Joints: Classification and Structure

Overview of Joint Types

Joints, also known as articulations, are connections between bones that allow for varying degrees of movement. They are classified both functionally and structurally, each with distinct characteristics and examples.

• Fibrous Joints: Sutures, syndesmosis, gomphosis

• Cartilaginous Joints: Symphysis, synchondrosis

• Synovial Joints: Hinge, pivot, saddle, condyloid (ellipsoidal), gliding, ball and socket

Example: The knee is a synovial hinge joint, while the pubic symphysis is a cartilaginous symphysis.

Additional info: The condition when two sutures grow closed is called synostosis.

Functional Classification of Joints

• Synarthroses: Immovable joints, no joint cavity (e.g., skull sutures)

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

• Diarthroses: Freely movable joints (e.g., most synovial joints)

Structural Classification of Joints

• Fibrous Joints: Bones joined by fibrous connective tissue, no joint cavity, minimal movement

• Cartilaginous Joints: Bones joined by cartilage, no joint cavity, slight movement

• Synovial Joints: Bones separated by a fluid-filled cavity, freely movable

Fibrous Joints

Types of Fibrous Joints

• Sutures: Interlocking edges held by sutural ligaments (periosteum). Example: Skull sutures.

• Synostosis: Fusion of two sutures due to age or pathology.

• Syndesmoses: Bones united by dense fibrous tissue, slight movement. Example: Distal tibia and fibula, interosseous membrane between radius and ulna.

• Gomphoses: Peg-and-socket joints. Example: Teeth and periodontal ligaments.

Cartilaginous Joints

Types of Cartilaginous Joints

• Synchondroses: Separated by hyaline cartilage, slight movement, often ossify with age. Example: Epiphyseal plates, costal cartilages.

• Symphyses: Bones joined by fibrocartilage disk, slight movement, cushioning. Example: Pubic symphysis, intervertebral discs.

Synovial Joints

General Features of Synovial Joints

Synovial joints are characterized by a fluid-filled cavity and allow for a wide range of movements. Their unique structure distinguishes them from other joint types.

• Articular Cartilage: Covers bone ends, reduces friction.

• Articular Capsule: Bilayered membrane; outer fibrous layer, inner synovial membrane.

• Synovial Cavity: Space containing synovial fluid.

• Synovial Fluid: Rich in hyaluronic acid, lubricates and nourishes joint.

• Ligaments: Reinforce the joint, may be internal or external to capsule.

• Menisci: Fibrocartilage disks reducing stress.

• Bursae and Tendon Sheaths: Sacs of synovial fluid reducing friction.

Movements of Synovial Joints

Planes of Movement

• Monaxial: Movement in one plane (e.g., hinge joints)

• Biaxial: Movement in two planes (e.g., condyloid joints)

• Multiaxial: Movement in all three planes (e.g., ball and socket joints)

Types of Movements

• Gliding: Flat surfaces slide across each other (e.g., intercarpal joints)

• Angular:

  • Flexion: Decreases joint angle

  • Extension: Increases joint angle

  • Dorsiflexion: Toes upward toward shin

  • Plantar Flexion: Toes pointed downward

  • Abduction: Limb moves away from midline

  • Adduction: Limb moves toward midline

• Circular:

  • Circumduction: Distal end describes a cone

  • Rotation: Bone pivots around its axis

• Special Movements:

  • Pronation: Palm faces downward

  • Supination: Palm faces upward

  • Inversion: Sole of foot inward

  • Eversion: Sole of foot outward

  • Protraction: Moving body part anteriorly

  • Retraction: Moving body part posteriorly

  • Elevation: Moving body part superiorly

  • Depression: Moving body part inferiorly

  • Opposition: Thumb touches other fingers

  • Reposition: Thumb returns to anatomical position

Types of Synovial Joints

Classification and Examples

Selected Synovial Joints and Common Disorders

Knee Joint

The knee is a complex ellipsoidal joint, often described as a hinge. It is stabilized by several ligaments and menisci.

• Ligaments: Medial collateral (MCL), lateral collateral (LCL), anterior cruciate (ACL), posterior cruciate (PCL)

• Menisci: Medial and lateral fibrocartilage disks

• Common Injury: The "unholy trio"—ACL, MCL, and medial meniscus—are often injured together

Shoulder Complex

• Glenohumeral Joint: Sacrifices stability for mobility; shallow glenoid cavity, stabilized by ligaments and muscles (infraspinatus, supraspinatus, teres minor, subscapularis)

• Acromioclavicular Joint: Gliding synovial joint, tightly held by ligaments

• Dislocation vs. Separation: Dislocation involves the humeral head leaving the glenoid cavity; separation involves disruption of the acromioclavicular joint

Elbow Joint

• Structure: Trochlea of humerus fits tightly with trochlear notch of ulna

• Ligaments: Ulnar collateral, radial collateral, annular ligament (holds head of radius)

• Common Injury: Dislocation of annular ligament

Hip Joint (Coxal)

• Structure: Ball and socket, anchored by strong ligaments (pubofemoral, ischiofemoral, iliofemoral)

• Movement: More restricted than shoulder

• Dislocation: Usually posterior and inferior

Temporomandibular Joint (TMJ)

• Structure: Combination gliding and ellipsoidal synovial joint with fibrocartilage disk

• Disorder: TMJ syndrome is the most common disorder

Talocrural (Ankle) Joint

• Structure: Medial and lateral malleoli attached to foot by strong ligaments

• Common Injury: Sprained ankles, usually due to inversion

• Tarsal Coalition: Abnormal fusion of tarsal bones (see slide for details)

Naming Joints

Principles of Joint Naming

Joints are typically named according to the bones they connect.

• Carpometacarpal Joint: Between carpals and metacarpals

• Radioulnar Joint: Between radius and ulna

• Atlantooccipital Joint: Between atlas and occipital bone

Summary Table: Joint Types and Examples