LDRT vs. Gel Injections (Hyaluronic Acid) for Arthritis

Gel injections are often the last stop before surgery. A doctor injects hyaluronic acid into the knee, the pain eases for a while, and when it wears off you talk about doing it again. Low-dose radiation therapy, or LDRT, is the option many patients hear about only after the injections stop helping. The two treatments have almost nothing in common apart from the problem they aim at.

What a Gel Injection Is

Hyaluronic acid is a substance your body makes on its own. In a healthy joint it sits in the cartilage and the joint fluid, where it works as a lubricant and a shock absorber. With age and osteoarthritis it thins out and breaks down. A gel injection, called viscosupplementation, puts a manufactured version of it back into the joint to restore some of that cushioning^[1].

These products are approved by the FDA as devices for pain from osteoarthritis of the knee, in patients who have not done well with simpler treatment. They are not FDA-approved for the hip, hand, or shoulder, and Medicare does not cover viscosupplementation for joints other than the knee^[2][3]. LDRT has no such single-joint limit. German guidance covers a range of joints, and US programs treat knees, hips, hands, shoulders, and feet^[4][5].

What a Course of Each Looks Like

A gel series is one to five injections into the knee, usually spaced a week apart, depending on the product. Synvisc and Euflexxa are three weekly injections. Hyalgan is five. Synvisc-One, Durolane, Gel-One, and Monovisc are a single injection^[2]. The injection is done in the office. The skin is cleaned and numbed, excess fluid may be drawn off first, and the gel goes directly into the joint space, sometimes with ultrasound or X-ray guidance to place the needle. You go home the same day, and the Arthritis Foundation advises avoiding jogging, heavy lifting, and prolonged standing for the first few days^[1].

An LDRT course is six short sessions of 0.5 Gy, a total of 3 Gy, given two or three times a week over two to three weeks^[4][5]. There is no needle into the joint, no incision, and no anesthesia. You lie on a treatment table while a machine delivers the dose from outside the body, and you go back to your normal day afterward. More visits than a single-shot gel product, fewer needles than any of them.

How Well Do Gel Injections Work?

A meta-analysis that mapped the effect over time, covering 54 trials and 7,545 participants, found the benefit over placebo appears by week four, peaks at week eight, and is still detectable but fading at week twenty-four^[6]. The Arthritis Foundation gives the patient version of the same timeline: improvement takes several weeks, relief lasts about six months, and a further course can be given six months later if the first one helped^[1].

The biggest review, published in the BMJ in 2022, pooled 169 trials with 21,163 participants. In the large placebo-controlled trials, the average pain advantage of gel over a placebo injection came to about 2 millimeters on a 100 millimeter pain scale, which the authors called clinically irrelevant. Serious adverse events were also somewhat more common in the gel groups, 3.7 percent against 2.5 percent, though the authors note those events were not all proven to be caused by the treatment^[7].

The American Academy of Orthopaedic Surgeons 2021 knee guideline says hyaluronic acid injections are not recommended for routine use in symptomatic knee osteoarthritis^[8]. The American College of Rheumatology conditionally recommends against them for the knee and strongly against them for the hip. Other bodies are softer: OARSI and the VA and Department of Defense guidelines conditionally allow them for the knee when pain persists despite other treatment^[9]. Some patients feel a gel series helped them, and the averages do not rule that out, but the average effect is small. The Arthritis Foundation also notes that people who are obese, over 65, or have severe arthritis are less likely to improve^[1].

And How Well Does LDRT Work?

LDRT has the opposite evidence problem: a large body of favorable observational data, and controlled trials that disagree with each other. In the newest and best of them, a randomized sham-controlled trial presented at ASTRO 2025, 70.3 percent of patients who got the standard 3 Gy dose were responders at four months, against 41.7 percent with sham treatment, and no treatment-related side effects were reported^[10]. Two older Dutch sham-controlled trials, in the knee and the hand, found no benefit at all, including at six and twelve months of follow-up^[11].

Where LDRT does look different from gel is duration. Gel relief is usually counted in months, about six of them^[1]. In the largest German knee cohort, 57 percent of the patients who responded to LDRT still had notable relief more than a year later^[12]. Gel works faster. A gel series peaks at about eight weeks^[6]. LDRT is usually judged at about three months, and our page on how long LDRT takes to work covers that timeline study by study.

Side by Side

• What it is: gel puts manufactured hyaluronic acid into the knee joint to supplement its lubricant^[1]. LDRT delivers a small radiation dose to the joint from outside the body to calm inflammation^[5].
• Which joints: gel is FDA-approved for the knee only, and Medicare covers no other joint^[2][3]. LDRT programs treat knees, hips, hands, shoulders, and other joints^[4].
• The visits: gel takes one to five weekly injections, depending on the product^[2]. LDRT takes six sessions over two to three weeks^[4].
• Needles: gel means a needle into the joint, every time. LDRT means no needle, no incision, and no anesthesia^[5].
• When it peaks: gel peaks at about eight weeks and fades by about six months^[6][1]. LDRT is judged at about three months, and responders in the German knee data often stayed better past a year^[12].
• Repeating it: a gel series can be repeated no sooner than six months, and Medicare requires proof the earlier series helped^[13]. A second LDRT series is considered at the three-month reassessment if the first response was incomplete^[5].
• Evidence verdict: gel has a large trial base that shows a small average effect, and a major guideline advises against routine use^[7][8]. LDRT has a smaller trial base that is split, one positive sham trial and two negative ones^[10][11].

Risks of Each

Gel injections are usually uneventful, but the joint has been entered with a needle, and the problems follow from that. Pain, swelling, stiffness, and a temporary arthritis flare around the injection are the common ones. Mayo Clinic lists joint pain, stiffness, and swelling among the more common effects, with bleeding and signs of infection among the less common ones^[14]. A stronger flare, sometimes called a pseudoseptic reaction, can bring severe pain and swelling in the days after a second or third injection and needs prompt medical care. The injections should not be given into a joint or skin area that is infected, and some products are made from rooster combs, which matters if you have an egg or poultry allergy^[1][2].

LDRT skips the needle risks because nothing enters the joint. In the ASTRO 2025 sham trial, no treatment-related side effects were reported at all^[10]. Its real risks are different in kind: a small theoretical cancer risk from any radiation dose, which our cancer risk page works through in numbers, and the chance that three months pass and it simply has not helped you.

Cost and Coverage

Both treatments can be covered, but the rules differ. Medicare covers gel injections for the knee only, and only when strict conditions are met: X-ray evidence of osteoarthritis, pain that interferes with function, at least three months of conservative treatment that did not work, and a cortisone injection that failed or cannot be used. A repeat series needs the same criteria again, proof the first series helped, and at least six months since it ended^[13]. Paying cash, the Arthritis Foundation puts out-of-pocket cost starting at about 300 dollars per injection^[1]. The Drugs.com cash price for one Synvisc syringe is about 417 dollars, or about 1,062 dollars for the three syringes in a standard course, and that is the drug alone, before office and injection fees^[15].

LDRT is billed as radiation oncology care, and Medicare and many commercial plans typically cover it, according to US program pages and clinician interviews. Coverage still varies by plan, and prior authorization is common, so the center should verify your benefits before the first session^[5]. No program publishes a single cash price for LDRT, so any exact dollar figure quoted without a benefits check is a guess. Our cost guide explains how to get your real number.

Which One Fits Which Patient

A gel series may still suit you if your problem joint is the knee, you want the treatment with the longest track record in US offices, and you accept a small average effect that lasts about six months^[6][1]. It is a weaker bet if your arthritis is severe, since severe disease is one of the groups less likely to improve^[1].

LDRT may be an option if gel helped for a few months and then stopped, if the joint is not a knee at all, if you want to avoid needles into the joint, or if you are trying to put off a replacement. Its evidence is promising, with dedicated guidance from radiation oncology societies, and with controlled trials that do not all agree^[10][11][4].

The two can be used one after the other. A gel series that bought you six decent months has not used anything up. It does not block LDRT later, and an LDRT course does not block surgery later either. What it should not do is repeat forever on habit, once the relief has clearly shrunk to a few weeks.

References:
1. Hyaluronic Acid Injections for Knee Pain. Arthritis Foundation (what hyaluronic acid does in the joint; procedure; relief about six months; out-of-pocket cost starting at about $300 per injection; obesity, age over 65, and severe arthritis less likely to improve). Arthritis Foundation
2. Sodium Hyaluronate policy, including FDA labeling table. UnitedHealthcare (device approval for knee osteoarthritis pain after failed conservative therapy; per-product schedules from one to five injections; avian-source products). UnitedHealthcare
3. Local Coverage Determination L39529. Centers for Medicare and Medicaid Services (viscosupplementation of joints other than the knee is not reasonable and necessary and is not covered). CMS
4. Ott OJ, et al. Guideline recommendations for the radiotherapy of painful degenerative skeletal disorders (DEGRO). Strahlentherapie und Onkologie. 2015;191:1-6 (single doses 0.5 to 1 Gy, total 3 to 6 Gy per series, across painful degenerative skeletal disorders). PDF
5. Low-Dose Radiation Therapy Making a Comeback for OA. Medscape, 2025 (six sessions of 0.5 Gy over two to three weeks; noninvasive; reevaluation at three months; Medicare coverage described by treating clinicians). Medscape
6. Bannuru RR, et al. Therapeutic trajectory following intra-articular hyaluronic acid injection in knee osteoarthritis: meta-analysis. Osteoarthritis and Cartilage. 2011;19(6):611-619 (54 trials, 7,545 participants; effect by week four, peak at week eight, residual at week twenty-four). PubMed
7. Pereira TV, et al. Viscosupplementation for knee osteoarthritis: systematic review and meta-analysis. BMJ. 2022;378:e069722 (169 trials, 21,163 participants; pain difference about 2 mm on a 100 mm scale in large trials; serious adverse events 3.7% vs 2.5%, not all proven caused by treatment). BMJ
8. Management of Osteoarthritis of the Knee (Non-Arthroplasty), Third Edition. American Academy of Orthopaedic Surgeons. 2021 (hyaluronic acid not recommended for routine use in symptomatic knee osteoarthritis, moderate strength). AAOS
9. Intra-articular hyaluronic acid (viscosupplementation) for osteoarthritis: is it effective? Australian Prescriber (guideline comparison: ACR conditional against for knee and strong against for hip; OARSI and VA/DoD conditional for knee). Australian Prescriber
10. Low-Dose Radiation Therapy Offers Substantial Relief to People With Painful Knee Osteoarthritis. ASTRO news release, 2025 (Kim BH, et al.; responders at four months 70.3% at 3 Gy vs 41.7% sham; no treatment-related toxicity). ASTRO
11. Low-Dose Radiation Therapy for Knee and Hand Osteoarthritis Questioned. RheumLive, reporting van den Ende et al., Lancet Rheumatology long-term follow-up of the Dutch sham trials (no difference at six and twelve months). RheumLive
12. Keller S, et al. Efficacy of low-dose radiotherapy in painful gonarthritis: experiences from German bicenter study. Radiation Oncology. 2013 (1,037 knee patients; 57% of responders with relief beyond one year). Springer
13. Local Coverage Determination L39260, hyaluronic acid injections for knee osteoarthritis. Centers for Medicare and Medicaid Services (coverage conditions, including three months of failed conservative therapy and failed or contraindicated steroid injection; repeat series only if the prior series helped, symptoms recurred, and at least six months elapsed). CMS
14. Hyaluronic Acid (Injection Route). Mayo Clinic (more common effects include joint pain, stiffness, and swelling; less common include bleeding and signs of infection). Mayo Clinic
15. Synvisc Prices, Coupons, Copay Cards and Patient Assistance. Drugs.com price guide (cash price about $417 for one 16 mg/2 mL syringe; about $1,062 for three syringes, drug cost only). Drugs.com