Anatomy & Physiology: Joints (Articulations) – Structure, Classification, and Movements

Joints: Introduction and Overview

Definition and Functions of Joints

• Joints (also called articulations) are sites where two or more bones meet.

• Functions of joints include:

  • Giving the skeleton mobility

  • Holding the skeleton together

• Joints are classified in two main ways:

  • Functional classification – based on the amount of movement allowed

  • Structural classification – based on the material binding bones together and the presence or absence of a joint cavity

Classification of Joints

Functional Classification of Joints

• Based on the amount of movement a joint allows.

• Three functional classifications:

  • Synarthroses – immovable joints

  • Amphiarthroses – slightly movable joints

  • Diarthroses – freely movable joints

Structural Classification of Joints

• Based on the material binding bones together and the presence or absence of a joint cavity.

• Three structural classifications:

  • Fibrous joints

  • Cartilaginous joints

  • Synovial joints

Fibrous Joints

General Features

• Bones are joined by dense fibrous connective tissue.

• No joint cavity is present.

• Most are synarthrotic (immovable), but the degree of movement depends on the length of connective tissue fibers.

• Three main types:

  • Sutures

  • Syndesmoses

  • Gomphoses

Sutures

• Rigid, interlocking joints found only in the skull.

• Immovable, providing protection for the brain.

• Short connective tissue fibers allow for growth during youth; in middle age, sutures ossify and fuse, becoming synostoses.

Syndesmoses

• Bones connected by ligaments (bands of fibrous tissue).

• Fiber length varies, so movement varies:

  • Little to no movement at the distal tibiofibular joint.

  • Large amount of movement at the interosseous membrane connecting the radius and ulna.

Gomphoses

• Peg-in-socket joints of teeth in alveolar sockets.

• The fibrous connection is the periodontal ligament.

Cartilaginous Joints

General Features

• Bones are united by cartilage.

• No joint cavity is present.

• Not highly movable.

• Two types:

  • Synchondroses

  • Symphyses

Synchondroses

• Bar or plate of hyaline cartilage unites the bones.

• Most are synarthrotic (immovable).

• Examples:

  • Epiphyseal plate joints in long bones of children

  • Joint between the first rib and the manubrium of the sternum

Symphyses

• Fibrocartilage unites the bones.

• Hyaline cartilage is also present as articular cartilage on bony surfaces.

• Strong, flexible amphiarthroses (slightly movable joints).

• Examples:

  • Intervertebral joints

  • Pubic symphysis

Synovial Joints

General Features

• Bones are separated by a fluid-filled joint cavity.

• All are diarthrotic (freely movable).

• Include almost all limb joints and most joints of the body.

Six Distinguishing Features of Synovial Joints

1. Articular cartilage: Hyaline cartilage covers opposing bone surfaces, preventing crushing of bone ends.

2. Joint (synovial) cavity: Small, fluid-filled potential space.

3. Articular (joint) capsule: Two layers – external fibrous layer (dense irregular connective tissue) and inner synovial membrane (loose connective tissue that makes synovial fluid).

4. Synovial fluid: Viscous, slippery filtrate of plasma and hyaluronic acid; lubricates and nourishes articular cartilage; contains phagocytic cells to remove microbes and debris.

5. Reinforcing ligaments: Capsular (thickened part of fibrous layer), extracapsular (outside capsule), and intracapsular (deep to capsule, covered by synovial membrane).

6. Nerves and blood vessels: Nerves detect pain, monitor joint position and stretch; capillary beds supply filtrate for synovial fluid.

Other Features of Some Synovial Joints

• Fatty pads: For cushioning between fibrous layer and synovial membrane or bone.

• Articular discs (menisci): Fibrocartilage separates articular surfaces to improve fit, stabilize joint, and reduce wear and tear.

• Bursae: Sacs lined with synovial membrane, containing synovial fluid; reduce friction where ligaments, muscles, skin, tendons, or bones rub together.

• Tendon sheaths: Elongated bursae wrapped completely around tendons subjected to friction.

Stabilizing Factors at Synovial Joints

• Shapes of articular surfaces (minor role).

• Ligament number and location (limited role).

• Muscle tendons that cross joint (most important):

  • Tendons are kept under tension by muscle tone, which is extremely important in reinforcing shoulder and knee joints and arches of the foot.

Movements at Synovial Joints

Range of Motion

• Nonaxial: Slipping movements only.

• Uniaxial: Movement in one plane.

• Biaxial: Movement in two planes.

• Multiaxial: Movement in or around all three planes.

Types of Movements

1. Gliding: One flat bone surface glides or slips over another similar surface (e.g., intercarpal joints, intertarsal joints, articular processes of vertebrae).

2. Angular movements:

   • Flexion: Decreases the angle of the joint.

   • Extension: Increases the angle of the joint.

   • Hyperextension: Extension beyond the normal range of motion.

   • Abduction: Movement away from the midline.

   • Adduction: Movement toward the midline.

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

3. Rotation: Turning of a bone around its own long axis (e.g., between C1 and C2 vertebrae, rotation of humerus and femur).

Muscle Attachments and Movements

• All muscles attach to bone or connective tissue at no fewer than two points:

  • Origin: Attachment to immovable bone

  • Insertion: Attachment to movable bone

• Muscle contraction causes insertion to move toward origin.

• Movements occur along transverse, frontal, or sagittal planes.

Summary Table: Characteristics of Body Joints

Joint Type	Structural Classification	Functional Classification	Movements Allowed	Examples
Fibrous: Suture	Fibrous	Synarthrosis	None	Sutures of the skull
Fibrous: Syndesmosis	Fibrous	Amphiarthrosis	Slight movement	Distal tibiofibular joint
Fibrous: Gomphosis	Fibrous	Synarthrosis	None	Tooth in alveolar socket
Cartilaginous: Synchondrosis	Cartilaginous	Synarthrosis	None	Epiphyseal plate, first sternocostal joint
Cartilaginous: Symphysis	Cartilaginous	Amphiarthrosis	Slight movement	Intervertebral discs, pubic symphysis
Synovial	Synovial	Diarthrosis	Freely movable	Shoulder, knee, hip, elbow

Additional info:

• For more detailed joint types and movement examples, consult a full anatomy textbook or lecture notes.