Chapter 8: Joints (Articulations) – Structured Study Notes

Joints (Articulations)

Definition of Articulations

An articulation (joint) is the location where two or more bones meet. Joints are essential for movement and provide structural support to the skeleton.

• Key Point: Joints allow varying degrees of movement, depending on their structure and function.

• Example: The knee joint enables walking and running, while skull sutures provide protection without movement.

Classification of Joints

Structural and Functional Classification

Joints are classified in two main ways:

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

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

Functional Types of Joints

Synarthroses, Amphiarthroses, Diarthroses

• Synarthroses: Immovable joints (e.g., skull sutures).

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

• Diarthroses: Freely movable joints (e.g., shoulder, hip).

Structural Types of Joints

Fibrous Joints

Fibrous joints are connected by dense connective tissue and lack a joint cavity. Most are immovable.

• Sutures: Found between skull bones; immovable (synarthrosis).

• Synostoses: Bones fuse together; immovable.

• Syndesmosis: Bones joined by ligaments or interosseous membrane; slightly movable (radius–ulna, tibia–fibula).

• Gomphosis: "Peg in socket" joint (teeth in alveolar sockets); immovable.

Cartilaginous Joints

Cartilaginous joints unite bones with cartilage and lack a joint cavity.

• Synchondroses: Hyaline cartilage; immovable (epiphyseal plates, costal cartilage).

• Symphyses: Fibrocartilage pad between bones; slightly movable (pubic symphysis, intervertebral discs).

Synovial Joints

Synovial joints have a fluid-filled cavity and are freely movable. Most joints in the body are synovial.

• Key Point: Synovial joints allow a wide range of movements.

General Structure of Synovial Joints

Components of Synovial Joints

• Articular Cartilage: Hyaline cartilage covers bone ends, reducing friction.

• Joint Cavity: Space filled with synovial fluid.

• Articular Capsule: Encloses the joint cavity; consists of an outer fibrous capsule (strength) and an inner synovial membrane (produces synovial fluid).

• Synovial Fluid: Lubricates, nourishes, and absorbs shock; contains hyaluronic acid.

• Reinforcing Ligaments: Support the joint; can be capsular, extracapsular, or intracapsular.

• Nerves and Blood Vessels: Supply the joint, monitor pain and position.

• Fat Pads: Cushion and protect; common in hip and knee.

• Articular Discs (Menisci): Fibrocartilage pads improve fit and stability.

Bursae and Tendon Sheaths

Accessory Structures

• Bursae: Flattened synovial sacs reduce friction between bone and soft tissues.

• Tendon Sheaths: Elongated bursae wrap around tendons, especially in wrists, hands, and feet.

Stability of a Joint

Factors Affecting Joint Stability

• Shape of Articular Surfaces: Determines range and type of movement (e.g., ball-and-socket vs hinge).

• Ligaments: Number and placement affect stability and mobility.

• Muscle Tone: Continuous low-level contraction stabilizes joints.

Joint Terminology

Origin and Insertion

• Origin: Fixed attachment of a muscle.

• Insertion: Movable attachment of a muscle.

Movements Allowed by Synovial Joints

Categories of Movement

• Gliding Movements: Sliding over flat surfaces (e.g., intercarpal joints).

• Angular Movements: Change angle between bones.

  • Sagittal Plane: Flexion (decreases angle), Extension (increases angle), Hyperextension (beyond anatomical position).

  • Frontal Plane: Abduction (away from midline), Adduction (toward midline), Lateral flexion (side-bending).

  • All Planes: Circumduction (circular movement).

• Rotation: Bone rotates around its axis (medial and lateral rotation).

Special Movements

• Supination / Pronation: Rotation of forearm.

• Dorsiflexion / Plantar flexion: Movement of foot.

• Inversion / Eversion: Movement of sole of foot.

• Protraction / Retraction: Movement of jaw or shoulder.

• Elevation / Depression: Raising or lowering body part.

• Opposition: Thumb movement to touch fingers.

Types of Synovial Joints

Classification and Examples

Type	Axes	Movement	Example
Plane (Gliding)	Nonaxial	Gliding	Intercarpal joints
Hinge	Uniaxial	Flexion, extension	Elbow
Pivot	Uniaxial	Rotation	Proximal radioulnar joint
Condyloid	Biaxial	Flexion, extension, abduction, adduction	Metacarpophalangeal joints
Saddle	Biaxial	Similar to condyloid	Thumb (carpometacarpal joint)
Ball-and-Socket	Multiaxial	All directions	Shoulder, hip

Specific Synovial Joints

Knee Joint

• Patellofemoral joint: Planar

• Tibiofemoral joint: Modified hinge

• Movements: Flexion and extension

• Structures: Menisci, patellar ligament, cruciate ligaments (anterior prevents forward sliding, posterior prevents backward sliding of tibia)

Glenohumeral (Shoulder) Joint

• Type: Ball-and-socket

• Movement: Circumduction

• Structures: Glenoid labrum, rotator cuff (SITS muscles: supraspinatus, infraspinatus, teres minor, subscapularis)

Elbow Joint

• Humeroulnar joint: Hinge (flexion and extension)

• Radioulnar joint: Pivot (supination and pronation)

Coxal (Hip) Joint

• Type: Ball-and-socket

• Movement: Circumduction

• Structures: Acetabular labrum, ligament of the head of the femur

Temporomandibular Joint (TMJ)

• Type: Modified hinge with gliding

• Movements: Elevation, depression, protraction, retraction

Joint Injuries

Common Joint Injuries

• Cartilage tears: Damage to joint cartilage

• Arthroscopy: Minimally invasive joint surgery

• Sprain: Ligament injury

• Strain: Muscle or tendon injury

• Dislocation: Bone forced out of joint

Inflammatory Conditions

Bursitis and Tendonitis

• Bursitis: Inflammation of a bursa

• Tendonitis: Inflammation of a tendon

Arthritis

Types of Arthritis

• Osteoarthritis: Degenerative joint disease; joint stiffness, pain, cartilage breakdown

• Rheumatoid Arthritis: Autoimmune disorder; inflammation, joint deformity

• Gouty Arthritis: Uric acid crystal buildup; sudden severe pain, swelling

• Lyme Disease: Bacterial infection; joint pain, inflammation