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 supported by the vertical sternum or breastbone (anteriorly) and the 12 thoracic vertebrae (posteriorly). The thoracic cage can also be described as an osteocartilaginous cage formed by the sternum, 12 pairs of ribs and costal cartilages, 12 thoracic vertebrae and the intervertebral (IV) discs interposed between them.
The thoracic cage, like skeletal tissue in most parts of the body, serves to support the thorax. It also functions to protect vital thoracic and abdominal internal organs from external forces. Additionally, it resists the negative internal pressures generated by the elastic recoil of the lungs and respiration-induced movements, provides attachment for and supports the weight of the upper limbs, as well as providing the anchoring attachment (origin) of many of the muscles that move and maintain the position of the upper limbs relative to the trunk.
Ossification And Development
During the intrauterine period of development, the rib cage begins to ossify and this process continues until approximately the 25th year of extra-uterine life. Of all the three groups of bones forming the thoracic cage, the vertebral and ribs ossification start by the end of the embryonic period (at approximately the 7th week of gestation). Sternal ossification begins during the 5th month of fetal development.
Although ossification is complete, on average, by age 25 years, progressive calcification of the costal cartilages can continue into old age. A notably significant development of the thoracic cage is expansion of the rib cage which contributes greatly to the broad shoulders observed particularly in males. In males, expansion of the rib cage is caused by the effects of testosterone hormone during puberty; thus males generally have broad shoulders and expanded thoraces, allowing them to inhale more air to supply their muscles with oxygen.
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, namely, the manubrium, body of sternum and the xiphoid process.
- The manubrium is a roughly trapezoidal bone. It is the widest and thickest of the three parts of the sternum. Its superior border has an easily palpated concave centre, called the jugular notch (or suprasternal notch). Lateral to the suprasternal notch are the clavicular notches which receive the sternal end (medial end) of the clavicle. In an articulated skeleton, the jugular notch is deepened by the medial ends of the left and right clavicles. The manubrium also articulates with the costal cartilage of the first rib – the synchondrosis of the first rib, as well as the superior half of the articular surface of the costal cartilage of the second rib. 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 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 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. Although often pointed in some individuals, the process may be blunt, bifid, curved or deflected to one side or anteriorly. The xiphoid process is small and cartilaginous in young people but gets ossified in adults older than age 40, with severe pains accompanying the process of ossification. Ossification of the xiphoid process in the elderly people may also cause fusion of the xiphoid process with the sternal body. 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.
The thoracic vertebrae are a group of 12 small bones that form the vertebral spine of the thorax. They are intermediate in size between those of the cervical region and lumbar region, to which they are also interposed, and increase in size from above downwards (fig. 3). 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.
Atypical thoracic vertebrae have “whole costal facets” in place of demifacets. The T1 vertebrae, T10, T11 and T12 are all atypical, having only single whole costal facets.
Ribs and Costal Cartilages
The ribs (derived from the Latin word costae) are curved, long bones connecting the sternum and most of the thoracic vertebrae (specifically T1-T10). They make up the highest number of bones forming the thoracic cage. They are remarkably light in weight yet highly resilient to pressure from within the thorax, e.g., pressure generated during inspiration.
There are three types of ribs, and all groups have a spongy interior containing bone marrow (hematopoietic tissue), which forms blood cells. All of the ribs that articulate (rib 1-10) with the sternum 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 cartilages of the first 8 ribs increase in length in descending order, with length then decreasing after the 8th. 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. The ribs can be divided into two groups, typical and atypical. The typical ribs include ribs 3 to 9th and have the following components:
- Wedge shaped head with two facets, separated by the crest of the head.
- Neck connecting the head with the body at the level of the tubercle.
- A tubercle at the junction of the neck and body and has a smooth articular part for articulating with the transverse process of the corresponding vertebra.
- A thin, flat and curved body (shaft), most markedly at the costal angle where the rib turns anterolaterally.
The atypical ribs are the 1st, 2nd, and the 10th – 12th ribs characterized with the following features:
- Have one or two facets, and a rough area on the superior surface, e.g, the tuberosity for serratus anterior of the second rib.
All 12 pairs of ribs can also be divided into the following types or groups:
- True (Vertebrocostal) ribs: The 1st to 7th ribs fall under this group, and they attach directly to the sternum through their own costal cartilages.
- False (vertebrochondral) ribs: The 8th, 9th, and 10th ribs are referred to as false ribs because their cartilages are connected to the costal cartilage of the rib above them; thus their connection with the sternum is indirect.
- Floating (vertebral, free) ribs: The 11th and 12th ribs are grouped as floating ribs. They articulate only to the vertebral column, thus hanging freely. Some authors group the 10th rib with the floating ribs; and the rudimentary cartilages of these floating ribs do not connect even indirectly with the sternum, instead they end in the posterior abdominal musculature.
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.
The joints forming the domed-shaped thoracic cage includes 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.
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.
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.
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.
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 joint. 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.