Anatomy of a Joint

In the human anatomy, a joint is the juncture of two or more bones. Bones come together inside a joint as part of a special design to help your body move fluidly and dynamically.

Most joints are mobile, although some—like the suture joints in the skull—don’t move. Other joints may move only slightly, like the vertebrae.

Mobile joints are the most common, and include:

  • Ball-and-socket joints enable multidirectional movements like backward, forward, and sideways. Examples of ball-and-socket joints are hips and shoulders.
  • Ellipsoidal joints enable multidirectional movements, but not rotational. They provide extension, flexion, abduction, adduction, circumduction movement. The wrist is an ellipsoidal joint.
  • Hinge joints enable bending and straightening movements only. Fingers, knees, elbows and toes are hinge joints.
    Pivot joints enable limited rotational motion around only a single axis, like the neck.
  • Plane joints provide gliding and basic rotational movements such as the acromioclavicular joint of the shoulder.
  • Saddle joints operate as extension, abduction, adduction, circumduction, and very slight rotation and flexion. The thumb is a good example of a saddle joint.

Your Joint Anatomy

Your joints are comprised of both bone and soft tissue, with every component playing a unique role in overall functionality and movement. In addition to the blood vessels that supply vital circulation to your joints—and the nerves that provide sensation and aid muscle movement—your joints’ anatomy includes:

  • Bones protect your organs, produce blood cells, store minerals, and aid joint mobility.`
  • Cartilage covers and protects your bones, reducing friction with its lubricated, smooth surface.
  • Ligaments connect bones together with strong, fibrous, elastic bands that help control joint movement.
  • Muscles contain protein and are responsible for the production of force and motion for your entire body.
  • Synovial membrane lines the joint. secures it within a joint capsule, and produces synovial fluid to lubricate the joint.
  • Synovial fluid provides joint lubrication.
  • Tendons secure muscles to bones with a tough connective tissue that helps to control the movement of the joint.
  • Bursae cushion joints against shock and friction, with their fluid-filled inner core and their positioning between bones and ligaments. Learn about bursitis (irritation of the bursae)

Joint Injury & Degeneration

Because your joints rotate, pivot, bend, flex, and extend—some while bearing the weight of your body—they are subject to unfortunate risks compounded by aging, injury, and wear and tear.

Common, general joint conditions include:

  • Fracture. Accident, trauma, or repetitive motion can cause fractures that affect the joint’s ability to bear weight and move normally
  • Overuse. Repetitive stress, attributable to prolonged activity or improper positioning of your body during activities, this is an extremely prevalent orthopedic issue
  • Osteoarthritis. Caused by damaged or deteriorated cartilage, osteoarthritis is the narrowing of space in your joint causing bone friction and pain. Learn more about osteoarthritis and other forms of arthritis
  • Tendon or ligament tear. When stretched too far, tendons and ligaments can stretch, pull, and tear – causing pain and swelling

What Causes Cracking and Popping Noises in Joints?

All cracks and pops are not equal. Some are not harmful. But some are. You need to know the difference:

  • The sound made when you crack your knuckles is actually an air bubble that forms by manipulating the joint in a certain way. The joint cannot be cracked again until the air from the bubble has been reabsorbed into the joint.
  • When ankles make a cracking noise or people crack their necks or backs, the sound you hear is ligaments and tendons that are passing over bumps on the bones.
  • Arthritic joints may often make cracking and grinding noises with joint movement – it’s roughness of the joint surface and loose bodies resulting from the loss of smooth cartilage.
  • A loud pop may be heard when torn cartilage in a joint or a loose piece of bone becomes entangled in joint surfaces and the joint gets stuck.