Joints and Articulations

Definition of Articulation

An articulation, or joint, is the location where two or more bones meet. Joints allow for movement and provide mechanical support to the skeleton.

• Key notes: Articulations are classified based on their structure and function.

• Categorization: Joints are categorized by the amount of movement they allow and the type of tissue that connects the bones.

Functional Classification of Joints

Joints are functionally classified by the degree of movement they permit:

• Synarthrosis: Immovable joints (e.g., sutures in the skull).

• Amphiarthrosis: Slightly movable joints (e.g., intervertebral discs).

• Diarthrosis: Freely movable joints (e.g., most limb joints).

Structural Classification of Joints

Joints are structurally classified by the material binding the bones and the presence or absence of a joint cavity:

• Fibrous Joints: Bones joined by dense connective tissue; no joint cavity.

  • Sutures: Found in the skull.

  • Syndesmoses: Bones connected by ligaments (e.g., distal tibiofibular joint).

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

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

  • Synchondroses: Hyaline cartilage unites bones (e.g., epiphyseal plates).

  • Symphyses: Fibrocartilage unites bones (e.g., pubic symphysis).

• Bony Joints: Bones fuse together (e.g., frontal bone in adults).

• Synovial Joints: Bones separated by a fluid-filled joint cavity; most common and movable type.

Synovial Joint Structure

Synovial joints have several key components:

• Articular Cartilage: Hyaline cartilage covering bone ends; reduces friction and absorbs shock. Found at the ends of long bones within synovial joints.

• Joint Capsule: Encloses the joint cavity; consists of an outer fibrous layer and an inner synovial membrane.

• Synovial Fluid: Viscous fluid within the joint cavity; lubricates, nourishes cartilage, and reduces friction.

  • Functions:

    1. Lubrication

    2. Nutrient distribution

    3. Shock absorption

• Accessory Structures: Support and protect the joint.

  • Bursa: Fluid-filled sacs that reduce friction between tissues.

    • Description: Small, synovial membrane-lined sacs.

    • Function: Cushion and reduce friction.

  • Fat Pads:

    • Description: Pads of adipose tissue.

    • Function: Protect articular cartilage and act as packing material.

  • Meniscus:

    • Description: Crescent-shaped fibrocartilage pads.

    • Function: Improve fit between bones, absorb shock, and distribute load.

  • Accessory Ligaments:

    • Description: Bands of dense connective tissue.

    • Function: Strengthen and support the joint.

Mobility in Joints

Joint mobility varies depending on structure and function. Synovial joints are the most mobile, while fibrous joints are the least.

• Key notes: Mobility is inversely related to stability; more mobile joints are less stable.

Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and the movements they allow:

1. Plane (Gliding) Joint: Flat surfaces; allows sliding movements (e.g., intercarpal joints).

2. Hinge Joint: Cylindrical surface fits into a trough; allows flexion and extension (e.g., elbow).

3. Pivot Joint: Rounded end fits into a ring; allows rotation (e.g., proximal radioulnar joint).

4. Condyloid (Ellipsoid) Joint: Oval articular surface fits into a depression; allows all angular motions (e.g., wrist).

5. Saddle Joint: Each surface has both concave and convex areas; allows more movement than condyloid (e.g., thumb carpometacarpal joint).

6. Ball-and-Socket Joint: Spherical head fits into a cup-like socket; allows multiaxial movement (e.g., shoulder, hip).

Types of Joint Movements

Movements at synovial joints are described relative to anatomical position:

• Gliding: Sliding movements between flat surfaces.

• Angular: Changes in angle between bones (flexion, extension, abduction, adduction).

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Rotation: Bone turns around its own axis.

Movements in Reference to Anatomical Position

• Flexion: Decreases the angle between bones (e.g., bending the elbow).

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

• Hyperextension: Extension beyond the anatomical position.

• Dorsiflexion: Lifting the foot upwards toward the shin.

• Plantar Flexion: Pointing the toes downward.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Circumduction: Circular movement of a limb.

• Rotation: Turning a bone around its axis.

• Pronation vs. Supination: Pronation is turning the palm downward; supination is turning the palm upward.

• Inversion vs. Eversion: Inversion is turning the sole inward; eversion is turning it outward.

• Protraction vs. Retraction: Protraction moves a body part forward; retraction moves it backward.

• Depression vs. Elevation: Depression lowers a body part; elevation raises it.

Joints of the Axial and Appendicular Skeleton

• Axial Skeleton Joints: Include intervertebral joints, costal joints, and skull sutures.

• Appendicular Skeleton Joints: Include shoulder, hip, knee, and elbow joints.

Intervertebral Disc Composition

Intervertebral discs are fibrocartilaginous cushions between vertebrae:

• Annulus Fibrosus: Outer ring of fibrocartilage; provides strength.

• Nucleus Pulposus: Gel-like center; provides elasticity and shock absorption.

Functions of Vertebral Ligaments

• Stabilize vertebral column

• Limit excessive movement

• Protect spinal cord

Shoulder Joint

• Also known as: Glenohumeral joint

• Range of motion: Most mobile joint in the body

• Type of joint: Ball-and-socket

• Key notes: Stability is provided by rotator cuff muscles and ligaments.

Hip Joint

• Type of joint: Ball-and-socket

• Key notes: Designed for weight-bearing and stability; less mobile than shoulder.

Elbow Joint

• True elbow flexion: Occurs between the humerus and ulna.

Knee Joint

The knee is a complex hinge joint supported by several structures:

• Lateral Collateral Ligament (LCL):

  • Name: LCL

  • Location: Lateral side of the knee

  • Function: Stabilizes the outer knee

• Medial Collateral Ligament (MCL):

  • Name: MCL

  • Location: Medial side of the knee

  • Function: Stabilizes the inner knee

• Anterior Cruciate Ligament (ACL):

  • Name: ACL

  • Location: Inside the knee joint

  • Function: Prevents anterior displacement of tibia

• Posterior Cruciate Ligament (PCL):

  • Name: PCL

  • Location: Inside the knee joint

  • Function: Prevents posterior displacement of tibia

• Menisci:

  • Types: Medial and lateral meniscus

  • Composed of: Fibrocartilage

  • Function: Cushion and stabilize the knee

Rheumatism

Rheumatism is a general term for disorders involving chronic, often intermittent pain affecting the joints and/or connective tissue. It includes conditions such as arthritis.

Summary Table: Joint Classifications

Key Equations

• Relationship between stability and mobility:

Additional info: Academic context and examples have been added to expand upon the original notes and provide a comprehensive study guide.