The clavicle, commonly called the collarbone, is a slender S-shaped bone that sits horizontally across the top of the chest, connecting the shoulder blade to the breastbone. It is the only bony link between the arm and the trunk, and its anatomy is more complex than its superficial position suggests. The bone’s unusual development, its role as a strut for shoulder movement, and the critical blood vessels and nerves running directly beneath it all make clavicle anatomy relevant well beyond the anatomy classroom.
Shape, Size, and the S-Curve
The clavicle is roughly the length of a pencil. A three-dimensional cadaveric study found the mean length to be about 159 mm, with male clavicles significantly longer (around 167 mm) and wider than female ones (around 151 mm).1PubMed Central. The anatomy of the clavicle: a three-dimensional cadaveric study The bone’s signature feature is its double curve: the medial (inner) two-thirds bows forward, while the lateral (outer) third sweeps backward. This creates the S-shape you can feel through your skin. Male clavicles are more curved than female ones in both regions, and left clavicles tend to be slightly longer than right ones.
At the medial end, the clavicle is rounded and thick where it meets the sternum at the sternoclavicular joint. At the lateral end, it flattens into a paddle-like shape where it articulates with the acromion of the shoulder blade at the acromioclavicular (AC) joint. That transition from round to flat is visible on any dried specimen and reflects the very different forces each end has to handle.
A Bone That Forms Unlike Any Other
The clavicle holds a special place in human embryology. It is the first bone in the body to begin hardening, and it does so through a process found in no other long bone: a combination of two different types of bone formation. The shaft develops directly from condensed tissue without a cartilage template, while the ends go through the cartilage-to-bone conversion that most long bones use exclusively.2Orthopaedics and Trauma. The enigmatic role and development of the clavicle Development begins from two separate hardening centers that extend outward from a cord of condensed cells running from the shoulder toward the breastbone.3Bone. A computational model of clavicle bone formation: A mechano-biochemical hypothesis
This dual origin has puzzled anatomists for over a century. It also has clinical relevance: conditions that disrupt one type of bone formation can affect the clavicle differently than the rest of the skeleton, as we will see with cleidocranial dysplasia later.
Muscles That Attach to the Clavicle
The clavicle serves as an anchor point for several large muscles. On the top and behind, the trapezius attaches to the posterior and partly upper surface. The sternocleidomastoid, the prominent neck muscle that turns your head, grips the inner upper portion. On the front, the pectoralis major and deltoid attach to the anterior surface.4PubMed Central. Insertion sites of the muscles attached to the clavicle: a cadaveric study of the clavicle This leaves a relatively bare strip along the upper back surface of the bone where no muscle attaches, a detail surgeons note because placing hardware there is less likely to interfere with muscle function.
Underneath, the small subclavius muscle runs along the bone’s inferior surface, acting partly as a cushion between the clavicle and the neurovascular structures below. Taken together, these muscle origins and insertions mean the clavicle is constantly being pulled in multiple directions at once, which explains why fractures there can be displaced so dramatically.
The Ligament System Holding It in Place
Two sets of ligaments keep the clavicle properly aligned. At the outer end, the coracoclavicular ligaments connect the underside of the clavicle to the coracoid process of the shoulder blade. These consist of the conoid ligament, which runs vertically from the back of the coracoid to a bony bump called the conoid tubercle on the clavicle’s underside, and the trapezoid ligament, which fans out more horizontally from the top of the coracoid to the trapezoid line on the clavicle’s underside.5PubMed Central. Qualitative and Quantitative Anatomic Descriptions of the Coracoclavicular and Acromioclavicular Ligaments: A Systematic Review Together, these two ligaments are the primary vertical stabilizers of the AC joint. When they tear, as in a severe shoulder separation, the clavicle rides upward.
A less well-known structure, the medial coracoclavicular ligament, has been identified as a distinct band running from the coracoid to the clavicle and subclavius sheath. Though it has less tensile strength than the main coracoclavicular ligaments, cutting it after those ligaments are already severed causes a significant additional increase in both upward and backward displacement of the clavicle.6PubMed Central. The medial coracoclavicular ligament: anatomy, biomechanics, and clinical relevance-a research study This matters for surgeons reconstructing shoulder separations, because ignoring this ligament could leave residual instability.
How the Clavicle Moves When You Raise Your Arm
People tend to think of the clavicle as a fixed bar, but it undergoes considerable motion during shoulder movement. When you raise your arm overhead, the clavicle elevates, retracts, and rotates backward along its long axis. One study measured roughly 11 to 15 degrees of elevation, 15 to 29 degrees of retraction, and 15 to 31 degrees of backward axial rotation during arm elevation, with substantial variation between individuals and planes of motion.7PubMed. Three-dimensional clavicular motion during arm elevation: reliability and descriptive data An MRI-based study found even larger values for retraction (about 31 degrees) and posterior axial rotation (about 33 degrees) during arm abduction.8PubMed. Three-dimensional clavicular and acromioclavicular rotations during arm abduction using vertically open MRI
At the AC joint itself, motion is smaller but still meaningful. During active shoulder abduction to 90 degrees, the joint undergoes about 15 degrees of upward rotation and around 7 degrees of posterior tilting.9PubMed Central. Three-dimensional acromioclavicular joint motions during elevation of the arm All of this motion is what allows your shoulder blade to rotate fully so you can reach overhead. If the clavicle’s length or alignment is disrupted, the shoulder blade cannot track properly.
Why Clavicle Shortening Matters
This biomechanical link is the reason surgeons care about restoring clavicle length after a fracture. Cadaver research has shown that when the clavicle is shortened by 10% or more, the shoulder blade loses its normal pattern of posterior tilting and external rotation during arm elevation.10PubMed. Effect of shortening deformity of the clavicle on scapular kinematics: a cadaveric study In vivo studies of patients with healed clavicle fractures that shortened beyond this threshold have confirmed the same finding.11PubMed. Effects of short malunion of the clavicle on in vivo scapular kinematics This abnormal scapular motion can contribute to shoulder impingement and weakness, which is one of the arguments for surgical fixation of significantly displaced midshaft clavicle fractures rather than letting them heal with overlap.
What Runs Beneath the Clavicle
Directly underneath the clavicle, separated by the thin subclavius muscle (only about 1 cm thick), lie some of the body’s most critical structures: the subclavian vein, the subclavian artery, and the brachial plexus, the nerve network that controls the entire arm. An anatomical study found that, measured from the sternal end, the subclavian vein sits at roughly 38% of clavicle length, the artery at about 45%, and the brachial plexus cords at about 55%.12PubMed. The relationship between the subclavian vessels and brachial plexus and the overlying clavicle: Anatomical study with application to plate osteosynthesis In practical terms, the neurovascular bundle runs below roughly the medial quarter through to the middle three-fifths of the clavicle. Expressing these distances as ratios of total clavicle length is useful because it accounts for the size difference between men and women.
This proximity is the reason clavicle fractures, while common and usually benign, occasionally become surgical emergencies. A sharply displaced fracture fragment can puncture the subclavian vein or compress the brachial plexus. And it is why surgeons placing plates and screws on the underside of the clavicle treat the medial half with particular caution.
The Costoclavicular Space and Thoracic Outlet Syndrome
The space between the clavicle and the first rib, called the costoclavicular space, is a bottleneck for the vessels and nerves heading to the arm. In thoracic outlet syndrome, this space narrows enough to compress those structures, causing pain, numbness, or vascular symptoms in the arm and hand. CT angiography of patients evaluated for this condition has shown that when the arm is moved into an overhead position, the costoclavicular space shrinks by an average of about 18 mm, which represents a roughly 56% reduction.13American Journal of Physical Medicine & Rehabilitation. Clavicle-Induced Narrowing of the Thoracic Outlet during Shoulder Abduction as Imaged by Computed Tomographic Angiography and Enhanced by Three-Dimensional Reformation In that same study, nearly half the patients showed significant subclavian artery compression and three-quarters showed significant vein compression in the overhead position.
Radiological studies have tried to define what a “normal” costoclavicular distance looks like across different body types and age groups to help clinicians identify pathological narrowing. Measurements below the fifth percentile, which correspond to distances under roughly half a centimeter in younger lean individuals, indicate a greater chance of thoracic outlet compression.14PubMed. Normal Costoclavicular Distance as a Standard in the Radiological Evaluation of Thoracic Outlet Syndrome in the Costoclavicular Space Older or heavier individuals had slightly larger cutoff values, likely reflecting age-related changes in the rib cage and soft tissue.
The Clavicle’s Blood Supply
For a bone that breaks more often than almost any other, knowing how it receives blood is relevant to understanding healing. Nearly all clavicles have at least one nutrient foramen, the tiny hole through which a feeding artery enters the bone. A meta-analysis found nutrient foramina in about 98% of specimens, most commonly as a single opening on the posterior surface of the middle third, with the artery entering at an angle directed toward the outer (acromial) end.15PubMed. The human clavicle’s nutrient foramen’s prevalence, number, location, and direction: A systematic review and meta-analysis Individual studies have found the foramen anywhere from about 38% to 44% of total clavicle length measured from the sternal end, which places it reliably in the middle third.16PubMed. Neurovascular foramina of the human clavicle and their clinical significance
Surgeons who plate clavicle fractures care about this because a plate placed directly over the nutrient foramen could compromise blood flow to the healing bone. Knowing that the foramen almost always sits on the posterior middle-third surface helps guide plate positioning.
Sex Differences and Forensic Identification
The clavicle is one of the most sexually dimorphic bones in the body, making it useful in forensic contexts where other identifying features are unavailable. Beyond sheer length, the mid-shaft circumference appears to be the single most telling measurement: one cadaveric study reported a mean mid-shaft circumference of about 39 mm in males versus 32 mm in females, with this trait showing the highest discriminating power of any clavicle measurement.17PubMed Central. Comparative morphometric analysis of the clavicle in male and female human cadavers: Implications for forensic identification Other research has found that a composite index combining length and circumference can correctly classify sex in roughly 86% of cases.18Translational Research in Anatomy. Variability in anatomical features of human clavicle: Its forensic anthropological and clinical significance
More recently, statistical shape modeling of CT-scanned clavicles has confirmed that about half of total shape variation is driven by length and midshaft width, with significant sex differences in overall size and curvature patterns.19PubMed Central. Statistical Shape Modeling of the Clavicle: Morphological Variation, Sex Differences, and Surgical Implications Interestingly, female and left clavicles tend to be straighter than male and right ones, while clavicular angles are among the least sexually dimorphic traits.
Forensic Age Estimation From the Medial Epiphysis
The clavicle is also one of the last bones in the body to finish growing. The medial epiphysis, the growth plate at the sternal end, begins fusing in the early teens and does not complete fusion until the late twenties.20Radiologic technology. Forensic age estimation using the medial clavicular epiphysis: A study review This long timeline spans the legally critical ages of 18 and 21, which is why forensic physicians often use CT scans of the medial clavicle to estimate whether an individual has reached the age of legal majority.
A systematic review of the technique found that individuals classified as having reached the most advanced stages of fusion (stages 4 and 5) were consistently 18 or older across all studies reviewed.21PubMed. Forensic age estimation using computed tomography of the medial clavicular epiphysis: a systematic review The 21-year threshold is trickier. One study found that bilateral stage 4 fusion achieved 100% specificity for age over 21, but relying on just one side could occasionally overestimate age, as one subject just shy of 21 had advanced fusion on one side only.22PubMed Central. Forensic age estimation of adolescents using computed tomography of the clavicles That individual was a physically demanding manual laborer, a reminder that heavy mechanical loading can accelerate skeletal maturation.
Cleidocranial Dysplasia
Perhaps the most dramatic demonstration of the clavicle’s unique biology is cleidocranial dysplasia, a genetic condition in which the clavicles are partially or completely absent. People with this condition can often bring their shoulders together in front of their chest, a party trick that is actually a sign of absent bony support. The condition results from mutations in the RUNX2 gene, a transcription factor essential for bone formation.23PubMed. Mutational analysis of RUNX2 gene in Chinese patients with cleidocranial dysplasia
Mouse studies have clarified why the clavicle is so sensitive to RUNX2 disruption. There appears to be a critical threshold: when functional RUNX2 levels drop to about 70% of normal, the clavicle and skull develop defects, but when levels stay above roughly 79%, the skeleton appears normal.24PubMed Central. A Runx2 threshold for the cleidocranial dysplasia phenotype This narrow threshold explains the variability seen in patients: different mutations reduce RUNX2 activity by different amounts, producing everything from mildly shortened clavicles to completely absent ones. The clavicle’s dual ossification process makes it especially vulnerable because both types of bone formation depend on adequate RUNX2 levels.
Distal Clavicle Osteolysis in Athletes
Young athletes, particularly weightlifters and overhead sport participants, can develop a puzzling condition in which the outer end of the clavicle gradually dissolves. Called atraumatic osteolysis of the distal clavicle, it is classified as a stress failure syndrome: the bone resorbs faster than it can remodel under repetitive loading.25PubMed. Atraumatic osteolysis of the distal clavicle. A review There is never a history of a single major injury to the AC joint. Instead, these athletes typically have a long training history with heavy strength work. The AC joint becomes the weakest link in the chain, and the distal clavicle pays the price. Treatment usually starts with activity modification, but persistent cases may require surgical excision of the damaged bone end.
Surgical Nerve Hazards on the Clavicle’s Surface
Even if the structures beneath the clavicle are safely avoided during surgery, the sensory nerves crossing over the bone are commonly at risk. The supraclavicular nerve, which provides skin sensation to the upper chest and shoulder region, crosses the clavicle in two to three branches. Nearly all specimens have a medial and lateral branch, and about half have an additional intermediate branch.26PubMed Central. The Anatomy of the Supraclavicular Nerve During Surgical Approach to the Clavicular Shaft Between the safe zones close to each joint, the location of these nerve branches is highly variable, making damage during surgical incisions difficult to avoid entirely.
Cadaveric mapping has tried to define safe corridors. One study identified two windows along the midshaft, at roughly 29–51% and 61–80% of clavicle length measured from the sternoclavicular joint, where incisions are less likely to encounter nerve branches.27PubMed. Safe zone of supraclavicular nerve during clavicle fixation and its anatomical variations, a cadaveric study Even so, numbness in the skin below the surgical scar is one of the most common complaints after clavicle plating, and patients are routinely warned about it beforehand.
Bilateral Asymmetry and Physical Activity
Your two clavicles are not identical, and the differences reflect how you use your body. Studies of bilateral asymmetry have found that clavicle shaft thickness responds to mechanical loading from habitual activity, meaning the dominant-side clavicle tends to be thicker (though not necessarily longer) in populations that perform heavy unilateral work.28PubMed. Patterns of clavicular bilateral asymmetry in relation to the humerus: variation among humans Length asymmetry, by contrast, does not track with activity level the same way, suggesting that clavicle length is more genetically constrained while cross-sectional dimensions are more responsive to environment.
Archaeological data adds a fascinating wrinkle. In a medieval English population, the dominant-side clavicle was actually shorter, consistent with the idea that compressive forces from heavy manual labor inhibit longitudinal growth while simultaneously stimulating the bone to thicken.29International Journal of Osteoarchaeology. Directional asymmetry in the human clavicle So the same mechanical stimulus that makes a clavicle wider can make it a few millimeters shorter, a trade-off between growth in different directions that plays out over years of repetitive loading during skeletal development.