Joint Classifications

Structural Classification of Joints

Joints, or articulations, are classified based on their structural characteristics and the presence or absence of a cavity. The three main structural classes are:

• Fibrous Joints: Bones joined by collagen fibers; no joint cavity.

• Cartilaginous Joints: Bones joined by cartilage; no joint cavity.

• Synovial Joints: Bones separated by a fluid-filled joint cavity.

Functional Classification (Movement)

• Synarthrosis — immovable (mostly fibrous)

• Amphiarthrosis — slightly movable (cartilaginous)

• Diarthrosis — freely movable (synovial)

Patterns

• Freely movable joints → appendicular skeleton

• Immovable/slightly movable → axial skeleton

• Less mobility = more stability

Fibrous Joints

Characteristics and Types

Fibrous joints are united by dense connective tissue (collagen fibers) and lack a joint cavity. Movement is minimal and depends on fiber length.

• Sutures: Short fibers interlock; found in the skull; synarthrotic (immobile). Allows brain growth in youth; later ossifies (synostosis).

• Syndesmoses: Bones connected by ligament. Short fibers (e.g., tibia-fibula) = little/no movement. Long fibers (e.g., radius-ulna) = more movement.

• Gomphoses: Peg-in-socket joint (e.g., tooth in alveolar socket); fibrous connection is the periodontal ligament.

Cartilaginous Joints

Characteristics and Types

Cartilaginous joints are united by cartilage and lack a joint cavity. Movement is slight to none.

• Synchondrosis: Bones united by hyaline cartilage; synarthrotic (immobile). Example: Epiphyseal plate, joint between first rib and manubrium.

• Symphysis: Bones united by fibrocartilage and hyaline cartilage; amphiarthrotic (slightly movable). Example: Intervertebral discs, pubic symphysis.

Synovial Joints

Characteristics and Structure

Synovial joints are the most common and freely movable joints in the body. They feature a fluid-filled joint cavity and specialized structures for movement and stability.

• Articular cartilage: Hyaline cartilage cushions and absorbs shock.

• Joint cavity: Small amount of synovial fluid; expands if fluid accumulates.

• Articular capsule: Two layers: Outer fibrous layer — dense irregular connective tissue. Inner synovial membrane — makes/secretes synovial fluid.

• Synovial fluid: Lubricates, nourishes cartilage, absorbs shock. Viscous, thins with activity; contains phagocytes.

• Reinforcing ligaments: Capsular — thickened part of capsule. Extracapsular — outside capsule. Intracapsular — deep to capsule (not inside cavity).

• Nerves & vessels: Pain detection, stretch, nutrient supply.

Additional Structures

• Fatty pads: Cushion between fibrous layer and synovial membrane or bone (hips, knees).

• Articular discs (menisci): Fibrocartilage wedges dividing synovial cavity; improve fit, increase stability, decrease wear (found in knees, jaws).

• Bursae: Flattened sacs lined with synovial membrane; reduce friction where ligaments, tendons, skin, or bone rub.

• Tendon sheaths: Elongated bursae wrapping around tendons; found in crowded tendon areas (e.g., wrist).

Example: Shoulder joint — subacromial bursa cushions humerus head; tendon sheath surrounds biceps tendon.

Joint Stability and Movements

Factors Affecting Stability

• Articular surfaces: Shape affects movement and minorly affects stability. Deep sockets and snug fits = more stable (e.g., hip joint).

• Ligaments: Unite bones and limit excessive motion. More ligaments = stronger joint. Stretched permanently if overstretched = instability. Over 6% of length = snapped ligament.

• Muscle tone: Most important stabilizer at most joints. Continuous low-level tension in ready muscles; tight joints. Critical for shoulder, knee, and foot arches.

Movements at Synovial Joints

• Nonaxial: Gliding (flat bones slide).

• Uniaxial: Movement in one plane.

• Biaxial: Movement in two planes.

• Multiaxial: Movement in/around all three planes.

General Types of Movements

• Gliding: Flat surfaces slide (intercarpal, intertarsal, vertebral facets).

• Angular movements: Change angle between bones.

  • Flexion: Decreasing angle, usually in sagittal plane.

  • Extension: Increasing angle, usually in sagittal plane.

  • Abduction: Limb moves away from body midline in frontal plane.

  • Adduction: Limb moves toward body midline in frontal plane.

  • Circumduction: Limb/finger moves describing a cone in space.

• Rotation: Bone turns around its longitudinal axis.

  • Medial: Rotating toward median plane (e.g., femur inward).

  • Lateral: Rotating away from median plane.

  • Examples: Shoulder, hip, C1-C2 vertebrae.

Additional info: The notes cover content from Chapter 5 (The Skeletal System), specifically the section on joint classification and synovial joint structure and function, which is essential for understanding movement and stability in the human body.