The Articular System: Joint Structure, Classification, Planes, and Degrees of Freedom

The Articular System: Joint Structure

• Bones: The Lever Portion of the Joint

  • Dictate the amount and direction of motion possible at a joint.

  • Muscles attach to bones through specialized connective tissues called tendons.

  • Types of Bones:

    • Flat bones

    • Irregular bones

    • Short bones

    • Long bones

• Cartilage: Tissue that Surrounds the Bones

  • Hyaline Cartilage (Articular Cartilage):

    • Covers the ends of bones within synovial joints.

    • Lacks its own nerve or blood supply, meaning it cannot repair itself if damaged.

  • Fibrocartilage:

    • Functions as a crucial shock absorber.

    • Decreases friction between the ends of bones.

    • Particularly important in weight-bearing bones to distribute forces.

  • Elastic Cartilage:

    • Primarily functions to maintain the shape of a given structure (e.g., ear, epiglottis).

• Soft Tissue Structures Supporting Joints

  • Muscle:

    • Provides the contractile force necessary for joint movement.

  • Tendon:

    • A strong band of connective tissue that connects muscle to bone.

  • Ligament:

    • A band of fibrous connective tissue primarily connecting bone to bone, providing stability.

  • Aponeurosis:

    • A broad, flat, tendinous sheath.

    • Found in areas where muscles attach to bone, but where the muscle itself cannot directly attach.

• Bursa

  • Pad-like sacs filled with synovial fluid.

  • Strategically located in areas subject to excessive friction, typically between tendons and bones or muscles and ligaments.

  • Their primary role is to reduce friction between two moving parts, allowing for smoother motion.

  • Types of Bursa:

    • Natural Bursa: Present as a normal anatomical structure.

    • Acquired Bursa: Develops in response to chronic friction in an area where a bursa would not normally be present.

Joint Classifications

• Joint Categories

  • Synarthrodial Joints:

    • Primarily designed for stability, allowing little to no movement.

    • Subdivisions include fibrous and cartilaginous joints.

  • Synovial Joints (Diarthrodial Joints):

    • Primarily designed for mobility, allowing a wide range of motion.

  • Fibrous Joints (Functional Classifications - Synarthrodial)

    • Synarthrosis:

      • True synarthrodial joints, exhibiting no movement.

      • Example: Suture joints between the bones of the skull.

    • Syndesmosis:

      • Classified as amphiarthrodial, allowing a small amount of motion.

      • Are ligamentous joints.

      • Example: The tibiofibular joint, connecting the tibia and fibula.

    • Gomphosis:

      • Synarthrodial joints, characterized by no movement.

      • Described as a "peg in socket" joint.

      • Example: The articulation of teeth within their sockets.

  • Cartilaginous Joints (Types of Synarthrodial Joints - Amphiarthrodial)

    • Hyaline Cartilage Joints (Synchondroses): Bones united by hyaline cartilage. Allow very little motion.

    • Fibrocartilaginous Joints (Symphyses): Bones united by fibrocartilage. Allow a small amount of motion.

• Synovial Joints (Diarthrodial Joints) - Characteristics

  • Characterized by no direct union between bone ends; rather, there is a joint cavity.

  • The joint cavity is filled with synovial fluid.

  • Generally less stable than synarthrodial joints, prioritizing mobility.

  • Examples: Shoulder, hip, knees, elbow.

  • Specific Types of Synovial Joints:

    • Ball and Socket Joints:

      • Allow movement in multiple planes.

      • Examples: Shoulder, hip.

    • Hinge Joints:

      • Allow primary movement in one plane (back and forth).

      • Examples: Elbow, knee.

    • Saddle Joints:

      • Articular surfaces are reciprocally concave and convex, resembling a saddle.

      • Example: The carpometacarpal joint of the thumb.

    • Pivot Joints:

      • Allow primarily rotational motion.

      • Example: The atlantoaxial joint (C1-C2 vertebrae) allowing head rotation.

    • Plane (Gliding) Joints:

      • Allow very little, usually gliding or sliding movement between flat surfaces.

      • Example: Intercarpal joints in the wrist.

    • Condyloid Joints:

      • An oval-shaped condyle fits into an elliptical cavity.

      • Permit movement in two planes (e.g., flexion/extension, abduction/adduction, circumduction).

      • Example: Metacarpophalangeal joints of the fingers.

Synovial Capsule and Fluid

• Synovial Capsule:

  • Outer Layer: A fibrous sleeve that is robust and provides significant strength to the joint.

  • Inner Layer: The synovial membrane, responsible for producing synovial fluid.

• Synovial Fluid:

  • A clear, viscous (thick) fluid.

  • Functions:

    • Lubricates articular cartilage, reducing friction during movement.

    • Provides shock absorption, cushioning the joint surfaces.

    • Supplies nutrition to the avascular articular cartilage.

Planes and Axes of Motion

• Cardinal Planes: Dividing the Body into Equal Parts

  • Sagittal Plane:

    • Divides the body vertically into right and left halves.

    • Movement occurring in this plane includes flexion and extension.

  • Frontal Plane (Coronal Plane):

    • Divides the body vertically into front (anterior) and back (posterior) parts.

    • Movement occurring in this plane includes abduction and adduction.

  • Transverse Plane (Horizontal Plane):

    • Divides the body horizontally into top (superior) and bottom (inferior) parts.

    • Movement occurring in this plane primarily involves rotational movements.

• Center of Gravity (COG):

  • The theoretical balance point of the body where its entire weight is considered to be equally distributed.

  • Located approximately anterior to the S2 vertebrae.

• Axes of Motion:

  • Sagittal Axis:

    • Runs through the joint from front to back (anterior to posterior).

    • Perpendicular to the frontal plane.

  • Frontal Axis (Coronal Axis):

    • Runs through the joint from side to side (medial to lateral).

    • Perpendicular to the sagittal plane.

  • Vertical Axis (Longitudinal Axis):

    • Runs through the joint from top to bottom (superior to inferior).

    • Perpendicular to the transverse plane.

• Combining Planes and Axes of Motion:

  • Sagittal Plane movements occur around the Frontal Axis:

    • Flexion and Extension.

  • Frontal Plane movements occur around the Sagittal Axis:

    • Abduction and Adduction.

  • Transverse Plane movements occur around the Vertical Axis:

    • Rotation movements.

Degrees of Freedom

• Describes how many planes and axes a joint can move in.

  • Nonaxial Joints:

    • Permit gliding or sliding motions (linear movement) rather than rotation around a specific axis.

    • These joints do not necessarily move in a plane around an axis in the traditional sense.

    • Example: Intercarpal joints of the wrist, where carpal bones glide past each other.

  • Uniaxial Joints:

    • Allow movement in 1 plane and around 1 axis (angular motion).

    • Examples:

      • Hinge Joints: Tibiofemoral (knee), Humeroulnar (elbow) joints.

      • Pivot Joints: Radioulnar joint (forearm pronation/supination), Atlantoaxial joint (C1-C2).

  • Biaxial Joints:

    • Allow movement in 2 planes and around 2 axes.

    • Example: The wrist joint (radiocarpal joint).

    • Types:

      • Condyloid joints

      • Saddle joints

  • Triaxial Joints:

    • Allow movement in 3 planes and around 3 axes.

    • Examples: The hip joint, the shoulder joint.

    • Type:

      • Ball and socket joints.

    • These are considered the most mobile joint type in the body.