Anatomy Textbook: Part 1 PDF

What is anatomy? Anatomy includes those structures that can be seen grossly (without the aid of magnification) and microscopically (with the aid of magnification). Typically, when used by itself, the term *anatomy* tends to mean gross or macroscopic anatomy---that is, the study of structures that c...

What is anatomy? Anatomy includes those structures that can be seen grossly (without the aid of magnification) and microscopically (with the aid of magnification). Typically, when used by itself, the term *anatomy* tends to mean gross or macroscopic anatomy---that is, the study of structures that can be seen without using a microscope. Microscopic anatomy, also called histology, is the study of cells and tissues using a microscope. Anatomy forms the basis for the practice of medicine. Anatomy leads the physician toward an understanding of a patient's disease, whether he or she is carrying out a physical examination or using the most advanced imaging techniques. Anatomy is also important for dentists, chiropractors, physical therapists, and all others involved in any aspect of patient treatment that begins with an analysis of clinical signs. The ability to interpret a clinical observation correctly is therefore the endpoint of a sound anatomical understanding. Observation and visualization are the primary techniques a student should use to learn anatomy. Anatomy is much more than just memorization of lists of names. Although the language of anatomy is important, the network of information needed to visualize the position of physical structures in a patient goes far beyond simple memorization. Knowing the names of the various branches of the external carotid artery is not the same as being able to visualize the course of the lingual artery from its origin in the neck to its termination in the tongue. Similarly, understanding the organization of the soft palate, how it is related to the oral and nasal cavities, and how it moves during swallowing is very different from being able to recite the names of its individual muscles and nerves. An understanding of anatomy requires an understanding of the context in which the terminology can be remembered. How can gross anatomy be studied? The term *anatomy* is derived from the Greek word *temnein* , meaning "to cut." Clearly, therefore, the study of anatomy is linked, at its root, to dissection, although dissection of cadavers by students is now augmented, or even in some cases replaced, by viewing prosected (previously dissected) material and plastic models, or using computer teaching modules and other learning aids such as virtual and augmented reality experiences. Anatomy can be studied following either a regional or a systemic approach. - With a **regional approach **, each *region *of the body is studied separately and all aspects of that region are studied at the same time. For example, if the thorax is to be studied, all of its structures are examined. This includes the vasculature, the nerves, the bones, the muscles, and all other structures and organs located in the region of the body defined as the thorax. After studying this region, the other regions of the body (i.e., the abdomen, pelvis, lower limb, upper limb, back, head, and neck) are studied in a similar fashion. - In contrast, in a **systemic approach **, each *system *of the body is studied and followed throughout the entire body. For example, a study of the cardiovascular system looks at the heart and all of the blood vessels in the body. When this is completed, the nervous system (brain, spinal cord, and all the nerves) might be examined in detail. This approach continues for the whole body until every system, including the nervous, skeletal, muscular, gastrointestinal, respiratory, lymphatic, and reproductive systems, has been studied. Each of these approaches has benefits and deficiencies. The regional approach works very well if the anatomy course involves cadaver dissection but falls short when it comes to understanding the continuity of an entire system throughout the body. Similarly, the systemic approach fosters an understanding of an entire system throughout the body, but it is very difficult to coordinate this directly with a cadaver dissection or to acquire sufficient detail. Important anatomical terms **The anatomical position** The anatomical position is the standard reference position of the body used to describe the location of structures ( [Fig. 1.1](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0010) ). The body is in the anatomical position when standing upright with feet together, hands by the side, and face looking forward. The mouth is closed and the facial expression is neutral. The rim of bone under the eyes is in the same horizontal plane as the top of the opening to the ear, and the eyes are open and focused on something in the distance. The palms of the hands face forward with the fingers straight and together and with the pad of the thumb turned 90 degrees to the pads of the fingers. The toes point forward. Fig. 1.1 The Anatomical Position, Planes, and Terms of Location and Orientation. Anatomical planes Three major groups of planes pass through the body in the anatomical position (see [Fig. 1.1](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0010) ). - **Coronal planes **are oriented vertically and divide the body into anterior and posterior parts. - **Sagittal planes **also are oriented vertically but are at right angles to the coronal planes and divide the body into right and left parts. The plane that passes through the center of the body dividing it into equal right and left halves is termed the **median sagittal plane.** - **Transverse, horizontal**, or **axial planes **divide the body into superior and inferior parts. Terms to describe location Anterior (ventral) and posterior (dorsal), medial and lateral, superior and inferior Three major pairs of terms are used to describe the location of structures relative to the body as a whole or to other structures (see [Fig. 1.1](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0010) ). - **Anterior **(or **ventral **) and **posterior **(or **dorsal **) describe the position of structures relative to the "front" and "back" of the body. For example, the nose is an anterior (ventral) structure, whereas the vertebral column is a posterior (dorsal) structure. Also, the nose is anterior to the ears and the vertebral column is posterior to the sternum. - **Medial **and **lateral **describe the position of structures relative to the median sagittal plane and the sides of the body. For example, the thumb is lateral to the little finger. The nose is in the median sagittal plane and is medial to the eyes, which are in turn medial to the external ears. - **Superior **and **inferior **describe structures in reference to the vertical axis of the body. For example, the head is superior to the shoulders and the knee joint is inferior to the hip joint. Proximal and distal, cranial and caudal, and rostral Other terms used to describe positions include proximal and distal, cranial and caudal, and rostral. - **Proximal **and **distal **are used with reference to being closer to or farther from a structure's origin, particularly in the limbs. For example, the hand is distal to the elbow joint. The glenohumeral joint is proximal to the elbow joint. These terms are also used to describe the relative positions of branches along the course of linear structures, such as airways, vessels, and nerves. For example, distal branches occur farther away toward the ends of the system, whereas proximal branches occur closer to and toward the origin of the system. - **Cranial **(toward the head) and **caudal **(toward the tail) are sometimes used instead of superior and inferior, respectively. - **Rostral **is used, particularly in the head, to describe the position of a structure with reference to the nose. For example, the forebrain is rostral to the hindbrain. Superficial and deep Two other terms used to describe the position of structures in the body are **superficial** and **deep**. These terms are used to describe the relative positions of two structures with respect to the surface of the body. For example, the sternum is superficial to the heart, and the stomach is deep to the abdominal wall. Superficial and deep can also be used in a more absolute fashion to define two major regions of the body. The superficial region of the body is external to the outer layer of deep fascia. Deep structures are enclosed by this layer. Structures in the superficial region of the body include the skin, superficial fascia, and mammary glands. Deep structures include most skeletal muscles and viscera. Superficial wounds are external to the outer layer of deep fascia, whereas deep wounds penetrate through it. Trans/non-binary anatomical terminology While anatomy is typically discussed in the sex-binary classification of female and male, many individuals do not fit into these models. These individuals include intersex, non-binary, and transgender people. In some areas of this text, relevant anatomical/clinical distinctions are made between "ciswomen and cismen" and "transwomen and transmen." "Cis" refers to individuals whose gender identity aligns with their assigned sex at birth, whereas "trans" refers to individuals whose gender identity does not align with their assigned sex at birth. "Non-binary" refers to an individual whose gender identity does not fit the binary model. Many trans or non-binary individuals receive a spectrum of gender-affirming care including hormones and surgery to meet their goals for gender transition that will alter their anatomy. The anatomical terminology in this text will reflect this (e.g., "In transwomen post-vaginoplasty"). These differences are clinically relevant and vary from cisgender individuals. Clinically, the anatomical terminology preferred by the patient should be used, which may include non-binary terms for classically gendered anatomy. In general, it is appropriate to use gender-inclusive anatomical language when engaging with non-binary/transgender patients. Some common examples are as follows: Preferred Term Instead Of ----------------- ---------------- Upper body Chest/Breast Erectile tissue Penis/Clitoris Gonads Testes/Ovaries Clinicians should not assume based on an individual's gender presentation which anatomical features are present. Acquiring a history of the patient's relevant anatomy (organ inventory) should be considered, including the patient's preferred terminology, to guide appropriate and sensitive care. Clinicians should base their clinical care on the present anatomy to diagnose, screen, and treat appropriately. Throughout this text, many anatomical features are still described using a gender-binary model (women and men). Anatomy has classically been taught in this way, which, for the time being, has some advantages in its use. First, it is important to understand that these gendered terms are conceptual models that help us approximate the realworld in order to more easily understand it, but they do not adequately describe all forms of variation. This includes biological variation in the anatomy that can be seen in intersex individuals or variations seen in persons receiving various degrees of gender-affirming care. Second, these terms continue to be used heavily in the literature, and so their use here helps maintain a degree of congruence with pre-existing information. The use of these terms is not intended to exclude individuals who do not fit sex-binary models. As we continue to develop our anatomical models, these terms may diminish in their use and be replaced by non-binary terminology such as "individuals with a prostate" or "individuals with a uterus." Whenever possible, it is best practice both scientifically and clinically to discuss the anatomy present without assumptions based on gender presentation. Body systems **Skeletal System** The skeleton can be divided into two subgroups, the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones of the skull (cranium), vertebral column, ribs, and sternum, whereas the appendicular skeleton consists of the bones of the upper and lower limbs ( [Fig. 1.14](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0075) ).   Fig. 1.14 The Axial Skeleton and the Appendicular Skeleton. The skeletal system consists of cartilage and bone. **Cartilage** Cartilage is an avascular form of connective tissue consisting of extracellular fibers embedded in a matrix that contains cells localized in small cavities. The amount and kind of extracellular fibers in the matrix vary depending on the type of cartilage. In heavy weight-bearing areas or areas prone to pulling forces, the amount of collagen is greatly increased and the cartilage is almost inextensible. In contrast, in areas where weight-bearing demands and stress are less, cartilage containing elastic fibers and fewer collagen fibers is common. The functions of cartilage are to: - support soft tissues - provide a smooth, gliding surface for bone articulations at joints, and - enable the development and growth of long bones. There are three types of cartilage: - hyaline---most common; matrix contains a moderate amount of collagen fibers (e.g., articular surfaces of bones); - elastic---matrix contains collagen fibers along with a large number of elastic fibers (e.g., external ear); - fibrocartilage---matrix contains a limited number of cells and ground substance amidst a substantial amount of collagen fibers (e.g., intervertebral discs). Cartilage is nourished by diffusion and has no blood vessels, lymphatics, or nerves. **Bone** Bone is a calcified, living, connective tissue that forms the majority of the skeleton. It consists of an intercellular calcified matrix, which also contains collagen fibers, and several types of cells within the matrix. Bones function as: - supportive structures for the body, - protectors of vital organs, - reservoirs of calcium and phosphorus, - levers on which muscles act to produce movement, and - containers for blood-producing cells. There are two types of bone, compact and spongy (trabecular or cancellous). Compact bone is dense bone that forms the outer shell of all bones and surrounds spongy bone. Spongy bone consists of spicules of bone enclosing cavities containing blood-forming cells (marrow). Classification of bones is by shape. - Long bones are tubular (e.g., humerus in upper limb; femur in lower limb). - Short bones are cuboidal (e.g., bones of the wrist and ankle). - Flat bones consist of two compact bone plates separated by spongy bone (e.g., skull). - Irregular bones are bones with various shapes (e.g., bones of the face). - Sesamoid bones are round or oval bones that develop in tendons. **In the clinic** Accessory and sesamoid bones These are extra bones that are not usually found as part of the normal skeleton but can exist as a normal variant in many people. They are typically found in multiple locations in the wrist and hands, ankles, and feet ( [Fig. 1.15](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0080) ). These should not be mistaken for fractures on imaging. Fig. 1.15 Accessory and Sesamoid Bones. (**A**) Radiograph of the ankle region showing an accessory bone (os trigonum). (**B**) Radiograph of the feet showing numerous sesamoid bones and an accessory bone (os naviculare). Sesamoid bones are embedded within tendons, the largest of which is the patella. There are many other sesamoids in the body, particularly in tendons of the hands and feet, and most frequently in flexor tendons of the thumb and big toe. Degenerative and inflammatory changes of, as well as mechanical stresses on, the accessory bones and sesamoids can cause pain, which can be treated with physiotherapy and targeted steroid injections, but in some severe cases it may be necessary to surgically remove the bone. Bones are vascular and are innervated. Generally, an adjacent artery gives off a nutrient artery, usually one per bone, that directly enters the internal cavity of the bone and supplies the marrow, spongy bone, and inner layers of compact bone. In addition, all bones are covered externally, except in the area of a joint where articular cartilage is present, by a fibrous connective tissue membrane called the periosteum, which has the unique capability of forming new bone. This membrane receives blood vessels whose branches supply the outer layers of compact bone. A bone stripped of its periosteum will not survive. Nerves accompany the vessels that supply the bone and the periosteum. Most of the nerves passing into the internal cavity with the nutrient artery are vasomotor fibers that regulate blood flow. Bone itself has few sensory nerve fibers. On the other hand, the periosteum is supplied with numerous sensory nerve fibers and is very sensitive to any type of injury. Developmentally, all bones come from mesenchyme by either intramembranous ossification, in which mesenchymal models of bones undergo ossification, or endochondral ossification, in which cartilaginous models of bones form from mesenchyme and undergo ossification. **In the clinic** Determination of skeletal age Throughout life the bones develop in a predictable way to form the skeletally mature adult at the end of puberty. Skeletal maturity tends to occur between the ages of 20 and 25 years. However, this may well vary according to geography and socioeconomic conditions. Skeletal maturity will also be determined by genetic factors and disease states. Up until the age of skeletal maturity, bony growth and development follows a typically predictable ordered state, which can be measured through either ultrasound, plain radiographs, or MRI scanning. Typically, the nondominant (left) hand is radiographed, and the radiograph is compared to a series of standard radiographs. From these images the bone age can be determined ( [Fig. 1.16](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0085) ). Traditionally, this comparison was done manually, but more recently, software packages employing artificial algorithms have automated this process. Fig. 1.16 A developmental series of radiographs showing the progressive ossification of carpal (wrist) bones from 3 (**A**) to 10 (**D**) years of age. In certain disease states, such as malnutrition and hypothyroidism, bony maturity may be slow. If the skeletal bone age is significantly reduced from the patient's true age, treatment may be required. In the healthy individual, the bone age accurately represents the true age of the patient. This is important in determining the true age of the subject. This may also have medicolegal importance. **In the clinic** Bone marrow transplants The bone marrow serves an important function. There are two types of bone marrow, red marrow (otherwise known as myeloid tissue) and yellow marrow. Red blood cells, platelets, and most white blood cells arise from within the red marrow. In the yellow marrow, a few white cells are made; however, this marrow is dominated by large fat globules (producing its yellow appearance) ( [Fig. 1.17](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0090) ). T1-weighted image in the coronal plane, demonstrating the relatively high signal intensity returned from the femoral heads and proximal femoral necks, consistent with yellow marrow. In this young patient, the vertebral bodies return an intermediate darker signal that represents red marrow. There is relatively little fat in these vertebrae, hence the lower signal return. From birth most of the body's marrow is red; however, as the subject ages, more red marrow is converted into yellow marrow within the medulla of the long and flat bones. Bone marrow contains two types of stem cells. Hemopoietic stem cells give rise to the white blood cells, red blood cells, and platelets. Mesenchymal stem cells differentiate into structures that form bone, cartilage, and muscle. There are a number of diseases that may involve the bone marrow, including infection and malignancy. In patients who develop a bone marrow malignancy (e.g., leukemia) it may be possible to harvest nonmalignant cells from the patient's bone marrow or cells from another person's bone marrow. The patient's own marrow can be destroyed with chemotherapy or radiation and the new cells infused. This treatment is bone marrow transplantation. **\ ** **In the clinic** Bone fractures Fractures occur in normal bone because of abnormal load or stress, in which the bone gives way ( [Fig. 1.18A](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0095) ). Fractures may also occur in bone that is of poor quality (osteoporosis); in such cases a normal stress is placed upon a bone that is not of sufficient quality to withstand this force and subsequently fractures. Fig. 1.18 Radiograph, lateral view, showing fracture of the ulna at the elbow joint ( **A **) and repair of this fracture ( **B **) using internal fixation with a plate and multiple screws. In children whose bones are still developing, fractures may occur across the growth plate or across the shaft. These shaft fractures typically involve partial cortical disruption, similar to breaking a branch of a young tree; hence they are termed "greenstick" fractures. After a fracture has occurred, the natural response is to heal the fracture. Between the fracture margins a blood clot is formed into which new vessels grow. A jelly-like matrix is formed, and further migration of collagen-producing cells occurs. On this soft tissue framework, calcium hydroxyapatite is produced by osteoblasts and forms insoluble crystals, and then bone matrix is laid down. As more bone is produced, a callus can be demonstrated forming across the fracture site. Treatment of fractures requires a fracture line reduction. If this cannot be maintained in a plaster of Paris cast, it may require internal or external fixation with screws and metal rods ( [Fig. 1.18B](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0095) ).  **In the clinic** Avascular necrosis Avascular necrosis is cellular death of bone resulting from a temporary or permanent loss of blood supply to that bone. Avascular necrosis may occur in a variety of medical conditions, some of which have an etiology that is less than clear. A typical site for avascular necrosis is a fracture across the femoral neck in an elderly patient. In these patients there is loss of continuity of the cortical medullary blood flow with loss of blood flow deep to the retinacular fibers. This essentially renders the femoral head bloodless; it subsequently undergoes necrosis and collapses ( [Fig. 1.19](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0100) ). In these patients it is necessary to replace the femoral head with a prosthesis. Fig. 1.19 Image of the hip joints demonstrating loss of height of the right femoral head with juxta-articular bony sclerosis and subchondral cyst formation secondary to avascular necrosis. There is also significant wasting of the muscles supporting the hip, which is secondary to disuse and pain. **In the clinic** Epiphyseal fractures As the skeleton develops, there are stages of intense growth typically around the ages of 7 to 10 years and later in puberty. These growth spurts are associated with increased cellular activity around the growth plate between the head and shaft of a bone. This increase in activity renders the growth plates more vulnerable to injuries, which may occur from dislocation across a growth plate or fracture through a growth plate. Occasionally an injury may result in growth plate compression, destroying that region of the growth plate, which may result in asymmetrical growth across that joint region. All fractures across the growth plate must be treated with care and expediency, requiring fracture reduction. **Joints** The sites where two skeletal elements come together are termed joints. The two general categories of joints ( [Fig. 1.20](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0105) ) are those in which: - the skeletal elements are separated by a cavity (i.e., **synovial joints **), and - there is no cavity and the components are held together by connective tissue (i.e., **solid joints **). Fig. 1.20 Joints. ( **A **) Synovial joint. ( **B **) Solid joint. Blood vessels that cross over a joint and nerves that innervate muscles acting on a joint usually contribute articular branches to that joint. **Synovial joints** Synovial joints are connections between skeletal components where the elements involved are separated by a narrow articular cavity ( [Fig. 1.21](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0110) ). In addition to containing an articular cavity, these joints have a number of characteristic features.   Fig. 1.21 Synovial Joints. ( **A **) Major features of a synovial joint. ( **B **) Accessory structures associated with synovial joints. First, a layer of cartilage, usually **hyaline cartilage** , covers the articulating surfaces of the skeletal elements. In other words, bony surfaces do not normally contact one another directly. As a consequence, when these joints are viewed in normal radiographs, a wide gap seems to separate the adjacent bones because the cartilage that covers the articulating surfaces is more transparent to X-rays than bone is. A second characteristic feature of synovial joints is the presence of a **joint capsule** consisting of an inner **synovial membrane** and an outer **fibrous membrane**. - The synovial membrane attaches to the margins of the joint surfaces at the interface between the cartilage and bone and encloses the articular cavity. The synovial membrane is highly vascular and produces synovial fluid, which percolates into the articular cavity and lubricates the articulating surfaces. Closed sacs of synovial membrane also occur outside joints, where they form synovial bursae or tendon sheaths. Bursae often intervene between structures, such as tendons and bone, tendons and joints, or skin and bone, and reduce the friction of one structure moving over the other. Tendon sheaths surround tendons and also reduce friction. - The **fibrous membrane **is formed by dense connective tissue and surrounds and stabilizes the joint. Parts of the fibrous membrane may thicken to form ligaments, which further stabilize the joint. Ligaments outside the capsule usually provide additional reinforcement. Another common but not universal feature of synovial joints is the presence of additional structures within the area enclosed by the capsule or synovial membrane, such as **articular discs** (usually composed of fibrocartilage), **fat pads**, and **tendons**. Articular discs absorb compression forces, adjust to changes in the contours of joint surfaces during movements, and increase the range of movements that can occur at joints. Fat pads usually occur between the synovial membrane and the capsule and move into and out of regions as joint contours change during movement. Redundant regions of the synovial membrane and fibrous membrane allow for large movements at joints. **Descriptions of synovial joints based on shape and movement** Synovial joints are described based on shape and movement: - based on the shape of their articular surfaces, synovial joints are described as plane (flat), hinge, pivot, bicondylar (two sets of contact points), condylar (ellipsoid), saddle, and ball and socket; - based on movement, synovial joints are described as uniaxial (movement in one plane), biaxial (movement in two planes), and multiaxial (movement in three planes). Hinge joints are uniaxial, whereas ball and socket joints are multiaxial. Specific types of synovial joints ( [Fig. 1.22 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0115)) - Plane joints---allow sliding or gliding movements when one bone moves across the surface of another (e.g., acromioclavicular joint)   Fig. 1.22 Various Types of Synovial Joints. ( **A **) Condylar (wrist). ( **B **) Gliding (radio-ulnar). ( **C **) Hinge (elbow). ( **D **) Ball and socket (hip). ( **E **) Saddle (carpometacarpal of thumb). ( **F **) Pivot (atlanto-axial). - Hinge joints---allow movement around one axis that passes transversely through the joint; permit flexion and extension (e.g., elbow \[humero-ulnar\] joint) - Pivot joints---allow movement around one axis that passes longitudinally along the shaft of the bone; permit rotation (e.g., atlanto-axial joint) - Bicondylar joints---allow movement mostly in one axis with limited rotation around a second axis; formed by two convex condyles that articulate with concave or flat surfaces (e.g., knee joint) - Condylar (ellipsoid) joints---allow movement around two axes that are at right angles to each other; permit flexion, extension, abduction, adduction, and circumduction (limited) (e.g., wrist joint) - Saddle joints---allow movement around two axes that are at right angles to each other; the articular surfaces are saddle shaped; permit flexion, extension, abduction, adduction, and circumduction (e.g., carpometacarpal joint of the thumb) - Ball-and-socket joints---allow movement around multiple axes; permit flexion, extension, abduction, adduction, circumduction, and rotation (e.g., hip joint) **Solid joints** Solid joints are connections between skeletal elements where the adjacent surfaces are linked together either by fibrous connective tissue or by cartilage, usually fibrocartilage ( [Fig. 1.23](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0120) ). Movements at these joints are more restricted than at synovial joints.  Fig. 1.23 Solid Joints. **Fibrous joints** include sutures, gomphoses, and syndesmoses. - **Sutures **occur only in the skull where adjacent bones are linked by a thin layer of connective tissue termed a *sutural ligament.* - **Gomphoses **occur only between the teeth and adjacent bone. In these joints, short collagen tissue fibers in the periodontal ligament run between the root of the tooth and the bony socket. - **Syndesmoses **are joints in which two adjacent bones are linked by a ligament. Examples are the ligamentum flavum, which connects adjacent vertebral laminae, and an interosseous membrane, which links, for example, the radius and ulna in the forearm. **Cartilaginous joints** include synchondroses and symphyses. - **Synchondroses **occur where two ossification centers in a developing bone remain separated by a layer of cartilage, for example, the growth plate that occurs between the head and shaft of developing long bones. These joints allow bone growth and eventually become completely ossified. - **Symphyses **occur where two separate bones are interconnected by cartilage. Most of these types of joints occur in the midline and include the pubic symphysis between the two pelvic bones, and intervertebral discs between adjacent vertebrae. **In the clinic** Degenerative joint disease Degenerative joint disease is commonly known as osteoarthritis or osteoarthrosis. The disorder is related to aging but not caused by aging. Typically there are decreases in water and proteoglycan content within the cartilage. The cartilage becomes more fragile and more susceptible to mechanical disruption ( [Fig. 1.24](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0125) ). As the cartilage wears, the underlying bone becomes fissured and also thickens. Synovial fluid may be forced into small cracks that appear in the bone's surface, which produces large cysts. Furthermore, reactive juxta-articular bony nodules are formed (osteophytes) ( [Fig. 1.25](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0130) ). As these processes occur, there is slight deformation, which alters the biomechanical forces through the joint. This in turn creates abnormal stresses, which further disrupt the joint. Fig. 1.24 This operative photograph demonstrates the focal areas of cartilage loss in the patella and femoral condyles throughout the knee joint.   Fig. 1.25 Osteoarthritis accounts for a large percentage of primary health care visits and is regarded as a significant problem. The etiology of osteoarthritis is not clear; however, osteoarthritis can occur secondary to other joint diseases, such as rheumatoid arthritis and infection. Overuse of joints and abnormal strains, such as those experienced by people who play sports, often cause one to be more susceptible to chronic joint osteoarthritis. Various treatments are available, including weight reduction, proper exercise, anti-inflammatory drug treatment, and joint replacement. **In the Clinic** Arthroscopy Arthroscopy is a technique of visualizing the inside of a joint using a small telescope placed through a tiny incision in the skin. Arthroscopy can be performed in most joints. However, it is most commonly performed in the knee, shoulder, ankle, and hip joints ( [Fig. 1.26](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0135) ). Fig. 1.26 Arthroscopic Procedure on Knee Joint. Arthroscopy allows the surgeon to view the inside of the joint and its contents. Notably, in the knee, the menisci and the ligaments are easily seen, and it is possible using separate puncture sites and specific instruments to remove the menisci and replace the cruciate ligaments. The advantages of arthroscopy are that it is performed through small incisions, it enables patients to quickly recover and return to normal activity, and it only requires either a light anesthetic or regional anesthesia during the procedure. **In the clinic** Joint replacement Joint replacement is undertaken for a variety of reasons. These predominantly include degenerative joint disease and joint destruction. Joints that have severely degenerated or lack their normal function are painful. In some patients, the pain may be so severe that it prevents them from leaving the house and undertaking even the smallest of activities without discomfort. Large joints are commonly affected, including the hip, knee, and shoulder. However, with ongoing developments in joint replacement materials and surgical techniques, even small joints of the fingers can be replaced. Typically, both sides of the joint are replaced; in the hip joint the acetabulum will be reamed, and a plastic or metal cup will be introduced. The femoral component will be fitted precisely to the femur and cemented in place ( [Fig. 1.27](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0140) ). Radiograph, Anteroposterior View, of the Pelvis After a Right Total Hip Replacement. There are additional significant degenerative changes in the left hip joint, which will also need to be replaced. Most patients derive significant benefit from joint replacement and continue to lead an active life afterward. In a minority of patients who have been fitted with a metal acetabular cup and metal femoral component, an aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL) may develop, possibly caused by a hypersensitivity response to the release of metal ions in adjacent tissues. These patients often have chronic pain and might need additional surgery to replace these joint replacements with safer models. **Skin and Fascias** Skin The skin is the largest organ of the body. It consists of the epidermis and the dermis. The epidermis is the outer cellular layer of stratified squamous epithelium, which is avascular and varies in thickness. The dermis is a dense bed of vascular connective tissue. The skin functions as a mechanical and permeability barrier, and as a sensory and thermoregulatory organ. It also can initiate primary immune responses. **Fascia** Fascia is connective tissue containing varying amounts of fat that separate, support, and interconnect organs and structures, enable movement of one structure relative to another, and allow the transit of vessels and nerves from one area to another. There are two general categories of fascia: superficial and deep. - Superficial (subcutaneous) fascia lies just deep to and is attached to the dermis of the skin. It is made up of loose connective tissue usually containing a large amount of fat. The thickness of the superficial fascia (subcutaneous tissue) varies considerably, both from one area of the body to another and from one individual to another. The superficial fascia allows movement of the skin over deeper areas of the body, acts as a conduit for vessels and nerves coursing to and from the skin, and serves as an energy (fat) reservoir. - Deep fascia usually consists of dense, organized connective tissue. The outer layer of deep fascia is attached to the deep surface of the superficial fascia and forms a thin fibrous covering over most of the deeper region of the body. Inward extensions of this fascial layer form intermuscular septa that compartmentalize groups of muscles with similar functions and innervations. Other extensions surround individual muscles and groups of vessels and nerves, forming an investing fascia. Near some joints the deep fascia thickens, forming retinacula. These fascial retinacula hold tendons in place and prevent them from bowing during movements at the joints. Finally, there is a layer of deep fascia separating the membrane lining the abdominal cavity (the parietal peritoneum) from the fascia covering the deep surface of the muscles of the abdominal wall (the transversalis fascia). This layer is referred to as **extraperitoneal fascia. **A similar layer of fascia in the thorax is termed the **endothoracic fascia.** **In the clinic** The importance of fascias Clinically, fascias are extremely important because they often limit the spread of infection and malignant disease. When infections or malignant diseases cross a fascial plain, a primary surgical clearance may require a far more extensive dissection to render the area free of tumor or infection. A typical example of the clinical importance of a fascial layer would be of that covering the psoas muscle. Infection within an intervertebral body secondary to tuberculosis can pass laterally into the psoas muscle. Pus fills the psoas muscle but is limited from further spread by the psoas fascia, which surrounds the muscle and extends inferiorly into the groin, pointing below the inguinal ligament. **In the clinic** Placement of skin incisions and scarring Surgical skin incisions are ideally placed along or parallel to lines of skin tension (Langer's lines) that correspond to the orientation of the dermal collagen fibers. They tend to run in the same direction as the underlying muscle fibers and incisions that are made along these lines tend to heal better with less scarring. In contrast, incisions made perpendicular to these lines are more likely to heal with a prominent scar and, in some severe cases, can lead to raised, firm, hypertrophic, or keloid, scars.