Joints and Skeletal Movement

Classification of Joints on the Basis of Structure and Function

Joints, responsible for movement and stability of the skeleton, can be classified based on structure or function.

Learning Objectives

Differentiate among the types of skeletal joints based on structure and function

Key Takeaways

Key Points

  • Fibrous joints contain fibrous connective tissue and cannot move; fibrous joints include sutures, syndesmoses, and gomphoses.
  • Cartilaginous joints contain cartilage and allow very little movement; there are two types of cartilaginous joints: synchondroses and symphyses.
  • Synovial joints are the only joints that have a space (a synovial cavity filled with fluid) between the adjoining bones.
  • The presence of synovial fluid and an articular capsule give synovial joints the greatest range of movement among the three joint types; however they are the weakest of the joint types.
  • Based on function, joints can be divided into synarthroses, amphiarthroses, and diarthroses.
  • Synarthrosis joints include fibrous joints; amphiarthrosis joints include cartilaginous joints; diarthrosis joints include synovial joints.

Key Terms

  • synovial fluid: a viscous, fluid found in the cavities of synovial joints whose main purpose is to reduce friction between the articular cartilage of synovial joints during movement
  • diarthrosis: a joint that can move freely in various planes
  • synarthrosis: immovable joint in which two bones are connected rigidly by fibrous tissue
  • amphiarthrosis: slightly movable joint in which the surfaces of bones are connected by ligaments or cartilage

Classification of Joints on the Basis of Structure and Function

The point at which two or more bones meet is called a joint or articulation. Joints are responsible for movement (e.g., the movement of limbs) and stability (e.g.,the stability found in the bones of the skull). There are two ways to classify joints: on the basis of their structure or on the basis of their function.

The structural classification divides joints into fibrous, cartilaginous, and synovial joints depending on the material composing the joint and the presence or absence of a cavity in the joint. The functional classification divides joints into three categories: synarthroses, amphiarthroses, and diarthroses.

Fibrous Joints

The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity, or space, present between the bones, so most fibrous joints do not move at all. There are three types of fibrous joints: sutures, syndesmoses, and gomphoses. Sutures are found only in the skull and possess short fibers of connective tissue that hold the skull bones tightly in place.

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Sutures: Sutures are fibrous joints found only in the skull.

Syndesmoses are joints in which the bones are connected by a band of connective tissue, allowing for more movement than in a suture. An example of a syndesmosis is the joint of the tibia and fibula in the ankle. The amount of movement in these types of joints is determined by the length of the connective tissue fibers. Gomphoses occur between teeth and their sockets; the term refers to the way the tooth fits into the socket like a peg. The tooth is connected to the socket by a connective tissue called the periodontal ligament. Fibrous joints classified as synarthroses, or immovable, include: sutures, gomphoses, and synchondroses

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Gomphoses: Gomphoses are fibrous joints between the teeth and their sockets.

Cartilaginous Joints

Cartilaginous joints are those in which the bones are connected by cartilage. There are two types of cartilaginous joints: synchondroses and symphyses. In a synchondrosis, the bones are joined by hyaline cartilage. Synchondroses are found in the epiphyseal plates of growing bones in children. In symphyses, hyaline cartilage covers the end of the bone, but the connection between bones occurs through fibrocartilage. Symphyses are found at the joints between vertebrae and between the pubic bones. Amphiarthroses allow only slight movement; therefore, either type of cartilaginous joint is an amphiarthrosis.

Synovial Joints

Synovial joints are the only joints that have a space between the adjoining bones. This space, referred to as the synovial (or joint) cavity, is filled with synovial fluid. Synovial fluid lubricates the joint, reducing friction between the bones and allowing for greater movement. The ends of the bones are covered with articular cartilage, a hyaline cartilage. The entire joint is surrounded by an articular capsule composed of connective tissue. This allows movement of the joint as well as resistance to dislocation. Articular capsules may also possess ligaments that hold the bones together. Synovial joints are capable of the greatest movement of the three structural joint types; however, the more mobile a joint, the weaker the joint. Knees, elbows, and shoulders are examples of synovial joints. Since they allow for free movement, synovial joints are classified as diarthroses.

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Synovial Joints: Synovial joints are the only joints that have a space or “synovial cavity” in the joint.

Movement at Synovial Joints

Synovial joints allow for many types of movement including gliding, angular, rotational, and special movements.

Learning Objectives

Differentiate among the types of movements possible at synovial joints

Key Takeaways

Key Points

  • Gliding movements occur as relatively flat bone surfaces move past each other, but they produce very little movement of the bones.
  • Angular movements are produced when the angle between the bones of a joint changes; they include flexion, extension, hyperextension, abduction, adduction, and circumduction.
  • Rotational movement involves moving the bone around its longitudinal axis; this can be movement toward the midline of the body (medial rotation) or away from the midline of the body (lateral rotation).
  • Special movements are all the other movements that cannot be classified as gliding, angular, or rotational; these movements include inversion, eversion, protraction, and retraction.
  • Other special movements include elevation, depression, supination, and pronation.

Key Terms

  • adduction: the movement of a bone toward the midline of the body
  • abduction: moving a bone away from the midline of the body
  • supination: the action of rotating the forearm so that the palm of the hand is turned up or forward
  • pronation: the action of rotating the forearm so that the palm of the hand is turned down or back

Movement at Synovial Joints

The range of movement allowed by synovial joints is fairly wide. These movements can be classified as: gliding, angular, rotational, or special movement.

Gliding Movement

Gliding movements occur as relatively flat bone surfaces move past each other. They produce very little rotation or angular movement of the bones. The joints of the carpal and tarsal bones are examples of joints that produce gliding movements.

Angular Movement

Angular movements are produced by changing the angle between the bones of a joint. There are several different types of angular movements, including flexion, extension, hyperextension, abduction, adduction, and circumduction. Flexion, or bending, occurs when the angle between the bones decreases. Moving the forearm upward at the elbow or moving the wrist to move the hand toward the forearm are examples of flexion. In extension, the opposite of flexion, the angle between the bones of a joint increases. Straightening a limb after flexion is an example of extension. Extension past the normal anatomical position is referred to as hyperextension. This includes moving the neck back to look upward or bending the wrist so that the hand moves away from the forearm.

Abduction occurs when a bone moves away from the midline of the body. Examples of abduction include moving the arms or legs laterally to lift them straight out to the side. Adduction is the movement of a bone toward the midline of the body. Movement of the limbs inward after abduction is an example of adduction. Circumduction is the movement of a limb in a circular motion, as in swinging an arm around.

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Angular and rotational movements: Synovial joints give the body many ways in which to move. (a)–(b) Flexion and extension motions are in the sagittal (anterior–posterior) plane of motion. These movements take place at the shoulder, hip, elbow, knee, wrist, metacarpophalangeal, metatarsophalangeal, and interphalangeal joints. (c)–(d) Anterior bending of the head or vertebral column is flexion, while any posterior movement of the head is extension. (e) Abduction and adduction are motions of the limbs, hand, fingers, or toes in the coronal (medial–lateral) plane of movement. Moving the limb or hand laterally away from the body, or spreading the fingers or toes, is abduction. Adduction brings the limb or hand toward or across the midline of the body or brings the fingers or toes together. Circumduction is the movement of the limb, hand, or fingers in a circular pattern, using the sequential combination of flexion, adduction, extension, and abduction motions. Adduction/abduction and circumduction take place at the shoulder, hip, wrist, metacarpophalangeal, and metatarsophalangeal joints. (f) Turning of the head side to side or twisting of the body is rotation. Medial and lateral rotation of the upper limb at the shoulder or lower limb at the hip involves turning the anterior surface of the limb toward the midline of the body (medial or internal rotation) or away from the midline (lateral or external rotation).

Rotational Movement

Rotational movement is the movement of a bone as it rotates around its longitudinal axis. Rotation can be toward the midline of the body, which is referred to as medial rotation, or away from the midline of the body, which is referred to as lateral rotation. Movement of the head from side to side is an example of rotation.

Special Movements

Some movements that cannot be classified as gliding, angular, or rotational are called special movements. Inversion involves moving the soles of the feet inward, toward the midline of the body. Eversion, the opposite of inversion, involves moving of the sole of the foot outward, away from the midline of the body. Protraction is the anterior movement of a bone in the horizontal plane. Retraction occurs as a joint moves back into position after protraction. Protraction and retraction can be seen in the movement of the mandible as the jaw is thrust outwards and then back inwards. Elevation is the movement of a bone upward, such as shrugging the shoulders, lifting the scapulae. Depression is the opposite of elevation and involves moving the bone downward, such as after the shoulders are shrugged and the scapulae return to their normal position from an elevated position. Dorsiflexion is a bending at the ankle such that the toes are lifted toward the knee. Plantarflexion is a bending at the ankle when the heel is lifted, such as when standing on the toes. Supination is the movement of the radius and ulna bones of the forearm so that the palm faces forward or up. Pronation is the opposite movement, in which the palm faces backward or down. Opposition is the movement of the thumb toward the fingers of the same hand, making it possible to grasp and hold objects.

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Special movements: (g) Supination of the forearm turns the palm upward in which the radius and ulna are parallel, while forearm pronation turns the palm downward in which the radius crosses over the ulna to form an “X.” (h) Dorsiflexion of the foot at the ankle joint moves the top of the foot toward the leg, while plantar flexion lifts the heel and points the toes. (i) Eversion of the foot moves the bottom (sole) of the foot away from the midline of the body, while foot inversion faces the sole toward the midline. (j) Protraction of the mandible pushes the chin forward, while retraction pulls the chin back. (k) Depression of the mandible opens the mouth, while elevation closes it. (l) Opposition of the thumb brings the tip of the thumb into contact with the tip of the fingers of the same hand.

Types of Synovial Joints

Synovial joints include planar, hinge, pivot, condyloid, saddle, and ball-and-socket joints, which allow varying types of movement.

Learning Objectives

Differentiate among the six categories of joints based on shape and structure

Key Takeaways

Key Points

  • Planar joints have bones with articulating surfaces that are flat or slightly curved, allowing for limited movement; pivot joints consist of the rounded end of one bone fitting into a ring formed by the other bone to allow rotational movement.
  • Hinge joints act like the hinge of a door; the slightly-rounded end of one bone fits into the slightly-hollow end of the other bone; one bone remains stationary.
  • Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone to allow angular movement along two axes.
  • Saddle joints include concave and convex portions that fit together and allow angular movement; ball-and-socket joints include a rounded, ball-like end of one bone fitting into a cup-like socket of another bone which allows the greatest range of motion.
  • Rheumatologists diagnose and treat joint disorders, which include rheumatoid arthritis and osteoporosis.
  • Immune cells enter joints and the synovium, causing cartilage breakdown, swelling, and inflammation of the joint lining, which breaks down cartilage, resulting in bones rubbing against each other, causing pain.

Key Terms

  • condyloid joint: consists of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone
  • ball-and-socket joint: consists of a rounded, ball-like end of one bone fitting into a cup-like socket of another bone, allowing the first segment to move around an indefinite number of axes which have one common center
  • rheumatoid arthritis: chronic, progressive disease in which the immune system attacks the joints; characterized by pain, inflammation and swelling of the joints, stiffness, weakness, loss of mobility, and deformity

Types of Synovial Joints

Synovial joints are further classified into six different categories on the basis of the shape and structure of the joint. The shape of the joint affects the type of movement permitted by the joint. These joints can be described as planar, hinge, pivot, condyloid, saddle, or ball-and-socket joints.

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Types of synovial joints: The six types of synovial joints allow the body to move in a variety of ways. (a) Pivot joints allow for rotation around an axis, such as between the first and second cervical vertebrae, which allows for side-to-side rotation of the head. (b) The hinge joint of the elbow works like a door hinge. (c) The articulation between the trapezium carpal bone and the first metacarpal bone at the base of the thumb is a saddle joint. (d) Planar (or plane) joints, such as those between the tarsal bones of the foot, allow for limited gliding movements between bones. (e) The radiocarpal joint of the wrist is a condyloid joint. (f) The hip and shoulder joints are the only ball-and-socket joints of the body.

Planar Joints

Planar joints have bones with articulating surfaces that are flat or slightly curved. These joints allow for gliding movements; therefore, the joints are sometimes referred to as gliding joints. The range of motion is limited and does not involve rotation. Planar joints are found in the carpal bones in the hand and the tarsal bones of the foot, as well as between vertebrae.

Hinge Joints

In hinge joints, the slightly-rounded end of one bone fits into the slightly-hollow end of the other bone. In this way, one bone moves while the other remains stationary, similar to the hinge of a door. The elbow is an example of a hinge joint. The knee is sometimes classified as a modified hinge joint.

Pivot Joints

Pivot joints consist of the rounded end of one bone fitting into a ring formed by the other bone. This structure allows rotational movement, as the rounded bone moves around its own axis. An example of a pivot joint is the joint of the first and second vertebrae of the neck that allows the head to move back and forth. The joint of the wrist that allows the palm of the hand to be turned up and down is also a pivot joint.

Condyloid Joints

Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone. This is also sometimes called an ellipsoidal joint. This type of joint allows angular movement along two axes, as seen in the joints of the wrist and fingers, which can move both side to side and up and down.

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Condyloid: The metacarpophalangeal joints in the finger are examples of condyloid joints.

Saddle Joints

Each bone in a saddle joint resembles a saddle, with concave and convex portions that fit together. Saddle joints allow angular movements similar to condyloid joints, but with a greater range of motion. An example of a saddle joint is the thumb joint, which can move back and forth and up and down; it can move more freely than the wrist or fingers.

Ball-and-Socket Joints

Ball-and-socket joints possess a rounded, ball-like end of one bone fitting into a cup-like socket of another bone. This organization allows the greatest range of motion, as all movement types are possible in all directions. Examples of ball-and-socket joints are the shoulder and hip joints.

The Role of Rheumatologists

Rheumatologists are medical doctors who specialize in the diagnosis and treatment of disorders of the joints, muscles, and bones. They diagnose and treat diseases such as arthritis, musculoskeletal disorders, osteoporosis, and autoimmune diseases such as ankylosing spondylitis and rheumatoid arthritis.

Rheumatoid arthritis (RA) is an inflammatory disorder that primarily affects the synovial joints of the hands, feet, and cervical spine. Affected joints become swollen, stiff, and painful. Although it is known that RA is an autoimmune disease in which the body’s immune system mistakenly attacks healthy tissue, the cause of RA remains unknown. Immune cells from the blood enter joints and the synovium, causing cartilage breakdown, swelling, and inflammation of the joint lining. Breakdown of cartilage results in bones rubbing against each other, causing pain. RA is more common in women than men; the age of onset is usually 40–50 years of age.

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Ball-and-socket: The shoulder joint is an example of a ball-and-socket joint.

Rheumatologists diagnose RA on the basis of symptoms (joint inflammation and pain), X-ray and MRI imaging, and blood tests. Arthrography, a type of medical imaging of joints, uses a contrast agent, such as a dye, that is opaque to X-rays. This allows the soft tissue structures of joints, such as cartilage, tendons, and ligaments, to be visualized. An arthrogram differs from a regular X-ray by showing the surface of soft tissues lining the joint in addition to joint bones. An arthrogram allows early degenerative changes in joint cartilage to be detected before bones become affected.

There is currently no cure for RA; however, rheumatologists have a number of treatment options available. Early stages can be treated by resting the affected joints, using a cane or joint splints, to minimize inflammation. When inflammation has decreased, exercise can be used to strengthen the muscles that surround the joint in order to maintain joint flexibility. If joint damage is more extensive, medications can be used to relieve pain and decrease inflammation. Anti-inflammatory drugs such as aspirin, topical pain relievers, and corticosteroid injections may be used. Surgery may be required in cases in which joint damage is severe.

Bone and Joint Disorders

The most common bone and joint disorder are types of arthritis.

Learning Objectives

Describe the causes and treatments for gout, rheumatoid arthritis, and osteoarthritis

Key Takeaways

Key Points

  • Arthritis is a common disorder of synovial joints that involves inflammation of the joint; there are a few major subtypes of this disorder.
  • The most common type of arthritis is osteoarthritis, which is associated with “wear and tear” of cartilage.
  • Gout is a form of arthritis resulting from the deposit of uric acid crystals within a body joint.
  • Rheumatoid arthritis is an autoimmune disease in which the joint capsule and synovial membrane become inflamed.

Key Terms

  • synovial joints: The most common type of joint in the body, which includes a joint cavity.
  • arthritis: A joint disorder that involves inflammation in one or more joints.

Synovial joints are the most common type of joint in the body. A key structural characteristic for a synovial joint that is not seen at fibrous or cartilaginous joints is the presence of a joint cavity.

Arthritis is a common disorder of synovial joints that involves inflammation of the joint. This often results in significant joint pain, along with swelling, stiffness, and reduced joint mobility. There are more than 100 different forms of arthritis. Arthritis may arise from aging, damage to the articular cartilage, autoimmune diseases, bacterial or viral infections, or unknown (probably genetic ) causes.
Arthritis is the most common cause of disability in the USA. More than 20 million individuals with arthritis have severe limitations in function on a daily basis.

Osteoarthritis

The most common type of arthritis is osteoarthritis, which is associated with aging and “wear and tear” of the articular cartilage. Risk factors that may lead to osteoarthritis later in life include injury to a joint; jobs that involve physical labor; sports with running, twisting, or throwing actions; and being overweight.

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Osteoarthritis of the Finger Joints

The formation of hard nobs at the middle finger joints (known as Bouchard’s nodes) and at the farther away finger joint (known as Heberden’s node) are a common feature of Osteoarthritis in the hands.

Osteoarthritis begins in the cartilage and eventually causes the two opposing bones to erode into each other. Osteoarthritis typically affects the weight-bearing joints, such as the back, knee and hip. There is no cure for osteoarthritis, but several treatments (surgery, lifestyle changes, medications) can help alleviate the pain.

Gout

Gout is a form of arthritis that results from the deposit of uric acid crystals within a body joint. Usually only one or a few joints are affected, such as the big toe, knee, or ankle. The attack may only last a few days, but could return to the same or another joint. Gout occurs when the body makes too much uric acid or the kidneys do not properly excrete it. A diet with excessive fructose has been implicated in raising the chances of a susceptible individual developing gout.

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Gout: Gout presenting in the metatarsal-phalangeal joint of the big toe: Note the slight redness of the skin overlying the joint.

Rheumatoid Arthritis

Other forms of arthritis are associated with various autoimmune diseases, bacterial infections of the joint, or unknown genetic causes. Autoimmune diseases like rheumatoid arthritis produce arthritis because the immune system of the body attacks the body joints.

In rheumatoid arthritis, the joint capsule and synovial membrane become inflamed. As the disease progresses, the articular cartilage is severely damaged or destroyed, resulting in joint deformation, loss of movement, and severe disability. The most commonly involved joints are the hands, feet, and cervical spine, with corresponding joints on both sides of the body usually affected, though not always to the same extent.

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Rheumatoid Arthritis: A untreated hand affected by rheumatoid arthritis.