Joints

Articulations (Joints)

Main Takeaway: Joints are classified by structure (what connects the bones) and function (how mobile the bones are). Understanding these classifications, the components of synovial joints, and specific joint mechanics is essential for understanding human movement and clinical pathologies.

I. Joint Classifications

Structural Classification (Anatomical)

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

2. Cartilaginous: Bones joined by cartilage; no joint cavity.

3. Synovial: Bones separated by a fluid-filled cavity.

Functional Classification (Physiological)

1. Synarthroses: Immobile (most stable).

2. Amphiarthroses: Slightly mobile.

3. Diarthroses: Freely mobile (usually synovial).

II. Fibrous Joints

• Gomphoses: Tooth to maxilla/mandible (synarthrosis).

• Sutures: Between skull bones (synarthrosis).

• Syndesmoses: Between parallel bones (e.g., radius/ulna, tibia/fibula) (amphiarthrosis).

III. Cartilaginous Joints

1. Synchondroses: Hyaline cartilage; usually synarthroses (e.g., epiphyseal plates, costochondral joints).

2. Symphyses: Fibrocartilage pads; amphiarthroses (e.g., intervertebral discs, pubic symphysis).

IV. Synovial Joints

Defined by a joint cavity filled with synovial fluid (a blood filtrate that lubricates and nourishes).

Anatomy

• Articular Capsule: Outer fibrous layer (dense regular connective tissue) and inner synovial membrane.

• Articular Cartilages: Hyaline cartilage covering surfaces to reduce friction and prevent bone-to-bone contact.

• Accessory Structures:

• Bursae: Fluid-filled sacs that cushion areas where tendons, ligaments, or bones rub.

• Tendon Sheaths: Elongated bursae wrapped around tendons.

• Fat Pads: Packing material for protection.

Classification by Shape & Movement

1. Plane: Gliding (uniaxial).

2. Hinge: Angular motion (uniaxial).

3. Pivot: Rotation (uniaxial).

4. Condylar: Two planes (biaxial).

5. Saddle: All movements (biaxial/multiaxial).

6. Ball-and-socket: All movements (multiaxial).

V. Joint Movements

• Gliding: Surfaces slide back-and-forth/side-to-side (e.g., carpals).

• Angular: Change in angle (flexion, extension, hyperextension, lateral flexion, abduction, adduction, circumduction).

• Rotational: Bone turns on its longitudinal axis (e.g., atlantoaxial "no" gesture, pronation/supination).

• Special: Depression/elevation, dorsiflexion/plantar flexion, inversion/eversion, protraction/retraction, opposition.

VI. Selected Articulations

• Temporomandibular Joint (TMJ): Diarthrotic hinge with an articular disc for shock absorption.

• Intervertebral Articulations: Symphyses between bodies; diarthrotic between articular processes.

• Sternoclavicular Joint: Diarthrotic saddle joint; contains an articular disc.

• Glenohumeral Joint: Ball-and-socket; highly mobile but less stable. Includes glenoid labrum (fibrocartilage lip), rotator cuff muscles, and various bursa.

• Elbow Joint: Hinge joint (humerus, ulna, radius). Includes collateral ligaments (ulnar/medial and radial/lateral) and the annular ligament (securing the radial head).

• Hip (Coxal) Joint: Ball-and-socket (femur and os coxae). Features acetabular labrum and reinforcing capsular ligaments.

• Knee Joint: Largest/most complex. Contains medial/lateral menisci (fibrocartilage wedges), collateral ligaments (fibular/lateral, tibial/medial), and cruciate ligaments (ACL/PCL).

• Note: The popliteal vein runs behind the knee and is susceptible to life-threatening aneurysms.

• Talocrural (Ankle) Joint: Hinge joint. Sprains are categorized by grade; grade 3/4 involves ligamentous tearing and potential bone involvement.