Chest Wall Anatomy PDF

Respiratory System The Chest Wall This begins a series of lectures on the anatomy of the thorax. We will cover mostly the cardiovascular and respiratory systems within this region but will touch on other aspects as well. There are 5 respiratory and 3 cardiov...

Respiratory System The Chest Wall This begins a series of lectures on the anatomy of the thorax. We will cover mostly the cardiovascular and respiratory systems within this region but will touch on other aspects as well. There are 5 respiratory and 3 cardiovascular lectures and 1 to deal with anything else. The plan is to start with the cardiovascular ones, however, as you will see this first lecture is part of the Respiratory set. The chest wall is essential for breathing. But we cannot get access to the cardiovascular structures until the chest wall is removed.  Respiratory System The Chest Wall Part 1: The Rib Cage So this lecture is divided into three parts, and in this 1st part we will consider the rib-cage.  Learning Outcomes After this lecture you should be able to: ▪ Describe the basic structure of the chest wall, including sternum, ribs, intercostal spaces and thoracic vertebrae ▪ Be familiar with the terms used to describe the thoracic cage ▪ Name the parts of a rib and know which ribs are typical and which are atypical ▪ Know which thoracic vertebrae are typical and which are atypical ▪ Name the joints of the thoracic cage, and indicate their classification ▪ Describe the basic arrangements of blood vessels, lymphatics and innervation of the chest. These are the learning outcomes for all parts of the lecture.  Learning Outcomes After this lecture you should be able to: ▪ Describe the basic structure of the chest wall, including sternum, ribs, intercostal spaces and thoracic vertebrae ▪ Be familiar with the terms used to describe the thoracic cage ▪ Name the parts of a rib and know which ribs are typical and which are atypical ▪ Know which thoracic vertebrae are typical and which are atypical ▪ Name the joints of the thoracic cage, and indicate their classification ▪ Describe the basic arrangements of blood vessels, lymphatics and innervation of the chest. But let’s focus on the first two for now. The learning outcomes for this part of the lecture are that afterwards you should be able to: Describe the basic structure of the chest wall, including sternum, ribs, intercostal spaces and thoracic vertebrae Be familiar with the terms used to describe the thoracic cage The remaining outcomes will be dealt with in subsequent sections of the lecture.  Extrinsic Muscles of the Chest Wall Pectoralis major Serratus anterior External Rectus oblique abdominis abdominis The thorax is the part of the body between the neck and the abdomen, partially encased by the ribs and containing the heart and lungs. It is also known as the chest (from the Ancient Greek (chista) kistē, meaning a box). The skin over the chest wall is thin. The superficial fascia is also thin, except in the region of the mammary gland in females (and some males). The Latin term for the chest is “pectus”. Hence the anterior chest is known as the pectoral region. The large muscles found there are the pectoral muscles (pectoralis major and minor). At the side of the chest there is a muscle attached to the upper 8 ribs that has a serrated appearance. This is the serratus anterior muscle. These two muscles are part of a group of muscles that can lift the ribcage upwards. This is part of the process of breathing in or inspiration. The muscles also however belong to a set of upper limb muscles, and hence elevating the chest is a secondary function. We say therefore that they are accessory muscles of inspiration. There are also muscles attached to the lower aspect of the chest wall, two of which are shown here. These are abdominal muscles first and foremost, but they also act to depress the chest and hence are accessory muscles of expiration.  Intrinsic Muscles of the Chest Wall All of the muscles outside of the rib-cage are known as ‘extrinsic’ muscles of the chest. Inside those, there are muscles that run between the ribs. These are ‘intrinsic’ muscles known as the intercostals, and there are three layers of them. Sandwiched between the 2nd and 3rd layer lie the main neurovascular bundle of the chest wall; the intercostal nerve, artery and vein. Inside the ribs, there is a layer of fascia called the endothoracic fascia (not shown on diagram). This fascia acts like a glue to adhere to a serous membrane called parietal pleura. The word ‘parietal’ comes from the Latin pariēs meaning “wall”. There is a second serous membrane covering the lungs and is called visceral pleura. Between these two serous membranes is a gap called the pleural cavity. Intercostal nerves supply the intrinsic muscles, parietal pleura, fascia and skin. The extrinsic muscles have their own nerve supply, and this is dealt with in other parts of the course.  The Sternum Manubrium sterni Sternal angle (of Louis) Body of sternum Xiphisternum (Xiphoid process) The rib cage comprises the sternum in front, the costal cartilages, ribs, and the thoracic vertebrae behind. The costal cartilages are in fact simply the unossified parts of the ribs. Ossification starts posteriorly but fails to complete the task. The sternum has three parts: the manubrium (or manubrium sterni to give it its full title), the body, and the xiphisternum (or xiphoid process). These parts are linked together by secondary cartilaginous joints. The reason for the names is based on what the old anatomists saw when looking at this bone. What they saw was a dagger. The manubrium is the handle of the dagger, and manus translates as hands, with a manubrium being a handle or holder. A body is just the main part and xiphos is a sharp point or sword in ancient Greek. The joint between the manubrium and the body is the sternal angle. It usually goes by the older name angle of Louis*. This joint forms an important hinge for movements of the rib cage. The movement as you can see is slight, yet this has a significant bearing on the mobility of the ribcage. In the elderly, this joint begins to cease, and this impedes breathing. *Discovered by a surgeon called Antoine Louis (1723-1792). The significance of the joint however was only revealed in the 19th century by Pierre-Charles Alexandre Louis. So the name "Angle de Louis" is apparently named after both doctors.  Rib Classification 1 2 3 True Ribs (1-7) 4 5 False Ribs (8-12) 6 Free-Floating Ribs (11-12) 7 11 8 9 12 10 Costal Margin There are 12 pairs of ribs. The first 7 ribs on each side are termed “true ribs” since they attach anteriorly to the sternum. All other ribs are known as “false ribs”. Ribs 8-10 have only anterior attachment to the cartilages above them, and ribs 11 and 12 have no anterior attachment at all and are hence known as free-floating ribs. The cartilages of the 10th rib up to the 7th rib make a continuous cartilaginous border known as the costal margin, and this demarcates the lowest part of the ribcage anteriorly.  The Thoracic Cage Joints 1st sternocostal joint (primary cartilaginous) 2nd-7th sternocostal joints (synovial) Chondrochondral joints (8-10) (synovial) The joints that the costal cartilages of the true ribs make with the sternum are called sternocostal joints. The 1st one is rather unique as it is a primary cartilaginous joint. Indeed, it is the only such joint in the entire adult skeleton. This joint type indicates that it is less mobile than the other joints which are all synovial. The 2nd sternocostal joint has a double facet, as it articulates both with the body of the sternum and the manubrium sterni. All of the remaining sternocostal joints have a single facet to articulate with the body. Ribs 8-10 articulate with the cartilages above them forming synovial joints called interchondral or chondrochondral joints. They are sometimes referred to erroneously as costochondral joints in some textbooks. “Costochondral” is a term that should be restricted to the union of the rib with the costal cartilage. Technically, that isn’t a joint at all, but simply the beginning of the unossified portion of the rib.  Rib Angulation There are twelve thoracic vertebrae to match the twelve pairs of ribs. The ribs attach posteriorly to those vertebrae and then course downwards and forwards, so that the ribcage of adults is arranged obliquely. The position of the ribs is largely produced by the effects of gravity, and by changes in the shape of the vertebral column during ageing. This obliquity reduces the amount of space inside the ribcage. This is the thoracic cavity. However, by lifting the ribcage upwards this space can be reclaimed. This is the process of thoracic breathing, since the size of the thoracic cavity is directly related to the volume of air that the lungs can inhale. This is only one of the ways we breathe, and we will discuss the full extent of the mechanics of breathing in a future lecture.  Rib Cage Support Sterno- cleidomastoid Scalenes: scalenus anterior scalenus medius scalenus posterior To prevent excessive displacement, the first rib is anchored by a group of muscles in the neck called the scalene muscles. These attach to the transverse process of some of the cervical vertebrae proximally, and to the 1st and 2 nd ribs distally. This group of muscles essentially counteract the effects of gravity on the ribcage, and resists the pull of any muscle trying to depress the 1st rib. As a consequence of their action, the first rib is held in a fairly static position. The action of these muscles is aided by the presence of a muscle of the head called sternocleidomastoid. This attaches to the sternum and clavicle distally and runs upwards to attach to the mastoid process of the skull superiorly. This muscle has more of a role to play for head movements, but can assist in stabilising the sternum for the purpose of breathing if required. Movement of the rib cage during inspiration is upwards towards the first rib, so it is important that this rib is kept stable.  Baby’s Rib Cage The thoracic cavity of young children, not being subject to these forces before they walk, have a rib cage which is more horizontal. This also changes the shape of the ribcage such that the costal margin is wider. This has an implication for breathing, as children of that age cannot use the ribs to expand the chest cavity. They rely solely upon the diaphragm for this function. The diaphragm is a bi-domed sheet of muscles attached across the lowest aspect of the ribcage.  Rib Cage of Adult and Newborn Occasionally though, children are born with a defective diaphragm. It has a complicated development, being composed of several parts which come together. It also begins life as a cervical (neck) structure and descends into its final position before birth. If the diaphragm fails to grow properly, or does not form in its normal position, then breathing may be impaired. Thankfully, usually defects are on one side only, and breathing may still continue normally on the unaffected side. However, this depends on the extent of the problem. Children with diaphragmatic malformations will need to have their breathing assisted by a machine.  Rib Fractures Ribs fracture easily in adults, whilst in children the bones are more flexible. Rather sadly, the baby whose X-ray you see on the left was the subject of abuse. The fractures of the bones on the left side show callous formation (shown by the arrow heads), which is an indication of healed fractures. The fractures shown on the right side of the body (shown by the arrows) indicate new fractures. You will be pleased to hear that the baby is now safe in a foster home. The fractured ends of the ribs of both children and adults are rarely displaced because the bones are held together by periosteum, connective tissues and muscles. Rib injuries usually heal rapidly. Occasionally, a fractured rib may cause a lung to collapse, but we’ll explore the reasons for that later.  Respiratory System The Chest Wall Part 2: Osteology Welcome to part 2 of this lecture on the chest wall. Here, we’ll take a closer look at some of the bones making up the ribcage.  Learning Outcomes After this lecture you should be able to: ▪ Describe the basic structure of the chest wall, including sternum, ribs, intercostal spaces and thoracic vertebrae ▪ Be familiar with the terms used to describe the thoracic cage ▪ Name the parts of a rib and know which ribs are typical and which are atypical ▪ Know which thoracic vertebrae are typical and which are atypical ▪ Name the joints of the thoracic cage, and indicate their classification ▪ Describe the basic arrangements of blood vessels, lymphatics and innervation of the chest. The learning outcomes for this section of the lecture are that afterwards you should be able to: Name the parts of a rib and know which ribs are typical and which are atypical Know which thoracic vertebrae are typical and which are atypical Name the joints of the thoracic cage, and indicate their classification The remaining outcome will be dealt with in Part 3.  Osteology: Typical Ribs Head neck Superior demi-facet Interarticular Crest Inferior demi-facet Tubercle Articular part (facet) Angle Non-articular (ligamentous) Subcostal groove Typical ribs are ribs 3-9 or possibly 10. Here we can see an example of a typical rib from behind. They have a head with two articular facets as each rib articulates with its own vertebra and the vertebra above. Distal to the head of course is a neck, and distal to that is a tubercle which has both an articular part and a non-articular part. The articular part makes a facet joint with the transverse process of the vertebra at that level. The non-articular part attaches ligaments which help to stabilise the joint. You needn’t learn the details of these ligaments. Beyond the tubercle lies the shaft. The shaft soon bends sharply and this is known as the angle of the rib. This demarcates the boundary of the back, since all true back muscles attach behind this line. The only other notable feature on the shaft is a groove inferiorly to house the neurovascular bundle. As we know the shaft becomes continuous with the costal cartilage anteriorly.  Osteology: Atypical Ribs 1st Short, strong, flat and very curved Has a tubercle for scalenus anterior Single facet on head Grooves for subclavian vessels 2nd Tubercle for scalenus posterior Tubercle for serratus anterior (10th) Only sometimes atypical May articulate with T10 vertebra only 11th and 12th Do not have tubercles and do not attach to anything anteriorly Only have single facet on their head Well, those were the features of a typical rib. An atypical rib is one that does not have all of the features of a typical rib or has some additional features. The 1st rib is flatter than other ribs and has grooves and tubercles not seen on the others. It has a single facet on its head, as it only articulates with the T1 vertebrae. The 2nd rib also has tubercles for the attachment of the scalene muscles. It does however has two facets on its head. The 10th rib is only sometimes atypical as it may occasionally articulate with the T10 vertebra only. The 11th and 12th ribs do not have tubercles and have no anterior attachments. Again, they only have a single facet on their head, as they only articulate with their own vertebra.  Articular Facets on Vertebra Superior facet Facet for rib tubercle for head of rib Inferior facet for rib below I’m aware that you have already completed a series of lectures on the vertebral column, so I will not go through the details of the vertebrae here. Suffice to say though, you should review and revise the parts of the thoracic vertebrae. Each typical rib articulates with the superior facet on the body at that level (e.g. the 5th rib articulates with the superior facet of the 5th thoracic vertebra). This is a costovertebral joint. It also makes another costovertebral joint with the inferior facet of the vertebra above. Lastly, the rib makes a joint with the transverse process of its own vertebra. This is a costotransverse joint. All of these joints are synovial and are mobile and are supported by a variety of ligaments.  Typical and Atypical Thoracic Vertebrae Atypical Vertebrae a. 1st b. 10th c. 11th d. 12th Like the ribs, some of the thoracic vertebrae are “typical” and some “atypical”. The atypical vertebrae are T1, T10, T11 and T12. Unlike the ribs, T2 is typical and T10 is atypical always. T10 only has one demi- facet on its body. Ribs 11 and 12 lack a costotransverse joint.  Costovertebral Joints The ribs move by rotating around their points of attachment to the thoracic vertebrae. The axis of this rotation is a line drawn between the costovertebral and costotransverse joints. However, not all ribs have the same rotation as the shape of the thoracic vertebrae change as they descend down the vertebral column. The upper vertebrae have an axis which is less than 45 degrees in the horizonal plane, whilst the lower ones have an angle greater than 45 degrees. The effect of this change is that the upper ribs move upwards and forwards as shown on the right hand side of this illustration, whilst the lower ribs move upwards and laterally. These are referred to as pump- handle and bucket-handle movements respectively.  Inspiration and Expiration Breathing can be sub-divided into inspiration (breathing in) and expiration (breathing out). This happens in response to changes in pressure between the air inside the lungs and atmospheric pressure. When the pressure inside is less than that outside air will be sucked in. When the pressure inside is greater than that outside air will be expelled. The pressure changes are brought about by changes in thoracic volume. Inspiration occurs when the thoracic volume is increased, and expiration occurs when the thoracic volume is reduced. The volume can be increased by three mechanisms. Firstly, this can occur by contraction of the diaphragm, which causes it to descend and hence increasing the supero-inferior dimension of the thoracic cavity.  Diaphragmatic Movements This illustration on the left shows the position of the diaphragm in maximal inspiration in red and maximal expiration in blue. The diaphragm is a bi-domed structure with two hemispheres. In normal or quiet breathing, the diaphragm will only need to descend about a cm or two to change the volume of the thoracic cavity sufficiently for effective gas exchange.  Pump Handle Movements The second method of increasing the thoracic cavity in inspiration is to move the upper ribs upwards and forwards, increasing the antero- posterior diameter of the thoracic cavity. This movement is referred to as a “pump-handle” mechanism as it is likened to the way water is drawn up from the well using the old village pumps.  Bucket Handle Movements The final way to increase the thoracic cavity during breathing is to lift the lower ribs outwards to the sides, so increasing the transverse diameter of the thorax. This is referred to as a “bucket-handle” mechanism.  Expiratory Movements Expiration is simply brought about by relaxation of the diaphragm and the muscles used to elevate the chest wall. It’s a little bit more complicated than that, but that’ll do for now.  Respiratory System The Chest Wall Part 3: Neurovasculature This is the final part of the lecture on the chest wall, and in this section we will consider its neurovasculature.  Learning Outcomes After this lecture you should be able to: ▪ Describe the basic structure of the chest wall, including sternum, ribs, intercostal spaces and thoracic vertebrae ▪ Be familiar with the terms used to describe the thoracic cage ▪ Name the parts of a rib and know which ribs are typical and which are atypical ▪ Know which thoracic vertebrae are typical and which are atypical ▪ Name the joints of the thoracic cage, and indicate their classification ▪ Describe the basic arrangements of blood vessels, lymphatics and innervation of the chest The final learning outcome for this lecture is that afterwards, you should be able to: Describe the basic arrangements of blood vessels, lymphatics and innervation of the chest.  Vasculature of the Chest Wall The thoracic wall receives a blood supply from at least three sources. Firstly, there is a supply to the pectoral region via branches of the axillary artery. Secondly, there is a supply from the front of the chest via a pair of internal thoracic arteries and lastly there are paired branches from the thoracic aorta. The internal thoracic gives rise to anterior intercostal arteries, whilst the aorta gives rise to posterior intercostal arteries. These anastomose with each other within the intercostal space, tucked away in the subcostal groove between the 2 nd and 3rd layer of intercostal muscles. Internal Thoracic (Mammary) Vessels Whilst the internal thoracic arteries arise as branches of the subclavian artery, the veins drain into the brachiocephalic veins at the root of the neck. Lower in the chest the veins lie either side of the artery as a venae comitantes. These unite before entering the brachiocephalic veins. The internal thoracic arteries also give branches internally to the diaphragm and pericardium of the heart, via pericardiacophrenic branches, and to the thymus gland which is situated anteriorly in the thoracic cavity. It terminates by forming the superior epigastric artery of the anterior abdominal wall, and the musculophrenic artery which follows the costal margin, supplying the peripheral part of the diaphragm.  Azygos Venous System The posterior venous drainage of the chest wall is via the azygos system. The word azygos means unpaired, as the main vein, the azygos vein drains into the superior vena cava which is situated on the right. It drains into the right atrium of the heart. The azygos vein receives blood from the entire posterior chest wall, with the exception of the upper 1-2 intercostal spaces, which drain instead to the brachiocephalic veins in the root of the neck. The venous drainage of the left side is split into two halves. Spaces 8- 11 drain to the hemiazygos vein, and spaces 3-7 drain to the accessory hemiazygos vein. These veins then shunt venous blood from left to right across the midline. The azygos vein joins the superior vena cava by the arch of the azygos at the plane of the sternal angle. The azygos system of veins is valveless, and the direction of the flow of blood is dependent upon the pressure changes during breathing.  Superficial Lymphatics of the Chest The lymphatic drainage of the chest wall may be divided into superficial and deep. The superficial drainage anteriorly is mostly concerned with the breast tissue. 75% of the lymphatic drainage of the breast tissue is to the anterior axillary lymph nodes lying along the inferior border of the pectoralis major muscle. The anterior axillary nodes are known as the ‘sentinel’ nodes since these are the first nodes to receive the lymphatic drainage of the breast. A sentinel is a word meaning a ‘guarding soldier’. The other 25% of the drainage is medially, via the internal thoracic (parasternal) nodes. Alternatively, some lymph may pass to the other breast, or pass with the lymphatics of the anterior abdominal wall. Superficial lymphatics of the posterior chest wall drain to the subscapular group of axillary lymph nodes. These are not shown on the illustration.  Lymphovenous Portals Ultimately all of the lymphatic vessels drain into the veins at the root of the neck in what are called lymphovenous portals. The left lymphovenous portal receives the left bronchomediastinal trunk, the thoracic duct, the jugular trunk from the left head and neck and the left subclavian trunk from the left upper limb. The lymphatic drainage of the left breast would reach either the subclavian trunk or the bronchomediastinal duct. The right lymphovenous portal receives the same from the right side of the body, but there is no thoracic duct on the right. This effectively means that the lymphatic drainage is rather unevenly split between left and right. The area indicated in light green on this illustration indicates the drainage to the right lymphatic duct, whilst the dark green area is where the drainage is mostly via the thoracic duct.  Deep Lymphatics of the Chest Bronchomediastinal Internal lymph trunk thoracic nodes Thoracic duct Brachiocephalic nodes Posterior intercostal nodes Tracheobronchial nodes Posterior mediastinal nodes Diaphragmatic nodes Here we can see a view of the lymphatics on the left side of the chest. Let’s colour code this with the anterior nodes in orange and the posterior nodes in blue. The anterior set comprise the internal thoracic nodes, the brachiocephalic nodes and the tracheobronchial nodes. All of these ultimately reach the bronchomediastinal lymph trunk. The posterior set comprise the posterior intercostal nodes and posterior mediastinal nodes. Ultimately, these drain to the thoracic duct. The lower nodes on the right side also drain into the thoracic duct. There are interconnections between these anterior and posterior sets. In this way, infection and cancers can spread from one territory to another. For example, spreading from the lungs to the posterior chest wall, and from here to the thoracic duct. Mostly though the drainage would be via the bronchomediastinal lymph trunks.  Nerve Supply of the Chest Wall The intercostal nerves are the anterior rami of the thoracic spinal nerves. The posterior rami supply the post-vertebral muscles of the chest, and the skin overlying these. The anterior rami supply the intrinsic muscles of the chest (intercostal muscles), and associated skin. Note the points at which the cutaneous nerves and vessels exit, posteriorly, laterally and anteriorly. The intercostal nerves also supply the parietal serous membrane surrounding the inside of the chest wall. The neurovascular bundle (vein, artery and nerve) lies in the subcostal groove, lying between the 2 nd and 3rd layers of muscles covering the space between the ribs. At least some of the bundles give collaterals lying along the lower margin of the space. Traditionally, it has always been considered safest to insert needles into the chest wall above a rib, so as to avoid the neurovascular bundle. However, this is where the collateral branches are, so perhaps the safest place is in the middle of the space. Indeed, if one needs to anaesthetise the nerves (intercostal nerve block), this should be done as far posteriorly as possible, but nerve blocks should be performed both above the rib below, and below the rib above. I hope that makes sense!  Dermatomes of the Chest Wall Each intercostal nerve supplies a dermatome. The skin of the chest above the plane of the sternal angle is supplied by C4. There is then no representation of C5-T1 as that has been used for the upper limb. Hence, the skin below the plane of the sternal angel is T2-T6 anteriorly. Remember once again that the nipple in males lies usually within the T4 dermatome. Dermatomes over the chest wall overlap, such that a given nerve is completely overlapped (half by the nerve above, and the other half by the nerve below). An intercostal nerve block is used to anaesthetise the skin, muscles and pleura, either to relieve pain or to permit clinical intervention. However, since dermatomes overlap, it may be necessary to anaesthetise more than one spinal nerve. If a cancer of the nipple was to be removed surgically, the T4 intercostal nerve would need to be blocked, but also T3 and T5 as well.  Respiratory System The Chest Wall And that concludes this particular block, and indeed the entire lecture on the chest wall. In the next lecture, we’ll consider the cardiovascular system.