This section discusses those structures relating to the kneecap that allow the knee to straighten and a person to stand upright and walk and run with no instability.
What is the patella?
The patella is the kneecap - the bone at the front of the knee.
It is unusual in being a bone within a tendon - the tendon of the quadriceps muscle that spans the region from the hip to the shinbone. It is very unusual in the body to find a bone inside a tendon (a 'sesamoid' bone), and what it means in reality is that any alteration of the direction of forces in the whole long quads muscle/tendon/bone complex has to affect the patella. Therefore many conditions, from the hip down to the foot, may result in pain experienced around the patella.
The extensor mechanism
The name given to that chain of structures that involves the patella and extends the knee is the 'extensor mechanism'.
Besides the patella, the extensor mechanism includes:
the quadriceps muscles
The quadriceps muscle has four muscle heads terminating at its lower end in a single tendon. The illustration on the right shows one head of the quadriceps muscle - the rectus femoris - and you can see the patella situated within its tendon before it inserts on the tibia.
The rectus femoris muscle attaches at its upper end to the pelvis and the top of the femur (thighbone), and at the lower end to the tibia (shinbone) at an important lumpy spot about 1.5 inches below the bottom of the patella - the 'tibial tubercle'.
We have left out the other three parts of the four-headed quadriceps muscle to make the illustration easier to understand.
the quadriceps tendon and the patellar tendon
Because the patella separates the tendon of the quadriceps muscles into two parts, the two parts have separate names. Above the patella the tendon is called the quadriceps tendon and below the patella it is called the patellar tendon (sometimes more correctly called the patellar ligament).
the tibial tubercle
This image on the right (an MRI scan with reverse contrast) shows the knee from the side, with the patella, quadriceps tendon and patellar tendon. The tibial tubercle (or tibial tuberosity) - the bony attachment site of the patellar tendon to the tibia - should align well with the groove where the patella glides and this alignment is calculated from CT or MRI scans as the tt-tg distance which is normally 9mm or less.
the trochlear groove of the femur
The trochlear groove is the groove in the femur bone along which the patella glides when the knee is bent and straightened. You an understand this best if we illustrate it from above, as if you were seated and looking down at your toes. Usually the outer (lateral) wall of the groove is higher than the inner (medial) wall, and the patella is contained within these walls. The undersurface of the patella is v-shaped and snugs into the groove. Although it is free to slide up and down the groove as the leg bends and straightens, it is not free to move from side to side as it is contained by the walls of this groove.
You may like to take a look at this video below, which has been taken during arthroscopic surgery, and which shows the patella from above as in the sketch just above the video.
You will see that as the knee is bent by the surgeon the patella engages in the groove of the femur and slips out of view.
[You may have also seen a curtain-like suprapatellar plica stretching horizontally across the field of view on the left. This is a different topic, but mentioned just so that you know. The shadowy stuff on the right is just some unimportant loose tissue floating into view.]
Fibrous restraints of the patella
The medial and lateral retinacula
The patella is restrained from moving sideways bytfibrous tethers on each side - the medial and lateral retinaculum. The most significant of these is the lateral retinaculum which may become excessively tight and cause problems.
The medial patellofemoral ligament (MPFL)
On the inner side of the patella another important guy rope is the medial patellofemoral ligament (MPFL) which may become damaged after a patellar dislocation event.
The fat pad
The fat pad is not part of the patellar mechanism but is intimately related to it anatomically. It acts as a fatty cushion protecting the underlying soft tissue structures from impacts to the patella.