Synovial Joints: Structure and Types

• Synovial joints are the most common and movable joint in the body
• They are enclosed by a synovial membrane

Synovial Joint Structure

• Articular cartilage is smooth, hyaline cartilage that covers the ends of bones
• Joint cavity is the space containing synovial fluid
• Synovial fluid lubricates and nourishes the articular cartilage
• The articular capsule has two layers: an outer fibrous capsule and an inner synovial membrane
• Reinforcing ligaments strengthen the joint and can be intrinsic, extrinsic, or capsular
• Nerves and blood vessels provide sensation and nutrients to the joint

Types of Synovial Joints

• Plane (gliding) joints have flat articular surfaces
• These joints facilitate short, gliding movements
• An example is intercarpal joints
• Hinge joints involve the cylindrical end of one bone fitting into a trough-shaped end of another
• Motion occurs along a single plane, allowing flexion and extension
• An example is the elbow joint
• Pivot joints feature a rounded end of one bone fitting into a ring formed by another bone and a ligament
• These joints allow rotational movement
• An example is the proximal radioulnar joint
• Condyloid (ellipsoidal) joints have an oval articular surface that fits into an oval depression
• These joints enable flexion, extension, abduction, and adduction
• An example is metacarpophalangeal joints (knuckles)
• Saddle joints have both concave and convex articular surfaces
• These joints allow greater freedom of movement than condyloid joints
• An example is the carpometacarpal joint of the thumb
• Ball-and-socket joints involve the spherical head of one bone fitting into a round socket of another
• These joints allow multi-axial movement, including flexion, extension, abduction, adduction, and rotation
• Examples include the shoulder and hip joints

Movements

• Flexion decreases the angle between bones
• Extension increases the angle between bones
• Abduction moves a limb away from the midline
• Adduction moves a limb toward the midline
• Rotation turns a bone around its own axis
• Circumduction is a circular movement that combines flexion, extension, abduction, and adduction

Stability Factors

• The shape of articular surfaces determines possible movements and contributes to stability
• Ligaments prevent excessive or undesirable movements, providing more stability with more ligaments
• Muscle tone maintains tension on tendons and ligaments, acting as a critical stabilizing factor

Common Synovial Joint Disorders

• Osteoarthritis is the degeneration of articular cartilage, leading to pain and stiffness
• Rheumatoid arthritis is an autoimmune disorder causing inflammation of the synovial membrane
• Bursitis is the inflammation of bursae (fluid-filled sacs that reduce friction)
• Tendonitis is the inflammation of the tendons around the joint