Joints: Classification and Structure

Functional Classes of Joints

Joints, or articulations, are classified functionally based on the amount of movement they allow. Understanding these classes is essential for analyzing joint function and pathology.

• Synarthroses: Immovable joints. Example: Sutures in the skull.

• Amphiarthroses: Slightly movable joints. Example: Intervertebral discs.

• Diarthroses: Freely movable joints. Example: Shoulder joint.

Structural Classes of Joints

Joints are also classified structurally based on 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. Example: Sutures, syndesmoses, gomphoses.

• Cartilaginous Joints: Bones joined by cartilage; no joint cavity. Example: Synchondroses (hyaline cartilage), symphyses (fibrocartilage).

• Synovial Joints: Bones separated by a fluid-filled joint cavity; most movable type. Example: Knee, elbow, shoulder.

Types of Fibrous Joints

• Sutures: Found between bones of the skull; immovable.

• Syndesmoses: Bones connected by ligaments; slight movement possible. Example: Distal tibiofibular joint.

• Gomphoses: Peg-in-socket joints. Example: Teeth in alveolar sockets.

Types of Cartilaginous Joints

• Synchondroses: Bones united by hyaline cartilage. Example: Epiphyseal plate in children.

• Symphyses: Bones united by fibrocartilage. Example: Pubic symphysis, intervertebral discs.

Synovial Joints: Structure and Function

Synovial Joint Features

Synovial joints are characterized by a joint cavity filled with synovial fluid, allowing free movement.

• Articular cartilage: Hyaline cartilage covering bone ends.

• Joint (articular) cavity: Space containing synovial fluid.

• Articular capsule: Two layers (fibrous outer layer and synovial inner membrane).

• Synovial fluid: Lubricates and nourishes cartilage.

• Reinforcing ligaments: Strengthen the joint.

• Nerves and blood vessels: Supply the joint tissues.

Additional Features in Synovial Joints

• Bursae: Fluid-filled sacs that reduce friction.

• Tendon sheaths: Elongated bursae that wrap around tendons.

Types of Synovial Joints

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

• Plane joints: Gliding movements. Example: Intercarpal joints.

• Hinge joints: Flexion and extension. Example: Elbow.

• Pivot joints: Rotation. Example: Proximal radioulnar joint.

• Condyloid joints: Flexion, extension, abduction, adduction. Example: Metacarpophalangeal joints.

• Saddle joints: Greater range of movement. Example: Thumb (carpometacarpal joint).

• Ball-and-socket joints: Multiaxial movement. Example: Shoulder, hip.

Joint Movements and Terminology

Types of Movements

Joints allow various movements, each with specific terminology.

• Flexion: Decreases the angle between bones.

• Extension: Increases the angle between bones.

• Hyperextension: Extension beyond anatomical position.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Dorsiflexion: Lifting the foot upwards.

• Plantar flexion: Pointing the toes downward.

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

• Rotation: Bone turns around its own axis.

• Supination: Turning palm upward.

• Pronation: Turning palm downward.

• Inversion: Turning sole of foot inward.

• Eversion: Turning sole of foot outward.

• Protraction: Moving a body part forward.

• Retraction: Moving a body part backward.

• Elevation: Lifting a body part.

• Depression: Lowering a body part.

• Lateral and medial excursion: Side-to-side movement (e.g., jaw).

Knee Joint: Structure and Injuries

Knee Articulations and Ligaments

The knee is a complex synovial joint involving multiple articulations and ligaments.

• Articulations: Femur-tibia, femur-patella.

• Ligaments: Cruciate ligaments (anterior and posterior), collateral ligaments (medial and lateral), patellar ligament.

During a lateral blow to the extended knee, the following structures are commonly injured:

• Medial collateral ligament (MCL)

• Medial meniscus

• Anterior cruciate ligament (ACL)

Major Synovial Joints

Shoulder, Elbow, and Hip Joints

• Shoulder (Glenohumeral) Joint: Ball-and-socket; greatest range of motion; prone to dislocation.

• Elbow Joint: Hinge joint; allows flexion and extension.

• Hip Joint: Ball-and-socket; stable due to deep socket and strong ligaments.

Temporomandibular Joint (TMJ)

• TMJ: Jaw joint; allows opening, closing, and side-to-side movement.

• TMJ Syndrome: Pain and dysfunction of the jaw joint and surrounding muscles.

Joint Injuries and Disorders

Strain vs. Sprain

• Strain: Injury to muscle or tendon.

• Sprain: Injury to ligament; involves stretching or tearing.

Dislocations occur when bones are forced out of alignment; they must be reduced (put back in place).

Common Joint Problems

Additional info:

• Joint injuries are common in athletes and may require imaging and physical therapy for diagnosis and treatment.

• Prevention includes proper warm-up, strengthening exercises, and protective equipment.