The kneecap, or patella, is a protective bone that sits at the front of the knee joint. It is the largest sesamoid bone in the human body, meaning it is embedded within a tendon rather than connecting two bones directly. The knee itself is the largest and most complex joint, designed to handle immense forces from walking, running, and jumping.
The Patella and Its Bony Connections
The patella’s primary function is to increase the mechanical advantage of the powerful quadriceps muscles, which straighten the leg. By acting as a fulcrum, the kneecap increases the distance between the quadriceps tendon and the axis of the knee joint, improving the muscle’s leverage. This structure allows a person to generate more force with less effort when extending the knee.
The back surface of the patella articulates with the bottom end of the femur, or thigh bone. It glides smoothly within a specialized depression on the femur called the patellofemoral groove. This sliding motion is crucial for the knee’s range of movement and joint health.
The femur meets the tibia, or shin bone, to form the main hinge of the knee joint. Although the patella does not directly touch the tibia, it is functionally connected to it. This bony relationship forms the patellofemoral joint, a sub-section of the larger knee joint structure.
The Crucial Role of Cartilage and Menisci
Allowing the patella to glide against the femur is a layer of specialized tissue called articular cartilage. This smooth, slick material covers the back of the patella and the ends of the femur and tibia. It acts as a low-friction bearing surface, enabling the bones to move smoothly.
The articular cartilage on the underside of the patella is thick, reaching up to six millimeters in depth, due to the high compressive forces it endures during activities like squatting. Within the main part of the joint, between the femur and the tibia, are the menisci. These are C-shaped wedges of tough, rubbery fibrocartilage that act as secondary shock absorbers.
The menisci deepen the relatively flat surface of the tibia, improving the fit between the leg bones. They distribute the load across the joint, absorbing between 30% and 55% of the compressive force, which protects the articular cartilage from excessive stress.
Connecting Tissues and Joint Lubrication
The patella is anchored by two primary connective tissues. The quadriceps tendon connects the thigh muscles to the top of the kneecap. Continuing from the bottom of the patella to the tibia is the patellar tendon, which is sometimes referred to as a ligament.
These two fibrous bands transmit the force of the quadriceps muscle, allowing the leg to straighten. Stability around the knee is also maintained by small, fluid-filled sacs called bursae. These structures are lined with a synovial membrane and are situated between bones and tendons to reduce friction during movement.
The entire joint is bathed in synovial fluid, a thick, viscous substance. This fluid acts as the joint’s natural lubricant, reducing friction and nourishing the surrounding cartilage. It fills the joint capsule, ensuring that the components glide and move smoothly.
Understanding Common Sources of Pain
Pain originating from beneath the kneecap frequently involves the patellofemoral joint and its surrounding structures. Patellofemoral Pain Syndrome (PFP), often called “runner’s knee,” is a general term for discomfort around or behind the patella that worsens with activities like squatting or climbing stairs. This condition is often related to poor tracking of the kneecap within the femoral groove, which irritates the soft tissues.
A more specific diagnosis is Chondromalacia Patellae, which refers to the softening or breakdown of the articular cartilage on the underside of the kneecap. While the cartilage itself does not contain nerves, its deterioration can lead to inflammation and pain in the underlying bone and surrounding joint lining.
Pain is also common in the patellar tendon itself, a condition known as patellar tendonitis or “jumper’s knee.” This tendon pain is an overuse injury caused by repeated stress from activities that involve forceful jumping or running. Abnormal alignment or chronic microtrauma from repetitive motion can contribute to the development of these issues.