Chapter 9: Joints – Structure, Classification, and Movements

Joints (Articulations)

Definition and Importance

Joints, also known as articulations, are the locations where two or more bones meet. They are essential for providing both stability and mobility to the skeleton, allowing for a wide range of movements while maintaining the integrity of the skeletal structure.

• Articulation: The anatomical term for a joint, derived from the Latin word articulus.

• Joints occur between bones, between cartilage and bones, and between teeth and bones.

• Joints are a compromise between strength and mobility.

• They are the weakest points of the skeleton and are designed to resist movements that could dislocate or misalign the bones.

Classification of Joints

Overview

Joints can be classified based on their structure or their function (degree of movement permitted).

Functional Classification

This classification is based on the amount of movement a joint allows:

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

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

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

Structural Classification

This classification is based on the material binding the bones together and the presence or absence of a joint cavity:

• Fibrous Joints: Bones are joined by dense fibrous connective tissue; no joint cavity. Most are immovable or only slightly movable.

• Cartilaginous Joints: Bones are united by cartilage; no joint cavity. Allow more movement than fibrous joints but less than synovial joints.

• Synovial Joints: Bones are separated by a fluid-filled joint cavity; all are freely movable (diarthroses).

Types of Fibrous Joints

• Sutures: Interlocking joints found only in the skull; allow for growth during youth and ossify in adulthood.

• Syndesmoses: Bones connected by ligaments; movement depends on the length of the connecting fibers (e.g., distal tibiofibular joint).

• Gomphoses: Peg-in-socket joints (e.g., teeth in their sockets, held by the periodontal ligament).

Types of Cartilaginous Joints

• Synchondroses: Bones united by hyaline cartilage (e.g., epiphyseal plates in children, joint between first rib and sternum).

• Symphyses: Bones united by fibrocartilage, providing strength and flexibility (e.g., intervertebral discs, pubic symphysis).

Synovial Joints

General Structure

Synovial joints are the most common and movable type of joint in the body. They have a complex structure that includes:

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

• Joint (synovial) cavity: Space filled with synovial fluid.

• Articular capsule: Double-layered capsule enclosing the joint cavity.

• Synovial fluid: Lubricates and nourishes the articular cartilage.

• Ligaments: Reinforce and strengthen the joint.

• Nerves and blood vessels: Supply the joint tissues.

• Articular discs (menisci): Pads of fibrocartilage that improve fit and stability (e.g., knee joint).

• Bursae and tendon sheaths: Fluid-filled sacs and elongated bursae that reduce friction between moving structures.

Types of Synovial Joints

Synovial joints are classified by the shape of their articulating surfaces and the movements they allow:

• Plane (gliding) joints: Allow sliding or gliding movements (e.g., intercarpal joints).

• Hinge joints: Permit flexion and extension (e.g., elbow, knee).

• Pivot joints: Allow rotation around a single axis (e.g., proximal radioulnar joint, atlas and axis).

• Condyloid (ellipsoid) joints: Permit movement in two planes (e.g., wrist joint).

• Saddle joints: Allow movement in two planes, with greater freedom than condyloid joints (e.g., thumb carpometacarpal joint).

• Ball-and-socket joints: Allow movement in multiple axes and planes (e.g., shoulder, hip).

Movements at Synovial Joints

Types of Movements

• Gliding: Flat bone surfaces slide past one another (e.g., between carpal bones).

• Angular Movements:

  • Flexion: Decreases the angle between bones (e.g., bending the elbow).

  • Extension: Increases the angle between bones (e.g., straightening the knee).

  • Abduction: Movement away from the midline (e.g., raising the arm sideways).

  • Adduction: Movement toward the midline (e.g., lowering the arm to the side).

  • Circumduction: Circular movement combining flexion, extension, abduction, and adduction (e.g., arm circles).

• Rotation: Turning a bone around its own long axis (e.g., turning the head side to side).

• Special Movements:

  • Elevation: Lifting a body part superiorly (e.g., shrugging shoulders).

  • Depression: Moving a body part inferiorly (e.g., lowering the jaw).

  • Protraction: Moving a body part anteriorly (e.g., jutting the jaw forward).

  • Retraction: Moving a body part posteriorly (e.g., pulling the jaw back).

  • Supination: Rotating the forearm so the palm faces anteriorly (radius and ulna are parallel).

  • Pronation: Rotating the forearm so the palm faces posteriorly (radius crosses over ulna).

  • Opposition: Moving the thumb to touch the tips of the other fingers.

  • Dorsiflexion: Lifting the foot upward toward the shin.

  • Plantar flexion: Pointing the toes downward.

  • Inversion: Turning the sole of the foot medially.

  • Eversion: Turning the sole of the foot laterally.

Knee Joint Example

The knee is the largest and most complex synovial joint in the body. It primarily functions as a hinge joint but also allows for slight rotation when flexed. The knee contains fibrocartilage menisci that improve joint stability and absorb shock. Ligaments such as the collateral and cruciate ligaments reinforce the joint and prevent excessive movement.

Summary Table: Joint Classification

Structural Type	Functional Type	Examples	Movement Allowed
Fibrous (Suture)	Synarthrosis	Skull sutures	Immovable
Fibrous (Syndesmosis)	Amphiarthrosis	Distal tibiofibular joint	Slightly movable
Fibrous (Gomphosis)	Synarthrosis	Teeth in sockets	Immovable
Cartilaginous (Synchondrosis)	Synarthrosis	Epiphyseal plate	Immovable
Cartilaginous (Symphysis)	Amphiarthrosis	Pubic symphysis, intervertebral discs	Slightly movable
Synovial	Diarthrosis	Shoulder, knee, hip	Freely movable

Key Terms and Definitions

• Articulation: A joint; the point of contact between bones or between bone and cartilage.

• Ligament: A band of dense connective tissue that connects bones and stabilizes joints.

• Synovial fluid: Viscous fluid within synovial joints that lubricates and nourishes the articular cartilage.

• Meniscus: A crescent-shaped fibrocartilage structure that improves joint fit and absorbs shock.

• Bursa: A small fluid-filled sac that reduces friction between moving structures in a joint.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.