The patella, commonly known as the kneecap, is a bone located at the front of the knee. This small, specialized bone plays a role in the mechanics of the lower limb.
Anatomy and Location of the Patella
The patella is positioned anterior to the knee joint, embedded within the quadriceps femoris muscle tendon. This unique placement classifies it as the largest sesamoid bone in the human body, meaning it is a bone formed within a tendon. Its shape is generally triangular, with the pointed apex directed downwards.
The base of the patella, its superior aspect, serves as the attachment point for the quadriceps tendon. Its inferior aspect connects to the tibial tuberosity via the patellar ligament. The posterior surface of the patella is covered with articular cartilage, up to 4 to 5 mm thick, facilitating smooth movement. This surface articulates with the trochlear groove of the femur, or thigh bone, forming the patellofemoral joint. This articulation allows for the patella’s guided movement during knee flexion and extension.
The Patella’s Role in Knee Movement
The patella contributes to the efficiency of knee movement, particularly during extension. It acts as a mechanical pulley, increasing the leverage of the quadriceps muscle by altering the angle at which the quadriceps tendon pulls on the tibia. This allows the quadriceps muscle to exert a greater turning force on the tibia with less effort, making activities like straightening the leg more efficient.
The bone also provides a protective shield for the anterior surface of the knee joint and underlying structures from direct impact. The patella helps to distribute compressive forces across the knee joint during movement. As the knee flexes, the contact area between the patella and the femur increases, spreading the load over a larger surface and reducing stress on specific points. This is noticeable in activities involving deep knee bending, where forces can reach up to six times half of the body weight.
Common Issues Involving the Patella
Problems with the patella often manifest as discomfort or pain around the front of the knee. One common condition is patellofemoral pain syndrome, frequently referred to as “runner’s knee” or “moviegoer’s knee”. This pain worsens with activities such as squatting, jumping, kneeling, or going up and down stairs, and can also be felt after prolonged sitting. It is associated with the degeneration of cartilage on the underside of the kneecap.
Another issue is patellar tracking disorder, where the kneecap does not move smoothly within the trochlear groove of the femur as the knee bends and straightens. This can lead to symptoms like a popping, grinding, slipping, or catching sensation in the knee. While the patella commonly shifts towards the outside of the leg, it can also move inward. This misalignment can stem from various factors, including muscle imbalances, tightness or looseness in surrounding tendons or ligaments, or even the shape of the patella itself.
In more severe instances, the patella can dislocate, meaning it completely slips out of its groove. This traumatic injury, which accounts for about 3% of knee injuries, occurs laterally and can result from high-force impacts or sudden twisting of the knee. Patellar dislocations are more prevalent in athletes, particularly young female athletes, and can cause pain, swelling, and an inability to bend or straighten the knee. These issues highlight the susceptibility of the patella to problems arising from overuse, structural variations, or direct trauma.