Platelet-rich plasma (PRP)

PRP concentrates platelets from your own blood and reinjects them into the joint, where degranulation releases growth factors — PDGF, TGF-β, VEGF, IGF-1 — that promote chondrocyte proliferation and calm synovial inflammation in the lab. Whether a transient growth-factor bolus changes cartilage biology in a loaded human knee is unproven: the one RCT that measured cartilage on MRI found no structural effect.

The trial that has to lead

RESTORE (JAMA, 2021) is the most rigorous test PRP has faced: 288 patients, three weekly leukocyte-poor injections versus saline, double-blind. At 12 months it was null on both primary endpoints — pain change −2.1 versus −1.8 (P=.17), medial tibial cartilage volume −1.4% versus −1.2% (P=.81) — and 29 of 31 secondary outcomes showed nothing either.

The wider placebo-controlled literature is kinder but not by much. A meta-analysis of 11 saline-controlled RCTs (1,616 patients) found a small pain benefit at 3 and 6 months that disappears by 12; the authors call its clinical relevance debatable. The standout positive trial is Patel 2013, where a roughly 10× concentration, WBC-filtered preparation beat saline on every WOMAC parameter — with one injection as good as two and benefit fading after 6 months.

Why it still rates promising

Against hyaluronic acid, PRP wins consistently: across 18 level-1 RCTs, PRP patients improved 44.7% on WOMAC versus 12.6% with HA, and a 5-year double-blind follow-up found fewer reinterventions with PRP at 24 months. The catch: HA itself performs near placebo (see the hyaluronic acid entry), and intra-articular saline outperforms oral placebo in network meta-analysis — beating HA is a low bar.

Preparation seems to matter. In network meta-analysis, leukocyte-poor PRP improved WOMAC versus both HA and placebo while leukocyte-rich PRP separated from neither — though this rests entirely on indirect comparison, and RESTORE used leukocyte-poor PRP and was still null.

The high-dose question, honestly

The site's map labels this "high-dose PRP" because dose is where the live question sits. What counts is absolute platelet count per course, not the concentration multiple. A review of 29 RCTs found positive trials averaged ~5.5 billion platelets versus ~2.3 billion in null trials; a dose meta-analysis puts the favorable threshold near 10 billion, clearer for function than pain. Dose advocates note RESTORE's commercial kit delivered roughly 2× concentration and call its platelet dose subtherapeutic.

Here is the honest status of that argument: the dose-response evidence is ecological — comparisons across trials, not within them. The only trial designed around an absolute dose (Bansal 2021, single 10-billion-platelet injection, benefit sustained at 12 months while the comparator faded) used HA as the comparator, not saline, and carries a published correction. A three-injection protocol beat single PRP in early OA, which fits cumulative dose but is confounded with injection frequency. Until a randomized dose-ranging trial against saline exists, "high dose fixes the null trials" is a hypothesis, not a finding.

Practical notes

Trials used 1–3 injections, usually weekly; benefit, where present, peaks at 3–6 months and wanes by 12. If following the dose hypothesis, the literature's target is ≥5–10 billion platelets per course. Costs are out-of-pocket in most systems, and AAOS gives PRP only a limited recommendation.

Safety

Autologous blood, so the main events are transient post-injection pain and swelling — more frequent with leukocyte-rich preparations (adverse-reaction odds ratio 5.63 versus HA, mostly self-limited). No serious safety signal in RCTs. Avoid NSAIDs around the injection window given theoretical platelet inhibition, and use caution with active infection, platelet disorders, or anticoagulation.