Joints: An Overview

Definition and Classification

Joints, also known as articulations, are sites where two or more bones meet. They are essential for movement and provide structural support. Joints are classified in two main ways:

• Structural classification: Based on the material binding the bones and the presence or absence of a joint cavity. There are three structural types: fibrous, cartilaginous, and synovial joints.

• Functional classification: Based on the degree of movement allowed. The three functional types are synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable).

Structural Classification of Joints

Fibrous Joints

Fibrous joints are connected by dense fibrous connective tissue and lack a joint cavity. Their movement depends on the length of connective fibers.

• Synarthroses: Mostly immovable joints.

• Amphiarthroses: Some are slightly movable.

Sutures

• Rigid, interlocking joints found only in the skull.

• Contain short connective tissue fibers that allow for expansion during youth.

• Ossify and fuse into immovable joints in middle age, protecting the brain.

Syndesmosis

• Bones are connected by ligaments (bands of connective tissue).

• Movement varies with fiber length: short fibers offer little/no movement (e.g., inferior tibiofibular joint), longer fibers allow more movement (e.g., interosseous membrane between radius and ulna).

Gomphosis

• Peg-in-socket fibrous joint.

• Periodontal ligament holds the tooth in its socket.

• Only examples are teeth in alveolar sockets.

Cartilaginous Joints

Cartilaginous joints are united by cartilage and lack a joint cavity.

• Synarthroses: Immovable joints.

• Amphiarthroses: Slightly movable joints.

Synchondroses

• Bar or plate of hyaline cartilage unites bones.

• Almost all are immovable.

• Examples: Epiphyseal plate (temporary joint), joint between first rib and sternum.

Symphyses

• Bones are united by fibrocartilage.

• Strong, slightly movable joints.

• Examples: Intervertebral joints, pubic symphysis.

Synovial Joints

Synovial joints are characterized by bones separated by a fluid-filled cavity. They are the most common joints in the limbs and are classified as diarthroses (freely movable).

• Six types, varying by movement and articular surface shape.

General Features of Synovial Joints

• Articular cartilage: Hyaline cartilage covers bone ends, preventing crushing.

• Joint (articular) cavity: Small, fluid-filled space unique to synovial joints.

• Articular capsule: Two layers—external fibrous layer (dense connective tissue) and inner synovial membrane (loose connective tissue, produces synovial fluid).

• Synovial fluid: Slippery filtrate of plasma and hyaluronic acid; lubricates and nourishes articular cartilage; contains phagocytic cells.

• Reinforcing ligaments: Strengthen and stabilize the joint.

• Nerves and blood vessels: Detect pain, monitor joint position/stretch, and supply filtrate for synovial fluid.

Other Features: Bursae and Tendon Sheaths

• Bursae: Bags of synovial fluid that reduce friction where ligaments, muscles, skin, tendons, or bones rub together (e.g., shoulder).

• Tendon sheaths: Elongated bursae wrapped around tendons subjected to friction (e.g., wrists).

Movements at Synovial Joints

Muscle Attachment and Movement

• All muscles attach to bone or connective tissue at no fewer than two points: origin (immovable bone) and insertion (movable bone).

• Muscle contraction causes insertion to move toward origin.

• Movements occur along transverse, frontal, or sagittal planes.

Range of Motion

• Nonaxial: Slipping movements only.

• Uniaxial: Movement in one plane.

• Biaxial: Movement in two planes.

• Multiaxial: Movement in or around all three planes.

Types of Movements

Gliding

• One flat bone surface slips over another.

• Examples: Intercarpal/intertarsal joints, articular processes of vertebrae.

Angular Movements

• Increase or decrease the angle between two bones.

• Flexion: Decreases angle of the joint.

• Extension: Increases angle of the joint.

• Hyperextension: Movement beyond anatomical position.

• Abduction: Movement away from the midline along the frontal plane.

• Adduction: Movement toward the midline along the frontal plane.

• Circumduction: Combination of flexion, abduction, extension, and adduction; distal end moves in a circle.

Rotation

• Turning of bone around its own long axis toward or away from midline.

• Medial rotation: Toward the midline.

• Lateral rotation: Away from the midline.

• Examples: Rotation between C1 and C2 vertebrae, rotation of humerus and femur.

Special Movements

• Supination and pronation: Rotation of radius and ulna. Supination: palms face anteriorly; radius and ulna are parallel. Pronation: palms face posteriorly; radius rotates over ulna.

• Dorsiflexion and plantar flexion: Dorsiflexion: bending foot toward shin. Plantar flexion: pointing toes.

• Inversion and eversion: Inversion: sole faces medially. Eversion: sole faces laterally.

• Protraction and retraction: Protraction: mandible juts out. Retraction: mandible pulled toward neck.

• Elevation and depression: Elevation: lifting body part superiorly. Depression: lowering body part.

• Opposition: Movement of thumb to touch tips of other fingers on the same hand or any grasping movement.

Types of Synovial Joints

Classification Based on Shape and Movement

Synovial joints are categorized by the shape of their articular surfaces and the movement they allow.

• Plane: Nonaxial, gliding movements. Example: intercarpal joints.

• Hinge: Uniaxial, flexion and extension. Example: elbow joint.

• Pivot: Uniaxial, rotation. Example: proximal radioulnar joint.

• Condylar: Biaxial, flexion/extension and abduction/adduction. Example: metacarpophalangeal joints.

• Saddle: Biaxial, similar to condylar but with greater range. Example: carpometacarpal joint of the thumb.

• Ball-and-socket: Multiaxial, all movements. Example: shoulder and hip joints.

The Knee Joint

Structure and Function

The knee is the largest and most complex joint in the body, consisting of three joints surrounded by a single cavity:

• Femoropatellar joint: Plane joint allowing gliding motion during knee flexion.

• Tibiofemoral joint: Lateral and medial joints between femoral condyles and menisci of tibia; hinge joint allowing flexion, extension, and some rotation when knee is partly flexed.

Ligaments and Bursae

• Anteriorly, the quadriceps tendon gives rise to three broad ligaments from patella to tibia: medial and lateral patellar retinacula flank the patellar ligament.

• There are approximately 12 bursae in the knee.

Extracapsular Ligaments

• Fibular and tibial collateral ligaments stabilize the knee and prevent rotation when extended.

Intracapsular Ligaments

• Located within the capsule but outside the synovial cavity.

• Anterior cruciate ligament (ACL): Attaches to anterior tibia, prevents forward sliding of tibia, and stops hyperextension.

• Posterior cruciate ligament (PCL): Attaches to posterior tibia, prevents backward sliding of tibia and forward sliding of femur.

Common Joint Pathology

Knee Injuries

• The knee absorbs significant vertical force but is vulnerable to horizontal blows.

• ACL tears are common in athletes who change direction rapidly.

• Sprains: Occur when reinforcing ligaments are stretched or torn.

• Dislocations: Occur when bones are forced out of alignment.

• Joint cracking: Results from pressure changes and air bubbles within the joint.

Arthritis

• Over 100 types of inflammatory or degenerative diseases that damage joints.

• Osteoarthritis: Cartilage is broken down faster than it is replaced; bone spurs may form.

• Rheumatoid arthritis: Autoimmune disease; inflammation of synovial membrane releases chemicals that destroy tissue, causing swelling, erosion of cartilage, and scar tissue formation.

Check Your Understanding

• The articular joint capsule contains a membrane that makes synovial fluid.

Summary Table: Structural and Functional Classification of Joints

Key Equations and Concepts

• Joint movement planes:

  • Transverse plane

  • Frontal plane

  • Sagittal plane

• Synovial fluid composition:

  • Plasma + hyaluronic acid

Additional info:

• Some content was expanded for clarity and completeness, including definitions and examples for each joint type and movement.

• Table was inferred for summary purposes.