Patella (kneecap)
Patella
The patella is the kneecap. It is a small triangular bone that sits in front of the knee joint, inside the tendon of the muscle that straightens the knee.
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Free 3D model of the patella. Right leg. Drag to rotate, zoom with + and − (or Ctrl + scroll), click a bone to highlight and name it.
Key points
- The patella is the kneecap, at the front of the knee.
- It is the largest sesamoid bone in the body, formed inside the quadriceps tendon.
- It articulates only with the femur.
- It gives the quadriceps more leverage to straighten the knee.
What is the patella?
The patella, or kneecap, is a small, flat, roughly triangular bone at the front of the knee. It is a sesamoid bone, which means it forms inside a tendon rather than standing on its own.
It is the largest sesamoid bone in the body. A newborn has a kneecap made of cartilage. It begins to turn into bone in early childhood.
Where is the patella?
It lies in front of the lower end of the femur, just above the joint line of the knee. Only skin and a thin fluid-filled sac cover its front surface.
It is easy to find. Sit with your leg straight and the thigh relaxed, then take hold of the hard disc at the front of the knee. In this position it can be shifted a little from side to side. When the knee bends, it is pulled firmly into its groove and no longer moves.
What are the parts of the patella?
The patella is shaped like a triangle with rounded corners. The broad upper edge is the base. The point at the bottom is the apex.
The front surface is rough and slightly domed. The back surface is smooth and, in life, covered with thick cartilage. A vertical ridge divides the back into two facets, a larger lateral facet and a smaller medial facet, which match the two sides of the groove on the femur.
What does the patella articulate with?
The patella articulates with one bone only, the femur. Its back surface slides in a groove on the front of the femur called the patellar surface. This is the patellofemoral joint, one part of the knee joint.
It does not touch the tibia. It is linked to the tibia by a strong band, the patellar ligament, but there is no joint between the two bones.
What attaches to the patella?
The tendon of the quadriceps femoris, the large muscle on the front of the thigh, attaches to the base and sides of the patella. Some of its fibres pass over the front of the bone.
From the apex the patellar ligament runs down to the tibial tuberosity, the bump at the top of the shin. Thin sheets of fibrous tissue on each side, the retinacula, help keep the patella centred. The model shows the bones only, so none of these soft tissues appear.
What does the patella do?
Its main job is to improve leverage. By holding the quadriceps tendon away from the centre of the knee, it lets the muscle straighten the knee with more force.
It also protects the front of the knee joint and gives the tendon a smooth, hard surface to glide on as the knee bends and straightens.
Common problems with the patella
Pain around or behind the kneecap is called patellofemoral pain syndrome and is common in runners. The patella can also dislocate, almost always toward the outer side of the knee.
A fall onto the knee can cause a patellar fracture. Pain at the apex, where the patellar ligament begins, is typical of patellar tendinopathy. In some people the bone forms in two pieces, called a bipartite patella, which is a normal variant. This page describes anatomy only.
Common questions
Is the patella a sesamoid bone?
Yes. It is the largest sesamoid bone in the body and lies in the quadriceps tendon.
Are babies born with kneecaps?
They are born with kneecaps made of cartilage. The cartilage starts to turn into bone in early childhood.
Does the patella touch the tibia?
No. It is connected to the tibia by the patellar ligament, but it forms a joint only with the femur.
Which way does the point of the patella face?
Downward. The pointed apex faces the shin and the broad base faces the thigh.
Is this 3D model of the patella free?
Yes. The interactive 3D model on this page is free to view and use for study, with no account or download needed. Its geometry comes from the open BodyParts3D data set (CC BY-SA).
About this free 3D model of the patella
The interactive 3D model on this page is free to use in your browser, with no sign-up. It is built from scan-based anatomy data, so every bone is a separate, correctly placed shape you can turn and inspect from any side.