6.4 Joints and Articulations

Joints and articulations are the unsung heroes of our skeletal system. They're the reason we can move, bend, and twist. Without them, we'd be stiff as boards! These connectors come in different types, each with its own unique structure and function.

From the immovable sutures in our skull to the freely movable ball-and-socket joints in our hips, joints are essential for our daily activities. They're not just about movement, though. Joints also provide stability, shock absorption, and even help distribute forces throughout our body.

Joint Classification by Structure and Function

Structural Classification

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• Joints are classified structurally based on the type of connective tissue that binds the bones together and the presence or absence of a joint cavity
• Structural classifications of joints include fibrous, cartilaginous, and synovial joints
• Fibrous joints are connected by dense regular connective tissue (sutures, gomphoses, and syndesmoses)
• Cartilaginous joints are connected by hyaline cartilage or fibrocartilage (synchondroses and symphyses)
• Synovial joints are characterized by the presence of a joint cavity and are the most common and most movable type of joint in the body

Functional Classification

• Functionally, joints are classified based on the degree of movement they permit: synarthrosis (immovable), amphiarthrosis (slightly movable), and diarthrosis (freely movable)
• Fibrous joints are functionally classified as synarthroses due to their limited mobility
• Cartilaginous joints are functionally classified as either synarthroses or amphiarthroses depending on the type of cartilage and the degree of movement allowed
• Synovial joints are functionally classified as diarthroses, allowing for a wide range of motion

Components of Synovial Joints

Articular Surfaces and Cartilage

• Synovial joints consist of the articulating bones, which are covered by articular cartilage
• Articular cartilage is a smooth, lubricated surface that enables movement and provides shock absorption
• The articular cartilage is avascular and receives nutrients from the synovial fluid

Joint Capsule and Synovial Membrane

• The articular capsule is a fibrous connective tissue that surrounds the joint, providing stability and a sealed environment for the synovial fluid
• The synovial membrane is a specialized connective tissue that lines the inner surface of the articular capsule
• The synovial membrane secretes synovial fluid, which lubricates the joint, nourishes the articular cartilage, and removes waste products

Accessory Structures

• Ligaments are dense regular connective tissue that connect bone to bone, providing stability and guiding movement
• Tendons are dense regular connective tissue that attach muscle to bone, transmitting the force generated by muscle contraction to the bones
• Bursae are fluid-filled sacs that reduce friction between moving structures, such as bones, muscles, and tendons
• Fat pads, such as the infrapatellar fat pad in the knee, provide cushioning and fill spaces within the joint

Movement Types in Joints

Gliding and Angular Movements

• Gliding movements occur between flat or slightly curved articular surfaces, such as in the carpals and tarsals
• Angular movements include flexion (decreasing joint angle), extension (increasing joint angle), abduction (moving away from the midline), adduction (moving toward the midline), and circumduction (combination of flexion, extension, abduction, and adduction)
• Angular movements occur in synovial joints such as the elbow (hinge joint), knee (modified hinge joint), and hip (ball-and-socket joint)

Rotation and Special Movements

• Rotation is a movement around a central axis, such as in the atlantoaxial joint (pivot joint) or the proximal radioulnar joint (pivot joint)
• Special movements are specific to certain joints and include:
  • Elevation and depression (scapula)
  • Protraction and retraction (mandible)
  • Inversion and eversion (ankle)
  • Dorsiflexion and plantar flexion (ankle)
  • Supination and pronation (forearm)

Ligaments and Tendons in Joint Function

Ligaments and Joint Stability

• Ligaments connect bone to bone and provide passive stability to the joint by limiting excessive or abnormal movements
• The arrangement and tautness of ligaments determine the type and range of motion permitted at a joint
• Examples of ligaments include the anterior cruciate ligament (ACL) and medial collateral ligament (MCL) in the knee, and the anterior talofibular ligament (ATFL) in the ankle

Tendons and Joint Movement

• Tendons connect muscle to bone and transmit the force generated by muscle contraction to the bones, enabling movement
• The strength and elasticity of tendons allow them to withstand the forces generated during movement and help maintain proper joint alignment
• Examples of tendons include the Achilles tendon (connecting the calf muscles to the calcaneus) and the rotator cuff tendons (connecting the rotator cuff muscles to the humerus)

Injury and Joint Dysfunction

• Injury to ligaments or tendons can result in joint instability, altered movement patterns, and increased risk of further injury
• Ligament sprains occur when a ligament is stretched or torn, leading to pain, swelling, and instability (ankle sprains)
• Tendon strains or tendinopathies involve micro-tears or inflammation of the tendon, causing pain and reduced function (rotator cuff tendinitis)
• Proper rehabilitation and strengthening exercises are essential for restoring joint stability and function after ligament or tendon injuries