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Patellofemoral Anatomy (part 1 of a course on Patellofemoral Pain)

Hi. I would like to introduce myself - I'm Ron Grelsamer, and I'm a specialist knee surgeon from New York. Welcome to my course on 'Pain around the Knee Cap'.

There are many topics in the knee about which I am passionate, but I think this tops my list. Why? Well clinicians make a lot of mistakes here and patients suffer for it. It seems a simple subject, but it is not. Correct diagnosis and treatment can make the difference between a cure and a wrecked knee.

Aimed at the level of patients, but also useful to clinicians, this course offers a comprehensive overview of the causes of pain around the knee cap, and what can be done to help patients suffering from knee cap pain. I have tried to strip it of 'med-speak' so that anyone can understand the topic, how it should be investigated and managed, where the pitfalls lie and how to avoid them.

One of the most common orthopaedic symptoms sending patients to their doctors or physiotherapists or athletes to their sports trainers is pain around the knee cap (peri-patellar pain).

Peri-patellar pain is no simple matter. There are many conditions in and around the knee cap which cause similar discomfort, but the conditions are in fact quite distinct, so need to be managed differently. Also, several disorders causing pain around the knee cap have nothing to do with the knee at all!

Being physically active does not ensure one will avoid peri-patellar pain. It may affect both athletes and 'couch potatoes' - in fact it is a major problem amongst athletes. Age offers no protection either - peri-patellar pain has a high incidence amongst adolescents, but occurs also in adults or the elderly. The different conditions causing the pain, however, tend to favour one or other age group.

Correctly diagnosed and evaluated, most conditions leading to peri-patellar pain can be effectively treated without resort to surgery.

One would think a problem as common and widespread as pain around the knee cap would occupy a high profile on medical and sports therapy courses, and all involved personnel would understand the issues and hurry each patient onto an effective resolution of their symptoms. But sadly that is not the case. A very large proportion of providers in primary care situations - family doctors, school physical instructors and sports trainers - are inadequately trained and woefully ignorant in this field.

Sufferers are frequently examined only cursorily. The first abnormal finding is often hailed as 'the diagnosis', but may have little to do with the cause of the pain. Referral to surgery is frequently inappropriate. Knees are frequently operated on when the problem is not originating in the knee at all. In the absence of anything obviously wrong inside the knee a surgical 'lateral release' is often done quite inappropriately. This may in itself lead to further problems and a rollercoaster ride leading to a lifetime of misery.

The situation is worsened by a muddle which exists in the words used to describe knee cap conditions.

So I am going to try and give you the sort of background a trainee doctor should have received.

Because I don't want you to feel overwhelmed by anatomy and anatomy terms, I'll introduce concepts gradually, and build up your vocabulary as you progress through the course. I have also built you a glossary of the more tricky words.

The patella

Start now by examining your own knee cap -

Sit up on a bed where you can rest your back comfortably, with your legs out straight in front of you. With the leg straight, feel the boundaries of the knee cap. The knee cap can normally be wiggled from side to side in this position, and there is a good reason for this - in this high position it is above the groove of the trochlea, i.e. the groove in the underlying thigh bone along which the knee cap slides during its excursion when the knee is bent. If you have dislocated your knee cap before, you will know to avoid doing this. Bend your knee and now you will note that it is no longer possible to wiggle the knee cap. This is because the knee cap has now entered the groove of the trochlea.

j-sign

Here is an illustration of the knee area, showing the femur bone, with its rounded end and trochlear groove, and the position of the knee cap when the knee is straight (extended) and bent (flexed).

[Reprinted with permission from R. Grelsamer, MD, The Adult Knee, Ed. Callaghan JJ, Rosenberg AG, Rubash HE, Simonian PT, Wickiewicz TL, Lippincott Williams & Wilkins, Philadelphia, Chapters 59-60, Patellar Instability & Patellar Pain pp 929-949, 2003.]

See how in the straight position the knee cap does not really make contact with the groove, and in fact is way above the joint line. You can confirm that on your own knee.

Then in the bent knee, the knee cap is guided into the groove.

On occasion, the groove is shallow or flat or even inverted. In other words, it is possible for the groove to be convex like a fried egg, in which case the knee cap can easily slip too far left or right (in which case the knee cap is said to be unstable).

Quadriceps muscle

Try squeezing your thigh. You can only feel the bone for a bit just above the knee joint. The rest is covered by muscles. The long bone of the thigh is the femur. The front of the femur is covered by four muscles that converge to the upper part of the knee cap, and, collectively, these are called the quadriceps (nickname: 'quads').

The 'quadriceps' muscle is the main muscle of the 'lap' and covers the whole of the front of the femur.

[The image on the left is reprinted with permission from R. Grelsamer, MD, The Adult Knee, Ed. Callaghan JJ, Rosenberg AG, Rubash HE, Simonian PT, Wickiewicz TL, Lippincott Williams & Wilkins, Philadelphia, Chapters 59-60, Patellar Instability & Patellar Pain pp 929-949, 2003.]

quads muscles

It is actually four muscles (quadri=4 ceps=heads):

  1. rectus femoris
  2. vastus medialis
  3. vastus lateralis
  4. vastus intermedius (not evident in the illustration)

They work together synergistically to straighten (extend) the knee.

Can you see the knee cap? What is its medical name?

Simple illustration demonstrates the bones in relation to one another. The fibula is on the outer aspect of the lower leg, so it helps you identify whether you are looking at a right or a left leg.

The patella is connected above to the rectus femoris and to the vastus intermedius via a broad, flat tendon called the quadriceps tendon. At the bottom end, the knee cap is attached to the tibia (shin bone) by way of the patellar tendon, occasionally called the patellar ligament . This whole structure of the quads muscle with its embedded patella is known as the extensor mechanism, and it stretches from the bony attachment on the tibia just below the knee (the 'tibial tubercle') to the top of the femur near the hip. When the quadriceps muscle contracts this whole structure - the extensor mechanism - is tautened and the leg extends (straightens).

All the bundles of the quadriceps muscle tend to pull the patella up towards the head, but whereas the rectus femoris and vastus intermedius pull straight up, the vastus lateralis pulls the patella up and out, so to speak, and the vastus medialis pulls up and in. [Lateral means away from the midline, medial means towards the midline. Thus the little toe is lateral to the big toe, and the big toe is medial to the second toe.]

Lateral retinaculum and tibial tubercle

Take particular note of the lateral retinaculum and tibial tubercle in the illustration, because several of the surgical procedures we will discuss involve these structures. The tibial tubercle is also known as the tibial tuberosity.

Patellar Tendon

Just a word here on terminology ....

The peculiar anatomy of the patella means the structure below it is generally called a tendon, though it may also be called a ligament. Bands of tissue connecting one bone to another are called ligaments while bands of tissue connecting muscles to bone are called tendons.

Since the biological rope we are discussing connects the kneecap to the shinbone, in one sense, it is a ligament. On the other hand, functionally speaking, this rope is but a continuation of the quadriceps muscles (and of the quadriceps tendon). In this sense, our rope is a tendon, thus the term 'patellar tendon'.

 

Updated: 11 Apr, 2013
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