Patella is a sesamoid bone, a bone that develops within a tendon, at the front of the knee. Commonly called the kneecap, it lies within the quadriceps tendon and articulates, or forms a joint, with the lower end of the femur, the thigh bone.
Where is the patella located?
The patella lies in front of the knee joint, between the quadriceps tendon above and the patellar ligament below. The quadriceps tendon connects the quadriceps femoris, the large muscle group at the front of the thigh, to the patella. The patellar ligament continues from the patella to the tibia, the larger bone of the lower leg.
A tendon connects muscle to another structure, usually bone, whereas a ligament usually connects bone to bone. Although called a ligament because it joins the patella to the tibia, the patellar ligament also continues the force-transmitting pathway of the quadriceps tendon.
The patella is close to the skin and can be felt at the front of the knee. Behind it lies the lower end of the femur; below and behind it lies the upper end of the tibia. The patella does not form a joint directly with the tibia.
What are the main parts of the patella?
The patella is roughly triangular, with a broad upper border and a pointed lower end. Its front surface is rough and slightly convex, meaning that it curves outwards. Its back surface includes a smooth joint surface and a lower non-articular area that gives attachment to the patellar ligament.
The terms anterior and posterior mean towards the front and towards the back, respectively. Medial means towards the body's midline, while lateral means away from that midline.
| Landmark | Position | Main anatomical feature |
|---|---|---|
| Base | Broad upper border | Receives the quadriceps tendon |
| Apex | Pointed lower end | Gives attachment to the patellar ligament |
| Anterior surface | Front of the bone | Rough surface crossed by tendon fibres |
| Posterior articular surface | Upper portion of the back surface | Contacts the femur and is covered by cartilage |
| Medial and lateral borders | Inner and outer edges | Give attachment to supporting soft tissues |
Like other bones of the skeletal system, the patella has a dense outer layer of compact bone surrounding spongy bone, an internal network of supporting bony struts. Its shape and internal structure allow it to transmit forces while remaining part of a moving tendon system.
Which joint does the patella form?
The patella forms the patellofemoral part of the knee joint with the distal femur, chiefly at its patellar surface. This surface includes a groove at the front of the lower femur. In deep flexion, the patella also contacts the femoral condyles. It guides the patella as the knee bends and straightens.
A vertical ridge divides most of the patella's posterior joint surface into medial and lateral facets. A facet is a defined area of a joint surface. The lateral facet is usually larger than the medial facet.
Articular cartilage, the smooth tissue covering bone at a joint, covers these surfaces. The patellofemoral joint shares the knee's joint capsule, the surrounding sleeve of connective tissue, and its synovial cavity, the space containing lubricating joint fluid.
As the knee flexes, or bends, the patella moves downwards relative to the femur and engages more deeply with the femoral groove. As the knee extends, or straightens, it moves upwards. The areas of contact between the patella and femur change through this movement.
What attaches to the patella?
The principal attachments link the thigh muscles to the tibia and help maintain the patella's position:
- The quadriceps tendon attaches to the base and extends over the anterior surface.
- The patellar ligament attaches to the apex and adjacent lower region, then passes to the tibial tuberosity, a bony prominence on the front of the upper tibia.
- The medial and lateral patellar retinacula attach along the sides. These fibrous expansions, formed partly by the quadriceps tendons, blend with the knee capsule and help stabilise the patella.
Below the patella, the infrapatellar fat pad lies behind the patellar ligament. This pad of fatty tissue occupies space within the knee capsule but outside the synovial cavity. In front of the patella, the prepatellar bursa is a small fluid-containing sac that allows the overlying tissues to move over the bone.
What does the patella do during knee movement?
The patella increases the mechanical advantage of the quadriceps by holding its tendon away from the knee's axis of rotation, the line around which the joint turns. This gives the muscle a more effective lever for straightening the knee.
The quadriceps femoris, part of the muscular system, supplies the force for this movement. That force passes through the quadriceps tendon, the patella and the patellar ligament to the tibia. Together, these structures form the knee's extensor mechanism, the linked arrangement that produces knee extension.
The patella also provides a broad, cartilage-covered surface through which the quadriceps tendon system bears against the femur. It forms a bony covering over the front of the knee joint.
How can you identify the patella in the 3D model?
In the 3D model next to this article, rotate to the front of the knee and select the left patella or right patella. Zoom in, isolate the selected bone, and rotate it to compare its front and back surfaces.
To explore the surrounding region, switch layers and hide or fade selected structures, then rotate and zoom to change your view. You can also isolate selected parts, pull parts apart, or cut a section along the sagittal, coronal or transverse plane. These controls offer different ways to explore the knee region.