What makes someone a realistic injection candidate

A scan showing knee osteoarthritis is not, on its own, a prescription for an injection. The question clinicians actually work through is more nuanced: does the structural picture on imaging, combined with everything else going on clinically, suggest that an injection is likely to help — and if so, which one?

Two inputs shape that answer. The first is radiographic: the Kellgren-Lawrence (KL) scale grades knee OA from 0 (normal) to 4 (severe bone-on-bone loss), and the grade gives a rough ceiling on how much certain injectables can realistically offer. The second input is the broader clinical picture — how long symptoms have been present, whether there is active inflammation, what treatments have already been tried, and whether factors such as alignment problems or comorbidities shift the risk-benefit calculation.

Neither input works in isolation. An expert-panel review of 17 real-world HA injection scenarios found only 6 rated clearly appropriate, illustrating that OA stage alone is insufficient to determine suitability. Equally, patients who have not yet attempted conservative measures — physiotherapy, weight management, oral anti-inflammatories — are generally not considered ready for injection, regardless of what the X-ray shows.

The sections below unpack each layer in turn, starting with what the KL grade itself tells you.

What the Kellgren-Lawrence scale actually measures

Radiology reports typically describe knee OA in terms of Kellgren-Lawrence (KL) grades — a five-point scale, 0 to 4, based on what a plain X-ray shows.

• Grade 0 — no changes; the joint looks structurally normal.
• Grade 1 — doubtful disease: very subtle bony spurs (osteophytes) may be visible, with no clear joint-space narrowing.
• Grade 2 — mild OA: definite osteophytes present; joint space may be slightly reduced.
• Grade 3 — moderate OA: multiple osteophytes, definite narrowing, early hardening (sclerosis) of the bone beneath the cartilage.
• Grade 4 — severe OA: large osteophytes, marked narrowing, dense sclerosis, and possible bony deformity — the pattern often described as 'bone on bone'.

One important caveat: KL grade measures structural changes, not how much pain someone is in. A person with KL 2 can be significantly disabled by symptoms, while someone with KL 3 may manage well day to day. The grade is not a verdict on how bad things feel.

What the grade does signal is the likely benefit ceiling for cartilage-targeting injectables. The more residual cartilage surface remains, the more an injection has to work with. That is why most injectable options are studied and used in grades 2–3, where some cartilage is still present.

X-ray grading has limits, too. It captures bone and joint-space width but misses soft-tissue detail. MRI adds information about cartilage thickness, meniscal integrity, and bone marrow changes — all of which can influence the choice of injection.

Which grades map to which injectable options

Each main injectable option has a preferred structural window — a grade range in which trial evidence is strongest and the likely benefit is highest.

Hyaluronic acid (viscosupplementation) has the broadest RCT base of the non-steroid options. A 2022 systematic review covering 38 trials and 5,025 patients found support for its use across KL grades 1–3. Grade 4 patients were notably absent from most of those trial populations, and excluding them appears to improve the measured effect size — a signal that residual cartilage surface matters for this class of injection. Whether HA offers meaningful benefit at grade 4 remains an open question without a solid evidence base.

Platelet-rich plasma (PRP) is similarly targeted at KL grades 2–3, where the structural environment is most receptive. Even within this preferred range, however, the evidence is mixed. The 2021 RESTORE randomised controlled trial (n=288, KL grades 2–3) found that three weekly PRP injections produced no significant improvement in overall pain scores or medial tibial cartilage volume compared with placebo at 12 months. Grade alone, therefore, does not predict a useful response.

Hydrogel injections (such as iPAAG/Arthrosamid) are positioned for KL 2–3 patients who have already tried and not responded adequately to prior treatments, but who still retain measurable joint space. Published data suggest effects that may persist for up to two years, though the evidence base is smaller than for HA.

Corticosteroids span the widest grade range — KL 1 through 4 — because they target inflammation rather than cartilage. At lower grades (KL 1–2), repeated injections carry a relative caution: some evidence links frequent steroid use to accelerated cartilage loss, so clinicians generally limit courses carefully. At KL 4, corticosteroids shift role entirely, serving as short-window pain relief rather than any form of structural support.

Bone-on-bone OA: what grade 4 changes about the conversation

Hearing 'bone on bone' can feel like a door closing. For patients at KL grade 4, however, the picture is more nuanced than that phrase implies — injections are not automatically ruled out, but the conversation with a specialist changes in character.

The fundamental shift is one of intent. At grades 2–3, injectable options can be used with a plausible disease-modifying or symptom-modifying rationale — there is cartilage surface to lubricate, reduce inflammation within, or potentially support. At grade 4, that rationale no longer holds cleanly. Hyaluronic acid depends on residual cartilage to function as a lubricant; with little or none remaining, its therapeutic basis is substantially weakened. PRP and other regenerative approaches show limited-to-no structural benefit in end-stage disease — no injection at this grade reverses or rebuilds cartilage loss.

What injections can offer at KL 4 is palliative pain relief — a meaningful bridge for patients who are waiting for joint replacement, who are medically unfit for surgery, or who prefer to defer the decision. Corticosteroids in particular retain a role here, providing short-window symptom control during acute flares. The goal, clearly framed, is quality of life management rather than structural support.

Patient-level factors also weigh more heavily at advanced grades. Some studies suggest that higher BMI and greater metabolic burden are associated with less favourable injection outcomes — considerations worth raising openly during assessment rather than treating as incidental.

Joint replacement remains the primary structural option for KL 4 disease. Whether and when to pursue it — and what role bridging injections play in the interim — is a decision that warrants specialist review rather than a general rule.

Clinical assessment beyond the X-ray

The X-ray gives a structural picture; the appointment fills in what the image cannot show. Before recommending a specific injectable — or any injection at all — a clinician builds a fuller picture through history, examination, and outcome scoring, because the same KL grade can present inside very different clinical contexts.

Physical examination adds dimensions that radiography cannot capture. Joint effusion (detected by a bulge test or patellar tap), joint-line tenderness, crepitus, and range of motion all speak to the inflammatory state of the joint at that moment. Varus or valgus alignment — the degree to which the knee angles inward or outward under load — indicates where stress is concentrated and whether alignment itself needs addressing before any injection will hold meaningful benefit.

Patient-reported scores and symptom pattern help quantify what examination alone cannot. WOMAC and KOOS questionnaires translate pain and functional limitation into tracked scores, establishing a baseline against which any treatment's effect can later be measured. Symptom character matters just as much: morning stiffness lasting under 30 minutes is characteristic of OA, whereas prolonged stiffness suggests an inflammatory arthritis — a distinction that can shift the recommendation from viscosupplementation toward a corticosteroid, or prompt specialist referral before any injection is given at all.

Treatment history completes the picture. A clinician will ask which injections were tried previously, at what frequency, and how the patient responded — because someone who received three hyaluronic acid courses with diminishing returns is in a materially different position from one who has never had an intra-articular injection. Whether conservative measures were genuinely sustained is assessed here rather than assumed. Metabolic health and body composition enter the same conversation: both are recognised response modifiers across the grade range, and may be discussed as part of preparing the conditions — physiological as well as structural — for the best chance of benefit.

Contraindications that override grade

Structural grade can point toward the right injectable — but certain medical factors override that guidance entirely, regardless of where someone sits on the KL scale.

The absolute bar is active infection. A confirmed joint infection, systemic bacteremia, or sepsis rules out any intra-articular injection at any grade — introducing a needle into an infected joint risks spreading and worsening the infection.

Several relative contraindications apply more selectively:

• Blood-thinning medication (anticoagulants) raises the risk of bleeding at the injection site and requires clinical review before any procedure.
• Uncontrolled diabetes is a particular concern with corticosteroids, which can cause significant blood-sugar spikes; a patient at KL 2 with unstable glucose control may be a less suitable corticosteroid candidate than a KL 3 patient whose diabetes is well managed.
• Active or recent malignancy is a relative contraindication for PRP and cellular therapies, given the growth-factor content of those products.
• Allergy to product components — including avian-derived hyaluronic acid preparations — is product-specific and unrelated to grade.
• Skin or soft-tissue infection overlying the planned injection site is a contraindication even when the joint itself is unaffected.

These factors do not appear on a radiology report, which is why the full picture — grade, clinical modifiers, current medications, and medical history together — needs to be reviewed by a specialist before any injection is confirmed. If you are at the stage of weighing options, an MSK assessment on amsk.co.uk can work through that full picture with you.

1. [1] Patient Age Is Not a Determinant of 12-Month Pain Response After SVF Injection for Knee OA (2026). (2026). https://doi.org/10.3390/medicina62061044 https://doi.org/10.3390/medicina62061044
2. [2] PRP vs Long-Acting Corticosteroid in KL Stage II-III Knee OA: WOMAC Outcomes (2024). (2024). https://doi.org/10.52783/jchr.v14.i6.6870 https://doi.org/10.52783/jchr.v14.i6.6870
3. [3] Role and Effectiveness of Intra-articular HA in Knee OA: Systematic Review (Cureus 2022). (2022). https://doi.org/10.7759/cureus.24503 https://doi.org/10.7759/cureus.24503
4. [4] RESTORE RCT: Intra-articular PRP vs Placebo in KL Grade 2-3 Knee OA (JAMA 2021). (2021). https://doi.org/10.1001/jama.2021.19415 https://doi.org/10.1001/jama.2021.19415
5. [5] Bone Marrow Stimulation + HA/Triamcinolone Combination in Moderate KOA (2025). (2025). https://doi.org/10.64252/64nw5642 https://doi.org/10.64252/64nw5642