Ribs

Ribs

The ribs are curved, flat bones which form the majority of the thoracic cage. They are extremely light, but highly resilient; contributing to their role in protecting the internal thoracic organs.

There are twelve pairs of ribs, all of which articulate with the vertebral column. However, only seven have a direct articulation with the sternum. As such, ribs can be allocated to one of three distinct types; true (vertebrosternal) ribs, false (vertebrochondral) ribs and floating (vertebral, free) ribs. Ribs one to seven are considered true ribs and attach directly to the sternum via their own costal cartilage.

Ribs eight to ten are the false ribs and are connected to the sternum indirectly via the cartilage of the rib above them. The final two pairs of ribs are floating ribs and the cartilage of these ribs tends to end within the abdominal musculature. These three types can then be classified as either typical or atypical.

Classification	Based on articulations – true ribs (1 to 7), false ribs (8 to 12) (inc. floating ribs (11 and 12) Based on structure – typical (3 to 9) and atypical (1, 2, 10, 11, and 12) ribs
Structure	Typical (3 to 9) and atypical (1, 2, 10, 11, and 12) ribs
Structure of typical ribs	Head (contains two articular facets), neck, tubercle (has an articular and non articular part), and body (curves at the costal angle, the internal surface contains the costal groove and it joins with the costal cartilage)
Structure of atypical ribs	First – widest, shortest, it has the sharpest curve and only one articular surface, contains two grooves for the subclavian vessels Second – it has two facets and a roughened tuberosity on its posterior surface Tenth, eleventh, twelfth – they have only one facet and neither a neck or tubercles
Ligaments	Costotransverse ligament (lateral and superior)
Muscles	External intercostals (elevate ribs), internal intercostals (depress ribs), innermost intercostals (depress ribs during forced expiration), subcostales (depress ribs), transversus thoracis (depress ribs), serratus posterior (elevates ribs), levatores costarum (elevate ribs)
Clinical	Chest drain insertion (superior border of rib), fractures, flail chest

In this article we shall consider the osteology and articulations of the ribs, as well ligamentous and muscular attachments, finishing with some clinical notes.

1. Typical ribs
2. Atypical ribs
3. Ligaments
4. Muscles attachments
5. Clinical notes
6. Sources

Typical ribs

Ribs three to nine are the ‘typical’ ribs and the major landmarks are the head, neck, tubercle and body.

Head

The head of each rib is wedge shaped and has two articular facets, which are separated by a wedge of bone, known as the crest of the head. The larger of the two facets is for articulation with the superior costal facet of its corresponding vertebral body, while the smaller of the two articulates with the inferior costal facet on the body of the superior vertebra.

Neck

The neck of the rib is generally unremarkable in terms of bony landmarks and is simply a flat piece of bone that connects the head of the rib with the body.

Tubercle

The tubercle is a bony prominence located at the junction between the neck and body which projects posteriorly. It consists of two parts, a smooth articular part which articulates with the transverse process of the associated vertebra and a roughened non articular part which forms the attachment of the costotransverse ligament.

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Body

The body, or shaft, of the rib is thin, flat and curved. The curve becomes most prominent at the costal angle, which is when the rib turns anterolaterally. The costal angle also marks the attachment for some of the deep back muscles to the ribs. The internal surface is concave and contains the costal groove which offers some protection and a path for the neurovascular bundle to follow. The body ends with a cup for the costal cartilage, which allows the rib to articulate with the sternum.

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Atypical ribs

The first, second and tenth to twelfth ribs are known as atypical and as such will be considered individually.

First rib

The first rib is the widest, shortest and has the sharpest curve of all the ribs. The head only articulates with the body of the T1 vertebra and therefore only one articulatory surface is present. As in the typical ribs, the tubercle has a facet for articulation with the transverse process of vertebrae. The superior surface is unique in that it is marked by two grooves that allow passage of the subclavian vessels. These grooves are separated by the scalene tubercle – to which the anterior scalene muscle attaches.

Second rib

The second rib is thinner and significantly longer than the first. There are two facets present on the head to allow articulation with the T1 (superior) and T2 (corresponding) vertebrae. Its main unusual feature is a roughened tuberosity on its superior surface, which forms part of the origin for serratus anterior

Tenth, eleventh, twelfth ribs

The tenth to twelfth ribs have only one facet on their heads, and as such only articulate with a single vertebra. Ribs eleven and twelve in particular are short and have no necks or tubercles.

Ligaments

The main ligamentous attachment to the ribs is the costotransverse ligament, this is a fairly complex ligament which is made up of three parts. The first part is known as ‘The’ costotransverse ligament and it fills the gap between the rib and the corresponding transverse process.

The next group of fibres are known as the lateral costotransverse ligament, which lies posteriorly and attaches the transverse process of the vertebra to the rib, just distal to the tubercle. The final part is the superior costotransverse ligament which is a two-layered ligament with the fibres orientated at right angles. This passes from the superior margin of the neck of the rib to the transverse process of the above vertebra.

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Muscles attachments

External intercostal muscles

There are eleven pairs of external intercostal muscles and these are the most superficial in the area. They extend from the lateral border of the costal grooves to the superior margins of the ribs below. The fibres of this muscle pass anteroinferiorly in an oblique manner and extend around the thoracic wall from the tubercles to the costal cartilages of the ribs. These muscles help to elevate the ribs in inspiration.

Internal intercostal muscles

The eleven pairs of internal intercostal muscles are found posterior to the external intercostals. These pass from the inferior edge of the costal groove to the superior margins of the ribs below. These muscle fibres extend in a posteroinferior direction and again pass in an oblique manner. These muscles are only present from the parasternal area to the angle of the ribs. Their action is to depress the ribs in forced expiration.

Innermost intercostal muscles

The fibres of these muscles extend in the same direction of the internal intercostal muscles. They extend from the medial edge of the costal groove to the medial surface of the rib below and are mainly present within the lateral thoracic wall. However these muscles are notable in that the neurovascular bundles pass through the opening between the internal and innermost intercostal muscle layers. These muscles assist with depression of the ribs in forced expiration.

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Subcostales

The subcostales muscles are located within the same plane as the innermost intercostals. They are unique in that they may span one or multiple ribs and become more numerous within the inferior regions of the posterior thoracic wall. They extend from the inner surface of one rib to the inner surface of either the next rib or even the one below that. This muscle assists the internal intercostal muscles.

Transversus thoracis

These muscles are found deep within the anterior thoracic wall. The transversus thoracic muscles originate from the posterior surface of the xiphoid process and the lower part of the body of the sternum. The fibres pass superolaterally to insert into the internal surface of costal cartilages of ribs two to six. This muscle assists in depression of the ribs.

Serratus posterior

This muscle is present posteriorly within the thoracic wall. It is split into superior and inferior fibres. The inferior fibres originate from the spinous processes of the T11 to L2 vertebrae and attach to the lower borders of ribs eight to twelve near the angle. The part of the muscle is thought to depress the ribs. The superior fibres originate from the spinous processes of the C7 to T3 vertebrae and attach to the superior borders of ribs two to four. This part of the muscle is thought to elevate the ribs.

Levatores costarum

Levatores costarum originates from the transverse processes of vertebrae C7 to T11 and attaches to the external surface between the tubercle and the angle of the rib below. Its action is to assist in elevation of the ribs.

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Clinical notes

Chest drain insertion

This is a commonly performed procedure and is necessary in cases of pneumothorax, pleural effusion, empyema and postoperatively following thoracic surgery. It is particularly important to place the tube in line with the superior border of the rib. This is because the neurovascular bundle (intercostal nerve, vein and artery) passes along the costal groove, which is located on the inferior border of a rib. This positioning minimises the risk of damage to surrounding structures.

Rib fractures

Fractures of the ribs tend to present with pain on respiration, coughing, laughing and most other chest movements. They are most typically caused by blows or crushing injuries. The weakest part of a rib is just anterior to its angle and as such this is the most common site of injury – however it is important to remember that a direct blow can lead to a fracture at any point.

Middle ribs are the most likely to fracture and are dangerous because the broken end can puncture numerous organs, such as the lungs. Fractures of the upper ribs are rare due to their relatively protected position, but if it occurs there can be damage to the brachial plexus. Additionally lower rib fractures can result in tearing of the diaphragm.

The main issues tend to arise when multiple ribs are fractured, for example after a severe trauma. In this case, if enough ribs are broken a flail segment, or flail chest, can occur – this is when the separated group of ribs moves in the opposite direction to chest wall expansion during inspiration, restricting breathing. If the segment is large enough assisted ventilation may be needed until the ribs have healed.

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