Articulations (Joints): Structure, Classification, and Movements

Articulations (Joints)

Introduction

Articulations, or joints, are anatomical structures that connect two bones. They are essential for movement, stability, and the overall function of the skeletal system. Joints can be classified by their structure or by their function (degree of movement allowed).

Classification of Joints by Function

Functional Types of Joints

• Diarthroses: Freely movable joints. Examples: Shoulder, knee, hip, elbow, interphalangeal, tarsal, and carpal joints.

• Amphiarthroses: Slightly movable joints. Examples: Intervertebral discs, costosternal joints, pubic symphysis.

• Synarthroses: Joints with little or no movement. Examples: Skull sutures, mental symphysis, teeth in sockets, 1st costosternal joint.

Classification of Joints by Structure

Structural Types of Joints

1. Synovial Joints

   • Bones are separated by a joint cavity filled with synovial fluid, enclosed in a fibrous joint capsule.

   • Highly movable; most common type in the limbs.

   • Examples: Shoulder, hip, elbow, knee, carpal, interphalangeal joints.

2. Fibrous Joints

   • Bones held together by collagenous fibers extending from the matrix of one bone into the matrix of the next.

   • No joint cavity.

   • Examples: Skull sutures, teeth in joints, distal radioulnar joints, tibiofibular joints.

3. Cartilaginous Joints

   • Bones held together by cartilage; no joint cavity.

   • Examples: Epiphyseal plates of long bones, costosternal joints, pubic symphysis, intervertebral discs.

Additional Joint Category

• Bony Joint (Synostosis): Immobile joints formed when two bones ossify together (merge or weld). Examples: Some cranial sutures, fused growth plates in long bones.

Detailed Types of Fibrous Joints (Synarthrosis)

1. Suture

• Immobile joints in the skull, visible as fissures through the compact bone.

• Types: Serrate (wavy lines, e.g., parietal/frontal bones), Lap/Squamous (diagonal line, e.g., temporal/parietal bones), Plane/Butt (straight line, e.g., paired maxillary and palatine bones).

2. Gomphosis

• Where teeth attach to bone via the fibrous periodontal ligament (collagen-based).

• Permits slight movement when biting; important for sensory feedback.

3. Syndesmosis

• Collagen fibers bind two bones, allowing more mobility than sutures or gomphoses, but still limited.

• Examples: Tibia-fibula at distal end, radius-ulna at distal ends.

Comparison Table: Tibia/Fibula vs. Radius/Ulna

Joint	Distance Between Bones	Mobility
Tibia/Fibula	Close together	Less mobile
Radius/Ulna	Farther apart	More mobile

Cartilaginous Joints (Amphiarthrosis)

Types

• Synchondoses: Bones joined by hyaline cartilage. Examples: Growth plate in long bones, first rib to sternum.

• Symphyses: Bones joined by fibrous cartilage. Examples: Pubic symphysis, intervertebral discs.

Developmental Note

• Infants and children have more joints than adults because some joints (e.g., growth plates, cranial sutures) fuse as development progresses.

Structure and Function of Synovial Joints

Main Structural Characteristics

1. Articular Cartilage: Hyaline cartilage covering bone surfaces at the joint, reducing friction and absorbing shock.

2. Articular Capsule: Two-layered capsule surrounding the joint.

   • External layer: Dense irregular connective tissue, continuous with periosteum.

   • Inner layer: Synovial membrane made of loose connective tissue, covering all internal joint surfaces except those covered by articular cartilage.

3. Joint (Synovial) Cavity: Potential space within the joint capsule and articular cartilage, filled with synovial fluid.

4. Synovial Fluid: Slippery fluid formed by filtration of blood through capillaries in the synovial membrane; lubricates and nourishes the joint.

5. Reinforcing Ligaments: Collagen-based connective tissue that stabilizes the joint; extra-stretchy ligaments can result in double-jointedness.

Types of Synovial Joints by Shape

• Ball & Socket Joints: Hip and shoulder; allow multiaxial movement.

• Hinge Joints: Ulna and femur, knee, fingers, toes; allow movement in one plane.

• Pivot Joints: Radius and ulna (forearm/elbow), first two vertebrae of spine; allow rotation.

• Saddle Joints: Thumb, sternum/clavicle; allow movement in two planes.

• Condyloid Joints: Fingers; allow movement in two planes.

• Gliding Joints: Carpal bones, patella-femur; bone surfaces are almost flat, allowing bones to slide past each other.

Complex Joints

• Some joints have characteristics of multiple joint types. Examples:

  • Elbow: Both hinge (ulna/humerus) and pivot (radius/ulna).

  • Knee: Similar to elbow, but stronger and less flexible.

  • Temporo-mandibular joint: Both lateral and dorsal/ventral movement.

Complex Synovial Joints

• Some joints contain an articular disc (e.g., jaw, sternoclavicular, ankle). In the knee, this is called a meniscus, which guides knee bones and absorbs shock.

Accessories to Synovial Joints

• Tendons: Collagen-based connective tissue connecting muscle to bone.

• Ligaments: Collagen-based connective tissue connecting bone to bone.

• Bursa: Fibrous sac holding synovial fluid, reducing friction around muscles and tendons. Bursitis: Inflammation of the bursa.

Movements of Various Joints

Shoulder (Ball & Socket) Joint Movements

• Abduction: Moving the arm away from the body (e.g., jumping jacks).

• Flexion: Decreasing the angle of the joint (e.g., raising the arm forward).

• Rotation: Moving the arm in a circular motion (e.g., sweeping a desk).

Other Movements

• Adduction: Returning the limb to the body’s midline.

• Elevation: Lifting a body part in the frontal plane (e.g., shrugging shoulders).

• Depression: Returning from elevation to anatomical position.

Clinical Notes

Gingivitis (Gum Disease)

• Bacterial infection destroys the ligament holding teeth in place; promoted by plaque.

Scurvy

• Chronic lack of vitamin C impairs collagen synthesis, weakening joints and delaying wound healing.

Summary Table: Joint Types and Examples

Type	Structural Classification	Functional Classification	Examples
Synovial	Joint cavity, synovial fluid	Diarthrosis	Shoulder, knee, hip
Fibrous	Collagen fibers, no cavity	Synarthrosis/Amphiarthrosis	Skull sutures, teeth, distal tibiofibular
Cartilaginous	Cartilage, no cavity	Amphiarthrosis	Pubic symphysis, intervertebral discs
Bony (Synostosis)	Ossified bone	Synarthrosis	Fused cranial sutures, growth plate fusion

Additional info: The notes provide a comprehensive overview of joint classification, structure, and function, suitable for Anatomy & Physiology students. Clinical relevance is included for conditions affecting joints and connective tissues.