Just about all long-time runners have heard the term “runner’s knee,” and probably most are at least aware that the kneecap is somehow involved. Many have even mastered the pronunciation of the most commonly used medical term for it – chondromalacia patella – while not really understanding what it means or how it develops. This month, I’ll try to explain this ailment in some detail and next time I’ll cover some of the newer ideas on treatment of it which is causing considerable excitement in the physical therapy community.
Chondromalacia means “softening of the cartilage.” The cartilage we’re talking about here is on the posterior (back) surface of the kneecap. This articular (joint) cartilage is like a TeflonR coating on the bone which provides a smooth, friction-free surface to allow for easier movement. As in all joints, one of the most important functions of this cartilage is to prevent pain from the movement of the bones against each other, since articular cartilage has no pain-sensing nerves, while bones do. Thus, when cartilage erodes, as it does in degenerative joint disease (arthritis), the exposed bone surface becomes painful as pressure is applied to it by weight-bearing or movement against an adjoining bone.
When we bend and straighten the knee, the patella slides in a groove at the end of the femur. The contacting surfaces of these two bones are normally a well-fit unit so that pressure is evenly distributed over the adjoining surfaces. You can visualize this (I hope) if you imagine yourself at the foot of a friend lying on a table, looking up toward the knee. With x-ray vision, you would see what looks like an inverted flying saucer (patella) just making it through a mountain valley (femoral groove) with only a little room to spare. Now, as long as the saucer flies straight and true through the valley, with maybe a brush or two with a treetop, the craft will avoid any damage to its bottom. But if it veers to the left or right….. #%$&#$@*^()&!!!
This is essentially what happens with runner’s knee. For one or more reasons (to be covered next time), the kneecap does not slide properly in the groove and pressure on its undersurface is either increased or distributed unevenly. This pressure can, over the long run, cause pain even without an erosion of the cartilage. In fact, most experts now believe that chondromalacia patella is just the end-stage of this process, with the pain starting some time before the cartilage has worn away. Initially, the discomfort is a dull, aching, throbbing kind, later progressing to a sharp, severe pain as the protective coating erodes and you have bone rubbing against bone. Symptoms are worse with increased activity (running, naturally) and especially with climbing or descending stairs, which in runners, of course, translates as hills. Sometimes, prolonged sitting with the knee bent will bring on the pain, earning this the term “movie-goers sign.” Swelling often accompanies the pain, further complicating the matter.
Next month, I’ll cover the underlying causes of “runner’s knee” and describe how we treat it.
