The patella is the kneecap, the bone sitting over the front of the knee. Plural = 'patellae'.
Page updated November 2023 by Dr Sheila Strover (Clinical Editor)
The patella from the side, showing its relationship to the femur bone and tibia bone. The patella is contained within the tendon of the quads muscle and the whole muscle-bone-tendon complex is known as the 'extensor mechanism'.
A stable patella is contained within the walls of the underlying groove of the femur.
The patella - or kneecap - is a key structure in the function of the knee.
The patella - a sesamoid bone
The patella (kneecap) is a 'sesamoid bone'. A sesamoid bone is a bone that develops in a muscle tendon, rather than being attached by ligaments to another bone. 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.
The extensor mechanism"
The patella is a 'sesamoid bone', contained within a tendon, and subject to the forces that pass through that tendon and its related muscles. 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 extendes, the forces go through all parts of the extensor mechanism.
Quadriceps muscle group
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 - the quadriceps tendon - that the patella develops.
Quadriceps tendon & Patellar tendon
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).
Tibial Tubercle
The place on the tibia bone where the tendon attaches is called the tibial tubercle or tibial tuberosity.
Other important concepts"
The Trochlear Groove
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'. This contact area between the back of the patella and the groove of the femur is called the patello-femoral joint or patello-femoral compartment.
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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 Patello-femoral Compartment
The patellofemoral joint forms one of the three '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.
Patellar Facets & 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
- Type I: the facets are concave, symmetrical and of equal size
- Type II: the medial facet is rather smaller than the lateral facet. The lateral facet is concave.
- Type III: the medial facet is markedly smaller than the lateral facet.
Peer-reviewed papers
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Quote:
"Evaluation and management of the painful patellofemoral joint in the athlete requires a thoughtful clinical evaluation including a comprehensive physical examination, standardized radiographs, and, often, advanced imaging such as MRI to evaluate the static and dynamic stabilizers as well as the articular surfaces of the joint."
Citation: Endo Y, Shubin Stein BE, Potter HG. Radiologic assessment of patellofemoral pain in the athlete. Sports Health. 2011 Mar;3(2):195-210. doi: 10.1177/1941738110397875. PMID: 23016009; PMCID: PMC3445133.
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Relevant material -
- Extensor mechanism
- Patellar tendon
- Patellar instability
- Q-angle
- Squinting patellae
- Suprapatellar pouch
Injury -
Imaging -
2023 - Primer -
2013 -
A simple explanation of the patella, and why it is important.