The natural lubricant of the human joint is called hyaluronic acid (HA). As part of a large, electrically charged sugar molecule, HA permits water to be absorbed and released from the cartilage surface of the joints. This provides durability and compressibility. Without HA, we would all grind to a halt.
Recent work has clarified the many reasons why when a surgeon injects HA into a painful or arthritic joint: It helps relieve symptoms. These reasons include both mechanical lubrication and biochemical effects. Here are 10 highlights of this research:
1. Friction: Friction is reduced by injecting the lubricant directly. (This effect, however, is limited by the fact that the HA is metabolized by the body in about three days on average.)
2. Permeability: HA is able to penetrate deeply into cartilage, directly affecting the cells that support cartilage called chondrocytes as well as into the synovium, or lining of the joint.
3. Cell behavior: HA suppresses the genetic expression of several inflammatory molecules, acting as an antiinflammatory and reducing joint swelling. It also reduces the production of degradative enzymes from the cells that break down cartilage — a process that accelerates arthritis.
4. Scarring: Scar formation, and subsequent loss of motion, is one of the most disabling aspects of any joint injury or surgical intervention. HA affects certain antibodies and cell receptors involved in cell adhesion, reducing the formation of scar tissue after injury or surgery.
5. Collagen formation: Excess collagen formation is another mechanism by which scar tissue forms after an injury. HA suppresses the gene expression for the specific types of collagen involved in the formation of scars, but not those collagens involved in the formation of normal articular cartilage.
6. Antioxidants: HA has been demonstrated to prevent chondrocyte cell death by promoting the expression of antioxidant enzymes.
7. Pain: After an ACL rupture or any tissue tear in the joint, inflammatory molecules associated with pain (the COX-2 enzyme) are reduced by the application of HA. The activity of neurotransmitters that stimulate the pain receptors is also reduced.
8. Long-lasting effects: Given that the bulk of injected HA is cleared from the joint in three days, its biologic and clinical effects are most likely due to the penetration of the lubricant into the tissues and its effect on the underlying cells. There is a natural increase in the production of HA after injection and an improvement in the physical properties of the synovial fluid.
9. Some people do not respond to HA injections. HA is divided in the marketplace between low- and high-molecular weight formulations. It is not clear which combinations provide the best balance between the physical effects of mechanical lubrication (high weight/cross-linked) and the biologic effects (low weight) of cell stimulation.
10. Our clinical experience is that the efficacy of HA injections can be improved by the addition of growth factors and stem cells to the injections. In our clinic, we combine the stimulation/anabolic factors with the HA into a single injection.
We are currently in, what I call, the “Anabolic Era” of sports medicine and arthritis care, a time when we have the tools to actually accelerate healing and regenerate tissues rather than just replace parts. Given the wide range of helpful effects of stimulatory injections, it seems likely that the use of these supplements will expand rapidly in clinical care. In fact, we are seeing a reduction in surgical volume as we guide patients in regenerative medicine.
Is a therapeutic version of the “Star Trek” medical tricorder far off? How about an MRI that treats, and not just images, body parts? Living in the Anabolic Era of sports medicine and arthritis care, we have the capacity to heal and regenerate parts so you may live long and prosper.
Dr. Kevin R. Stone is an orthopedic surgeon at The Stone Clinic and chairman of the Stone Research Foundation in San Francisco.