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Sternum

The sternum, commonly known as the breastbone, is a long, flat, partially T-shaped bone that forms the central anterior portion of the thoracic cage in humans.^[1] It consists of three primary segments: the superior manubrium, the elongated central body (or mesosternum), and the inferior cartilaginous xiphoid process, which gradually ossifies with age.^[1] The sternum measures approximately 17 cm in length in adults and serves as a critical attachment point for the clavicles and the costal cartilages of the first seven ribs, forming the sternoclavicular and sternocostal joints.^[1] Through these articulations, it contributes to the structural integrity of the chest wall, protecting vital mediastinal organs such as the heart and major blood vessels.^[2] Functionally, the sternum plays a key role in respiration by providing a stable yet flexible anchor for rib movements, allowing the thoracic cavity to expand during inhalation as the sternum displaces anteriorly and superiorly.^[2] Its development involves endochondral ossification, beginning in utero with multiple ossification centers that fuse progressively; fusion between the manubrium and body occurs variably in late adulthood, often remaining incomplete or after age 40, while the xiphoid process may remain partially cartilaginous until the fourth or fifth decade of life, with full ossification occurring variably by age 40 to 60.^[1] Blood supply to the sternum is derived primarily from the internal thoracic artery and its perforating branches, with venous drainage via accompanying veins and lymphatic flow toward axillary and internal thoracic nodes.^[1] Clinically, the sternum is a vital surgical landmark, often accessed via median sternotomy for procedures like coronary artery bypass grafting or thymectomy, due to its central position overlying the heart.^[1] Congenital anomalies, such as cleft sternum (incidence of 1 in 50,000 to 100,000 births) or pectus excavatum (1 in 400 to 1,000 births), can affect its form and function, potentially leading to respiratory or cardiac complications.^[1] Additionally, its dimensions and ossification patterns aid in forensic anthropology for sex estimation, though age determination is less reliable due to individual variability.^[1]

Anatomy

Manubrium

The manubrium is the broad, superior segment of the sternum, characterized by a thick, trapezoidal shape that widens superiorly.^[3] In adults, it measures approximately 5 cm in length (superoinferior dimension), with a width of about 5 cm at its broadest superior point.^[4]^[5] Positioned at the level of the third and fourth thoracic vertebrae, it forms the uppermost portion of the anterior thoracic wall.^[1] The anterior surface of the manubrium is smooth and slightly convex transversely, providing minimal attachment sites for soft tissues.^[6] In contrast, the posterior surface is rougher and concave, facilitating attachments for muscles such as the sternohyoid and sternothyroid.^[7] At its superior border lies the jugular notch, also known as the suprasternal notch, a U-shaped depression that serves as a palpable landmark between the clavicles.^[8] The lateral borders feature smooth depressions for the clavicular articulations and costal facets. The manubrium articulates superiorly and laterally with the medial ends of the clavicles via the sternoclavicular joints, which are reinforced by ligaments for stability.^[1] It also connects to the costal cartilages of the first two ribs: a full facet for the first rib on each side and a demifacet for the superior half of the second rib.^[8] Inferiorly, it joins the body of the sternum at the manubriosternal joint, a secondary cartilaginous synchondrosis.^[3] Vascular supply to the manubrium is provided by medial horizontal branches of the internal thoracic arteries, which arise from the subclavian arteries and course along the posterior aspect of the sternum.^[1] These branches ensure nutrient delivery to the bone and adjacent structures.

Body

The body of the sternum, also known as the mesosternum or gladiolus, forms the central, elongated portion of the sternum and is its longest segment. It articulates superiorly with the manubrium at the manubriosternal joint.^[1] This flat bone is roughly quadrilateral in outline, longer and narrower than the manubrium, and typically measures about 10 cm in length in adults, with a width that increases inferiorly to a maximum of around 4 cm near the level of the fifth rib.^[9] Developmentally derived from four separate sternebrae, the body fuses into a single unit by early adulthood, marked by three horizontal transverse ridges that delineate the former segments.^[10] The anterior surface is nearly flat and slightly convex, directed forward and upward, featuring these faint transverse ridges that indicate the sites of sternebrae fusion; it also provides attachment for muscles such as the pectoralis major.^[11] The posterior surface is slightly concave, with less prominent transverse lines and small demifacets for articulation with the costal cartilages of the ribs.^[11] Nutrient foramina on the posterior surface allow entry of vascular branches from the internal thoracic arteries to supply the bone.^[1] Along its lateral borders, the body bears seven costal notches that articulate with the costal cartilages of ribs 2 through 7, forming the sternocostal joints; these notches are shallow superiorly and progressively deepen inferiorly to accommodate the rib attachments more securely.^[1]

Xiphoid process

The xiphoid process, also known as the xiphisternum, is the smallest and most inferior component of the sternum, forming a small, cartilaginous structure at birth that typically measures 2 to 5 cm in length.^[12]^[13] It is primarily composed of hyaline cartilage proximally and elastic cartilage distally, often exhibiting a triangular or pointed shape that is directed inferiorly or slightly posteriorly, though it attaches to the inferior border of the sternal body via the xiphisternal joint, a symphysis that may ossify with age.^[12]^[13] Shape variations are common, including bifid (forked), perforated (with foramina), elongated, curved, or broad forms, which do not typically impact function but can influence palpation.^[12] This structure serves as a key insertion point for several abdominal muscles, including the rectus abdominis anteriorly and the diaphragm posteriorly, contributing to trunk stability and aiding in the anchoring of the linea alba at its tip.^[12] Ossification patterns vary widely, beginning in childhood with centers appearing between ages 5 and 18; it typically progresses variably, with completion occurring as late as 40 to 60 years and remaining incomplete in some adults.^[12]^[13] The xiphisternal joint allows limited mobility in youth, but ossification reduces this flexibility over time. In many cases, ossification is incomplete throughout life, preserving a degree of flexibility.^[13] Clinically, the xiphoid process is readily palpable in the epigastric region at the level of the T10 vertebra, serving as a critical anatomical landmark for procedures such as cardiopulmonary resuscitation (CPR), where hand placement is positioned just superior to it to avoid fracture, and for pericardiocentesis or other subxiphoid injections.^[12]^[13] Its superficial position and variability necessitate careful assessment during physical examinations to distinguish it from potential pathologies like xiphoid syndrome, which involves localized pain and swelling.^[12]

Joints and ligaments

The sternoclavicular joint is a saddle-type synovial joint that articulates the medial end of the clavicle with the clavicular notch of the manubrium and the first costal cartilage, featuring an intra-articular fibrocartilaginous disc that divides the joint cavity and permits limited multidirectional movement.^[14] This disc attaches superiorly to the clavicle and inferiorly to the first costal cartilage, enhancing stability while allowing approximately 35° of elevation-depression, 70° of protraction-retraction, and 45° of axial rotation.^[14] The joint is primarily stabilized by the anterior and posterior sternoclavicular ligaments, which extend from the clavicle to the manubrium and prevent excessive displacement, as well as the costoclavicular ligament, which anchors the clavicle to the first rib and provides the strongest restraint against superior and anteroposterior translation.^[15] An additional interclavicular ligament connects the medial ends of both clavicles over the manubrium, reinforcing superior stability.^[15] The costosternal joints, also known as sternocostal joints, connect the costal cartilages of the first seven ribs to the lateral margins of the sternum, with the first joint typically forming a synchondrosis and the second through seventh as synovial plane joints enclosed by fibrous capsules.^[16] These joints facilitate thoracic cage expansion during respiration, with the second joint often featuring an intra-articular ligament that partially divides its cavity.^[16] Stability is provided by the radiate sternocostal ligaments, which fan out from each costal cartilage to the sternum's anterior and posterior surfaces, supplemented by intra-articular ligaments in select joints.^[17] Intrastemal joints unite the segments of the sternum, including the manubriosternal joint—a symphysis between the manubrium and body at the sternal angle—and the xiphisternal joint between the body and xiphoid process, both typically cartilaginous unions reinforced by intersternebral ligaments and periosteal fibers.^[16] The manubriosternal joint allows slight anteroposterior and vertical mobility, while the xiphisternal joint exhibits variable fibrocartilaginous characteristics that permit limited movement, often ossifying into a synostosis by middle age.^[18] These connections are supported by fibrous discs or hyaline cartilage discs that maintain alignment during thoracic excursions.^[16] Biomechanically, the sternum's joints exhibit limited mobility to safeguard thoracic viscera, with ligaments such as the costoclavicular and radiate sternocostal types constraining excessive translation and rotation during upper limb and respiratory movements, thereby distributing forces across the thoracic ring.^[19] This design ensures the sternum acts as a stable anchor for the ribs, minimizing displacement risks while accommodating the 13 articulations per thoracic level.^[19]

Development and variations

Embryonic development

The sternum originates from paired bands of mesenchymal tissue derived from the parietal layer of the lateral plate mesoderm in the ventral body wall, forming around the sixth week of gestation.^[1] These sternal bands, initially located lateral to the midline, consist of concentrations of mesenchymal cells that condense and migrate medially toward the developing heart tube.^[1] By the seventh week, the mesenchyme within these bands undergoes chondrification to form a primary cartilaginous model, divided into three main segments—the future manubrium, body, and xiphoid process—comprising up to six cartilaginous sternebrae.^[1] The paired bands then fuse progressively in a craniocaudal direction along the midline, starting around the ninth week, to create a single sternal plate by the tenth week; this fusion process establishes the foundational structure that will later ossify.^[1]^[20] Ossification of the sternum proceeds via endochondral mechanisms, beginning in utero. The manubrium develops its primary ossification center around the fifth fetal month (approximately 20 weeks gestation), followed by the first sternebra between 5 and 7 months in utero, with additional centers appearing progressively in the body segments.^[1]^[21] The sternebrae of the sternal body typically initiate ossification around birth or shortly thereafter, enveloped by growth plates that facilitate expansion.^[22] Neural crest cells contribute to the development of the sternoclavicular joint, particularly through the interclavicular mesenchyme that connects the forming manubrium to the clavicles; this mesenchyme shares an origin with neural crest-derived portions of the clavicle undergoing endochondral ossification.^[23] Fusion of the ossified sternebrae occurs gradually during childhood and adolescence, typically completing by the late teens to early twenties, while the xiphoid process ossifies and fuses later, often remaining partially cartilaginous into adulthood.^[1]^[24]

Anatomical variations

The sternum exhibits several anatomical variations arising from differences in ossification and fusion during development. One common variation is the sternal foramen, a perforation typically resulting from incomplete fusion of the sternebrae in the lower third of the sternal body. This oval or round defect has a prevalence of approximately 5-8% in the general population, though reported rates range from 2.5% to 13.8% across studies, and it measures about 6 mm in average diameter.^[25]^[26] Asymmetry in the sternum can manifest in the costal notches and sternebrae fusion lines. The costal notches, which articulate with the costal cartilages of ribs 2-7, may show displacement, such as the second costal notch being misplaced (prevalence ~2.6%) or the seventh notch shifted (prevalence ~13.2%), often more frequently on the right side. Fusion lines between sternebrae, normally complete by age 25, can remain visible or irregular in some individuals, leading to non-fusion of segments and subtle asymmetric contours in the sternal body.^[27]^[26] Variations in the xiphoid process include differences in elongation, curvature, and shape. The process may elongate beyond 4 cm, exhibit ventral or dorsal deflection, or adopt hook-like, reverse "S," or V-shaped forms, with bifid or trifid endings occurring in about 32.8% and 4.6% of cases, respectively. These morphological differences, such as broad, thin, or curved structures, are observed in up to 37% of individuals for double-ended forms.^[28]^[26] Suprasternal ossicles, small accessory bones in the jugular notch, represent a rare variation with a prevalence of 1.5-6.9%, though higher rates up to 15.5% have been reported in specific populations; they appear as unilateral, bilateral, or midline pyramidal or ovoid structures.^[26]^[27] Population differences influence the incidence of these variations. Sternal foramina show higher prevalence in South American (up to 13.9%) and African groups compared to European or Asian populations, with some studies noting elevated rates in indigenous Bolivians (12.8-13.4%). Suprasternal ossicles are more common in Asian populations (up to 50%) than in Europeans (12.2%), while costal notch displacements and xiphoid trifurcations also vary by ethnicity, with the latter more frequent in Asians (7%). Gender trends are inconsistent but often indicate slightly higher rates of foramina and ossicles in males.^[29]^[27]^[25]

Function

Respiratory role

The sternum functions as the central anchor for the rib cage, allowing elevation of the ribs through the costosternal joints during inspiration, which expands the thoracic cavity to facilitate air intake.^[1] This attachment via costal cartilages to the second through seventh ribs enables the dynamic movements essential for respiration, with the sternum's stability providing a fixed point for rib articulation.^[1] The stability of the sternum supports the characteristic bucket-handle motion of the lower ribs, which increases the transverse diameter of the thorax, and the pump-handle motion of the upper ribs, which elevates the sternum anteriorly to enhance the anterior-posterior diameter.^[30] These motions are driven by the contraction of external intercostal muscles, allowing the ribs to pivot outward and upward relative to the sternum. In synergy with the diaphragm, which descends to increase the vertical dimension of the thorax, and the intercostal muscles, the sternum elevates the anterior chest wall, collectively optimizing lung expansion during quiet breathing.^[30] The xiphoid process further aids this process by serving as an attachment site for the diaphragm, integrating sternal mechanics with primary respiratory musculature.^[1] In pathological conditions such as ankylosing spondylitis, inflammation and fusion at the costosternal joints restrict sternal and rib mobility, leading to reduced chest expansion and impaired ventilation.^[31] Patients exhibit diminished chest wall motion, with expansions limited to approximately 4 cm at key intercostal spaces, correlating with lower functional residual capacity (around 58% of predicted) and vital capacity (85% of predicted), thereby compromising overall respiratory efficiency.^[31]

Protective role

The sternum forms the anterior wall of the thoracic cavity, serving as a rigid shield that protects vital structures such as the heart, lungs, and great vessels from external trauma and blunt force impacts.^[1] This protective barrier is essential in absorbing and dissipating energy during collisions or falls, preventing direct injury to the mediastinal contents and adjacent pulmonary tissues.^[1] By integrating with the rib cage, the sternum creates a bony enclosure that maintains the integrity of the thoracic compartment under mechanical stress.^[32] As an attachment site for key pectoral muscles, including the pectoralis major, the sternum contributes to upper limb stability and overall postural support.^[1] These muscular connections enable the sternum to anchor forces that stabilize the shoulder girdle during arm movements and upright posture, reducing strain on the spine and facilitating balanced weight distribution across the torso.^[33] This role enhances functional stability, allowing efficient load-bearing in daily activities and preventing compensatory imbalances that could lead to musculoskeletal issues.^[34] The sternum plays a critical role in load distribution during physical activity by resisting compressive forces transmitted through the upper body.^[35] It helps redistribute mechanical stresses from the ribs and spine, minimizing localized pressure on thoracic vertebrae and underlying organs during bending, lifting, or impact scenarios.^[35] This biomechanical function is supported by the sternum's flat, compact structure, which provides enhanced rigidity for protection while keeping overall mass low—an evolutionary adaptation seen in mammalian thoracic evolution for efficient skeletal support without excessive weight.^[36] In terms of biomechanical strength, the intact sternum can withstand substantial compressive forces before fracturing, as demonstrated in cadaveric studies simulating blunt trauma.^[37]

Clinical significance

Trauma and fractures

Trauma to the sternum typically arises from blunt or penetrating forces impacting the anterior chest wall, such as those encountered in high-energy events, rendering its relatively thin bony structure vulnerable to injury.^[38] Sternal fractures are common in motor vehicle accidents, which account for 40-70% of cases, with isolated sternal fractures occurring in 3-8% of chest traumas overall.^[39]^[40] These fractures are classified into types including transverse (the most common, typically at the sternal body), comminuted, and depression fractures.^[38]^[41] Patients often present with symptoms such as localized chest pain, crepitus upon palpation, and dyspnea, and these injuries are associated with myocardial contusion in 5-20% of cases.^[38]^[42]^[43] Diagnosis is primarily achieved through chest X-ray, which detects most fractures on lateral views, or computed tomography (CT) for detailed assessment of fracture extent and associated injuries; severity can be evaluated using the sternal fracture index alongside imaging findings.^[38]^[44]^[45] Treatment for most sternal fractures is conservative, emphasizing pain control with analgesics, respiratory monitoring to prevent complications like pneumonia, and observation, reserved for cases without instability or significant associated injuries.^[38]^[46]

Surgical interventions

The median sternotomy is the standard surgical approach for accessing the thoracic cavity during open-heart procedures, involving a longitudinal midline incision from the sternal notch to the xiphoid process, followed by division of the sternum with a saw and closure using stainless steel wires passed through drill holes in the bone halves.^[47] This technique provides optimal exposure for cardiac surgeries such as coronary artery bypass grafting and valve replacements, used in the majority of open-heart operations, though minimally invasive approaches have increased to over 30% in certain procedures like aortic valve replacement as of the 2010s, with further growth in robotic-assisted techniques reaching 10-20% in specialized centers by 2025.^[48]^[49] Bone marrow biopsy and aspiration at the sternum, performed via a needle inserted into the manubrium or body, have largely been supplanted by posterior iliac crest sites in modern practice because of the higher risk of complications at the sternal location, including potential cardiac or vascular injury from bone penetration.^[50] The posterior iliac crest offers a safer alternative with lower rates of serious adverse events, such as hemorrhage or organ damage, making it the preferred site for routine hematologic diagnostics.^[51] For traumatic sternal fractures, particularly unstable transverse or comminuted types that fail conservative management, surgical fixation employs techniques such as stainless steel wiring to approximate fracture fragments or rigid plating systems like titanium locks to restore stability and prevent paradoxical chest wall motion.^[52] Wiring involves passing multiple loops around the bone edges for simple transverse fractures, while plating uses self-tapping screws to secure contoured plates across the fracture line in more complex cases, promoting faster healing and reducing respiratory compromise.^[53] Common risks associated with sternal surgeries include deep sternal wound infection (occurring in 0.5-5% of median sternotomy cases), dehiscence (0.3-1%), and non-union (up to 34% in high-risk patients), often exacerbated by factors like obesity, diabetes, or prolonged ventilation.^[54] Postoperative pain management typically involves multimodal analgesia, including opioids, nonsteroidal anti-inflammatory drugs, and intercostal nerve blocks, to mitigate chronic discomfort from tissue retraction and hardware.^[55] Since the 1990s, advances in minimally invasive techniques have reduced the reliance on full median sternotomy, with partial upper or lower sternotomies—such as J- or L-shaped incisions limited to the manubrium and proximal body—enabling procedures like isolated aortic valve replacement through smaller exposures that decrease blood loss, hospital stay, and recovery time while maintaining comparable outcomes.^[48] These approaches, often combined with endoscopic assistance, have expanded to over 20% of eligible cardiac surgeries at specialized centers, prioritizing patient cosmesis and faster return to function.^[56]

Congenital conditions

Congenital conditions of the sternum arise primarily from disruptions in the normal embryonic fusion of the sternal bars along the midline, leading to structural defects that can range from cosmetic concerns to life-threatening exposures of thoracic organs.^[57] These anomalies occur due to incomplete fusion between the 6th and 10th weeks of gestation, resulting in varying degrees of sternal clefting or deformities.^[58] Sternal cleft, also known as bifid sternum, is a rare congenital defect characterized by a failure of midline fusion of the sternal primordia, with an estimated incidence of 1 in 50,000 to 100,000 live births.^[59] It accounts for approximately 0.15% of all chest wall deformities and is more common in females.^[60] The condition is classified by severity and extent: type A (superior partial cleft, the most common, involving only the manubrium); type B (subtotal cleft, extending to the lower sternum); type C (total cleft, complete longitudinal split); and type D (thoracoabdominal extension, often the most severe).00112-X/pdf) Most cases are isolated, but up to 72% are associated with other anomalies, including cardiac or diaphragmatic defects.^[57] Ectopia cordis frequently accompanies sternal cleft, particularly in complete or type D forms, where the heart is partially or fully exposed outside the thorax due to defects in the pericardium, diaphragm, and abdominal wall.^[61] This severe manifestation occurs in about 1 in 126,000 live births and is often linked to chromosomal abnormalities or syndromic conditions.^[62] Surgical correction is typically performed in infancy to cover the heart and restore chest integrity, often involving multidisciplinary approaches with cardiothoracic and plastic surgeons.^[63] Pectus excavatum and pectus carinatum represent common sternal deformities, affecting approximately 1 in 400 to 1,000 births, with a male predominance of 3:1 to 4:1.^[64] Pectus excavatum involves posterior depression of the sternum, potentially compressing the heart and lungs, while pectus carinatum features anterior protrusion.^[58] These are thought to result from uneven growth of costal cartilage relative to the sternum during puberty, though genetic factors contribute in up to 40% of cases.^[65] Genetic associations include rare syndromes such as pentalogy of Cantrell, which combines sternal cleft, ectopia cordis, diaphragmatic hernia, pericardial defect, and omphalocele, with an incidence of less than 1 in 100,000 births and a poor prognosis without early intervention.^[61] Other linked conditions may involve chromosomal anomalies like trisomy 18.^[66] Management of sternal cleft focuses on primary closure within the first 3 months of life, when the chest wall is most pliable, using techniques like sliding chondroplasty or prosthetic interposition for larger defects to achieve midline approximation without tension.^[57] For ectopia cordis, staged repairs in the neonatal period aim to reposition the heart and close the defect, with survival rates improving to over 50% in specialized centers.^[62] Pectus deformities are addressed cosmetically and functionally; the Nuss procedure, a minimally invasive repair for excavatum, involves inserting a convex bar to remodel the sternum, typically removed after 2-3 years, with low recurrence rates of 2-5%.^[67] Pectus carinatum often responds to bracing in adolescents, with surgical options like the Ravitch procedure reserved for refractory cases.^[58]

Comparative anatomy

In mammals

The sternum in mammals exhibits considerable variation in shape, size, and segmentation, reflecting adaptations to diverse locomotor demands and body plans. In carnivores such as dogs, the sternum typically comprises eight fused sternebrae, presenting a laterally compressed and medially constricted form that supports agile quadrupedal movement.^[68] This configuration, with a narrow xiphoid cartilage, facilitates efficient thoracic flexibility during predation and pursuit. In contrast, herbivores like the horse and ox possess a sternum of seven sternebrae, often elongated anteroposteriorly and featuring a keeled or canoe-shaped profile in the horse, which enhances ventral stability for weight-bearing in grazing postures.^[68] The ox sternum transitions from lateral compression to flattening, providing a broad base for pectoral muscle attachments that aid in sustained locomotion and load distribution.^[68] Compared to quadrupedal mammals, the human sternum is notably simplified, consisting of three primary elements—the manubrium, body, and xiphoid process—rather than the six to eight sternebrae common in other species, which may relate to the reduced mechanical demands of bipedal posture.^[69] Specialized adaptations further diversify sternal morphology; for instance, bats possess a prominent keel on the sternum, serving as a robust anchor for the enlarged pectoral flight muscles essential for powered aerial locomotion.^[70] In monotremes, the sternum is markedly reduced and segmented, paralleling evolutionary modifications to the shoulder girdle and reflecting their basal mammalian position with retained cartilaginous elements.^[71] Ossification of the mammalian sternum originates from ventral mesoderm, undergoing endochondral formation similar across species, but fusion timing of sternebrae varies with body size and developmental pace.^[72] In smaller mammals like rodents, ossification centers appear perinatally and fuse rapidly, whereas in larger forms such as equids, the process extends into later growth stages to accommodate prolonged skeletal maturation.^[73] Specific examples illustrate these patterns: the canine sternum features eight distinct segments that fuse early to form a unified structure supporting dynamic thoracic excursions,^[74] while the equine sternum culminates in a prominent bony xiphoid process, the terminal sternebra that ossifies last and contributes to abdominal wall anchorage.^[68]

In non-mammalian vertebrates

In non-mammalian vertebrates, the sternum and its homologs exhibit significant variation, reflecting adaptations to diverse locomotor and respiratory demands, with evolutionary origins tracing back to dermal bone precursors in early tetrapods that transitioned to more integrated endochondral structures in derived forms.^[75] The interclavicle and gastralia—ventral abdominal ribs—served as key precursors in basal tetrapods, providing ventral support before the development of a unified sternum.^[76] In fish, no true sternum exists; instead, the pectoral girdle includes a coracoid bar or process that supports the pectoral fins and anchors associated musculature, functioning as a structural analog to the tetrapod sternum by bracing the girdle against the body wall.^[77] This arrangement remains attached to the skull in many species, differing from the axial integration seen in tetrapods.^[77] Amphibians typically possess a cartilaginous sternum or lack a fully ossified one, often integrated with the coracoid bones of the pectoral girdle to form a flexible ventral support for the ribcage.^[78] In frogs like Rana esculenta, the sternum appears as a dotted cartilaginous element amid the epicoracoids and omosternum, emphasizing pliability suited to jumping and aquatic transitions.^[78] Salamanders, such as Cryptobranchus alleganiensis, feature a thin, cartilaginous sternum that attaches to the body wall via thickenings, with the coracoid similarly delicate and unossified for minimal rigidity.^[79] Reptiles show further diversification, with the interclavicle and gastralia persisting as dermal elements in many lineages, while a true sternum—often large and cartilaginous or bony—develops in most but is absent in snakes and turtles.^[80] In snakes, the complete lack of a sternum accommodates their elongate, limbless body, relying instead on expanded ribs for ventral enclosure.^[80] Turtles incorporate sternal elements into the plastron, fusing them with gastralia for protective armor.^[80] Birds represent a specialized extreme, with the sternum prominently featuring a keel, or carina, that projects ventrally to maximize attachment area for flight muscles like the pectoralis.^[81] This carina, formed by fused sternal plates, ossifies early in embryonic development—beginning in the diaphysis and extending outward—to provide robust support for powered flight, as seen in species like quail where midline fusion occurs progressively.^[82] In flying birds, the carina extends nearly the full length of the sternum, contrasting with reduced forms in flightless ratites.^[82]

In arthropods

In arthropods, the sternum is not a true bone but rather the ventral portion of a body segment, formed by sclerotized plates called sternites that constitute part of the chitinous exoskeleton.^[83] These sternites, along with the dorsal tergites and lateral pleurites, create a segmented protective covering for the body.^[84] Unlike the internal ossification seen in vertebrate sterna, arthropod sternites originate from ectodermal epidermal cells that secrete the cuticle as an extracellular matrix.^[85] Growth occurs through periodic molting (ecdysis), where the old exoskeleton is shed and replaced by a new one, allowing expansion before hardening.^[86] The primary function of sternites is to shield the ventral nerve cord and internal viscera from mechanical damage and desiccation, while also providing attachment sites for muscles and articulation points with the pleurae on either side.^[83] In insects, for instance, the thoracic sterna support the coxae of the legs and contribute to the rigid structure of the thorax, where they connect via pleural sutures to the dorsal notum (tergum), sometimes forming fused elements in specialized taxa for enhanced stability during flight or locomotion.^[87] In crustaceans like crabs, the sternum forms the ventral boundary of the branchial chamber, enclosing the gills and facilitating respiration by maintaining a protected space for water flow over the respiratory structures.^[88] This arrangement allows for flexibility in movement while ensuring compartmentalization of vital organs. Developmentally, sternites arise from the ectoderm during embryogenesis, with the underlying hypodermis (epidermal layer) depositing layers of chitin, proteins, and sometimes minerals to form the rigid sclerites.^[89] Molting is hormonally regulated by ecdysteroids, enabling juveniles to increase in size across instars, a process absent in the continuous, remodeling growth of endoskeletal elements.^[90] Evolutionarily, arthropod sternites exemplify convergent adaptation for ventral protection, paralleling the role of vertebrate sterna in safeguarding thoracic contents, but achieved through external plating rather than internal mineralization.^[91]

Nomenclature and history

Etymology

The term "sternum" derives from the New Latin sternum, borrowed from the Latin sternum meaning "breastbone," which in turn originates from the Ancient Greek stérnon (στέρνον), denoting "chest," "breast," or "breastbone."^[92] In ancient Greek literature, including Homeric epics, stérnon primarily referred to the male chest as the seat of emotions and strength, while stêthos (στέθος) more specifically indicated the breast or breastbone in anatomical contexts.^[92] The Hippocratic Corpus, a collection of ancient medical texts attributed to Hippocrates and his followers from the 5th–4th centuries BCE, employed stérnon to describe the chest region, often in discussions of thoracic injuries and affections, marking an early medical usage of the term for the structure now known as the sternum.^[93] The nomenclature for the sternum's subdivisions emerged during the Renaissance, with anatomist Andreas Vesalius providing detailed descriptions in his seminal 1543 work De humani corporis fabrica. The manubrium, the superior handle-like portion, derives its name from the Latin manubrium meaning "handle," reflecting its shape and position.^[94] The xiphoid process, the inferior cartilaginous extension, originates from the Greek xiphoeidḗs (ξιφοειδής), meaning "sword-shaped," due to its pointed form resembling a sword tip.^[12] Vesalius distinguished these parts—manubrium, body (or gladiolus, from Latin for "small sword"), and xiphoid—as the three components of the sternum, correcting earlier misconceptions from Galen and standardizing their identification through dissection-based observations.^[95] The modern anatomical terminology for the sternum was formalized with the adoption of the Basle Nomina Anatomica (BNA) in 1895 by the German Anatomical Society, which established sternum as the standard Latin term for international use, alongside its subdivisions, to promote uniformity in medical education and literature.^[96] This standardization persisted through subsequent revisions, such as the Nomina Anatomica (1955), Terminologia Anatomica (1998), and its second edition (2019), ensuring consistent nomenclature for the sternum and its parts.^[97]^[98] In English, the sternum is commonly called the "breastbone," a term evoking the protective "breastplate" metaphor from biblical translations, particularly Ephesians 6:14 in the King James Version, where the "breastplate of righteousness" symbolizes moral defense over the vital chest area, paralleling the bone's role in safeguarding the heart and lungs.

Historical perspectives

The study of the sternum dates back to ancient times, with the Greek physician Galen (c. 129–c. 216 CE) providing one of the earliest detailed descriptions during his anatomical dissections in the 2nd century. Galen referred to it as the "breastbone" (στέρνον in Greek), portraying it as a flat, elongated structure composed of seven bony segments that formed the anterior wall of the thorax, protecting vital organs like the heart and lungs.^[99] His observations, drawn primarily from animal dissections due to restrictions on human cadaver use, emphasized its role in supporting the rib cage and facilitating respiration, though they included inaccuracies such as overestimating the number of segments based on comparisons to primate anatomy.^[100] During the Renaissance, Andreas Vesalius advanced the understanding of the sternum through direct human dissections, challenging Galen's errors in his seminal 1543 work De Humani Corporis Fabrica. Vesalius accurately illustrated the sternum as consisting of three main parts—the manubrium, body (gladiolus), and xiphoid process—using detailed woodcut engravings that depicted its segmented, cartilaginous nature and articulations with the clavicles and costal cartilages.^[101] These illustrations not only corrected the segmental count to reflect human anatomy but also highlighted its ossification patterns, marking a shift toward empirical observation over classical authority and influencing subsequent anatomical texts.^[102] In the 19th century, anatomists conducted studies on the developmental aspects of the sternum, particularly its ossification from multiple centers within the cartilaginous sternum, beginning in utero and continuing into adolescence, providing insights into age-related fusion and variations. The discovery of X-rays by Wilhelm Conrad Röntgen in 1895 revolutionized sternal diagnostics, enabling non-invasive visualization of fractures for the first time; by the early 1900s, radiographs were routinely used to assess sternal integrity in trauma cases, improving accuracy over palpation and reducing surgical exploration. The 20th century saw surgical innovations centered on the sternum, with median sternotomy emerging as a standard approach. In the 1930s, American surgeon Alfred Blalock popularized the technique through his 1936 performance of the first successful transsternal thymectomy, splitting the sternum longitudinally to access the mediastinum, which not only treated thymic tumors but also yielded unexpected benefits for myasthenia gravis patients. This method, refined for cardiac and thoracic procedures, transformed access to the anterior chest while highlighting the sternum's biomechanical role in load-bearing.^[103] Advancements in imaging since the 1970s further refined sternal analysis, with computed tomography (CT), invented by Godfrey Hounsfield in 1971, and magnetic resonance imaging (MRI), developed in the mid-1970s, allowing three-dimensional visualization of congenital variations and post-traumatic changes.^[104] These modalities enabled precise mapping of ossification irregularities and soft-tissue interactions, surpassing plain radiography in detecting subtle anomalies. Post-2000 research has increasingly incorporated computational biomechanics, using finite element modeling to simulate sternal stresses during fixation after sternotomy, informing prosthetic designs and predicting fracture risks under dynamic loads.^[37] During the Islamic Golden Age (8th–14th centuries), scholars like Avicenna (Ibn Sina) advanced knowledge of thoracic anatomy, including the sternum's structure and role in protecting vital organs, building on Greek texts and incorporating dissections where permitted, influencing global anatomical understanding.^[105]

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