemporal nerve.

The orbital surface forms the lateral part and some of the inferior part of the bony orbit. The zygomatic nerve passes through the zygomatic-orbital foramen on this surface. The lateral palpebral ligament attaches to a small protuberance called the orbital tubercle.

Processes

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Each zygomatic bone is diamond-shaped and composed of three processes with similarly named associated bony articulations: frontal, temporal, and maxillary. Each process of the zygomatic bone forms important structures of the skull.

The orbital surface of the frontal process of the zygomatic bone forms the anterior lateral orbital wall, with usually a small paired foramen, the zygomaticofacial foramen opening on its lateral surface. The temporal process of the zygomatic bone forms the zygomatic arch along with the zygomatic process of the temporal bone, with a paired zygomaticotemporal foramen present on the medial deep surface of the bone. The orbital surface of the maxillary process of the zygomatic bone forms a part of the infraorbital rim and a small part of the anterior part of the lateral orbital wall.[1]

Orbital process

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The orbital process is a thick, strong plate, projecting backward and medialward from the orbital margin. Its antero-medial surface forms, by its junction with the orbital surface of the maxilla and with the great wing of the sphenoid, part of the floor and lateral wall of the orbit. On it are seen the orifices of two canals, the zygomatico-orbital foramina; one of these canals opens into the temporal fossa, the other on the malar surface of the bone; the former transmits the zygomaticotemporal, the latter the zygomaticofacial nerve.

  • Its postero-lateral surface, smooth and convex, forms parts of the temporal and infratemporal fossae.
  • Its anterior margin, smooth and rounded, is part of the circumference of the orbit.
  • Its superior margin, rough, and directed horizontally, articulates with the frontal bone behind the zygomatic process.
  • Its posterior margin is serrated for articulation, with the great wing of the sphenoid and the orbital surface of the maxilla.

At the angle of junction of the sphenoidal and maxillary portions, a short, concave, non-articular part is generally seen; this forms the anterior boundary of the inferior orbital fissure: occasionally, this non-articular part is absent, the fissure then being completed by the junction of the maxilla and sphenoid, or by the interposition of a small sutural bone in the angular interval between them.

Borders

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The antero-superior or orbital border is smooth, concave, and forms a considerable part of the circumference of the orbit.

The antero-inferior or maxillary border is rough, and bevelled at the expense of its inner table, to articulate with the maxilla; near the orbital margin it gives origin to the quadratus labii superioris.

The postero-superior or temporal border, curved like an italic letter f, is continuous above with the commencement of the temporal line, and below with the upper border of the zygomatic arch; the temporal fascia is attached to it.

The postero-inferior or zygomatic border affords attachment by its rough edge to the masseter.

Articulations

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The zygomatic bone articulates with the frontal bone, sphenoid bone, and paired temporal bones, and maxillary bones.

Development

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The zygomatic bone is generally described as ossifying from three centers—one for the malar and two for the orbital portion; these appear about the eighth week and fuse about the fifth month of fetal life.

Mall describes it as being ossified from one center which appears just beneath and to the lateral side of the orbit.

After birth, the bone is sometimes divided by a horizontal suture into an upper larger, and a lower smaller division.

In some quadrumana the zygomatic bone consisted of two parts, an orbital and a malar.

Society and culture

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The high cheekbones of model Natasha Poly (left) and Abraham Lincoln (right)

Zygomatic arches, also known as high cheek bones, are considered physically attractive in some cultures, in both males and females.[2][3]

Ancient Chinese sculptures of goddesses typically have a "broad forehead, raised eyebrows, high cheekbones, and large, sensuous mouth".[4] Similarly, many depictions of Qin warriors in the Terracotta Army are depicted with "broad foreheads, high cheekbones, large eyes, thick eyebrows, and stiff beards."[4]

For this reason some individuals undergo cheek augmentation, a form of cosmetic surgery.[5]

Animals

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The zygomatic is homologous to the jugal bone of other tetrapods.

Non-mammalian vertebrates

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Diagram showing homologous bones of the skulls of a Monitor lizard and a Crocodile. Jugal bone labelled Ju, in pale green, at centre left.
This jugal bone from an Edmontosaurus is over three feet (1 meter) long

In non-mammalian vertebrates, the zygomatic bone is referred to as the jugal bone, since these animals have no zygomatic arch. It is found in most reptiles, amphibians, and birds. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species.

This bone is considered key in the determination of general traits of the skull, as in the case of creatures, such as dinosaurs in paleontology, whose entire skull has not been found. In coelacanths and early tetrapods the bone is relatively large. Here, it is a plate-like bone forming the lower margin of the orbit and much of the side of the face. In ray-finned fishes it is reduced or absent, and the entire cheek region is generally small. The bone is also absent in living amphibians.[6]

With the exception of turtles, the jugal bone in reptiles forms a relatively narrow bar separating the orbit from the inferior temporal fenestra, of which it may also form the lower boundary. The bone is similarly reduced in birds. In mammals, it takes on broadly the form seen in humans, with the bar between the orbit and fenestra vanishing entirely, and only the lower boundary of the fenestra remaining, as the zygomatic arch.[6]

Additional images

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  • Left zygomatic bone, malar surface
    Left zygomatic bone, malar surface
  • Left zygomatic bone, temporal surface
    Left zygomatic bone, temporal surface
  • Left infratemporal fossa
    Left infratemporal fossa

See also

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This article uses anatomical terminology.

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Zygomatic bone

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from Grokipedia
The zygomatic bone, also known as the cheekbone or malar bone, is a paired, that forms the prominence of the and defines the anterior and lateral portions of the face. The term "zygomatic" derives from the zygōma (ζυγόμα), meaning "yoke" or "bar," alluding to the it helps form. "Malar" derives from Latin mala, meaning "". It is roughly quadrangular in shape and contributes to the skeletal framework of the , cheeks, and parts of the temporal and infratemporal fossae. Located in the upper lateral part of the face, each zygomatic bone protrudes laterally to create the cheek's contour and articulates with the frontal, maxillary, temporal, and sphenoid bones.

Overview and Terminology

Location and Description

The zygomatic bone, also known as the zygoma or malar bone, is a paired situated in the upper and lateral part of the face. It forms the prominence of the , contributes to the lateral wall and floor of the , and participates in the boundaries of the temporal and infratemporal fossae. This bone articulates with the frontal, sphenoid, temporal, and maxillary bones, thereby integrating into the lateral aspect of the and providing structural continuity across the . The zygomatic bone exhibits a quadrangular or diamond-shaped morphology, consisting of a central body from which three processes—frontal, temporal, and maxillary—extend, with approximate dimensions of 3 to 4 cm in length, 2 cm in height, and up to 1 cm in thickness at certain points. Its external surface is covered by compact cortical , while the interior contains cancellous for metabolic support, and it features foramina that facilitate vascular ingress for blood supply.

Etymology and Synonyms

The term "zygomatic" derives from the zygōma, meaning "" or "bar," which in turn stems from zygon, referring to a used to join draft animals; this nomenclature reflects the bone's function in bridging the cranial and skeletons near the and temporal region. The Latin designation os zygomaticum emerged later, attributed to the 17th-century anatomist Jean Riolan the Younger, emphasizing its yoke-like connective role in human anatomy. Common synonyms for the zygomatic bone include malar bone, derived from the Latin māla (cheek), highlighting its contribution to the cheek's prominence; cheekbone, a descriptive term; and jugal bone, from Latin iugum (), primarily used in for homologous structures in vertebrates. The bone is occasionally simply called the zygoma, though this term more precisely denotes the formed by the temporal process of the zygomatic bone and the of the . Historically, the zygomatic bone was first described in detail by the Roman physician (c. 129–200 AD) in his Elementary Course on Bones, where he referred to it as the zygoma while delineating its sutures and relations to adjacent facial structures. In modern anatomical nomenclature, the Terminologia Anatomica (1998, updated 2019) officially designates it as os zygomaticum, standardizing its use in international medical and scientific contexts.

Gross Anatomy

Surfaces

The zygomatic bone features three distinct surfaces: the lateral (malar), medial (temporal), and orbital surfaces, each contributing to specific anatomical relations within the facial skeleton. The lateral surface, also termed the malar surface, is convex and positioned subcutaneously, forming the prominent contour of the cheek. This surface is smooth and provides attachment sites for facial muscles, including the origin of the zygomaticus major muscle from its anterolateral aspect and the zygomaticus minor muscle from a more posteromedial portion adjacent to the zygomaticomaxillary suture. It is perforated by the zygomaticofacial foramen, which transmits the zygomaticofacial nerve (a sensory branch of the maxillary division of the trigeminal nerve) along with accompanying zygomaticofacial artery and vein branches. The medial surface, known as the temporal surface, is concave and oriented toward the temporal and infratemporal fossae, forming part of their lateral boundaries. This surface is rougher in texture compared to the lateral aspect and includes the zygomaticotemporal , through which the (another branch of the from the maxillary division) emerges to supply sensation to the temple region. The orbital surface is smooth, slightly concave, and constitutes a significant portion of the orbit's anterolateral floor and anterior lateral wall, articulating with the frontal, maxillary, and sphenoid bones. It contributes to the infraorbital rim and supports the orbital contents, including the origin nearby, while occasionally featuring a zygomatico-orbital foramen that communicates with internal canals leading to the zygomaticofacial and zygomaticotemporal foramina.

Processes

The zygomatic bone features three principal processes extending from its central body: the frontal, temporal, and maxillary processes, which provide key structural extensions for the facial skeleton. The frontal process is a robust projection that extends superiorly and slightly posteriorly from the upper margin of the zygomatic bone, delineating the lateral aspect of the orbital margin. It possesses a thick structure with a serrated superior margin and includes distinct surfaces: an orbital surface facing inward toward the orbit and a temporal surface oriented laterally. On the orbital surface, Whitnall's tubercle appears as a small bony prominence situated just within the orbital opening, approximately 11 mm below the superior border, and is observed in approximately 95% of skulls. The temporal process arises from the lower half of the zygomatic bone as an elongated, flattened extension that projects posteriorly and slightly superiorly in a gently arched configuration. Its posterior terminus is characterized by an oblique and serrated margin, enhancing its structural integrity as a posterior extension. The maxillary process originates from the anterosuperior angle of the , extending inferiorly and anteriorly as a pointed, anteromedial projection that contributes to the inferolateral orbital margin and the orbital floor. This process maintains continuity posteriorly with the bone's orbital surface, integrating seamlessly with the surrounding morphology.

Borders and Articulations

The zygomatic bone possesses four distinct borders that delineate its quadrilateral shape and facilitate its connections within the facial skeleton. The anteroinferior border, also known as the maxillary border, articulates with the maxilla through the zygomaticomaxillary suture, forming a key junction in the infraorbital region. The posterosuperior border, or temporal border, is sinuous and convex superiorly while concave inferiorly, contributing to the formation of the zygomatic arch as it meets the temporal bone. The orbital border, situated superiorly, is smooth and concave, defining the inferolateral margin of the orbit and separating the orbital and lateral surfaces of the bone. The lateral border, referred to as the zygomaticofrontal border, extends along the frontal process and joins the frontal bone, establishing the superolateral orbital rim. The zygomatic bone articulates with four adjacent cranial bones via fibrous sutures, which are predominantly serrated to enhance stability. It connects superiorly to the at the frontozygomatic suture, a serrated that reinforces the lateral orbital wall. Laterally, it joins the through the zygomaticotemporal suture, forming a sutured bony bridge known as the via the temporal process of the zygomatic bone and the of the . Inferiorly, the zygomaticomaxillary suture links it to the in a serrated , supporting the midfacial . Additionally, it articulates posteriorly with the greater wing of the via the sphenozygomatic suture, another serrated connection that contributes to orbital integrity. These borders and articulations have significant anatomical implications, particularly in defining the positions of foramina and sites for muscle attachments. The anteroinferior and lateral borders accommodate the zygomaticofacial foramen, which transmits the zygomaticofacial nerve and vessels, while the posterosuperior border features the zygomaticotemporal foramen for the corresponding nerve branch. Muscle origins along these edges include the on the posteroinferior aspect of the and the zygomaticus major and minor on the lateral border, influencing and mastication.

Function

Structural and Biomechanical Roles

The zygomatic bone serves as a critical structural in the , providing vertical support that connects the midfacial region to the and facilitates the transmission of masticatory forces from the to the skull base. This role is essential for maintaining the integrity of the facial framework during , where forces generated by the masseter and temporalis muscles are dissipated through the zygomaticomaxillary complex. The bone's robust , including its integration with the , enables it to act as a load-bearing pillar within the broader facial , which comprises vertical and horizontal struts that collectively resist deformation under mechanical stress. Biomechanically, the zygomatic bone and its arch are adapted to withstand a combination of compressive, tensile, and shear stresses during mastication and lateral impacts. The arch, formed by the temporal process of the zygomatic bone and the zygomatic process of the temporal bone, experiences bending moments that place its superior aspect in tension and inferior aspect in compression, optimizing force distribution to prevent localized failure. This configuration allows the structure to resist lateral forces effectively, such as those from side-to-side jaw movements, while minimizing strain propagation to adjacent cranial elements. In terms of load-bearing, the zygomatic bone contributes to the orbital floor and lateral wall, providing support that prevents inferior displacement or herniation of orbital contents under vertical loading. Quantitative analyses reveal variations in that enhance structural strength, with higher cortical bone thickness observed in the compared to trabecular regions, correlating with increased resistance to compressive loads. Finite element models of the zygomatic pillar during simulated masseter contraction demonstrate peak von Mises stresses concentrated at the sutural interfaces, such as the zygomaticofrontal and zygomaticomaxillary sutures, under typical masticatory forces, underscoring the bone's role in stress dissipation through these articulations. These models confirm that the zygomatic bone's geometry efficiently redirects forces away from vulnerable areas, maintaining skeletal stability.

Contributions to Facial Form and Movement

The zygomatic bone plays a pivotal role in defining the aesthetic contours of the midface, particularly through its contribution to the prominence of the cheekbones, which are widely regarded as a marker of attractiveness in diverse cultural contexts. High cheekbones, formed by the lateral projection of the zygomatic bone, create a sculpted appearance that enhances and perceived youthfulness, influencing standards across Western, Asian, and other global populations where pronounced midfacial structure is often idealized. This prominence not only provides structural definition but also supports the overlying soft tissues, contributing to the overall harmony of facial features that is subjectively evaluated in social and evolutionary terms. In terms of muscle attachments, the zygomatic bone serves as a critical origin point for several essential to expression and mastication. The muscles arise from the anterolateral surface of the zygomatic bone, enabling the elevation of the upper lip and corner of the mouth to produce smiling and other positive facial expressions, thereby facilitating and social interaction. Additionally, the medial surface and provide attachment for the , a primary adductor that originates from the inferior border of the arch and , powering the forceful closure of the during and supporting efficient mastication. The bone further contributes to facial movement by forming part of the structural framework that supports the (TMJ), where its arch and articulations with the temporal and maxillary bones provide stability for mandibular excursions during jaw opening, closing, and lateral movements. As the lateral rim of the , the zygomatic bone also aids in eye protection by reinforcing the bony enclosure against lateral impacts and maintaining the position of the globe during dynamic facial actions like blinking or head turning. These roles underscore its integration with biomechanical support for coordinated facial dynamics. Evolutionarily, variations in the exhibit , with males typically displaying larger and more robust structures compared to females, a trait linked to displays of physical prowess and mate attraction in ancestral populations. This dimorphism, evident in greater projection and mass of the in men, reflects adaptations in craniofacial morphology that may have enhanced visual signaling in social and reproductive contexts, while population-specific differences further highlight its role in diverse evolutionary histories.

Development and Embryology

Ossification Centers

The zygomatic bone forms through , a in which develops directly from mesenchymal without a cartilaginous intermediate. The primary emerges in the body of the bone, positioned in the below and lateral to the developing , during the eighth prenatal week (approximately the 56th day of embryonic development). This center initially appears as a small ossification, which rapidly expands to encompass the main body and extend into the forming processes. Some embryological studies describe the possibility of additional centers, with up to three contributing to the bone's formation: one primary center for the malar (body) portion and two secondary centers associated with the orbital process. These secondary centers arise shortly after the primary one and fuse into a single cohesive structure by around the 22nd week of . The main center expands by the third prenatal month, with the orbital portion elongating significantly to form the upper margins of the . Histologically, begins with of mesenchymal cells into a fibrous , followed by differentiation into osteoprogenitor cells and osteoblasts that deposit matrix along vascular channels. Blood vessels invade the ossifying tissue, supplying nutrients and enabling the recruitment of osteoclasts for remodeling, resulting in the formation of woven that later matures into lamellar . This vascular pattern supports the rapid expansion of the bone's surfaces and processes. The primary center and any accessory ones integrate by late fetal stages, establishing the bone's basic architecture at birth. Continued growth and remodeling through , driven by functional forces from mastication and facial musculature, lead to full structural maturity, with the bone achieving adult size and density by late teens.

Embryonic Development and Variations

The zygomatic bone originates from the of the first , which is primarily populated by cells that migrate from the dorsal during the fourth week of . These -derived cells form the core of the maxillary prominence, a key facial process that gives rise to the zygomatic bone through subsequent differentiation and patterning. The influence of cells ensures the integration of the zygomatic bone into the broader craniofacial complex, supporting its role in orbital and maxillary architecture. Key developmental stages begin with mesenchymal condensation around 5-6 weeks of , marking the initial aggregation of cells destined to form the zygomatic anlage. regulation plays a critical role in positioning and identity specification of derivatives, including the first arch components that contribute to the zygomatic bone. This genetic control establishes proximodistal and dorsoventral axes, ensuring proper alignment with adjacent structures like the and . Developmental variations of the zygomatic bone are uncommon but can include or , often linked to disruptions in migration or arch patterning. In , a mandibulofacial dysostosis, symmetric of the zygomatic bones is a hallmark feature, resulting from mutations in the TCOF1 gene and affecting approximately 1 in 50,000 live births. Total of the zygomatic bone has been reported in severe cases of this syndrome, leading to significant midfacial underdevelopment. Facial asymmetry occurs in 12-37% of individuals evaluated for orthodontic concerns, often as a mild congenital variation without syndromic association, and may involve the zygomatic region. Modern insights emphasize the role of BMP and FGF signaling pathways in craniofacial patterning, particularly in regulating proliferation and differentiation within the first . BMP signaling promotes ventral identity in the maxillary prominence, while FGF ligands from the overlying drive mesenchymal growth and essential for zygomatic precursor formation. These pathways interact antagonistically to fine-tune skeletal element size and shape, with disruptions contributing to hypoplastic variations.

Clinical Significance

Fractures and Trauma

The zygomaticomaxillary complex (ZMC) fracture represents the most prevalent injury involving the zygomatic bone, typically resulting from blunt that disrupts the bone's articulations with the , , and . These fractures often manifest as tripod or tetrapod patterns, where the bone detaches at three or four key suture lines, including the zygomaticofrontal, zygomaticomaxillary, infraorbital, and zygomaticotemporal sutures, leading to displacement of the zygomatic body. Common mechanisms of ZMC fractures include high-impact lateral forces from assaults, falls, motor vehicle accidents, or sports-related injuries, which exploit the bone's lateral prominence and relative fixation points to cause inward rotation and posterior displacement. Such forces often exceed the biomechanical tolerance of the zygomatic buttresses, resulting in a characteristic "tripod" configuration when three primary attachments fail, though tetrapod variants incorporate additional infraorbital rim involvement. Symptoms of ZMC fractures frequently include periorbital ecchymosis, due to orbital volume increase, from extraocular muscle entrapment or fat herniation, and secondary to impingement. Classification systems such as Rowe and Williams, which categorize fractures based on stability after elevation (e.g., stable arch-only versus unstable comminuted types), and Knight and North, which delineate six groups by fracture line extent and displacement (e.g., Group IV for classic ), aid in assessing severity and guiding initial management. Complications from untreated or improperly managed ZMC fractures encompass , which can produce persistent facial asymmetry and altered mastication, as well as heightened infection risk owing to the fracture's proximity to the and potential for sinus communication. Additional risks include chronic sensory deficits from involvement and orbital sequelae like persistent if orbital floor disruption is overlooked. Iatrogenic damage is a rare complication of surgical intervention, with sensory recovery occurring in the majority of affected cases within months.

Surgical Interventions and Imaging

Imaging techniques play a crucial role in diagnosing and planning interventions for the zygomatic bone. Computed tomography (CT) scans are considered the gold standard for evaluating zygomatic fractures, providing detailed multiplanar views and enabling three-dimensional (3D) reconstructions to assess displacement and involvement of adjacent structures. Plain X-rays, such as the Waters or submentovertex views, serve as an initial screening tool to identify gross abnormalities in the and complex, though they lack the precision of CT for complex cases. has limited utility in primary bone evaluation but can assess involvement or complications such as impingement in select cases. Surgical interventions for zygomatic bone injuries primarily involve open reduction and (ORIF) to restore anatomical alignment and function. The Gillies temporal approach provides access to the through a small incision in the temporal , minimizing visible scarring and allowing for elevation and fixation of depressed fractures. An intraoral approach, often via a maxillary vestibular incision, enables direct access to the zygomaticomaxillary buttress for reduction and plating without external scars, reducing risks of injury. miniplates are widely used for stabilization due to their , strength, and resistance to , typically applied at two or three points (frontozygomatic suture, infraorbital rim, and zygomaticomaxillary buttress) to ensure rigid fixation. In cosmetic procedures, zygoma reduction (malarplasty) addresses prominent or asymmetric zygomatic bones by and setback, often guided by preoperative simulations to achieve facial harmony. Augmentation with implants, such as custom alloplastic materials, enhances zygomatic projection in cases of , improving aesthetic contours. Recent advances since 2020 include 3D-printed surgical guides, which enhance precision in both reduction malarplasty and implant placement by aligning with patient-specific anatomy derived from CT data, reducing operative time and improving outcomes. As of 2025, 3D-printed bioceramic implants have emerged for reconstructing zygomatic bone defects, offering promising efficacy and safety in clinical trials. Surgical outcomes for zygomatic interventions demonstrate high success rates, with patient satisfaction reaching approximately 98% in restoring facial aesthetics and function following ORIF.

Comparative Anatomy

In Mammals

In mammals, the zygomatic bone is homologous to the jugal bone observed in other tetrapods, serving as a key component of the craniofacial that has evolved to support diverse feeding and structural adaptations across species. This homology underscores its conserved role in forming the , which provides anchorage for masticatory muscles and contributes to orbital protection. Notable variations in form occur in response to ecological niches; for instance, in herbivores like , the zygomatic bone is enlarged and robust, extending anteriorly to form a prominent crest that enhances attachment sites for the , enabling sustained grinding of fibrous during . In contrast, aquatic mammals such as cetaceans exhibit a reduced zygomatic bone, often diminished to a slender rod-like structure in odontocetes like the , reflecting adaptations to streamlined skulls and reduced masticatory demands in a suction-feeding . Among , the zygomatic bone is particularly prominent, forming a broad arch that widens the and supports expansive temporalis and s, as seen in robust forms like baboons and apes where it bolsters lateral breadth for powerful biting. Sexual dimorphism is evident in carnivores, with males typically displaying larger and more pronounced zygomatic arches compared to females, likely tied to agonistic behaviors and greater bite capabilities.

In Non-Mammalian Vertebrates

In non-mammalian vertebrates, the zygomatic bone's homologue is the jugal bone, a dermal element that originated in early gnathostomes during the period as part of the circumorbital series of bones, initially associated with the opercular apparatus for cover support in . This bone provided lateral covering to the region and structural reinforcement to the emerging apparatus in primitive osteichthyans, such as sarcopterygians, where it separated from the preopercular bone to form a distinct plate beneath the . In subsequent evolution, the jugal became integrated into the skull, modifying its role from opercular involvement to primary contributions in facial bracing and orbital framing, while retaining dermal origins traceable to early assemblages. In fish and amphibians, the jugal is typically absent or vestigial, reflecting its reduction in aquatic and semi-aquatic forms where robust facial buttressing is less critical. For instance, in modern anurans (frogs), the jugal is entirely lost, resulting in no contribution to the orbital wall, which is instead bounded primarily by the maxilla and squamosal; this absence simplifies the skull and aligns with the loss of other dermal elements like the postorbital and lacrimal. Early tetrapods, such as Devonian amphibians, show nascent dermal bone origins for the jugal, but it remains rudimentary compared to later forms, emphasizing its evolutionary transition from fish-like suborbital plates. Among reptiles, the jugal is prominently developed as a key component of the lateral wall, articulating with the anteriorly, postorbital dorsally, and quadratojugal posteriorly to form the infratemporal and postorbital bars. In squamates ( and snakes), the jugal exhibits elongation along its ventral process, supporting jaw adduction by bracing the quadrate and facilitating kinetic movements, though it is reduced or absent in some snake lineages. These adaptations highlight phylogenetic differences, with the reptilian jugal prioritizing mechanical reinforcement over the lighter configurations seen elsewhere. In birds, the jugal adopts a specialized, lightweight morphology as a thin, strut-like rod forming the jugal bar, which spans the ventral margin of the cranium from the to the quadrate without contributing to a full . This reduction minimizes weight for flight while maintaining orbital stability and transmitting forces from the beak to the braincase, differing markedly from the triradiate, load-bearing form in basal reptiles. In some advanced avian lineages, the jugal fuses proximally with the quadratojugal, further streamlining the structure for .

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