BIO 121 Lecture on Articulations

Articulations (Joints) Overview

Joint Structure

• Articulations: Articulations, or joints, are specialized connections between bones that play a critical role in determining the direction and distance of movement, referred to as Range of Motion (ROM). The structure and type of joint significantly influence its functionality, with different joint types providing various degrees of mobility and stability.

• Note: It is important to note that joint strength typically decreases as mobility increases; therefore, highly mobile joints like the shoulder are more prone to injuries.

• Example: The shoulder joint exemplifies high mobility due to its wide range of motion but comes with an increased risk of dislocation and other injuries due to its structural composition.

Functional Categories of Joints

• Synarthrosis (no movement):

• Synarthrosis joints allow no movement due to their structure; bony edges are either extremely close together or interlocked in such a way that they offer a high degree of stability.

• Example:

  • Suture: These are found exclusively between the skull bones, where the bony surfaces interlock and are secured by dense fibrous connective tissue, providing strength and protection to the brain.

• Amphiarthrosis (little movement):

• Amphiarthrosis joints permit a small amount of movement while retaining their structural integrity. They are typically connected by collagen fibers or cartilage, which allows for flexibility while maintaining strength.

• Examples:

  • Syndesmosis: This type involves bones that are connected by a ligament, such as the distal joint between the tibia and fibula.

  • Symphysis: This joint type has a wedge or pad of fibrocartilage between the bones, illustrated by the pubic symphysis that allows for slight movement during childbirth.

• Diarthrosis (free movement):

• Diarthrosis joints, or synovial joints, are characterized by their ability to allow extensive movement. These joints are crucial for mobility and are further categorized based on their type of movement.

• Subdivided by type of motion:

  • Monaxial: Movement occurs in one plane, exemplified by joints such as the elbow and ankle.

  • Biaxial: Movement occurs in two planes, as seen in the joints of the ribs and wrist.

  • Triaxial: Movement occurs in three planes, demonstrated by joints like the shoulder and hip, allowing for rotational movement and a broad range of activities.

Structural Categories of Joints

• Fibrous:

• Suture: Interlocked bones that create a synarthrotic joint, preventing any movement.

• Gomphosis: A unique fibrous connection that anchors teeth into their bony sockets, securing dental stability.

• Cartilaginous:

• Synchondrosis: This is defined as a rigid cartilaginous bridge connecting bones.

  • Examples: Common examples include epiphyseal cartilage found in growing bones and the joint between the first ribs and the sternum, which assists in respiratory movement.

• Synostosis: A fusion of two bones, rendering them immovable.

  • Examples: Notable instances include the coronal suture of the skull and the epiphyseal lines that form when growth plates close.

• Synovial:

• Comprised of articulating bones separated by a joint cavity filled with synovial fluid, synovial joints are the most mobile type and are crucial for bodily movement.

• Common structures include:

  • Articular Cartilage: This smooth tissue covers the ends of bones in synovial joints, reducing friction and absorbing shock.

  • Articular Capsule: A fibrous enclosure surrounding the joint, containing the synovial membrane which produces synovial fluid.

  • Synovial Fluid: A viscous fluid that lubricates and nourishes the cartilage surfaces within the joint.

Synovial Joint Structures

• Accessory Structures: These elements enhance joint functionality:

• Meniscus: A fibrocartilage pad that stabilizes joints and helps to evenly distribute weight across the joint surfaces.

• Fat Pads: Localized adipose tissue that cushions joints during movement and helps protect the underlying structures.

• Accessory Ligaments: Strong, supportive tissues that help stabilize and strengthen the joint against dislocations.

• Bursae: These are small, fluid-filled sacs located throughout the joint that serve to minimize friction between moving parts.

Stabilizing Factors of Synovial Joints

• Several factors contribute to the stability and prevent injury in synovial joints:

• Collagen Fibers: Strong fibers within the joint capsule and surrounding ligaments limit the range of motion and provide structural support.

• Articulating Surface Shapes: Customized shapes of bones and the presence of menisci restrict movement in certain directions to prevent dislocation.

• Surrounding Structures: Other bones, muscles, and fat pads contribute to joint stability, while tendons can either facilitate or restrict specific movements.

Common Joint Injuries

• Examples of common joint injuries include:

• Dislocation (luxation): A significant injury resulting in the displacement of bones at a joint, typically involving damage to articular cartilage, ligaments, and joint capsules.

• Subluxation: A partial dislocation in which the joint surfaces remain partially in contact.

• Sprain: An injury that typically involves overstretching or tearing of ligaments, leading to joint instability and pain.

• Strain: This type of injury affects muscles or tendons, resulting from excessive force or stretching beyond capacity.

Types of Dynamic Motion of the Skeleton

• Linear Motion (gliding): Involves sliding movements that occur between flat surfaces of bones.

• Angular Motion: Encompasses movements that change the angle between two bones:

• Decreasing angle is known as flexion; increasing angle is known as extension.

• Occasionally, a movement that exceeds the normal extent of extension is termed hyperextension.

• Rotation: This includes movements such as left/right rotation, medial/lateral rotation, and variations like pronation/supination.

• Special Movements: These are unique movements of certain body parts:

• Dorsiflexion: Flexion of the foot at the ankle, bringing the toes upward.

• Plantarflexion: Movement of the foot that points the toes away from the leg.

• Opposition: A specific movement of the thumb towards the palm, allowing for gripping actions.

• Lateral Flexion: Side bending movement of the spine.

• Protraction and Retraction: Protraction involves moving a body part forward, while retraction brings it back.

• Elevation: Moving a body part upwards.

• Depression: The downward movement of a body part, opposing elevation.