Two injections, two very different recovery rationales

A gel injection in the knee is not a single treatment — and the aftercare that follows depends entirely on which gel was used and what it was designed to do.

Arthrosamid® is a polyacrylamide hydrogel injected for knee osteoarthritis. It integrates into the synovial membrane — the joint's inner lining — where it acts as a long-term cushioning scaffold. It does not repair cartilage, and because it is non-biodegradable, it remains in the joint indefinitely.

ChondroFiller® is a biodegradable collagen hydrogel injected to address focal articular cartilage defects. It scaffolds the migration of the body's own cells into a localised area of cartilage damage, gradually remodelling over 12–24 months until the scaffold is fully resorbed and replaced by new tissue.

Both are delivered as outpatient, ultrasound-guided injections with no hospital admission. Beyond that, their aftercare timelines diverge substantially — because they act on different structures through different biological processes.

If you are unsure which injection you received, check with your clinician before acting on any of the guidance below.

The first 14 days after Arthrosamid: why rest is non-negotiable

The single most important instruction after an Arthrosamid® injection is straightforward: rest deliberately for the first 14 days. Clinicians sometimes phrase this as being 'deliberately lazy' — and the wording is intentional.

The reason is biological. Histopathological data from animal models show that synovial cells begin invading the iPAAG gel at around days 10–14, with a stable sub-synovial layer established by approximately day 30. Disrupting this early window with sustained knee loading risks preventing the gel from anchoring securely within the joint lining — which would undermine the treatment before it has taken hold.

In the first two to three days, some pain, soreness, swelling, and stiffness around the knee is expected and normal. This is the joint's natural inflammatory response as it settles after the procedure, not a sign that something has gone wrong. Paracetamol and ibuprofen are appropriate during this period; ibuprofen and other NSAIDs are typically continued for up to seven days to reduce inflammation and lower infection risk. TENS (transcutaneous electrical nerve stimulation) can be used as an additional measure if discomfort is persistent.

For the remainder of the 14-day window, light walking and routine daily movement are generally acceptable. What should be avoided is sustained or high-impact knee loading — anything that places prolonged mechanical stress on the joint before the gel has had sufficient time to integrate. Think of this period less as enforced inactivity and more as purposeful protection of an implant that is still finding its footing.

Returning to activity after Arthrosamid: what to expect beyond week two

Once the 14-day integration window has passed, the question patients most commonly ask is a practical one: when will I actually feel better, and what can I reasonably do?

Pain reduction with Arthrosamid® tends to emerge gradually rather than at a fixed point. Most patients report meaningful improvement from around week four — a timeline consistent with the biological process of gel anchoring and early synovial remodelling. The first fortnight is therefore not a period for judging whether the treatment is working; benefit arrives after integration, not during it.

Longer-term data are encouraging. A five-year RCT extension involving 58 patients who completed the full follow-up found sustained WOMAC pain improvement of −16.2 points at year five, with no adverse events attributed to the implant during years one to five. A separate 24-month cohort of 314 knees confirmed durable patient-reported benefit across multiple outcome measures. Importantly, activity resumed after gel integration does not appear to compromise the implant. Because Arthrosamid® is non-biodegradable, there is no second phase of protected loading to anticipate — once the initial 14 days are complete, progressive return to normal movement follows symptom tolerance.

What published evidence does not specify is an exact milestone for returning to running, sport, or strenuous exercise. No protocol currently defines these thresholds for Arthrosamid® patients specifically; decisions should be guided by the treating clinician based on individual symptom response and function, rather than assumed from a fixed schedule.

The protect phase after ChondroFiller: weeks one to six

ChondroFiller® follows a more structured recovery path than Arthrosamid® — one that resembles the phased aftercare associated with cartilage repair surgery, though without the hospital admission or arthroscopic wound to manage.

The three phases are: Protect (weeks one to six), Strengthen (weeks six to twelve), and Return to Activity thereafter. This section concerns the first.

What limited weight-bearing means in practice

During weeks one to six, the priority is protecting the collagen scaffold while it stabilises. In practical terms this means avoiding prolonged standing, ascending and descending stairs more than necessary, and any activity that places sustained compressive load through the knee — running, gym work, heavy lifting, and impact sport are all off the agenda at this stage. Light controlled movement is not only permitted but encouraged: gentle, assisted range-of-motion exercises help maintain joint health without destabilising the scaffold. Crutches may be recommended to assist weight-bearing in the early days.

The biology behind the restriction

The collagen gel sets within three to five minutes of injection, but that rapid hardening is only the beginning. Over the following days and weeks, host cells — chondrocytes and stem cells — begin migrating into the matrix. Ex vivo data confirm that by day 14, cell DNA content within the scaffold increases 2.4-fold, reflecting active early recruitment. Full cartilage maturation then continues over the first twelve months, with the scaffold progressively resorbed and replaced by the patient's own tissue by one to two years.

Because ChondroFiller® is biodegradable — unlike Arthrosamid® — the scaffold is at its most structurally vulnerable in this early remodelling window. Engaging a physiotherapist from the outset, even within the protect phase, helps maintain muscle tone around the knee and prepares the joint for the strengthening work ahead.

Rebuilding strength and returning to activity after ChondroFiller

From around week six, the focus shifts decisively toward rebuilding. Physiotherapy during this strengthen phase — typically spanning weeks six to twelve — targets quadriceps recovery, joint stability, and a careful reintroduction of low-impact movement. Cycling, swimming, and progressive walking programmes are the standard foundations, chosen because they load the knee through a controlled range without the impact forces that could still disturb maturing cartilage.

Clinical data give a useful benchmark for what recovery looks like over time. Across manufacturer-sponsored and independent studies, IKDC functional scores improve by approximately 30 points at twelve months — a shift that reflects meaningful restoration of knee function rather than marginal gain. MRI findings (MOCART scores) from a randomised study confirm that scaffold maturation continues to progress visibly through to week 52, even as patient-reported symptoms are already improving well before that point. These are population-level benchmarks, not individual guarantees; patients with larger defects or lower pre-injection fitness may move through each phase more slowly.

Beyond week twelve, current published guidance does not specify milestones for returning to running, contact sport, or high-impact activity. The evidence base for the injectable format's later-stage rehab remains limited, and clinicians acknowledge this gap openly. In practice, the transition back to full activity is determined at clinical follow-up, using symptom response and functional assessment — not a fixed calendar date — as the decision criteria.

The underlying biology supports patience: the scaffold continues filling the defect through twelve months and is fully replaced by native tissue over one to two years. Recovery after ChondroFiller® is a gradual biological process, and the strengthen phase marks its most active, visible stage.

When to seek advice and how to book an assessment

Knowing when to call the clinic is as important as following the aftercare plan itself. Contact your clinical team if pain or swelling worsens significantly after the first 72 hours, if the knee shows signs of infection — spreading warmth, redness, or a fever — or if the expected benefit has not begun to emerge by around weeks four to six for Arthrosamid® or weeks six to eight for ChondroFiller®. Most patients will not need to make that call, but acting promptly if something does not feel right is always the right instinct.

How quickly any individual progresses will depend on factors your clinician will already have discussed with you at your pre-injection assessment; these are best reviewed in that clinical context rather than benchmarked against a general timeline. What applies to everyone is that neither injection removes the need for structured physiotherapy — it remains central to getting the best result from either treatment.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. Book an assessment at lincolnshireknee.co.uk.

Two injections, two very different biological clocks — understanding which one yours is running on is the starting point for a recovery that actually holds.

1. [1] Sustained symptom relief and safety over five years following a single intra-articular injection of 2.5% polyacrylamide hydrogel in patients with knee osteoarthritis. (2025). https://doi.org/10.55563/clinexprheumatol/bsper8 https://doi.org/10.55563/clinexprheumatol/bsper8
2. [2] IMPLANTATION OF CHONDROFILLER LIQUID® AS A SCAFFOLD MATERIAL FOR THE TREATMENT OF CHONDRAL LESIONS OF THE KNEE JOINT. (2024). https://doi.org/10.5272/jimab.2024304.5936 https://doi.org/10.5272/jimab.2024304.5936
3. [3] Controlled, randomized multicenter study to compare compatibility and safety of ChondroFiller liquid with microfracturing of patients with focal cartilage defects of the knee joint. (2016). https://doi.org/10.5348/VNP05-2016-1-OA-1 https://doi.org/10.5348/VNP05-2016-1-OA-1
4. [4] Development of an Ex Vivo Osteochondral Biomimetic Platform for Mechanistic Investigation of Cartilage Regeneration. (2025). https://doi.org/10.3390/ijms262311759 https://doi.org/10.3390/ijms262311759
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