The sternum is the flat, blade-shaped bone running down the center of your chest, connecting most of your ribs through cartilage and forming the front wall of the ribcage. It sits just beneath the skin, which is why you can feel it easily by pressing a finger to the middle of your chest. Despite being one of the most accessible bones in the body, the sternum does far more than simply close off the front of the thorax. It protects the heart and major blood vessels directly behind it, plays an active mechanical role every time you breathe, houses bone marrow used in medical diagnostics, and has become one of the most commonly divided bones in modern surgery.
Three Parts, One Bone
The sternum has three distinct segments. The top portion, called the manubrium, is a roughly trapezoidal plate that articulates with the collarbones and the first two pairs of ribs. Below it lies the body (sometimes called the gladiolus or corpus sterni), which is the longest segment and connects to ribs three through seven via strips of costal cartilage. At the bottom sits the xiphoid process, a small cartilaginous or bony projection that serves as an attachment point for abdominal muscles and the diaphragm. In younger people the xiphoid is mostly cartilage; it gradually ossifies over the course of decades, and in many adults it never fully turns to bone.
Where the manubrium meets the body, the bone angles slightly forward, creating a palpable ridge called the sternal angle (also known as the angle of Louis). Clinicians use this landmark constantly because it sits at the level of the second rib, making it a reliable starting point for counting ribs during a physical exam. The sternal angle also marks the approximate level where the trachea divides into the left and right bronchi.
How the Sternum Helps You Breathe
The sternum is not a passive shield. During breathing, it moves upward and forward as the ribs expand, a motion sometimes described as a “pump handle.” An in-vivo imaging study measured the sternum’s upward displacement at roughly 18 to 19 millimeters over a full inspiratory effort, with the sternal angle varying by about four and a half degrees on average.1PubMed. In-vivo analysis of sternal angle, sternal and sternocostal kinematics in supine humans during breathing That might not sound like much, but it is enough to meaningfully change the volume of the thoracic cavity with each breath. The joints where the ribs attach to the sternum (sternocostal joints) allow small rotations that decrease gradually from the lower ribs to the upper ones, giving the chest wall its characteristic accordion-like expansion.
The sternum’s contribution to thoracic stiffness is also substantial. When researchers tested cadaveric thoracic spines and systematically released different structures, releasing the sternum and its rib connections accounted for about 42% of total sagittal range of motion, more than the contribution of facet joints alone.2Spine. The Role of the Sternum, Costosternal Articulations, Intervertebral Disc, and Facets in Thoracic Sagittal Plane Biomechanics In practical terms, the sternum and costal cartilage work together as a major structural brace for the upper body. Anything that compromises their integrity, whether a fracture, a surgical cut, or a congenital malformation, changes how the chest moves and bears load.
How the Sternum Forms Before Birth
The sternum does not begin as a single midline structure. Early in embryonic development, two bars of cartilage form separately on either side of the chest. These sternal bars migrate toward each other as the body wall grows inward, eventually meeting and fusing along the midline. Studies of human embryos show that the bilateral bars begin fusing after about seven weeks of gestation, during what embryologists call Carnegie Stage 20.3PubMed Central. Development of the ventral body wall in the human embryo If this midline fusion fails, the result is a sternal cleft, a rare congenital anomaly in which part or all of the sternum remains open, leaving the heart covered only by skin and soft tissue.4PubMed Central. Isolated V-Shaped Sternal Cleft – A Rare Chest Wall Malformation These cases are uncommon enough that they show up mostly as individual case reports, but they illustrate just how critical that embryonic migration step is.
Pectus Excavatum and Pectus Carinatum
The two most recognized congenital chest wall deformities both center on the sternum. In pectus excavatum (sometimes called funnel chest), the sternum and adjacent costal cartilage are pushed inward, creating a visible depression. In pectus carinatum (pigeon chest), the sternum juts outward. Both are thought to involve abnormal growth or structural weakness in the costal cartilage rather than a defect in the sternum itself, though the exact cause remains debated. Leading theories point to overgrowth or metabolic disturbances of the cartilage, and these mechanisms may overlap rather than being mutually exclusive.5PubMed Central. Pectus excavatum and carinatum: a narrative review of epidemiology, etiopathogenesis, clinical features, and classification
At the cellular level, researchers have found that cartilage cells (chondrocytes) from people with these deformities show reduced expression of a gene called ASIC2, which encodes an acid-sensing ion channel. When exposed to low pH, their chondrocytes mount a weaker calcium response than normal cells do, and that response deteriorates further on repeated exposure.6PubMed Central. Abnormal response of costal chondrocytes to acidosis in patients with chest wall deformity The molecular chain between that finding and the visible deformity is still being worked out, but it suggests that the cartilage in affected individuals is biochemically different, not just mechanically overgrown.
Beyond appearance, these conditions carry functional consequences. Adolescents with pectus excavatum or pectus carinatum tend to have lower body weight, poorer posture, and reduced physical function compared to peers without chest wall deformities. Those with pectus excavatum also report lower scores on some measures of psychosocial well-being and quality of life, while pectus carinatum appears to affect posture even more than pectus excavatum does.7PubMed. Comparison of physical functions and psychosocial conditions between adolescents with pectus excavatum, pectus carinatum and healthy controls Interestingly, the severity of the deformity itself does not correlate well with how impaired a person feels. A person’s awareness and self-consciousness about the condition appears to track more closely with functional and psychosocial outcomes than the actual depth of the dent or prominence of the protrusion.
Sternal Fractures
Sternal fractures most commonly result from blunt force to the chest, typically a seatbelt injury in a motor vehicle collision or a direct blow during contact sports. The fracture usually occurs in the body of the sternum, at or just below the sternal angle, where the bone is thinnest. Pain with breathing, tenderness over the fracture site, and sometimes visible swelling or bruising are the hallmarks. Most sternal fractures are treated without surgery. Rest, pain management, and time are generally sufficient, though physicians monitor for associated injuries because the force required to break the sternum often causes damage to structures behind it, particularly cardiac contusion or thoracic spine injuries.
One important distinction: sternal fractures caused by CPR (cardiopulmonary resuscitation) are a separate clinical category. Chest compressions during CPR frequently crack the sternum, especially in older adults, but these fractures are expected consequences of a life-saving procedure and are managed differently from traumatic ones.8PubMed Central. Traumatic Sternal Fractures can be Safely Treated Conservatively – A 13-Year Retrospective Cohort Study
Median Sternotomy and Its Complications
If you have ever seen a scar running down the center of someone’s chest, you have seen the aftermath of a median sternotomy. This approach, in which the sternum is sawed in half lengthwise, is the standard way to access the heart for open-heart surgery. Although the technique was first described in 1897, it was essentially ignored for six decades until surgeons rediscovered its value in the late 1950s. With the rise of coronary artery bypass grafting in the late 1960s, it became one of the most commonly performed surgical incisions in the world.9PubMed. Median sternotomy
After surgery, the two halves of the sternum are wired back together with stainless steel cerclage wires. Healing takes roughly six to eight weeks under ideal circumstances, and most patients are told to avoid heavy lifting or pulling motions during that window. Complications, while relatively uncommon, can be serious. In one series of 2,400 consecutive sternotomies, roughly 6% of patients developed superficial wound separation, about 3% had deeper dehiscence (the sternal halves pulling apart without infection), and a small fraction developed mediastinitis, a deep infection of the space behind the sternum.10PubMed. Superficial wound dehiscence after median sternotomy: surgical treatment versus secondary wound healing Deep sternal wound infections are divided into superficial infections confined to the soft tissue and deep infections that involve the bone and mediastinum, and the distinction matters because deep infections require much more aggressive treatment.11PubMed Central. Wound Healing Complications After Sternotomy—Causes, Prevention, and Treatment—A New Look at an Old Problem On imaging, complications range from hardware-related issues like broken or migrated wires in the acute phase to bone-related problems like nonunion or chronic infection months later.12PubMed. Poststernotomy Imaging: Pictorial Review of Expected Postsurgical Findings and Complications
Bone Marrow Aspiration From the Sternum
The sternum is one of the sites where bone marrow can be sampled, though the iliac crest (hip bone) is more commonly used today. Sternal aspiration is performed using a specialized needle with an adjustable guard that limits how deeply the needle can penetrate. The puncture is made just off the midline, at the level of the second or third intercostal space. The guard is typically set to allow only about 5 millimeters of penetration beyond the front surface of the bone, and the operator pushes through the cortex using a careful twisting motion until resistance drops, indicating entry into the marrow cavity.13PubMed Central. Professional accountability in a sternal bone marrow aspiration: a forensic case report
The reason for all this caution is that the sternum is thin, and the structures behind it are unforgiving. If the needle passes entirely through the bone, it enters the mediastinum, where the ascending aorta and heart lie directly behind the sternal body. Research measuring sternal thickness has shown that this risk is higher in older individuals with shorter stature, whose sterna tend to be thinner.14PubMed. Risk factors concerning sternal bone marrow aspiration and patient safety in Japan Fatalities from sternal puncture are rare but have been documented since the procedure was introduced in 1927, with reported deaths attributed to cardiac tamponade caused by the needle lacerating a great vessel.15JAMA Surgery. Acute Cardiac Tamponade, a Complication of Sternal Marrow Aspiration These risks are a major reason why most clinicians now prefer the iliac crest for routine marrow biopsies.
Infections That Reach the Sternum
Most anterior chest wall pain that sends people to a doctor turns out to be costochondritis, an inflammation of the cartilage connecting the ribs to the sternum. Costochondritis is typically caused by physical strain, repetitive movement, or heavy coughing, and it resolves with anti-inflammatory medication. It is not an infection. True infectious costochondritis, where bacteria invade the cartilage and potentially the sternum itself (sternal osteomyelitis), is far less common and usually develops through a different route: spread from a postoperative wound, a neighboring abscess, or, more rarely, through the bloodstream. One documented case involved a man with diabetes who developed osteomyelitis of the lower sternum from a Pseudomonas aeruginosa infection that did not originate from surgery or adjacent tissue, an unusual presentation that required imaging to identify the extent of bone and cartilage involvement.16PubMed Central. Infectious Costochondritis With Sternal Osteomyelitis The takeaway: chest wall pain is almost never an infection, but persistent pain with swelling or drainage warrants a closer look.
Tumors of the Sternum
Primary tumors originating in the sternum are uncommon. In a series spanning more than six decades at a major center, 54 patients were identified with primary malignant sternal tumors. The most common type by far was chondrosarcoma, a cancer arising from cartilage, which accounted for roughly half the solid tumors. Osteosarcoma was the next most frequent. Most patients presented with a palpable mass, and about half also had sternal pain.17PubMed. Predictors of survival in malignant tumors of the sternum Treatment of sternal tumors often requires wide resection, removing the affected portion of the sternum along with a margin of healthy tissue, which raises the question of how to rebuild the chest wall afterward.
That challenge has driven some remarkable advances. Surgeons have begun using 3D-printed titanium prostheses custom-shaped to match the patient’s anatomy. In one reported case, a patient underwent extensive resection for a chest wall sarcoma and received a titanium implant designed from their own CT scan data.18European journal of cardio-thoracic surgery : official journal of the European Association for Cardio-thoracic Surgery. Tridimensional titanium-printed custom-made prosthesis for sternocostal reconstruction A similar approach was used in a patient who required sternal excision for metastatic breast cancer, with the 3D-printed prosthesis reported to deliver both structural integrity and a good cosmetic result.19PubMed. Novel Sternal Reconstruction With Custom Three-Dimensional-Printed Titanium PoreStar Prosthesis These are still individual case reports rather than routine procedures, but they represent a plausible near-future direction for chest wall reconstruction.
Sternoclavicular Joint Injuries
The sternoclavicular joint, where the collarbone meets the manubrium, is one of the most overlooked joints in the body, partly because injuries there are relatively rare and partly because the joint is hard to image on standard X-rays. When trauma does dislocate the inner end of the clavicle, the direction matters enormously. Anterior dislocations (the clavicle shifts forward) are far more common and usually benign. Posterior dislocations push the clavicle backward into the space behind the sternum, where it can compress or damage the trachea, esophagus, or major blood vessels. MRI studies of injured sternoclavicular joints found articular disc damage in about 80% of cases and posterior ligament injuries in roughly 40%.20PubMed. MR imaging of the sternoclavicular joint following trauma Cases of posterior clavicular subluxation showed impingement on mediastinal structures even when the clavicle was not fully dislocated, which explains why emergency physicians treat posterior sternoclavicular dislocations with more urgency than anterior ones.
The Sternum as a Forensic Clock
Forensic anthropologists have long been interested in the sternum because its ossification pattern changes with age. The three segments of the sternum begin as separate cartilaginous pieces in childhood and fuse gradually over the course of adulthood. In theory, the degree of fusion at the manubriosternal joint (between the manubrium and the body) and the xiphisternal joint (between the body and the xiphoid process) should correlate with age. In practice, the relationship is messier than it looks.
A study of CT scans from a population in northern India found that manubrium-to-body fusion explained about 40% of the variance in age, with younger individuals showing no fusion and partial or complete fusion becoming more common in older individuals. The same study confirmed sex-based differences in ossification timing.21Journal of Indian Academy of Forensic Medicine. Radiological Age Estimation from Manubriosternal and Xiphisternal Joints But a study of 461 sterna from a South African population painted a much less encouraging picture: the majority of sterna remained unfused throughout adult life, and complete fusion was observed in individuals as young as 25 as well as much older ones. Logistic regression accuracy for age estimation was only about 63%, leading the researchers to call sternal fusion “highly variable” and unreliable for adult age estimation in that population.22PubMed. Forensic age-at-death estimation from the sternum in a black South African population
Japanese researchers have tried to improve accuracy by scoring not just sternal joint fusion but also the ossification of costal cartilage at its sternal attachment points, using 3D post-mortem CT. They found that ossification at the sternal end of the costal cartilages had the highest correlation with age, while manubriosternal fusion had no significant correlation at all in their sample. The best composite score yielded a standard error of about 12 to 15 years depending on sex, which limits its precision but can still help narrow a wide age window.23Legal Medicine. Age estimation from ossification of sternum and true ribs using 3D post-mortem CT images in a Japanese population The emerging picture is that sternal fusion alone is too variable across populations to serve as a standalone age indicator, but when combined with costal cartilage ossification scores, it can contribute to a reasonable estimate.
The Sternalis Muscle and Other Anatomical Surprises
Some people have a muscle that does not appear in standard anatomy textbooks. The sternalis muscle is a thin, vertical band of muscle that runs along the surface of the sternum, parallel to it, within the soft tissue of the anterior chest wall. It is present in a minority of people and is usually found incidentally on imaging or during surgery. Its clinical relevance is subtle but real: on a mammogram, the sternalis can mimic a breast mass, leading to unnecessary follow-up if the radiologist is unfamiliar with the variant.24PubMed. Sternalis muscle, what every anatomist and clinician should know During thoracic surgery, encountering an unexpected sternalis muscle can complicate and prolong the procedure.25PubMed Central. Sternalis Muscle: A Cadaveric Case Report of a Rare Variant With Multiple Branching The muscle has no clearly established function, and its embryological origin is debated, but awareness of it matters for anyone reading a chest image or operating in the anterior thoracic region.
How the Sternum Evolved
The sternum has a surprisingly long evolutionary pedigree. In the lineage that eventually led to mammals, a segmented sternum appeared far earlier than previously appreciated. A nearly complete skeleton of Gorgonops torvus, a predatory synapsid from the Permian period (roughly 260 million years ago), preserves an articulated sternum consisting of a large plate-like element plus three smaller flat segments. This arrangement of a broad anterior element with smaller posterior “sternebrae” looks strikingly similar to the mammalian pattern of a manubrium plus sternal body segments, although the precise homologies remain uncertain.26Scientific Reports. The earliest segmental sternum in a Permian synapsid and its implications for the evolution of mammalian locomotion and ventilation
More recent developmental work suggests that the mammalian sternum is not a single evolutionary invention but a composite structure. The manubrium appears to incorporate the ancestral interclavicle, a midline bone found in reptiles and early synapsids, while the sternebrae of the body represent a developmentally distinct set of skeletal elements. These two components were originally independent structures that merged over evolutionary time.27Current Biology. The therian sternum is a composite structure including the ancestral interclavicle This dual origin helps explain why the manubrium and the body of the sternum remain partially separate throughout life in many adults, joined only at the manubriosternal joint rather than fusing completely.
Why Bird Sterna Look So Different
If you have ever carved a chicken, you have noticed the prominent ridge running down the center of the breastbone. That ridge, the sternal keel, is the defining feature of the avian sternum and exists for one reason: to anchor the massive flight muscles. A large-scale comparative study of bird sterna found that species using forelimb propulsion (flying, or in some cases “flying” underwater like penguins) have sterna roughly a third larger than non-flying birds of the same body mass.28PubMed Central. The relationship between sternum variation and mode of locomotion in birds Within flying species, deeper keels correlated with slower but more powerful flight styles, while more robust caudal (rear) borders of the sternum were associated with faster flapping. Narrower sterna, meanwhile, were linked to running ability. The avian sternum, in other words, is not just structurally different from the mammalian version but functionally tuned to each species’ mode of locomotion, making it one of the most informative bones in comparative vertebrate anatomy.