The patella (kneecap) is a 'sesamoid bone'.

extensor mechanism

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

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.

 

skyline patellar X-ray patella from above

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

patellofemoral joint

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

Wiberg classification of the patella

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.

 

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