It’s one of the most common questions I hear from patients considering PRP therapy: how many injections will I need? Three is the number you’ll see cited most often in online resources and even in some research protocols. But I want to give you a more honest and nuanced answer — one that I think reframes the question in a way that’s actually more useful.
The number of injections isn’t really the right variable to focus on. The more important question is: how much total platelet dose is being delivered to the joint?
If you understand that distinction, it changes how you think about choosing a PRP provider — and it explains why what we do at this practice is clinically different from a standard protocol.
If you’re new to PRP, start with my overview of PRP therapy for knee pain before reading further.
Why Total Platelet Dose Matters More Than Injection Number
The research literature on PRP for knee osteoarthritis has long struggled with inconsistent results — some studies show strong benefit, others show modest or no benefit. For years, the leading explanation was injection number or PRP “type.” But a growing body of evidence points to a different culprit: total platelet dose.
A 2021 study by Bansal et al. published in Scientific Reports addressed this directly. The investigators found that 10 billion platelets was the critical threshold for achieving long-term chondroprotective benefit — with meaningful improvements in pain, function, and WOMAC scores sustained out to one year. Studies delivering fewer platelets than this threshold showed substantially weaker and less durable results. (PMID: 33597586)
A 2025 systematic review by Berrigan et al. in Arthroscopy reinforced this conclusion. Examining 29 studies, they found that studies showing positive outcomes at 6 and 12 months used on average more than twice the platelet dose of studies that showed no benefit — approximately 5,500 × 10⁶ platelets in successful studies versus 2,300 × 10⁶ in unsuccessful ones. The authors concluded that better clinical outcomes from PRP for knee OA may be directly related to a greater platelet dose.
> “Studies showing positive PRP outcomes at 6 and 12 months used more than twice the platelet dose of studies that showed no benefit.” — Berrigan et al., Arthroscopy, 2025
This matters enormously when interpreting PRP research — and when choosing a provider. A study showing that “three injections didn’t work” may be a study that delivered an insufficient total dose across all three injections. Three low-concentration injections do not equal one high-concentration injection.
What This Means in Our Practice
As a Regenexx network provider, I use preparation protocols that target 14 to 20x platelet concentration above your baseline blood platelet count. Standard bedside centrifuge systems used at many physician offices produce 3 to 5x concentration.
The practical implication: a single injection prepared to 14–20x concentration may deliver more total platelets to the joint than two or three injections prepared at 3–5x concentration. That’s not a marketing claim — it follows directly from the math and from the dose-response research above.
For most of my patients, I do not start by planning a fixed series of injections. I start by ensuring each injection delivers an adequate platelet dose, assess the patient’s response, and repeat as needed based on how they’re doing. The goal is to deliver sufficient biological stimulus in as few treatment sessions as possible — not to hit an arbitrary injection count.
How Platelet Concentration Compares: A Practical Guide
| Preparation System | Typical Concentration | Typical Platelet Dose (per injection) | Series Needed to Reach 10B threshold? |
|---|---|---|---|
| Standard bedside kit | 3–5x | ~1.5–2.5 billion | Likely requires 4–6 injections |
| Mid-tier system | 6–9x | ~3–5 billion | Requires 2–3 injections |
| Regenexx protocol | 14–20x | ~8–12+ billion | Often achieved in 1–2 injections |
Estimates based on average baseline platelet count of ~200,000/µL in a 55–70 kg adult.
When I Do Recommend Repeat Injections
Repeat treatment isn’t off the table — it’s just driven by clinical response rather than a pre-set protocol.
After a patient completes their initial injection and has had 6 to 8 weeks for the full response to develop, I reassess. The questions I’m working through:
- Has there been meaningful improvement in pain and function?
- Is the response holding, or beginning to wane?
- Does the MRI show bone marrow lesions or subchondral pathology that intraarticular PRP alone may not fully address?
- Would BMAC stem cell therapy be a more appropriate next step for this patient’s anatomy and disease severity?
For patients who respond well, many do fine with a single high-dose injection and return only when symptoms begin to return — often 12 to 18 months later. For patients with partial response or more complex pathology, a second injection or transition to BMAC may be the right move. You can read more about that decision in my post on avoiding knee replacement with BMAC therapy.
Bone Marrow-Derived PRP: A Clinically Distinct Option
One option worth knowing about — and one that most patients never hear discussed — is bone marrow-derived PRP. Rather than drawing PRP from peripheral blood, this preparation is drawn from the same bone marrow aspirate used for BMAC. It carries a distinct cytokine and growth factor profile, including higher concentrations of certain anti-inflammatory mediators and mesenchymal stem cell-derived factors not present in blood-derived PRP.
For patients already undergoing a BMAC procedure — for bone marrow lesions, for example — this approach allows us to treat both the subchondral bone with BMAC delivered intraosseously and the intraarticular joint environment with bone marrow-derived PRP, from a single aspirate, in one procedure.
This is part of what makes the Regenexx approach different in practice. Learn more about BMAC therapy and how it’s used at our Miami practice.
When Will I Feel Results?
PRP is not a quick fix. Most patients begin to notice meaningful improvement 4 to 8 weeks after injection, with continued gains over 3 to 6 months as tissue remodeling progresses. The increased soreness that occurs in the 2 to 5 days following injection is normal — it reflects the inflammatory healing response being activated, not a problem with the treatment.
I ask patients not to draw final conclusions about efficacy until at least 6 to 8 weeks after their injection. Evaluating PRP at one or two weeks is like leaving a loaf of bread in the oven for five minutes and deciding it didn’t work.
Frequently Asked Questions
If your PRP is higher concentration, why do some studies recommend three injections?
Most studies recommending three injections were designed using lower-concentration preparation systems where three injections were needed to accumulate adequate total platelet dose. When a single injection already delivers 10 billion or more platelets, the rationale for automatically repeating three times largely disappears. The goal is dose — not a specific number of procedures.
Can I start with one injection and add more if needed?
Yes — and that’s essentially how I practice. Start with a well-prepared, high-dose injection, allow adequate time to assess the response, and repeat if and when the clinical picture calls for it. This is a more individualized approach than committing to a series before you’ve seen how you respond.
How does PRP injection feel?
Mild to moderate discomfort at injection. Most patients experience 2 to 5 days of increased soreness in the knee as the healing response activates. This is expected and manageable with rest, ice, and Tylenol. Most patients resume normal activity within a week.
Can PRP be combined with BMAC in the same session?
Yes — and this is often the optimal approach for patients with both intraarticular pathology and subchondral bone involvement. Intraarticular PRP addresses the joint space environment while BMAC delivered intraosseously under fluoroscopic guidance targets bone marrow lesions. Both can be done from a single bone marrow aspirate using the bone marrow-derived PRP approach described above.
How is Regenexx PRP different from standard PRP?
Concentration, preparation standardization, and delivery precision. Regenexx protocols target 14–20x platelet concentration versus 3–5x for standard systems. All injections are performed under imaging guidance — ultrasound or fluoroscopy — to confirm accurate placement. And the preparation is lab-based rather than bedside, with quality control that bedside kits cannot match.
When is PRP not the right choice?
PRP is most effective for mild to moderate knee OA with preserved joint space. End-stage arthritis with severe bone-on-bone changes is generally not an ideal PRP candidate — those patients may be better served by BMAC or, if regenerative options have been exhausted, a surgical consultation. A thorough review of your imaging is always the starting point.
If you’re dealing with knee pain and want to understand whether PRP is the right fit for your situation — and what a properly dosed, image-guided injection actually involves — I’d encourage you to schedule a consultation at our Miami practice. We’ll review your imaging, your history, and your goals and build a plan around your specific anatomy and disease stage.
Schedule online through ZocDoc or contact our office directly.
Learn more: PRP for knee pain | BMAC stem cell therapy | Bone marrow lesions | Regenexx provider Miami | About Dr. Bonner
References
- Bansal H, et al. Platelet-rich plasma (PRP) in osteoarthritis (OA) knee: Correct dose critical for long term clinical efficacy. Sci Rep. 2021. PMID: 33597586
- Berrigan WA, et al. A greater platelet dose may yield better clinical outcomes for platelet-rich plasma in the treatment of knee osteoarthritis: a systematic review. Arthroscopy. 2025. PMID: 38513880
- Bensa A, et al. Corticosteroid injections for knee osteoarthritis offer clinical benefits similar to hyaluronic acid and lower than platelet-rich plasma: a systematic review and meta-analysis. EFORT Open Rev. 2024. PMID: 39222336
This post is for educational purposes only and does not constitute medical advice. Results from regenerative treatments vary by individual, and not all patients are candidates. Please schedule a consultation to discuss your specific condition and treatment options.