For such a small bone, the patella (kneecap) seems to cause a lot of problems.
Pain under the patella (or around it) carries many names: patellofemoral pain syndrome, retropatellar pain syndrome, runner's knee, jumper's knee, chondromalacia patella among others. And it also carries frustration for both clinicians and patients.
For decades, patella pain has been explained and treated by Structuralism - a school of thought that believes the genesis of musculoskeletal complaints is from one or more biomechanical abnormalities. For patella pain, the biomechanical abnormalities include a laterally tracking patella, weak medial quadriceps, tight hamstrings, tight iliotibial band, tight calf muscles, weak or tight hip rotator muscles and over pronation of the foot. A Structuralist view would then be to set the mechanics "right" and symptoms would subside.
But, there's a problem with this view. No one has perfect biomechanics. Most people do not train muscles, work on flexibility, for example, enough to alter or maintain a biomechanical homeostasis and they don't know their biometrics (strength, flexibility, motion, power, balance, etc.) well enough or at all to direct their training toward any specific deficit. So, most people go through life with a variety of "abnormalities" and are just fine until one day (or over a series of days) something happens and suddenly pain appears under the kneecap.
The reason a person hurts under the kneecap is because the force applied to tissue(s) is greater than what the tissue can withstand. There are two primary sources of retro-patellar pain from tissue over load: sub-chondral bone and synoivum. I'll cover sub-chondral bone today (there are sources of pain outside the knee that can create knee pain but that's another day).
The over load can occur from a high force, blunt trauma such as a fall on the knee or a lower load, sustained force such as sitting in a three hour movie with your knees bent to ninety degrees. Function occurs across a certain range of loads or a "physiologic zone". When you move too far outside this zone, past the edge of your physiologic function, you often develop symptoms. The cartilage under the patella serves as a force attenuator. It slows the transmission of loads into the underlying bone much like the bumper on your car protects the frame from collision. When the cartilage is too soft, the force travels into the bone at a higher rate. Whether the overload is from blunt trauma of a fall or the sustained overload of a movie, the bone under the cartilage of the patella may experience force greater than its ability to manage it and the result is you hurt (cartilage has no nerve endings so it can't be a pain generator). In fact, there is actually an increase in metabolic activity in the bone similar to a stress fracture with resultant swelling. The increase in pressure activates pain nerve fibers.*
When you get up from the movie for example, your knee may hurt for a period of time then go away. So, from a Structuralist perspective, does that mean your biomechanics improved? No. It means that your tissues were able to return to a state of biochemcial equilibrium. The time for this to occur varies depending on how "fit" your tissues are, how far over your physiologic edge you were pushed and what you chose to do following the movie (move around or walk next door and sit through a meal with your knees bent to ninety degrees again).
I'm not suggesting though that biomechanics are not important to address. Weakness of the hip external rotators. for example, allows the femur to rotate inward and create focal loading points under the kneecap. But, failure to also address the tissues at risk (in this case both cartilage and bone under the patella) often results in a short term resolution of pain with long term recidivism.
Haing once been a Structuralist, I understand its allure. You can sometimes see the biomechanical deviations, feel them, test them and the urge to direct all of your attention to the mechanical variants is exceptionally strong. Sometimes, it works. Most of the time, it doesn't.
Dye SF, Chew MH: The use of scintigraphy to detect increased
osseous metabolic activity about the knee. J Bone Joint Surg
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