articulations

Articulations

  • Definition:

    • Articulation (joint) is a place of contact between bones, cartilage, or between bones and teeth.

Classification of Joints

  • Classification can be based on:

    • Function:

    • Extent of movement
      y - Structure:

    • Presence or absence of joint cavity

    • Types of connective tissue involved

Functional Classifications

  1. Synarthrosis:

    • Definition: Immobile joint.

    • Characteristics: No movement allowed.

  2. Amphiarthrosis:

    • Definition: Slightly mobile joint.

    • Characteristics: Allow limited movement.

  3. Diarthrosis:

    • Definition: Freely movable joint.

    • Characteristics: Wide range of movement.

Structural Classifications

  1. Fibrous:

    • Definition: No joint cavity; held together by dense regular connective tissue.

    • Categories:

      • Gomphosis: Peg in socket; found between tooth and alveolar process; Synarthrosis (immobile).

      • Sutures: Seams found between certain bones of the skull; Synarthrosis (immobile); can become synostoses due to fusion over time.

      • Syndesmoses: Parallel bones held by long strands of dense regular connective tissue (e.g., intraosseous membrane between radius-ulna or tibia-fibula); Amphiarthrosis (slight mobility).

  2. Cartilaginous:

    • Definition: No joint cavity; cartilage between bones (hyaline or fibrocartilage).

    • Categories:

      • Synchondroses: Contain hyaline cartilage; Synarthrosis (immobile); found at locations like epiphyseal plates and costochondral joints.

      • Symphyses: Contain fibrocartilage; Amphiarthrosis (slight mobility); Locations include pubic symphysis and intervertebral joints.

  3. Synovial:

    • Definition: Contains a joint cavity filled with synovial fluid; articulating surfaces are enclosed in a joint capsule; bones are connected by ligaments.

    • Characteristics:

      • Ends of bones covered with articular cartilage; joint cavity separates articulating bones; joint enclosed by an articular capsule lined by synovial membrane containing synovial fluid.

    • Categories of Synovial Joints by Movement:

      • Plane: Diarthrosis; simplest, allowing side-to-side movements; examples include intercarpal and intertarsal joints.

      • Hinge: Diarthrosis; convex to concave articulation; examples include elbow, knee, and interphalangeal joints.

      • Pivot: Diarthrosis; rounded surface within a ring; examples include atlantoaxial and proximal radioulnar joints.

      • Condylar: Diarthrosis; convex to concave articulation; example includes metacarpophalangeal joints.

      • Saddle: Diarthrosis; greater range of motion; example includes carpometacarpal joints.

      • Ball-and-Socket: Diarthrosis; spherical head in an articular cup; examples include coxal and glenohumeral joints.

Functions of Synovial Fluid

  • Definition: A viscous, oily substance formed by synovial membrane cells and blood plasma filtrate.

  • Functions of Synovial Fluid:

    • Lubrication of joints.

    • Nourishment of chondrocytes.

    • Shock absorption.

Components of Synovial Joints

  • Articular Cartilage:

    • Type: Hyaline cartilage.

    • Function: Reduces friction, absorbs compression, and prevents damage; lacks perichondrium.

  • Articular Capsule:

    • Outer Fibrous Layer: Dense connective tissue that strengthens the joint.

    • Inner Synovial Membrane: Specialized connective tissue that produces synovial fluid.

  • Ligaments:

    • Definition: Dense regular connective tissue that connects bone to bone; stabilizes, strengthens, and reinforces joints.

    • Types:

    • Intrinsic Ligaments: Thickenings of the articular capsule; may be intra- or extra-capsular.

    • Extrinsic Ligaments: Physically separate from the joint capsule.

  • Accessory Structures:

    • Tendons: Dense regular connective tissue that binds muscle to bone and serves as the source of joint movement.

    • Bursa: Fibrous sac filled with synovial fluid, lined by synovial membrane; alleviates friction; tendon sheath is an elongated bursa wrapped around a tendon in areas of high friction.

    • Articular Discs: Small and round (meniscus) structures that provide shock absorption and additional support for smooth movement.

    • Fat Pads: Peripherally distributed fat that serves as packing material and protection.

Classifications by Movement

  1. Uniaxial Joint:

    • Bone moves in only one plane/axis.

    • Examples: Hinge joints (e.g., elbow, knee) and pivot joints (e.g., atlantoaxial joint).

  2. Biaxial Joint:

    • Bone moves in two planes/axes.

    • Examples: Condylar joints (e.g., metacarpophalangeal joints) and saddle joints.

  3. Multiaxial Joint:

    • Bone moves in multiple planes/axes.

    • Examples: Ball-and-socket joints (e.g., shoulder and hip).

Types of Movements in Synovial Joints

  1. Gliding Movements:

    • Articulating surfaces slide past one another in back-and-forth or side-to-side motions; limited movement but allows multiple directions.

  2. Angular Movements:

    • Flexion: Decreasing the angle between two bones in the anterior/posterior plane.

    • Extension: Increasing the angle between two bones in the anterior/posterior plane.

    • Lateral Flexion: Bending the vertebral column in a lateral direction along the coronal plane.

    • Abduction: Lateral movement of a body part away from the midline.

    • Adduction: Medial movement of a body part towards the midline.

    • Circumduction: Proximal end of an appendage remains stationary while the distal end makes a circular motion.

  3. Rotational Movements:

    • Lateral Rotation: Turning anterior bone surface laterally.

    • Medial Rotation: Turning posterior surface of the bone medially.

    • Pronation: Rotation of the forearm resulting in palm facing posteriorly or inferiorly.

    • Supination: Rotation of the forearm resulting in palm facing anteriorly or superiorly.

  4. Special Movements:

    • Depression: Movement inferiorly.

    • Elevation: Movement superiorly.

    • Dorsiflexion: Ankle joint movement "up"; dorsum moves toward the anterior surface of the leg.

    • Plantar Flexion: Ankle movement "down"; plantar surface moves toward the posterior surface of the leg.

    • Eversion: Twisting the foot to turn the sole outward.

    • Inversion: Twisting the foot to turn the sole inward.

    • Protraction: Anterior movement of a body part from anatomical position.

    • Retraction: Posterior movement of a body part from anatomical position.

    • Opposition: Thumb movement crossing the palm towards fingers for grasping.

Biomechanics of Synovial Joints

  • Biomechanics Definition: Application of mechanical principles to biology.

  • Key Terminology:

    • Lever: An elongated, rigid object (long bone) that rotates around a fixed point (joint).

    • Fulcrum: Fixed point (joint).

    • Effort: The force applied at one arm (point of the lever) (muscle).

    • Effort Arm: Part of the lever from fulcrum to point of effort.

    • Resistance: Opposition to movement located at another point.

    • Resistance Arm: Part of the lever from fulcrum to point of resistance.

  • Movement occurs if effort exceeds resistance (ER).

Lever Classes

  1. First Class Lever: (E - F - R)

    • Effort (E), Fulcrum (F), Resistance (R).

  2. Second Class Lever: (E - R - F)

    • Effort (E), Resistance (R), Fulcrum (F).

  3. Third Class Lever: (F - E - R)

    • Fulcrum (F), Effort (E), Resistance (R).

Specific Joints

  1. Temporomandibular Joint (TMJ):

    • Articulation: Head of the mandible with temporal bone.

    • Structural Classification: Hinge and plane.

    • Movements: Mandible depression and elevation, gliding (both lateral and protraction).

    • Note: Only mobile joint of the skull.

  2. Glenohumeral Joint:

    • Articulation: Head of humerus with glenoid cavity of scapula.

    • Structural Classification: Ball and socket.

    • Movements: Abduction, adduction, circumduction, flexion, extension, lateral and medial rotation.

    • Note: Most mobile joint in the body.

Aging and Joint Health

  • Arthritis: Damage to articular cartilage.

    • Osteoarthritis: Caused by the wear and tear of articular cartilage.

    • Rheumatoid Arthritis: An autoimmune disorder leading to synovial fluid inflammation, affecting the articular cartilage and bone.

  • Joint Health: Moderate exercise helps in:

    • Allowing for synovial fluid movement

    • Nourishing cartilage.

    • Strengthening joint stabilizing muscles.