Four years on, thirty-five of thirty-seven knees. None replaced.

BMAC — BONE MARROW ASPIRATE CONCENTRATE

BMAC, bone marrow aspirate concentrate, is marrow drawn from the iliac crest (the top of the hip bone) under local anesthetic. It is concentrated and injected in one visit. It brings growth factors. It also brings a cell population and an anti-inflammatory protein profile. In one series of advanced grade III and IV knees, thirty-five of thirty-seven were still better four years later. None had been replaced.

Four years after a single course, thirty-five of thirty-seven bone-on-bone knees were still better. Not one had gone to a replacement. That is the reason this procedure is worth the harvest.

What is a BMAC injection?

BMAC is bone marrow aspirate concentrate. A BMAC injection and a BMAC procedure are the same thing seen from two ends. The harvest and the delivery happen in one appointment. You will also see bone marrow concentrate, marrow aspirate concentrate, or just a marrow harvest. All of them mean what is set out below.

The abbreviation is not unique to medicine. That is why a search for it turns up community media stations, a university access center and several sports conferences. If you came looking for one of those, this is not it.

Do BMAC injections work?

Most orthobiologic evidence comes from early disease, in people who were going to do fairly well anyway. The most striking BMAC series did the opposite. It treated Kellgren–Lawrence grade III and IV knees. Those are advanced joints with real structural damage. It followed them for four years.

Thirty-five of thirty-seven knees improved on both function and pain scores. The gains began in the first year and were still climbing at year two. Not one of those patients went on to a prosthesis (an artificial joint) during the follow-up period. These are people routinely told the only option left is replacement. For them, that is a considerable finding.

A separate two-year prospective series in grade II–III knees found the same direction of travel. It tracked cartilage imaging along with the symptom scores. It did not rely on how people felt alone.

How does BMAC compare with PRP?

The comparison that matters to anyone choosing is BMAC against PRP. It has been tested head to head. A single-center trial followed 175 patients across grades II to IV for twelve months. It compared marrow concentrate, platelet-rich plasma and hyaluronic acid. BMAC produced the most substantial improvement in quality of life. The biggest gains were in physical health and mobility, and they held up consistently. PRP beat hyaluronic acid on some measures. BMAC beat both.

That fits inside a wider picture that is now well established. A Level I systematic review and meta-analysis pooled twenty-seven randomized trials. It covered over a thousand patients. It found PRP and BMAC both outperform hyaluronic acid. A network meta-analysis went further. It found PRP, BMAC and hyaluronic acid all outperform corticosteroid injection at six months and beyond. That matters here. Many people arrive after three steroid shots and are told there is nothing left.

The fair summary: PRP is an excellent treatment. It remains the sensible starting point for many joints. Where more is needed, marrow is the step with the evidence behind it. That comparison is worked through in detail on PRP compared with BMAC.

What is actually in the aspirate

Marrow is drawn from the iliac crest under local anesthetic. Then it is spun to concentrate it. What comes back is not one cell type but a mix. It holds mesenchymal stromal cells and their progenitors (early-stage cells). It holds platelets and their growth factors. It also holds signaling proteins, including interleukin-1 receptor antagonist. That protein works against one of the main inflammatory drivers in an arthritic joint.

That last part is one reason marrow acts differently from plasma. PRP delivers a burst of growth factors. Marrow delivers that plus a cell population and an anti-inflammatory protein profile. That is the case, based on how it works, for why it holds up better in joints that are further along.

Two indications where marrow is the specific answer

Osteonecrosis of the femoral head. This is not an arthritis problem. It is bone in the hip ball dying from lost blood supply. It moves on to collapse. Core decompression augmented with marrow concentrate has been studied as a way to save the joint. Studies report progression-free survival and conversion-to-replacement rates. Those rates support doing it early, before the head collapses. There is no plasma version of this. If osteonecrosis is on your imaging, marrow is the conversation.

Rotator cuff repair. When a cuff is being repaired in surgery, a systematic review of marrow augmentation reports better healing and function and lower retear rates. That is a question for the surgeon doing the repair. It is not a replacement for them. But it is a real, separate use. It is set out with the rest of the shoulder picture on PRP for rotator cuff tendinopathy.

The risk conversation, corrected

Marrow harvesting is more than a blood draw. This page will not pretend otherwise. But the common belief that it is meaningfully riskier than other injections does not survive the data. A systematic review and meta-analysis pooled six randomized trials with 860 patients. It found the complication rate for BMAC statistically indistinguishable from comparator injections. The rates were roughly 42% against 41%, a difference of no significance. Knee effusion (fluid in the knee) was the most common event. The number needed to harm was 152.

What stays true is worth saying exactly. An infection seeded at or near bone is a more serious problem than a soft-tissue infection. It takes longer to treat and has worse consequences. The frequency is not higher; the stakes if it happens are. That is why sterile technique and the antibiotic decision are handled differently for a marrow harvest. It is a reason for care, not avoidance. Everything on the general risks page applies as written.

How painful is a BMAC procedure?

The iliac crest is numbed and marrow is drawn through a needle. You will feel pressure. You will feel a brief, deep pulling as the marrow is drawn. That part is unpleasant, and it is short. Afterward, expect the harvest site to ache for several days, sometimes more than the treated joint does.

Processing happens in the room at the same visit. Nothing is shipped and nothing is stored. The injection into the target joint is placed under ultrasound or fluoroscopic guidance, like everything here. The recovery timeline applies, with a second sore site added.

Why cell yield varies so much, and what that means for you

This is the honest limit, and as with fat, it points somewhere useful. The cell content of marrow varies hugely between people. Age lowers it. So does smoking. So, importantly, does insulin resistance and chronic metabolic inflammation. It degrades the marrow niche the same way it degrades every other repair process the body is asked to run.

Two people can have the same procedure done the same way and get very different biology. A clinic that ignores that and blames the treatment has mixed up the product with the terrain that makes it. Reading the metabolic picture before deciding is not an upsell here. It is what gives the harvest a fair chance of being worth the discomfort you paid for.

Who is a good candidate for BMAC?

• Advanced degeneration: grade III and IV knees. That is where the four-year data was earned, and where plasma has less to work with.
• A joint that responded to PRP but not enough. The response was real and partial, but short of what was needed.
• Osteonecrosis of the femoral head. Here marrow is the specific answer, not a general one.
• Subchondral bone involvement. Here, marrow lesions (damage in the bone under the cartilage) are driving the pain, not the joint surface.
• Anyone putting off a replacement who wants the years in between to be worth something. See PRP compared with surgery.

It does not suit someone who expects it to replace an arthroplasty (joint replacement). It does not suit anyone unwilling to have a second spot made sore. Decide both of those before the appointment, not during it.

What people ask before a marrow harvest

Is BMAC better than PRP?

On the head-to-head comparison, yes for the harder joints. A 175-patient trial across grades II to IV found BMAC produced the greatest quality-of-life gains. It came out ahead of both PRP and hyaluronic acid. For an earlier-stage joint, PRP is often the sensible starting point, because it does a good deal with far less to endure.

Does the harvest hurt?

There is pressure and a brief, deep pulling during the draw. It is truly unpleasant and truly short. The ache afterward lasts a few days. It is often worse than the joint that was treated.

Is it riskier than a normal injection?

Not by frequency. Pooled randomized data puts the complication rate at about 42% against 41% for comparator injections. There was no significant difference. Effusion was the most common event. What differs is how bad an infection near bone would be. That is why technique and antibiotic choices are handled differently.

Is this a stem cell treatment?

It contains stromal cells, but nothing is cultured, expanded or counted. Calling it stem cell therapy oversells what is in the syringe. The difference is drawn on PRP compared with stem cell treatments.

How many treatments will I need?

Usually one course. The lasting results above came from single treatments. That is the practical argument for accepting the harvest instead of repeating a series.

Is BMAC done in one visit?

Yes. Marrow is drawn from the iliac crest, the top of the hip bone, under local anesthetic. It is concentrated in the room and injected into the target joint at the same visit. Nothing is shipped and nothing is stored. The injection is placed under ultrasound or fluoroscopic guidance. Expect two sore spots afterward: the treated joint and the harvest site. The harvest site can ache for several days.

How long after a BMAC injection do you feel results?

Improvement builds slowly. In the four-year series of advanced grade III and IV knees, gains in pain and function began within the first year. They were still climbing at year two. In the first days, expect the harvest site to ache, sometimes more than the treated joint. The same recovery timeline as PRP applies, with a second sore site added.

Why do BMAC results vary from person to person?

The cell content of marrow varies a great deal between people. Age lowers it, and so do smoking, insulin resistance and chronic metabolic inflammation. Two people can have the same procedure done the same way and get very different biology. That is why we read your metabolic picture before deciding. It gives the harvest a fair chance of being worth the discomfort.

Related reading

• PRP compared with BMAC
• Microfat and Lipogems
• PRP compared with stem cell treatments
• Metabolic health and tissue repair
• PRP for knee osteoarthritis
• Risks and side effects