Patella anatomy: the kneecap, its surfaces, attachments and function

Also called

Kneecap

What it is

The kneecap, a triangular sesamoid bone in the tendon of quadriceps femoris.

Location

Lies in front of the knee, between the quadriceps tendon above and the patellar ligament below, articulating with the femur.

Function

Protects the knee joint and increases the leverage of the quadriceps.

Body system

Skeletal · paired (left and right)

FMA ID

FMA24487 (Foundational Model of Anatomy)

Key facts

• It is the largest sesamoid bone in the body.

Clinical relevance

Most patellar dislocations are lateral.

The patella is a thick, flat, triangular bone. Its broad upper edge is the base, its pointed lower end is the apex, and it has medial and lateral borders, a front (anterior) surface and a back (posterior) articular surface. Inside, it is made of dense spongy (trabecular) bone covered by a thin layer of compact bone. (Kenhub: Patella.)

The quadriceps tendon attaches to the base and continues down over the front surface. The apex points downward and gives rise to the patellar ligament, which runs to the tibial tuberosity. Small openings in the middle third of the front surface let arteries into the bone. (Kenhub: Patella.)

Only the upper two-thirds of the back surface take part in the joint with the femur; the lower part, near the apex, is non-articular. The lateral articular area is usually larger than the medial one. (Kenhub: Patella.)

The articular surface is divided by a vertical ridge into a medial and a lateral facet. There is also a small extra 'odd' facet along the far medial edge. (Kenhub: Knee joint.)

The medial facet faces the medial condyle of the femur and the lateral facet faces the lateral condyle. Together they glide in the patellofemoral groove, the patellar surface at the front of the lower femur. (TeachMeAnatomy: The Patella.)

The patella articulates only with the femur. It does not touch the tibia, even though its tendon pulls on the tibia. (OpenStax Anatomy and Physiology 2e, 8.4: Bones of the Lower Limb.)

The patellar ligament is the continuation of the quadriceps tendon below the patella. It attaches to the tibial tuberosity, so the pull of the quadriceps reaches the tibia through the patella. (TeachMeAnatomy: The Knee Joint.)

The medial and lateral patellar retinacula are expansions of vastus medialis and vastus lateralis. These sheets form the medial and lateral patellar retinacula, blend with the patellar ligament on each side of the patella. (Kenhub: Knee joint.)

Thin patellomeniscal ligaments also run from the lower patella to the front horns of the medial and lateral menisci. (Kenhub: Knee joint.)

The patella lifts the quadriceps tendon away from the front of the knee. This increases the leverage of the quadriceps, so the muscle can straighten (extend) the knee more efficiently, and it also stops the tendon rubbing against the lower femur. (OpenStax Anatomy and Physiology 2e, 8.4: Bones of the Lower Limb.)

Kenhub describes the patella as acting like a pulley that increases the moment arm of the quadriceps, while the femur rolls and glides beneath it during flexion and extension. (Kenhub: Knee joint.)

The patella also shields the front of the knee joint from direct blows. (TeachMeAnatomy: The Patella.)

The femur runs down and inward from the hip to the knee, while the tibia is vertical. The Q-angle measures this lateral deviation and is normally about 10 to 15 degrees. It is usually larger in females because their pelvis is wider. A larger Q-angle increases the sideways (lateral) pull on the patella when the quadriceps contracts. (OpenStax Anatomy and Physiology 2e, 8.4: Bones of the Lower Limb.)

Three things resist that lateral pull: the horizontal lower fibres of vastus medialis, the forward projection of the lateral femoral condyle, and tension in the medial patellar retinaculum. (Kenhub: Patella.)

The patella is supplied by the genicular arteries: the superior and inferior medial and lateral genicular arteries and the descending genicular artery. Around the patella they form a peripatellar anastomosis, a ring of connected vessels that also supplies the knee joint. (Kenhub: Patella.)

The patella ossifies from several centres that appear between about 3 and 6 years of age and soon fuse together. (Kenhub: Patella.)

If one of these centres fails to fuse, the result is a bipartite patella: an extra piece of bone joined to the main patella by fibrocartilage. It is found in about 0.6 to 2% of adults, is up to nine times more common in males, and is bilateral in about half of cases. In the Saupe classification, about 75% of fragments sit at the upper outer (superolateral) corner, 20% on the lateral margin and 5% at the lower pole. Most are found by chance on X-ray, and only about 2% become painful. (Loewen A, et al. Arthroscopic Management for Bipartite Patella: A Systematic Review. Orthop J Sports Med. 2021.)

In patellar dislocation the patella slips out of its groove on the femur. It accounts for about 3% of knee injuries, and most dislocations are lateral, usually after a sudden twist or a high-force impact. (TeachMeAnatomy: The Patella.)

Dislocation is common in young athletes, particularly young women. Wasting of vastus medialis, for example after a long period of bed rest, makes lateral dislocation more likely. The apprehension test checks for previous dislocation. (Kenhub: Patella.)

Patellar fractures are caused by a direct blow or a sudden strong contraction of the quadriceps. When the bone breaks across, the quadriceps tendon pulls the upper fragment up and the patellar ligament pulls the lower fragment down, so the pieces separate. (TeachMeAnatomy: The Patella.)

Patellofemoral pain syndrome (runner's knee) is the most common overuse injury in runners. It causes a dull ache around and deep to the front of the knee, is more common in females, and is linked with abnormal tracking of the patella toward the lateral side. (OpenStax Anatomy and Physiology 2e, 8.4: Bones of the Lower Limb.)

In Sinding-Larsen-Johansson syndrome, repeated pulling of the patellar ligament makes the apex of the patella tender. It is most common in active children aged 12 to 14. Tapping the patellar ligament produces the knee-jerk (patellar) reflex, which tests the L2 to L4 segments through the femoral nerve. (Kenhub: Patella.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Great saphenous vein · cardiovascular
• Anterior tibial recurrent artery · cardiovascular
• Popliteal artery · cardiovascular
• Lateral superior genicular artery · cardiovascular
• Inferior lateral genicular artery · cardiovascular
• Medial superior genicular artery · cardiovascular
• Popliteal vein · cardiovascular
• Inferior medial genicular artery · cardiovascular
• Descending genicular artery · cardiovascular
• Middle genicular artery · cardiovascular