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Causes
By Mayo Clinic staffIn the simplest terms, a joint occurs wherever two bones come together. But that definition doesn't begin to convey the complexity of joints, which provide your body with flexibility, support and a wide range of motion.
You have four types of joints: fixed, pivot, ball-and-socket and hinge. Your knees are hinge joints, which, as the name suggests, work much like the hinge of a door, allowing the joint to move backward and forward. Your knees are the largest and heaviest hinge joints in your body. They're also the most complex. In addition to bending and straightening, they twist and rotate. This makes them especially vulnerable to damage, which is why they sustain more injuries on average than do other joints.
A closer look at your knees
Your knee joint is essentially four bones held together by ligaments. Your thighbone (femur) makes up the top part of the joint, and two lower leg bones, the tibia and the fibula, comprise the lower part. The fourth bone, the patella, slides in a groove on the end of the femur.
Ligaments are large bands of tissue that connect bones to one another. In the knee joint, four main ligaments link the femur to the tibia and help stabilize your knee as it moves through its arc of motion. These include the collateral ligaments along the inner (medial) and outer (lateral) sides of your knee and the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), which cross each other as they stretch diagonally from the bottom of your thighbone to the top of your shinbone.
Other structures in your knee include:
- Tendons. These fibrous bands of tissue connect muscles to bones. Your knee has two important tendons, which make it possible for you to straighten or extend your leg: the quadriceps tendon, which connects the long quadriceps muscle on the front of your thigh to the patella, and the patellar tendon, which connects the patella to the tibia.
- Meniscus. This C-shaped cartilage, which curves around the inside and outside of your knee, cushions your knee joint.
- Bursae. A number of these fluid-filled sacs surround your knee. They help cushion your knee joint so that ligaments and tendons slide across it smoothly.
Normally, all of these structures work together smoothly. But injury and disease can disrupt this balance, resulting in pain, muscle weakness and decreased function.
Some common causes of knee pain and injuries include:
- A blow to the knee, either from contact during sports, a fall or a car accident
- Repeated stress or overuse, which may occur from playing sports or if your work or hobby requires doing the same activity over and over again
- Sudden turning, pivoting, stopping, cutting from side to side, which happens frequently during certain sports
- Awkward landings from a fall or from jumping during sports, such as basketball
- Rapidly growing bones, which are especially prone to injury during sports
- Degeneration from aging
