What’s the cause?
When you lose the range of motion of your joint (flexibility), the ordinarily congruent articular surface is compromised and leads to edge-loading of the articular cartilage overlying the bone ends. Think of it like having two dinner plates stacked on top of one another. When perfectly stacked, the surface area of contact of the plates is maximized, meaning that the load on the top plate can be evenly distributed over the lower plate. However, if you slide one of the plates over so that the edges overlap, the force transfer from the top plate into the bottom plate becomes concentrated at the points of contact. This focuses all the load through the edge rather than evenly distributing it over a broad surface area. The same thing that happens when you lose flexibility. When your flexibility is limited, a smaller area of the joint surface is subject to the loads applied during activity, so maintaining your flexibility is key to keeping your joints healthy.
Joint contractures and joint stiffness must be overcome to restore native congruence to joint articulations and maximize the surface area of contact with impact, no matter what joint is involved. The knee joint has its greatest volume at 30º of flexion. Joint effusions will often move into a flexed position which subjects the posteromedial or posterolateral structures to contraction and stiffening.
Back to our dinner plate example, the edge loading puts more force through the articular cartilage matrix and causes disintegration of the matrix, which dissolves into the joint fluid.
These nanomolecular fragments of the articular cartilage matrix stimulate inflammatory cells that line the knee joint or any other synovial joint in the body, causing a painful inflammatory response.
Ultimately, the muscles and tendons that are not elongating properly due to their limited excursion from loss of flexibility become stiff due to scar tissue formation. Biochemical changes occur throughout the musculoskeletal tissue causing degeneration. This degeneration affects the fibrous tissues' ability to contract or relax and is worsened by disuse.
The increased load will be concentrated in the bone due to the limited joint excursion. That leads to changes in the bone under the cartilage surface, known as the subchondral bone. The body responds by building up additional bone to support the cartilage. Unfortunately, this additional bone can become thick and stiff. The concave or tibial side of the joint is usually the first to thicken and become stiff because of how impact loading affects the joint. Once the tibial side becomes stiff, it changes how the joint functions. At the point of impact, the load will reflect to the convex femoral subchondral bone, and the same process occurs.
The third stage involves both sides of the joint compartment becoming stiff and losing elasticity or the amount of force the bone is able to absorb. If it cannot be absorbed, it will pass on to the cartilage surface, where the matrix can become compromised.
The soft tissue structures about the joint absorb impact under normal loading conditions and should be optimized to alleviate symptoms associated with joint arthritis.
What can help?
Fully stretching your soft tissues around your joints and maintaining your flexibility regularly will keep your joints as healthy as they can be and should be done routinely. As we age, this becomes particularly important because our tissues lose water, and we become less flexible. Decreased flexibility due to age is due to biochemical changes that occur in tissue collagen with age. Adding a daily stretching routine can be easily accomplished in very little time, and the payoff can be significant. Whether biking, doing yoga, pool exercise classes, or any other activity that uses your entire body, you will soon find that your body still works pretty well with the right attention to detail. Physical therapists and athletic trainers can be an excellent resource for anyone who wants to improve their range of motion and strength of their joints. Often, simple stretching exercises can relieve pain from disuse that has been present for years. Of course, it takes time and dedication, but the reward potential is too much to ignore, particularly in the setting of knee joint arthritis.