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

BMAC — BONE MARROW ASPIRATE CONCENTRATE

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

What BMAC stands for, and the other names it goes by

BMAC is bone marrow aspirate concentrate. A BMAC injection and a BMAC procedure are the same thing described from different ends — the harvest and the delivery happen in one appointment. You will also see bone marrow concentrate, marrow aspirate concentrate, or simply a marrow harvest. All describe what is set out below.

The abbreviation is not unique to medicine, which is why searching it returns community media stations, a university access center and several athletic conferences. If you arrived looking for one of those, this is not it.

The result that makes the case

Most orthobiologic evidence is gathered in early disease, in people who were going to do reasonably well anyway. The most striking BMAC series did the opposite: it treated Kellgren–Lawrence grade III and IV knees — advanced, structurally compromised joints — and followed them for four years.

Thirty-five of thirty-seven knees improved on both function and pain scores, with gains that began in the first year and were still climbing at year two. Not one of those patients went on to a prosthesis during the follow-up period. In a population routinely told the only remaining option is replacement, that is a considerable finding.

A separate two-year prospective series in grade II–III knees found the same direction of travel, and tracked cartilage imaging alongside the symptom scores rather than relying on how people felt alone.

Head to head, marrow does more than plasma

The comparison that matters to anybody choosing is BMAC against PRP, and it has been run directly. In a single-center trial of 175 patients across grades II to IV, comparing marrow concentrate, platelet-rich plasma and hyaluronic acid over twelve months, BMAC produced the most substantial improvement in quality of life — particularly physical health and mobility — and did so consistently. PRP beat hyaluronic acid on some measures; BMAC beat both.

That sits inside a wider picture that is now well established. A Level I systematic review and meta-analysis of twenty-seven randomized trials and over a thousand patients found PRP and BMAC both outperform hyaluronic acid. A network meta-analysis found PRP, BMAC and hyaluronic acid all outperform corticosteroid injection at six months and beyond — which matters given how many people arrive here having had steroid three times and been told there is nothing left.

The reasonable summary: PRP is an excellent treatment and remains the sensible starting point for many joints. Where more is needed, marrow is the step that has 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 and spun to concentrate it. What comes back is not a single cell type but a mixed population: mesenchymal stromal cells and their progenitors, platelets and their growth factors, and a range of signaling proteins including interleukin-1 receptor antagonist, which acts against one of the central inflammatory drivers in an arthritic joint.

That last component is part of why marrow behaves differently from plasma. PRP delivers a growth-factor bolus. Marrow delivers that plus a cell population and an anti-inflammatory protein profile, which is the mechanistic argument 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 dying from lost blood supply, and it progresses to collapse. Core decompression augmented with marrow concentrate has been studied as a joint-preserving operation, reporting progression-free survival and conversion-to-replacement rates that support doing it early, before the head collapses. There is no plasma equivalent to this. If osteonecrosis is on your imaging, marrow is the conversation.

Rotator cuff repair. Where a cuff is being surgically repaired, systematic review of marrow augmentation reports improved healing and function and lower retear rates. That is a question for the surgeon performing the repair rather than an alternative to them — but it is a real, distinct use, and it is set out alongside the rest of the shoulder picture on PRP for rotator cuff tendinopathy.

The risk conversation, corrected

Marrow harvesting is more than a blood draw and this page will not pretend otherwise. But the received wisdom that it is meaningfully riskier than other injections does not survive the data. A systematic review and meta-analysis of six randomized trials covering 860 patients found the complication rate for BMAC statistically indistinguishable from comparator injections — roughly 42% against 41%, a difference of no significance — with knee effusion the most common event and a number needed to harm of 152.

What remains true, and is worth stating precisely rather than vaguely: an infection seeded at or near bone is a more serious problem than a soft-tissue infection, with longer treatment and worse consequences. The frequency is not higher; the stakes if it happens are. That is why the sterile technique and the antibiotic decision are handled differently for a marrow harvest, and it is a reason for care rather than avoidance. Everything on the general risks page applies as written.

What the harvest is like, honestly

The iliac crest is anesthetized and marrow is drawn through a needle. You will feel pressure, and a brief deep pulling sensation as the aspiration is taken — 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, as everything here is. 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 limitation and, as with fat, it points somewhere useful. The cell content of marrow varies enormously between people. Age reduces it. So does smoking. So, importantly, does insulin resistance and chronic metabolic inflammation, which degrades the marrow niche in the same way it degrades every other repair process this body is being asked to run.

Two people can have identical procedures performed identically and receive materially different biology. A clinic that ignores that and blames the treatment has confused the product with the terrain producing 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 it.

Who this suits

• Advanced degeneration — grade III and IV knees, where the four-year data was earned and where plasma has less to work with.
• A joint that responded to PRP but not enough — real, partial, and short of what was needed.
• Osteonecrosis of the femoral head, where marrow is the specific answer rather than a general one.
• Subchondral bone involvement, where marrow lesions rather than the joint surface are driving the pain.
• Anyone deferring a replacement and wanting the intervening years to be worth something — see PRP compared with surgery.

It does not suit somebody expecting it to replace an arthroplasty, and it does not suit anybody unwilling to have a second site made sore. Both of those are worth deciding before the appointment, not during it.

What people ask before a marrow harvest

Is BMAC better than PRP?

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

Does the harvest hurt?

There is pressure and a brief deep pulling sensation during the aspiration, which is genuinely unpleasant and genuinely short. The ache afterward lasts a few days and is commonly 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, with no significant difference and effusion the most common event. What differs is severity if an infection occurs near bone, which is why technique and antibiotic decisions are handled differently.

Is this a stem cell treatment?

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

How many treatments will I need?

Usually one course. The durability data above came from single treatments, which is the practical argument for accepting the harvest rather than repeating a series.

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