The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately 6 inches (15 cm) long[1] that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavicle is the only long bone in the body that lies horizontally.[2] Together with the shoulder blade, it makes up the shoulder girdle. It is a palpable bone and, in people who have less fat in this region, the location of the bone is clearly visible. It receives its name from Latinclavicula 'little key' because the bone rotates along its axis like a key when the shoulder is abducted. The clavicle is the most commonly fractured bone. It can easily be fractured by impacts to the shoulder from the force of falling on outstretched arms or by a direct hit.[3]
The collarbone is a thin doubly curved long bone that connects the arm to the trunk of the body.[4] Located directly above the first rib, it acts as a strut to keep the scapula in place so that the arm can hang freely. At its rounded medial end (sternal end), it articulates with the manubrium of the sternum (breastbone) at the sternoclavicular joint. At its flattened lateral end (acromial end), it articulates with the acromion, a process of the scapula (shoulder blade), at the acromioclavicular joint.
Right clavicle—from below, and from above
Left clavicle—from above, and from below
The rounded medial region (sternal region) of the shaft has a long curve laterally and anteriorly along two-thirds of the entire shaft. The flattened lateral region (acromial region) of the shaft has an even larger posterior curve to articulate with the acromion of the scapula. The medial region is the longest clavicular region as it takes up two-thirds of the entire shaft. The lateral region is both the widest clavicular region and thinnest clavicular region. The lateral end has a rough inferior surface that bears a ridge, the trapezoid line, and a slight rounded projection, the conoid tubercle (above the coracoid process). These surface features are attachment sites for muscles and ligaments of the shoulder.
It can be divided into three parts: medial end, lateral end, and shaft.
The medial end is also known as the sternal end. It is quadrangular and articulates with the clavicular notch of the manubrium of the sternum to form the sternoclavicular joint.[5] The articular surface extends to the inferior aspect for articulation with the first costal cartilage.
The lateral end is also known as the acromial end. It is flat from above downward. It bears a facet that articulates with the shoulder to form the acromioclavicular joint. The area surrounding the joint gives an attachment to the joint capsule. The anterior border is concave forward and the posterior border is convex backward.[6]
The shaft is divided into two main regions, the medial region, and the lateral region. The medial region is also known as the sternal region, it is the longest clavicular region as it takes up two-thirds of the entire shaft. The lateral region is also known as the acromial region, it is both the widest clavicular region and thinnest clavicular region.
The lateral region of the shaft has two borders and two surfaces.
the anterior border is concave forward and gives origin to the deltoid muscle.
the posterior border is convex and gives attachment to the trapezius muscle.
the inferior surface has a ridge called the trapezoid line and a tubercle; the conoid tubercle for attachment with the trapezoid and the conoid ligament, part of the coracoclavicular ligament that serves to connect the collarbone with the coracoid process of the scapula.
The collarbone is the first bone to begin the process of ossification (laying down of minerals onto a preformed matrix) during development of the embryo, during the fifth and sixth weeks of gestation. However, it is one of the last bones to finish ossification at about 21–25 years of age. Its lateral end is formed by intramembranous ossification while medially it is formed by endochondral ossification. It consists of a mass of cancellous bone surrounded by a compact bone shell. The cancellous bone forms via two ossification centres, one medial and one lateral, which fuse later on. The compact forms as the layer of fascia covering the bone stimulate the ossification of adjacent tissue. The resulting compact bone is known as a periosteal collar.
The shape of the clavicle varies more than most other long bones. It is occasionally pierced by a branch of the supraclavicular nerve. In males the clavicle is usually longer and larger than in females. A study measuring 748 males and 252 females saw a difference in collarbone length between age groups 18–20 and 21–25 of about 6 and 5 mm (0.24 and 0.20 in) for males and females respectively.[9]
The left clavicle is usually longer and weaker than the right clavicle.[8][10]
The levator claviculae muscle, present in 2–3% of people, originates on the transverse processes of the upper cervical vertebrae and is inserted in the lateral half of the clavicle.
It serves as a rigid support from which the scapula and free limb are suspended; an arrangement that keeps the upper limb away from the thorax so that the arm has maximum range of movement. Acting as a flexible, crane-like strut, it allows the scapula to move freely on the thoracic wall.
Relation of Brachial Plexus with the ClavicleCovering the cervicoaxillary canal, it protects the neurovascular bundle that supplies the upper limb.
Transmits physical impacts from the upper limb to the axial skeleton.
A vertical line drawn from the mid-clavicle called the mid-clavicular line is used as a reference in describing cardiac apex beat during medical examination. It is also useful for evaluating an enlarged liver, and for locating the gallbladder which is between the mid-clavicular line and the transpyloric plane.
Clavicle fractures (colloquially, a broken collarbone) occur as a result of injury or trauma. The most common type of fractures occur when a person falls horizontally on the shoulder or with an outstretched hand. A direct hit to the collarbone will also cause a break. In most cases, the direct hit occurs from the lateral side towards the medial side of the bone. The most common site of fracture is the junction between the two curvatures of the bone, which is the weakest point.[11] This results in the sternocleidomastoid muscle lifting the medial aspect superiorly, which can perforate the overlying skin.
The clavicle first appears as part of the skeleton in primitive bony fish, where it is associated with the pectoral fin; they also have a bone called the cleithrum. In such fish, the paired clavicles run behind and below the gills on each side, and are joined by a solid symphysis on the fish's underside. They are, however, absent in cartilaginous fish and in the vast majority of living bony fish, including all of the teleosts.[12]
The earliest tetrapods retained this arrangement, with the addition of a diamond-shaped interclavicle between the base of the clavicles, although this is not found in living amphibians. The cleithrum disappeared early in the evolution of reptiles, and is not found in any living amniotes, but the interclavicle is present in most modern reptiles, and also in monotremes. In modern forms, however, there are a number of variations from the primitive pattern. For example, crocodilians and salamanders lack clavicles altogether (although crocodilians do retain the interclavicle), while in turtles, they form part of the armoured plastron.[12]
The interclavicle is absent in marsupials and placental mammals. In many mammals, the clavicles are also reduced, or even absent, to allow the scapula greater freedom of motion, which may be useful in fast-running animals.[12]
Though a number of fossil hominin (humans and chimpanzees) clavicles have been found, most of these are mere segments offering limited information on the form and function of the pectoral girdle. One exception is the clavicle of AL 333x6/9 attributed to Australopithecus afarensis which has a well-preserved sternal end. One interpretation of this specimen, based on the orientation of its lateral end and the position of the deltoid attachment area, suggests that this clavicle is distinct from those found in extant apes (including humans), and thus that the shape of the human shoulder dates back to less than 3 to 4 million years ago. However, analyses of the clavicle in extant primates suggest that the low position of the scapula in humans is reflected mostly in the curvature of the medial portion of the clavicle rather than the lateral portion. This part of the bone is similar in A. afarensis and it is thus possible that this species had a high shoulder position similar to that in modern humans.[13]
In dinosaurs, the main bones of the pectoral girdle were the scapula (shoulder blade) and the coracoid, both of which directly articulated with the clavicle. The clavicle was present in saurischian dinosaurs but largely absent in ornithischian dinosaurs. The place on the scapula where it articulated with the humerus (upper bone of the forelimb) is called the glenoid. The clavicles fused in some theropod dinosaurs to form a furcula, which is the equivalent to a wishbone.[14]
In birds, the clavicles and interclavicle have fused to form a single Y-shaped bone, the furcula or "wishbone" which evolved from the clavicles found in coelurosaurian theropods.[citation needed]
^Shane Tubbs, R.; Loukas, Marios; Slappey, John B.; McEvoy, William C.; Linganna, Sanjay; Shoja, Mohammadali M.; Jerry Oakes, W. (9 July 2007). "Surgical and clinical anatomy of the interclavicular ligament". Surgical and Radiologic Anatomy. 29 (5): 357–360. doi:10.1007/s00276-007-0219-z. PMID17563831.
^Standring, Susan (2016). Gray's anatomy: the anatomical basis of clinical practice .Digital version (41st ed.). Philadelphia, Pa.: Elsevier. p. 892. ISBN9780702052309.
^Kaur, H; Harjeet Sahni, D (January 2002). "Length and curves of the clavicle in Northwest Indians". Journal of Anatomical Society of India. 51 (2): 199–209.
^A. Bernat, T. Huysmans, F. Van Glabbeek, J. Sijbers, J. Gielen, and A. Van Tongel (2014). "The anatomy of the clavicle: A Three-dimensional Cadaveric Study". Clinical Anatomy. 27 (5): 712–723. doi:10.1002/ca.22288. PMID24142486. S2CID23982787.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ abcRomer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 184–186. ISBN978-0-03-910284-5.
^Larson, Susan G. (2009). "Evolution of the Hominin Shoulder: Early Homo". In Grine, Frederick E.; Fleagle, John G.; Leakey, Richard E. (eds.). The First Humans – Origin and Early Evolution of the Genus Homo. Vertebrate Paleobiology and Paleoanthropology. Springer. p. 66. doi:10.1007/978-1-4020-9980-9. ISBN978-1-4020-9979-3.
^Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300. ISBN1-4051-3413-5.
The clavicle, commonly known as the collarbone, is a long, sigmoid-shaped bone that forms part of the pectoral girdle and serves as the only direct osseous connection between the upper limb and the axial skeleton.[1] The term "clavicle" derives from Latin clavicula, meaning "little key" (diminutive of clavis, "key"), due to the bone's axial rotation resembling the action of a key during shoulder abduction; the adjectival form is "clavicular".[2][3] In ancient Greek, the collarbone was called κλείς (kleís), which also meant "key," "bolt," or "bar," likely reflecting its function in securing the shoulder joint.[2] No direct connection to Greek mythology exists. Positioned horizontally across the superior aspect of the thorax, it articulates medially with the manubrium of the sternum at the sternoclavicular joint and laterally with the acromion of the scapula at the acromioclavicular joint.[1] This dual articulation enables the clavicle to act as a structural brace, transmitting forces from the upper extremity to the trunk while allowing a wide range of shoulder motion.[4]Structurally, the clavicle features a medial two-thirds that is convex anteriorly and a lateral one-third that is concave anteriorly, creating its characteristic S-shape; unlike most long bones, it lacks a medullary cavity and is instead filled with cancellous bone surrounded by a dense cortical shell.[1] Its superior surface is smooth to accommodate overlying skin and subcutaneous tissues, while the inferior surface bears muscular attachments and impressions such as the costal tuberosity for the costoclavicular ligament.[4] The bone develops through a unique combination of intramembranous ossification at the lateral end and endochondral ossification at the medial end, making it the first bone to begin ossifying in the developing embryo around the fifth to sixth week of gestation.[1] Blood supply to the clavicle is provided primarily by periosteal branches from the suprascapular, thoracoacromial, and internal thoracic arteries, as it has no central nutrient artery.[1]Functionally, the clavicle plays a pivotal role in shoulder girdle stability and mobility by elevating the scapula, protecting underlying neurovascular structures like the brachial plexus and subclavian vessels, and facilitating movements such as arm elevation and rotation.[1] It also disperses mechanical loads during upper limb activities, preventing excessive stress on the shoulder joint.[4] Clinically, clavicle fractures are common, accounting for 2.6–10% of all fractures (about 5% in adults), and are among the most frequent in children and young adults, often due to falls or direct trauma, which can compromise shoulder function if not properly managed.[5][6] Disorders of the associated sternoclavicular and acromioclavicular joints may lead to pain, instability, or limited range of motion, highlighting the bone's importance in overall upper limb health.[1]
Anatomy
Overview
The clavicle, also known as the collarbone, is the only long bone in the human body that lies horizontally, serving as a key strut connecting the upper limb to the axial skeleton as part of the pectoral girdle.[1] It is a sigmoid-shaped bone that runs between the sternum anteriorly and the scapula laterally, forming the anterior aspect of the shoulder girdle and providing structural support for arm movement.[1] The bone exhibits a double curve, with a medial convexity and lateral concavity when viewed from above, allowing it to accommodate the underlying vascular and muscular structures.[1]In adults, the clavicle measures an average length of 14-16 cm, varying by sex and population, with males typically having longer and thicker bones than females.[7] A slight asymmetry is common, with the left clavicle often longer than the right by an average of 2-4 mm, though differences exceeding 10 mm are rare and may have clinical implications.[8]The blood supply is primarily periosteal, derived from branches of the suprascapular and thoracoacromial arteries, supplemented by the internal thoracic artery, without a central nutrient artery or medullary cavity.[9]
Medial two-thirds
The medial two-thirds of the clavicle, comprising the thicker proximal segment, features a triangular cross-section that transitions laterally into a more flattened profile, with anterior convexity to accommodate the bone's load-bearing role. This regional morphology enhances structural integrity near the trunk, distinguishing it from the distal portion.[10]At its medial extremity, known as the sternal end, the clavicle presents an enlarged, quadrangular shape with a roughened articular surface that forms the sternoclavicular joint. This joint articulates with the clavicular notch of the manubrium sterni through a fibrocartilaginous intra-articular disc, providing a saddle-like synovial interface reinforced by a fibrous capsule.[11][12][13]On the inferior surface of this medial segment lies the costal tuberosity, a prominent roughened area serving as the primary attachment site for the costoclavicular ligament, which anchors the clavicle to the first rib and contributes to joint stability. Adjacent to this, nutrient foramina—typically one or more openings for vascular entry—are located predominantly on the inferior surface, alongside shallow grooves accommodating branches of the suprascapular and internal thoracic arteries for periosteal nourishment, as the clavicle lacks a true medullary cavity.[1][14][1]The overall thickness of the medial two-thirds, which is greater than in the lateral portion (averaging 1.44 cm anteroposteriorly at its narrowest in adults), combined with its anterior convexity, confers resistance to compressive and medial-directed forces transmitted from the upper limb to the axial skeleton.[15][1]
Lateral one-third
The lateral one-third of the clavicle is characterized by a flattened cross-section, distinguishing it from the more cylindrical medial portion, and presents a concave anterior surface that contributes to the bone's overall S-shaped curvature.[16][17]At its lateral extremity, known as the acromial end, the clavicle broadens into a flattened structure featuring a broad, oval articular facet directed obliquely downward and laterally; this facet articulates with the medial aspect of the acromion process of the scapula to form the acromioclavicular joint.[16][1] On the inferior surface near this end, two prominent tubercles are present: the laterally positioned trapezoid tubercle, a roughened ridge for the attachment of the trapezoid ligament, and the medially located conoid tubercle, a more conical projection serving as the insertion site for the conoid ligament; together, these form the coracoclavicular ligament complex that stabilizes the acromioclavicular joint.[16][1]The superior surface of the lateral one-third is roughened, providing attachment sites for muscles and ligaments; anteriorly, it bears an impression for the origin of the anterior deltoid muscle, while posteriorly it features an impression for the insertion of the trapezius muscle.[16][1][18] Medially on this superior surface, a rough impression accommodates the interclavicular ligament, which connects the clavicles across the superior aspect of the sternum.[1] Due to its relatively thinner and flattened morphology compared to the medial two-thirds, this region is particularly susceptible to bending stresses, especially under loads transmitted from the upper limb.[16][19]
Shaft
The shaft of the clavicle exhibits a characteristic sigmoid or S-shaped curvature, with the medial portion displaying anterior convexity and the lateral portion showing anterior concavity.[1] This double curvature facilitates optimal transmission of compressive and tensile forces from the upper limb to the axial skeleton, distributing loads efficiently while protecting underlying neurovascular structures.[20]In cross-section, the shaft transitions from a prismatic shape medially, where it is more rounded and robust, to a tubular midportion, and finally to a flattened profile laterally.[21] These variations in geometry contribute to the bone's mechanical strength, with the prismatic medial section providing greater resistance to bending forces and the flattened lateral section accommodating attachments and movements at the shoulder.[22]The superior surface of the shaft is smooth and subcutaneous, allowing unobstructed passage of the skin and the platysma muscle.[23] In contrast, the inferior surface features a prominent subclavian groove that accommodates the subclavius muscle, along with a nutrient foramen typically located in the midshaft region for vascular entry into the medullary canal.[1]
Articulations and ligaments
The clavicle articulates medially with the manubrium of the sternum and the first costal cartilage at the sternoclavicular (SC) joint, forming the only true articulation between the upper limb and the axial skeleton.[12] This saddle-shaped, diarthrodial synovial joint features convex anteroposterior and concave vertical articular surfaces on the clavicle, allowing multiaxial motion including up to 35° of elevation-depression, 70° of anteroposterior gliding, and 45° of axial rotation.[12] Laterally, the clavicle connects with the acromion process of the scapula at the acromioclavicular (AC) joint, a diarthrodial plane synovial joint that facilitates gliding movements essential for shoulder abduction, flexion, and scapular rotation.[24] Both joints are lined with fibrocartilage and contain intra-articular discs that enhance stability and distribute compressive forces.[1]The SC joint is reinforced by several ligaments that provide anteroposterior, vertical, and medial stability. The anterior sternoclavicular ligament spans from the anterosuperior surface of the medial clavicle to the anterior edge of the manubrium and first costal cartilage, preventing excessive superior displacement.[25] The posterior sternoclavicular ligament, extending from the posterior clavicle to the posterior manubrium, serves as the primary restraint against anteroposterior translation.[12] The interclavicular ligament connects the medial ends of both clavicles superior to the joint, continuous with the deep cervical fascia, and facilitates medial traction to maintain alignment during arm movements.[25] Additionally, the costoclavicular ligament, a short, cone-shaped structure with anterior and posterior laminae, anchors the inferior medial clavicle to the first rib and its costal cartilage, acting as the main vertical stabilizer and transmitting forces from the upper limb to the thorax.[25]At the AC joint, stability is maintained by capsular and extracapsular ligaments that resist horizontal and vertical shear. The acromioclavicular ligament complex includes superior, inferior, anterior, and posterior bands; the superior and posterior components are the strongest, providing primary horizontal stability by limiting anterior-posterior translation between the clavicle and acromion.[24] The coracoclavicular ligament, located inferior to the AC joint, comprises two parts: the trapezoid ligament (anterolateral, fan-shaped, attaching the distal clavicle to the coracoid process) and the conoid ligament (posteromedial, conical ligament attaching to the coracoid process); which together offer vertical stability and prevent superior clavicle displacement under load.[24] The coracoacromial ligament, a triangular extracapsular band bridging the coracoid process to the acromion, further supports the joint by forming part of the coracoacromial arch and resisting superior humeral head migration.[24] These ligaments collectively enable the AC joint to transmit forces from the upper extremity to the axial skeleton while accommodating the wide range of shoulder girdle motions.[1]
Development and Variation
Embryological development
The clavicle originates from the mesoderm of the lateral plate, specifically at somite levels 1–14 during early embryonic development.[26] It is the first bone to begin ossification in the human fetus, with primary ossification centers appearing between 5 and 6 weeks of gestation.[27] These two membranous primary centers—one medial and one lateral—emerge in a condensed rod of mesenchyme along the future shaft and fuse approximately one week later, forming the initial bony structure.[28]Unlike most long bones, which form through endochondral ossification involving a cartilage model, the clavicle primarily develops via intramembranous ossification, where mesenchymal tissue directly differentiates into bone without an intermediate cartilaginous stage.[29] This process begins in the diaphysis and proceeds outward, establishing the S-shaped curvature by around 9 weeks through angulation primarily at the medial center.[30] The ends of the clavicle, however, incorporate endochondral elements, with cartilage developing at both the sternal and acromial aspects to facilitate later articulations.[31]The sternal end of the clavicle receives a notable contribution from neural crest cells, particularly post-otic neural crest, which forms part of the endochondral bone and connective tissue in this region, anchoring head-derived structures to the shoulder girdle.[32] Secondary ossification centers appear during adolescence: the medial (sternal) center between 15 and 20 years of age, and the lateral (acromial) center between 11 and 19 years, with fusion completing between 21 and 25 years in females and 23 and 30 years in males.[33][34][1]At birth, the fetal clavicle measures approximately 41 mm in length, having grown logarithmically from earlier stages through periosteal apposition along the shaft.[35] This mechanism of circumferential deposition continues postnatally, contributing to the bone's elongation and thickening.[29]
Postnatal changes
The clavicle undergoes significant postnatal growth, primarily through elongation at its ends, with a steady increase in length of approximately 8.4 mm per year from birth until age 12 in both sexes.[36] This growth accounts for the majority of the bone's final length, with about 80% achieved by age 9 in females and age 12 in males, after which the rate slows considerably.[37] The process is driven by endochondral ossification at the medial and lateral epiphyseal plates, contributing to the bone's adaptation to increasing body size and shoulder girdle demands during childhood.[7]
Recent longitudinal radiographic studies have revealed that earlier cross-sectional analyses underestimated late growth potential. In males, clavicular lengthening continues substantially beyond age 16: approximately 3.2 mm/year (2.4%/year) from ages 16-19, and 1.7 mm/year (1.1%/year) from ages 20-25, yielding a total increase of about 17.5 mm (over 10% of final clavicle length) from age 16 to 25 in some cases. Growth remains ongoing in many individuals up to age 25, with remodeling potential persisting. This prolonged window is due to the medial clavicular physis being among the last epiphyseal plates to fuse in the body, typically between 23 and 30 years in males (later than most long bones).[38][39]Epiphyseal fusion occurs progressively, beginning at the lateral end around ages 16-19 and completing at the medial end last, typically between 21 and 25 years in females and 23 and 30 years in males.[40][41] This delayed medial fusion reflects the clavicle's prolonged growth potential compared to other long bones, allowing continued remodeling into early adulthood.[42] Incomplete fusion prior to these ages can be observed radiographically and is utilized in forensic age estimation.[43]Remodeling of the clavicle involves adaptive changes in shape and structure influenced by mechanical loading from muscle attachments and upper limb activity, resulting in increased S-shaped curvature and robusticity over time.[38] In contrast, disuse conditions such as paralysis lead to boneatrophy, characterized by reduced cortical thickness and density due to diminished Wolff's law-driven loading.[44] These changes highlight the clavicle's sensitivity to biomechanical stimuli throughout postnatal life.Sex differences emerge prominently from adolescence, with male clavicles becoming longer (mean adult length 15-16 cm vs. 14-15 cm in females) and thicker, reflecting higher growth rates post-puberty (5.4 mm/year in males vs. 2.6 mm/year in females after age 12).[36] This dimorphism arises from hormonal influences on ossification and remodeling, leading to greater overall robusticity in males.[45]In senescence, the clavicle experiences age-related bone loss, particularly after age 50, where osteoporosis reduces mineral density and increases fragility fracture risk, often from low-trauma falls.[46] This heightened vulnerability contributes to higher morbidity in elderly populations, with postmenopausal women at particular risk due to accelerated resorption.[47]
Anatomical variations
The human clavicle commonly exhibits bilateral length asymmetry, with the left side typically longer than the right by an average of 1.8 to 4.1 mm across sexes and populations.[48][49] This difference is statistically significant (p < 0.001), and approximately 30% of individuals show an asymmetry exceeding 5 mm, while 2-3% exceed 10 mm.[48][49] Such variations, potentially influenced by hand dominance or sex (with males displaying greater absolute lengths and differences), can complicate assessments of fracture displacement during surgical planning, as assuming symmetry may overestimate or underestimate shortening.[48][49]Bifurcation of the clavicular shaft represents a rare congenital anatomical variation, often manifesting as a partial or complete fork, particularly in the lateral third or acromial region.[50][51] Documented in isolated case reports, this anomaly may arise from aberrant ossification during development and can mimic traumatic fractures on imaging, potentially leading to misdiagnosis.[52][53] Its prevalence remains undocumented due to its infrequency, but it carries clinical relevance in differential diagnosis of shoulder girdle anomalies.Accessory epiphyseal ossicles at the sternal end of the clavicle occur when the medial epiphysis fails to fuse fully with the shaft, a process that normally completes by age 23-25 years.[1] These ossicles, derived from delayed or incomplete ossification of the thin sternal epiphysis (which contributes up to 80% of clavicular length), are uncommon and may present as separate bony fragments visible on radiographs, occasionally causing pain or instability at the sternoclavicular joint.[1][54] Their presence can be mistaken for fractures or tumors, necessitating careful imaging evaluation.Foramina in the clavicular shaft, primarily nutrient foramina for vascular supply, are present in nearly all specimens (prevalence 97-100%), with a single foramen observed in 71% of cases, typically located on the posterior surface of the middle third and directed toward the acromial end.[55][56] Multiple foramina (two or more) occur in 21-29% of clavicles, and these openings transmit the nutrient artery, supporting intraosseous blood flow.[55][57] Less commonly, vascular foramina may appear, contributing to overall porosity variations that influence surgical drilling or plating.Supernumerary ossicles at the acromial end of the clavicle are exceedingly rare, often reported as extra bony processes or unfused secondary ossification centers near the lateral epiphysis, which ossifies even later than the medial end.[58][59] Case studies describe prominent projections extending from the acromial facet, potentially altering acromioclavicular joint mechanics and predisposing to impingement or degenerative changes.[58] Population studies indicate morphological differences in clavicular curvature and dimensions across races, with Asian individuals (e.g., Chinese) showing shorter, less curved clavicles compared to Caucasians, though specific prevalence of acromial ossicles remains unquantified and may vary ethnically.[60][61] These variants underscore the need for population-specific anatomical data in orthopedic interventions.
Functions
Structural support
The clavicle functions as a structural strut connecting the thorax to the upper limb, maintaining the integrity of the shoulder girdle by preventing the scapula and arm from collapsing medially into the thoracic cage under the weight of the upper extremity.[62] This strut-like configuration positions the scapula superiorly and laterally relative to the rib cage, thereby elevating it to facilitate full arm abduction and enhance overall shoulder mobility.[1] By acting in this manner, the clavicle ensures the pectoral girdle remains suspended and stable, allowing efficient upper limb function without compromising the thoracic structure.[63]In terms of force transmission, the clavicle serves as a conduit for both compressive and tensile loads generated by the upper limb, directing these forces toward the sternum and axial skeleton to distribute impact and maintain girdle stability.[1] Its S-shaped curvature and cortical bone microstructure are adapted to handle tensile stresses and torsional forces effectively, though less optimally for pure compression, which helps dissipate energy from activities like weight-bearing or impact.[19] This biomechanical role protects the underlying upper extremity from excessive strain while transmitting loads efficiently across the shoulder complex.[63]The pectoral girdle, including the clavicle, is suspended from the axial skeleton primarily through ligaments such as the sternoclavicular and costoclavicular ligaments, which anchor it to the sternum and first rib for added structural support.[63] Additionally, the coracoclavicular ligaments connect the clavicle to the scapula, further stabilizing the girdle against displacement.[1] This ligamentous suspension contributes to the protection of the thoracic outlet by shielding the brachial plexus and subclavian vessels as they traverse from the trunk to the upper limb, preventing compression or injury in this vulnerable region.[63]
Muscular attachments
The clavicle serves as an attachment site for several muscles that contribute to the mobility and stability of the shoulder girdle and neck. These attachments are distributed across its superior, inferior, anterior, and posterior surfaces, influencing movements such as head rotation, shoulder elevation, and arm flexion.[1]On the medial superior surface, the sternocleidomastoid muscle inserts via its clavicular head, enabling rotation of the head to the opposite side and ipsilateral lateral bending, as well as bilateral flexion of the head.[1] This attachment spans approximately 23 mm in mediolateral length on average, based on cadaveric measurements.[64] Laterally on the superior surface, the deltoid muscle originates from the anterior aspect of the lateral third, facilitating shoulder flexion and abduction.[1] Cadaveric studies indicate this origin covers about 46 mm mediolaterally.[64]The posterior superior surface hosts the insertion of the trapezius muscle, which stabilizes the scapula and elevates the shoulder during upper limb movements.[1] This broad attachment extends roughly 53 mm mediolaterally and aids in scapular retraction.[64] Anteriorly, the medial half of the clavicle provides origin for the clavicular head of the pectoralis major muscle, which drives flexion, horizontal adduction, and internal rotation of the humerus.[1] This site measures around 70 mm in mediolateral extent in anatomical dissections.[64]Inferiorly, the subclavius muscle originates from the subclavian groove, acting to depress the shoulder and pull the clavicle anteroinferiorly for enhanced stability during arm elevation.[1] Additionally, the sternohyoid muscle originates from the medial inferior surface, contributing to depression of the hyoid bone during swallowing and speech.[1] These inferior attachments underscore the clavicle's role in anchoring muscles that support both thoracic and cervical functions.[1]
Biomechanics in movement
The clavicle plays a pivotal role in shoulder kinematics by facilitating multiplanar motion at the sternoclavicular (SC) and acromioclavicular (AC) joints, enabling the scapula to position optimally for upper limb elevation and reach. During shoulder movements, the clavicle undergoes elevation and depression primarily at the SC joint, with a total range of 30-45 degrees, allowing the lateral end of the clavicle to rise up to 10 cm during full arm elevation while depression contributes about 3 cm of excursion.[65][66] This vertical translation supports scapular upward rotation and helps maintain glenohumeral joint congruence throughout the arc of motion.In the horizontal plane, the clavicle enables protraction and retraction, totaling approximately 35 degrees, which is essential for scapular positioning during forward reaching or pulling actions. Protraction moves the scapula anteriorly around the thoracic wall, while retraction pulls it posteriorly, coordinating with trapezius and rhomboid muscle actions to stabilize the shoulder girdle. Additionally, axial rotation of the clavicle reaches up to 50 degrees posteriorly during arm elevation, occurring mainly at the SC joint to align the AC joint for continued humeral abduction beyond 90 degrees.[67] These motions are guided by supporting ligaments such as the costoclavicular and interclavicular ligaments, which limit excessive translation.Within the scapulohumeral rhythm—the coordinated sequence of glenohumeral and scapulothoracic movements—the clavicle contributes significantly to full arm abduction, rotating 30-50 degrees posteriorly to accommodate the final phase of humeral elevation to 180 degrees. This rotation, combined with 15 degrees of additional elevation, ensures the scapula upwardly rotates by about 60 degrees overall, preventing impingement and optimizing force transmission from the upper limb to the axial skeleton. Muscle contributions, including the sternocleidomastoid for elevation and pectoralis minor for protraction, further modulate these kinematics without altering the clavicle's primary role in load distribution.Biomechanically, the clavicle experiences varying stress during dynamic shoulder activities, with bending moments peaking laterally due to its cantilever-like configuration from the fixed SC joint. During arm swings or abduction, torque along the longitudinal axis can reach 2.4 Nm, primarily from inertial forces and muscular pulls, concentrating compressive and shear stresses at the distal third.[68] This lateral predominance of moments underscores the clavicle's vulnerability in high-velocity motions, though its S-shaped curvature helps dissipate forces across the shaft.
Clinical Significance
Fractures and injuries
Clavicle fractures represent a significant portion of skeletal injuries, accounting for 2% to 10% of all fractures and occurring at a rate of approximately 1 in 1000 people per year.[6] They are the most common fracture in children, comprising 44% to 66% of pediatric shoulder fractures, and exhibit a bimodal distribution in adults, with peaks in males under 25 years from sports-related trauma and over 55 years from falls.[69] The primary mechanism involves indirect force from a fall onto the lateral shoulder, which accounts for about 87% of cases, while direct blows or falls on an outstretched hand are less frequent.[6]Fractures are classified primarily by anatomical location using the Allman system, which divides them into three groups: Group I (middle third, 69% of cases, often simple transverse patterns), Group II (distal third, 28%, frequently associated with acromioclavicular joint dislocation), and Group III (medial third, 3%, often intra-articular involving the sternoclavicular joint).[6] For distal fractures, the Neer classification provides further detail: Type I (minimal displacement lateral to the coracoclavicular ligaments, stable), Type II (displacement due to coracoclavicular ligament disruption, unstable), and Type III (intra-articular extension into the acromioclavicular joint, comminuted).[6] Middle third fractures, the most prevalent, are typically simple and transverse, while lateral and medial variants carry higher instability risks.[69]Complications arise in a minority of cases but can be severe, including neurovascular injuries such as brachial plexus damage or subclavian vessel disruption from fragment displacement, pneumothorax from apical lung penetration, and nonunion rates of 6% for midshaft fractures (higher at 28% to 44% for distal types).[6] Malunion is more common but often asymptomatic, though it may affect shoulder cosmesis or function.[69]Initial management prioritizes conservative approaches for nondisplaced or minimally displaced fractures, using a sling or figure-of-eight brace for 4 to 6 weeks to promote union, which typically occurs in 18 to 28 weeks.[6] Open reduction and internal fixation (ORIF) with plates and screws is indicated for significantly displaced, comminuted, or open fractures, particularly those with shortening greater than 2 cm or neurovascular compromise, to restore anatomy and reduce nonunion risk.[69]
Congenital and acquired disorders
Congenital disorders of the clavicle primarily involve developmental anomalies that result in structural abnormalities present at birth. Cleidocranial dysplasia (CCD), also known as cleidocranial dysostosis, is an autosomal dominant skeletal disorder characterized by hypoplastic or absent clavicles, delayed closure of fontanels, and dental anomalies.[70] This condition arises from heterozygous mutations in the RUNX2 gene, which encodes a transcription factor essential for osteoblast differentiation and bone formation.[71] In affected individuals, the clavicles may be partially or completely absent, leading to increased shoulder mobility but potential cosmetic and functional concerns.[72] Approximately 30% of CCD cases lack identifiable RUNX2 mutations, suggesting possible regulatory or other genetic factors.[70]Another rare congenital condition is pseudarthrosis of the clavicle, defined as a failure of fusion of the primary ossification centers during embryonic development, resulting in a non-union at the mid-clavicle present at birth.[73] This anomaly typically manifests as a painless, mobile mass over the clavicle, more commonly affecting the right side in females and the left in males, with an estimated incidence of 1 in 17,481 live births based on large cohort studies.[74] Over 400 cases have been reported worldwide as of 2023, and it is often unilateral, though bilateral involvement occurs in about 10% of instances.[74] The condition does not typically cause pain or functional impairment in infancy but may lead to asymmetry or pseudarthrosis progression with growth if untreated.[73]Acquired disorders of the clavicle encompass non-traumatic pathologies that develop postnatally, including infections, degenerative changes, and neoplastic processes. Osteomyelitis of the clavicle is an uncommon bone infection, accounting for less than 1% of all osteomyelitis cases, often resulting from hematogenous spread or contiguous extension from adjacent infections, though it can rarely follow trauma.[75] In children, it may present with swelling and fever, while in adults, chronic forms can mimic tumors or fractures; Staphylococcus aureus is the most frequent pathogen.[76] Prompt diagnosis via imaging and culture is crucial, as untreated cases can lead to sequestrum formation or chronic sinus tracts.[75]Degenerative arthritis affects the sternoclavicular (SC) and acromioclavicular (AC) joints, with prevalence increasing after age 40 due to cumulative mechanical stress and age-related cartilage loss.[77] In the AC joint, radiographic evidence of osteoarthritis is found in 54-57% of individuals over 50, often causing anterior shoulder pain exacerbated by overhead activities.[78] Similarly, SC joint osteoarthritis is associated with shorter clavicle length and manifests as medial clavicular pain or swelling, impacting up to 20-30% of older adults with shoulder complaints.[79] These changes involve joint space narrowing, osteophyte formation, and subchondral sclerosis, contributing to reduced range of motion.[79]Tumors of the clavicle are infrequent, representing less than 1% of primary bone neoplasms, and can significantly alter bone density through lytic or sclerotic patterns. Primary malignancies such as osteosarcoma, though rare in the clavicle (accounting for less than 1% of all osteosarcomas), produce osteoid matrix and typically affect adolescents, leading to aggressive bone destruction and soft tissue extension.[80] Metastatic lesions, more common in adults, originate from primaries like breast, lung, or prostate cancer and cause osteolytic changes that decrease bone density, predisposing to pathologic fractures.[81] Blastic metastases, such as from prostate carcinoma, increase density via reactive bone formation.[81] Early detection is vital, as clavicular involvement often indicates advanced disease.[81]
Diagnostic imaging and treatment
Diagnostic imaging of the clavicle primarily relies on radiography as the initial modality due to its accessibility and ability to detect fractures effectively. Standard anteroposterior (AP) views of the clavicle and shoulder provide essential visualization of the bone's alignment and any displacement, forming the cornerstone for initial assessment in acute injuries.[82] For evaluating the acromioclavicular (AC) joint specifically, the Zanca view—a 10-15° cephalic tilt AP projection—enhances clarity of the distal clavicle and joint space by reducing overlap from the acromion.[83] Axial views may supplement these to assess sternoclavicular involvement, though they are less routinely used.[84]Advanced imaging techniques are reserved for cases requiring detailed evaluation of fracture complexity or associated soft tissues. Computed tomography (CT) scans offer multiplanar reconstruction to precisely delineate fracture patterns, fragment displacement, and intra-articular extension in complex or comminuted fractures, particularly when planning surgical intervention.[85] Magnetic resonance imaging (MRI) is valuable for assessing soft tissue injuries, such as ligamentous damage or neurovascular involvement, without radiation exposure, though it is not first-line due to cost and availability.[85] Ultrasound serves as a dynamic, non-invasive tool for detecting ligament tears around the AC joint, allowing real-time assessment of joint stability and effusion, with high sensitivity in acute settings.[86]Treatment of clavicle fractures and related disorders emphasizes restoring function while minimizing complications, with options stratified by injury severity. Nonoperative management with sling immobilization is standard for nondisplaced fractures, but surgical fixation is indicated for significantly displaced midshaft fractures (shortening >20 mm, >100% displacement, or skin tenting) to prevent nonunion and maintain shoulder mechanics. As of 2025, recent practice favors minimally invasive techniques such as intramedullary nailing to reduce soft tissue disruption and improve recovery times.[84] Intramedullary nailing involves inserting a flexible or rigid nail through the fracture site to achieve stable, minimally invasive fixation, offering reduced soft tissue disruption and quicker recovery compared to open techniques, particularly in simple transverse or short oblique patterns.[87] Locking plate fixation, using precontoured plates applied superiorly or anteriorly, provides rigid stability for comminuted or unstable fractures, with indications including polytrauma or open injuries, though it carries a higher risk of implant prominence.[87] For distal clavicle fractures with coracoclavicular ligament disruption, hook plates or coracoclavicular reconstructions may be employed to restore joint stability.[88]Rehabilitation follows immobilization or surgery to promote healing and restore range of motion, typically beginning within days to weeks depending on stability. Pendulum exercises, involving gentle forward flexion and circular arm swings while leaning forward, are initiated early to prevent shoulder stiffness without stressing the fracture site, progressing to active-assisted and strengthening phases over 6-12 weeks.[89]Physical therapy protocols emphasize gradual loading, with full return to activities often achieved by 3-6 months post-treatment.[90]
Comparative Anatomy
In mammals
The clavicle is present in most mammalian species, where it serves as a key component of the pectoral girdle, providing structural support to the forelimbs by linking the scapula to the sternum and enabling efficient weight transmission during locomotion.[91] In terrestrial mammals, it facilitates a range of movements, from quadrupedal gait to suspensory behaviors, but is notably absent or highly reduced in certain aquatic lineages, such as cetaceans (whales and dolphins), where the loss accommodates streamlined body forms and enhanced scapular mobility for propulsion through water.[91]In quadrupedal mammals, the clavicle is typically shorter and more curved compared to those in more upright or suspensory species, enhancing stability and load-bearing during horizontal locomotion; for example, in dogs (Canis familiaris), it is represented by a vestigial fibrous ligament, which minimizes interference with the forelimb's protraction and retraction.[92] This morphology contrasts with the more robust forms in other quadrupeds like cats, where a slender, curved clavicle still provides limited bracing but allows greater flexibility in predatory movements.[92]Among primates, the clavicle exhibits notable elongation, particularly in species adapted to brachiation—arm-swinging locomotion—where it acts as a strut to position the scapula laterally and support overhead arm suspension; gibbons (Hylobatidae), for instance, possess exceptionally long clavicles relative to body size, which correlates with their specialized arboreal lifestyle.[93] In humans (Homo sapiens), the clavicle adopts a unique horizontal orientation, with a pronounced S-shaped curvature that aligns the shoulders laterally, optimizing the glenohumeral joint for overhead reaching and tool use in bipedal posture.[93]Marsupials display variations in the pectoral girdle, retaining a well-developed clavicle similar to other therians for forelimb support, but uniquely featuring epipubic bones in the pelvic girdle that project anteriorly to stabilize the pouch.[94]
In non-mammalian vertebrates
In non-mammalian vertebrates, the clavicle exhibits diverse morphologies as a dermal bone contributing to the pectoral girdle, varying by group and often adapted to locomotion or habitat. In fish, particularly teleosts like zebrafish, the clavicle forms a dermal element of the pectoral girdle, emerging early in development alongside the cleithrum to support the fin skeleton and integrate with branchiostegal rays for gill protection and buoyancy.[95] This structure arises from neural crest-derived mesenchyme, highlighting its evolutionary role in stabilizing the appendage base before tetrapod transitions.[26]Amphibians show further reduction of the clavicle, often absent or vestigial in adults, with remnants primarily in larval stages such as tadpoles where precursors contribute to the developing girdle before metamorphosis. In modern anurans like frogs, any persisting clavicle is small and paired, forming part of a simplified dermal ossification alongside a reduced cleithrum, reflecting adaptations to jumping and aquatic-terrestrial shifts.[96] Urodeles (salamanders) typically lack clavicles entirely in adulthood, emphasizing the girdle's minimal role in weight-bearing compared to endochondral elements like the scapulocoracoid.[97]Among reptiles, the clavicle is generally present as a paired dermal bone, articulating with an interclavicle to form a robust ventral girdle that anchors forelimb muscles and provides structural support, as seen in lizards where it appears as a large, flat, triangular plate.[98] In crocodilians, however, clavicles are absent, leaving the interclavicle as a small, median dermal element anterior to the sternum that aids in terrestrial locomotion by stabilizing the shoulder region without direct clavicular contribution.[97] This configuration contrasts with other reptiles, where the full dermal complex (clavicles plus interclavicle) enhances girdle rigidity for sprawling gait.[99]Birds represent a specialized case, with the clavicles fused midline into the furcula or "wishbone," a Y- or U-shaped structure that springs to store elastic energy during flight downstrokes and reinforces the thoracic skeleton against aerodynamic stresses.[100] In flight-capable species, the furcula articulates with the sternum and coracoids for enhanced stability, but it is reduced or absent in ratites like ostriches, correlating with flightlessness and reliance on powerful hindlimb propulsion.[101]
Evolutionary history
The clavicle originated as a dermal bone in Devonian fishes, where it functioned primarily to support the gill apparatus and contribute to the protective dermal skeleton of the pectoral region. Fossil evidence from early gnathostomes, such as those from the Late Devonian, indicates that the clavicle-like elements, including the cleithrum, formed part of the branchial basket and aided in stabilizing the opercular series during respiration.[102] This pharyngeal derivation underscores the clavicle's role in an aquatic context before the evolution of terrestrial locomotion.[103]During the tetrapod transition in the Late Devonian and Early Carboniferous, paired clavicles emerged in stem tetrapods and early amphibians, adapting to support weight-bearing on terrestrial substrates. In fossils like those of Ichthyostega and Acanthostega, the clavicles formed a robust coracoid-clavicle-interclavicle complex that braced the forelimbs against the body, facilitating propulsion and load distribution during shallow-water or mudflat ambulation.[104] This structural innovation marked a shift from the sprawling, fin-based locomotion of sarcopterygians to the more upright, limb-driven gait of early tetrapods, with the clavicles providing anchorage for emerging shoulder musculature.[26]In the synapsid lineage leading to mammals, the clavicle underwent significant elongation and curvature changes to accommodate the transition from sprawling to parasagittal limb postures during the Permian and Triassic periods. Non-mammalian synapsids, such as dimetrodonts and cynodonts, exhibited progressively S-shaped clavicles that enhanced forelimb protraction and supported the shift toward more erect postures, enabling efficient terrestrial movement.[105] By the origin of crown-group mammals in the Late Triassic, this elongation stabilized the glenoid fossa against the axial skeleton, optimizing force transmission in upright gaits.[106]Among dinosaurs, clavicles were retained in theropod lineages, often fusing into a furcula that stiffened the pectoral girdle for predatory agility; for instance, a Y-shaped furcula is documented in Tyrannosaurus rex, aiding in forelimb stabilization during locomotion.[107] In contrast, clavicles were absent or reduced in many sauropodomorphs, particularly titanosauriforms, correlating with their wide-gauge, columnar limb posture that relied less on clavicular bracing.[99]In human evolution, clavicles in the Homo genus, including Homo erectus, exhibit lengths within modern human variation, supporting a laterally oriented shoulder that facilitates arm motions critical for hunting and tool use, with high-speed throwing capacity dating back approximately 2 million years. Fossil evidence from Middle Pleistocene Homo specimens shows clavicles similar to modern lengths.[108]
Position of collarbone (shown in red). Animation.
Shape of collarbone (left). Animation.
3D image
Pectoral girdle—front
Diagram of the human shoulder joint, front view
Diagram of the human shoulder joint, back view
Muscles of the neck. Anterior view.
Clavicle
Media related to Clavicle at Wikimedia Commons
