What a cortisone injection does inside the knee

Most patients asking about a cortisone injection for the knee want to know two things: how quickly it will work, and whether it will actually make a difference. The short answer to both is encouraging — but with important caveats worth understanding before the procedure.

A cortisone injection delivers a synthetic corticosteroid — typically triamcinolone, betamethasone, or methylprednisolone — directly into the knee joint space. This is a meaningful distinction from a tablet or an intramuscular injection: the drug acts locally on the synovial lining and surrounding tissue rather than circulating through the body at high concentrations. By suppressing the inflammatory mediators responsible for swelling, heat, and pain inside the joint, the injection can produce noticeable relief within a few days, with most patients reaching peak benefit at around two to four weeks.

The primary clinical use is symptomatic knee osteoarthritis (KOA), a degenerative condition affecting an estimated 240 million people worldwide and ranked among the leading causes of disability globally. For many of those patients, corticosteroid injection offers one of the fastest-onset pain-relief options currently available, at relatively low cost and with a straightforward outpatient procedure.

One nuance worth naming honestly: a published meta-analysis of knee OA injection trials found that saline placebo injections also produce meaningful and surprisingly long-lasting symptomatic improvement. This does not undermine the case for corticosteroid — the pharmacological anti-inflammatory effect is real and accounts for the greater part of what patients experience — but it does suggest that the act of injecting fluid into an inflamed knee joint carries some non-specific benefit of its own. Setting realistic expectations around this is part of good pre-injection counselling.

How much relief to expect — and how long it lasts

The question patients most often ask in the days after booking an injection is straightforward: how long will it last? The honest evidence-based answer is that relief is real, arrives quickly, and has a predictable shelf life — typically measured in weeks to months rather than years.

Multiple randomised trials confirm that pain scores (VAS) and functional scores (WOMAC) improve significantly within the first two to four weeks, which is the period when corticosteroid injections outperform most alternatives, including platelet-rich plasma (PRP). Head-to-head data show corticosteroid is statistically superior to PRP at both two weeks and one month. The picture reverses from two months onward: PRP produces better clinical outcomes at two, three, six, and twelve months post-injection. This cross-over pattern maps the relief curve precisely — cortisone gets there faster; other options hold on longer.

For most people with knee osteoarthritis, the main benefit window sits between approximately two and twelve weeks, with sustained relief beyond three to six months not consistently demonstrated in the trial data. It is worth noting that the typical trial population has early-to-moderate disease; patients with end-stage, bone-on-bone OA are largely excluded, so outcomes in that group are less predictable and should be discussed individually with a clinician.

Understanding this timeline is practically useful. The benefit window is well-suited to a specific purpose: enabling a patient to engage more effectively with physiotherapy, settle an acute flare, or create space to weigh a longer-term treatment decision — and planned with that goal in mind, two to three months of meaningful pain relief is a genuine clinical resource.

The structural safety question: cartilage, meniscus, and repeat injections

Reasonable concern about joint damage from repeated steroid injections deserves a straight answer — and the evidence is genuinely mixed, which is exactly why it belongs in any honest pre-injection conversation.

On single-injection safety, the most directly relevant data come from a matched-cohort study of 186 pairs drawn from the Osteoarthritis Initiative dataset. A single intra-articular corticosteroid injection did not accelerate cartilage volume loss — a reassuring finding for patients considering one-off or occasional use. The same study, however, found a transient, statistically significant reduction in medial meniscal thickness (p = 0.006) and joint space width (p = 0.011) in the year of injection compared to matched controls. The clinical significance of that meniscal change is not yet established, but it is a disclosure-worthy finding for patients planning repeat treatment.

For repeated injections, the signal is more concerning. A 2023 meta-analysis of six studies (n = 1,437 participants) found that intra-articular corticosteroid injections were associated with significantly greater odds of knee cartilage structural worsening compared to controls (OR 2.01, 95% CI 1.18–3.44). The direct causal pathway is not fully established, but the association is statistically robust across sensitivity analyses.

Critically, no published guideline has defined a proven safe annual injection frequency. The widely cited convention of three injections per year per knee is clinical practice rather than a trial-derived threshold. There is no evidence base that precisely maps where cumulative structural risk begins.

What patients should do with this information is straightforward: discuss injection frequency explicitly with a clinician rather than treating repeat injections as automatically routine. If relief is shortening with each course, that is a signal to reassess the overall treatment plan.

Systemic side effects and the pre-surgery timing rule

Telling your care team the right things before and after an injection is as important as the injection itself — and two risks in particular warrant a brief, practical conversation.

The most immediately relevant systemic effect is a transient rise in blood glucose, which typically occurs within 24 to 72 hours of the injection and generally settles within a few days. For patients managing type 2 diabetes or using insulin, this is not a reason to avoid treatment, but it is a reason to monitor blood sugar closely after the injection and to notify a GP or diabetes nurse in advance so they can advise on any temporary adjustment to management.

Repeated courses carry a small additional risk of adrenal suppression, and — as NHS clinical guidance documents — infection susceptibility may increase with higher cumulative steroid doses. Local infection at the injection site itself is recognised but rare when the procedure is performed under appropriate conditions.

The operationally critical rule concerns surgical timing. Current evidence supports a minimum four-week delay between a corticosteroid injection and any arthroscopic knee procedure, because the peri-operative infection risk is measurably elevated in that early window. The same caution applies for patients being worked up for total knee arthroplasty: pre-operative injections have been associated with increased post-replacement infection risk, making disclosure of recent injections an important step before any elective knee surgery is scheduled.

None of these risks make cortisone injection high-risk for the majority of patients — they are manageable with straightforward precautions and transparent communication with the treating team.

When cortisone isn't enough: the next options in sequence

Cortisone's predictable short shelf-life means the injection rarely represents a final answer — it is usually the first step in a sequence that a clinician and patient work through together.

Intra-articular hyaluronic acid (viscosupplementation) is typically the next intra-articular option. HA injections aim to supplement the knee's natural joint fluid and provide medium-term symptom relief. Whether a single injection or a course of multiple injections is optimal remains contested: a systematic review of eleven studies found no consistent difference between regimens, though earlier meta-analysis data suggest multi-dose courses of two to four, or five or more injections are more reliably superior to saline than single-dose schedules.

Platelet-rich plasma (PRP) offers a different biological rationale — rather than suppressing inflammation, it delivers concentrated growth factors with the aim of supporting tissue healing. Its benefit profile extends considerably further along the timeline than cortisone's, which is worth factoring in when planning an injection sequence for a patient managing years of knee OA rather than weeks of acute flare. Guideline opinion is genuinely divided: OARSI supports PRP for knee OA; AAOS and ACR currently oppose it, citing the level of evidence. That debate remains live, and patients are entitled to know it.

Combination approaches are an underused option. Trial data in 150 knee OA patients show that adding oral duloxetine to an intra-articular corticosteroid plus HA injection achieved superior pain control at 24 weeks compared to injection alone — pointing to central sensitisation as a component of knee OA pain that local treatment alone does not fully address.

Physiotherapy, structured exercise, and weight management run as a parallel track throughout all of these stages. They are evidence-based complements to injection therapy, not afterthoughts.

Genicular nerve radiofrequency ablation (RFA) is a procedural bridge for patients who have not responded adequately to cortisone and HA and are not yet surgical candidates. It targets the sensory nerves supplying the knee rather than the joint surface itself. In a small reported case series, endoscopic RFA following failed image-guided RFA achieved greater than 80% reduction in VAS pain scores at both six and twelve months. When intra-articular and procedural options have been exhausted, unicompartmental or total knee replacement becomes the appropriate next consideration for suitable patients.

Getting a structured assessment at Lincolnshire Knee

Three questions tend to guide the clinical conversation after a cortisone injection: Has the relief been adequate and durable? Are structural changes accumulating that should influence repeat injection planning? And is the next step another injection — or something further along the pathway?

Answering the third question accurately requires more than a symptom score. It requires a clinical assessment of cartilage and meniscal status, ideally with MRI analysis that can quantify the structural picture — particularly medial compartment change — rather than estimate it. That information shifts the conversation from a repeat prescription to a considered decision: whether a further cortisone injection is still appropriate, whether viscosupplementation or PRP is a better fit for the current disease stage, or whether a joint preservation or replacement discussion is the more honest next step.

No single treatment decision needs to be made before that assessment. The goal is a clear, personalised plan that reflects where the knee actually is, not where it was at the last GP appointment. Consultations are available at Sleaford (NG34) and Grantham (NG31).

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

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2. [2] Waiting one month after an intra-articular corticosteroid injection for performing a knee, shoulder, and hip arthroscopy could minimize the risk for post-operative infection. (2024). https://doi.org/10.1016/j.arthro.2024.02.027 https://doi.org/10.1016/j.arthro.2024.02.027
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