Chapter 8 - Joints PDF

8 by first asking Joints 8.1 How are joints classified? In this chapter, you will learn that Joints determine how bones move relative to each o...

8 by first asking Joints 8.1 How are joints classified? In this chapter, you will learn that Joints determine how bones move relative to each other then asking 8.6 What happens when things go wrong? then exploring and finally, exploring Developmental Aspects 8.2 Fibrous joints 8.3 Cartilaginous 8.4 Synovial joints of Joints joints looking closer at focusing on Movement of 8.5 Selected synovial joints synovial joints The graceful movements of ballet dancers and the rough-and-tumble grapplings of football players demonstrate 8.1 Joints are classified into the great variety of motion allowed by joints, or articula- three structural and three functional tions—the sites where two or more bones meet. Our joints have two fundamental functions: They give our skeleton mobil- categories ity, and they hold it together, sometimes playing a protective Learning Outcomes role in the process. N Define joint, or articulation. N Classify joints by structure and by function. Joints are classified by structure and by function. The structural CAREER CONNECTION classification focuses on the material binding the bones together and whether or not a joint cavity is present. Structurally, there are fibrous, cartilaginous, and synovial joints (Table 8.1 on p. 255). Only synovial joints have a joint cavity. The functional classification is based on the amount of move- ment allowed at the joint. On this basis, there are synarthroses (sin0ar-thro9sēz; syn = together , arthro = joint ), which are immovable joints; amphiarthroses (am0fe-ar-thro9sēz; amphi = on both sides ), slightly movable joints; and Play a video to learn how diarthroses (di0ar-thro9sēz; dia = through, apart ), or freely the chapter content is used movable joints. Freely movable joints predominate in the appen- in a real healthcare setting dicular skeleton (limbs). Immovable and slightly movable joints @ Mastering A&P > Study Area. are largely restricted to the axial skeleton. This localization of functional joint types makes sense because the less movable the joint, the more stable it is likely to be. 251  252 UNIT 2 Covering, Support, and Movement of the Body (a) Suture (b) Syndesmosis (c) Gomphosis Joint held together with very short, Joint held together by a ligament. “Peg in socket” fibrous joint. Periodontal interconnecting fibers, and bone edges Fibrous tissue can vary in length, but ligament holds tooth in socket. interlock. Found only in the skull. is longer than in sutures. Socket of Suture Fibula alveolar line process Tibia Root of tooth 8 Fibrous connective Ligament Periodontal tissue ligament Figure 8.1 Fibrous joints. In general, fibrous joints are immovable, and synovial joints interlock, and the junction is completely filled by a minimal are freely movable. However, cartilaginous joints have both rigid amount of very short connective tissue fibers that are con- and slightly movable examples. Since the structural categories tinuous with the periosteum ( p. 178). The result is nearly are more clear-cut, we will use the structural classification in this rigid splices that knit the bones together, yet allow the skull to discussion, indicating functional properties where appropriate. expand as the brain grows during youth. During middle age, the fibrous tissue ossifies and the skull bones fuse into a single Check Your Understanding unit. At this stage, the closed sutures are more precisely called 1. What functional joint class contains the least-mobile joints? synostoses (sin0os-to9sēz), literally, “bony junctions.” Because 2. How are joint mobility and stability related? movement of the cranial bones would damage the brain, the For answers, see Answers Appendix. immovable nature of sutures is a protective adaptation. Syndesmoses 8.2In fibrous joints, the bones are In syndesmoses (sin0des-mo9sēz), the bones are connected connected by fibrous tissue exclusively by ligaments (syndesmos = ligament), cords or bands of fibrous tissue. The amount of movement allowed at Learning Outcome a syndesmosis depends on the length of the connecting fibers. N Describe the general structure of fibrous joints. Name and Although the connecting fibers are always longer than those give an example of each of the three common types of in sutures, they vary quite a bit in length. If the fibers are short fibrous joints. (as in the ligament connecting the distal ends of the tibia and In fibrous joints, the bones are joined by the collagen fibers fibula, Figure 8.1b), little or no movement is allowed, a char- of connective tissue. No joint cavity is present. The amount acteristic best described as “give.” If the fibers are long [as in of movement allowed depends on the length of the connective the ligament-like interosseous membrane connecting the radius tissue fibers. Most fibrous joints are immovable, although a and ulna ( Figure 7.29, p. 232)], a large amount of movement few are slightly movable. The three types of fibrous joints are is possible. sutures, syndesmoses, and gomphoses. Gomphoses Sutures A gomphosis (gom-fo9sis) is a peg-in-socket fibrous joint Sutures, literally, “seams,” occur only between bones of (Figure 8.1c). The only example is the articulation of a tooth the skull (Figure 8.1a). The wavy articulating bone edges with its bony alveolar socket. The term gomphosis comes from  Chapter 8 Joints 253 (a) Synchondroses Bones united by hyaline cartilage Sternum (manubrium) Epiphyseal plate (temporary Joint between first rib hyaline cartilage and sternum (immovable) joint) (b) Symphyses Bones united by fibrocartilage 8 Body of vertebra Fibrocartilaginous intervertebral disc (sandwiched between Pubic symphysis hyaline cartilage) Figure 8.2 Cartilaginous joints. the Greek gompho, meaning “nail” or “bolt,” and refers to the In cartilaginous joints (kar0tĭ-laj9ĭ-nus), the articulating bones way teeth are embedded in their sockets (as if hammered in). are united by cartilage. Like fibrous joints, they lack a joint cav- The fibrous connection in this case is the short periodontal ity and are not highly movable. The two types of cartilaginous ligament (Figure 23.12, p. 886). joints are synchondroses and symphyses. Synchondroses Check Your Understanding A bar or plate of hyaline cartilage unites the bones at a 3. To what functional class do most fibrous joints belong? synchondrosis (sin0kon-dro9sis; “junction of cartilage”). Virtually 4. PREDICT Babies are sometimes diagnosed with a condition all synchondroses are synarthrotic (immovable). called craniosynostosis. Using the definition you just learned, The most common examples of synchondroses are the describe what you think this condition is and predict what epiphyseal plates in long bones of children (Figure 8.2a). problems it might cause. Epiphyseal plates are temporary joints and eventually become For answers, see Answers Appendix. synostoses. Another example of a synchondrosis is the immov- able joint between the costal cartilage of the first rib and the manubrium of the sternum (Figure 8.2a). 8.3 In cartilaginous joints, the bones Symphyses A joint where fibrocartilage unites the bones is a symphysis are connected by cartilage (sim9fĭ-sis; “growing together”). Since fibrocartilage is compress- Learning Outcome ible and resilient, it acts as a shock absorber and permits a limited N Describe the general structure of cartilaginous joints. amount of movement at the joint. Even though fibrocartilage is the Name and give an example of each of the two common main element of a symphysis, hyaline cartilage is also present in types of cartilaginous joints. the form of articular cartilages on the bony surfaces. Symphyses  254 UNIT 2 Covering, Support, and Movement of the Body are amphiarthrotic joints that provide strength with flexibility. Examples include the intervertebral joints and the pubic symphy- sis of the pelvis (Figure 8.2b, and see Table 8.3 on pp. 262–263). Check Your Understanding 5. MAKE CONNECTIONS Evan is 25 years old. Would you expect to find synchondroses at the ends of his femur? Explain. (Hint: See Chapter 6, p. 187.) For answers, see Answers Appendix. Ligament Synovial joints have a fluid-filled 8.4 Joint cavity (contains joint cavity synovial fluid) Learning Outcomes Articular (hyaline) cartilage N Describe the structural characteristics of synovial joints. N Compare the structures and functions of bursae and Fibrous tendon sheaths. layer 8 N List three natural factors that stabilize synovial joints. Synovial N Name and describe (or perform) the common body membrane Articular capsule movements. (secretes N Name and provide examples of the six types of synovial synovial fluid) joints based on the movement(s) allowed. Synovial joints (si-no9ve-al; “joint eggs”) are those in which Periosteum the articulating bones are separated by a fluid-containing joint cavity. This arrangement permits substantial freedom of move- ment, and all synovial joints are freely movable diarthroses. Figure 8.3 General structure of a synovial joint. Nearly all joints of the limbs—indeed, most joints of the body—fall into this class. viscous, egg-white consistency (ovum = egg) due to hya- General Structure luronic acid secreted by cells in the synovial membrane, but Synovial joints have six distinguishing features (Figure 8.3): it thins and becomes less viscous during joint activity. Synovial fluid, which is also found within the articu- Articular cartilage. Glassy-smooth hyaline cartilage covers lar cartilages, provides a slippery, weight-bearing film that the opposing bone surfaces as articular cartilage. These thin reduces friction between the cartilages. Without this lubri- (1 mm or less) but spongy cushions absorb compression placed cant, rubbing would wear away joint surfaces and excessive on the joint and thereby keep the bone ends from being crushed. friction could overheat and destroy the joint tissues. The syn- Joint (articular) cavity. The joint cavity is a feature that ovial fluid is forced from the cartilages when a joint is com- is unique to synovial joints. It contains a small amount of pressed; then as pressure on the joint is relieved, synovial synovial fluid. The joint cavity is a potential space—it is fluid seeps back into the articular cartilages like water into a normally almost nonexistent. However, it can expand if fluid sponge, ready to be squeezed out again the next time the joint accumulates (as happens during inflammation). is loaded (put under pressure). This process, called weep- Articular capsule. The joint cavity is enclosed by a two- ing lubrication, lubricates the free surfaces of the cartilages layered articular capsule, or joint capsule. The tough and nourishes their cells. (Remember, cartilage is avascular.) external fibrous layer is composed of dense irregular Synovial fluid also contains phagocytic cells that rid the joint connective tissue that is continuous with the periostea of the cavity of microbes and cellular debris. articulating bones. It strengthens the joint so that the bones Reinforcing ligaments. Synovial joints are reinforced and are not pulled apart. The inner layer of the joint capsule is a strengthened by a number of bandlike ligaments. Most of- synovial membrane composed of loose connective tissue. ten, these are capsular ligaments, which are thickened parts Besides lining the fibrous layer internally, it covers all inter- of the fibrous layer. In other cases, they remain distinct and nal joint surfaces that are not hyaline cartilage. The synovial are found outside the capsule (as extracapsular ligaments) membrane’s function is to make synovial fluid. or deep to it (as intracapsular ligaments). Since intracapsu- Synovial fluid. A small amount of slippery synovial fluid lar ligaments are covered with synovial membrane, they do occupies all free spaces within the joint capsule. This fluid is not actually lie within the joint cavity. formed largely by the filtration of blood flowing through the Some people said to be “double-jointed” amaze the rest capillaries in the synovial membrane. Synovial fluid has a of us by placing both heels behind their neck. However, they  Chapter 8 Joints 255 Table 8.1 Summary of Joint Classes STRUCTURAL CLASS STRUCTURAL CHARACTERISTICS TYPES MOBILITY Fibrous Adjoining bones united by collagen fibers Suture (short fibers) Immobile (synarthrosis) Syndesmosis (longer fibers) Slightly movable (amphiarthrosis) and immobile Gomphosis (periodontal ligament) Immobile Cartilaginous Adjoining bones united by cartilage Synchondrosis (hyaline cartilage) Immobile Symphysis (fibrocartilage) Slightly movable Synovial Adjoining bones covered with articular Plane Saddle Freely movable (diarthrosis; cartilage, separated by a joint cavity, and movements depend on the Condylar Pivot enclosed within an articular capsule lined shapes of the joint surfaces) with synovial membrane Hinge Ball-and-socket have the normal number of joints. It’s just that their joint cap- “crescents”), extend inward from the articular capsule and par- sules and ligaments are more stretchy and loose than average. tially or completely divide the synovial cavity in two (see the Nerves and blood vessels. Synovial joints are richly sup- menisci of the knee in Figure 8.13b, e, and f). Articular discs plied with sensory nerve fibers that innervate (supply) the improve the fit between articulating bone ends, making the joint 8 capsule. Some of these fibers detect pain, as anyone who has more stable and minimizing wear and tear on the joint surfaces. suffered joint injury is aware, but most monitor joint position Besides the knees, articular discs occur in the jaw and a few and stretch. Monitoring joint stretch allows the nervous sys- other joints (see notations in the Structural Type column in tem to sense our posture and body movements (see p. 491). Table 8.3 on pp. 262–263). Synovial joints are also richly supplied with blood vessels, most of which supply the synovial membrane. There, exten- Bursae and Tendon Sheaths sive capillary beds produce the blood filtrate that is the basis Bursae and tendon sheaths are not strictly part of synovial of synovial fluid. joints, but they are often found closely associated with them Besides the basic components just described, certain syno- (Figure 8.4). Essentially bags of lubricant, they act as “ball vial joints have other structural features. Some, such as the hip bearings” to reduce friction between adjacent structures dur- and knee joints, have cushioning fatty pads between the fibrous ing joint activity. Bursae (ber9se; “purse”) are flattened fibrous layer and the synovial membrane or bone. Others have discs sacs lined with synovial membrane and containing a thin film or wedges of fibrocartilage separating the articular surfaces. of synovial fluid. They occur where ligaments, muscles, skin, Where present, these articular discs, or menisci (mĕ-nis9ki; tendons, or bones rub together. A tendon sheath is essentially an elongated bursa that wraps completely around a tendon subjected to friction, like a bun Acromion of scapula around a hot dog. They are common where several tendons are crowded together within narrow canals (in the wrist, for example). Subacromial bursa Fibrous layer of articular capsule A bursa is a fluid-filled sac that decreases friction where a A tendon ligament (or other structure) Articular sheath is an would rub against a bone. cartilage elongated Bursa rolls fluid-filled Joint cavity and lessens sac that containing friction. wraps synovial fluid around a Synovial tendon to membrane decrease friction. Fibrous layer Tendon of Humerus long head Humerus head of biceps rolls medially as brachii muscle arm abducts. (a) Frontal section through the right shoulder joint (b) Enlargement of (a) Figure 8.4 Bursae and tendon sheaths.  256 UNIT 2 Covering, Support, and Movement of the Body Factors Influencing the Stability Table 8.2 Movements at Synovial Joints of Synovial Joints MOVEMENT DEFINITION Joints, particularly synovial joints, are the weakest parts of the Gliding Sliding the flat surfaces of two bones across skeleton. Nonetheless, their structure resists various forces, each other such as crushing or tearing, that threaten to force them out of Angular Movements alignment. Because these joints are constantly stretched and Flexion Decreasing the angle between two bones, compressed, they must be stabilized so that they do not dislo- usually in the sagittal plane cate (come out of alignment). The stability of a synovial joint Extension Increasing the angle between two bones, depends chiefly on three factors: usually in the sagittal plane The shapes of the articular surfaces Abduction Moving a limb away from the body midline in the frontal plane The number and positioning of ligaments Adduction Moving a limb toward the body midline in Muscle tone the frontal plane Circumduction Moving a limb or finger so that it describes Articular Surfaces a cone in space The shapes of articular surfaces determine what movements are Rotation Turning a bone around its longitudinal axis possible at a joint, but surprisingly, articular surfaces play only Medial rotation Rotating toward the median plane a minor role in joint stability. Many joints have shallow sock- 8 Lateral rotation Rotating away from the median plane ets or noncomplementary articulating surfaces (“misfits”) that actually hinder joint stability. But when articular surfaces are large and fit snugly together, or when the socket is deep, stabil- ity is vastly improved. The ball and deep socket of the hip joint provide the best example of a joint made extremely stable by Range of motion allowed by synovial joints varies from the shape of its articular surfaces. nonaxial movement (gliding movements only) to uniaxial movement (movement in one plane) to biaxial movement Ligaments (movement in two planes) to multiaxial movement (move- ment in or around all three planes of space and axes). Range of The capsules and ligaments of synovial joints unite the bones motion varies greatly. In some people, such as trained gymnasts and prevent excessive or undesirable motion. As a rule, or acrobats, range of joint movement may be extraordinary. The the more ligaments a joint has, the stronger it is. However, ranges of motion at the major joints are given in the far right when other stabilizing factors are inadequate, undue tension column of Table 8.3 on pp. 262–263. is placed on the ligaments and they stretch. Stretched liga- There are three general types of movements: gliding , ments stay stretched, like taffy, and a ligament can stretch angular movements, and rotation. The most common body only about 6% of its length before it snaps. As a result, when movements allowed by synovial joints are described next, ligaments are the major means of bracing a joint, the joint is illustrated in Figures 8.5–8.7, and summarized in Table 8.2. not very stable. Gliding Movements Muscle Tone Gliding occurs when one f lat, or nearly f lat, bone surface For most joints, the muscle tendons that cross the joint are the glides or slips over another (back-and-forth and side-to-side; most important stabilizing factor. These tendons are kept under Figure 8. 5) without appreciable angulation or rotation. tension by the tone of their muscles. (Muscle tone is defined as Gliding occurs at the intercarpal and intertarsal joints, and low levels of contractile activity in relaxed muscles that keep the muscles healthy and ready to react to stimulation.) Mus- cle tone is extremely important in reinforcing the shoulder and knee joints and the arches of the foot. Gliding occurs when the flat surfaces of two bones slide across each other. Movements Allowed by Synovial Joints Every skeletal muscle of the body is attached to bone or other connective tissue structures at no fewer than two points. Body movement occurs when muscles contract across joints. Those movements can be described in directional terms relative to the lines, or axes, around which the body part moves and the planes of space along which the movement occurs, that is, along the transverse, frontal, or sagittal plane. (See Chapter 1, p. 15, to review these planes.) Figure 8.5 Gliding movements allowed by synovial joints.  Chapter 8 Joints 257 Flexion and extension Extension Hyperextension occur in the sagittal plane. Flexion Hyper- Flexion extension Extension Flexion decreases Extension the angle between Extension increases two bones the angle between two bones Flexion Extension Hyperextension Flexion 8 Hyperextension is extension past the anatomical position. (a) Flexion, extension, and hyperextension at the shoulder and knee between the f lat articular processes of the verte- brae (see Table 8.3). (b) Flexion, extension, and hyperextension of the neck and vertebral column Angular Movements Angular movements ( Figure 8. 6) increase or Abduction and adduction decrease the angle between two bones. These move- occur in the frontal ments may occur in any plane of the body and (coronal) plane. include flexion, extension, hyperextension, abduc- tion, adduction, and circumduction. Flexion Flexion (flek9shun) is a bending move- ment, usually along the sagittal plane, that decreases the angle of the joint and brings the articulating bones closer together. Examples include bending the head forward on the chest ( Figure 8. 6b ) and bending the body trunk or the knee from a straight Abduction moves a limb away from to an angled position (Figure 8.6a and b). As a less the median obvious example, the arm is flexed at the shoul- (midsagittal) plane. der when the arm is lifted in an anterior direction (Figure 8.6a). Adduction moves a Circumduction moves limb toward the the distal end of a Extension Extension is the reverse of flexion and median (midsagittal) limb in a circle. plane. occurs at the same joints. It involves movement along the sagittal plane that increases the angle between the articulating bones and typically straightens a flexed limb or body part. Examples include straight- ening a flexed neck, body trunk, elbow, or knee (Fig- ure 8.6a and b). Continuing such movements beyond (c) Abduction, adduction, and circumduction of the upper limb at the shoulder the anatomical position is called hyperextension (Figure 8.6a and b). Figure 8.6 Angular movements allowed by synovial joints.  258 UNIT 2 Covering, Support, and Movement of the Body Abduction Abduction (“moving away”) is movement of a Special Movements limb away from the midline or median plane of the body, along Certain movements do not fit into any of the above categories the frontal plane. Raising the arm or thigh laterally is an exam- and occur at only a few joints. Some of these special move- ple of abduction (Figure 8.6c). For the fingers or toes, abduc- ments are illustrated in Figure 8.8. tion means spreading them apart. In this case the “midline” is the third finger or second toe. Notice, however, that lateral Supination and Pronation The terms supination (soo0pĭ- bending of the trunk away from the body midline in the frontal na9shun; “turning backward”) and pronation (pro-na9shun; plane is called lateral flexion, not abduction. “turning forward”) refer to the movements of the radius around the ulna (Figure 8.8a). Rotating the forearm laterally so that the Adduction Adduction (“moving toward”) is the opposite of palm faces anteriorly or superiorly is supination. In the ana- abduction, so it is the movement of a limb toward the body tomical position, the hand is supinated and the radius and ulna midline or, in the case of the digits, toward the midline of the are parallel. hand or foot (Figure 8.6c). In pronation, the forearm rotates medially and the palm faces Circumduction Circumduction (Figure 8.6c) is moving a posteriorly or inferiorly. Pronation moves the distal end of the limb so that it describes a cone in space (circum = around ; radius across the ulna so that the two bones form an X. This is duco = to draw ). The distal end of the limb moves in a cir- the forearm’s position when we are standing in a relaxed man- cle, while the point of the cone (the shoulder or hip joint) is ner. Pronation is a much weaker movement than supination. more or less stationary. A baseball pitcher winding up to throw A trick to help you keep these terms straight: A pro basket- a ball is actually circumducting their pitching arm. Because ball player pronates their forearm to dribble the ball. 8 circumduction consists of flexion, abduction, extension, and Opposition The saddle joint between metacarpal I and the adduction performed in succession, it is the quickest way to trapezium allows a movement called opposition of the thumb exercise the many muscles that move the hip and shoulder ball- (Figure 8.8b). This movement is the action taken when you and-socket joints. touch your thumb to the tips of the other fingers on the same hand. It is opposition that makes the human hand such a fine Rotation tool for grasping and manipulating objects. Rotation is the turning of a bone around its own long axis. It is the only movement allowed between the first two cervical Dorsiflexion and Plantar Flexion of the Foot The up-and- vertebrae and is common at the hip (Figure 8.7) and shoul- down movements of the foot at the ankle are given more spe- der joints. Rotation may be directed toward the midline or cific names (Figure 8.8c). Lifting the foot so that its superior away from it. For example, in medial rotation of the thigh, the surface approaches the shin is dorsiflexion (corresponds to femur’s anterior surface moves toward the median plane of the wrist extension), whereas depressing the foot (pointing the toes) body; lateral rotation is the opposite movement. is plantar flexion (corresponds to wrist flexion). Inversion and Eversion Inversion and eversion are special movements of the foot (Figure 8.8d). In inversion, the sole of Rotation turns a bone the foot turns medially. In eversion, the sole faces laterally. around its own long axis. Elevation and Depression Elevation means lifting a body part superiorly (Figure 8.8e). For example, the scapulae are elevated when you shrug your shoulders. Moving the elevated part inferiorly is depression. During chewing, the mandible is Rotation alternately elevated and depressed. Protraction and Retraction Nonangular anterior and poste- rior movements in a transverse plane are called protraction and retraction, respectively (Figure 8.8f). The mandible is protracted when you jut out your jaw and retracted when you bring it back. Lateral rotation Types of Synovial Joints Although all synovial joints have structural features in com- Medial rotation mon, they do not have a common structural plan. Based on the shape of their articular surfaces, which in turn determine the movements allowed, synovial joints can be classified fur- ther into six major categories—plane, hinge, pivot, condylar (or ellipsoid), saddle, and ball-and-socket joints. The proper- ties of these joints are summarized in Focus on Synovial Joints Figure 8.7 Rotational movements allowed by synovial joints. (Focus Figure 8.1) on pp. 260–261. (Text continues on p. 263.)  Chapter 8 Joints 259 Pronation rotates Supination rotates the radius over the radius and ulna the ulna. parallel. Opposition brings the tip of the thumb in contact with any finger on the same hand. P S 8 (a) Pronation (P) and supination (S) of the forearm (b) Opposition of the thumb Dorsiflexion moves the superior surface of the foot toward the shin (as in standing on your heel). Inversion turns Eversion turns the sole of the the sole of the foot medially. foot laterally. Plantar flexion moves the superior surface of the foot farther away from the shin (as in pointing your toes). (c) Dorsiflexion and plantar flexion of the foot (d) Inversion and eversion of the foot Protraction Retraction moves the moves the Elevation moves Depression moves mandible mandible the mandible up the mandible down forward. back. (closing the mouth). (opening the mouth). (e) Elevation and depression of the mandible (f) Protraction and retraction of the mandible Figure 8.8 Special body movements allowed by synovial joints. FOCUS FIGURE 8.1 Synovial Joints Six types of synovial joint shapes determine the movements that can occur at a joint. (a) Plane joint Nonaxial movement Metacarpals Flat articular surfaces Gliding Carpals Examples: Intercarpal joints, intertarsal joints, joints between vertebral articular surfaces (b) Hinge joint Uniaxial movement Humerus Medial/lateral axis Cylinder Trough Ulna Flexion and extension Examples: Elbow joints, interphalangeal joints (c) Pivot joint Uniaxial movement Vertical axis Sleeve (bone and ligament) Ulna Axle (rounded bone) Rotation Radius Examples: Proximal radioulnar joints, atlantoaxial joint 260  (d) Condylar joint Biaxial movement Medial/ Anterior/ Phalanges lateral posterior axis axis Oval articular Metacarpals surfaces Flexion and extension Adduction and abduction Examples: Metacarpophalangeal (knuckle) joints, wrist joints (e) Saddle joint Biaxial movement Medial/ Anterior/ lateral posterior axis axis Articular Metacarpal I surfaces are both concave and convex Adduction and abduction Flexion and extension Trapezium Example: Carpometacarpal joints of the thumbs (f) Ball-and-socket joint Multiaxial movement Cup Medial/lateral Anterior/posterior Vertical axis (socket) axis axis Scapula Spherical head (ball) Humerus Flexion and extension Adduction and abduction Rotation Examples: Shoulder joints and hip joints 261  Table 8.3 Structural and Functional Characteristics of Body Joints ILLUSTRATION JOINT ARTICULATING BONES STRUCTURAL TYPE* FUNCTIONAL TYPE; MOVEMENTS ALLOWED Skull Cranial and facial Fibrous; suture Synarthrotic; no movement bones Temporo- Temporal bone of skull Synovial; modified Diarthrotic; gliding and uniaxial rotation; mandibular and mandible hinge† (contains slight lateral movement, elevation, articular disc) depression, protraction, and retraction of mandible Atlanto-occipital Occipital bone of skull Synovial; condylar Diarthrotic; biaxial; flexion, extension, lateral and atlas flexion, circumduction of head on neck Atlantoaxial Atlas (C 1) and Synovial; pivot Diarthrotic; uniaxial; rotation of the head axis (C 2 ) Intervertebral Between adjacent Cartilaginous; Amphiarthrotic; slight movement vertebral bodies symphysis Intervertebral Between articular Synovial; plane Diarthrotic; gliding processes Costovertebral Vertebrae (transverse Synovial; plane Diarthrotic; gliding of ribs processes or bodies) 8 and ribs Sternoclavicular Sternum and clavicle Synovial; shallow Diarthrotic; multiaxial (allows clavicle to saddle (contains move in all axes) articular disc) Sternocostal Sternum and rib 1 Cartilaginous; Synarthrotic; no movement (first) synchondrosis Sternocostal Sternum and ribs 2–7 Synovial; double plane Diarthrotic; gliding Acromio- Acromion of scapula Synovial; plane Diarthrotic; gliding and rotation of scapula clavicular and clavicle (contains articular on clavicle disc) Shoulder Scapula and humerus Synovial; ball-and- Diarthrotic; multiaxial; flexion, extension, (glenohumeral) socket abduction, adduction, circumduction, rotation of humerus Elbow Ulna (and radius) with Synovial; hinge Diarthrotic; uniaxial; flexion, extension of humerus forearm Proximal Radius and ulna Synovial; pivot Diarthrotic; uniaxial; pivot (convex head of radioulnar radius rotates in radial notch of ulna) Distal Radius and ulna Synovial; pivot Diarthrotic; uniaxial; rotation of radius radioulnar (contains articular around long axis of forearm to allow disc) pronation and supination Wrist Radius and proximal Synovial; condylar Diarthrotic; biaxial; flexion, extension, carpals abduction, adduction, circumduction of hand Intercarpal Adjacent carpals Synovial; plane Diarthrotic; gliding Carpometacarpal Carpal (trapezium) Synovial; saddle Diarthrotic; biaxial; flexion, extension, of digit I and metacarpal I abduction, adduction, circumduction, (thumb) opposition of metacarpal I Carpometacarpal Carpal(s) and Synovial; plane Diarthrotic; gliding of metacarpals of digits II–V metacarpal(s) Metacarpo- Metacarpal and Synovial; condylar Diarthrotic; biaxial; flexion, extension, phalangeal proximal phalanx abduction, adduction, circumduction of (knuckle) fingers Interphalangeal Adjacent phalanges Synovial; hinge Diarthrotic; uniaxial; flexion, extension of (finger) fingers  Chapter 8 Joints 263 Table 8.3 (continued) ILLUSTRATION JOINT ARTICULATING BONES STRUCTURAL TYPE* FUNCTIONAL TYPE; MOVEMENTS ALLOWED Sacroiliac Sacrum and hip bone Synovial; plane Diarthrotic in child; amphiarthrotic in adult in childhood, (more movement during pregnancy) increasingly fibrous in adult Pubic symphysis Pubic bones Cartilaginous; Amphiarthrotic; slight movement (enhanced symphysis during pregnancy) Hip (coxal) Hip bone and femur Synovial; ball-and- Diarthrotic; multiaxial; flexion, extension, socket abduction, adduction, rotation, circumduction of thigh Knee Femur and tibia Synovial; modified Diarthrotic; biaxial; flexion, extension of leg, (tibiofemoral) hinge† (contains some rotation allowed in flexed position articular discs) Knee Femur and patella Synovial; plane Diarthrotic; gliding of patella (femoropatellar) Superior Tibia and fibula Synovial; plane Diarthrotic; gliding of fibula 8 tibiofibular (proximally) Inferior Tibia and fibula Fibrous; syndesmosis Synarthrotic; slight “give” during tibiofibular (distally) dorsiflexion Ankle Tibia and fibula with Synovial; hinge Diarthrotic; uniaxial; dorsiflexion, and talus plantar flexion of foot Intertarsal Adjacent tarsals Synovial; plane Diarthrotic; gliding; inversion and eversion of foot Tarsometatarsal Tarsal(s) and Synovial; plane Diarthrotic; gliding of metatarsals metatarsal(s) Metatarso- Metatarsal and Synovial; condylar Diarthrotic; biaxial; flexion, extension, phalangeal proximal phalanx abduction, adduction, circumduction of great toe Interpha- Adjacent phalanges Synovial; hinge Diarthrotic; uniaxial; flexion, extension langeal (toe) of toes *Fibrous joints indicated by orange circles ( ); cartilaginous joints by blue circles ( ); synovial joints by purple circles ( ). † These modified hinge joints are structurally bicondylar. Table 8.3 summarizes the characteristics of joints in the body. Check Your Understanding 6. How do bursae and tendon sheaths improve joint function? 7. Generally speaking, what factor is most important in stabilizing synovial joints? 8. On the basis of movement allowed, which of the following joints are uniaxial? Hinge, (a) (b) condylar, saddle, pivot. 9. Name the category (according to its shape and movement allowed) of each of the synovial joints labeled a–d at right. 10. APPLY Hwan bent over to pick up a dime. What movement was occurring at his hip joint and between his index finger and thumb? 11. DRAW Sketch the general structure of a synovial joint. Label the articular cartilages, the articular capsule, and the joint cavity. For answers, see Answers Appendix. (c) (d) A CLOSER LOOK CLINICAL Joints: From Knights in Shining Armor to Bionic Humans The technology for fashioning joints in strong cobalt and titanium alloys, and the medieval suits of armor developed over number of knee replacements exceeds the centuries. The technology for creating the number of hip replacements. prostheses (artificial joints) used in medicine Replacements are now available for many today developed, in relative terms, in a other joints, including fingers, elbows, and flash—less than 70 years. Unlike the joints in shoulders. Most such operations are done medieval armor, which was worn outside the to reduce pain and restore about 80% of body, today’s artificial joints must function original joint function. Total hip and knee inside the body. The history of joint prostheses replacements last more than 20 years. dates to the 1940s and 1950s, when One common mode of failure is that World War II and the Korean War left large the prosthesis works loose from the bone numbers of wounded who needed artificial over time. A solution for this problem is to limbs. Today, more than 1 million Americans use a cementless prosthesis, which allows per year receive a total joint replacement, the bone to grow into its surface, fixing it mostly because of the destructive effects of in place. For this to happen, a precise fit osteoarthritis or rheumatoid arthritis. in the prosthesis and the bone must be To produce durable, mobile joints requires achieved. Prostheses can now be produced substances that are strong, nontoxic, on 3-D printers, so they can be customized and resistant to the corrosive effects of to exactly fit the patient. In addition, surgical organic acids in blood. In 1963, Sir John A hip prosthesis. robots can help enhance the precision of Charnley, an English orthopedic surgeon, this surgery. revolutionized the therapy of arthritic hips proved to be exceptionally strong and Joint replacement therapy has come of with an artificial hip design that is still in use relatively problem free. Hip prostheses were age. However, exciting new techniques that today. His device consisted of a metal ball followed by knee prostheses, but not until call on the ability of the patient’s own tissues on a stem and a cup-shaped polyethylene 10 years later did smoothly operating total to regenerate aim to make metal prostheses plastic socket anchored to the pelvis by knee joint replacements become a reality. obsolete. These techniques offer hope for a methyl methacrylate cement. This cement Today, the metal parts of the prostheses are more permanent solution in the future. 8.5 Five examples illustrate the diversity of synovial joints Learning Outcome N Describe the jaw, shoulder, elbow, hip, and knee joints in terms of articulating bones, anatomical characteristics of the joint, movements allowed, and joint stability. In this section, we examine five joints in detail: temporomandibular (jaw), shoulder, elbow, hip, and knee joints. All have the six distinguishing characteristics of synovial joints, and we will not discuss these common features again. Instead, we will empha- size the unique structural features, functional abilities, and, in certain cases, functional weaknesses of each of these joints. Temporomandibular Joint The temporomandibular joint (TMJ), or jaw joint, is a modified hinge joint. It lies just anterior to the ear (Figure 8.9). At this joint, the condylar process of the man- dible articulates with the inferior surface of the squamous part of the temporal bone ( p. 208). The mandible’s condylar process is egg shaped, whereas the articular sur- face of the temporal bone has a more complex shape. Posteriorly, it forms the concave mandibular fossa; anteriorly it forms a dense knob called the articular tubercle. The lateral aspect of the loose articular capsule that encloses the joint is thickened into a lateral ligament. Within the capsule, an articular disc divides the synovial cavity into superior and inferior compartments (Figure 8.9a, b). Two distinct kinds of movement occur at the TMJ. First, the concave inferior disc surface receives the condylar process of the mandible and allows the familiar 264  Chapter 8 Joints 265 hinge-like movement of depressing and elevating the man- foods such as nuts or hard candies. The superior compart- dible while opening and closing the mouth (Figure 8.9a, b). ment also allows this joint to glide from side to side. As Second, the superior disc surface glides anteriorly along with the posterior teeth are drawn together during grinding, the the condylar process when the mouth is opened wide. This mandible moves with a side-to-side movement called lateral anterior movement braces the condylar process against the excursion (Figure 8.9c). This lateral jaw movement is unique articular tubercle, so that the mandible is not forced through to mammals and it is readily apparent in horses and cows as the thin roof of the mandibular fossa when one bites hard they chew. Articular disc Mandibular fossa Articular tubercle Articular tubercle Zygomatic process Infratemporal fossa Mandibular fossa Superior joint cavity External acoustic meatus Articular capsule 8 Articular Synovial capsule membranes Ramus of mandible Condylar process of Lateral mandible ligament Ramus of Inferior joint mandible cavity (a) Location of the joint in the skull (b) Enlargement of a sagittal section through the joint Superior view Outline of the mandibular fossa (c) Lateral excursion: lateral (side-to-side) movements of the mandible Figure 8.9 The temporomandibular indicated by black arrows. The inferior compartment lets the condylar process move (jaw) joint. Orange arrows show bone compartment of the joint cavity allows the forward to brace against the articular tubercle movement. In (b), note that the two parts of condylar process of the mandible to rotate in when the mouth opens wide, and also allows the joint cavity allow different movements, opening and closing the mouth. The superior lateral excursion of this joint (c).  266 UNIT 2 Covering, Support, and Movement of the Body HOMEOSTATIC CLINICAL IMBALANCE 8.1 Dislocations of the TMJ occur more readily than any other joint dislocation because of the shallow socket in the joint. Even a deep yawn can dislocate it. This joint almost always dislocates anteriorly, the condylar process of the mandible ending up in a skull region called the infratemporal fossa (Figure 8.9a). In such cases, the mouth remains wide open. To realign a dislocated TMJ, the physician places their thumbs in the pa- tient’s mouth between the lower molars and the cheeks, and then pushes the mandible inferiorly and posteriorly. At least 5% of Americans experience painful TMJ disorders, the most common symptoms of which are pain in the ear and face, tenderness of the jaw muscles, popping sounds when the mouth opens, and joint stiffness. Usually caused by painful spasms of the chewing muscles, TMJ disorders often occur in people who grind their teeth; how- ever, it can also result from jaw trauma or from poor occlusion (closing together) of the teeth. Treatment usually focuses on getting the jaw muscles to relax by using massage, muscle-relaxant drugs, heat or cold, or stress reduction techniques. For tooth grinders, use of a bite plate during sleep may be recommended. 8 Shoulder (Glenohumeral) Joint In the shoulder joint, stability has been sacrificed to provide the most freely mov- ing joint of the body. The shoulder joint is a ball-and-socket joint. The large hemi- spherical head of the humerus fits in the small, shallow glenoid cavity of the scapula ( pp. 228–229), like a golf ball sitting on a tee (Figure 8.10). Although the gle- noid cavity is slightly deepened by a rim of fibrocartilage, the glenoid labrum ( labrum = lip ), it is only about one-third the size of the humeral head and contributes little to joint stability (Figure 8.10d and e). The articular capsule enclosing the joint cavity (from the margin of the glenoid cavity to the anatomical neck of the humerus) is remarkably thin and loose, qualities that contribute to this joint’s freedom of movement. The few ligaments reinforcing the shoulder joint are located primarily on its anterior aspect. The superiorly located coracohumeral ligament (kor9ah-ko-hu9mer-ul) provides the only strong thickening of the capsule and helps support the weight of the upper limb (Figure 8.10c). Three glenohumeral ligaments (glĕ0no-hu9mer-ul) strengthen the front of the cap- sule somewhat but are weak and may even be absent (Figure 8.10c, d). Muscle tendons that cross the shoulder joint contribute most to this joint’s stability. The “superstabilizer” is the tendon of the long head of the biceps brachii muscle of the arm (Figure 8.10c). This tendon attaches to the superior margin of the glenoid labrum, travels through the joint cavity, and then runs within the intertubercular sulcus of the humerus. It secures the head of the humerus against the glenoid cavity. Four other tendons (and the associated muscles) make up the rotator cuff. This cuff encircles the shoulder joint and blends with the articular capsule. The four muscles of the rotator cuff are the subscapularis, supraspinatus, infraspinatus, and teres minor (see Figure 10.15, pp. 355–356). The rotator cuff can be severely stretched when the arm is vigorously circumducted; this is a common injury of baseball pitchers. HOMEOSTATIC CLINICAL IMBALANCE 8. 2 One price of mobility in the shoulder is that shoulder dislocations are common inju- ries. Because the structures reinforcing this joint are weakest anteriorly and inferiorly, the head of the humerus easily dislocates forward and downward. The glenoid cavity provides poor support when the humerus is rotated laterally and abducted, as when a football player uses the arm to tackle an opponent or a bicycle commuter hits the ground during an accident. These situations may cause shoulder dislocations, as do blows to the top and back of the shoulder.  Chapter 8 Joints 267 Acromion of scapula Glenoid labrum Coracoacromial ligament Synovial cavity Subacromial of the glenoid bursa cavity containing synovial fluid Fibrous layer of articular capsule Articular cartilage Tendon sheath Synovial membrane Fibrous layer of Tendon of articular capsule long head of biceps brachii muscle Humerus (a) Frontal section through right shoulder joint (b) Cadaver photo corresponding to (a) 8 Acromion Acromion Coracoid Coracoacromial process ligament Coracoid process Subacromial Articular capsule Articular bursa capsule reinforced by glenohumeral Glenoid cavity Coracohumeral ligaments ligament Glenoid labrum Transverse Subscapular Tendon of long humeral bursa head of biceps ligament brachii muscle Tendon of the subscapularis Glenohumeral Tendon sheath ligaments muscle Tendon of the Tendon of subscapularis long head Scapula muscle of biceps brachii Scapula muscle Posterior Anterior (c) Anterior view of right shoulder joint capsule (d) Lateral view of socket of right shoulder joint, humerus removed Acromion (cut) Rotator cuff muscles Glenoid (cut) cavity of scapula Glenoid labrum Capsule of shoulder joint (opened) Head of humerus (e) Posterior view of an opened right shoulder joint Figure 8.10 The shoulder joint.  268 UNIT 2 Covering, Support, and Movement of the Body Articular capsule Synovial Humerus membrane Humerus Anular Synovial cavity ligament Articular cartilage Radius Fat pad Lateral Coronoid process epicondyle Tendon of Tendon of triceps brachialis muscle Articular muscle capsule Ulna Bursa Radial collateral Trochlea ligament Articular cartilage Olecranon of the trochlear notch Ulna (a) Median sagittal section through right elbow (lateral view) (b) Lateral view of right elbow joint Articular 8 capsule Humerus Anular Humerus Anular ligament ligament Medial Coronoid epicondyle process Medial Radius epicondyle Articular Ulnar capsule collateral ligament Radius Coronoid process of ulna Ulnar Ulna collateral Ulna ligament (c) Cadaver photo of medial view of right elbow (d) Medial view of right elbow Figure 8.11 The elbow joint. Elbow Joint and the radial collateral ligament, a triangular ligament on the lateral side (Figure 8.11b, c, and d). Additionally, tendons Our upper limbs are flexible extensions that permit us to reach of several arm muscles, such as the biceps and triceps, cross the out and manipulate things in our environment. Besides the shoul- elbow joint and provide security. der joint, the most prominent of the upper limb joints is the The radius is a passive “onlooker” in the angular elbow elbow. The elbow joint provides a stable and smoothly operat- movements. However, its head rotates within the anular liga- ing hinge that allows flexion and extension only (Figure 8.11). ment during supination and pronation of the forearm. Within the joint, both the radius and ulna articulate with the condyles of the humerus, but it is the close gripping of the trochlea by the ulna’s trochlear notch that forms the “hinge” Hip Joint and stabilizes this joint (Figure 8.11a). A relatively lax articular The hip (coxal) joint, like the shoulder joint, is a ball-and-socket capsule extends inferiorly from the humerus to the ulna and joint. It has a good range of motion, but not nearly as wide as the radius, and to the anular ligament (an9u-lar) encircling the shoulder’s range. Movements occur in all possible planes but are head of the radius (Figure 8.11b, c). limited by the joint’s strong ligaments and its deep socket. Anteriorly and posteriorly, the articular capsule is thin and The hip joint is formed by the articulation of the spherical allows substantial freedom for elbow flexion and extension. head of the femur with the deeply cupped acetabulum of the hip However, side-to-side movements are restricted by two strong bone (Figure 8.12) ( p. 269). The depth of the acetabulum is capsular ligaments: the ulnar collateral ligament medially, enhanced by a circular rim of fibrocartilage called the  Chapter 8 Joints 269 Hip (coxal) bone Acetabular Articular cartilage labrum Ligament of the head of the femur Synovial Acetabular labrum (ligamentum teres) membrane Femur Ligament of the head of the femur (ligamentum teres) Head of femur Articular capsule (cut) Synovial cavity Articular capsule (a) Frontal section through the right hip joint (b) Photo of the interior of the hip joint, lateral view 8 Iliofemoral ligament Anterior inferior Iliofemoral iliac spine ligament Ischium Ischiofemoral ligament Pubofemoral ligament Greater Greater trochanter trochanter of femur (c) Posterior view of right hip joint, capsule in place (d) Anterior view of right hip joint, capsule in place Figure 8.12 The hip joint. acetabular labrum (as0ĕ-tab9u-lar) (Figure 8.12a, b). Because These ligaments are arranged in such a way that they the diameter of the labrum is smaller than that of the head of the “screw” the femur head into the acetabulum when a person femur, the femur cannot easily slip out of the socket, and hip stands up straight, thereby providing stability. dislocations are rare. The ligament of the head of the femur, also called the liga- The thick articular capsule extends from the rim of the ace- mentum teres, is a flat intracapsular band that runs from the tabulum to the neck of the femur and completely encloses the femur head to the lower lip of the acetabulum (Figure 8.12a, b). joint. Several strong ligaments reinforce the capsule of the hip This ligament is slack during most hip movements, so it is not joint. These include: important in stabilizing the joint. In fact, its mechanical func- The iliofemoral ligament (il0e-o-fem9o-ral), a strong tion (if any) is unclear, but it does contain an artery that helps V-shaped ligament anteriorly (Figure 8.12c, d) supply the head of the femur. Damage to this artery may lead to severe arthritis of the hip joint. The pubofemoral ligament (pu0bo-fem9o-ral), a triangular Muscle tendons that cross the joint and the bulky hip and thigh thickening of the inferior part of the capsule (Figure 8.12d) muscles that surround it contribu