Overview
This section explains how joints in the human body are classified, focusing on both structural and functional types. It describes the criteria for each classification and provides examples to illustrate the different types of joints.
Structural Classification of Joints
- Joints, or articulations, are locations where adjacent bones or bone and cartilage come together to form a connection.
- Structural classification is based on how the bones are joined:
- Fibrous joints: Bones are united by fibrous connective tissue. These joints do not have a joint cavity.
- Cartilaginous joints: Bones are joined by either hyaline cartilage or fibrocartilage. There is no joint cavity.
- Synovial joints: The articulating surfaces of the bones are not directly connected but instead meet within a fluid-filled joint cavity. This cavity contains lubricating fluid, allowing for free movement. Synovial joints are the most common type in the body.
Functional Classification of Joints
- Functional classification is determined by the degree of movement allowed between the bones:
- Synarthrosis: Immobile joints that provide strong connections between bones.
- Amphiarthrosis: Joints that allow limited movement.
- Diarthrosis: Freely movable joints that permit a wide range of motion.
- The functional classification often depends on the joint's location and structure:
- Fibrous joints can be either synarthroses (immobile) or amphiarthroses (slightly mobile).
- Cartilaginous joints can also be synarthroses or amphiarthroses.
- All synovial joints are functionally classified as diarthroses (freely movable).
Types of Functional Joints and Examples
- Synarthrosis:
- Immobile or nearly immobile joints.
- Provide strong unions, important for protecting internal organs.
- Examples: Sutures between skull bones, manubriosternal joint (between the manubrium and body of the sternum).
- Amphiarthrosis:
- Joints with limited mobility.
- Examples: Intervertebral discs (cartilaginous joints between vertebrae, made of fibrocartilage), pubic symphysis (cartilaginous joint between the right and left pubic bones).
- These joints provide stability and allow for small movements that can add up to larger movements in the body.
- Diarthrosis:
- Freely movable joints, mainly found in the appendicular skeleton (limbs).
- Allow for extensive movement and are divided based on the number of axes of motion:
- Uniaxial joints: Movement in one plane (e.g., elbow joint, which allows bending and straightening).
- Biaxial joints: Movement in two planes (e.g., metacarpophalangeal or knuckle joints, which allow bending/straightening and spreading fingers).
- Multiaxial joints: Movement in three planes, including rotation (e.g., shoulder and hip joints, which allow movement in anterior-posterior, medial-lateral, and rotational directions).
Subtypes of Diarthrosis (Synovial) Joints
- Uniaxial joints: Permit movement around a single axis or in one anatomical plane. Example: Elbow joint.
- Biaxial joints: Allow movement in two different planes. Example: Knuckle (metacarpophalangeal) joints, which allow both flexion/extension and abduction/adduction.
- Multiaxial joints: Allow movement in three anatomical planes and rotation. Examples: Shoulder and hip joints, which permit a wide range of limb movements, including rotation.
Key Terms & Definitions
- Joint (articulation): The location where two bones or bone and cartilage come together.
- Fibrous joint: A joint where bones are united by fibrous connective tissue.
- Cartilaginous joint: A joint where bones are joined by cartilage, either hyaline or fibrocartilage.
- Synovial joint: A joint where bones meet within a fluid-filled cavity, allowing free movement.
- Synarthrosis: An immobile joint.
- Amphiarthrosis: A slightly movable joint.
- Diarthrosis: A freely movable joint.
- Uniaxial joint: A joint that allows movement in one plane.
- Biaxial joint: A joint that allows movement in two planes.
- Multiaxial joint: A joint that allows movement in three planes and rotation.
- Intervertebral disc: A pad of fibrocartilage between vertebrae, providing limited movement and strong union.
- Pubic symphysis: A cartilaginous joint joining the pubic regions of the right and left hip bones.
Action Items / Next Steps
- Review diagrams and images that illustrate examples of synarthrosis, amphiarthrosis, and diarthrosis joints, such as sutures in the skull, intervertebral discs, and the hip joint.
- Practice identifying the structural and functional classifications of joints in laboratory settings or on exams.
- Familiarize yourself with the anatomical planes (transverse, frontal, sagittal) to better understand the axes of movement in diarthrotic joints.