Important patellofemoral concepts
A discussion of various anatomical concepts relating to the kneecap.
In this second part of the course, I am going to outline some important concepts which need to be absorbed in order to understand what the clinician is looking for during the investigation of pain around the knee cap. The first is the .Q-angle'.
In more ways than one, the patellar mechanism, anatomically speaking, 'rounds a corner'. When seen from the front, the quadriceps, patella and patellar tendon form a sharp angle. This is called the 'Q-angle'.
The Q-angle is the angle formed between two lines -
It gives an idea of the angulation of femur bone to tibial tubercle, and the consequent stresses which may be contributing to bowstringing - i.e. pulling the patella to the outer (lateral) side of its groove.
In some people, this angle (called the quadriceps or Q-angle) is particularly pronounced and it contributes to the overall malalignment (abnormal movement) of the patella. Because of this unique setup, every time the quadriceps muscles contract, the patella tends to be pulled to the outside part of the knee . This is termed the bowstring effect.
[Illustrations reprinted with permission, The Adult Knee, Chapters 59-60, Lippincott Williams & Wilkins, 2003.]
There are other parts of the anatomy that either exacerbate or counter the bowstring effect.
For example, the lateral retinaculum is a tough, broad band of tissue attached to the entire outside ('lateral') part of the knee cap. Predictably, it tends to pull the patella to the outside.
On the inside ('medial') aspect of the patella lies the medial retinaculum. It tends to be weaker than its lateral counterpart and receives significant assistance from the part of the vastus medialis muscle called the vastus medialis obliquus (also known familiarly as the VMO).
The VMO is actually the lower portion of the vastus medialis that we discussed above. People with mal-alignment of their patella are commonly cursed with a VMO that is deficient: The fibers are too vertical and they don't attach themselves far enough down on the knee cap. The consequence of this suboptimal setup is a VMO that does not adequately balance the bowstring effect. This leads the patella to be pushed up against the lateral wall of the trochlea or to be pushed out of the trochlea altogether.
In cross section, the patella looks like a triangle. It lies snug in the trochlea. In other words, relative to the horizontal normally it is not tilted. In some patients, however, it is indeed tilted, always with the 'lateral' side down.
This is most commonly associated with an excessively tight lateral retinaculum and is the most common form of knee cap mal-alignment. Whether the tight lateral retinaculum is the cause or the result of the tilt remains a subject of debate.
The cartilaginous underbelly of the patella features 'facets', i.e. angled surfaces. If the doctor palpates (feels) these facets by curling his fingers around the patella, this should not be painful. 
This manoeuvere, on the other hand, is painful in patients whose patellae are poorly aligned.
[Reprinted with permission, The Adult Knee, Chapters 59-60, Lippincott Williams & Wilkins, 2003.]
The combination of tilt and tender facets is, in my opinion, evidence that part of the problem is coming from the knee cap. It's that simple.
Tenderness of the lateral facet in combination with patellar tilt should suggest to the doctor that the tilt is of clinical significance (many patients have tilt that is asymptomatic).
With your leg straight see if you can feel where the joint line is a few centimeters below the bottom of the knee cap. Refer to the illustrations to help you. Bend and straighten while you 'palpate'. See if you can feel the two rounded 'knuckle' ends (the 'condyles') of the femur just above the joint line. Palpate as far as you can the under-surface of the knee cap. Any tenderness?
Let's consider just one more anatomical concept for now and then take a little break from the subject. People are always surprised at just how big a knee can swell after an injury when it can fill up with blood or joint fluid (effusion). This is because the cavity of the joint is not confined to the narrow gap between the bones.
The cavity of the knee extends half a hand up above the knee cap and the space can easily accommodate two or three of cups of fluid. This extension of the joint cavity up above the patella is called the supra-patellar pouch and it exists to lubricate movement during flexion (bending) and extension (straightening).
The knee comprises two (or one could even say three) joints -
The patella changes its relation to the femur as the knee bends. In extension (leg straight) it occupies a position above the trochlear groove of the femur, but moves firmly into the groove as the knee flexes (bends) and is drawn into a position over the joint line.
Its articular (joint) cartilage is the thickest and softest in the human body.
It is inherently unstable and is held in place by many finely tuned soft tissues.
The existence of a quadriceps (Q) angle leads to a tendency of the patella to move laterally as the quadriceps muscles are tightened.
This lateral movement is resisted by the higher wall of the trochlea on the lateral side, and, conversely, encouraged by a high Q angle.