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.
In this article we shall consider the osteology and articulations of the ribs, as well ligamentous and muscular attachments, finishing with some clinical notes.
In this section we shall consider individually the characteristics of both typical and atypical ribs.
Ribs three to nine are the ‘typical’ ribs and the major landmarks are the head, neck, tubercle and body.
The head of each ribs 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 rib.
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.
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.
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.
The first, second and tenth to twelfth ribs are known as atypical and as such will be considered individually.
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.
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.
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.
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.
Muscles attached to the ribs
- External intercostal muscles – There are eleven pairs of external intercostal muscles and these are the most superficial in the area. These 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.
- 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 costal cartilages of ribs three to six. This muscle assists in depression of the ribs.
- Serratus posterior – This muscle is present posteriorly within the thoracic wall. The muscle 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 – This muscle originates from the spinous processes of vertebrae C7 to T11 and attaches to the posterior surface and angle of the rib below. Its action is to assist in elevation of the ribs.