Joints (Articulations) of the Human Body

Joints, or articulations, are connections between bones that allow for varying degrees of movement and stability. They are classified based on their structure and function, which determines their mobility and strength.

Structural Classification of Joints

• Fibrous Joints

  • Held together by dense connective tissue.

  • Bones are in close contact.

  • Examples: Sutures of the skull, syndesmosis between tibia and fibula, gomphosis (teeth in jaw).

• Cartilaginous Joints

  • Held together by cartilage (hyaline or fibrocartilage).

  • Examples: Epiphyseal plates (synchondrosis), pubic symphysis, intervertebral discs (symphysis).

• Synovial Joints

  • Have a complex structure with a joint capsule and synovial fluid.

  • Permit the widest range of movement but are the weakest type of joint.

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

Functional Classification of Joints

• Synarthrosis (Immovable Joint)

  • Bones may interlock and are extremely strong.

  • Examples: Sutures, gomphosis, synchondrosis.

• Amphiarthrosis (Slightly Movable Joint)

  • Articulating bones connected by collagen fibers or cartilage.

  • Weaker than synarthrotic joints but stronger than diarthrotic joints.

  • Examples: Syndesmosis, symphysis.

• Diarthrosis (Freely Movable Joint)

  • Permits the widest range of movement.

  • Is the weakest type of joint.

  • All synovial joints are diarthrotic.

Types of Fibrous Joints

• Syndesmosis

  • Bones bound by a sheet (interosseous membrane) or bundle (interosseous ligament) of dense connective tissue.

  • Slightly flexible (amphiarthrotic).

  • Example: Distal tibiofibular joint.

• Sutures

  • Only found between flat bones of the skull.

  • Thin layer of connective tissue connects the bones.

  • Immovable (synarthrotic).

• Gomphosis

  • Cone-shaped bony process in a bony socket.

  • Holds teeth into the jaw bones.

  • No movement (synarthrotic joint).

Types of Cartilaginous Joints

• Synchondrosis

  • Bands of hyaline cartilage unite the bones.

  • Some are temporary (e.g., epiphyseal plates, which become synostosis after ossification).

  • Some are permanent (e.g., joint between manubrium and first rib).

  • Immovable (synarthrotic).

• Symphysis

  • Bones are connected by a pad of fibrocartilage.

  • Allow limited movement (amphiarthrotic).

  • Examples: Pubic symphysis, intervertebral discs.

Structure of Synovial Joints

• Diarthrotic (freely movable), typically found at the ends of long bones (e.g., limbs).

• Bones are separated by articular cartilage and synovial fluid.

• Joint Capsule: Sac enclosing articulating ends of bones, consisting of:

  • Outer fibrous layer

  • Inner synovial membrane (secretes synovial fluid)

• Synovial Fluid: Lubricates, cushions shocks, and supports chondrocytes.

• Joint capsule is continuous with the periosteum of the bones.

Joint Accessories

• Bursa

  • Synovial fluid-filled pocket.

  • Forms in connective tissue where tendons or ligaments rub against other tissues.

• Fat Pads

  • Masses of adipose tissue covered by synovial membrane.

  • Fill in spaces created as bones move.

• Meniscus

  • Pad of fibrocartilage between articulating bones.

  • May subdivide synovial cavity, channel flow of synovial fluid, and allow for variations in shapes of articular surfaces.

• Ligaments

  • Support, strengthen, and reinforce joints.

  • Capsular (intrinsic) ligaments: Thickenings of the joint capsule.

  • Extrinsic ligaments: Not part of the capsule; can be extracapsular (e.g., patellar ligament) or intracapsular (e.g., cruciate ligaments).

Mobility vs. Strength in Joints

• A joint cannot be both highly mobile and very strong.

• Movement beyond the normal range of motion causes damage (e.g., dislocation/luxation).

• Such injuries are painful due to the presence of surrounding nerves.

Types of Movement in Joints

• Gliding

• Angular Motion

• Circumduction

• Rotation

• Movements are only partial and never without limit.

Note: The shapes of articulating bones lead to anatomical classification of synovial joints.

Types of Synovial Joints

• Ball-and-Socket Joint

  • Partially spherical end lies in a socket.

  • Allows for multidirectional movement and rotation.

  • Examples: Shoulder joint, hip joint.

• Condylar (Ellipsoidal) Joint

  • Allows movement in two planes: adduction/abduction and flexion/extension.

  • Elliptical shape ('knob in cup').

  • Examples: Radiocarpal joints, metacarpophalangeal joints 2-5.

• Gliding (Plane) Joint

  • Flat or nearly flat articular surfaces glide past one another.

  • Examples: Acromioclavicular and sternoclavicular joints, intercarpal/intertarsal joints, vertebrocostal joints, sacroiliac joints.

• Hinge Joint

  • Allows movement in only one plane (flexion and extension).

  • Examples: Elbow, knee, ankle, interphalangeal joints.

• Pivot Joint

  • Allows only rotary movement around a single axis.

  • Examples: Atlantoaxial joint (between C1 and C2 vertebrae), proximal radioulnar joints.

• Saddle Joint

  • Bones forming the joint are shaped like a saddle with the other bone resting on it like a rider on a horse.

  • Provides stability while allowing more mobility than a hinge or gliding joint.

  • Example: First carpometacarpal joint (thumb).

Summary Table: Structural and Functional Classification of Joints

Key Terms

• Synarthrosis: Immovable joint.

• Amphiarthrosis: Slightly movable joint.

• Diarthrosis: Freely movable joint.

• Synovial Fluid: Viscous fluid within synovial joints that reduces friction and nourishes cartilage.

• Meniscus: Crescent-shaped fibrocartilage pad in some synovial joints.

• Bursa: Fluid-filled sac that reduces friction between tissues.

Example: Clinical Application

• Dislocation (Luxation): Occurs when bones are forced out of alignment in a joint, often causing pain and requiring medical intervention.

• Arthritis: Inflammation of joints, often affecting synovial joints due to their high mobility and complex structure.