Patella

Written by Dr Sheila Strover on March 14, 2025

The patella is the kneecap, the bone sitting over the front of the knee. Plural = 'patellae'.

Illustration of a jumping patient, with an inset showing the knee bones from the side, and how the patella is embedded in the patellar tendon.
The patella is contained within the tendon of the quads muscle, which is tethered to the tibia bone at the bottom pole.
Illustration of the patellofemoral joint, as if the patient were seated and looking down at their right knee.
The walls of the underlying groove of the femur help to stabilise the patella.

The patella - a sesamoid bone

The patella (kneecap) is a 'sesamoid bone'. A sesamoid bone is one that develops inside a muscle tendon, rather than being attached by ligaments to another bone.

The presence of a patella in humans suggests its importance in upright stance, as many mammals have no patella, and some have small ones as well as other small sesamoid bones associated with the knee joint.  Sometimes a person is born without a patella, or only a tiny one.

The tendon in which the patella develops is the tendon of the quadriceps muscle, the big muscle that makes up the bulk of your 'lap'. The whole structure is often referred to as the extensor mechanism.

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The extensor mechanism

  • The back of the patella is covered with smooth, shiny white joint cartilage and is in contact with the cartilage of the femur within a groove known as the 'trochlear groove'.
  • The contact area between the back of the patella and the groove of the femur is called the patello-femoral joint or patello-femoral compartment.
    When this groove is abnormal in shape, the condition is known as 'trochlear dysplasia', and this may be associated with instability of the patella.
  • The part of the tendon below the patella is called the patellar tendon, while the part above is called the quads tendon, but it is all part of a single functional structure. When the knee straightens or extends, the forces go through all parts of the extensor mechanism.
  • The quadriceps muscle at its upper end is actually comprised of four heads- hence the name quadri-ceps, and attach to the hip and the femur bone. This illustration shows for simplicity just the one head of the quadriceps - the most superficial of the four muscles, and which is called the rectus femoris.
  • At the lower end of the muscle group the four heads fuse into a common tendon, a strong and fibrous structure that attaches to the tibia bone about three centimetres below the bottom of the patella. It is in this tendon that the patella develops. The bit of tendon above the sesamoid patella is known as the quadriceps tendon and the bit below the patella is known as the patellar tendon (or sometimes it is called the patellar ligament).
  • The place on the tibia bone where the tendon attaches is called the tibial tubercle or tibial tuberosity.
Illustration of the whole extensor mechanism, showing the quads muscle attachments on pelvis and tibia.

Because of its situation within a tendon, the patella is subject to the forces that pass through that tendon, and...

"....Evaluation and management of the painful patellofemoral joint in the athlete requires a thoughtful clinical evaluation....to evaluate the static and dynamic stabilizers as well as the articular surfaces of the joint...."

Yoshimi Endo et al

The patellofemoral surgeon has a lot to consider...

"....There are currently approximately 12 human genetic disorders....that regularly include abnormal, reduced or absent patellae (hypoplasia or aplasia)...."

Mark E Samuels et al

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The Patellofemoral Joint

The key articulating area is the contact area between the back of the patella and the underlying femur, into which it snugs.

Here, the back of the patella is covered with smooth, shiny white joint cartilage and is in contact with the smooth, shiny cartilage of the femur within a groove known as the 'trochlear groove'.

The contact area between the back of the patella and the groove of the femur is called the patello-femoral joint or patello-femoral 'compartment'.

When this groove is abnormal in shape, the condition is known as 'trochlear dysplasia', and this may be associated with instability of the patella.

Illustration of the patellofemoral joint, as if the patient were looking down at their own knee.

The patellofemoral compartment

This patellofemoral compartment forms one of the three articulating compartments of the knee.

The red circle on this illustration marks the patellofemoral joint, but also indicates what we mean by 'patellofemoral compartment' [the other two compartments being 'tibio-femoral']. The patella is tethered via its tendon to the tibia bone, but of course the femur moves with walking and the patella glides throughout the length of the groove.

An illustration showing an oblique view of the knee joint, particularly illustrating how the patella snugs into its underlying groove on the femur.

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Patellar facets and patellar dysplasia

The flattened areas at the back of the patella - where the patella makes contact with the femur - are called the 'facets'.

In a normal patella they are roughly equal in size, but when they are very different the patella looks distorted and the condition known as patellar dysplasia.

Wiberg Classification of patellar shape

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Illustration to show the normal patellar shape, and the variations known as Wiberg variants.

A-Z Keywords - Extensor mechanism, Patellar tendon, Patellar instability, Q-angle, Squinting patellae, Suprapatellar pouch, Patellar fracture, Patellar tendon rupture, Quads tendon rupture, Patella baja, Merchant view, Rosenberg view

Primer - 2017 - The Patella - A Primer

Contributions by experts -

Video - What is the patella?

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