chapter 8 articulation

Joints: Classification and Structure

Classification of Joints

Joints, or articulations, are connections between bones that allow for varying degrees of movement. They are classified both structurally and functionally.

• Structural Classification:

  • Bony (Synostosis): Immovable joints where two bones fuse (e.g., frontal bone).

  • Fibrous: Bones joined by dense connective tissue; little or no movement (e.g., sutures of the skull).

  • Cartilaginous: Bones joined by cartilage; slight movement (e.g., intervertebral discs between vertebrae).

  • Synovial: Bones separated by a fluid-filled cavity; freely movable (e.g., shoulder, knee, elbow joints).

• Functional Classification:

  • Synarthrotic: Immovable joints (e.g., sutures of the skull).

  • Amphiarthrotic: Slightly movable joints (e.g., intervertebral discs).

  • Diarthrotic: Freely movable joints (e.g., shoulder, knee joints).

Types of Fibrous Joints

Fibrous joints are connected by dense connective tissue and allow minimal movement. The main types include:

• Sutures: Found between skull bones; minimal movement due to tight connection.

• Gomphoses: Peg-in-socket joints (e.g., teeth in alveolar sockets of maxilla and mandible).

• Syndesmoses: Bones connected by ligaments; allow more movement than sutures (e.g., distal joint between tibia and fibula).

Types of Cartilaginous Joints

Cartilaginous joints unite bones with cartilage and allow limited movement. The two main types are:

• Synchondroses: Bones joined by hyaline cartilage (e.g., epiphyseal plate in children’s long bones).

• Symphyses: Bones joined by fibrocartilage; amphiarthrotic (e.g., intervertebral discs between vertebrae, pubic symphysis).

Structure of Synovial Joints

Synovial joints are the most movable type of joint in the body. They have a complex structure that allows for a wide range of motion.

• Articular Cartilage: Covers the ends of bones, reducing friction and absorbing shock.

• Synovial Cavity: Space filled with synovial fluid, allowing free movement.

• Articular Capsule: Double-layered structure enclosing the joint cavity.

• Synovial Fluid: Lubricates the joint, nourishes cartilage, and removes waste.

• Ligaments: Connect bone to bone, providing stability.

• Nerves and Blood Vessels: Supply sensation and nutrition to the joint.

• Associated Structures:

  • Tendons: Connect muscle to bone.

  • Bursae: Fluid-filled sacs reducing friction between tissues.

  • Tendon Sheaths: Tubular bursae that wrap around tendons.

Characteristics of Synovial Joints

• Diarthrotic: Freely movable.

• Components: Articular cartilage, synovial cavity, synovial fluid, articular capsule, ligaments, nerves, and blood vessels.

Muscle Attachment: Origin and Insertion

• Origin: The fixed attachment point of a muscle, usually proximal.

• Insertion: The movable attachment point, usually distal; moves during muscle contraction.

Movements of Synovial Joints

Synovial joints allow a variety of movements, each with specific terminology:

• Gliding: Sliding motion between flat surfaces (e.g., intercarpal joints).

• Flexion: Decreases the angle between bones.

• Extension: Increases the angle between bones.

• Hyperextension: Extension beyond the normal range.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Supination: Rotating the forearm to turn the palm up ("hold a soup").

• Pronation: Rotating the forearm to turn the palm down ("pour a soup").

• Elevation: Lifting a body part upward (e.g., closing the mouth).

• Depression: Moving a body part downward (e.g., opening the mouth).

• Protraction: Moving a body part forward (e.g., mandible).

• Retraction: Moving a body part backward (e.g., mandible).

• Opposition: Moving the thumb to touch the fingertips.

• Dorsiflexion: Lifting the foot upward (standing on heels).

• Plantar Flexion: Pointing the foot downward (standing on tiptoes).

Classes of Synovial Joints

Synovial joints are further classified by the shapes of their articulating surfaces and the types of movement they allow. The main types are:

Joint Type	Movement	Example
Plane (Gliding)	Sliding or gliding	Intercarpal joints
Hinge	Flexion and extension	Elbow, knee
Condyloid (Ellipsoid)	Flexion, extension, abduction, adduction, circumduction	Wrist joint
Pivot	Rotation	Proximal radioulnar joint
Saddle	Flexion, extension, abduction, adduction, circumduction	Thumb (first carpometacarpal joint)
Ball and Socket	All movements, including rotation	Shoulder, hip

Structure of the Knee Joint

The knee joint is a complex synovial joint that allows for flexion, extension, and slight rotation. It is stabilized by several ligaments and contains menisci for shock absorption.

• Articular Cartilage: Covers the femoral and tibial surfaces.

• Menisci: Two fibrocartilage pads that improve fit and absorb shock.

• Ligaments: Include the anterior and posterior cruciate ligaments (ACL, PCL), medial and lateral collateral ligaments (MCL, LCL).

• Synovial Membrane: Lines the joint capsule and secretes synovial fluid.

• Bursae: Reduce friction between structures.

Additional info: The knee is the largest and most complex synovial joint in the body, making it susceptible to injury, especially in athletes.