Primary knee problems are ones in which structures in or surrounding the knee joint are the problem. You may also have knee pain secondary to a problem with your ankle, hip or low back region. We will largely be considering primary knee pain in this blog.
You may also have referred pain, where there is little to no issue in the knee at all, but rather, the body is confused as to where the problem is, so just kind of takes a best guess as to the source of the pain or dysfunction.
Anterior (front) knee pain
The patella, or kneecap, looks as though it is floating in mid-air in the diagram above. It is actually embedded in the common tendon for the quadriceps group of muscles (or 'quads') on the front of the thigh, as seen on the right. It helps them to get better leverage when straightening out the knee.
If the balance of these muscles is uneven, the patella will be pulled off to one side (normally towards the outside of the thigh), creating uneven pressure and wear on part of the underside of the kneecap. This is known as patellar maltracking, and is commonly an issue in juveniles/adolescents and seniors as well as runners, particularly long-distance.
This can also increase the strain on the patellar tendon, leading to patellar tendinopathy, or "jumper's knee".
So what about pain on the sides, or back, of the knee?
Soft-tissue injury is probably the most common cause, with the hamstrings being a common source, especially with any sport or hobby involving sprinting. A muscle called popliteus is also a good candidate, and will likely be the topic of a future blog post. LaPrade et al (2010) called the popliteus tendon the "fifth major ligament of the knee", or, as I like to call it, the fifth elastic band.
A Baker's Cyst is a fluid-filled swelling in the back of the knee. It is often due to an underlying problem with the knee, or inflammatory reaction (such as in rheumatoid arthritis, a meniscal tear, or due to a 'loose body' such as from osteoarthritis). It may cause no symptoms at all, but if it is putting pressure on another structure, such as the popliteal vein, it can cause pain or discomfort.
LAPRADE R. et al, (2010) Analysis of the static function of the popliteus tendon in evaluation of an anatomic reconstruction: “the fifth ligament of the knee”, Am J Sports Med., 38:543