Joints

Chapter 08: Joints

8.1 Classification of Joints

• Definition of Joints (Articulations)

  • Sites where two or more bones meet.

• Functions of Joints

  • Provide mobility to the skeleton.
  • Hold the skeleton together.

• Classification of Joints

  • Structural Classification: Based on the material binding the joints and the presence of a joint cavity.

  • Fibrous Joints:

    • Bones joined by collagen fibers; no joint cavity present.

  • Cartilaginous Joints:

    • Bones united by cartilage; no joint cavity present.

  • Synovial Joints:

    • Characterized by a fluid-filled joint cavity between articulating bones.

  • Functional Classification: Based on the movement the joint allows.

  • Synarthroses: Immovable joints.

  • Amphiarthroses: Slightly movable joints.

  • Diarthroses: Freely movable joints.

  • Note: Diarthroses predominate in the appendicular skeleton (limbs).

8.2 Fibrous Joints

• Definition: Bones joined by dense fibrous connective tissue without a joint cavity.

• Most fibrous joints are immovable (synarthrotic).

• Three Types of Fibrous Joints:

  • Sutures:

  • Immovable joints found in the skull where bone edges interlock.

  • Allow for growth in youth through short connective tissue fibers.

  • Sutures ossify and fuse in middle age, becoming immovable (synostoses) for brain protection.

  • Syndesmoses:

  • Fibrous joints connected exclusively by ligaments made of fibrous connective tissue.

  • Movement: Variable based on fiber length.

    • Short fibers: Minimal movement (e.g., inferior tibiofibular joint).
    • Longer fibers: Allow slight movement (e.g., interosseous membrane connecting radius and ulna).

  • Gomphoses:

  • Peg-in-socket joints found in the teeth and alveolar sockets.

  • The periodontal ligament is the fibrous connection holding the tooth in the socket.

8.3 Cartilaginous Joints

• Definition: Bones united by cartilage, similar to fibrous joints without a joint cavity.

• Not highly movable.

• Two Types of Cartilaginous Joints:

  • Synchondroses:

  • Unite bones with a bar or plate of hyaline cartilage.

  • Most are synarthrotic (immovable).

  • Examples:

    • Temporary epiphyseal plate joints becoming synostoses after plate closure.
    • Cartilage of the first rib connected to the manubrium of the sternum.

  • Symphyses:

  • Unite bone with fibrocartilage, forming strong, amphiarthrotic (slightly movable) joints.

  • Examples:

    • Intervertebral joints.
    • Pubic symphysis.

8.4 Synovial Joints

• Definition: Joints that separate bones with a fluid-filled joint cavity, making all diarthrotic (freely movable).
• Include almost all limb joints.
• Characteristics of Synovial Joints:
  • Six general features.
  • Presence of bursae and tendon sheaths.
  • Stability is influenced by three factors:
  • Shape of articular surfaces.
  • The number and location of ligaments.
  • Muscle tone, which reinforces the joint's stability.
  • Allow various types of movements and can be classified into six different types.

8.4.1 General Structure of Synovial Joints

• Features:
  • Articular Cartilage: Hyaline cartilage covering bone ends to prevent crushing.
  • Joint (Synovial) Cavity: Small, fluid-filled space unique to synovial joints.
  • Articular Capsule: Two layers thick:
  • External Fibrous Layer: Dense irregular connective tissue.
  • Inner Synovial Membrane: Loose connective tissue secreting synovial fluid.
  • Synovial Fluid: Viscous fluid consisting of plasma filtrate and hyaluronic acid.
  • Functions: Lubricates and nourishes articular cartilage; contains phagocytic cells that remove microbes and debris.
  • Reinforcing Ligaments:
  • Capsular: Thickened parts of the fibrous layer.
  • Extracapsular: Ligaments outside the capsule.
  • Intracapsular: Located deep to the capsule, covered by synovial membrane.
  • Nerves and Blood Vessels:
  • Nerves detect pain, monitor joint position and stretch.
  • Capillary beds supply filtrate for synovial fluid.

8.4.2 Additional Features of Some Synovial Joints

• Fatty Pads: Cushioning structures between the fibrous layer of the capsule and synovial membrane or bone.
• Articular Discs (Menisci): Fibrocartilage structures that improve the fit of bone ends, stabilize joints, and reduce wear and tear.

8.4.3 Bursae and Tendon Sheaths

• Bursae: Fluid-filled sacs decreasing friction where ligaments, muscles, tendons, or bones rub together; closely associated but not strictly part of synovial joints.
• Tendon Sheaths: Elongated bursae wrapped around tendons subjected to friction.

8.4.4 Factors Influencing Stability of Synovial Joints

• Three Key Factors:
  • Shape of Articular Surfaces: Minor role; shallow surfaces are less stable than ball-and-socket designs.
  • Ligament Number and Location: Limited role; more ligaments generally enhance strength.
  • Muscle Tone: Most critical; keeps tendons taut across joints and reinforces the shoulder and knee joints and foot arches.

8.4.5 Movements Allowed by Synovial Joints

• Range of Motion: Defined by four categories of movement:

  • Nonaxial Movement: Gliding movements only.
  • Uniaxial Movement: Movement in one plane.
  • Biaxial Movement: Movement in two planes.
  • Multiaxial Movement: Movement in or around all three planes.

• Types of Movements:

  • Gliding Movements:

  • Defined as one flat bone surface gliding over another.

  • Examples include intercarpal joints, intertarsal joints, and between vertebral articular processes.

  • Angular Movements:

  • Increase or decrease the angle between two bones, often along the sagittal plane.

  • Includes:

    • Flexion: Decreases the angle of a joint.
    • Extension: Increases the angle of a joint.
    • Hyperextension: Movement beyond the anatomical position.

  • Additional Angular Movements:

  • Abduction: Movement away from the midline in the frontal plane.

  • Adduction: Movement toward the midline in the frontal plane.

  • Circumduction: Movement where a limb describes a cone in space, combining flexion, abduction, extension, and adduction.

  • Rotation: Turning of a bone around its long axis; can be medial (toward midline) or lateral (away from midline).

  • Special Movements:

  • Supination: Palms face anteriorly/superiorly, with radius and ulna parallel.

  • Pronation: Palms face posteriorly, with the radius rotating over the ulna.

  • Opposition: Thumb movement towards tips of fingers on the same hand.

  • Dorsiflexion: Bending foot toward the shin.

  • Plantar Flexion: Pointing toes.

  • Inversion: Sole of the foot faces medially.

  • Eversion: Sole of the foot faces laterally.

  • Elevation: Lifting the body part superiorly (e.g., moving mandible up).

  • Depression: Lowering the body part (e.g., moving mandible down).

  • Protraction: Movement of the body part in the lateral plane (e.g., mandible juts out).

  • Retraction: Movement of the body part back toward the neck (e.g., mandible pulled back).

• Table 8.2 Movements at Synovial Joints:

  • Gliding: Sliding flat surfaces of two bones across each other.
  • Angular Movements:
  • Flexion: Decreasing angle between two bones, usually in the sagittal plane.
  • Extension: Increasing angle between two bones, usually in the sagittal plane.
  • Abduction: Moving a limb away from the body midline in the frontal plane.
  • Adduction: Moving a limb toward the body midline in the frontal plane.
  • Circumduction: Moving a limb or finger to describe a cone in space.
  • Rotation: Turning a bone around its longitudinal axis; can be medial or lateral.

8.5 Types of Synovial Joints

• Classification: Based on the shape of articular surfaces and the movement capability, categorized into six types:
  • Plane Joints: Flat articular surfaces; nonaxial movement (gliding).
  • Hinge Joints: Cylinder trough shape; uniaxial movement (flexion and extension).
  • Pivot Joints: Sleeve (bone and ligament) and axle (rounded bone); uniaxial movement (rotation).
  • Condylar (or Ellipsoid) Joints: Oval articular surfaces; biaxial movement (flexion, extension, adduction, and abduction).
  • Saddle Joints: Articular surfaces are both concave and convex; biaxial movement.
  • Ball and Socket Joints: Cup-shaped socket; multiaxial movement (flexion, extension, adduction, abduction, rotation).

8.6 Selected Synovial Joints

• Diversity in Synovial Joints: Despite general features, some joints display unique structural features, abilities, and weaknesses.
• Five Main Synovial Joints:
  • Temporomandibular Joint (TMJ): Modified hinge joint allowing hinge and gliding movement.
  • Most easily dislocated joint in the body.
  • Shoulder Joint (Glenohumeral Joint): Most freely moving joint; ball-and-socket structure with reinforcement from muscle tendons and ligaments.
  • Elbow Joint: Hinge joint allowing flexion and extension, characterized by trochlear notch of ulna articulating with trochlea of humerus.
  • Hip Joint (Coxal Joint): Strong ball-and-socket joint with deep acetabulum allowing stable weight bearing.
  • Knee Joint: Largest and most complex joint, formed by three joints within a single cavity allowing flexion, extension, and some rotation.

8.7 Disorders of Joints

• Types of Joint Damage: Joints are prone to injury, inflammation, and degeneration.
• Common Joint Injuries:
  • Sprains: Stretching or tearing of ligaments, common in ankle, knee, and lumbar back.
  • Dislocations (Luxations): Bones forced out of alignment, often with sprains and inflammation.
  • Cartilage tears (Torn Menisci): Result from compression and shear stress; typically remain torn due to lack of vascular supply.

Common Joint Injuries

• Cartilage Tears:
  • Usually healed with arthroscopic surgery; partial meniscus removal can stabilize but reduce mobility, while complete removal risks osteoarthritis.
• Sprains:
  • Reinforcing ligaments may need surgical intervention if torn completely; otherwise, rest, ice, and time promote healing.
• Dislocations: Require reduction to treat; can result in subluxation (partial dislocation).

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation due to friction or impact; treated with rest and anti-inflammatory drugs if severe.
• Tendonitis: Inflammation of tendon sheaths often due to overuse; treatment mirrors that of bursitis.
• Arthritis: Broad category of >100 inflammatory/degenerative diseases damaging joints, leading to pain, stiffness, and swelling.
  • Common types include osteoarthritis (OA), rheumatoid arthritis (RA), and gouty arthritis.
  • Osteoarthritis (OA): Most prevalent; irreversible and degenerative, leading to cartilage breakdown.
  • Rheumatoid Arthritis (RA): Chronic autoimmune condition with bilateral pain and joint swelling; affects more females.
  • Gouty Arthritis: Caused by needle-like uric acid crystal deposition; affects primarily males and often impacts the big toe.
• Lyme Disease: Caused by tick bites leading to symptoms that may evolve into joint arthritis; treated with antibiotics.

8.8 Summary of Key Points

• Joints play crucial roles in mobility and stability of the skeleton.
• Understanding types of joints and their movements is essential for anatomy and physiology.
• Knowledge of joint disorders aids in prevention and management strategies in clinical practice.