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Does Cartilage Regeneration Work?

A review of knee cartilage “patching” with autologous chondrocyte implantation (ACI)

Paul Ingraham • 10m read
Photo of man in shorts holding knee, presumably because it hurts.

“Autologous chondrocyte implantation” (ACI) is implanting a “patch” of your own cartilage cells, usually into a knee. This treatment depends on some biotech wizardry owned by a big company, and big biotech companies make a lot of people a bit nervous. For better or for worse, this is Big Medical Science — it takes many resources to develop and deliver a technique like this, so it’s only available1 as MACI®, the brand name of an ACI product used to repair knee cartilage defects, a technology owned by Vericel (it started out as CARTICEL®, owned by Genzyme). The M is for “matrix.”

The idea is to take a small sample of your very own cartilage cells (chondrocytes) and grow them into a surplus population of several million spares on a scaffolding (matrix) that a surgeon implants back into your knee. This is somewhat like tossing some grass seed on a rough patch of lawn, only it requires a bunch of folks with advanced degrees, and it costs a lot more. Work is probably still being done on the ideal formula for getting the implanted cells to stay put and make new cartilage. The limitations of ACI today will probably not be the limitations of the ACI of tomorrow.

Here’s a video of a surgeon performing the procedure (back when it was a Genzyme product):

Genzyme Autologous Chondrocyte Implantaion (Genzyme Procedure) for Chondral Defect of the Knee  1:36

For my readers — particularly many of you who have chronic patellofemoral pain syndrome (kneecap pain) — I’ve got good news and bad news about MACI. It’s both amazing and disappointing.

Relationship to other cell and stem cell therapies

Stem cells are generic cells that do not yet have a job. In theory, they can become what we need them to be, which is a potentially powerful tool in medicine. Stem cell therapy is a broad concept in regenerative medicine, and it is a hot hot hot topic right now.

Stem cell therapy is identical in spirit to the other two main regenerative therapies in musculoskeletal and pain medicine: platelet-rich plasma and cartilage regeneration. Neither PRP or ACI is technically a stem cell therapy — they use mature, specialized cells, so they are just cell therapies.

But regeneration is the goal of all of these methods, and the topic of stem cell therapy overlaps so much with PRP and ACI that they are practically the same thing with regards to safety, efficacy, and the concerns of skeptical consumers and regulators. They are biologically intriguing treatments that might amount to something someday — after all, we know regeneration is possible, thanks to salamanders! — but it’s a depressing mess so far, instead of being inspiring and promising. These treatments are all being rushed to market in the same way, all sold as high-tech medicine to desperate consumers long before the science is done.

The good news about chondrocyte implantation

On the bright side, there seems to be little doubt that the MACI procedure really can grow fresh, shiny new cartilage. Which is cool! I defy anyone to be unimpressed by that. It’s a little bit of true regeneration, just like a salamander.

Cartilage just doesn’t fix easily. In the past, trying to repair degrading cartilage with surgery has involved comparatively primitive methods: polishing it (debridement), which is notoriously ineffective; trying to stimulate healing with stresses, such as creating tiny fractures in the underlying bone (microfracture), or drilling or abrading it (drilling/abrasion); transplanting chunks of bone and cartilage from somewhere (or someone) else (autograft/allograft). None of these techniques has a particularly good track record, and some of them (debridement) are basically surgical snake oil. By contrast, implanting cells seems awfully clever and advanced.

And the patient gets “sustained and clinically meaningful improvement in pain and function,” according to American Journal of Sports Medicine in January of 20092 (plus other, similar research results reviewed below). It all sounds pretty great, and makes me want to buy stock!

Chondrocytes: the cells that build and maintain cartilage: The sparseness of chondrocytes scattered throughout cartilage mean that your joints are “plagued by inherent limited healing potential.”

Chondrocytes: the cells that build & maintain cartilage

The sparseness of chondrocytes scattered throughout cartilage mean that your joints are “plagued by inherent limited healing potential.”

The bad news about chondrocyte implantation

On the other hand, as always, you’re going to sober up after reading the fine print — as always. The MACI website has a lot of fine print, just like drug ads in magazines. And you can learn some interesting things from that small print! The most interesting is that, although ACI is a treatment for regrowing cartilage, it’s not actually intended for arthritis per se.

The original version of MACI (back when it was CARTICEL) was “not indicated for the treatment of cartilage damage associated with generalized osteoarthritis.” As of 2018, MACI is intended for “cartilage defects” — not “arthritis,” a term that is conspicuously missing from the reasons to have the procedure. In fact, severe arthritis is actually a major reason to avoid MACI — it’s contraindicated — because arthritis is actually one of the side effects of the procedure. Yes, you read that right: this cartilage repair treatment can actually cause arthritis in some patients.3

Er, really? That’s quite a kick in the optimism. This is definitely not what most candidates want to hear. But it’s true: by design, ACI is for younger patients with cartilage that has been damaged by injury. Why? Rehabilitation is not quick or risk-free. Even with a large batch of fresh chondrocytes, growing cartilage is slooooooow. There are also numerous potential complications, such as overgrowth of the implant — eek, it’s alive, and it’s taking over my knee! And all possible complications are more likely for older patients.

Who knew implanting things in a body could go poorly for some people?! Imagine that.

So this procedure is actually not appropriate for the majority of (older) people with a lot of stubborn anterior knee pain, or even for large numbers of younger patients whose cartilage just isn’t rough enough to justify it.

In a word, bummer.

Genzyme’s original slogan for CARTICEL was remarkably restrained because of the various problems (the worst of which I haven’t even brought up yet): “the only treatment that uses your own cells to regenerate your knee cartilage that may offer a long-term solution for a knee cartilage injury.” May offer. May. Hmm …

Nicer cartilage is just not the same thing as cured knee pain

If only wishing made it so.

The <cite>American Journal of Sports Medicine</cite> has published three studies of AIC so far this year.

The American Journal of Sports Medicine has published three studies of AIC so far this year.

There have been three prominent studies of chondrocyte implantation this year that I know of, all of them in American Journal of Sports Medicine. The largest was the STAR study, referenced above, published in January: the “Study of the Treatment of Articular Repair,” which was the source of the rather glowing results quoted above. Also in January, Kon et al found it was not only “satisfactory” but produced “better clinical results and sport activity resumption” than the microfracture technique;4 and Gobbi et al concludes that the treatment resulted in “a significant decline” in lesions and pain and is a “viable” treatment.5

Indeed, ACI looks promising. Results from STAR and Kon et al seem to pretty decisively show that it is almost certainly an improvement over older techniques. However, there is an important caveat that would be easy for the casual student to miss …

All three studies show dramatic improvement in the patients’ knee cartilage, but they each also showed that patients did not recover completely from their knee pain. For instance, Gobbi et al’s subjects went from an athletic activity (Tegner) score of about 2.5 to about 4.75 — a level of activity that does not even include “recreational sports.” We’re talking about patients who are barely running or even playing beer league softball.

Or consider Kon et al’s subjects, who went from a subjective score of 41 originally to 70 five years later. That’s a nice improvement, but still well short of feeling fantastic. How “cured” of a chronic pain would you feel if you were only at 70% of healthy status? Most athletes don’t want to operate at much less than “110%” — 70% is still pretty broken as far as most seriously active people are concerned.

The STAR study had similar results: across the board, by several measures, results were a heck of a lot better than nothing, but also well short of full recovery.

So, patients who were treated with ACI definitely improved, and that is not an unimportant result. But cured? Not hardly! Despite the fact that their cartilage looks much nicer than it did, these patients are still not exactly good to go. Unfortunately, there are still more reasons for concern.

Does MACI power a placebo?

Could the MACI benefits found by researchers have been in the mind? It seems unlikely at first glance — this is cold, hard surgery we’re dealing with, and the results were strongly positive, “statistically significant” and all that. Unfortunately, a placebo effect is not an unreasonable consideration after a closer look. The value of uncontrolled studies of knee pain surgeries (and many other surgeries) should always be suspect in principle, but especially since the surprising 2002 discovery (by Moseley et al) that a surgical placebo can act strongly on patients.6 For more about that fascinating bit of science, see Knee Debridement is a Completely Ineffective Procedure Since then, many other orthopedic surgeries have failed to beat a placebo.7

Kon et al’s study was compared only to an older cartilage repair technique, and Kon et al did not compare their patients to anything at all. So neither of these surgeries was compared to a placebo, and that’s definitely a problem. Bizarre as it sounds, it’s actually quite plausible that the substantial benefits of the surgery were due to placebo.

The STAR study was larger, and it looked only at patients who had already failed to get relief from a previous, non-ACI knee surgery — a kind of compromise. If these patients already had surgery that had no effect, placebo or otherwise, it’s safer to assume that the ACI benefits were “real.” It’s safer, but still not safe: the cool-factor of a fancy new surgery can probably power a superior placebo. Remember, it’s been nicely proven that people like the taste of crappy wine when they think it’s the expensive stuff — they almost certainly think the price and über-modern cache of ACI means it must be more effective … and that absolutely has an effect on patient hopes, dreams, and placebos.

Although this is no more proven than anything else, it really is possible that the new surgery is no better than the old one: just better at generating a robust placebo effect. Crazy, eh?

But the bottom line is always that patellofemoral pain is multifaceted

An even more obvious explanation for the imperfect results of the surgeries is this: the condition of your cartilage is only one of many factors in anterior knee pain. Patellar bone fatigue and synovial membrane irritation, for instance, are two other major candidates — and likely involved to some degree in many cases of knee pain.

Related Reading

Most of the knee pain information on is about patellofemoral syndrome and iliotibial band syndrome, the two most common runner’s knee conditions, typically causing pain on the front and side of the knee respectively (learn more about the difference). Arthritis is not my usual subject matter, although I do keep tabs on it (particularly because of the way it is routinely confused with patellofemoral pain).

Articles about other kinds of regenerative medicine:

About Paul Ingraham

Headshot of Paul Ingraham, short hair, neat beard, suit jacket.

I am a science writer in Vancouver, Canada. I was a Registered Massage Therapist for a decade and the assistant editor of for several years. I’ve had many injuries as a runner and ultimate player, and I’ve been a chronic pain patient myself since 2015. Full bio. See you on Facebook or Twitter., or subscribe:

What’s new in this article?

Five updates have been logged for this article since publication (2009). All updates are logged to show a long term commitment to quality, accuracy, and currency. more Like good footnotes, update logging sets apart from most other health websites and blogs. It’s fine print, but important fine print, in the same spirit of transparency as the editing history available for Wikipedia pages.

I log any change to articles that might be of interest to a keen reader. Complete update logging started in 2016. Prior to that, I only logged major updates for the most popular and controversial articles.

See the What’s New? page for updates to all recent site updates.

2020 — Added a footnote to clarify the source of claims about autologous chondrocyte implantation side effects.

2019 — New section, “Relationship to other cell and stem cell therapies.”

2018 — Updated references to CARTICEL and Genyzme to MACI and Vericel, and a bunch of other miscellaneous minor improvements.

2016 — Added citation to Louw 2016. Converted all inline citations to footnotes.

2016 — Miscellaneous minor improvements and updates.

2009 — Publication.


  1. It’s a big world, perhaps there are other options, but as of 2018, this is the only FDA-approved product of this kind available in the USA.
  2. Zaslav K, Cole B, Brewster R, et al. A prospective study of autologous chondrocyte implantation in patients with failed prior treatment for articular cartilage defect of the knee: results of the Study of the Treatment of Articular Repair (STAR) clinical trial. Am J Sports Med. 2009 Jan;37(1):42–55. PubMed 18927254 ❐
  3. Although this side effect is actually well-known, the claim seems startling on its face. The primary citation for this is the STAR study, Zaslav et al. That said, the adverse events associated with ACI are widely reported. For instance, reports multiple relevant and common harms, deriving the information from the STAR study and also “the Cartilage Repair Registry, the Swedish Series, and post-marketing adverse event reporting.” Adverse events include arthrofibrosis/joint adhesions, chondromalacia or chondrosis, and cartilage injury. These are not technically “arthritis” — that word was a simplification — but they are collectively extremely similar to it in practice. The cartilage can get damaged, and these are very much non-rare complications, with rates ranging from 5-16% of patients.
  4. Kon E, Gobbi A, Filardo G, et al. Arthroscopic second-generation autologous chondrocyte implantation compared with microfracture for chondral lesions of the knee: prospective nonrandomized study at 5 years. Am J Sports Med. 2009 Jan;37(1):33–41.
  5. Gobbi A, Kon E, Berruto M, et al. Patellofemoral Full-Thickness Chondral Defects Treated With Second-Generation Autologous Chondrocyte Implantation Results at 5 Years’ Follow-up. Am J Sports Med. 2009 Jun;37(6):083–1092.
  6. Moseley JB, O’Malley K, Petersen NJ, et al. A controlled trial of arthroscopic surgery for osteoarthritis of the knee. N Engl J Med. 2002 Jul 11;347(2):81–8. PubMed 12110735 ❐ PainSci Bibliography 56845 ❐

    In this landmark and fascinating study, people with osteoarthritis improved equally well regardless of whether they received a real surgical procedure or a sham, which is a particularly striking example of the placebo effect and implies that belief can have an effect even on a “mechanical” knee problem. From the abstract: “In this controlled trial involving patients with osteoarthritis of the knee, the outcomes after arthroscopic lavage or arthroscopic debridement were no better than those after a placebo procedure.”

    In 2008, these findings were fully supported by a Cochrane Collaboration review (Laupattarakasem) which concluded that “there is ‘gold’ level evidence that arthoscopic debridement has no benefit,” and by New England Journal of Medicine (Kirkley) which reported that “surgery for osteoarthritis of the knee provides no additional benefit to optimized physical and medical therapy.”

    (See more detailed commentary on this paper.)

  7. Louw A, Diener I, Fernández-de-Las-Peñas C, Puentedura EJ. Sham Surgery in Orthopedics: A Systematic Review of the Literature. Pain Med. 2016 Jul. PubMed 27402957 ❐ PainSci Bibliography 53458 ❐

    This review of a half dozen good quality tests of four popular orthopedic (“carpentry”) surgeries found that none of them were more effective than a placebo. It’s an eyebrow-raiser that Louw et al could find only six good (controlled) trials of orthopedic surgeries at all — there should have been more — and all of them were bad news.

    The surgeries that failed their tests were:

    • vertebroplasty for osteoporotic compression fractures (stabilizing crushed verebtrae)
    • intradiscal electrothermal therapy (burninating nerve fibres)
    • arthroscopic debridement for osteoarthritis (“polishing” rough arthritic joint surfaces)
    • open debridement of common extensor tendons for tennis elbow (scraping the tendon)

    Surgeries have always been surprisingly based on tradition, authority, and educated guessing rather than good scientific trials; as they are tested properly, compared to a placebo (a sham surgery), many are failing. This review of the trend does a great job of explaining the problem. This is one of the best academic citations to support the claim that “sham surgery has shown to be just as effective as actual surgery in reducing pain and disability.” The need for placebo-controlled trials of surgeries (and the damning results) is explored in much greater detail — and very readably — in the excellent book, Surgery: The ultimate placebo, by Ian Harris.


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