Synovial joints are the most common and flexible joints in the body. They allow for smooth, pain-free movement thanks to their unique structure, which includes articular cartilage, a joint capsule, and synovial fluid.

These joints come in six types, each with its own shape and range of motion. From the simple sliding of plane joints to the wide-ranging movement of ball-and-socket joints, synovial joints are essential for our daily activities and overall mobility.

Synovial Joint Structure and Function

Components of synovial joints

• Articular cartilage covers the articulating surfaces of bones
  • Hyaline cartilage provides smooth, low-friction surface for joint movement
  • Lacks blood vessels and nerves, receiving nutrients from synovial fluid (diffusion)
• Joint capsule surrounds the joint, providing stability and support
  • Consists of outer fibrous capsule and inner synovial membrane
• Synovial membrane lines the inner surface of the joint capsule
  • Secretes synovial fluid
  • Composed of type A synoviocytes (macrophage-like cells that remove debris) and type B synoviocytes (fibroblast-like cells that secrete synovial fluid components)
• Synovial fluid is a clear, viscous fluid within the joint cavity
  • Lubricates and nourishes the articular cartilage
  • Contains hyaluronic acid (viscosity), lubricin (lubrication), and nutrients
• Subchondral bone is the layer of bone beneath the articular cartilage
  • Provides structural support and shock absorption

Roles of accessory structures

• Ligaments are bands of dense regular connective tissue
  • Connect bones to bones, providing stability and limiting excessive joint movement (collateral ligaments)
• Tendons are bands of dense regular connective tissue
  • Connect muscles to bones, transmitting muscle forces to enable joint movement (quadriceps tendon)
• Menisci are crescent-shaped fibrocartilaginous structures found in some synovial joints
  • Improve joint stability and congruency, distribute load, and absorb shock (medial and lateral menisci of the knee)
• Bursae are fluid-filled sacs lined with synovial membrane
  • Reduce friction between moving structures such as bones, tendons, muscles, and skin (subacromial bursa in the shoulder, prepatellar bursa in the knee)
• Fat pads are adipose tissue within or around some joints
  • Provide cushioning and fill space (infrapatellar fat pad in the knee joint)

Classification of Synovial Joints

Types of synovial joints (synovial joint classification)

1. Plane (gliding) joints have flat or slightly curved articulating surfaces

   • Allow sliding or gliding movements (intercarpal joints in the wrist, intertarsal joints in the ankle, acromioclavicular joint in the shoulder)

2. Hinge joints have a cylindrical surface of one bone fitting into a trough-shaped surface of another

   • Allow flexion and extension movements in one plane (elbow joint, interphalangeal joints in fingers and toes, ankle joint)

3. Pivot joints have a rounded surface of one bone articulating with a ring formed by another bone and a ligament

   • Allow rotational movements (atlantoaxial joint between C1 and C2 vertebrae, proximal radioulnar joint in the forearm)

4. Condyloid (ellipsoidal) joints have an oval-shaped condyle of one bone fitting into an elliptical cavity of another

   • Allow flexion, extension, abduction, adduction, and circumduction movements (metacarpophalangeal joints in the hand, radiocarpal joint in the wrist)

5. Saddle joints have articulating surfaces with both concave and convex regions, resembling a saddle

   • Allow flexion, extension, abduction, adduction, and circumduction movements (carpometacarpal joint of the thumb)

6. Ball-and-socket joints have a spherical head of one bone fitting into a cup-like cavity of another

   • Allow flexion, extension, abduction, adduction, rotation, and circumduction movements (shoulder joint, hip joint)

Joint Mechanics and Function

Range of motion and joint stability

• Range of motion refers to the extent of movement possible at a joint
  • Influenced by joint type, ligament flexibility, and muscle strength
• Joint stability is maintained through a balance of static and dynamic stabilizers
  • Static stabilizers include joint capsule, ligaments, and bony structures
  • Dynamic stabilizers involve muscles and tendons

Joint lubrication and movement

• Synovial fluid provides lubrication through various mechanisms
  • Boundary lubrication: Lubricin forms a thin layer between articulating surfaces
  • Fluid film lubrication: Synovial fluid forms a thin film between surfaces during movement
• Arthrokinematics describes the precise movements of joint surfaces
  • Includes rolling, sliding, and spinning motions between articulating surfaces