Knee pain is one of the most common reasons for visits to orthopedic physicians. It can originate from:

• Within the knee joint
• Around the knee (e.g., tendons, ligaments, muscles)
• Referred pain (e.g., from the spine)

Causes of knee pain within the joint include:

• Regular wear and tear related to aging
• Chronic conditions (e.g., arthritis)
• Sudden injury (e.g., a fall or sports injury)

The extra weight of obesity also increases the stress on joints, including the knees.

The knee is the joint that enables the leg to bend and extend. It connects the thigh bone (femur) in the upper part of the leg with the shin bone of the lower leg (tibia). The kneecap (patella) is the small bone that fits on the grooved ends of the femur and forms the top of the knee. The fibula, the other bone in the lower leg, joins the tibia before it reaches the knee.

Ligaments and tendons connect the bones and muscles that make the knee move. Ligaments connect the bones. Four ligaments join the femur and tibia at the knee. Along the sides of the knee, the medial collateral ligament (MCL) on the inside of the leg and the lateral collateral ligament (LCL) on the outside of the leg provide stability. Two ligaments cross the knee diagonally to join the femur and tibia – the anterior cruciate ligament (ACL) on the front of the knee and the posterior cruciate ligament (PCL) on the back. The ACL limits rotation and forward motion of the tibia. The PCL limits backward movement of the tibia. An ACL tear is a common sports injury.

Tendons connect muscle to bones. The quadriceps tendon connects the quadriceps muscle of the thigh to the patella and covers the patella. The patellar tendon connects the patella to the tibia. Technically, this is a ligament because it connects two bones (patella and tibia), but it is usually referred to as a tendon.

Soft tissues and fluids cushion the bones around the knee to enable smooth movement. Tissue called cartilage covers the ends of the bones where they adjoin other parts of the leg. Two pads of cartilage called menisci (one is called a meniscus) separate the femur and tibia. Fluid-filled sacs called bursae also provide cushioning between bones and tendons or ligaments.

The coordination of these bones, ligaments, tendons and cartilage enable the knee to bend the leg and support the body’s weight. Many conditions and injuries can cause knee pain, including overuse, chronic wear and tear on the joint, disease and inflammation.