Classification and Structure of Joints in the Human Body 5.0

Joints: Classification and Structure

Introduction to Joints

Joints, also known as articulations, are points where two or more bones meet. They play a crucial role in holding the skeleton together and enabling body movement. Joints are classified based on their function and structure.

• Function: The movement allowed by the joint.

• Structure: The type of tissue that connects the bones.

Functional Classification of Joints

Types of Functional Joints

• Synarthroses: Immovable joints. "Syn" = together

• Amphiarthroses: Slightly movable joints. "Amphi" = both

• Diarthroses: Freely movable joints. "Dia" = through, apart

Structural Classification of Joints

Overview

Joints are structurally classified based on the type of connective tissue and the presence or absence of a joint cavity. The three main types are:

• Fibrous Joints

• Cartilaginous Joints

• Synovial Joints

Structural Joints - Fibrous

Fibrous joints are connected by dense connective tissue and do not have a joint cavity. They are generally immovable or allow very limited movement.

Types of Fibrous Joints

Type	Description	Movement(s)	Examples	Helpful Tips
Sutures	Seams between skull bones.	Immovable	Skull sutures	"Syn" = together
Gomphoses	Peg-in-socket fibrous joint.	Immovable	Tooth in its bony socket	"Peg" = tooth
Syndesmoses	Bones connected by ligaments or interosseous membranes.	Variable	Distal tibiofibular joint, radius/ulna	"Des" = band (think ligaments connecting)

Structural Joints - Cartilaginous

Cartilaginous joints are united by cartilage and lack a joint cavity. They allow more movement than fibrous joints but less than synovial joints.

Types of Cartilaginous Joints

Type	Description	Movement(s)	Examples	Helpful Tips
Synchondroses	Bones united by hyaline cartilage.	Immovable	Epiphyseal plate, joint between 1st rib and sternum	"Chondro" = cartilage, "Syn" = together, "Physis" = growth
Symphyses	Bones united by fibrocartilage.	Limited movement	Intervertebral discs, pubic symphysis	"Sym" = together

Structural Joints - Synovial

Synovial joints are characterized by the presence of a synovial (fluid-filled) cavity. They are all diarthrotic (freely movable) and include almost all limb joints.

Anatomy of Synovial Joints

1. Articular capsule: Surrounds both bones on all sides, creating the synovial cavity.

   • Superficial fibrous layer: Provides strength.

   • Deep synovial membrane: Makes the synovial fluid.

2. Synovial cavity: A fluid-filled space between the bones.

3. Synovial fluid: Lubricates the joint and allows friction-free movement.

4. Articular cartilage: Covers the ends of the bones, reducing friction and absorbing shock.

5. Reinforcing ligaments: Stabilize the joint.

   • Capsular ligaments: Thicker layers of the fibrous portion of the articular capsule.

   • Extracapsular ligaments: Superficial to the articular capsule.

   • Intracapsular ligaments: Inside the articular capsule.

Other Synovial Joint Structures

• Menisci: Fibrocartilage discs that improve the fit between bones and minimize wear and tear on joint surfaces.

• Bursae: Sacs of synovial fluid located between ligaments, muscles, bones, or tendons to reduce friction.

• Tendon sheaths: Elongated bursae that wrap around a tendon to reduce friction.

Clinical Implications

Why We Study Anatomy

• Bursitis: Inflammation of the bursa due to blow or friction. Example: Water on the knee.

• Tendonitis: Inflammation of the tendon sheath due to overuse. Example: Student's elbow.

Stability and Mobility of Joints

For diarthrotic joints, joint stability and mobility depend on:

1. Muscle tone: Keeps tendons taut as they cross joints; most important for strengthening a joint.

2. Ligament number and location: Have a limited role.

3. Shape of the articular (joint) surface: Has a minor role.

Hypermobility and Ehlers-Danlos Syndrome

• Hypermobility: Being "double-jointed"; increased range of motion at joints.

• Ehlers-Danlos syndrome: A genetic disorder affecting connective tissue, primarily the skin, joints, and blood vessel walls.

Types of Synovial Joints and Movements

Uniaxial Synovial Joints

Pivot Joint

• Movement in one plane or a single direction (rotation).

• Examples:

  • Joint between the atlas vertebra (C1) and axis vertebra (C2).

  • Joint between the humerus and radius.

• Action: Bone rotates along its long axis, toward the midline, or away from it.

• Supination: Turning the palms anteriorly or upward.

• Pronation: Turning the palms posteriorly or downward.

Hinge Joint

• Movement in one plane or a single direction.

• Example: Joint between the humerus and ulna.

• Actions: Flexion and extension.

Angular Movements

• Flexion: Decreases the angle between bones.

• Extension: Increases the angle between bones.

Biaxial Synovial Joints

Condylar & Saddle Joints

• Movements in two planes or directions.

• Examples:

  • Wrist joint or between palm and fingers.

  • Joint between the thumb and palm.

• Actions: Flexion/extension, abduction/adduction.

Angular Movements

• Abduction: Movement along the frontal plane, away from the midline.

• Adduction: Movement along the frontal plane, toward the midline.

Multiaxial Synovial Joints

Ball-&-Socket Joints

• Movements in many planes or directions.

• Examples: Shoulder joint, hip joint.

• Actions: May perform many actions, including flexion/extension, abduction/adduction, and rotation.

Special Movements

• Protraction: Moving a body part forward in the lateral plane.

• Retraction: Moving a body part back in the lateral plane.

• Elevation: Moving the body part superiorly.

• Depression: Moving the body part inferiorly.

Special Movements of the Foot

• Dorsiflexion: The foot's movement toward the knee.

• Plantar flexion: The foot's movement toward the plantar surface.

• Inversion: Turning the sole towards the midline.

• Eversion: Turning the sole away from the midline.