The core candidacy picture

The question most patients arrive with — 'could an injection help my knee?' — is really three questions rolled into one: has osteoarthritis been confirmed on imaging, has conservative care had a fair trial, and is the joint still at a stage where an injection can do something useful?

All three need to be satisfied. A diagnosis based on symptoms alone is not enough; an X-ray or MRI is needed to confirm OA and, crucially, to show how much joint space remains. That last point matters because every main injectate — whether a corticosteroid, hyaluronic acid, or a newer hydrogel — requires some remaining cartilage surface and joint space to act on. When the joint has reached end-stage 'bone-on-bone' narrowing, the structural conditions for a meaningful injectable effect are largely gone.

The middle ground — mild-to-moderate joint-space narrowing — is where injections are most likely to add real value, provided the other criteria are met: persistent moderate-to-severe pain that limits everyday activity, and at least three months of first-line conservative care (exercise, weight management, and oral analgesics) that has not brought sufficient relief.

Even within this profile, candidacy is not a simple yes or no. Which injection makes clinical sense depends on the specific picture — the pattern of symptoms, whether there is active inflammation, and what prior treatments have and have not achieved.

Why conservative care comes first

'Failed conservative care' is the clinical threshold that unlocks injection consideration — but it carries a specific meaning. It does not mean attending a handful of physiotherapy sessions and finding them uncomfortable, or taking ibuprofen occasionally without clear benefit. It means consistent, sustained adherence to a structured programme — guided exercise, appropriate weight management, and regular analgesia — with pain that remains functionally limiting throughout.

Patients who have not yet completed a structured exercise programme are generally not at the point of injection assessment. That ordering matters because injections work best as an adjunct to rehabilitation, not a replacement for it. The primary aim is to reduce acute pain enough to make productive physiotherapy possible — an injection that settles a flare can unlock weeks of exercise that would otherwise have been too painful to attempt.

A practical illustration of the pathway: structured physiotherapy for three months, persistent pain that still interferes with daily activity, specialist review — and at that point, injection therapy enters the conversation as a way to support further progress, not to substitute for it.

Matching the injection type to your situation

Assuming injection therapy is appropriate, the next question is which type — and the answer depends on the specific clinical picture rather than a one-size preference.

Corticosteroid (steroid) injection

The strongest indication is an acute flare accompanied by visible or palpable swelling, warmth, or confirmed synovitis. In that setting, joint aspiration is often carried out first, followed by an intra-articular corticosteroid to damp down inflammation rapidly. A meta-analysis of 11 randomised trials (842 patients) found clinically meaningful pain relief — reaching the threshold that patients actually notice — at up to six weeks. Benefit at three months is statistically detectable but falls below that meaningful threshold, and at six months there is no significant difference from placebo. Steroid injections are therefore most useful for settling an acute episode, not for sustained symptom control.

Hyaluronic acid (viscosupplementation)

For patients with mild-to-moderate disease who have not responded to oral analgesia or short-term steroid treatment, hyaluronic acid acts differently — restoring some of the lubricating and shock-absorbing properties that OA progressively depletes from synovial fluid. Its effect builds more slowly but may extend over several months, which suits patients looking for a longer window of relief to support rehabilitation.

PRP (platelet-rich plasma)

PRP is sometimes promoted for younger or more active patients with mild-to-moderate degeneration. The RESTORE trial — a well-designed RCT of 288 participants with KL grade 2–3 disease — found no significant benefit over placebo for pain or cartilage volume at 12 months. Current guidelines do not recommend PRP for knee OA on the basis of available evidence.

Arthrosamid® and emerging biologics

Arthrosamid® (a non-absorbable polyacrylamide hydrogel) is a single-injection option supported by a 24-month observational cohort of 269 patients, though no randomised controlled trial has been published. Bone-marrow-derived stem cell injections have shown encouraging results in a small RCT (24 participants), with improved patient-reported scores and MRI-confirmed cartilage preservation at 12 months. Both remain earlier-stage options, most relevant for patients who have not responded adequately to conventional injectates and are not yet at the threshold for surgical discussion.

How disease severity changes the picture

Radiology reports often refer to Kellgren–Lawrence (KL) grading — a 1-to-4 scale that describes how much structural change is visible on an X-ray. Grades 1 and 2 reflect early narrowing with most of the joint space intact; grade 3 shows more marked narrowing with some bone-edge changes; grade 4 — the 'bone-on-bone' description patients sometimes encounter — indicates that very little cartilage space remains between the joint surfaces.

This grading matters to injection candidacy because most injectates need some remaining joint space and cartilage surface to act on. At grades 2–3 there is enough structure for treatments such as hyaluronic acid (the main viscosupplement used in practice) to restore lubrication and cushioning. At grade 4, that target is largely gone, and viscosupplementation is unlikely to offer meaningful benefit.

Advanced disease does not, however, close every door. Corticosteroid injections can still play a role at grade 4 — reducing pain and swelling sufficiently to support quality of life or to help a patient prepare physically and mentally for surgery. The treatment goal simply shifts from medium-term symptom management to short-term comfort and surgical readiness.

Imaging is one input, not the whole picture. Some patients with grade 3 or 4 findings on X-ray report relatively manageable symptoms, while others with less severe radiographic changes are significantly limited. A consultant weighs both the structural findings and the patient's lived experience before recommending the next step — which, for persistent grade 4 disease, typically means a referral for surgical assessment.

When injections are not appropriate

Not everyone with knee OA is suitable for injection therapy — and some situations are firm barriers, while others simply require a more careful conversation with a specialist.

Absolute contraindications

The following factors rule out intra-articular injection regardless of agent:

• Active infection at or near the target joint — including septic arthritis, bacteraemia, or cellulitis overlying the knee
• Intra-articular fracture at the injection site
• Known allergy to the specific injectate being considered
• A joint prosthesis already in place in the affected knee

Relative contraindications — individual risk–benefit weighing required

These do not automatically rule out treatment, but a specialist will need to assess whether the benefit justifies the risk for the individual patient:

• Uncontrolled diabetes — relevant specifically to corticosteroids, which can cause a temporary but clinically significant blood glucose rise. This concern does not apply in the same way to hyaluronic acid or hydrogel injections.
• Anticoagulant therapy or significant bleeding disorders — particularly relevant where joint aspiration is planned alongside the injection.
• Planned total knee replacement within three to six months — corticosteroid injections in this window may carry a small elevated risk of periprosthetic infection post-surgery.
• Active unhealed wounds, glaucoma, severe osteoporosis, or poorly controlled blood pressure — the NHS flags these as reasons for extra caution with corticosteroid injections specifically.

Relative contraindications are factors to be weighed, not automatic refusals. Where one agent is not appropriate, a different injectate may still be suitable — the risk profile is not uniform across all injection types.

What a suitability assessment looks like

Arriving at a specialist appointment with a clear picture of your symptom history makes the consultation more useful. Assessment typically covers three areas.

Clinical history. The specialist will ask how long pain has been present, which activities it limits, and exactly what has been tried — including the duration and doses of any analgesics, physiotherapy, and previous injections. This establishes whether conservative care has had a genuine trial.

Examination and imaging. The joint is assessed for warmth, effusion, and range of movement. If imaging is not recent or was taken non-weight-bearing, updated X-rays may be requested: weight-bearing views show joint space more accurately and can meaningfully change the assessment of disease severity.

Deciding on suitability. The choice of injectate — or the decision that none is appropriate — is reached in discussion with the patient, weighing disease stage, symptom pattern, prior treatment responses, and safety factors. Patients should expect questions about current medications, recent infections, and any planned surgery.

Suitability is not fixed at a single point in time. A contraindication that applies today — poorly controlled blood glucose, for example — may resolve, opening options that were unavailable at first assessment. Before attending, it is worth bringing any previous imaging, a current medication list, and a rough timeline of what has already been tried. These three things alone give a specialist the starting material to reach a clear recommendation rather than a deferred one.

1. [1] Intra-Articular Injection of Bone Marrow–Derived MSCs in Knee OA: RCT — Cell Transplantation. (2025). https://doi.org/10.1177/09636897241303275 https://doi.org/10.1177/09636897241303275
2. [2] Effect of Intra-articular PRP vs Placebo in Knee OA: RESTORE RCT — JAMA. (2021). https://doi.org/10.1001/jama.2021.19415 https://doi.org/10.1001/jama.2021.19415
3. [3] Intra-articular corticosteroid injections vs placebo: systematic review and meta-analysis — Knee Surgery & Sports Traumatology. (2024). https://doi.org/10.1002/ksa.12057 https://doi.org/10.1002/ksa.12057