Joints

Joints

  • Rigid elements of the skeleton meet at joints or articulations.
  • Greek root arthro means joint
  • Structure of joints enables resistance to crushing, tearing, and other forces.

Classifications of Joints

  • Joints can be classified by function or structure
  • Functional classification is based on amount of movement
    • Synarthroses—immovable; common in axial skeleton
    • Amphiarthroses—slightly movable; common in axial skeleton
    • Diarthroses—freely movable; common in appendicular skeleton (all synovial joints)
  • Structural classification is based on:
    • Material that binds bones together
    • Presence or absence of a joint cavity
    • Structural classifications include:
      • Fibrous
      • Cartilaginous
      • Synovial

Summary of Joint Classes

  • Fibrous:
    • Structural Characteristics: Adjoining bones united by collagenic fibers
    • Types:
      • Suture (short fibers)
      • Syndesmosis (longer fibers)
      • Gomphosis (periodontal ligament)
    • Mobility: Immobile (synarthrosis), Slightly mobile (amphiarthrosis) and immobile
  • Cartilaginous:
    • Structural Characteristics: Adjoining bones united by cartilage
    • Types:
      • Synchondrosis (hyaline cartilage) - Immobile
      • Symphysis (fibrocartilage) - Slightly movable
  • Synovial:
    • Structural Characteristics: Adjoining bones separated by a joint cavity, covered with articular cartilage, and enclosed within an articular capsule lined with synovial membrane
    • Types:
      • Plane
      • Hinge
      • Pivot
      • Condylar
      • Saddle
      • Ball-and-socket
    • Mobility: Freely movable (diarthrosis); movements depend on design of joint

Fibrous Joints

  • Bones are connected by fibrous connective tissue
  • Do not have a joint cavity
  • Most are immovable or slightly movable
  • Types
    • Sutures
    • Syndesmoses
    • Gomphoses

Sutures

  • Bones are tightly bound by a minimal amount of fibrous tissue
  • Occur only between the bones of the skull
  • Allow bone growth so the skull can expand with brain during childhood
  • Fibrous tissue ossifies in middle age
  • Synostoses—closed sutures

Syndesmoses

  • Bones are connected exclusively by ligaments
  • Amount of movement depends on length of fibers
  • Tibiofibular joint—immovable synarthrosis
  • Interosseous membrane between radius and ulna
    • Freely movable diarthrosis

Gomphoses

  • Tooth in a socket
  • Connecting ligament-the periodontal ligament

Cartilaginous Joints

  • Bones are united by cartilage
  • Lack a joint cavity
  • Two types
    • Synchondroses
    • Symphyses

Synchondroses

  • Hyaline cartilage unites bones
  • Epiphyseal plates
  • Joint between first rib and manubrium

Symphyses

  • Fibrocartilage unites bones; resists tension and compression
  • Slightly movable joints that provide strength with flexibility
  • Intervertebral discs
  • Pubic symphysis
  • Hyaline cartilage—present as articular cartilage

Synovial Joints

  • Most movable type of joint
  • All are diarthroses
  • Each contains a fluid-filled joint cavity

General Structure of Synovial Joints

  • Articular cartilage
    • Ends of opposing bones are covered with hyaline cartilage
    • Absorbs compression
  • Joint (articular) cavity
    • Unique to synovial joints
    • Cavity is a potential space that holds a small amount of synovial fluid
  • Articular capsule—joint cavity is enclosed in a two-layered capsule
    • Fibrous layer—dense irregular connective tissue, which strengthens joint
    • Synovial membrane—loose connective tissue
      • Lines joint capsule and covers internal joint surfaces
      • Functions to make synovial fluid
  • Synovial fluid
    • A viscous fluid similar to raw egg white
    • A filtrate of blood
    • Arises from capillaries in synovial membrane
    • Contains glycoprotein molecules secreted by fibroblasts
    • Weeping lubrication—Pressure on joints squeezes synovial fluid into and out of articular cartilage
  • Reinforcing ligaments
    • Often are thickened parts of the fibrous layer
    • Sometimes are extracapsular ligaments—located outside the capsule
    • Sometimes are intracapsular ligaments—located internal to the capsule
  • Richly supplied with sensory nerves
    • Detect pain
    • Most monitor how much the capsule is being stretched
  • Have a rich blood supply
    • Most supply the synovial membrane
    • Extensive capillary beds produce basis of synovial fluid
    • Branches of several major nerves and blood vessels

Synovial Joints with Articular Discs

  • Some synovial joints contain an articular disc
  • Occur in the temporomandibular joint and at the knee joint
  • Occur in joints whose articulating bones have somewhat different shapes

How Synovial Joints Function

  • Synovial joints—lubricating devices
  • Friction could overheat and destroy joint tissue
  • Are subjected to compressive forces
  • Fluid is squeezed out as opposing cartilages touch
  • Cartilages ride on the slippery film

Bursae and Tendon Sheaths

  • Bursae and tendon sheaths are not synovial joints
  • Closed bags of lubricant
  • Reduce friction between body elements
  • Bursa—a flattened fibrous sac lined by a synovial membrane
  • Tendon sheath—an elongated bursa that wraps around a tendon

Movements Allowed by Synovial Joints

  • Three basic types of movement
    • Gliding—one bone across the surface of another
    • Angular movement—movements change the angle between bones
    • Rotation—movement around a bone's long axis

Gliding Joints

  • Flat surfaces of two bones slip across each other
  • Gliding occurs between:
    • Carpals
    • Articular processes of vertebrae
    • Tarsals

Angular Movements

  • Increase or decrease angle between bones
  • Movements involve:
    • Flexion and extension
    • Abduction and adduction
    • Circumduction

Rotation

  • Involves turning movement of a bone around its long axis
  • The only movement allowed between atlas and axis vertebrae
  • Occurs at the hip and shoulder joints

Special Movements

  • Elevation-lifting a body part superiorly
  • Depression-moving the elevated part inferiorly
  • Protraction-nonangular movement anteriorly
  • Retraction-nonangular movement posteriorly
  • Supination—forearm rotates laterally, palm faces anteriorly
  • Pronation—forearm rotates medially, palm faces posteriorly
    • Brings radius across the ulna
  • Opposition—thumb moves across the palm to touch the tips of other fingers
  • Inversion and eversion
    • Special movements at the foot
    • Inversion—turns sole medially
    • Eversion—turns sole laterally
  • Dorsiflexion and plantar flexion
    • Up-and-down movements of the foot
    • Dorsiflexion—lifting the foot so its superior surface approaches the shin
    • Plantar flexion—depressing the foot, elevating the heel

Synovial Joints Classified by Shape

  • Plane joint
    • Articular surfaces are flat planes
    • Short gliding movements are allowed
    • Intertarsal and intercarpal joints
    • Movements are nonaxial
    • Gliding does not involve rotation around any axis
  • Hinge joints
    • Cylindrical end of one bone fits into a trough on another bone
    • Angular movement is allowed in one plane
    • Elbow, ankle, and joints between phalanges
    • Movement is uniaxial—allows movement around one axis only
  • Pivot joints
    • Classified as uniaxial—rotating bone turns only around its long axis
    • Examples
      • Proximal radioulnar joint
      • Joint between atlas and axis
  • Condylar (or ellipsoid) joints
    • Allow moving bone to travel
      • Side to side—abduction–adduction
      • Back and forth—flexion–extension
    • Classified as biaxial—movement occurs around two axes
  • Saddle joints
    • Each articular surface has concave and convex surfaces
    • Classified as biaxial joints
    • 1st carpometacarpal joint is a good example
    • Allows opposition of the thumb
  • Ball-and-socket joints
    • Spherical head of one bone fits into round socket of another
    • Classified as multiaxial—allow movement in all axes
    • Shoulder and hip joints are examples

Factors Influencing Stability of Synovial Joints

  • Articular surfaces
    • Shapes of articulating surfaces determine movements possible
    • Seldom play a major role in joint stability
    • Exceptions that do provide stability
      • Hip joint, elbow joint, and ankle
  • Ligaments
    • Capsules and ligaments prevent excessive motions
    • Ligaments on the medial or inferior side of a joint
      • Prevent excessive abduction
    • Lateral or superiorly located ligaments
      • Resist adduction
    • Ligaments on the anterior side of a joint
      • Resist extension and lateral rotation
    • Ligaments on the posterior side of a joint
      • Resist flexion and medial rotation
    • The more ligaments, usually the stronger and more stable the joint
  • Muscle tone
    • Helps stabilize joints by keeping tension on tendons
    • Is important in reinforcing:
      • Shoulder and knee joints
      • Supporting joints in arches of the foot

Selected Synovial Joints

  • Sternoclavicular joint
    • Is a saddle joint
    • Four ligaments surround the joint
      • Anterior and posterior sternoclavicular ligaments
      • Interclavicular ligament
      • Costoclavicular ligament
    • Performs multiple complex movements
  • Temporomandibular joint
    • Is a modified hinge joint
    • The head of the mandible articulates with the temporal bone
    • Lateral excursion is a side-to-side movement
    • Two surfaces of the articular disc allow
      • Hingelike movement
      • Gliding of superior surface anteriorly
  • Shoulder (glenohumeral) joint
    • The most freely movable joint lacks stability
    • Articular capsule is thin and loose
    • Muscle tendons contribute to joint stability
  • Glenohumeral Joint
    • The rotator cuff is made up of four muscles and their associated tendons
      • Subscapularis
      • Supraspinatus
      • Infraspinatus
      • Teres minor
    • Rotator cuff injuries are common shoulder injuries
  • Elbow joint
    • Allows flexion and extension
    • Articulation of the humerus with the trochlear notch of the ulna forms the hinge
    • Tendons of biceps and triceps brachii provide stability
  • Wrist Joint
    • Stabilized by numerous ligaments
    • Composed of radiocarpal and intercarpal joint
      • Radiocarpal joint—joint between the radius and proximal carpals (the scaphoid and lunate)
        • Allows for flexion, extension, adduction, abduction, and circumduction
      • Intercarpal joint—joint between the proximal and distal rows or carpals
        • Allows for gliding movement
  • Hip joint
    • A ball-and-socket structure
    • Movements occur in all axes
    • Limited by ligaments and acetabulum
    • Head of femur articulates with acetabulum
    • Stability comes chiefly from acetabulum and capsular ligaments
    • Muscle tendons contribute somewhat to stability
    • The actions of the muscles that cross the hip include flexion, extension, abduction, adduction, lateral rotation and medial rotation.
  • Knee joint
    • The largest and most complex joint
    • Acts primarily as a hinge joint
    • Has some capacity for rotation when leg is flexed
    • Structurally considered compound and bicondyloid
    • Two fibrocartilage menisci occur within the joint cavity
      • Femoropatellar joint—shares the joint cavity
        • Allows patella to glide across the distal femur
    • Capsule of the knee joint
      • Covers posterior and lateral aspects of the knee
      • Covers tibial and femoral condyles
      • Does not cover the anterior aspect of the knee
        • Anteriorly covered by three ligaments
          • Patellar ligament
          • Medial and lateral patellar retinacula
    • Ligaments of the knee joint
      • Become taut when knee is extended
      • These extracapsular and capsular ligaments are
        • Fibular and tibial collateral ligament
        • Oblique popliteal ligament
        • Arcuate popliteal ligament
    • Intracapsular ligaments
      • Cruciate ligaments
        • Cross each other like an “X”
        • Each cruciate ligament runs from the proximal tibia to the distal femur
          • Anterior cruciate ligament
          • Posterior cruciate ligament
        • Cruciate ligaments—prevent undesirable movements at the knee
          • Anterior cruciate ligament—prevents anterior sliding of the tibia
          • Posterior cruciate ligament—prevents forward sliding of the femur or backward displacement of the tibia
  • Ankle joint
    • A hinge joint between:
      • United inferior ends of tibia and fibula
      • The talus of the foot
      • Allows the movements dorsiflexion and plantar flexion only
    • Medially and laterally stabilized by ligaments
      • Medial (deltoid) ligament
      • Lateral ligament
      • Inferior ends of tibia and fibula are joined by ligaments
        • Anterior and posterior tibiofibular ligaments

Disorders of Joints

  • Structure of joints makes them prone to traumatic stress
  • Function of joints makes them subject to friction and wear
  • Affected by inflammatory and degenerative processes

Joint Injuries

  • Torn cartilage—common injury to meniscus of knee joint
  • Sprains—ligaments of a reinforcing joint are stretched or torn
  • Dislocation—occurs when the bones of a joint are forced out of alignment

Inflammatory and Degenerative Conditions

  • Bursitis—inflammation of a bursa due to injury or friction
  • Tendonitis—inflammation of a tendon sheath
  • Arthritis—describes over 100 kinds of joint- damaging diseases
    • Osteoarthritis—most common type of “wear and tear” arthritis
    • Rheumatoid arthritis—a chronic inflammatory disorder
    • Gouty arthritis (gout)—uric acid buildup causes pain in joints
    • Lyme disease—inflammatory disease often resulting in joint pain

The Joints Throughout Life

  • Synovial joints develop from mesenchyme
  • By week 8 of fetal development, joints resemble adult joints
    • Outer region of mesenchyme becomes fibrous joint capsule
    • Inner region becomes the joint cavity
  • During youth—injury may tear an epiphysis off a bone shaft
  • Advancing age—osteoarthritis becomes more common
  • Exercise—helps maintain joint health