The patella, commonly known as the kneecap, is a specialized bone located at the front of the knee joint. It rests within the tendon of the powerful quadriceps muscle group, situated where the thigh bone (femur) meets the lower leg bones. The patella forms the patellofemoral joint where it glides against the femur. Its primary role is to ensure the smooth, efficient operation of the knee’s extension mechanism and act as a protective shield for the joint.
The Unique Structure of the Patella
The patella is structurally unique because it is classified as the largest sesamoid bone in the human body. A sesamoid bone is distinguished by its characteristic of being embedded within a tendon rather than connecting to other bones via ligaments. In this case, the patella is encased in the quadriceps tendon, which connects the thigh muscles to the tibia, or shin bone.
The bone possesses a triangular shape, with the pointed apex directed downward toward the leg. Its posterior surface, which faces the femur, is covered with one of the thickest layers of articular cartilage found anywhere in the body. This extensive cushioning is necessary to absorb the high compressive forces that travel through the joint during movement. This posterior surface articulates with a specific depression on the femur called the trochlear groove, allowing the patella to track, or slide, smoothly as the knee bends and straightens.
How the Kneecap Optimizes Movement
The mechanical function of the kneecap is to act as a fulcrum, which is a lever arm for the quadriceps muscle group. Without the patella, the quadriceps tendon would rest directly against the femur, reducing the effectiveness of the muscle. By holding the tendon further away from the center of the knee’s rotation, the patella effectively increases the distance of the lever arm.
This increase in leverage significantly boosts the mechanical advantage of the quadriceps muscle. A greater mechanical advantage means the muscle can generate more force with the same amount of effort, which is noticeable when straightening the leg against resistance, such as climbing stairs or standing up. The patella contributes most significantly to the extension torque, or rotational force, especially during the final 30 degrees of leg extension.
Common Causes of Kneecap Pain and Injury
The mechanics that make the patella so efficient also expose it to high stresses, making it vulnerable to several common conditions that cause pain. Patellofemoral pain syndrome (PFPS), frequently referred to as “runner’s knee,” is the most common diagnosis involving the kneecap. This condition is characterized by a dull, aching pain felt around or beneath the patella, which worsens with activities that load the joint, like running, jumping, or prolonged sitting. PFPS often results from overuse, muscle imbalances, or poor alignment that causes excessive friction between the patella and the femoral groove.
Another common issue is patellar tracking disorder, which occurs when the kneecap does not glide centrally within the trochlear groove. Instead, the patella may shift too far laterally, or to the outside, a condition known as maltracking or patellar subluxation. This abnormal movement can lead to painful rubbing and the premature wearing down of the cartilage on the posterior patella surface. In severe cases, a direct blow or a sudden, forceful twisting motion of the leg can cause an acute patellar dislocation, where the kneecap completely slips out of the groove, usually to the outside of the knee.
Traumatic injuries include patellar fractures, which typically happen from a direct, high-impact fall onto the bent knee or a car accident dashboard injury. The patella can break into multiple fragments, potentially disrupting the quadriceps tendon and the extensor mechanism. Patellar tendonitis, or “jumper’s knee,” is an overuse injury involving inflammation or small tears in the patellar tendon, located between the bottom of the kneecap and the shinbone.