The knee is where this treatment has the most to stand on.

PRP FOR THE KNEE

PRP for knee osteoarthritis targets pain and function, not the X-ray. The evidence is broadly favorable at six to twelve months. It will not regrow cartilage. The honest answer depends most on stage. A knee with joint space left and a knee that is bone-on-bone are two different problems wearing the same diagnosis.

What is actually happening in an arthritic knee

The familiar story is that cartilage wears out. That is not wrong, but it is the least interesting part. Joint cartilage has no blood supply and almost no ability to repair itself. So once it thins, it does not thicken again. What changes is everything around it. The synovium (the joint lining) becomes inflamed. It starts making catabolic (tissue-breaking) cytokines, mainly IL-1β and TNF-α. Those drive matrix metalloproteinases, enzymes that break down what cartilage is left. The subchondral bone (just under the cartilage) remodels and becomes a pain source on its own. The joint capsule stiffens.

This is why osteoarthritis stopped being called a wear-and-tear disease. Now it is called an inflammatory one. It is an active process in living tissue, not erosion.

Why some knees degrade faster than others, with the numbers

Mechanical load explains part of the variation, but nothing like all of it. The rest is body-wide. And it has been measured at a scale that is hard to argue with.

A prospective cohort followed 256,364 UK Biobank participants who had no osteoarthritis at baseline for a median of eleven years. It staged each person on cardiovascular-kidney-metabolic health from 0 to 4. Compared with stage 0, people at stage 4 developed knee osteoarthritis at a hazard ratio of 2.67 (95% CI 2.44 to 2.93). Two details in that paper matter more than the headline. First, the association was stronger in people under 60. Second, some people carried both a high genetic risk score and stage 4 metabolic disease. In them the hazard ratio reached 4.00, with a positive additive interaction between the two.

You did not pick your genes. You have some say over the metabolic stage. And the two multiply instead of simply adding. That is the argument for treating the terrain in a forty-five-year-old knee, not a seventy-five-year-old one.

The mechanism is not mysterious. Fat tissue is endocrine tissue, meaning it releases hormones. In a person carrying visceral fat (fat packed around the organs), it sends leptin, resistin and IL-6 into the bloodstream. Those reach the synovium no matter what the knee is doing. That is why osteoarthritis shows up in joints that bear no weight in metabolically unwell people. A purely mechanical model cannot explain that. Alongside it, insulin resistance impairs chondrocyte (cartilage cell) function. Meanwhile advanced glycation end-products stiffen the collagen network. That makes cartilage more brittle under the same load.

The blood-pressure finding that is not about weight

The obvious pushback is that metabolic illness travels with body weight. And body weight loads the knee. So the evidence that counts is the evidence that holds up after weight is adjusted for.

A meta-analysis of 26 observational studies covering 97,960 people found hypertension (high blood pressure) associated with knee osteoarthritis. The odds ratio was 1.62 (95% CI 1.32 to 1.98). It rose to 1.89 for radiographic knee osteoarthritis (arthritis seen on X-ray). Some studies adjusted for body mass index. In just those, the link with radiographic disease held at 1.42 (1.13 to 1.78). It was especially strong in women (2.27).

And here is the control that makes the point. The same analysis found no significant association at the hand (1.19, 0.92 to 1.53). The hand carries no body weight at all. If this were simply load, the hand would not be the joint that comes out clean. The authors read it as pointing toward a blood-vessel contribution to cartilage failure. That is the same story of blood flow to the bone under the cartilage that runs through everything else on this site.

What is actually modifiable, and by how much

A link is only useful if something moves. A network meta-analysis pooled 13 randomized trials in knee osteoarthritis, with 2,800 people. It compared seven weight-loss plans. Every one beat control on weight change. On pain, only diet plus exercise separated from control. And meta-regression set the threshold plainly. Meaningful pain relief was expected once a patient reached at least a 7% reduction in body weight.

Two honest notes on that. The approach with the largest weight loss was psychological support with diet and exercise, at 11.2 kg. It was not statistically superior to control for pain. So this is not a simple more-is-better relationship. And 7% is a target, not a promise. What it does is turn a vague instruction into a number you can work toward. That is a different conversation from being told to lose weight.

Then there is the driver that shows up on no panel: whether you can afford to take load off the joint. Take a knee that stands on concrete for nine hours. It has a different outlook than the same knee in someone who can sit down. No injection changes that math. It is worth saying plainly, because patients are routinely blamed for a path their working life decided. What we measure, and what we do with it, is on the metabolic health page.

Does PRP regrow cartilage in the knee?

PRP will not regrow cartilage. Anyone who tells you otherwise is selling something. What it appears to do in a knee is shift the environment inside the joint. It damps down the IL-1β-driven catabolic signaling. It influences how synovial cells behave. And it improves the thickness and slipperiness of the joint fluid. The effect people report is less pain and better function, sometimes for many months. That happens in a joint whose X-ray has not changed at all.

That difference matters when you decide what success would mean. If your goal is a better X-ray, this is the wrong treatment. If it is climbing stairs without planning the route first, it may not be.

How effective is PRP for knee osteoarthritis?

Knee osteoarthritis has the largest PRP research base of any condition. It is broadly favorable for pain and function at six to twelve months. Several comparisons suggest it lasts beyond what a corticosteroid injection offers. The trials also vary widely. Preparation protocols differ between trials so much that they are not really studying the same treatment. Platelet concentrations vary several-fold. And leukocyte (white blood cell) content, which plausibly matters a great deal in a joint, is often not reported at all.

The steady signal is that earlier-stage disease responds better. The steady gap is that trial patients are healthier, metabolically, than the people who actually walk in.

That gap is the subject of why your insurer will not pay for this. It is the single most useful thing to understand before deciding.

Which stages of knee arthritis respond best to PRP?

Knee arthritis on X-ray is graded Kellgren–Lawrence 1 through 4. That number predicts how a joint responds to an injection inside the joint better than pain score, age or how long you have had symptoms. It is the first thing worth knowing about your own knee. Most people have never been told it.

• Grade 1–2 — joint space mostly intact, bone spurs (osteophytes) minor. This is where the trial evidence is strongest. It is also where a good response most often lasts a year or longer.
• Grade 3 — space narrowed but present, cartilage thinned but not gone. Worth treating, with a smaller expected effect and a plan to repeat it.
• Grade 4 — bone touching bone, usually with deformity. Structure will not be restored here, and nobody should tell you otherwise. What treatment does at this stage is buy time and cut how much medication it takes to get through a day. A blinded placebo-controlled trial was done just in grade IV knees. The people in it were unwilling to have a replacement or had a reason to put one off. It found serial PRP reached a clinically meaningful improvement in pain and function at three and six months. Saline did not.

That last point is worth separating from the usual advice, because the usual advice is wrong about it. Grade 4 is not an automatic exit from this clinic. Most people at that stage are not choosing between an injection and a replacement. They are choosing between an injection and another year of rising pain medication while they wait, put it off, or are told they are too young. Buying that year back, at a lower medication burden, is a real clinical goal. It is not a consolation prize. Where a replacement is wanted and appropriate, it remains the definitive operation for the structure. We will say so plainly.

Three things then move the decision either way. A corticosteroid injection that worked and then stopped working is a favorable sign, not a failure. It shows the joint is the pain source, which is more than most people arrive knowing. Mechanical locking or a large unstable meniscal tear is not an arthritis problem. It needs a different conversation. And an uncontrolled blood sugar gets addressed before the knee does. That is a sequence, not a refusal, for the reasons set out on metabolic health and tissue repair.

The one person this does not suit at any grade is the person who expects the injection to be the whole treatment. The quadriceps (thigh muscle) work is not optional, and it is not a formality.

What a knee replacement actually costs

People are usually offered a choice between an injection that might not work and an operation that will. That is not the real comparison, and the figures below are the reason. They are not arguments against a knee replacement. They are the math that should be in the room when you decide when.

What is counted	Reported figure
Moderate to severe pain at 1 year	25%
Moderate to severe pain at ~5 years	28%
Dissatisfied with the result at 5 years	12.7%
Still taking opioids at 12 months	21%
Stiffness needing manipulation under anesthesia	1.3–5.8%
Deep periprosthetic infection	1.08%
Venous thromboembolism at 30 days	1.4%
Returned to sport at 3 months / 6 months	18.7% / 70%
Lifetime revision risk if replaced at age 46–50	22.4%
Implant surviving at 10 years	~96%

Two of those rows do most of the work. Pain does not reliably end. A quarter of replaced knees still hurt at a year. At roughly five years the figure is 28%. So it does not fade with time the way people are led to expect. None of it counts as a surgical complication. No infection, no loosening, nothing a registry records. It is simply what a meaningful minority of patients live with afterward. To be fair, the traffic is not all one way. One mixed-methods study found many people with lasting pain at a year did report improvement by five to seven years.

And the implant is most finite in the people who are told to wait. A knee replaced between 46 and 50 carries a 22.4% lifetime risk of revision, about one in four. Revision is a larger operation with worse results than the original. That is the actual content of “you are too young.” It is a real problem. It is a reason to control the timing, not a reason to accept whatever timing you are handed. How the two options compare.

How many PRP injections does a knee need?

Most knee protocols are one to three injections spaced three to four weeks apart, placed under ultrasound guidance. Blind knee injections miss the joint more often than clinicians like to admit. That is especially true in a knee with no effusion (no extra fluid). A preparation placed in the fat pad instead of the joint is a wasted draw.

Alongside it we look at your metabolic panel. That is because the cartilage you are asking to behave better is fed by the same circulation as everything else. We also look at your loading pattern, because quadriceps strength is one of the few things that reliably changes the path of an arthritic knee. Neither of those is a consolation prize handed over at discharge. They are the part that decides whether the window the injection opens stays open.

How soon can I walk after a PRP injection in my knee?

Expect a flare for two to four days. A knee joint is a closed space, and the inflammatory response has nowhere to go. So it is often more noticeable here than in a tendon. Walking is fine and encouraged. Impact and deep loaded bending wait. Most people notice change between weeks four and eight. The fuller answer arrives at three to six months.

The recovery timeline in detail →

Where this sits against pain medication

A knee that hurts constantly is a common reason people end up on a daily opioid. It is one of the cases where the drug is doing real work and also steadily losing ground. The aim here is not to take anything away from you. It is to widen the window in which you can move, sleep and load the leg. That is because quadriceps strength and sleep quality both feed back into how much pain the joint makes. Neither improves while you are immobile. Where that works, dose questions usually answer themselves. Where it does not, we say so.

Around a knee injection, our physiotherapy program for PRP and joint care rebuilds the thigh and hip muscles that take load off the cartilage.

Frequently asked questions about PRP for the knee

Is PRP better than a cortisone shot for knee arthritis?

For short-term relief, cortisone often works faster. The trade is durability and tissue effect. Repeated corticosteroid exposure is associated with cartilage loss over time. That is a poor bargain in a joint you plan to keep. The full comparison is here.

How long does it last?

Reported benefit commonly runs six to twelve months, sometimes longer. Repeat treatment is normal, not a sign of failure. What to expect from a course of treatment. The platelets are gone in days; what holds is what they signaled.

Will it let me avoid a knee replacement?

Sometimes it delays one. It does not reverse structural disease. A knee that needs replacing will still need replacing. How PRP and surgery actually compare.

Does my weight matter?

It matters through two separate routes, and the second one surprises people. Load is the obvious one. The other is that fat tissue acts as an endocrine (hormone) organ. It sends inflammatory signals to the joint whatever the knee is doing. The useful number is this. In the pooled randomized data, meaningful pain relief was expected at a 7% reduction in body weight. Diet plus exercise was the combination that separated from control. That is a target, not a lecture. What we measure. Metabolic terrain is the ceiling on the result, and it is measurable.

What are the downsides of PRP injections in the knee?

PRP will not regrow cartilage or change your X-ray. So a knee that needs replacing will still need it. Expect a flare for two to four days, because the knee is a closed space. The effect is smaller in a grade 3 knee. Repeat treatment is normal, and your insurer will not pay for it. It also asks something of you: the quadriceps work is not optional.

What is the best injection for knee osteoarthritis?

It depends on what you need the shot to do. Cortisone often works faster for short-term relief. But repeated steroid exposure is associated with cartilage loss over time. PRP has the largest evidence base in the knee. It is broadly favorable for pain and function at six to twelve months. Several comparisons suggest it lasts longer than a steroid. Your arthritis grade predicts the response better than your pain score, your age or how long the knee has hurt.

Does PRP work for bone-on-bone knee arthritis?

It will not restore the joint, and nobody should tell you it will. At grade 4, where bone touches bone, treatment buys time. It lowers how much medication it takes to get through a day. A blinded placebo-controlled trial in grade IV knees found serial PRP gave a meaningful improvement in pain and function at three and six months. Saline did not.

What should I avoid after a PRP injection in my knee?

Walking is fine and encouraged. Impact and deep loaded bending wait while the two-to-four-day flare settles. Most people notice change between weeks four and eight. The fuller answer arrives at three to six months. Do not skip the quadriceps work. It is the part that decides whether the window the injection opens stays open.

How painful is a PRP injection in the knee?

A knee is one of the easier places to have PRP. The skin is numbed, the joint capsule has room for the volume, and the injection itself is brief. What follows is an ache of fullness rather than sharpness, and acetaminophen is the right tool for it, because an anti-inflammatory blunts the signal the injection was given to start. Tendons around the knee are a different experience: the patellar tendon is among the injections people rate as most uncomfortable. How every injection site compares.

Related reading

• Why a pain clinic asks to see your bloodwork
• How platelet-rich plasma works
• PRP compared with cortisone
• PRP compared with hyaluronic acid injections
• Bone marrow aspirate concentrate
• Am I a candidate?
• When the knee was injured rather than worn out

There is no magic in a series of three — the number follows the tissue and the first response.