As discussed in the following article: "What Does Arthritis Look Like? An Illustrated Guide," the most common cause for chronic, persistent knee pain is osteoarthritis.
While the medical science of osteoarthritis is a bit complicated, the result is simple.
The articular cartilage of the knee, which when healthy is like the smooth white shiny surface of the end a chicken bone, roughens and appears like crabmeat.
This coarsening of the articular cartilage causes painful friction with joint movement
Over time, microscopic bits of cartilage break off and float around in the synovial fluid.
Synovial fluid is the natural lubricating liquid in the knee and is perhaps the most slippery biological fluid on the planet. Like engine oil, with time and wear it loses its lubricating properties.
So not only do the surfaces of the knee lose their ball bearing like smoothness, but the synovial fluid breaks down and fails as a lubricant.
Science has not yet invented an easy restoration for the articular cartilage, but naturally derived lubrication fluids are available. The use of these is known as viscosupplementation.
It's a bit like putting STP into a squeaky hinge.
Many patients who believe they just must live with their knee pain or have surgery find gratifying relief with viscosupplementation injections.
Most are pleasantly surprised to learn that with modern techniques, the procedure is no more than a bit uncomfortable.
Many carriers including Medicare cover most of the cost of viscosupplemention as frequently as every 6 months.