Anatomy and Physiology Chapter 9 Notes PDF
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These notes cover the classification of joints, including fibrous, cartilaginous, and synovial joints, along with their structural and functional characteristics. It details the different types of joints, their distinguishing features, and example articulations.
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CHAPTER 9 Classification of Joints Joint/articulation – place of contact between bones, between bone/cartilage or between bones and teeth Bones articulate with each other at a joint. arthrology - the scientific study of joints Joints are classified by both their structural...
CHAPTER 9 Classification of Joints Joint/articulation – place of contact between bones, between bone/cartilage or between bones and teeth Bones articulate with each other at a joint. arthrology - the scientific study of joints Joints are classified by both their structural characteristics and their functional characteristics (the movements they allow) Joints are categorized structurally on the basis of whether a space occurs between the articulating bones and the type of connective tissue that binds the articulating surfaces of the bones: o A fibrous joint ▪ has no joint cavity ▪ occurs where bones are held together by dense regular connective tissue. o Cartilaginous ▪ joint has no joint cavity ▪ occurs where bones are joined by cartilage. o A synovial joint ▪ has a joint cavity (filled with a lubricating fluid) that separates the articulating surfaces of the bones. ▪ The articulating surfaces are enclosed within a connective tissue capsule, ▪ the bones are attached to each other by various ligaments. Joints are classified functionally based on the extent of movement they permit. o A synarthrosis ▪ an immobile joint. ▪ Two types of fibrous joints and one type of cartilaginous joint are synarthroses. o An amphiarthrosis ▪ a slightly mobile joint. ▪ One type of fibrous joint and one type of cartilaginous joint are amphiarthroses. o A diarthrosis ▪ is a freely mobile joint. All synovial joints are diarthroses. The motion permitted at a joint ranges o from no movement ▪ such as where some skull bones interlock at a suture, o to extensive movement, ▪ such as that seen at the shoulder, where the humerus articulates with the scapula. The structure of each joint determines both its mobility and its stability. There is an inverse relationship between mobility and stability in articulations. The more mobile a joint, the less stable the joint. In contrast, the less mobile the joint, the more stable the joint is. If a joint is immobile, it has maximum stability. Fibrous Joints Articulating bones in fibrous joints are connected by dense regular connective tissue Fibrous joints have no joint cavity o they lack a space between the articulating bones. Most fibrous joints are immobile or at most only slightly mobile; o their primary function is to hold together two bones. Examples o articulations of the teeth in their sockets, o sutures between skull bones, o articulations between either the radius and ulna or the tibia and fibula. three types of fibrous joints: gomphoses, sutures, and syndesmoses Gomphoses A gomphosis resembles a “peg in a socket.” The only gomphoses in the human body are the articulations of the roots of individual teeth with the alveolar processes (sockets) of the mandible and the maxillae. A tooth is held firmly in place by fibrous periodontal membranes. This joint is immobile is functionally classified as a synarthrosis. o This is why braces can be painful and take so long to correctly position the teeth Orthodontists reposition these normally immobile joints through the use of clamps, bands, rings, and braces. In response to the mechanical stressors, osteoblasts and osteoclasts work together to modify the alveolar process o This results in the remodeling of the joints and the slow repositioning of the teeth. Sutures fibrous joints found only between certain bones of the skull. functionally classified as synarthroses since they are immobile joints. have distinct, interlocking, usually irregular edges o increases their stability o decrease the number of fractures at these articulations. Joins bones, Permits the skull to grow (by new bone being deposited at these sutures) as the brain increases in size during childhood. In older adults, the dense regular connective tissue in the suture becomes ossified, fusing the skull bones together. When the bones completely fuse across the suture line, the obliterated sutures are now called synostoses Syndesmoses fibrous joints in which articulating bones are joined by long strands of dense regular connective tissue only. allow for slight mobility o they are classified functionally as amphiarthroses. found between the radius and ulna o and between the tibia and fibula. The shafts of the two articulating bones are bound by a broad, ligamentous sheet called an interosseous membrane (or interosseous ligament). o provides a pivot where the radius and ulna (or the tibia and fibula) can move relative to one another. Cartilaginous Joints have cartilage between the articulating bones. Like fibrous joints, cartilaginous joints also lack a joint cavity. They may be either immobile or slightly mobile. The cartilage found between the articulating bones is either hyaline cartilage or fibrocartilage two types of cartilaginous joints o synchondroses o symphyses Synchondroses An articulation in which bones are joined by hyaline cartilage is called a synchondrosis Functionally, all synchondroses are immobile classified functionally as synarthroses. The hyaline cartilage of epiphyseal plates in children forms synchondroses that bind the epiphyses and diaphysis of long bones o When the hyaline cartilage stops growing, bone replaces the cartilage and a synchondrosis no longer exists The spheno-occipital synchondrosis is found between the body of the sphenoid and the basilar part of the occipital bone. o This synchondrosis typically fuses between 18 and 25 years of age, making it a useful tool for assessing the age of the skull Other examples of synchondroses involve costal cartilage. o The costochondral joint, the joint between each bony rib and its respective costal cartilage, is a synchondrosis. o the attachment of the first rib to the sternum by costal cartilage (called the first sternocostal joint) is another synchondrosis. o Here, the first rib and its costal cartilage are united firmly to the manubrium of the sternum to provide stability to the rib cage. o (Note that the sternocostal joints between the sternum and the costal cartilage of ribs 2–7 are synovial joints, and thus are not synchondroses.) Symphyses A symphysis has a pad of fibrocartilage between the articulating bones The fibrocartilage resists both compression and tension stresses and acts as a resilient shock absorber. All symphyses are amphiarthroses o they allow slight mobility. Example o pubic symphysis - located between the right and left pubic bones. ▪ In pregnant females, the pubic symphysis becomes more mobile to allow the pelvis to change shape slightly as the fetus passes through the birth canal. o intervertebral joints, ▪ bodies of adjacent vertebrae are both separated and united by intervertebral discs. ▪ Individual intervertebral discs allow only slight movements between the adjacent vertebrae; however, the collective movements of all the intervertebral discs afford the spine considerable flexibility. Synovial Joints freely mobile articulations (i.e., diarthroses). Most of the commonly known joints in the body are synovial joints, o glenohumeral (shoulder) joint, o temporomandibular joint, o elbow joint, and o knee joint. Distinguishing Features and Anatomy of Synovial Joints bones in a synovial joint are separated by a space called a joint cavity. Functionally, all synovial joints are classified as diarthroses because all are freely mobile. Often, the terms synovial joint and diarthrosis are equated. All synovial joints include several basic features: o an articular capsule, o a joint cavity, o synovial fluid, o articular cartilage, o ligaments, o nerves, and blood vessels (figure 9.4). Each synovial joint is composed of a double- layered capsule called the articular capsule, or joint capsule. Its outer layer is called the fibrous layer, and the inner layer is a synovial membrane (or synovium) The fibrous layer is formed from dense connective tissue. o It strengthens the joint to prevent the bones from being pulled apart. o The synovial membrane is a specialized type of connective tissue, the cells of which help produce and secrete synovial fluid. o This membrane covers all the internal joint surfaces not covered by cartilage and lines the articular capsule. All articulating bone surfaces in a synovial joint are covered by a thin layer of hyaline cartilage called articular cartilage. o This cartilage has numerous functions: ▪ reduces friction in the joint during movement, acts as a cushion to absorb compression placed on the joint, and prevents damage to the articulating ends of the bones. This special hyaline cartilage lacks a perichondrium mature cartilage is avascular, so it does not have blood vessels to bring nutrients to and remove waste products from the cartilage. o Thus, if cartilage is damaged, its avascularity is correlated with lack of or delayed healing of the tissue. The repetitious compression and expansion that occurs during exercise is vital to maintaining healthy articular cartilage because this action enhances its obtaining nutrition and its waste removal. Only synovial joints house a joint cavity (or articular cavity), o a space that permits separation of the articulating bones. o The articular cartilage and synovial fluid (described next) within the joint cavity together reduce friction as bones move at a synovial joint. Synovial fluid is a viscous, oily substance located within a synovial joint. It is a product of both the synovial membrane cells and the filtrate formed from blood plasma. Synovial fluid has three functions: o Lubricates. ▪ Synovial fluid lubricates the articular cartilage on the surface of articulating bones (in the same way that oil in a car engine lubricates the moving engine parts). o Nourishes the chondrocytes. ▪ The relatively small volume of synovial fluid must be circulated continually to provide nutrients to and remove wastes from articular cartilage’s chondrocytes. ▪ Whenever movement occurs at a synovial joint, the combined compression and re-expansion of the articular cartilage circulates the synovial fluid into and out of the cartilage matrix. o Acts as a shock absorber ▪ Synovial fluid distributes stresses and force evenly across the articular surfaces when the pressure in the joint suddenly increases. Ligaments o are composed of dense regular connective tissue, o connect one bone to another bone. o function to stabilize, strengthen, and reinforce most synovial joints. o Intrinsic ligaments represent thickenings of the articular capsule itself. ▪ include extracapsular ligaments outside the articular capsule and intracapsular ligaments within the articular capsule. ▪ Extrinsic ligaments are outside of, and physically separate from, the articular capsule. The specific intrinsic and extrinsic ligaments are specific to each type of joint (e.g., knee, shoulder). All synovial joints have numerous blood vessels to transport oxygen and nutrients to the tissue, and to remove wastes. o They also have many sensory receptors that innervate the articular capsule and associated ligaments. ▪ these sensory receptors include proprioceptors that detect the movement, stretch, and positioning of the joint ▪ By monitoring stretching at a joint, the nervous system can detect changes in our posture and adjust body movements. ▪ Joints also contain nociceptors that detect painful stimuli in the joint, which provides us with sensory input regarding possible injury to the joint Tendons o composed of dense regular connective tissue o not part of the synovial joint itself. o attaches a muscle to a bone. o When a muscle contracts, the tendon from that muscle moves the bone to which it is attached, thus causing movement at the joint. o Tendons help stabilize joints because they pass across or around a joint to provide mechanical support, and sometimes they limit the range or amount of movement permitted at a joint. Synovial joints usually have bursae and fat pads as accessory structures in addition to the main components just described. o A bursa is a fibrous, saclike structure that contains synovial fluid and is lined internally by a synovial membrane o There are numerous bursae in the body, and they are associated with most synovial joints and are where bones, ligaments, muscles, skin, or tendons overlie each other and rub together. o Bursae may be either connected to the joint cavity or completely separate from it. o They alleviate the friction resulting from the various body movements, such as where a tendon or ligament rubs against bone. o An elongated bursa called a tendon sheath wraps around a tendon where there may be excessive friction. ▪ Tendon sheaths are especially common in the confined spaces of the wrist and ankle Fat pads are often distributed along the periphery of a synovial joint. o They act as packing material and provide some protection for the joint. o Often, fat pads fill the spaces that form when bones move and the joint cavity changes shape Classification of Synovial Joints there are three major anatomic planes (coronal, sagittal, and transverse). Synovial joints are classified by the shapes of their articulating surfaces and the types of movement they allow along these planes. Movement of a bone at a synovial joint may be described in one of three ways: o A joint is said to be uniaxial if the bone moves in just one plane or axis. o A joint is biaxial if the bone moves in two planes or axes. o A joint is multiaxial or triaxial if the bone moves in multiple planes or axes. All synovial joints are diarthroses, as mentioned, but some are more mobile than others. From least mobile to most freely mobile, the six specific types of synovial joints are o plane joints o hinge joints o pivot joints o condylar joints o saddle joints o ball-and-socket joints. plane joint o also called a planar or gliding joint o the simplest synovial articulation and the least mobile type of diarthrosis. o a uniaxial joint because it usually allows only limited side-to-side movements in a single plane, and because there is no rotational or angular movement with this joint. o The articular surfaces of the bones are flat, or planar. o Examples of plane joints include the intercarpal and intertarsal joints (the joints between the carpal bones and tarsal bones, respectively). hinge joint o formed by the convex surface of one articulating bone fitting into a concave depression on the other bone in the joint. o Movement is confined to a single axis, like the movement seen at the hinge of a door ▪ so a hinge joint is considered a uniaxial joint. o An example is the elbow joint. ▪ The trochlear notch of the ulna fits directly into the trochlea of the humerus, so the forearm can be moved only anteriorly toward the arm or posteriorly away from the arm. ▪ Other hinge joints occur in the knee and the finger (interphalangeal [IP]) joints. pivot joint o uniaxial joint in which one articulating bone with a rounded surface fits into a ring formed by a ligament and another bone. o The first bone rotates on its longitudinal axis relative to the second bone. o An example is the proximal radioulnar joint, where the rounded head of the radius pivots along the ulna and permits the radius to rotate. o Another example is the atlantoaxial joint between the first two cervical vertebrae (i.e., the atlas and axis). ▪ The rounded dens of the axis fits snugly against an articular facet on the anterior arch of the atlas. ▪ This joint pivots when you shake your head “no.” Condylar joints o also called condyloid or ellipsoid joints o are biaxial joints with an oval, convex surface on one bone that articulates with a concave articular surface on the second bone of the joint. o Biaxial joints can move in two axes, such as back-and forth and side to side. o Examples ▪ metacarpophalangeal (MP) joints of fingers 2 through 5. ▪ The MP joints are commonly referred to as knuckles. ▪ Examine your hand and look at the movements along the MP joints; you can flex and extend the fingers at this joint, which is one axis of movement. ▪ You also can move your fingers apart from one another and move them closer together, which is the second axis of movement. A saddle joint o named because the articular surfaces of the bones have convex and concave regions that resemble the shape of a saddle. o This biaxial joint allows a greater range of movement than either a condylar or hinge joint. o The carpometacarpal joint of the thumb (between the trapezium, which is a carpal bone, and the first metacarpal) is an example of a saddle joint. ▪ This joint permits the thumb to move toward the other fingers so that we can grasp objects. Ball-and-socket joints o multiaxial joints in which the spherical articulating head of one bone fits into the rounded, cuplike socket of a second bone. o Examples ▪ coxal (hip) ▪ glenohumeral (shoulder) joints. ▪ The multiaxial nature of these joints permits movement in three planes. ▪ The ball-and-socket joint is considered the most freely mobile type of synovial joint. The Movement of Synovial Joints Four types of motion occur at synovial joints: gliding motion, angular motion, rotational motion, and special movements (motions that occur only at specific joints) Gliding Motion Gliding o a simple movement in which two opposing surfaces slide slightly back-and-forth or side- to-side with respect to one another. o the angle between the bones does not change, and only limited movement is possible in any direction. o typically occurs along plane joints, such as between the carpals or the tarsals. Angular Motion either decreases or increases the angle between two bones. These movements may occur at many of the synovial joints. They include the following specific types: o flexion and extension, o lateral flexion, o abduction and adduction, o circumduction Flexion o movement in an anteriorposterior (AP) plane of the body that decreases the angle between the bones. o Bones are brought closer together as the angle between them decreases. o Examples ▪ the bending of the fingers toward the palm to make a fist ▪ the bending of the forearm toward the arm at the elbow ▪ flexion at the shoulder when the arm is raised anteriorly ▪ flexion of the neck when the head is bent anteriorly and you look down at your feet. Extension o The opposite of flexion o movement in an anterior-posterior (AP) plane that increases the angle between the articulating bones. o Extension is a straightening action that occurs in an AP plane. o Straightening the fingers after making a clenched fist o straightening the forearm until it projects directly away from the anterior side of your body Hyperextension o the extension of a joint beyond its normal range of motion. o may occur if someone has extensively mobile joints or an injury at the joint. Lateral flexion o occurs when the trunk of the body moves in a coronal plane laterally away from the body. o occurs primarily between the vertebrae in the cervical and lumbar regions of the vertebral column Abduction o means to move away, o the lateral movement of a body part away from the body midline. o occurs when either the arm or the thigh is moved laterally away from the body midline. o Abduction of either the fingers or the toes means that you spread them apart, away from the longest digit that acts as the midline. o Abducting the wrist (also known as radial deviation) involves pointing the hand and fingers laterally, away from the body. Adduction o The opposite of abduction o means to move toward. o This is the medial movement of a body part toward the body midline. o Adduction occurs when the raised arm or thigh is brought back toward the body midline, or in the case of the digits, toward the midline of the hand. o Adducting the wrist (also known as ulnar deviation) involves pointing the hand and fingers medially, toward the body. Circumduction o a sequence of movements in which the proximal end of an appendage remains relatively stationary while the distal end makes a circular motion o The resulting movement makes an imaginary cone shape. o This is demonstrated when you draw a circle on the blackboard. o The shoulder remains stationary while your hand moves. ▪ The tip of the imaginary cone is the stationary shoulder, while the rounded “base” of the cone is the circle made by the hand. o Circumduction is a complex movement that occurs as a result of a continuous sequence of flexion, abduction, extension, and adduction Rotational Motion Rotation o A pivoting motion in which a bone turns on its own longitudinal axis o Rotational movement occurs at the atlantoaxial joint, which pivots when you rotate your head to gesture “no.” o Some limb rotations are described as either away from the median plane or toward it. ▪ For example, lateral rotation (or external rotation) turns the anterior surface of the femur or humerus laterally, ▪ medial rotation (or internal rotation) turns the anterior surface of the femur or humerus medially. Pronation o the medial rotation of the forearm so that the palm of the hand is directed posteriorly or inferiorly. o The radius and ulna are crossed to form an X Supination occurs when the forearm rotates laterally so that the palm faces anteriorly or superiorly. o In the anatomic position, the forearm is supinated. 9.5d Special Movements Some movements occur only at specific joints and do not readily fit into any of the functional categories previously discussed. These special movements include o depression and elevation, o dorsiflexion and plantar flexion, o eversion and inversion, o protraction and retraction, o opposition and reposition. Depression is the inferior movement of a part of the body. o Examples include ▪ opening your mouth (by depressing your mandible) to chew food and the movement of your shoulders in an inferior direction. Elevation is the superior movement of a body part. o Examples of elevation include ▪ the superior movement of the mandible while closing the mouth and the movement of the shoulders in a superior direction (shrugging your shoulders). Dorsiflexion and plantar flexion are limited to the ankle joint Dorsiflexion o occurs when the talocrural (ankle) joint is bent such that the dorsum (superior surface) of the foot and the toes moves toward the leg. o This movement occurs when you dig in your heels, and it prevents your toes from scraping the ground when you take a step. Plantar flexion o a movement of the foot at the talocrural joint so that the toes point inferiorly. o When a ballerina is standing on tiptoes, the ankle joint is in full plantar flexion. Eversion and inversion are movements that occur at the intertarsal joints of the foot only Eversion o the sole of the foot turns to face laterally or outward, inversion o the sole of the foot turns medially or inward during inversion o (eversion is foot pronation, whereas inversion is foot supination.) Protraction o the anterior movement of a body part from anatomic position, as when jutting your jaw anteriorly at the temporomandibular joint or hunching the shoulders anteriorly by crossing the arms. o In the latter case, the clavicles move anteriorly due to movement at both the acromioclavicular and sternoclavicular joints. Retraction o the posteriorly directed movement of a body part from the anatomic position. opposition o At the carpometacarpal joint, the thumb moves toward the palmar tips of the fingers as it crosses the palm of the hand. o It enables the hand to grasp objects and is the most distinctive digital movement in humans. o The opposite movement is called reposition. Synovial Joints and Levers When analyzing synovial joint movement and muscle contraction, anatomists often compare the movement to the mechanics of a lever; this practice of applying mechanical principles to biology is known as biomechanics. Terminology of Levers Lever o an elongated, rigid object that rotates around a fixed point called the fulcrum o A seesaw is a familiar example of a lever. o Levers have the ability to alter the speed and distance of movement produced by a force, the direction of an applied force, and the force strength. Movement occurs when an effort applied to one point on the lever exceeds a resistance located at some other point. The part of a lever from the fulcrum to the point of effort is called the effort arm, and the lever part from the fulcrum to the point of resistance is the resistance arm. In the body, a long bone acts as a lever, a joint serves as the fulcrum, and the effort is generated by a muscle attached to the bone. Types of Levers Three classes of levers are found in the human body: o first-class o second-class o and third-class First-Class Levers o has a fulcrum in the middle, between the effort (force) and the resistance. o An example of a first-class lever is a pair of scissors. ▪ The effort is applied to the handle of the scissors while the resistance is at the cutting end of the scissors. ▪ The fulcrum (pivot for movement) is along the middle of the scissors, between the handle and the cutting ends. o In the body, an example of a first-class lever is the atlanto-occipital joint of the neck ▪ the muscles on the posterior side of the neck (effort) pull inferiorly on the nuchal lines of the skull and oppose the tendency of the head (resistance) to tip anteriorly. Second-Class Levers o The resistance in a second-class lever is between the fulcrum and the applied effort. o A common example of this type of lever is lifting the handles of a wheelbarrow, allowing it to pivot on its wheel at the opposite end and lift a load in the middle. ▪ The load weight is the resistance, and the upward lift on the handle is the effort. ▪ A small force can balance a larger weight in this type of lever, because the effort is always farther from the fulcrum than the resistance. o Second-class levers are rare in the body, but one example occurs when the foot is plantar flexed so that a person can stand on tiptoe. o The contraction of the calf muscle causes a pull superiorly by the calcaneal tendon attached to the heel (calcaneus). Third-Class Levers o A third-class lever is noted when the effort is applied between the resistance and the fulcrum, as when picking up a small object with a pair of forceps. o Third-class levers are the most common levers in the body. o A third-class lever is found at the elbow where the fulcrum is the joint between the humerus and ulna, the effort is applied by the biceps brachii muscle at its attachment to the radius, and the resistance is provided by any weight in the hand or by the weight of the forearm itself. o The mandible acts as a third-class lever when you bite with your incisors on a piece of food. ▪ The temporomandibular joint is the fulcrum, and the temporalis muscle exerts the effort, whereas the resistance is the item of food being bitten Features and Anatomy of Selected Joints Both the axial skeleton and appendicular skeleton exhibit many more joints than are individually discussed here. Temporomandibular Joint The temporomandibular joint (TMJ) is the articulation formed at the point where the head of the mandible articulates with the temporal bone o specifically, the articular tubercle of the temporal bone anteriorly and the mandibular fossa posteriorly. o This small, complex articulation is the only mobile joint between bones in the skull The temporomandibular joint has several unique anatomic features. o A loose articular capsule surrounds the joint and promotes an extensive range of motion. ▪ It contains an articular disc, which is a thick pad of fibrocartilage separating the articulating bones and extending horizontally to divide the synovial cavity into two separate chambers. ▪ As a result, the TMJ is really two synovial joints—one between the temporal bone and the articular disc, and a second between the articular disc and the mandible. Several ligaments support the TMJ. o The sphenomandibular ligament (an extracapsular ligament) is a thin band that extends anteriorly and inferiorly from the sphenoid to the medial surface of the mandibular ramus. o the temporomandibular ligament (or lateral ligament) is composed of two short bands that extend inferiorly and posteriorly from the articular tubercle of the temporal bone to the mandible. The temporomandibular joint functions as a hinge during mandibular depression and elevation while chewing. It also glides slightly forward during protraction of the mandible for biting, and glides slightly from side to side to grind food between the teeth during chewing. Shoulder Joint The joints associated with movement at the shoulder include the o sternoclavicular joint, o acromioclavicular joint, o glenohumeral joint. Sternoclavicular Joint o a saddle joint formed by the articulation between the manubrium of the sternum and the sternal end of the clavicle o A fibrocartilaginous articular disc partitions the sternoclavicular joint into two parts and forms two separate synovial cavities. ▪ As a result, a wide range of movement is possible, including depression, elevation, and circumduction of the clavicle at this joint. o Support and stability are provided to this articulation by the fibers of the articular capsule and by multiple extracapsular ligaments, such as the sternoclavicular and costoclavicular ligaments. o This anatomic arrangement makes the sternoclavicular joint very stable. o If you fall on an outstretched hand so that force is applied to the joint, the clavicle will fracture before this joint dislocates. Acromioclavicular Joint o The acromioclavicular joint is a plane joint between the acromion of the scapula and the lateral end of the clavicle o A fibrocartilaginous articular disc lies within the joint cavity between these two bones. o This joint works with both the sternoclavicular joint and the glenohumeral joint to give the upper limb a full range of movement. o Several ligaments provide great stability to this joint. o The articular capsule is strengthened superiorly by an acromioclavicular ligament. o In addition, a very strong coracoclavicular ligament binds the clavicle to the coracoid process of the scapula. ▪ If this ligament is torn, the acromion and clavicle no longer align properly Glenohumeral (Shoulder) Joint o commonly referred to as the shoulder joint. o It is a ball-and-socket joint formed by the articulation of the head of the humerus and the glenoid cavity of the scapula o It permits the greatest range of motion of any joint in the body, and so it is both the most unstable joint in the body and the one most frequently dislocated. The fibrocartilaginous glenoid labrum encircles and covers the surface of the glenoid cavity. A relatively loose articular capsule attaches to the surgical neck of the humerus. The glenohumeral joint has several major ligaments. o The coracoacromial ligament extends across the space between the coracoid process and the acromion. o The large coracohumeral ligament is a thickening of the superior part of the articular capsule. ▪ It extends from the coracoid process to the humeral head. o The glenohumeral ligaments are three thickenings of the anterior portion of the articular capsule. ▪ These ligaments are often indistinct or absent and provide only minimal support. In addition, the tendon of the long head of biceps brachii is within the articular capsule and helps stabilize the humeral head in the joint. Unlike other joints in the body (where ligaments provide most of the support to a joint), the ligaments of the glenohumeral joint provide little support. o Instead, most of the glenohumeral joint’s strength is due to the rotator cuff muscles surrounding it o The rotator cuff muscles (i.e., subscapularis, supraspinatus, infraspinatus, and teres minor) work as a group to hold the head of the humerus in the glenoid cavity. ▪ The tendons of these muscles encircle the joint (except for its inferior portion) and fuse with the articular capsule. ▪ Because the inferior portion of the joint lacks support from rotator cuff muscles, this area is weak and is the most likely site of injury. Bursae help decrease friction at the specific places on the shoulder where both tendons and large muscles extend across the articular capsule. o The shoulder has a relatively large number of bursae. Elbow Joint The elbow joint is a hinge joint composed of two articulations o (1) the humeroulnar joint, ▪ where the trochlea of the humerus articulates with the trochlear notch of the ulna, and o (2) the humeroradial joint ▪ where the capitulum of the humerus articulates with the head of the radius Both joints are enclosed within a single articular capsule The elbow is an extremely stable joint for several reasons. o First, the articular capsule is relatively thick, and thus effectively protects the articulations. o Second, the bony surfaces of the humerus and ulna interlock very well, and thus provide a solid bony support. o Finally, multiple strong supporting ligaments help reinforce the articular capsule. o Because of the trade-off between stability and mobility, the elbow joint is very stable but is not as mobile as some other joints, such as the glenohumeral joint. The elbow joint has two main supporting ligaments. o The radial collateral ligament (or lateral collateral ligament) is responsible for stabilizing the joint at its lateral surface; ▪ it extends from the lateral epicondyle of the humerus to the head of the radius. o The ulnar collateral ligament (or medial collateral ligament) stabilizes the medial side of the joint and extends from the medial epicondyle of the humerus to both the coronoid process and the olecranon of the ulna. ▪ This ligament may be torn with repetitive use of the joint (e.g., as seen with some baseball pitchers). The torn ligament may be replaced with another body tendon to restore elbow function, in a procedure known as Tommy John surgery. In addition, the elbow joint has an anular ligament that surrounds the neck of the radius and binds the proximal head of the radius to the ulna. o The anular ligament helps hold the head of the radius in place. Despite the support from the capsule and ligaments, the elbow joint is subject to damage from severe impacts or unusual stresses. o For example, if you fall on an outstretched hand and the elbow joint is partially flexed, the posterior stress on the ulna combined with contractions of muscles that extend the elbow may break the ulna at the center of the trochlear notch. o Sometimes dislocations result from stresses to the elbow. ▪ This is particularly true when growth is still occurring at the epiphyseal plate, so children and teenagers may be prone to humeral epicondyle dislocations or fractures. Hip Joint The hip joint, also known as the coxal joint, is the articulation between the head of the femur and the relatively deep, concave acetabulum of the os coxae A fibrocartilaginous acetabular labrum further deepens this socket. The hip joint’s more extensive bony architecture is therefore much more substantial and more stable than that of the glenohumeral joint. Conversely, the hip joint’s increased stability means that it is less mobile than the glenohumeral joint. The hip joint must be more stable (and thus less mobile) because it supports the body weight. The hip joint is secured by a strong articular capsule, several ligaments, and a number of powerful muscles. o The articular capsule extends from the acetabulum to the trochanters of the femur, enclosing both the femoral head and neck. ▪ This arrangement prevents the head from moving away from the acetabulum. o The ligamentous fibers of the articular capsule reflect (i.e., fold over) around the neck of the femur ▪ These reflected fibers, called retinacular fibers, provide additional stability to the capsule. Located within the retinacular fibers are retinacular arteries (branches of the deep femoral artery), which supply almost all of the blood to the head and neck of the femur. The articular capsule is reinforced by three spiraling intracapsular ligaments: o The iliofemoral ligament is a Y-shaped ligament that provides strong reinforcement for the anterior region of the articular capsule. o o The ischiofemoral ligament is spiral-shaped and posteriorly located. o The pubofemoral (pyū′bō-fem′ŏ-răl) ligament is a triangular thickening of the capsule’s inferior region. o All of these spiraling ligaments become taut when the femur is extended at the hip joint, so the hip joint is most stable in the extended position. Another tiny ligament, the ligament of head of femur, also called the ligamentum teres, originates along the acetabulum. o Its attachment point is the fovea of the head of the femur. o This ligament does not provide stability to the joint; rather, it typically contains a small artery that supplies the head of the femur. The combination of a deep bony socket, a strong articular capsule, supporting ligaments, and muscular padding gives the hip joint its stability. Movements possible at the hip joint include o Flexion o Extension o Abduction o Adduction o Circumduction o medial and lateral rotation of the femur. Knee Joint The knee joint is the largest and most complex diarthrosis of the body It is primarily a hinge joint, but when the knee is flexed, it is also capable of slight rotation and lateral gliding. Structurally, the knee is composed of two separate articulations: o (1) The tibiofemoral joint is between the condyles of the femur and the condyles of the tibia, o (2) the patellofemoral joint is between the patella and the patellar surface of the femur. The knee joint has an articular capsule that encloses only the medial, lateral, and posterior regions of the knee joint. The articular capsule does not cover the anterior surface of the knee joint; o rather, the quadriceps femoris muscle tendon passes over the knee joint’s anterior surface. ▪ The patella is embedded within this tendon, and the patellar ligament extends beyond the patella and continues to where it attaches on the tibial tuberosity of the tibia. ▪ Thus, there is no single unified capsule in the knee, nor is there a common joint cavity. Posteriorly, the capsule is strengthened by several popliteal ligaments On either side of the knee joint are two collateral ligaments that become taut on extension and provide additional stability to the joint. o The fibular collateral ligament (lateral collateral ligament) reinforces the lateral surface of the joint. ▪ This ligament extends from the femur to the fibula and prevents hyperadduction of the leg at the knee. (In other words, it prevents the leg from moving too far medially relative to the thigh transpose punctuation). o The tibial collateral ligament (medial collateral ligament) reinforces the medial surface of the knee joint. ▪ This ligament runs from the femur to the tibia and prevents hyperabduction of the leg at the knee. (In other words, it prevents the leg from moving too far laterally relative to the thigh.) ▪ This ligament also attaches to the medial meniscus of the knee joint, so an injury to the tibial collateral ligament usually affects the medial meniscus as well. Deep to the articular capsule and within the knee joint itself are a pair of C-shaped fibrocartilage pads positioned on the condyles of the tibia. o These pads are called the medial meniscus and the lateral meniscus. ▪ They partially stabilize the joint medially and laterally, ▪ act as cushions between articular surfaces, ▪ continuously change shape to conform to the articulating surfaces as the femur moves. Two cruciate ligaments are deep to the articular capsule of the knee joint. o They limit the anterior and posterior movement of the femur on the tibia. o These ligaments cross each other in the form of an X, hence the name cruciate (which means cross). ▪ The anterior cruciate ligament (ACL) extends from the posterior femur to the anterior side of the tibia. When the knee is extended, the ACL is pulled tight and prevents hyperextension of the leg at the knee joint. The ACL prevents the tibia from moving too far anteriorly relative to the femur. ▪ The posterior cruciate ligament (PCL) attaches from the anteroinferior femur to the posterior side of the tibia. The PCL becomes taut on flexion, and so it prevents hyperflexion of the leg at the knee joint. The PCL also prevents posterior displacement of the tibia relative to the femur. Humans are bipedal, meaning that we walk on two feet. o An important aspect of bipedal locomotion is the ability to “lock” the knees in the extended position and stand erect without tiring the leg muscles. o At full extension, the tibia rotates laterally so as to tighten the anterior cruciate ligament and squeeze the menisci between the tibia and femur. o Muscular contraction by the popliteus muscle unlocks and flexes the knee joint. Talocrural (Ankle) Joint The talocrural (ankle) joint is a highly modified hinge joint that permits both dorsiflexion and plantar flexion of the foot at the ankle joint. It includes two articulations within one articular capsule. o One articulation is between the distal end of the tibia and the talus; o the other is between the distal end of the fibula and the lateral aspect of the talus o The medial and lateral malleoli of the tibia and fibula, respectively, form extensive medial and lateral margins and prevent the talus from sliding side-to-side. The talocrural joint includes several distinctive anatomic features. o Its articular capsule covers the distal surfaces of the tibia, the medial malleolus, the lateral malleolus, and the talus. ▪ A multipart deltoid ligament (or medial ligament) binds the tibia to the foot on the medial side. This ligament prevents overeversion of the foot. It is incredibly strong and rarely tears; in fact, it typically will pull off the medial malleolus before it ever tears ▪ A much thinner, multipart lateral ligament binds the fibula to the foot on the lateral side. This ligament prevents overinversion of the foot. It is not as strong as the deltoid ligament and is prone to sprains and tears. o Two tibiofibular ligaments (anterior and posterior) bind the tibia to the fibula. Development and Aging of the Joints Joints start to form by the sixth week of development and progressively differentiate during the fetal period. In the area of future fibrous joints, the mesenchyme around the developing bone differentiates into dense regular connective tissue, in cartilaginous joints it differentiates either into fibrocartilage or hyaline cartilage. The development of the synovial joints is more complex. o The most laterally placed mesenchyme forms the articular capsule and supporting ligaments of the joint. o Just medial to this region, the mesenchyme forms the synovial membrane, which then starts secreting synovial fluid into the joint cavity. o The centrally located mesenchyme may be reabsorbed or can form menisci or articular discs, depending upon the type of synovial joint. Prior to the closure of the epiphyseal plates, some injuries to a young person may result in subluxation or fracture of an epiphysis, with potential adverse effects on the future development and health of the joint o the bone may not reach its potential full length, or the individual may develop arthritic-like changes in the joint. Arthritis is a rheumatic (i.e., referring to the joints or muscles) disease that involves damage to articular cartilage o The primary problem that develops in an aging joint is osteoarthritis, also known as degenerative arthritis. o The damage can have various causes, but it usually results from cumulative wear and tear at the joint surface. Just as the strength of a bone is maintained by continual application of stress, the health of joints is directly related to moderate exercise. o Exercise compresses the articular cartilages, causing synovial fluid to be squeezed out of the cartilage and then pulled back inside the cartilage matrix o This flow of fluid gives the chondrocytes within the cartilage the nourishment required to maintain their health. o Exercise also strengthens the muscles that support and stabilize the joint. ▪ However, extreme exercise should be avoided, because it aggravates potential joint problems and may worsen osteoarthritis.