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Gel Injections (Hyaluronic Acid)

A healthy knee joint has up to four milliliters of joint fluid within the joint capsule. Inside the joint capsule, specialized cells known as synoviocytes produce the components of this joint fluid: hyaluronic acid (which is also one of the main components of cartilage), salts, collagen, and different proteins. Hyaluronic acid (HA) gives the joint fluid its viscous, slippery quality. It acts like a shock absorber and lubricant in your knee joint and is needed to help the joint work properly, allowing the cartilage surfaces that coat your bones to glide on each other smoothly. When cartilage wears off, such as in the case of osteoarthritis, more fluid is produced in an attempt to improve the gliding of the surfaces and also to diminish the inflammation.

Joints affected by osteoarthritis typically have a lower concentration of hyaluronic acid in their joint fluid than healthy joints and therefore less protection against joint friction and impact. This condition further accelerates the joint degeneration process, setting in place a vicious cycle. A gel injection may artificially supplement the joint fluid’s natural viscosity—this is known as viscosupplementation.

By injecting an engineered normal fluid component, doctors hope to temporarily lubricate the knee joint, thereby decreasing pain, improving function, and perhaps even slowing the degeneration process. Interestingly, viscosupplementation seems to work, but not for the reasons initially expected. The injected fluid doesn’t stick around for long in your joint—it’s frequently cleared within a day or two. Even so, pain relief can last months following a single injection or sequential HA injections. Several researchers have found that viscosupplementation provided a clinical improvement in pain and function compared to all other treatment modalities for up to twenty-six weeks, with significant improvements when compared to pre-injection. It is thought that viscosupplementation works because of its anti-inflammatory actions. Briefly, in osteoarthritic knees, HA that is normally in the joint binds to proteins that promote inflammation and tissue damage. Injecting large amounts of HA may clear the knee of these inflammatory molecules, alleviating the pain symptoms that result from its presence.

Several versions of hyaluronan gel injections are used to treat the symptoms of knee osteoarthritis. Their effectiveness is closely related to the preparation. The natural fluid of the knee is extremely viscous (heavy). Gel products that are similar in viscosity to natural joint fluid are thought to be more effective. Still, not all heavy gels are equal since the natural hyaluronan is only one molecule, and sometimes engineered HAs are composed of multiple smaller pieces of HA put together to make it heavier. Indeed, a single chain HA (one piece) has been reported to be more effective. The processing technique is also important: it can be produced by the extraction of avian-derived virus molecules (AD-HA) or through bacterial processes of biological fermentation (Bio-HA). The latter is considered to be less likely to cause an immune response, which leads to swelling in about one out of two hundred fifty patients.  Additionally, there are two common ways to do this: one injection or multiple injections given every week or every other week. A recent review of several studies showed that multiple injection regimens provided better pain relief than single injections. They also showed that viscosupplementation was generally deemed safe with few to no treatment-related adverse effects reported across studies. The most common side effects of viscosupplementation are pain, swelling, and inflammation at the site of injection. Infrequently, these symptoms are pronounced enough to require immediate medical care. Additionally, a small number of people have an allergic reaction to the injected material. You should let our team know if you have allergies to eggs, bird feathers, or other bird products, because some viscosupplementation injections may spur an allergic reaction. Finally, although it is highly unlikely, any injection into the knee carries a risk of infection.

When the pain relief wears off, you may get another series of injections. There should be at least six months in between the initial injection and the second round (although no studies have assessed this time frame). Viscosupplementation treatments can be repeated one or more times but may not work indefinitely. As with cortisone injections, when osteoarthritis is more severe, there may be less symptomatic relief.

At a Glance

Dr. Jorge Chahla

  • Triple fellowship-trained sports medicine surgeon
  • Performs over 500 surgeries per year
  • Professor of orthopedic surgery at Rush University
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