The Thorax - Bones
What is the thoracic spine?
The thoracic spine is the second segment of the vertebral column, located between the cervical and lumbar vertebral segments.
It consists of twelve vertebrae, which are separated by intervertebral discs.
Along with the sternum and ribs, the thoracic spine forms part of the thoracic cage.
This bony structure helps protect the internal viscera – such as the heart, lungs and oesophagus.
What do thoracic vertebrae look like?
The thoracic vertebrae have four features which distinguish them from other vertebrae:
The vertebral body is heart-shaped.
Presence of demi-facets on the sides of each vertebral body – these articulate with the heads of the ribs.
Presence of costal facets on the transverse processes – these articulate with the tubercles of the ribs. They are present on T1-T10 only.
The spinous processes are long and slant inferiorly. This offers increased protection to the spinal cord, preventing an object such as a knife entering the spinal canal.
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The superior and inferior costal facets are located on the sides of each vertebral body.
They consist of cartilage lined depressions, which articulate with the heads of the ribs.
The superior facet articulates with the head of the adjacent rib, and the inferior facet articulates with the head of the rib below.
In the majority of the vertebrae (T2-T9) these facets are demi-facets. There are some atypical vertebrae that possess whole facets.
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The atypical thoracic vertebrae display variation in the size, location and number of their superior and inferior costal facets.
T1 – Superior facet is not a demifacet, as this is the only vertebrae to articulate with the 1st rib.
T10 – A single pair of whole facets is present which articulate with the 10th rib. These facets are located across both the vertebral body and the pedicle.
T11 and T12 – Each have a single pair of entire costal facets, which are located on the pedicles.
Understanding the Rib Cage
What are the ribs?
The ribs are a set of twelve bones which form the protective ‘cage’ of the thorax. They articulate with the vertebral column posteriorly, and terminate anteriorly as cartilage (known as costal cartilage).
As part of the bony thorax, the ribs protect the internal thoracic organs. They also have a role in breathing – during chest expansion the ribcage moves to permit lung inflation.
There are two classifications of ribs – atypical and typical. The typical ribs have a generalised structure, while the atypical ribs have variations on this structure.
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The typical rib consists of a head, neck and body:
The head is wedge shaped, and has two articular facets separated by a wedge of bone. One facet articulates with the numerically corresponding vertebrae, and the other articulates with the vertebrae above.
The neck contains no bony prominences, but simply connects the head with the body. Where the neck meets the body there is a roughed tubercle, with a facet for articulation with the transverse process of the corresponding vertebrae.
The body, or shaft of the rib is flat and curved. The internal surface of the shaft has a groove for the neurovascular supply
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Ribs 1, 2, 10 11 and 12 can be described as ‘atypical’ – they have features that are not common to all the ribs.
Rib 1 is shorter and wider than the other ribs. It only has one facet on its head for articulation with its corresponding vertebrae (there isn’t a thoracic vertebrae above it). The superior surface is marked by two grooves, which make way for the subclavian vessels.
Rib 2 is thinner and longer than rib 1, and has two articular facets on the head as normal. It has a roughened area on its upper surface, where the serratus anterior muscle attaches.
Rib 10 only has one facet – for articulation with its numerically corresponding vertebrae.
Ribs 11 and 12 have no neck, and only contain one facet, which is for articulation with their corresponding vertebrae.
What is the sternum?
The sternum (or breastbone) is a flat bone located at the anterior aspect of the thorax. It lies in the midline of the chest and has a ‘T’ shape.
As part of the bony thoracic wall, the sternum helps protect the internal thoracic viscera – such as the heart, lungs and oesophagus.
The sternum can be divided into three parts; the manubrium, body and xiphoid process. In children, these elements are joined by cartilage. The cartilage ossifies to bone during adulthood.
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The manubrium is the most superior portion of the sternum. It is trapezoid in shape.
The superior aspect of the manubrium is concave, producing a depression known as the jugular notch – this is visible underneath the skin. Either side of the jugular notch, there is a large fossa lined with cartilage. These fossae articulate with the medial ends of the clavicles, forming the sternoclavicular joints.
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On the lateral edges of the manubrium, there is a facet (cartilage lined depression in the bone), for articulation with the costal cartilage of the 1st rib, and a demifacet (half-facet) for articulation with part of the costal cartilage of the 2nd rib.
Inferiorly, the manubrium articulates with the body of the sternum, forming the sternal angle. This can be felt as a transverse ridge of bone on the anterior aspect of the sternum. The sternal angle is commonly used as an aid to count ribs, as it marks the level of the 2nd costal cartilage.
The body is flat and elongated – the largest part of the sternum. It articulates with the manubrium superiorly (manubriosternal joint) and the xiphoid process inferiorly (xiphisternal joint).
The lateral edges of the body are marked by numerous articular facets (cartilage lined depressions in the bone). These articular facets articulate with the costal cartilages of ribs 3-6. There are smaller facets for articulation with parts of the second and seventh ribs – known as demifacets.
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The xiphoid process is the most inferior and smallest part of the sternum. It is variable is shape and size, with its tip located (roughly) at the level of the T10 vertebrae. The xiphoid process is largely cartilaginous in structure, and completely ossifies late in life – around the age of 40.
In some individuals, the xiphoid process articulates with part of the costal cartilage of the seventh rib.
What is cartilage?
Cartilage is a strong, flexible connective tissue found throughout the body. Its main functions are:
Absorbing shock of joints
Reducing friction/lubrication of joints
Supporting structures/helping them to keep their shape (e.g. ears, nose)
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Hyaline cartilage is the most common type of cartilage in your body. It lines your joints and caps the ends of your bones.
Hyaline cartilage at the ends of your bones is sometimes referred to as articular cartilage.
Hyaline cartilage is slippery and smooth which helps your bones move smoothly past each other in your joints. It’s flexible but strong enough to help your joints hold their shape.
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Fibrocartilage is what its name sounds like: tough cartilage made of thick fibers. It’s the strongest and least flexible of the three types. It’s tough enough to hold parts of your body in place and absorb impacts.
We’ll look at an important piece of fibrocartilage in the Knee Joint.
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Elastic cartilage is your most flexible cartilage. It supports parts of your body that need to bend and move to function. Elastic cartilage can bounce back to its original shape, even after a strong force. Your ear is made of elastic cartilage. It can bend and move without hurting you before returning to its usual shape.
Costal cartilage?
The costal cartilage is segments of cartilage that connect the sternum to the ribs and help to extend the ribs into a forward motion.
This cartilage also contributes to elasticity within the walls of the thorax, allowing the chest to expand during respiration.
There are twelve costal cartilage sections.
Each has two cartilages, extremities, and borders.
Seven pairs of the costal cartilage are connected to the sternum.
Two of the costal cartilage sections are pointed, ending in the walls of the abdomen (11th and 12th).
Three pairs of costal cartilage are articulated (connected) with the preceding ribs (8 - 10th).
The anterior (front) surfaces are convex, while the posterior (rear) surfaces are concave.
The superior section is concave, while the inferior is convex.
The eleventh and twelfth costal cartilage segments are pointed and are free of attachments. Once a person reaches age 65 and beyond, their costal cartilage becomes prone to superficial ossification, or hardening into a bony substance.
What are the joints of the thoracic spine?
The joints of the thoracic spine can be divided into two groups – those that are present throughout the vertebral column, and those unique to the thoracic spine.
There are two types of joints present throughout the vertebral column:
Between vertebral bodies – adjacent vertebral bodies are joined by intervertebral discs, made of fibrocartilage. This is a type of cartilaginous joint, known as a symphysis.
Between vertebral arches – formed by the articulation of superior and inferior articular processes from adjacent vertebrae. It is a synovial type joint.
The articulations between the vertebrae and the ribs are unique to the thoracic spine.
For each rib, there are two separate articulations on the posterior aspect – costovertebral and costotransverse.
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Each costovertebral joint consists of the head of the rib articulating with:
Superior costal facet of the corresponding vertebra
Inferior costal facet of the superior vertebra
Intervertebral disc separating the two vertebrae
Within this joint, the intra-articular ligament of head of rib attaches the rib head to the intervertebral disc.
Only slight gliding movements can occur at these joints, due to the close articulation of their components.
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The costotransverse joints are formed by the articulation of transverse processes of a thoracic vertebra and the tubercle of the adjacent rib. They are present in all vertebrae except T11 and T12.
The anterior attachment of the ribs vary:
Ribs 1-7 attach independently to the sternum.
Ribs 8 – 10 attach to the costal cartilages superior to them.
Ribs 11 and 12 do not have an anterior attachment and end in the abdominal musculature. Because of this, they are sometimes called ‘floating ribs’.
Where are the thoracic discs?
There are 24 intervertebral discs in the spine. Of those, 12 are located in the thoracic spine.
Each thoracic disc sits between two vertebrae to provide cushioning and shock absorption while preventing the vertebrae from grinding against each other.
Thoracic discs tend to be thinner than cervical discs and lumbar discs, which may contribute to the thoracic spine’s relative lack of mobility compared to the neck and lower back.
Another distinguishing feature of the thoracic discs is that all but the bottom two interface with ribs.
Each thoracic disc is comprised of the following:
Outer layer (annulus fibrosus). This durable exterior of the disc is comprised of tough collagen fibers to help distribute major loads placed on the spine and protect the disc’s soft interior.
Inner core (nucleus pulposus). This jelly-like interior is a loose network of fibers floating in a mucoprotein gel. The disc’s inner core provides more cushioning and movement between adjacent vertebrae than the outer layer.
Intervertebral discs are the largest structures in the human body that do not have blood vessels. A super-thin structure between the disc and vertebra, called a vertebral end plate, provides diffusion so that nutrients can get into the disc.
