Viscoelastic Injections for Osteoarthritis- Do They Work?

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A recent article in the New England Journal of Medicine brought additional scrutiny on the use of viscoelastic supplementation such as hyaluronic acid injections. In order to understand why these products are being used, let’s first talk about how they are supposed to work.

The joint is filled with synovial fluid. This fluid is unique in that it offers both lubrication and shock absorption for the knee.

A key component of synovial fluid is Hyaluronic Acid (HA). HA is largely responsible for the shock-absorbing and lubricating qualities of synovial fluid. Inflamed knees, such as those suffering from osteoarthritis, tend to break down HA much quicker than normal knees, and clear the smaller HA molecules faster. While a normal knee may keep HA around for 20 hours at a time, an inflamed arthritic knee breaks it down almost twice as fast.

The theory of viscoelastic supplementation is that injection of HA or HA like molecules will lead to increased viscoelascatic properties of the knee joint fluid allowing it to be better at shock absorption and lubrication. The problem with that theory is that the arthritic knee degrades half of the HA in the first 11-12 hours, and then an additional half of what’s left over the next 11-12 hours, etc. So while the injected HA may be present in the knee for the first 12-24 hours, after that point it’s presence in the knee is negligible.

If the HA is degraded so quickly, how is it supposed to work? Proponents of viscoelastic supplementation argue that the HA injection returns the knee to a state of “homeostasis” or normalcy, which increases the knees own production of native HA. That being said, there is no evidence to at this point that the proposed mechanism is actually what happens.

First, this section comes with a disclaimer that this is a highly contested topic. The New England Journal of Medicine article suggests that HA injections have “at best a small benefit”, and at worst have no additional benefit when compared to a placebo. A number of positive meta-analyses show a small effect of these injections, however, most of these do not compare to placebo injections. Other meta-analyses that do compare the injections to placebo have found no difference between the two groups.

While the benefits of these injections are debatable, the costs and risks have been well documented. The NEJM article quotes a course of injections to cost approximately $500, this cost is likely variable. A number of minor side effects have been reported including local joint pain and swelling. More serious side effects include a pseudoseptic reaction that occurs in 1-3% if patients. This reaction mimics and infected joint, but does so in the absence of an infection. True joint infections after HA injections have been reported but are exceedingly rare.

Since the benefits are less than clear, and costs and risks are well defined, many professional associations have moved away from recommending the use of HA injections in the management of OA of the knee. The most recent AAOS guidelines state “We cannot recommend using hyaluronic acid for patients with symptomatic osteoarthritis of the knee.” The Osteoarthritis Research Society International (OARSI) similarly, do not recommend the use of HA in the treatment of hip or knee OA. So why do doctors still use these injections?

The argument for the use of HA injections is that there are certain patients who do not have great options available to them in terms of management of their OA. These may be patients that are too young to receive a total knee replacement, but are faced with significant pain and limitations that have not responded to conservative management such as weight loss and non-steroidal anti-inflammatory medications. For those patients, HA may offer the chance (however small) to push off joint replacement a little further down the road, when ideally they may be a better candidate.

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