Joints (Articulations)

Introduction to Joints

Joints, or articulations, are the locations where two bones meet and where body movement occurs. The structure of a joint determines its function, and there is a tradeoff between strength and mobility: the more mobile a joint, the less stable it tends to be.

Classification of Joints

Structural and Functional Classification

Joints are classified by both their anatomical structure and their functional range of motion.

• Structural Classification: Fibrous, Cartilaginous, Bony, Synovial

• Functional Classification: Synarthrosis (immovable), Amphiarthrosis (slightly movable), Diarthrosis (freely movable)

Synarthrosis (Immovable Joints)

Synarthroses are very strong joints where the edges of bones may touch or interlock. They may be fibrous or cartilaginous.

• Suture: Found only between bones of the skull; bones interlock and are bound by dense fibrous connective tissue.

• Gomphosis: Binds teeth to bony sockets via the periodontal ligament.

• Synchondrosis: Rigid cartilaginous bridge between two bones (e.g., between vertebrosternal ribs and sternum).

• Synostosis: Created when two bones fuse (e.g., metopic suture of frontal bone).

Amphiarthrosis (Slightly Movable Joints)

Amphiarthroses allow more movement than synarthroses but are stronger than diarthroses. They may be fibrous or cartilaginous.

• Syndesmosis: Bones connected by a ligament.

• Symphysis: Bones connected by fibrocartilage.

Diarthrosis (Freely Movable Joints)

Diarthroses are synovial joints, which are freely movable and found at the ends of long bones.

Synovial Joints

Basic Structure of Synovial Joints

Synovial joints are surrounded by a joint capsule and contain a synovial membrane. Synovial fluid fills the joint cavity, and articular cartilage covers the articulating surfaces to prevent direct contact between bones.

• Synovial Fluid: Lubricates, distributes nutrients, and absorbs shock.

• Accessory Structures: Cartilages (meniscus), fat pads, ligaments, tendons, bursae.

Accessory Structures of Synovial Joints

• Meniscus: Fibrocartilage pad between opposing bones.

• Fat Pads: Adipose tissue covered by synovial membrane, protecting articular cartilages.

• Ligaments: Support and strengthen joints; sprains occur when ligaments are torn.

• Tendons: Attach muscles around the joint.

• Bursae: Small pockets of synovial fluid that cushion areas where tendons or ligaments rub against other tissues.

Factors Stabilizing Synovial Joints

• Collagen fibers of joint capsule and ligaments

• Shapes of articulating surfaces and menisci

• Other bones, muscles, or fat pads

• Tendons attached to articulating bones

Movements at Synovial Joints

Planes and Types of Movement

Movements are described by the plane or direction and the relationship between structures. Joints may be monaxial (1 plane), biaxial (2 planes), or triaxial (3 planes).

Types of Movement

• Gliding: Two flat surfaces slide past each other (e.g., carpal bones).

• Angular: Includes flexion, extension, hyperextension, abduction, adduction, and circumduction.

• Rotational: Includes medial and lateral rotation, pronation, and supination.

• Special Movements: Inversion, eversion, dorsiflexion, plantar flexion, opposition, reposition, protraction, retraction, depression, elevation, lateral flexion.

Gliding Movement

Gliding occurs when two flat surfaces slide past each other.

Angular Movement

Angular movements change the angle between articulating bones.

• Flexion: Decreases angle between bones.

• Extension: Increases angle between bones.

• Hyperextension: Extension past anatomical position.

• Abduction: Movement away from longitudinal axis.

• Adduction: Movement toward longitudinal axis.

• Circumduction: Circular movement without rotation.

Rotational Movement

Rotation occurs around the longitudinal axis of the body or limb.

• Medial Rotation: Internal rotation toward the long axis.

• Lateral Rotation: External rotation away from the body.

• Pronation: Rotates forearm so that radius rolls across ulna; palm faces posteriorly.

• Supination: Turns palm anteriorly; forearm is supinated in anatomical position.

Special Movements

• Inversion: Twists sole of foot medially.

• Eversion: Twists sole of foot laterally.

• Dorsiflexion: Flexion at ankle (lifting toes).

• Plantar Flexion: Extension at ankle (pointing toes).

• Opposition: Movement of thumb toward palm or other fingers.

• Reposition: Opposite of opposition.

• Protraction: Anterior movement in horizontal plane.

• Retraction: Pulling back.

• Depression: Moving a structure inferiorly.

• Elevation: Moving a structure superiorly.

• Lateral Flexion: Bending vertebral column to the side.

Classification of Synovial Joints

Types of Synovial Joints

Synovial joints are classified by the shape of their articulating surfaces and the type of movement they allow.

Intervertebral Joints

Structure and Function

Intervertebral joints include synovial joints between articular processes and symphyses between vertebral bodies. Intervertebral discs separate vertebral bodies and consist of anulus fibrosus (outer fibrocartilage) and nucleus pulposus (gelatinous core).

Intervertebral Ligaments

• Ligamenta flava: Connect laminae of adjacent vertebrae.

• Posterior longitudinal ligament: Connects posterior surfaces of vertebral bodies.

• Anterior longitudinal ligament: Connects anterior surfaces of vertebral bodies.

• Interspinous ligaments: Connect spinous processes.

• Supraspinous ligament: Connects tips of spinous processes (C7 to sacrum).

Elbow and Knee Joints

Elbow Joint

The elbow is a hinge joint involving the humerus, radius, and ulna. The humero-ulnar joint is the largest and strongest, while the humeroradial joint is smaller.

• Biceps brachii muscle: Controls elbow motion.

• Elbow ligaments: Radial collateral, anular, ulnar collateral.

Knee Joint

The knee is a complex hinge joint transferring weight from femur to tibia. It has three articulations: two femur–tibia and one patella–femur.

• Menisci: Fibrocartilage pads at femur–tibia articulations.

• Major ligaments: Patellar, popliteal, cruciate, tibial collateral, fibular collateral.

Shoulder and Hip Joints

Shoulder Joint (Glenohumeral Joint)

The shoulder is a ball-and-socket diarthrosis between the head of the humerus and the glenoid cavity of the scapula. It has the greatest range of motion and is frequently dislocated.

• Glenoid labrum: Rim of fibrocartilage deepening the socket.

• Ligaments: Acromioclavicular, coracoclavicular, coraco-acromial, coracohumeral, glenohumeral.

• Rotator cuff muscles: Supraspinatus, infraspinatus, teres minor, subscapularis.

• Bursae: Subdeltoid, subcoracoid, subacromial, subscapular.

Hip Joint

The hip is a strong ball-and-socket diarthrosis between the head of the femur and the acetabulum of the hip bone. The acetabular labrum increases depth and seals in synovial fluid.

• Ligaments: Iliofemoral, pubofemoral, ischiofemoral, transverse acetabular, ligament of the femoral head.

Effects of Aging on Joints

Degenerative Changes and Clinical Problems

Aging leads to degenerative changes in joints, including rheumatism (pain and stiffness), arthritis (joint inflammation), osteoarthritis (wear and tear), rheumatoid arthritis (immune attack), and gouty arthritis (uric acid crystals).

• Joint immobilization reduces synovial fluid flow and can cause arthritis symptoms.

• Bone mass decreases, bones weaken, and fracture risk increases with age.

Integration with Other Systems

Functional Relationships

The skeletal system interacts with other body systems. Bone remodeling involves osteoblasts (formation) and osteoclasts (recycling). Factors affecting bone balance include age, physical stress, hormones, mineral uptake, and genetics.

• Muscles attach to bones for movement.

• Endocrine system controls bone growth and metabolism.

• Digestive and urinary systems supply minerals for bone growth.

• The skeleton serves as a mineral reserve.

Summary Table: Joint Classification