Joints of the Skeletal System

Overview

Joints, also known as articulations, are the sites where two or more bones meet. They play a crucial role in providing mobility and stability to the skeletal system. This section covers the classification of joints, their structural and functional types, the movements they allow, and common joint disorders.

Classification of Joints

Structural Classification

Joints are classified based on the material binding the bones and the presence or absence of a joint cavity.

• Fibrous Joints: Bones joined by dense connective tissue (CT); no joint cavity.

• Cartilaginous Joints: Bones connected by cartilage; no joint cavity.

• Synovial Joints: Bones separated by a fluid-filled joint cavity.

Functional Classification

Joints are also classified by the degree of movement they permit:

• Synarthroses: Immovable joints.

• Amphiarthroses: Slightly movable joints.

• Diarthroses: Freely movable joints.

Fibrous Joints

Characteristics

Fibrous joints are connected by dense connective tissue and lack a joint cavity, allowing minimal or no movement.

• Suture: Found between bones of the skull; bones interlock and are filled with connective tissue. In adulthood, these may ossify to form synostoses.

• Gomphosis: The joint between a tooth and its socket in the jawbone, held together by the periodontal ligament.

• Syndesmosis: Bones are bound by a sheet or cord of connective tissue (interosseous membrane). Examples include the tibiofibular and radioulnar joints.

Cartilaginous Joints

Characteristics

Cartilaginous joints are connected by cartilage and lack a joint cavity. They allow slight movement.

• Synchondroses: Bones joined by a band of hyaline cartilage. These can be temporary (e.g., epiphyseal plate in children) or permanent (e.g., costal cartilage). In adults, some may ossify to become synostoses.

• Symphyses: Articular surfaces of bones are covered with hyaline cartilage and attached to fibrocartilage. Examples include the pubic symphysis and intervertebral discs.

Synovial Joints

Key Structures

Synovial joints are the most movable type of joint and are characterized by the presence of a joint cavity filled with synovial fluid.

• Articular Cartilage: Hyaline cartilage covering the ends of bones.

• Joint Capsule: Composed of an external fibrous layer and an internal synovial membrane.

• Joint Cavity: Space filled with synovial fluid.

• Synovial Fluid: Contains glycosaminoglycans (GAGs), proteoglycans, and phagocytic cells; nourishes cartilage and provides lubrication (weeping lubrication).

• Reinforcing Ligaments: Strengthen the joint capsule and stabilize the joint.

• Blood Vessels & Nerves: Provide nutrients and proprioceptive feedback.

Accessory Structures

• Meniscus: Fibrocartilage pads that improve fit and stability of the joint.

• Bursae: Fibrous sacs containing synovial fluid that reduce friction.

Types of Synovial Joints

Classification by Shape and Movement

Synovial joints are classified based on the shapes of their articulating surfaces and the types of movement they allow.

• Ball-and-Socket Joint: Spherical head fits into a concave cavity; movement in all planes and rotation (e.g., hip, shoulder).

• Condyloid (Ellipsoid) Joint: Oval condyle fits into an elliptical cavity; movement in two planes (e.g., metacarpophalangeal joints).

• Gliding (Plane) Joint: Flat or slightly curved surfaces allow sliding and twisting movements (e.g., wrist, ankle, vertebrae).

• Hinge Joint: Convex surface fits into a concave surface; movement in one plane (e.g., elbow, phalanges).

• Pivot Joint: Cylindrical surface protrudes into a ring formed by bone and ligament; rotational movement around a central axis (e.g., atlantoaxial joint in neck).

• Saddle Joint: Both surfaces are concave and convex; movement in two planes (e.g., thumb carpometacarpal joint).

Types of Joint Movements

Basic Movements

• Flexion: Decreases the angle between bones (e.g., bending the knee).

• Extension: Increases the angle between bones (e.g., straightening the knee).

• Hyperextension: Extension beyond the normal range (e.g., bending the wrist backward).

• Dorsiflexion: Flexion of the ankle so the foot approaches the shin.

• Plantar Flexion: Extension of the ankle so the foot moves away from the shin.

• Abduction: Movement away from the midline of the body.

• Adduction: Movement toward the midline of the body.

• Rotation: Moving a bone around its long axis.

• Circumduction: Moving a body part so its end follows a circular path.

• Supination: Rotating the palm to face anteriorly.

• Pronation: Rotating the palm to face posteriorly.

• Eversion: Sole of foot faces laterally.

• Inversion: Sole of foot faces medially.

• Protraction: Forward movement in a transverse plane.

• Retraction: Backward movement in a transverse plane.

• Elevation: Lifting or moving superiorly along a frontal plane.

• Depression: Lowering an elevated part to its original position.

Common Joint Injuries and Disorders

Joint Injuries

• Sprains: Stretching or tearing of connective tissue associated with the joint (common in spine, ankle, knee).

• Dislocations: Bones are forced out of normal position in a joint cavity; often associated with sprains (shoulders, fingers).

• Bursitis: Inflammation of a bursa due to overuse or stress (heel, knee, elbow).

• Tendonitis: Inflammation of tendon sheaths; similar causes, symptoms, and treatment as bursitis.

Arthritis

• Rheumatoid Arthritis (RA): Autoimmune disorder causing inflammation and thickening of the synovial membrane, damaged cartilage, and possible bone fusion (ankylosis); systemic illness with severe pain.

• Gout: Increased uric acid levels lead to urate crystal deposition in joints (commonly the great toe).

• Lyme Disease: Bacterial infection (from deer tick/black-legged tick) affecting joints.

• Osteoarthritis (OA): Degenerative disease associated with aging; articular cartilage deteriorates, bone thickens, and bony spurs may form, restricting movement. Most commonly affects hinge, spine, and weight-bearing joints. Risk factors include heredity, joint injury, overuse, obesity, sex, and physical activity.

Concept Check

• Synchondroses in Adults: Synchondroses (e.g., epiphyseal plates) typically ossify in adulthood, so they are not found at the ends of the femur in individuals in their mid-twenties.

• Distinguishing OA vs. RA: OA is characterized by localized joint pain and stiffness, often in weight-bearing joints, with bony spurs and cartilage degeneration. RA presents with systemic symptoms, joint swelling, and possible ankylosis due to autoimmune inflammation.

• Properties of Synovial Joints: Synovial joints are distinguished by a joint cavity, articular cartilage, synovial fluid, and accessory structures such as ligaments and bursae, allowing for a wide range of movements.

Summary Table: Joint Classification

Key Equations

While joint movement is primarily anatomical, some biomechanical equations may be relevant:

• Torque (Joint Movement):

*Additional info: Biomechanical equations are included for academic completeness, though not directly referenced in the original notes.*