Rib Cage
The rib cage is a closed chain that involves many joints and muscles. The anterior border of the rib cage is the sternum, the lateral borders are the ribs, and the posterior border is formed by the thoracic vertebrae. The superior border of the rib cage is formed by the jugular notch of the sternum, by the superior borders of the first costocartilages, and by the first ribs and their contiguous first thoracic vertebra.
The inferior border of the rib cage is formed by the xiphoid process, the shared costocartilage of ribs 7 through 10, the inferior portions of the 11th and 12th ribs, and the 12th thoracic vertebra (see Fig. 5–1).
The sternum is an osseous protective plate for the heart and is composed of the manubrium, body, and xiphoid process (Fig. 5–3). The manubrium and the
Figure 5–3 The sternum is composed of the manubrium, the body of the sternum, and the xiphoid process. The costal notches for the chondrosternal joints are also evident in this anterior view.
Figure 5–4 The costal facets on the typical thoracic vertebrae are found on the superior and inferior aspects of the posterior body and the anterior transverse processes.
Figure 5-2 A. A right thoracic scoliosis (named for the side of the convexity) of 52° shows the evident rib distortion that results from accompanying rotation of the involved vertebrae. There is also a lumbar curve of 32°. B. The bodies of the thoracic vertebrae in a right scoliosis typically rotate to the right, resulting in posterior displacement of the right transverse process and the attached right rib, as well as anterior displacement of the opposite transverse process and left rib.
A
Sternum
Right
B
Jugular notch
Costal cartilage of 1st rib Manubrium
Manubriosternal joint
Body of sternum
Xiphoid process 7th costal
notch 4th costal
notch 2nd costal
notch
body of the sternum form a dorsally concave angle of approximately 160°. The xiphoid process often angles dorsally from the body of the sternum and may be diffi- cult to palpate.
There are 12 thoracic vertebrae that make up the poste- rior aspect of the rib cage. One of the unique aspects of the typical thoracic vertebra is that the vertebral body and transverse processes have six costal articulating surfaces, four on the body (a superior and an inferior costal facet, or demifacet, on each side) and one costal facet on each
Superior costal facet
Inferior costal facet Transverse
costal facet
transverse process (Fig. 5–4). The rib cage also includes 12 pairs of ribs. The ribs are curved flat bones that gradually increase in length from rib 1 to rib 7 and then decrease in length again from rib 8 to rib 12.9The posteriorly located head of each rib articulates with contiguous thoracic vertebral bodies. The costal tubercles of ribs 1 to 10 also articulate with the transverse processes of thoracic verte- brae (Fig. 5–5). Anteriorly, ribs 1 to 10 are joined either directly or indirectly to the sternum through their costal cartilages (Fig. 5–6). Ribs 1 through 7 are classified as ver- tebrosternal (or “true”) ribs because each rib, through its costocartilage, attaches directly to the sternum. The costocartilage of ribs 8 through 10 articulates with the cos- tocartilage of the superior rib, indirectly articulating with the sternum via rib 7. These ribs are classified as verte- brochondral (or “false”) ribs. The 11th and 12th ribs are
called vertebral (or “floating”) ribs because they have no anterior attachment to the sternum.9
Articulations of the Rib Cage
The articulations that join the bones of the rib cage include the manubriosternal, xiphisternal, costovertebral, cos- totransverse, costochondral, chondrosternal, and the interchondral joints.
Manubriosternal and Xiphisternal Joints
The manubrium and the body of the sternum articulate at the manubriosternal joint (see Fig. 5–3). This joint is also known as the sternal angle or the angle of Louis and is
readily palpable as a horizontal ridge at the level of the second ribs’ anterior attachments.5,10The manubriosternal joint is a synchondrosis. The manubriosternal joint has a fibrocartilaginous disc between the hyaline cartilage–
covered articulating ends of the manubrium and the body of the sternum—it is structurally similar to the symphysis pubis of the pelvis. Ossification of the manubriosternal joint occurs in elderly persons.10,11 The xiphoid process joins the inferior aspect of the sternal body at the xiphis- ternal joint. The xiphisternal joint is also a synchondrosis and tends to ossify by 40 to 50 years of age.12
Costovertebral Joints
The typical costovertebral joint is a synovial joint formed by the head of the rib, two adjacent vertebral bodies, and the interposed intervertebral disc. Ribs 2 through 9 have typical costovertebral joints, inasmuch as the heads of these ribs each have two articular facets, or so-called demifac- ets10,13 (see Fig. 5–5). The demifacets are separated by a ridge called the crest of the head of the rib. The small, oval, and slightly convex demifacets of the ribs are called the superior and inferior costovertebral facets. Adjacent thoracic vertebrae have facets corresponding to those of the heads of the ribs that articulate with them. The heads of ribs 2 through 9 fit snugly into the angle formed by the ad- jacent vertebral demifacets and the intervening disc. Each rib’s superior facet articulates with the inferior facet of the vertebrae above it. Each rib’s inferior facet articulates with the superior facet of its own numbered vertebrae (Fig. 5–7).
Take rib 4 as an example: the rib’s superior facet articulates with vertebral body above (T3), and the rib’s inferior facet articulates with the superior facet of the vertebral body be- low (T4). Ribs 1, 10, 11, and 12 are atypical ribs because they articulate with only one vertebral body and are num- bered by that body.9,12,13The costovertebral facets of T10 through T12 are located more posteriorly on the pedicle of the vertebra.10
The typical costovertebral joint is divided into two cavi- ties by the interosseous or intra-articular ligament.12,13
Costal tubercle
Outer surface
of rib Inner surface
of rib
Crest of the head Superior
facet of head
Inferior facet of head
Site of articulation with costal cartilage Neck of the rib
Body of the rib
Anterior Posterior
Head of the rib Figure 5–5 The typical rib (ribs 2 through 9) is
a curved flat bone. The posteriorly located head of the rib has superior and inferior facets that are separated by a ridge called the crest of the head.
The superior and inferior facets (also known as demifacets) articulate, respectively, with the infe- rior and superior costal facets on the bodies of adjacent vertebrae. The neck of the rib extends from the head of the rib to the costal tubercle.
The facet on the costal tubercle articulates with the transverse process of the corresponding vertebra; the rib articulates anteriorly to the sternum via costocartilages.
Figure 5–6 In this anterior view of the rib cage, the ribs articu- late with the costal cartilages. The ribs join the costal cartilages at the costochondral joints. The costal cartilages of the first through the seventh ribs articulate directly with the sternum through the chondrosternal joints. The costal cartilages of the 8th through the 10th ribs articulate indirectly with the sternum through the costal cartilages of the adjacent superior rib at the interchondral joints.
transverse processes of the vertebrae and slightly convex costal tubercles on the corresponding ribs. This allows slight rotation movements between these segments. At the costotransverse joints of approximately T7 through T10, both articular surfaces are flat and gliding motions predom- inate. Ribs 11 and 12 do not articulate with their respective transverse processes of T11 or T12.
The costotransverse joint is surrounded by a thin, fibrous capsule. Three major ligaments support the costotransverse joint capsule. These are the lateral costotransverse liga- ment, the costotransverse ligament, and the superior costotransverse ligament (Fig. 5–9). The lateral costo- transverse ligament is a short, stout band located between the lateral portion of the costal tubercle and the tip of the corresponding transverse process.13,14The costotransverse ligament is composed of short fibers that run within the This ligament extends from the crest of the head of the rib
to attach to the anulus fibrosus of the intervertebral disc.10,13 The radiate ligament is located within the capsule, with firm attachments to the anterolateral portion of the capsule.
The radiate ligament has three bands: the superior band, which attaches to the superior vertebra; the intermediate band, which attaches to the intervertebral disc; and the inferior band, which attaches to the inferior vertebra9,10,12 (see Fig. 5–7). A fibrous capsule surrounds the entire artic- ulation of each costovertebral joint.
The atypical costovertebral joints of ribs 1, 10, 11, and 12 are more mobile than the other, more typical costovertebral joints because the rib head articulates with only one verte- bra. The interosseous ligament is absent in these joints;
therefore, they each have only one cavity.13The radiate lig- ament is present in these joints, with the superior band still attached to the superior vertebra. Both rotation and gliding motions occur at all of the costovertebral joints.14
Costotransverse Joints
The costotransverse joint is a synovial joint formed by the articulation of the costal tubercle of the rib with a costal facet on the transverse process of the corresponding vertebra13(Fig. 5–8). There are 10 pairs of costotransverse joints articulating vertebrae T1 through T10 with the rib of the same number. The costotransverse joints on T1 through approximately T6 have slightly concave costal facets on the
Figure 5–7 A lateral view of the costovertebral joints and ligaments. The three bands of the radiate ligament reinforce the costovertebral joints. The superior and inferior bands of the radiate ligament attach to the joint capsule (removed) and to the superior and inferior vertebral bodies, respectively. The intermediate band attaches to the intervertebral disc. The middle costovertebral joint is shown with the radiate ligament bands removed to reveal the intra-articular ligament that attaches the head of the rib to the anulus fibrosus.
Radiate ligament
Costotransverse ligament
Lateral costotransverse
ligament
Superior costotransverse
ligament
Figure 5–8 A superior view of the costovertebral and costo- transverse joints shows the capsuloligamentous structures on the right. The joint capsules and ligaments are removed on the left to show the articulating surfaces.
Figure 5–9 Ligaments supporting the costotransverse joint, including (1) the costotransverse ligament, (2) the lateral costo- transverse ligament, and (3) the superior costotransverse ligament.
costotransverse foramen between the neck of the rib poste- riorly and the transverse process at the same level.10,13The superior costotransverse ligament runs from the crest of the neck of the rib to the inferior border of the cranial trans- verse process.
Costochondral and Chondrosternal Joints
The costochondral joints are formed by the articulation of the 1st through 10th ribs anterolaterally with the costal cartilages (see Fig. 5–6). The costochondral joints are syn- chondroses.10 The periosteum and the perichondrium are continuous, giving support to the union. The costochondral joints have no ligamentous support.
The chondrosternal joints are formed by the articulation of the costal cartilages of ribs 1 through 7 anteriorly with the sternum (see Fig. 5–6). Rib 1 attaches to the lateral facet of the manubrium, rib 2 is attached via two demifacets at the manubriosternal junction, and ribs 3 through 7 articulate with the lateral facets of the sternal body. The chondroster- nal joints of ribs 1, 6, and 7 are synchondroses. The chon- drosternal joints of ribs 2 through 5 are synovial joints.
The chondrosternal joints of ribs 1 through 7 have cap- sules that are continuous with the periosteum and support the connection of the cartilage as a whole.13 Ligamentous support for the capsule includes anterior and posterior radiate costosternal ligaments. The sternocostal liga- ment is an intra-articular ligament, similar to the intra-artic- ular ligament of the costovertebral joint, that divides the two demifacets of the second chondrosternal joint.9,12,13The chon- drosternal joints may ossify with age.9 The costoxiphoid ligament connects the anterior and posterior surfaces of the seventh costal cartilage to the front and back of the xiphoid process.
Interchondral Joints
The costal cartilages of ribs 7 through 10 each articulate with the cartilage immediately above them to form the in- terchondral joints. For ribs 8 through 10, this articulation forms the rib’s only connection to the sternum, albeit indi- rectly (see Fig. 5–6). The interchondral joints are synovial joints and are supported by a capsule and interchondral
ligaments. The interchondral articulations, like the chon- drosternal joints, tend to become fibrous and fuse with age.
A B
Upper Ribs
Lower Ribs
ConceptCornerstone 5-1
Rib Cage Summary
In summary, ribs 1 through 10 articulate posteriorly with the vertebral column by two synovial joints (the costover- tebral and costotransverse joints) and anteriorly through the costocartilages to the manubriosternum, either di- rectly or indirectly. These joints form a closed kinematic chain in which the segments are interdependent and mo- tion is restricted. These articulations and their associated ligamentous support give the thoracic cage the stability necessary to protect internal organs and yet provide enough flexibility to maximize function.13Ribs 11 and 12 have a single costovertebral joint, no costotransverse joint, and no attachment anteriorly to the sternum.
These ribs form an open kinematic chain, and the motion of these ribs is less restricted.
Kinematics of the Ribs and Manubriosternum
The movement of the rib cage is an amazing combination of complex geometrics that are governed by (1) the types and angles of the articulations, (2) the movement of the manubriosternum, and (3) the elasticity of the costal cartilages.
The costovertebral and costotransverse joints are me- chanically linked, with a single axis of motion for elevation and depression passing through the centers of both joints.6,12–14 The length, shape, and downward angle of each rib is unique, and therefore, the axis of rotation for each rib is slightly different. The axes of rotation for the up- per ribs lie closest to the frontal plane, allowing the motion of those ribs to occur predominantly in the sagittal plane.
The axes of rotation for the lower ribs move toward the sagittal plane, allowing the motion of those ribs to be closer to the frontal plane (Fig. 5–10). The axes of rotation for
Figure 5–10 A. The common axis of motion for the upper ribs passes through the centers of the costovertebral and costotransverse joints and lies nearly in the frontal plane. B. The axis through the costovertebral and costo- transverse joints for the lower ribs lies closer to the sagittal plane.
ribs 11 and 12 pass through the costovertebral joint only, because there is no costotransverse joint present. The axes of rotation for these last two ribs also lie close to the frontal plane.
The first rib is unique because its anterior articulation is larger and thicker than that of any other rib.9The first costal cartilage is stiffer than the other costocartilages. Also, the first chondrosternal joint is cartilaginous (synchondrosis), not synovial, and therefore is firmly attached to the manubrium.
Finally, the first chondrosternal joint is just inferior and pos- terior to the sternoclavicular joint. For these reasons, there is very little movement of the first rib at its attachment on the manubrium. Posteriorly, the costovertebral joint of the first rib has a single facet, which increases the mobility at that joint. During inspiration, the costovertebral joint moves superiorly and posteriorly, elevating the first rib.
Ribs 2 through 7, which are attached to the body of the sternum, increase in length and mobility the more caudal the rib. In these upper ribs, most of the movement occurs at the anterior aspect of the rib, given the nearly coronal axis at the vertebrae. The costocartilage rotates upward, becom- ing more horizontal with inspiration.13 The movement of the ribs pushes the sternum ventrally and superiorly. The manubrium moves less than the body of the sternum because the shortest and least mobile first rib is attached to the manubrium. This discrepancy in rib length increases the movement of the body of the sternum in relation to the manubrium and results in slight movement at the manubriosternal joint.10The greatest effect of the motion of the upper ribs and sternum is the increase in the antero- posterior (A-P) diameter of the thorax. This combined rib and sternal motion that occurs in a predominantly sagittal plane has been termed the “pump-handle” motion of the thorax (Fig. 5–11).
Elevation of ribs 8 through 10 occurs about an axis of motion lying more towards the sagittal plane. The lower ribs have a more angled shape (downward obliquity increases from rib 1 to rib 10) and an indirect attachment
anteriorly to the sternum. These factors allow the lower ribs more motion at the lateral aspect of the rib cage. The great- est effect of the elevation of the lower ribs is the increase in the transverse diameter of the lower thorax. This motion that occurs in a more frontal plane has been termed the
“bucket-handle” motion of the thorax (Fig. 5–12).
There is a gradual shift in the orientation of the ribs’ axes of motion from cephalad to caudal; therefore, the interme- diate ribs demonstrate a transitional zone, with qualities of both types of motion.9,12–15 The 11th and 12th ribs each have only one posterior articulation with a single vertebra and no anterior articulation to the sternum; therefore, they do not participate in the closed-chain motion of the thorax.
Figure 5–11 Elevation of the upper ribs at the costovertebral and costotransverse joints results in anterior and superior movement of the sternum (and accompanying torsion of the costal cartilages), referred to as the “pump-handle” motion of the thorax.
Figure 5–12 Elevation of the lower ribs at the costovertebral and costotransverse joints results in a lateral motion of the rib cage, referred to as “bucket-handle” motion of the thorax.
only the primary inspiratory muscles are needed for venti- lation. During active or forced breathing that occurs with, for example, increased activity or pulmonary pathologies, accessory muscles of both inspiration and expiration are recruited to help meet the increased demand for ventilation.
The ventilatory muscles are most accurately classified as either primary or accessory muscles of ventilation.
A muscle’s action during the ventilatory cycle, especially the action of an accessory muscle, is neither simple nor ab- solute, which makes the categorizing of ventilatory muscles as either inspiratory muscles or expiratory muscles inaccu- rate and misleading.
Primary Muscles of Ventilation
The primary muscles are those recruited for quiet venti- lation. These include the diaphragm, the intercostal Figure 5–13 The rib cage volume changes in scoliosis.
A. Normal ribcage. B. With scoliosis, the restriction is asymmet- rically distributed, with the concave side of the thorax (left) de- creasing in volume and the convex side (right) increasing in vol- ume.17(From Tobin MJ: Respiratory muscles in disease. Clin Chest Med 9:263, 1988, with permission.)
Continuing Exploration 5-1:
Effects of Scoliosis on the Rib Cage
The single axis of motion of the ribs is through the cos- tovertebral and costotransverse joints. Therefore, changes in the alignment of these joints will change the mobility of the thorax. In scoliosis, the thoracic vertebrae not only laterally deviate but also rotate, altering the alignment of the costovertebral and costotransverse articulating sur- faces (see Figs. 5–2A and B). The rib cage volume changes asymmetrically in scolioss, with the concave side of the thorax showing anterior rib distortion, a decrease in ver- tebral height, a narrowing of the intercostal spaces, and a decrease in lung volume. The convex side shows posterior rib distortion, widened intercostals spaces and an in- creased lung volume.16Figure 5–13A is a view of a normal thorax in a 4-year-old. Figure 5–13B is a view of the tho- rax of a 4-year-old with a congenital thoracic scoliosis and shows an extreme vertebral rotation and the accompany- ing rib distortion. The amount of ventilatory dysfunction found in patients with scoliosis depends on the angle of the deformity, the length of the deformity, the region of the deformity, the amount of rotation of the deformity, and the age at onset.16–18
CASE APPLICATION
Rib Distortion
in Scoliosis case 5–1
Mary is seen by an orthopedic physician, who confirms the measurement of her midthoracic scoliotic curve at 40°. This degree of scoliotic angulation in the midthoracic region is likely to be accompanied by rotation of the involved verte- brae and a decrease in her pulmonary reserve. This may be a contributing factor in Mary’s shortness of breath while playing tennis.
A
B
Muscles Associated With the Rib Cage
The muscles that act on the rib cage are generally referred o as the ventilatory muscles. The ventilatory muscles are striated skeletal muscles that differ from other skeletal muscles in a number of ways: (1) they have increased fatigue resistance and greater oxidative capacity; (2) they contract rhythmically throughout life rather than episodically; (3) they work primarily against the elastic properties of the lungs and airway resistance rather than against gravitational forces; (4) neurological control of these muscles is both voluntary and involun- tary; and (5) the actions of these muscles are life- sustaining.
Any muscle that attaches to the chest wall has the po- tential to contribute to ventilation. The recruitment of muscles for ventilation is related to the type of breathing being performed.19In quiet breathing that occurs at rest,