Do PRP injections work for knee arthritis? Pain yes, cartilage no

PRP injections do reduce knee arthritis pain, and the relief outlasts the gel injection they are usually tested against. What they do not do is regrow cartilage, which is the thing most people are hoping for.

Platelet-rich plasma, or PRP, is a patient's own blood spun in a centrifuge to concentrate the platelets, then injected back into the joint. Pooled results from 18 randomised trials covering 1,608 adults, average age 58, found knee scores for pain, stiffness and everyday function improved by 44.7% after PRP against 12.6% after hyaluronic acid, the gel injection that has been the standard alternative for years [6]. A separate pooling of 20 randomised trials put PRP ahead of the gel on pain at six and twelve months, and on function from the first month onward [1].

The benefit is slow to arrive and then holds. A one-year trial randomly assigned 200 adults with mild to moderate knee arthritis, average age 57 and about seven in ten of them women, to one of four injections: PRP, a related plasma preparation, hyaluronic acid, or ozone [13]. Ozone worked fastest and faded first, beating the other three at two months, overtaken by all of them by six months. At twelve months only the two plasma treatments were still ahead of the gel and the ozone [13].

What PRP does not do is rebuild the joint. Researchers pooled every study that scanned knees before and after injection, 14 studies covering 1,099 people, and found no measurable gain in cartilage, the smooth tissue capping the bone ends that wears thin in arthritis [11]. Four of those studies, covering 187 people, showed no increase in cartilage thickness; three randomised trials, covering 112 people, showed no difference in overall cartilage against no injection at all [11]. The authors put it bluntly: PRP should not be given with the aim of growing cartilage back [11].

Side effects look unremarkable. Across those 20 randomised trials PRP caused no more adverse events than the gel injection [1], and a review of the randomised evidence described it as a safe treatment [2]. That has not been enough to move the guidelines. The American Academy of Orthopaedic Surgeons pulled together 54 studies, 36 rated high quality and 18 moderate, comparing PRP against dummy injections, paracetamol, anti-inflammatory painkillers, steroid injections, exercise and several other injections, and marked where the evidence ran out alongside the potential harms and costs [3]. As of 2023, the leading clinical practice guidelines still did not recommend PRP for knee arthritis [5].

The working theory is that PRP calms the joint rather than resurfaces it. Platelets are the blood cells that clump together at a wound, and concentrated into an injection they release growth factors that act on the inflammation and tissue breakdown going on inside an arthritic knee [4].

One trial measured that directly. In 150 adults randomly assigned to PRP or hyaluronic acid and followed for a year, two of the proteins that drive joint inflammation fell in the PRP group within a month and did not in the gel group [12]. The same trial found PRP patients could walk 120 metres further in six minutes without pain after a year, against 4 metres further for the gel group [12].

Whether PRP also pushes cartilage cells to multiply is contested, and results from lab dishes and animals have not carried over to people [4]. That is the most likely explanation for pain improving while the scans stay flat [4][11].

There is no standard recipe, which is why one clinic's PRP is not the same product as another's. Platelet concentration, whether the white blood cells are filtered out, and whether the platelets are activated first all vary between preparations, and that variation is a large part of why the trials are hard to compare [8]. Two findings point at what matters: the trial that held the platelet count at 10 billion per injection saw its benefit last a full year [12], and in the pooled randomised evidence the version with white blood cells filtered out scored better than the version that leaves them in [6].

The underlying trials are not strong. Reviewers describe them as low to moderate methodological quality using preparations that differ from study to study, which is why the pooled averages here carry less weight than the tidy numbers suggest [2][8]. "PRP" names a process, not a defined product [8].

The two largest poolings both compare PRP against hyaluronic acid rather than against a dummy injection [1][6]. That shows PRP beats that particular alternative. It does not separate how much of the relief comes from the platelets and how much from having a needle put into the knee.

Who this evidence does not cover:

• People with severe, bone-on-bone arthritis. The trials enrolled mild to moderate disease [13], and reviewers note that younger patients with less advanced wear may be the ones who respond [2].
• Anyone hoping for structural repair. The scanning evidence says no [11].
• Other joints and other uses. This is knee osteoarthritis. Hip evidence amounted to three randomised trials as of 2017 [2], and nothing here speaks to shoulders, tendons or scalps.
• Anyone asking about year two. Follow-up in most of these trials stops at twelve months [2][6].

No trial has shown that PRP slows the arthritis itself [5].

Longevity and regenerative clinics list PRP among their joint services, as do sports medicine and orthopaedic practices. The outline of a first visit is the same everywhere: blood is drawn from the arm, spun in a centrifuge to concentrate the platelets, and injected into the joint [4][12]. Trials rarely used a single shot, and the one-year four-arm trial gave two injections three weeks apart [13].

The question worth putting to a clinic is what is actually in their preparation: how concentrated the platelets are, and whether the white blood cells are filtered out. Those are the two variables separating the trials that reported year-long benefit from the ones that did not [6][8][12].