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Aimee Lake, DPT, PRPC
The structure of a synovial joint goes like this:
All of these elements are designed for maximal smooth movement and shock absorption at the joint.
The disease process starts at the cartilage. As cartilage ages it softens. The collagen fibers begin to lose their support structure. The cartilage begins to looked shredded, a process called fibrillation. As a result of all these changes the cartilage loses it’s ability to act as a shock absorber for the joint. This process happens to all synovial joints as we age. However it becomes pathological when it occurs prematurely, progresses abnormally quickly, or causes significant pain.
As the cartilage breakdown continues the normal forces acting on the joint result in increased softening and destruction of the articular surfaces. This process is followed by generalized increased cellular activity, which leads to an inflammatory process within the joint. Then the inflammation causes further breakdown of the cartilage.
As cartilage is lost, the subchrondral bone (the ends of the bones that make up the joint) are exposed. The joint space on an xray starts to look narrower. This essentially means that the subchondral bone (which is the bone underneath the cartilage) becomes the new articulating surface of the joint. You may have heard someone refer to very arthritic joints as being “bone-on-bone”. This is another way of saying that all of the cartilage has been lost and the two bones are simply rubbing against each other. Bone has a lot of nerve endings in it losing the protective cartilage surface means that the very tender bone underneath can become quite painful as it is exposed to abnormal forces.
The result of all this loss of impact-absorbing structures at the joint can cause cysts to form in the ends of the bones around the joint. Cysts needs bloods supply and more blood coming into the small joint space can cause extra painful pressure in the area.
Sometimes xrays will show that a particular joint has lost a lot of its cartilage and the bony surfaces are quiet close together aka bone-on-bone. But the person does not report pain or loss of function at the joint. Meanwhile other joints can look quite good on xray with only mild changes noted and yet the person can reports significant pain and difficulty using the joint. Scientists are still not sure of all the reasons for this.
Osteoarthritis affects 16% of US adults according to the CDC. This is up from 13% in 2005 and up from 6% in early industrial times (defined as adults who died between 1904 and 1940). These statistics controlled for age and BMI meaning that the doubling of osteoarthritis isn’t because we are living longer or tend to be more over-weight then adults in the early 1900’s were.
The current theory suggests that physical inactivity may be the primary reason that the incidence of osteoarthritis has more than doubled in just a few generations. Joints that are moved less have thinner cartilage and more inflammation according to the above study. It is also suggested that the modern diet rich in refined carbohydrates may play a role.
Studies in runners have shown repeatedly that regular running actually decreases the incidence of developing knee osteoarthritis. Now we can see this may simply be because runners are more active which means their knee joints get more movement than knees of sedentary individuals. It could also be for some other reasons that we haven’t totally figured out yet.
In our next post we will talk about what happens when your knee arthritis gets very, very bad.