Synovial Joints Overview

Synovial Joints

• Six Features:
  • Articular Cartilage: Glassy-smooth, hyaline cartilage that covers the bone surfaces. It acts as a spongy cushion, absorbing compression and preventing bone crushing.
  • Joint (Articular) Cavity: Unique to synovial joints. It contains synovial fluid and is a potential space, expanding only when fluid accumulates.
  • Articular Capsule: A double-layered capsule that encloses the joint cavity.
    • Fibrous Layer: Tough outer layer composed of dense irregular connective tissue, continuous with the periosteum. It strengthens the joint to prevent bone separation.
    • Synovial Membrane: Inner layer composed of loose connective tissue lining the fibrous layer and covering non-cartilage joint surfaces. It produces synovial fluid.
  • Synovial Fluid: Viscous, egg-white consistency due to hyaluronic acid. Secreted by cells in the synovial membrane, it thins during joint activity.
    • Acts as a lubricant, reducing friction between cartilages.
    • Enables "weeping lubrication": synovial fluid is squeezed out and reabsorbed during joint compression and relaxation.
    • Nourishes cartilage cells.
    • Contains phagocytic cells to remove microbes and debris.
  • Reinforcing Ligaments: Bandlike ligaments that reinforce and strengthen the joint.
    • Capsular Ligaments: Thickened portions of the fibrous layer.
    • Extracapsular Ligaments: Distinct ligaments found outside the capsule.
    • Intracapsular Ligaments: Ligaments deep to, but not actually within, the joint cavity because they're covered by synovial membrane.
• Movement:
  • Synovial joints are freely movable, allowing for a considerable range of motion.
• Stability:
  • Ligaments: Provide significant support and stability.
  • Muscle Tendons: Muscles crossing the joint contribute by pulling the bones together.
  • Joint Shape: The shape of the articulating bone surfaces influences joint movement.
• Individuals described as "double-jointed" have the same number of joints as others. However, their ligaments are more flexible and their joints have a greater range of motion. This can lead to greater joint instability.

Synovial Joint Stability

• Synovial Joints are the weakest points in the skeleton and require stability to prevent dislocation
• Stability is dependent on:
  • Shapes of articular surfaces
  • Number and positioning of ligaments
  • Muscle tone

Types of Synovial Joints

• Plane: Flat articular surfaces allow for gliding movements
• Hinge: Uni-axial for flexion and extension
• Pivot: Uni-axial for rotation
• Condylar: Bi-axial, allowing for flexion, extension, abduction, and adduction
• Saddle: Bi-axial, allowing for flexion, extension, abduction, and adduction
• Ball-and-Socket: Multiaxial, with flexion, extension, abduction, adduction, rotation, and Circumduction

Special Body Movements

• Gliding: Sliding across surfaces
• Flexion: Decreasing the angle between two bones
• Extension: Increasing the angle between two bones
• Abduction: Moving a limb away from midline
• Adduction: Moving a limb towards the midline
• Circumduction: Moving a limb in a cone shape
• Rotation: Turning a bone on its longitudinal axis
• Medial Rotation: Towards the median plane
• Dorsiflexion and Plantar Flexion: Foot movements
• Inversion and Eversion: Foot movements
• Protraction and Retraction: Mandible movements
• Elevation and Depression: Mandible movements

Structural and Functional Characteristics of Body Joints

• Skull: Fibrous suture, no movement.
• Temporomandibular: Synovial, modified hinge, gliding, rotation, lateral movement, elevation, depression, protraction, and retraction
• Atlanto-occipital: Synovial, condylar, flexion, extension, lateral flexion, circumduction.
• Atlantoaxial: Synovial, pivot, rotation of the head.
• Intervertebral: Cartilaginous symphysis, slight movement.
• Intervertebral articular: Synovial plane, gliding.
• Costovertebral: Synovial plane, gliding of ribs.
• Sternoclavicular: Synovial shallow saddle, multiaxial movement of the clavicle.
• Sternocostal (first): Cartilaginous synchondrosis, no movement.
• Sternocostal (2-7): Synovial, double plane, gliding.
• Acromioclavicular: Synovial plane, gliding, rotation of scapula on clavicle.
• Shoulder (Glenohumeral): Synovial ball-and-socket, multiaxial, flexion, extension, abduction, adduction, rotation, and circumduction of the humerus.
• Elbow: Synovial hinge, flexion and extension of the forearm.
• Proximal Radioulnar: Synovial pivot, rotation of head of radius.
• Distal Radioulnar: Synovial pivot, rotation of the radius, allowing for pronation and supination.
• Wrist: Synovial condylar, flexion, extension, abduction, adduction, circumduction of the hand.
• Intercarpal: Synovial plane, gliding.
• Carpometacarpal of digit I (thumb): Synovial saddle, flexion, extension, abduction, adduction, circumduction, opposition.
• Carpometacarpal of digits II-V: Synovial plane, gliding of metacarpals.
• Metacarpophalangeal (knuckle): Synovial condylar, flexion, extension, abduction, adduction, circumduction of fingers.
• Interphalangeal (finger): Synovial hinge, flexion, extension of fingers.
• Sacroiliac: Synovial plane in childhood, becoming fibrous in adulthood.
• Pubic Symphysis: Cartilaginous symphysis, allows for slight movement.
• Hip (Coxal): Synovial ball-and-socket, multiaxial, flexion, extension, abduction, adduction, rotation, circumduction of the thigh.
• Knee (Tibiofemoral): Synovial modified hinge, flexion and extension of the leg, rotation in flexed position
• Knee (Femoropatellar): Synovial plane, gliding of the patella.
• Superior Tibiofibular: Synovial plane, gliding of fibula.
• Inferior Tibiofibular: Fiirous syndesmosis, giving for dorsiflexion.
• Ankle: Synovial hinge, dorsiflexion and plantar flexion of the foot.
• Intertarsal: Synovial plane, gliding, inversion, and eversion of the foot.
• Tarsometatarsal: Synovial plane, gliding of metatarsals.
• Metatarsophalangeal: Synovial condylar, flexion extension, abduction, adduction, circumduction of the great toe.
• Interphalangeal (toe): Synovial hinge, flexion, extension of toes.

Joint Structure and Function

• Bursae and tendon sheaths improve joint function by reducing friction between moving parts.
• The most important factor in stabilizing synovial joints is the strength and arrangement of ligaments and muscles around the joint.

Types of Synovial Joints

• Uniaxial joints allow movement in one plane, examples are hinge and pivot joints.
• Condylar and saddle joints are biaxial joints, allowing movement in two planes.

The Shoulder Joint

• The shoulder joint is a ball-and-socket joint, with the head of the humerus fitting into the glenoid cavity of the scapula.
• The glenoid labrum, a rim of fibrocartilage, deepens the glenoid cavity slightly but does not contribute significantly to joint stability.
• The articular capsule of the shoulder joint is thin and loose, allowing for free movement.
• The coracohumeral ligament and glenohumeral ligaments provide some reinforcement, but the primary stability comes from the muscles surrounding the joint, particularly the tendon of the long head of the biceps brachii muscle.

The Hip Joint

• The hip joint is a ball-and-socket joint, with the head of the femur fitting into the acetabulum of the hip bone.
• The acetabulum is significantly deeper than the glenoid cavity, providing greater stability to the hip joint.
• The acetabular labrum, a rim of fibrocartilage, deepens the acetabulum further and improves stability.
• The articular capsule of the hip joint is strong and reinforced by three ligaments: the iliofemoral ligament, the ischiofemoral ligament, and the pubofemoral ligament.
• The ligament of the head of the femur (ligamentum teres) is a small ligament that helps stabilize the hip joint.
• The hip joint sacrifices some mobility for stability.

Test your knowledge about synovial joints, their unique features, and functions. The quiz covers aspects like articular cartilage, joint cavity, articular capsule, and synovial fluid. Understand how these components work together to facilitate movement and reduce friction.