Chapter 9: Joints Key Words and Topics

Chapter 9: Joints Key Words and Topics Make certain that you can define, and use in context, each of the terms listed below, and that you understand the significance of each of the concepts. 1. Define the term “articulation” (joint). • articulation or arthrosis A joint, also called an articulation or arthrosis is a point of contact between two bones, between bone and cartilage, or between bone and teeth. When we say one bone articulates with another bone, we mean that the bones form a joint. 2. Classify joints on the basis of structure and function. • structural classification -The structural classification of joints is based on two criteria: (1) the presence or absence of a space between the articulating bones, called a synovial cavity, and (2) the type of connective tissue that binds the bones together. Structurally, joints are classified as one of the following types: • fibrous joints- There is no synovial cavity, and the bones are held together by dense irregular connective tissue that is rich in collagen fibers. • cartilaginous joints- There is no synovial cavity, and the bones are held together by cartilage. • synovial joints- The bones forming the joint have a synovial cavity and are united by the dense irregular connective tissue of an articular capsule, and often by accessory ligaments. • functional classification • synarthrosis- An immovable joint. The plural is synarthroses. • Amphiarthrosis-A slightly movable joint. The plural is amphiarthroses. • Diarthrosis- A freely movable joint. The plural is diarthroses. All diarthroses are synovial joints. They have a variety of shapes and permit several different types of movements. 3. Describe the structure and functions of the three types of fibrous joints. • fibrous joints (see above for definition) • suture A suture (SOO‐chur; sutur = seam) is a fibrous joint composed of a thin layer of dense irregular connective tissue; sutures occur only between bones of the skull. An example is the coronal suture between the parietal and frontal bones (Figure 9.1a). The irregular, interlocking edges of sutures give them added strength and decrease their chance of fracturing. Sutures are joints that form as the numerous bones of the skull come in contact during development. They are immovable or slightly movable. In older individuals, sutures are immovable (synarthroses), but in infants and children they are slightly movable (amphiarthroses) (Figure 9.1b). Sutures play important roles in shock absorption in the skull. • Some sutures, although present during growth of the skull, are replaced by bone in the adult. Such a suture is called a synostosis (sin′‐os‐TŌ‐sis; os‐ = bone), or bony joint—a joint in which there is a complete fusion of two separate bones into one. For example, the frontal bone grows in halves that join together across a suture line. Usually they are completely fused by age 6 and the suture becomes obscure. If the suture persists beyond age 6, it is called a frontal or metopic suture (me‐TŌ‐ pik; metopon = forehead). A synostosis is classified as a synarthrosis because it is immovable. • Syndesmosis A syndesmosis (sin′‐dez‐MŌ‐sis; syndesmo‐ = band or ligament) is a fibrous joint in which there is a greater distance between the articulating surfaces and more dense irregular connective tissue than in a suture. The dense irregular connective tissue is typically arranged as a bundle (ligament), allowing the joint to permit limited movement. One example of a syndesmosis is the distal tibiofibular joint, where the anterior tibiofibular ligament connects the tibia and fibula (Figure 9.1c, left). It permits slight movement (amphiarthrosis). • Gomphosis Another example of a syndesmosis is called a gomphosis (gom‐FŌ‐sis; gompbo‐ = bolt or nail) or dentoalveolar joint, in which a cone‐shaped peg fits into a socket. The only examples of gomphoses in the human body are the articulations between the roots of the teeth and their sockets (alveoli) in the maxillae and mandible (Figure 9.1c, right). The dense irregular connective tissue between a tooth and its socket is the thin periodontal ligament (membrane). A healthy gomphosis permits no movement (synarthrosis). Inflammation and degeneration of the gums, periodontal ligament, and bone is called periodontal disease. • interosseous membrane The final category of fibrous joint is the interosseous membrane (in′‐ter‐OS‐ē‐us), which is a substantial sheet of dense irregular connective tissue that binds neighboring long bones and permits slight movement (amphiarthrosis). There are two principal interosseous membrane joints in the human body. One occurs between the radius and ulna in the forearm (see Figure 8.6) and the other occurs between the tibia and fibula in the leg (Figure 9.1d). 4. Describe the structure and functions of the two types of cartilaginous joints. • cartilaginous joints Like a fibrous joint, a cartilaginous joint (kar′‐ti‐LAJ‐i‐nus) lacks a synovial cavity and allows little or no movement. Here the articulating bones are tightly connected by either hyaline cartilage or fibrocartilage (see Table 4.6). The two types of cartilaginous joints are synchondroses and symphyses. • Synchondrosis A synchondrosis (sin′‐kon‐DRŌ‐sis; chondro‐ = cartilage) is a cartilaginous joint in which the connecting material is hyaline cartilage. An example of a synchondrosis is the epiphyseal (growth) plate that connects the epiphysis and diaphysis of a growing bone (Figure 9.2a). A photomicrograph of the epiphyseal plate is shown in Figure 6.7b. Functionally, a synchondrosis is immovable (synarthrosis). When bone elongation ceases, bone replaces the hyaline cartilage, and the synchondrosis becomes a synostosis, a bony joint. Another example of a synchondrosis is the joint between the first rib and the manubrium of the sternum, which also ossifies during adult life and becomes an immovable (synarthrosis) synostosis, or bony joint (see Figure 7.22b) • Symphysis A symphysis (SIM‐fi‐sis = growing together) is a cartilaginous joint in which the ends of the articulating bones are covered with hyaline cartilage, but a broad, flat disc of fibrocartilage connects the bones. All symphyses (plural) occur in the midline of the body. The pubic symphysis between the anterior surfaces of the hip bones is one example of a symphysis (Figure 9.2b). This type of joint is also found at the junction of the manubrium and body of the sternum (see Figure 7.22) and at the intervertebral joints between the bodies of vertebrae (see Figure 7.20a). A portion of the intervertebral disc is composed of fibrocartilage. A symphysis is a slightly movable joint (amphiarthrosis). 5. Describe the structure of synovial joints. • synovial joints Synovial joints (si‐NŌ‐vē‐al) have certain characteristics that distinguish them from other joints. The unique characteristic of a synovial joint is the presence of a space called a synovial (joint) cavitybetween the articulating bones (Figure 9.3). Because the synovial cavity allows considerable movement at a joint, all synovial joints are classified functionally as freely movable (diarthroses). The bones at a synovial joint are covered by a layer of hyaline cartilage called articular cartilage. The cartilage covers the articulating surface of the bones with a smooth, slippery surface but does not bind them together. Articular cartilage reduces friction between bones in the joint during movement and helps to absorb shock • synovial (joint) cavity (see above) • articular cartilage (joint cartilage) • articular capsule A sleevelike articular (joint) capsule surrounds a synovial joint, encloses the synovial cavity, and unites the articulating bones. The articular capsule is composed of two layers, an outer fibrous membrane and an inner synovial membrane (Figure 9.3a). • fibrous capsule The fibrous membrane usually consists of dense irregular connective tissue (mostly collagen fibers) that attaches to the periosteum of the articulating bones. In fact, the fibrous membrane is literally a thickened continuation of the periosteum between the bones. The flexibility of the fibrous membrane permits considerable movement at a joint, while its great tensile strength (resistance to stretching) helps prevent the bones from dislocating. The fibers of some fibrous membranes are arranged as parallel bundles of dense regular connective tissue that are highly adapted for resisting strains • ligament The strength of these fiber bundles, called ligaments (liga‐ = bound or tied), is one of the principal mechanical factors that hold bones close together in a synovial joint. Ligaments are often designated by individual names. The inner layer of the articular capsule, the synovial membrane, is composed of areolar connective tissue with elastic fibers. At many synovial joints the synovial membrane includes accumulations of adipose tissue, called articular fat pads. An example is the infrapatellar fat pad in the knee (see Figure 9.15c). A “double‐jointed” person does not really have extra joints. Individuals who are double‐jointed have greater flexibility in their articular capsules and ligaments; the resulting increase in range of motion allows them to entertain fellow partygoers with activities such as touching their thumbs to their wrists and putting their ankles or elbows behind their necks. Unfortunately, such flexible joints are less structurally stable and are more easily dislocated. • synovial membrane (see below) • synovial fluid The synovial membrane secretes synovial fluid (ov‐ = egg), a viscous, clear or pale yellow fluid named for its similarity in appearance and consistency to uncooked egg white. Synovial fluid consists of hyaluronic acid secreted by fibroblastlike cells in the synovial membrane and interstitial fluid filtered from blood plasma. It forms a thin film over the surfaces within the articular capsule. Its functions include reducing friction by lubricating the joint, absorbing shocks, and supplying oxygen and nutrients to and removing carbon dioxide and metabolic wastes from the chondrocytes within articular cartilage. (Recall that cartilage is an avascular tissue, so it does not have blood vessels to perform the latter function.) Synovial fluid also contains phagocytic cells that remove microbes and the debris that results from normal wear and tear in the joint. When a synovial joint is immobile for a time, the fluid becomes quite viscous (gel‐like), but as joint movement increases, the fluid becomes less viscous. One of the benefits of warming up before exercise is that it stimulates the production and secretion of synovial fluid; more fluid means less stress on the joints during exercise. We are all familiar with the cracking sounds heard as certain joints move, or the popping sounds that arise when a person pulls on his fingers to crack his knuckles. According to one theory, when the synovial cavity expands, the pressure inside the synovial cavity decreases, creating a partial vacuum. The suction draws carbon dioxide and oxygen out of blood vessels in the synovial membrane, forming bubbles in the fluid. When the fingers are flexed (bent) the volume of the cavity decreases and the pressure increases; this bursts the bubbles and creates cracking or popping sounds as the gases are driven back into solution. • accessory ligaments and articular discs Many synovial joints also contain accessory ligaments called extracapsular ligaments and intracapsular ligaments (see Figure 9.15d). Extracapsular ligaments lie outside the articular capsule. Examples are the fibular and tibial collateral ligaments of the knee joint. Intracapsular ligaments occur within the articular capsule but are excluded from the synovial cavity by folds of the synovial membrane. Examples are the anterior and posterior cruciate ligaments of the knee joint. Inside some synovial joints, such as the knee, crescent‐shaped pads of fibrocartilage lie between the articular surfaces of the bones and are attached to the fibrous capsule. These pads are called articular discs(Fibrocartilage pad between articular surfaces of bones of some synovial joints. Also called a meniscus) or menisci (me‐NIS‐sī or me‐NIS‐kī; singular is meniscus). Figure 9.15d depicts the lateral and medial menisci in the knee joint. The discs bind strongly to the inside of the fibrous membrane and usually subdivide the synovial cavity into two spaces, allowing separate movements to occur in each space. As you will see later, separate movements also occur in the respective compartments of the temporomandibular joint (TMJ) (see Exhibit 9.A). The functions of the menisci are not completely understood but are known to include the following: (1) shock absorption; (2) a better fit between articulating bony surfaces; (3) providing adaptable surfaces for combined movements; (4) weight distribution over a greater contact surface; and (5) distribution of synovial lubricant across the articular surfaces of the joint. • nerve and blood supply The nerves that supply a joint are the same as those that supply the skeletal muscles that move the joint. Synovial joints contain many nerve endings that are distributed to the articular capsule and associated ligaments. Some of the nerve endings convey information about pain from the joint to the spinal cord and brain for processing. Other nerve endings respond to the degree of movement and stretch at a joint, such as when a physician strikes the tendon below your kneecap to test your reflexes. The spinal cord and brain respond by sending impulses through different nerves to the muscles to adjust body movements. Although many of the components of synovial joints are avascular, arteries in the vicinity send out numerous branches that penetrate the ligaments and articular capsule to deliver oxygen and nutrients. Veins remove carbon dioxide and wastes from the joints. The arterial branches from several different arteries typically merge around a joint before penetrating the articular capsule. The chondrocytes in the articular cartilage of a synovial joint receive oxygen and nutrients from synovial fluid derived from blood; all other joint tissues are supplied directly by capillaries. Carbon dioxide and wastes pass from chondrocytes of articular cartilage into synovial fluid and then into veins; carbon dioxide and wastes from all other joint structures pass directly into veins. 6. Describe the structure and functions of bursae and tendon sheaths. • bursa (plural is bursae) The various movements of the body create friction between moving parts. Saclike structures called bursae (BER‐sē = purses; singular is bursa) are strategically situated to alleviate friction in some joints, such as the shoulder and knee joints (see Figures 9.12 and 9.15c). Bursae are not strictly part of synovial joints, but they do resemble joint capsules because their walls consist of an outer fibrous membrane of thin, dense connective tissue lined by a synovial membrane. They are filled with a small amount of fluid that is similar to synovial fluid. Bursae can be located between the skin and bones, tendons and bones, muscles and bones, or ligaments and bones. The fluid ‐filled bursal sacs cushion the movement of these body parts against one another. • tendon sheaths Structures called tendon sheaths also reduce friction at joints. Tendon (synovial) sheaths are tubelike bursae that wrap around certain tendons that experience considerable friction as they pass through tunnels formed by connective tissue and bone. The inner layer of a tendon sheath, the visceral layer, is attached to the surface of the tendon. The outer layer, known as the parietal layer, is attached to bone (see Figure 11.18a). Between the layers is a cavity that contains a film of synovial fluid. A tendon sheath protects all sides of a tendon from friction as the tendon slides back and forth. Tendon sheaths are found where tendons pass through synovial cavities, such as the tendon of the biceps brachii muscle at the shoulder joint (see Figure 9.12c). Tendon sheaths are also found at the wrist and ankle, where many tendons come together in a confined space (see Figure 11.18a), and in the fingers and toes, where there is a great deal of movement (see Figure 11.18). 7. Describe the types of movements that can occur at synovial joints. • types of movements at synovial joints • gliding Gliding is a simple movement in which relatively flat bone surfaces move back‐and‐forth and from side‐to‐side with respect to one another (Figure 9.4). There is no significant alteration of the angle between the bones. Gliding movements are limited in range due to the structure of the articular capsule and associated ligaments and bones; however, these sliding movements can also be combined with rotation. The intercarpal and intertarsal joints are examples of articulations where gliding movements occur. • angular In angular movements, there is an increase or a decrease in the angle between articulating bones. The major angular movements are flexion, extension, lateral flexion, hyperextension, abduction, adduction, and circumduction. These movements are discussed with respect to the body in the anatomical position (see Figure 1.5). • flexion Flexion and extension are opposite movements. In flexion (FLEK‐shun; flex‐ = to bend) there is a decrease in the angle between articulating bones; • extension in extension (eks‐TEN‐shun; exten‐ = to stretch out) there is an increase in the angle between articulating bones, often to restore a part of the body to the anatomical position after it has been flexed. Both movements usually occur along the sagittal plane. All of the following are examples of flexion (as you have probably already guessed, extension is simply the reverse of these movements): • Bending the head toward the chest at the atlanto‐occipital joint between the atlas (the first vertebra) and the occipital bone of the skull, and at the cervical intervertebral joints between the cervical vertebrae (Figure 9.5a) • Bending the trunk forward at the intervertebral joints • Moving the humerus forward at the shoulder joint, as in swinging the arms forward while walking (Figure 9.5b) • Moving the forearm toward the arm at the elbow joint between the humerus, ulna, and radius (Figure 9.5c) • Moving the palm toward the forearm at the wrist or radiocarpal joint between the radius and carpals, as in the upward movement when doing wrist curls (Figure 9.5d) • Bending the digits of the hand or feet at the interphalangeal joints between phalanges • Moving the femur forward at the hip joint between the femur and hip bone, as in walking (Figure 9.5e) • Moving the heel toward the buttock at the tibiofemoral joint between the tibia, femur, and patella, as occurs when bending the knee (Figure 9.5f) • lateral flexion Although flexion and extension usually occur along the sagittal plane, there are a few exceptions. For example, flexion of the thumb involves movement of the thumb medially across the palm at the carpometacarpal joint between the trapezium and metacarpal of the thumb, as when you touch your thumb to the opposite side of your palm (see Figure 11.18g). Another example is movement of the trunk sideways to the right or left at the waist. This movement, which occurs along the frontal plane and involves the intervertebral joints, is called lateral flexion (Figure 9.5g). • hyperextension Continuation of extension beyond the anatomical position is called hyperextension (hī‐per‐ek‐STEN‐shun; hyper‐ = beyond or excessive). Examples of hyperextension include: • Bending the head backward at the atlanto‐occipital and cervical intervertebral joints (Figure 9.5a) • Bending the trunk backward at the intervertebral joints • Moving the humerus backward at the shoulder joint, as in swinging the arms backward while walking (Figure 9.5b) • Moving the palm backward at the wrist joint (Figure 9.5d) • Moving the femur backward at the hip joint, as in walking (Figure 9.5e) Hyperextension of hinge joints, such as the elbow, interphalangeal, and knee joints, is usually prevented by the arrangement of ligaments and the anatomical alignment of the bones. • Abduction Abduction (ab‐DUK‐shun; ab‐ = away; ‐duct = to lead) is the movement of a bone away from the midline; • Adduction adduction (ad‐DUK‐shun; ad‐ = toward) is the movement of a bone toward the midline. Both movements usually occur along the frontal plane. Examples of abduction include moving the humerus laterally at the shoulder joint, moving the palm laterally at the wrist joint, and moving the femur laterally at the hip joint (Figure 9.6a–c). The movement that returns each of these body parts to the anatomical position is adduction (Figure 9.6a–c). The midline of the body is not used as a point of reference for abduction and adduction of the digits. In abduction of the fingers (but not the thumb), an imaginary line is drawn through the longitudinal axis of the middle (longest) finger, and the fingers move away (spread out) from the middle finger (Figure 9.6d). In abduction of the thumb, the thumb moves away from the palm in the sagittal plane (see Figure 11.18g). Abduction of the toes is relative to an imaginary line drawn through the second toe. Adduction of the fingers and toes returns them to the anatomical position. Adduction of the thumb moves the thumb toward the palm in the sagittal plane (see Figure 11.18g). • Circumduction Circumduction (ser‐kum‐DUK‐shun; circ‐ = circle) is movement of the distal end of a body part in a circle (Figure 9.7). Circumduction is not an isolated movement by itself but rather a continuous sequence of flexion, abduction, extension, adduction, and rotation of the joint (or in the opposite order). It does not occur along a separate axis or plane of movement. Examples of circumduction are moving the humerus in a circle at the shoulder joint (Figure 9.7a), moving the hand in a circle at the wrist joint, moving the thumb in a circle at the carpometacarpal joint, moving the fingers in a circle at the metacarpophalangeal joints (between the metacarpals and phalanges), and moving the femur in a circle at the hip joint (Figure 9.7b). Both the shoulder and hip joints permit circumduction. Flexion, abduction, extension, and adduction are more limited in the hip joints than in the shoulder joints due to the tension on certain ligaments and muscles and the depth of the acetabulum in the hip joint (see Exhibits 9.B and 9.D). • Rotation 8. In rotation (rō‐TĀ‐shun; rota‐ = revolve), a bone revolves around its own longitudinal axis. One example is turning the head from side to side at the atlanto‐axial joint (between the atlas and axis), as when you shake your head “no” (Figure 9.8a). Another is turning the trunk from side to side at the intervertebral joints while keeping the hips and lower limbs in the anatomical position. In the limbs, rotation is defined relative to the midline, and specific qualifying terms are used. If the anterior surface of a bone of the limb is turned toward the midline, the movement is called medial (internal) rotation. You can medially rotate the humerus at the shoulder joint as follows: Starting in the anatomical position, flex your elbow and then move your palm across the chest (Figure 9.8b). You can medially rotate the femur at the hip joint as follows: Lie on your back, bend your knee, and then move your leg and foot laterally from the midline. Although you are moving your leg and foot laterally, the femur is rotating medially (Figure 9.8c). Medial rotation of the leg at the knee joint can be produced by sitting on a chair, bending your knee, raising your lower limb off the floor, and turning your toes medially. If the anterior surface of the bone of a limb is turned away from the midline, the movement is called lateral (external) rotation (see Figure 9.8b, c).