The patella is a shield-shaped bone situated at the front of the knee joint. It is positioned within the tendon of the quadriceps femoris muscle group, which runs along the front of the thigh. This unique placement makes the patella an integral component of the knee’s extensor mechanism, connecting the thigh muscles to the lower leg bone. The patella moves along a groove in the femur, or thigh bone, during knee flexion and extension.
Anatomical Structure of the Patella
The patella is distinguished as the largest sesamoid bone in the human body, meaning it is a bone embedded within a tendon. Its overall shape is triangular, with the broad base positioned superiorly and the pointed apex facing inferiorly. This base provides the attachment site for the quadriceps tendon.
The apex anchors the patellar ligament, which connects down to the tibia. The posterior surface, which faces the femur, is covered by a layer of articular cartilage. This is the thickest cartilage found anywhere in the body, capable of reaching up to six millimeters in depth. This resilient tissue facilitates smooth gliding and helps manage the compressive forces that occur as the knee bends.
Essential Roles of the Kneecap
The patella’s primary function is to enhance the mechanical advantage of the quadriceps muscle. By holding the quadriceps tendon away from the axis of the knee joint, the patella effectively lengthens the lever arm of the extensor mechanism. This allows the quadriceps to generate greater force for leg extension, increasing the muscle’s efficiency by up to 40 to 50 percent.
The patella works as an anatomical pulley, redirecting the line of pull of the quadriceps tendon. This redirection minimizes friction between the tendon and the femur. It also serves a protective role, acting as a bony shield for the cartilage and structures of the underlying knee joint from direct physical impact.
Specific Conditions Affecting the Patella
Patellofemoral Pain Syndrome (PFPS)
A common source of anterior knee discomfort is Patellofemoral Pain Syndrome (PFPS), sometimes called “Runner’s Knee.” This condition presents as a dull, aching sensation located around or beneath the kneecap. The pain often intensifies during activities that place high stress on the joint, such as squatting, running downhill, or ascending and descending stairs.
Patellar Tendinitis
Patellar tendinitis, known as “Jumper’s Knee,” involves inflammation or breakdown of the patellar tendon just below the kneecap. Athletes who engage in sports requiring repeated jumping and forceful knee extension, like basketball or volleyball, commonly experience this localized tenderness. Symptoms include pain directly at the lower pole of the patella, accompanied by stiffness or mild swelling.
Patellar Dislocation
Patellar dislocation or subluxation occurs when the kneecap slides partially or completely out of the trochlear groove of the femur, usually shifting laterally. A complete dislocation is an acutely painful event often accompanied by a visible deformity, severe swelling, and immediate inability to bear weight. Instability can become chronic if stabilizing ligaments, such as the medial patellofemoral ligament, are stretched or torn.
Patellar Fracture
A patellar fracture is a break in the bone, typically caused by a direct fall onto the knee or a forceful, sudden contraction of the quadriceps muscle. A person with a displaced fracture will experience severe pain, bruising, and an inability to straighten the leg against gravity. This inability to perform a straight leg raise indicates that the extensor mechanism has been compromised.
Treatment and Recovery Methods
Evaluation of patellar issues begins with a physical examination to assess pain location and range of motion, often followed by diagnostic imaging. X-rays are typically used to identify fractures or alignment abnormalities, while a magnetic resonance imaging (MRI) scan provides detailed images of soft tissues like tendons, ligaments, and cartilage. A computed tomography (CT) scan assesses complex fracture patterns or bony anatomy.
Conservative management is the initial approach for most patellar conditions, including tendinitis and non-displaced fractures. This involves the RICE protocol—Rest, Ice, Compression, and Elevation—to manage acute pain and swelling. Physical therapy focuses on strengthening the quadriceps and hip muscles to improve patellar tracking and stability.
For patellar tendinitis, rehabilitation emphasizes controlled, progressive loading of the tendon through eccentric exercises. More serious injuries, such as unstable patellar fractures, may require surgical intervention to realign and secure the bone fragments. Chronic patellar instability may necessitate ligament reconstruction procedures to restore stability and prevent future dislocations. Recovery timelines vary widely, but all rehabilitation protocols aim to restore full range of motion and functional muscle strength.