Joint Classifications and Synovial Joint Structure

Joint Classifications

Structural Classification of Joints

Joints, or articulations, are classified based on the material binding the bones and the presence or absence of a joint 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 of Joints

Joints are also classified by the degree of movement they allow:

• Synarthrosis: Immovable (mostly fibrous joints).

• Amphiarthrosis: Slightly movable (mostly cartilaginous joints).

• Diarthrosis: Freely movable (synovial joints).

Patterns of Mobility

• Freely movable joints are typically found in the appendicular skeleton.

• Immovable/slightly movable joints are more common in the axial skeleton.

• Less mobility generally means greater stability.

Joint Classification Table

Structural Class	Structural Characteristic	Types	Mobility
Fibrous	Adjoining bones united by collagen fibers	Suture (short fibers) Syndesmosis (long fibers) Gomphosis (periodontal ligament)	Immobile (synarthrosis) Slightly movable (amphiarthrosis) and immobile Immobile (synarthrosis)
Cartilaginous	Adjoining bones united by cartilage	Synchondrosis (hyaline cartilage) Symphysis (fibrocartilage)	Slightly immobile Slightly movable
Synovial	Adjoining bones covered by articular cartilage; enclosed within a joint cavity lined by synovial membrane	Plane Condylar Saddle Pivot Hinge Ball-and-socket	Freely movable (diarthrosis; movement depends on the shapes of joint surfaces)

Fibrous Joints

Characteristics and Types

Fibrous joints are connected by dense connective tissue, mainly collagen. They lack a joint cavity and allow little to no movement, depending on fiber length.

• Sutures: Short fibers interlock bones (e.g., skull); synarthrotic (immovable).

• Synostosis: Ossified suture in adulthood.

• Syndesmoses: Bones connected by ligaments; movement depends on fiber length. Examples: Tibia-fibula (little movement), radius-ulna (more movement).

• Gomphoses: Peg-in-socket joints (e.g., teeth in alveolar sockets); immovable.

Example: The periodontal ligament anchors teeth to the jaw.

Cartilaginous Joints

Characteristics and Types

Cartilaginous joints unite bones with cartilage and lack a joint cavity. They allow slight movement.

• Synchondroses: Bones united by hyaline cartilage; usually synarthrotic. Example: Epiphyseal plate in children, joint between first rib and manubrium.

• Symphyses: Bones united by fibrocartilage and hyaline cartilage; amphiarthrotic (slightly movable). Examples: 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 complex supporting structures.

• Articular cartilage: Hyaline cartilage covers bone ends, cushions, and absorbs shock.

• Joint cavity: Contains synovial fluid, which lubricates and nourishes the joint.

• Articular capsule: Encloses the joint cavity; consists of an outer fibrous layer (dense irregular CT) and an inner synovial membrane (secretes synovial fluid).

• Reinforcing ligaments: Strengthen and stabilize the joint; may be capsular (thickened part of capsule), extracapsular (outside capsule), or intracapsular (deep to capsule).

• Nerves and blood vessels: Provide pain detection, stretch sensation, and nutrient supply.

Additional Features

• Fatty pads: Found in hips and knees; cushion between fibrous layer and synovial membrane or bone.

• Articular discs (menisci): Fibrocartilage wedges that improve fit and stability; found in knees, jaws, and a few other joints.

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

• Tendon sheaths: Elongated bursae that wrap around tendons in crowded areas (e.g., wrist).

Example: The subacromial bursa cushions the shoulder joint, and the tendon sheath surrounds the biceps tendon.

Joint Stability and Movements

Factors Affecting Stability

• Articular surfaces: Shape affects movement and stability; deep sockets and snug fits (e.g., hip joint) are more stable.

• Ligaments: Unite bones and limit excessive motion; more ligaments = stronger joint, but stretched ligaments can lead to instability.

• Muscle tone: Most important stabilizer; continuous low-level tension keeps joints tight, especially in 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 past each other (e.g., intercarpal, intertarsal, vertebral facets).

• Angular Movements: Change the angle between bones:

  • Flexion: Decreases angle, usually in sagittal plane.

  • Extension: Increases angle, usually in sagittal plane.

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

  • Adduction: Limb moves toward midline in frontal plane.

  • Circumduction: Limb/finger moves in a circular motion.

• Rotation: Bone turns around its longitudinal axis.

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

  • Lateral rotation: Rotating away from the median plane.

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

Key Terms

• Synarthrosis: Immovable joint

• Amphiarthrosis: Slightly movable joint

• Diarthrosis: Freely movable joint

• Articular cartilage: Hyaline cartilage covering bone ends

• Synovial fluid: Lubricating fluid in joint cavity

• Bursa: Fluid-filled sac reducing friction

• Tendon sheath: Elongated bursa around tendon

Additional info:

• Synovial fluid is viscous and thins with activity, containing phagocytes for cleaning debris.

• Capsular ligaments are part of the joint capsule, while extracapsular and intracapsular ligaments are outside or deep to the capsule, respectively.