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Thoracic cage

The thoracic cage (rib cage) is the skeleton of the thoracic wall. It is formed by the 12 thoracic vertebrae, 12 pairs of ribs and associated costal cartilages and the sternum.

The thoracic cage takes the form of a domed bird cage with the horizontal bars formed by ribs and costal cartilages. It is supported by the vertical sternum (anteriorly) and the 12 thoracic vertebrae (posteriorly). 

The thoracic cage, like skeletal tissue in most parts of the body, functions to support thorax and protect the vital structures within it (e.g. heart, lungs, aorta, etc). Its rigid structure allows it to be an attachment point for many muscles of the upper body and to support the weight of the upper limbs. The thoracic cage also facilitates the act of breating by resisting the negative pressure generated by the elastic recoil of the lungs and respiration-induced movements. 

Key facts about the thoracic cage
Definition The thoracic cage (rib cage) is the skeleton of the thoracic wall. 
Structural components 12 thoracic vertebrae with their intervertebral discs, 12 pairs of ribs and their associated costal cartilages and sternum
Intercostal spaces Named according to the rib forming the superior border and contain intercostal muscles, vessels, and nerves
Joints Xiphisternal: xiphoid process and body of sternum
Intervertebral: between vertebrae
Sternochondral: sternum and costal cartilages
Sternoclavicular: manubrium and clavicles
Manubriosternal: manubrium and body of sternum
Costochondral: costal cartilage and rib
Costovertebral: formed by the ribs and bodies of the vertebrae.
Interchondral: joining the costal cartilages to one another
Functions Provides support, stability for the upper part of the body, protection for thorax and thoracic organs, facilitates breathing, attachment for the muscles 
Contents
  1. Sternum
  2. Thoracic vertebrae
  3. Ribs and costal cartilages
  4. Intercostal spaces
  5. Joints
    1. Costochondral joints
    2. Interchondral joints
    3. Sternocostal joints
    4. Costovertebral joints
  6. Clinical notes 
    1. Dislocation of costochondral joint
    2. Dislocation of sternocostal and interchondral joints
    3. Flail chest
    4. Supernumerary ribs
  7. Sources
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Sternum

The sternum (derived from the Greek word, sternon meaning chest) is a flat, elongated bone forming the middle of the anterior part of the thoracic cage. 

The sternum consists of three parts:

  • The manubrium
  • The body of sternum
  • The xiphoid process

The manubrium is a roughly trapezoidal bone. It is the widest and thickest of the three parts of the sternum. The manubrium forms articulations with the clavicle, body of the sternum, first rib and superior half of the second rib. 

The body of the sternum is longer, narrower and thinner than the manubrium. It is interposed between the manubrium and the xiphoid process, and is located at the level of the T5 – T9 vertebrae. On the lateral borders, the body articulates with the costal cartilages of the second to seventh ribs, and forms a xiphisternal joint at its junction with the xiphoid process.

The manubrium and body of the sternum lie in slightly different planes superiorly and inferiorly to their junction, the manubriosternal joint; hence, their junction forms a projecting sternal angle (of Louis).

The xiphoid process is the smallest and most variable part of the sternum. It is thin, elongated and lies at the level of the T10 vertebra. The xiphoid process is an important landmark in the median plane, indicating the inferior limit of the central part of the thoracic cavity. This inferior limit corresponds to the xiphisternal joint, and it is also the site of the infrasternal angle (subcostal angle) of the inferior thoracic aperture. Additionally, the xiphoid process is a midline marker for the superior limit of the liver, the central tendon of the diaphragm, and the inferior border of the heart.

Test your knowledge on the sternum with the following quiz!

Thoracic vertebrae

The thoracic vertebrae are a group of 12 irregular bones that form the thoracic portion of the vertebral spine. 

According to their structure, the thoracic vertebrae can be typical and atypical. They are mostly typical vertebrae in that they are independent, have bodies, vertebral arches, and seven processes for muscular and articular connections. Most of them also have costal facets on their transverse processes for articulation with the tubercles of ribs. They are also characterized with bilateral costal facets (demifacets) on their bodies, and long, inferiorly slanting spinous processes.

In contrast, atypical thoracic vertebrae (T1, T10, T11, T12) have “whole costal facets” in place of demifacets. 

Learn the bones of the thoracic cavity with the following study unit:

Ribs and costal cartilages

The ribs are curved, flat bones connecting the sternum and most of the thoracic vertebrae (specifically T1-T10). They are remarkably light in weight yet highly resilient to pressure from within the thorax, e.g., pressure generated during inspiration.

All of the ribs that articulate with the sternum (rib 1-10) are prolonged anteriorly, with their attached costal cartilages with which they articulate with the sternum. These costal cartilages also contribute to the elasticity of the thoracic wall, providing a flexible attachment for their anterior or distal ends.

The first seven (and sometimes the 8th) cartilages attach directly and independently to the sternum. The 8th, 9th and 10th cartilages articulate with the costal cartilages just superior to them, forming a continuous, articulated, cartilaginous costal margin of the rib cage. According to their appearance, the ribs can be divided into two groups: typical and atypical.

Reinforce your knowledge about the ribs with this quiz:

Intercostal spaces

The bones forming the thoracic cage are arranged in a pattern that allows some space between them. Those spaces are referred to as the intercostal spaces . The intercostal spaces separate the ribs and their costal cartilages from one another and allow smooth expansion of the cage during inspiration. The spaces are named according to the rib forming the superior border of the space, for example, the 4th intercostal space lies between the 4th rib and 5th rib; therefore, there are 11 intercostal spaces in the rib cage.

Intercostal spaces are occupied by intercostal muscles and membranes, 11 intercostal nerves and two sets (main and collateral) of intercostal blood vessels also identified by the same number assigned to the intercostal space. Below the 12th rib, is referred to as the subcostal space and the anterior ramus of the spinal nerve T12 runs through this space, and it is thus referred to as the subcostal nerve.

Have you been wondering whether you should use 3D anatomy tools to learn about the thoracic cage? Here's why you should think again. 

Joints

The joints forming the domed-shaped thoracic cage include the:

  • Xiphisternal joint – xiphoid process and body of sternum
  • Intervertebral joints – between vertebrae
  • Sternochondral joints – sternum and costal cartilages
  • Sternoclavicular joints – manubrium and clavicles
  • Manubriosternal joints – manubrium and body of sternum
  • Costochondral joints – costal cartilage and rib
  • Costovertebral joints – formed by the ribs and bodies of the vertebrae.
  • Interchondral joints – joining the costal cartilages to one another.

Some of the above joints are briefly described below.

Costochondral joints

The costochondral joints are the articulations between each rib and its costal cartilage. It is a hyaline cartilaginous type of joint. The articulation is between the cup-shaped depression in the sternal end of a rib and the lateral end of a costal cartilage. The rib and its cartilage are firmly bound together by the continuity of the periosteum of the rib with the perichondrium of the cartilage. No movement normally occurs at these joints.

Interchondral joints

The interchondral joints are plane synovial joints between the adjacent borders of the 6th and 7th, 7th and 8th, and 8th and 9th costal cartilages. The joints are usually strengthened by interchondral ligaments, and they also have synovial cavities that are enclosed by joint capsules.

Sternocostal joints

These are joints formed by the lateral borders of the sternum and the costal cartilages of the 1st to 7th ribs, and sometimes the 8th rib. The first pair of costal cartilages articulate with the manubrium by means of a thin dense layer of tightly adherent fibrocartilage interposed between cartilage and the manubrium, the synchondrosis of the first rib. The second to seventh pairs of costal cartilages articulate with the sternum at synovial joints with fibrocartilaginous articular surfaces on both the chondral and sternal aspects, allowing movement during respiration. The sternocostal joints are also referred to as sternochondral joints.

Costovertebral joints

Articulation of the left and right ribs with the vertebral column (thoracic vertebrae) complete the domed birdcage-like thoracic cage posteriorly. The costovertebral joints are synovial joints, and they are richly surrounded by joint capsule. Of all the joints of the rib cage, these joints have the largest amount of ligaments crossing and stabilizing them.

Most of the ribs are attached firmly to the intervertebral (IV) discs by intra-articular ligament within the joints. Fanning around from the anterior margin of the heads of the ribs to the sides of the bodies of vertebrae and the IV discs between them is a radiate sternocostal ligament. Also crossing these joints is the costotransverse ligament passing from the neck of the rib to the transverse process, and a lateral costotransverse ligament passing from the tubercle of the rib to the tip of the transverse process. These ligaments strengthen the anterior and posterior aspects of the joints respectively. There is a superior costotransverse ligament which may be divided into a strong anterior costotransverse ligament and a weak posterior costotransverse ligament, both joining the crests of the neck of the ribs to the transverse processes superior to each of the ribs.

Master the costovertebral joints with our tailored quiz!

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