At a joint, bones meet other bones, and they’re supported by a collection of soft tissues that hold them together, cushion them, and allow them to move. These tissues include cartilage, ligaments, tendons, a fluid-filled capsule, nerves, and small cushioning sacs called bursae. Each plays a distinct role in keeping the joint stable, mobile, and protected from wear.
Bones Form the Foundation
Every joint starts with at least two bones coming together. The shape of those bone ends determines what kind of movement the joint allows. A ball-and-socket arrangement at the hip permits rotation in multiple directions, while a hinge shape at the elbow limits movement to bending and straightening. Some joints, like those between the bones of your skull, don’t move at all. In those cases, the bones are connected by dense fibrous tissue rather than the more complex structures found in movable joints.
Cartilage Caps the Bone Ends
Where bones meet inside a movable joint, their ends are coated in a layer of hyaline cartilage, the most common type of cartilage in the body. This coating is slippery and smooth, allowing bones to glide past each other without grinding. It’s also flexible enough to absorb impact while being firm enough to help the joint hold its shape.
Articular cartilage is mostly water, making up 60 to 80 percent of its weight. The rest is a network of collagen fibers (15 to 22 percent) and proteoglycans, large molecules that help cartilage retain moisture and resist compression. This high water content is what gives cartilage its ability to cushion forces during movement. When cartilage wears down over time, bones lose that protective buffer, which is the underlying problem in osteoarthritis.
Ligaments and Tendons
Ligaments are tough, slightly elastic bands of tissue that connect bone to bone across a joint. Their job is stability. They prevent excessive movement that could damage the joint, acting like reinforcing straps that keep the bones aligned during motion. The knee alone has four major ligaments holding its three bones in place.
Tendons serve a different purpose. These strong, non-elastic cords connect muscles to bones, transmitting the force your muscles generate so that bones actually move. Without tendons, your muscles could contract all they wanted and nothing would happen at the joint. Both ligaments and tendons are made primarily of collagen, but their roles are complementary: ligaments restrict movement, tendons create it.
The Joint Capsule and Synovial Fluid
Most freely movable joints are enclosed in a joint capsule, a two-layered sleeve that surrounds the entire joint. The outer layer is made of tough, fibrous tissue that protects the joint’s interior and contains nerve cells and blood vessels. The inner layer is the synovial membrane, a thin lining that produces synovial fluid.
Synovial fluid is the joint’s built-in lubricant. It fills the small space between the bone ends, reducing friction even further than cartilage alone can manage. A healthy knee contains only about 3 to 4 milliliters of this fluid, roughly less than a teaspoon. Smaller joints hold even less, sometimes just a few drops. Despite the tiny volume, this fluid is essential. It also delivers nutrients to cartilage, which has no blood supply of its own and depends on synovial fluid soaking in during movement.
Bursae Provide Extra Cushioning
Bursae are small, fluid-filled sacs positioned at points where tissues would otherwise rub against each other. They sit between tendons and bone, between ligaments and bony prominences, or between bone and the overlying skin. You’ll find them in high-friction areas like the shoulder, elbow, hip, and kneecap. When a bursa becomes inflamed, usually from repetitive motion or pressure, the result is bursitis, a common source of joint pain that’s actually occurring outside the joint itself.
Nerves That Sense Position and Movement
Joints contain specialized nerve endings embedded in the capsule, ligaments, and surrounding connective tissue. These sensors are what allow you to know where your body is in space without looking, a sense called proprioception. Different types of receptors handle different information. Some detect the direction and speed of movement, firing rapidly when a joint changes position. Others respond to pressure and the angle of the joint, providing a slower, ongoing signal. A third type senses twisting forces.
Joints also contain free nerve endings that serve as pain receptors. These fire when ligaments, tendons, or the joint capsule itself are damaged, producing the sharp or aching pain you feel after a sprain or strain. Together, these sensors give your brain a constant stream of data about what every joint is doing, allowing you to coordinate movement, maintain balance, and pull back before pushing a joint past its safe range.
How It All Works Together
A working joint is a collaboration. Bones provide the rigid structure. Cartilage prevents those bones from grinding against each other. Synovial fluid lubricates and nourishes the cartilage. The joint capsule seals everything in and keeps the fluid where it belongs. Ligaments prevent the bones from sliding too far in any direction. Tendons channel muscle force into controlled movement. Bursae protect the soft tissues from friction at pressure points. And nerves feed your brain the information it needs to coordinate all of this without conscious effort. When any one component breaks down, whether through injury, inflammation, or age-related wear, the others are affected too, which is why joint problems rarely involve just one tissue in isolation.