Movement by Design

Joints are the places where two or more bones meet. Some joints are built mainly for stability, while others allow large ranges of movement. Together, they turn the skeleton from a rigid framework into a moving mechanical system that can walk, run, bend, reach, chew, breathe, and protect the organs.

These six are the major synovial joints, which are the body's most freely movable joints.

But not every joint is designed for large movement. There are also two other important structural groups.

Fibrous joints

Fibrous joints connect bones with strong fibrous connective tissue and usually permit very little movement.

The best example is the sutures of the skull. These joints lock the skull bones together, creating a strong protective casing around the brain. Another fibrous joint is found between the lower ends of the tibia and fibula.

Their purpose is primarily strength and stability rather than mobility.

Cartilaginous joints

In these joints, bones are connected by cartilage.

Examples include the intervertebral discs between the vertebrae, the pubic symphysis in the pelvis, and the connection between the ribs and parts of the sternum.

They allow a limited amount of movement while also absorbing pressure and distributing forces. The cartilage between spinal vertebrae, for example, allows the spine to bend while cushioning the bones from repeated compression.

What keeps movable joints running smoothly?

The most important joint lubricant is synovial fluid.

Inside a synovial joint is a small enclosed space called the joint cavity. Its inner lining, the synovial membrane, produces synovial fluid.

Synovial fluid is a thick, slippery substance containing water, proteins and molecules such as hyaluronic acid. It performs several important jobs:

Lubricates the surfaces of the joint

Reduces friction between moving bones

Helps nourish the articular cartilage

Carries away some metabolic waste

Helps absorb and distribute mechanical forces

The ends of the bones are also covered with extremely smooth articular cartilage. Cartilage is not itself a fluid, but it works together with synovial fluid to create an exceptionally low-friction surface.

When you bend your knee, for example, the femur and tibia are not normally grinding directly against each other. Their cartilage-covered surfaces move across a thin layer of synovial fluid.

Other substances also contribute

Water is essential because cartilage contains a large amount of water. When pressure is placed on cartilage, fluid shifts within its matrix and helps distribute the load. When the pressure decreases, fluid moves back into the tissue.

Some joints also contain specialized structures filled or surrounded by synovial fluid. Bursae are small fluid-filled sacs that reduce friction where tendons, muscles or skin move over bones. The shoulder, knee, hip and elbow contain several important bursae.

Why joints are so important to the skeleton

The skeleton would be extraordinarily limited without joints. Bones provide the framework, but joints provide controlled movement.

Muscles pull on bones through tendons. The joint determines where and how that bone is allowed to move. In this way, bones, muscles, tendons, ligaments, cartilage and joints operate as one mechanical system.

Consider the difference between the hip and knee. The hip's ball-and-socket design allows the leg to swing forward, backward, sideways and rotate. The knee's hinge-like structure restricts much of that motion so the leg can remain relatively stable while supporting body weight.

Joints also help the skeleton perform four major jobs: mobility, allowing walking and manipulation of objects; stability, keeping bones correctly aligned; force distribution, absorbing and transferring stresses generated by gravity and movement; and protection, allowing structures such as the skull, rib cage and vertebral column to remain strong without being completely rigid.

A healthy joint is therefore much more than a connection between two bones. It is a precisely engineered biological interface where bone, cartilage, ligaments, membranes, muscles and lubricating fluid cooperate to turn the skeleton into a living system of motion.