Sternum

The sternum, a rather unremarkable flat bone situated squarely in the middle of the anterior rib cage, serves as the anchor point for a significant portion of our skeletal structure. You might call it the breastbone, if you’re feeling particularly quaint. It’s essentially a long, flat bone that bridges the gap, connecting the ribs via their respective cartilages and thus forming the very front of the rib cage . Its primary function, beyond its structural contribution, is to act as a rather robust shield, protecting vital organs like the heart , lungs , and the major blood vessels that snake through our chests. If you were to sketch it, it vaguely resembles a necktie – a detail that always struck me as rather absurd. It’s one of the larger, longer flat bones in the body, which, frankly, isn’t saying much. Anatomically, it’s divided into three distinct regions: the manubrium, the body, and the xiphoid process . The word itself, “sternum,” is a rather dry translation from the Ancient Greek στέρνον ( stérnon ), meaning simply “chest.” Charming, isn’t it?

Structure

The sternum itself is a rather narrow, unequivocally flat bone. It occupies the central, anterior portion of the chest . At its apex, it plays host to the clavicles , more commonly known as collarbones. The edges of the sternum, meanwhile, engage in a rather intimate union with the costal cartilages of the first two pairs of ribs . For those with a penchant for the truly obscure, the inner surface of the sternum also provides attachment for the sternopericardial ligaments . And yes, the sternocleidomastoid muscle also finds a connection point at its superior aspect.

As I mentioned, the sternum is comprised of three primary sections, ordered from superior to inferior:

• Manubrium: The broad, upper segment.
• Body (also known as the gladiolus): The elongated middle section.
• Xiphoid process : The small, inferior tip.

In its natural, unadulterated state, the sternum doesn’t sit perfectly upright. It’s angled obliquely, sloping downwards and forwards. Its anterior surface is noticeably convex, while the posterior side is concave. It’s broad at the top, somewhat T-shaped, then narrows considerably where the manubrium meets the body. It widens again slightly before tapering down to its lower extremity. Curiously, it tends to be longer in males than in females. A subtle, yet persistent difference.

Manubrium

The manubrium, a rather grand name for a ‘handle’ (Latin for ‘handle’), is the broad, uppermost part of the sternum. Its shape is best described as quadrangular , narrowing as it descends. At its superior, broadest point, one finds the suprasternal notch , also known as the jugular notch. It’s that little dip you can feel between your collarbones. Flanking this notch are the right and left clavicular notches , where the clavicles make their rather important connection.

The manubrium’s primary relationships involve articulation with the body of the sternum, the clavicles, and the cartilages of the first 1.5 pairs of ribs . The inferior border, characterized by its oval shape and roughness, is covered with a thin layer of cartilage, preparing it for its union with the body. The lateral borders are marked superiorly by a depression, destined for the first costal cartilage . Inferior to that, a small facet exists, which, when paired with a similar facet on the superior angle of the body, creates a distinct notch for the reception of the second rib’s costal cartilage. Between the depression for the first costal cartilage and the demi-facet for the second, a narrow, curved edge slopes downwards and medially. Furthermore, the superior sternopericardial ligament anchors the pericardium to the posterior surface of the manubrium.

Body

The body, or gladiolus if you prefer a touch of the dramatic, is the longest segment of the sternum. It’s flat, and for all intents and purposes, possesses only an anterior and a posterior surface. The anterior surface is smooth, directed upwards and forwards, and subtly marked by three transverse ridges. These ridges correspond to the points where the bone articulates with the third, fourth, and fifth articular depressions. The pectoralis major muscle attaches to its lateral aspects. Occasionally, near the junction of the third and fourth parts of the body, a small opening, the sternal foramen, might be observed. Its size and shape are, predictably, variable. The posterior surface, in contrast, is slightly concave and also bears three transverse lines, though they are less pronounced than their anterior counterparts. From the lower part of this posterior surface, the transversus thoracis muscle originates.

The sternal angle , also known as the angle of Louis, marks the junction of the body and the manubrium. This palpable prominence is where the sternum juts out the most. However, its distinctness can vary; in some individuals, it might be concave or even rounded. This angle is a crucial landmark for clinicians, as it’s the precise point where the second rib attaches.

Each lateral border, at its superior aspect, presents a small facet. This facet, in conjunction with a similar one on the manubrium, forms the complete articulation for the cartilage of the second rib. Inferior to this, four distinct angular depressions await the cartilages of the third, fourth, fifth, and sixth ribs. The inferior angle of the body possesses a small facet that, when combined with a corresponding facet on the xiphoid process, creates a notch for the cartilage of the seventh rib. These articular depressions are separated by curved interarticular intervals that diminish in length from superior to inferior, mirroring the intercostal spaces. It’s worth noting that most of the cartilages of the true ribs articulate with the sternum at the lines where its primitive component segments originally fused. This fusion process is more readily observed in other vertebrates, where the segments remain distinct for a longer period.

The superior border of the body is oval and articulates with the manubrium at the aforementioned sternal angle. The inferior border is narrower and articulates with the xiphoid process .

Xiphoid process

At the very bottom of the sternum lies the xiphoid process , a small, pointed structure. It’s important to note that improperly performed chest compressions during cardiopulmonary resuscitation can lead to this process fracturing and embedding itself into the liver, a complication that can result in fatal hemorrhage. A rather grim reminder of the fragility of even the most seemingly solid structures.

The sternum itself is composed of highly vascular tissue, encased within a thin layer of compact bone. This compact bone is thickest in the manubrium, specifically between the articular facets for the clavicles . The inferior sternopericardial ligament, a rather persistent connective tissue, attaches the pericardium to the posterior aspect of the xiphoid process.

Joints

The cartilages of the superior five ribs establish their connections with the sternum at the sternocostal joints . The right and left clavicular notches, as previously mentioned, articulate with their respective clavicles. The costal cartilage of the second rib’s articulation with the sternum at the sternal angle makes this landmark particularly useful for anatomical orientation.

The transversus thoracis muscle, innervated by one of the intercostal nerves , attaches superiorly to the posterior surface of the lower sternum. Its inferior attachment is to the internal surface of costal cartilages two through six, and its action is to depress the ribs.

Development

The development of the sternum is a fascinating, albeit somewhat complex, process. It originates from two cartilaginous bars, one on the left and one on the right, which are connected to the cartilages of the ribs on each side. These two bars eventually fuse along their midline to form a single cartilaginous sternum. This cartilaginous precursor then undergoes ossification from six distinct centers: one for the manubrium, four for the body, and one for the xiphoid process .

The ossification centers typically appear in the intervals between the articular depressions for the costal cartilages . Their appearance follows a general timeline: in the manubrium and the first segment of the body, during the sixth month of fetal life; in the second and third segments of the body, during the seventh month; in the fourth segment, during the first year after birth; and finally, in the xiphoid process, between the ages of five and eighteen years.

These centers emerge in the upper parts of the segments and proceed gradually downwards. Interestingly, there are occasional instances of two small episternal centers appearing, one on either side of the jugular notch . These are thought to be vestigial remnants, echoes of the episternal bones found in monotremes and lizards.

It’s not uncommon for some of the sternal segments to develop from multiple centers, with variations in number and position being quite common. The first segment, for instance, may arise from two, three, or even six centers. When two centers are present, they are usually stacked vertically, with the upper one being larger. The second segment rarely has more than one center. The third, fourth, and fifth segments are often formed from two laterally placed centers. The imperfect union of these lateral centers can explain the rare occurrence of the sternal foramen, a small hole, or the vertical fissure that occasionally bisects this part of the bone, leading to a congenital malformation known as fissura sterni. These conditions are, in essence, a consequence of the way the cartilaginous sternum is initially formed.

Even more rarely, the upper end of the sternum might exhibit a fissure. The fusion of the various centers of the body typically commences around puberty and proceeds in an inferior to superior direction. By the age of 25, complete union is generally achieved. It’s worth noting that sternum development concludes earlier in females than in males.

The xiphoid process may fuse with the body of the sternum before the age of thirty, though this is more frequently observed after forty. Conversely, it can remain ununited even in old age. In advanced years, the manubrium and body may occasionally fuse, though this bony union is often superficial, with the central portion of the intervening cartilage remaining unossified.

The body of the sternum is, in essence, the product of the fusion of four segments, referred to as sternebrae.

Variations

A certain percentage of the population, ranging from 2.5% to 13.5%, exhibits a foramen in the lower third of the sternal body, known as a sternal foramen . In exceedingly rare instances, multiple such foramina can be observed. Fusion of the manubriosternal joint, the articulation between the manubrium and the body, occurs in approximately 5% of individuals. Small ossicles, termed episternal ossicles , may also be present posterior to the superior end of the manubrium. Another variation, the suprasternal tubercle, forms when these episternal ossicles fuse with the manubrium.

Clinical significance

Bone marrow biopsy

Given that the sternum contains bone marrow , it is sometimes utilized as a site for bone marrow biopsy . This is particularly relevant for patients with a high BMI (those who are obese or significantly overweight), as excess tissue can make access to more traditional biopsy sites, such as the pelvis , challenging.

Sternal opening

A relatively uncommon congenital disorder of the sternum, sometimes classified as an anatomical variation , is the sternal foramen. This is a single, round hole present from birth, typically located off-center to the right or left, and commonly found within the second, third, and fourth segments of the sternal body. Congenital sternal foramina can be mistaken for bullet holes, a rather unsettling comparison. While usually asymptomatic , they can pose a problem if acupuncture in the area is contemplated.

Sternal fracture

Fractures of the sternum are not particularly common. They typically result from significant trauma, such as the driver’s chest impacting the steering column in a car accident . A sternal fracture is usually comminuted, meaning the bone is broken into multiple pieces. The most frequent site for these fractures is at the sternal angle . Some research suggests that repeated punches or sustained beatings to the sternal region, sometimes referred to as “breastbone punches,” have also led to sternal fractures. Such injuries have been documented in contact sports like hockey and football. Crucially, sternal fractures are often accompanied by underlying injuries, such as pulmonary contusions , or bruising of the lung tissue.

Dislocation

A manubriosternal dislocation, a separation of the manubrium from the body of the sternum, is rare and typically a consequence of severe trauma. It can, however, also occur following minor trauma in individuals with a pre-existing condition like arthritis.

Sternotomy

In certain surgical procedures, particularly cardiothoracic surgery , the sternum is deliberately cut open, a procedure known as a median sternotomy , to provide access to the thoracic organs. Surgical fixation of the sternotomy is typically achieved using wire cerclage or a plate and screw system. Complications associated with sternotomy, while not frequent, can be severe, occurring in approximately 0.5% to 5% of cases. These complications can lead to increased mortality rates, necessitate reoperation, and carry a mortality risk as high as 40%. Common issues include dehiscence (the reopening of the surgical wound) and sternal non-union (failure of the bone to heal properly), often exacerbated by lateral forces generated during post-operative activities like coughing or sneezing.

Resection

In cases of malignancy, the sternum can be completely removed, a procedure termed resection. This is typically part of a radical surgical approach, sometimes accompanied by a mediastinal lymphadenectomy .

Bifid sternum or sternal cleft

A bifid sternum represents an extremely rare congenital abnormality where the sternum fails to fuse completely. This results in a sternal cleft , a visible split in the sternum that is apparent at birth and usually presents without other symptoms.

Other animals

In the broader context of vertebrate anatomy, the sternum is a flat bone situated in the anterior midline of the rib cage . It is endochondral in origin, meaning it develops from cartilage. The sternum likely first emerged in early tetrapods as an extension of the [pectoral girdle]; it is notably absent in fish . In amphibians and reptiles , it typically presents as a shield-shaped structure, often composed entirely of cartilage . It is absent in both turtles and snakes . In birds , the sternum is a relatively large bone and characteristically features a prominent keel , a projection to which the powerful flight muscles are attached. It is only in mammals , including humans, that the sternum adopts the elongated, segmented form we observe.

Arthropods

In arthropod anatomy, the term “sternum” refers to the ventral portion of a segment within the thorax or abdomen .

Etymology

The English term “sternum” is a direct translation of the Ancient Greek word στέρνον ( sternon ). The ancient Greek writer Homer used στέρνον to refer specifically to the male chest, while στῆθος ( stithos ) was used for the chest of both sexes. The Greek physician Hippocrates also employed στέρνον for the chest, and στῆθος for the breastbone. It was the Greek physician Galen who first used στέρνον with its modern meaning of breastbone.

The sternum, as the solid bony part of the chest, can be conceptually linked to the Ancient Greek words στερεός ( stereós ) and στερρός ( sterrós ), both meaning firm or solid. The English term “breastbone” is perhaps more akin to the Latin “os pectoris,” derived from the classical Latin “os” (bone) and “pectus” (chest or breast). It’s worth noting that “pectus” in classical Latin could also refer to the breastbone itself, adding a layer of linguistic complexity.