Classification and Structure of Articulations (Joints) in Anatomy & Physiology

Classification of Articulations

Introduction to Articulations

Articulations, also known as joints or arthroses, are the locations where two or more bones meet. Joints are essential for movement and stability in the skeletal system. There are two primary methods for classifying joints: functional and structural.

• Functional Classification: Based on the degree of movement permitted by the joint.

• Structural Classification: Based on the material binding the bones together and the presence or absence of a joint cavity.

Functional Classification of Joints

1. Synarthroses (Immovable Joints)

Synarthroses are joints where bones are in direct contact and are held together by dense connective tissue (CT) or cartilage. These joints allow little or no movement.

• Suture: Found in the skull; bones are tightly bound by dense CT.

• Gomphosis: A peg-in-socket joint, such as the teeth held in jaw sockets by the periodontal ligament (dense CT).

• Synchondrosis: Bones held together by hyaline cartilage, e.g., epiphyseal plate in growing children.

2. Amphiarthroses (Slightly Movable Joints)

Amphiarthroses allow limited movement and are held together by fibrocartilage or dense CT.

• Symphysis: Bones joined via fibrocartilage, e.g., pubic symphysis, intervertebral discs.

• Syndesmosis: Bones held together by an interosseous ligament (dense CT), e.g., between tibia & fibula.

3. Diarthroses (Freely Movable Joints)

Diarthroses are freely movable joints, typically characterized by a joint cavity and are also known as synovial joints.

• Examples include the shoulder, hip, and knee joints.

Structural Classification of Joints

1. Fibrous Joints

• Sutures: Immovable joints found in the skull.

• Gomphosis: Peg-in-socket joints, e.g., teeth in jaw.

• Syndesmosis: Bones connected by ligaments, allowing slight movement.

2. Cartilaginous Joints

• Synchondrosis: Bones joined by hyaline cartilage.

• Symphysis: Bones joined by fibrocartilage.

3. Synovial Joints

• Characterized by a joint cavity filled with synovial fluid, allowing free movement.

Types of Synovial Joints

Overview

Synovial joints are classified based on the shapes of their articulating surfaces and the types of movement they allow.

Type	Movement	Example
Ball & Socket	Triaxial (multiaxial): movement in all planes plus rotation	Shoulder, hip
Condyloid/Ellipsoidal	Biaxial: movement in two planes but no rotation	Metacarpophalangeal joints
Planar/Gliding	Usually flat articulating surfaces; allows slight sliding movement	Intercarpal, intertarsal joints
Hinge	Uniaxial: allows movement in one plane	Elbow, phalanges
Pivot	Uniaxial: allows rotational movement	Atlantoaxial joint (C1 to C2)
Saddle	Biaxial: allows movement in 2 planes	Carpometacarpal joint of thumb

Movements at Synovial Joints

Types of Movements

Synovial joints allow a variety of movements, which are essential for daily activities and complex motions.

• Flexion: Decreasing the angle between bones (e.g., bending the knee).

• Extension: Increasing the angle between bones (e.g., straightening the knee).

• Hyperextension: Extension beyond the normal range.

• Dorsiflexion: Flexing the foot upward.

• Plantar Flexion: Pointing the foot downward.

• Abduction: Moving a limb away from the midline.

• Adduction: Moving a limb toward the midline.

• Circumduction: Moving a limb in a circular motion.

• Rotation: Moving around an axis.

• Lateral/External Rotation: Moving away from the midline.

• Medial/Internal Rotation: Moving toward the midline.

• Pronation: Rotating the forearm so the palm faces downward.

• Supination: Rotating the forearm so the palm faces upward.

• Inversion: Turning the sole of the foot inward.

• Eversion: Turning the sole of the foot outward.

• Protraction: Moving a part forward.

• Retraction: Moving a part backward.

• Elevation: Raising a part (e.g., shrugging shoulders).

• Depression: Lowering a part.

• Opposition: Touching the thumb to the tip of another finger.

Synovial Joint Structure

Main Components

Synovial joints have a complex structure that allows for free movement and stability.

• Articular Cartilage: Hyaline cartilage covering the articulating surfaces of bones, reducing friction and absorbing shock.

• Joint (Articular) Capsule: Encloses the joint, strengthens it, and prevents excessive movement.

• Fibrous Capsule: Outer layer of dense CT, continuous with the periosteum.

• Synovial Membrane: Inner layer that secretes synovial fluid for lubrication.

• Meniscus: Fibrocartilage disc that partitions some joint cavities.

• Bursa: Synovial membrane fluid-filled sacs that reduce friction.

• Tendon Sheath: Modified bursa that surrounds a tendon, aiding movement.

Knee Joint Anatomy

Structure and Ligaments

The knee is a complex hinge joint formed by the articulation of the femur, tibia, and patella. It is stabilized by several ligaments and menisci.

• Ligaments Associated with the Joint Capsule:

  • Patellar Ligament: Connects the patella to the tibial tuberosity.

  • Popliteal Ligaments: Oblique ligaments that connect the femur, tibia, and fibula.

  • Medial/Lateral Collateral Ligaments (MCL/LCL): Provide medial and lateral stability.

• Ligaments Inside the Joint Capsule:

  • Anterior Cruciate Ligament (ACL): Prevents anterior displacement of the tibia.

  • Posterior Cruciate Ligament (PCL): Prevents posterior displacement of the tibia.

• Main Bursae: Suprapatellar, prepatellar, and infrapatellar bursae reduce friction.

Arthropathies (Joint Diseases)

Overview

Arthropathies are diseases affecting the joints, often resulting in pain, inflammation, and loss of function.

• Arthritis: General term for joint inflammation. Types include:

  • Osteoarthritis: Degenerative joint disease associated with aging and wear.

  • Rheumatoid Arthritis: Autoimmune disease causing destruction of articular cartilage.

  • Gout: Caused by accumulation of uric acid crystals in joints.

  • Septic Arthritis: Infection of the joint.

  • Psoriatic Arthritis: Associated with psoriasis.

• Bursitis: Inflammation of a bursa.

• Tendinitis: Inflammation of a tendon.

• Dislocation/Luxation: Displacement of bones at a joint.

• Sprain: Damage to ligaments or tendons.

• Strain: Injury to muscle or tendon.

• Torn Cartilage: Damage to cartilage, often requiring surgery.

• Ankylosis: Loss of joint mobility.

Treatments

• NSAIDs: Non-steroidal anti-inflammatory drugs for pain relief.

• Corticosteroids: Used for severe inflammation (oral or injection).

Summary Table: Joint Types and Examples

Structural Type	Functional Type	Example
Fibrous (Suture)	Synarthrosis	Skull sutures
Fibrous (Syndesmosis)	Amphiarthrosis	Distal tibiofibular joint
Cartilaginous (Symphysis)	Amphiarthrosis	Pubic symphysis
Synovial (Ball & Socket)	Diarthrosis	Shoulder, hip

Key Equations and Terms

• Range of Motion (ROM): The degree of movement that occurs at a joint.

• Synovial Fluid: Viscous fluid that lubricates synovial joints.

Additional info: The notes have been expanded to include definitions, examples, and context for each joint type and movement, as well as a summary table for clarity.