If you are considering hydrogel injections for knee osteoarthritis, one of the first questions is how long the benefit may last. That matters because relief is not just about the day of the injection. It is about whether your knee feels easier to trust, walk on, and live with over time.

What hydrogel injections are designed to do

When people talk about hydrogel injections in the knee, they are often referring to Arthrosamid, a non-biodegradable polyacrylamide hydrogel used for the symptomatic treatment of knee osteoarthritis. Regenesis describes it as a long-lasting, single-injection option, while published clinical studies describe sustained improvements after one treatment.

The shortest honest answer is this: current evidence suggests that hydrogel injections can help for much longer than a few weeks or a few months, with benefits shown at one year, beyond two years in many patients, and up to five years in longer-term follow-up studies. That does not mean every patient gets the same result for the same length of time, but it does mean this is generally positioned as a longer-acting option rather than a quick temporary fix.

One reason this question can be confusing is that people often mean two different things when they ask how long the injection “lasts”. Sometimes they mean how long the material stays in the joint. Sometimes they mean how long pain relief and functional improvement continue. Those are not exactly the same thing. The hydrogel itself is designed to remain in the joint, but the more useful question for patients is how long the clinical benefit lasts.

What the evidence says about how long they last

In the shorter term, the evidence is already fairly encouraging. A 2024 study reported that a single injection remained effective one year after treatment, with improvements in WOMAC pain, stiffness, physical function, and patient global assessment. That matters because it suggests the benefit is not just an early post-injection effect that fades quickly.

The more striking data come from the longer follow-up studies. A 2025 five-year extension study reported sustained and statistically significant improvements in WOMAC pain, stiffness, physical function, and quality of life after one injection. Arthrosamid’s own 2025 materials also referenced five-year efficacy data and ten-year safety follow-up data being presented, which supports the idea that the treatment is being studied as a long-duration option.

That is why clinics often describe hydrogel injections as lasting around three to five years in appropriate patients. Regenesis says a single Arthrosamid injection can provide pain relief for up to three to five years, and other clinical sources describe benefits lasting beyond two years in the majority of patients, with some reporting improvement out to five years. Those timelines are useful, but they should still be read as ranges rather than guarantees.

When you may start to notice improvement

It is also worth noting that improvement is not usually instant. Regenesis says many patients begin to notice improvement within four to six weeks, and some sources describe the full benefit as becoming clearer over roughly six to twelve weeks as the hydrogel integrates into the synovial lining. So, even though hydrogel injections may last for years in some cases, the knee often improves gradually rather than overnight.

Of course, no injection lasts the same amount of time for every knee. The severity of the osteoarthritis, the amount of inflammation in the joint, age, activity levels, body weight, and general knee function can all affect how long the benefit feels meaningful. A patient with moderate symptoms and good joint function may experience the result differently from someone with more advanced joint damage and daily functional limitation. This is an inference based on the range-based outcomes reported in clinical and clinic sources.

A realistic way to think about the timeline

There is also a sensible note of caution. NICE has said Arthrosamid was not selected for Health Technology Evaluation guidance because the panel considered there to be insufficient evidence. That does not mean the treatment does not work. It means the evidence, while encouraging in several studies, is still not as settled as some patients may assume from marketing language alone.

So, how long do hydrogel injections last in your knee? The fairest answer is that current evidence supports benefit lasting at least one year, often beyond two years, and in some patients up to three to five years after a single injection. The exact duration depends on the person, the severity of the arthritis, and how well the knee responds. If knee osteoarthritis is affecting your mobility or confidence in daily life, a proper assessment can help you understand whether a hydrogel injection is likely to be a sensible option for you.