Bones of the Thorax

Overview
The skeletal structure of the thorax safeguards essential organs and serves as an anchor for the muscles involved in breathing. This section presents the primary bony elements of the thorax and explains their roles in support, protection, and respiratory movement.

Key Structures
This section examines the ribs, sternum, and thoracic vertebral column, describing their anatomical characteristics, joint connections, and how they contribute to thoracic motion and ventilation.

Clinical Relevance and Learning Focus
An understanding of thoracic skeletal anatomy is crucial for evaluating traumatic injuries, analysing chest imaging, and appreciating the mechanics of respiration. This section aids in recognising fractures, structural abnormalities, and degenerative conditions.

The Sternum

Contents

The sternum, commonly known as the breastbone, is a flat bone situated on the anterior aspect of the thorax. It lies along the midline of the chest and has a characteristic T-shaped appearance.

As a component of the bony thoracic wall, the sternum plays a protective role for underlying thoracic organs, including the heart, lungs, and oesophagus.

This article explores the osteology of the sternum, including its individual parts, joint articulations, and relevant clinical considerations.

Fig 1 – Anatomical position of the sternum within the thorax.

Parts of the Sternum

The sternum is composed of three distinct regions: the manubrium, the body, and the xiphoid process. In children, these components are connected by cartilage, which gradually ossifies into bone during adulthood.

Manubrium

The manubrium forms the uppermost portion of the sternum and has a trapezoidal shape.

Its superior border is concave, creating a visible depression called the jugular notch. On either side of this notch are large, cartilage-lined fossae that articulate with the medial ends of the clavicles, forming the sternoclavicular joints.

Along the lateral margins of the manubrium are articular surfaces for rib attachment: a full facet for the costal cartilage of the first rib and a demifacet for part of the costal cartilage of the second rib.

Inferiorly, the manubrium joins the body of the sternum at the manubriosternal joint, forming the sternal angle. This palpable transverse ridge on the anterior chest wall is an important surface landmark, commonly used to identify the level of the second costal cartilage when counting ribs.

Body

The body of the sternum is the longest and largest segment. It is flat and elongated, articulating superiorly with the manubrium at the manubriosternal joint and inferiorly with the xiphoid process at the xiphisternal joint.

Its lateral borders contain multiple cartilage-lined articular facets that articulate with the costal cartilages of ribs three to six. Smaller demifacets are also present for partial articulation with the second and seventh ribs.

Xiphoid Process

The xiphoid process is the smallest and most inferior part of the sternum. Its shape and size are highly variable, and its tip is typically located at the level of the T10 vertebra.

This structure is predominantly cartilaginous in early life and usually undergoes complete ossification later in adulthood, often around the age of 40. In some individuals, the xiphoid process also articulates with part of the costal cartilage of the seventh rib.

Fig 2 – The parts of the sternum and their articulations.

Clinical Relevance

Fractures of the Sternum

Sternal fractures most commonly result from severe blunt trauma to the chest, such as that sustained during motor vehicle collisions. Although relatively rare, these injuries are often serious.

The sternum typically fractures into multiple fragments, producing a comminuted fracture. The most frequent fracture site is the manubriosternal joint, where the manubrium meets the body of the sternum. Despite the extent of bony disruption, displacement of fracture fragments is uncommon due to stabilising attachments from the pectoral muscles.

Sternal fractures are associated with a high mortality rate, estimated between 25% and 45%. This elevated risk is not usually caused by the fracture itself, but rather by concurrent injuries to the heart and lungs that occur during the same traumatic event. Consequently, patients with suspected sternal fractures must be carefully assessed for associated visceral injury. Common diagnostic investigations include X-ray imaging, computed tomography (CT), and ultrasound.

Fig 3 – Lateral chest X-ray demonstrating a displaced sternal fracture.

The Ribs

Contents

The ribs consist of twelve pairs of bones that form the rigid yet flexible framework of the thoracic cage. Posteriorly, they articulate with the vertebral column, while anteriorly they continue as costal cartilage.

As components of the bony thorax, the ribs provide protection for the organs within the chest. They also play an essential role in breathing, moving during expansion of the thoracic cavity to allow the lungs to inflate.

This article examines rib anatomy, focusing on their structural features, joint connections, and clinical relevance.

Fig 1 – Overview of the ribs and their associated costal cartilages.

Rib Structure

Ribs are classified into two groups: typical and atypical. Typical ribs share a common structural pattern, whereas atypical ribs display variations from this standard form.

Typical Ribs

A typical rib is composed of three main parts: the head, neck, and body.

• Head: The head is wedge-shaped and contains two articular facets separated by a ridge of bone. One facet articulates with the vertebra of the same number, while the other articulates with the vertebra above.
• Neck: The neck is a short segment without prominent features that connects the head to the body. At the junction between the neck and body is a roughened tubercle with an articular facet that articulates with the transverse process of the corresponding vertebra.
• Body (shaft): The body of the rib is flattened and curved. Along its internal surface runs a costal groove, which houses and protects the intercostal nerves and vessels.

Fig 2 – Bony landmarks of a typical rib.

Atypical Ribs

Ribs 1, 2, 10, 11, and 12 are considered atypical due to distinctive structural features.

• Rib 1: Shorter and broader than the others, rib 1 has a single articular facet on its head, as there is no vertebra above it. Its superior surface contains two grooves for passage of the subclavian vessels.
• Rib 2: Longer and thinner than rib 1, rib 2 has two articular facets on its head. Its upper surface includes a roughened area for attachment of the serratus anterior muscle.
• Rib 10: Possesses only one articular facet on its head for articulation with the corresponding vertebra.
• Ribs 11 and 12: These ribs lack a neck and have a single articular facet for articulation with their respective vertebrae.

Articulations

Most ribs form joints both posteriorly and anteriorly.

Posterior Articulations

All twelve ribs articulate posteriorly with the vertebral column, forming two joints:

• Costotransverse joint: Between the tubercle of the rib and the transverse costal facet of the corresponding vertebra.
• Costovertebral joint: Between the head of the rib and the superior costal facet of the corresponding vertebra, as well as the inferior costal facet of the vertebra above.

Fig 3 – Posterior articulations between a typical rib and its corresponding thoracic vertebrae.

Anterior Articulations

Anterior rib attachments vary according to rib number:

• Ribs 1–7: Attach directly to the sternum via their costal cartilages.
• Ribs 8–10: Attach indirectly to the sternum by joining the costal cartilage of the rib above.
• Ribs 11 and 12: Have no anterior attachment and terminate within the abdominal musculature. These are commonly referred to as floating ribs.

Clinical Relevance

Rib Fractures

Rib fractures most frequently affect the middle ribs and usually result from direct trauma or crushing injuries. A major concern with rib fractures is damage to surrounding soft tissues from sharp bone fragments. Structures at particular risk include the lungs, spleen, and diaphragm.

When two or more fractures occur in at least two adjacent ribs, the affected segment of the chest wall becomes mechanically unstable and moves independently of the rest of the thorax. This produces paradoxical movement during respiration, a condition known as flail chest. Flail chest limits effective expansion of the rib cage and reduces blood oxygenation. Management typically involves stabilisation of the fractured ribs to prevent abnormal movement.

Fig 4 – Chest radiograph showing multiple rib fractures consistent with flail chest.

The Thoracic Spine

Contents

The thoracic spine is the second region of the vertebral column and lies between the cervical and lumbar segments. It is composed of twelve vertebrae, separated from one another by intervertebral discs.

Together with the ribs and sternum, the thoracic spine forms the thoracic cage. This rigid framework provides protection for vital internal organs, including the heart, lungs, and oesophagus.

This article reviews the anatomy of the thoracic vertebrae, focusing on their distinguishing features, joints, supporting ligaments, and clinical relevance.

Fig 1.0 – Overview of thoracic spine anatomy.

Characteristic Features

Thoracic vertebrae possess several defining features that distinguish them from cervical and lumbar vertebrae:

• The vertebral bodies are heart-shaped.
• Demi-facets are present on the sides of the vertebral bodies, allowing articulation with the heads of the ribs.
• Costal facets are found on the transverse processes and articulate with the rib tubercles; these are present from T1 to T10 only.
• The spinous processes are long and slope inferiorly, increasing protection of the spinal cord by limiting direct posterior access to the spinal canal.

Fig 2 – Lateral view of a typical thoracic vertebra.

Superior and Inferior Costal Facets

Superior and inferior costal facets are located on the lateral aspects of the vertebral bodies. These cartilage-lined surfaces articulate with the heads of the ribs: the superior facet articulates with the rib of the same number, while the inferior facet articulates with the rib below.

In most thoracic vertebrae (T2–T9), these are demi-facets. However, certain vertebrae are atypical and possess complete facets instead.

Atypical Vertebrae

Some thoracic vertebrae differ in the number, size, or position of their costal facets:

• T1: Has a complete superior facet, as it is the only vertebra to articulate with the first rib.
• T10: Possesses a single pair of complete costal facets that articulate with the tenth rib; these extend across both the vertebral body and pedicle.
• T11 and T12: Each has a single pair of full costal facets located on the pedicles.

Joints

The joints of the thoracic spine can be divided into those common to the entire vertebral column and those unique to the thoracic region.

Joints Present Throughout the Vertebral Column

Two main joint types are found throughout the spine:

• Intervertebral joints: Adjacent vertebral bodies are connected by intervertebral discs composed of fibrocartilage. These are cartilaginous joints known as symphyses.
• Zygapophysial (facet) joints: Formed between the superior and inferior articular processes of adjacent vertebrae; these are synovial joints.

Joints Unique to the Thoracic Spine

Articulations between the ribs and vertebrae are specific to the thoracic spine. Each rib forms two joints:

• Costovertebral joint: The head of the rib articulates with the superior costal facet of its corresponding vertebra, the inferior costal facet of the vertebra above, and the intervening intervertebral disc. The intra-articular ligament of the head of the rib anchors the rib head to the disc. Movement at this joint is limited to slight gliding.
• Costotransverse joint: Formed between the transverse process of a thoracic vertebra and the tubercle of the corresponding rib. These joints are present from T1 to T10 and absent at T11 and T12.

Fig 3 – Articulations between a rib and its corresponding thoracic vertebra.

Ligaments

Numerous ligaments provide stability to the thoracic spine.

Ligaments Present Throughout the Vertebral Column

• Anterior and posterior longitudinal ligaments: Run the length of the vertebral column, covering the vertebral bodies and intervertebral discs.
• Ligamentum flavum: Connects the laminae of adjacent vertebrae.
• Interspinous ligaments: Join adjacent spinous processes.
• Supraspinous ligament: Connects the tips of the spinous processes.
  (In the cervical region, the interspinous and supraspinous ligaments thicken to form the nuchal ligament.)

Fig 4 – Ligaments associated with the vertebral column.

Ligaments Unique to the Thoracic Spine

Several smaller ligaments support the costovertebral joints:

• Radiate ligament of the head of the rib: Spreads from the rib head to the adjacent vertebral bodies and intervertebral disc.
• Costotransverse ligament: Connects the neck of the rib to the transverse process.
• Lateral costotransverse ligament: Extends from the transverse process to the rib tubercle.
• Superior costotransverse ligament: Runs from the upper border of the rib neck to the transverse process of the vertebra above.

Clinical Relevance

Thoracic Kyphosis

Kyphosis refers to an exaggerated curvature of the thoracic spine, producing a rounded or “hunched” appearance of the back. In younger individuals, it may result from poor posture, wedge-shaped vertebrae (as seen in Scheuermann’s disease), or abnormal vertebral fusion during development.

In adults, kyphosis commonly develops secondary to disease. The most frequent cause is osteoporosis, a condition characterised by reduced bone density, particularly in older individuals. Loss of bone strength reduces the spine’s ability to support body weight, leading to progressive thoracic curvature.

Fig 5 – Diagram illustrating thoracic kyphosis.