Why knee cartilage cannot regrow on its own

Knee cartilage cannot regrow — and understanding why shapes everything that follows in this article.

Articular (hyaline) cartilage, the glassy tissue covering the ends of the knee's bones, contains no blood vessels and no nerve endings. Because it is avascular, it cannot mount the inflammatory healing response that vascularised tissue uses to repair itself after injury. The cells within it — chondrocytes — are locked inside a dense protein matrix and have almost no ability to migrate into a breach or multiply in sufficient numbers to close a defect. Once the surface is damaged, it stays damaged.

Focal chondral defects are far more common than most patients realise: they are found in up to 60% of knees at arthroscopy, with 5–10% classed as full-thickness. Left untreated, these lesions are progressive — the damaged area tends to enlarge and the surrounding cartilage deteriorates, increasing the risk of premature osteoarthritis.

No current surgical procedure reverses this biology. STACI, OATS, OCA, and MACI each work by restoring surface congruity and providing a biological environment that may support cartilage-like tissue; none regenerates native hyaline cartilage from scratch. The honest clinical goals are pain relief, improved function, and delay of joint replacement — and those goals, set against this biology, are the frame within which each procedure in this article should be read.

What these procedures are actually trying to do

Three shared aims unite STACI, OATS, and OCA: restore the joint surface, reduce pain, and buy time before a knee replacement becomes unavoidable. All three are designed for focal, isolated cartilage defects — patches of damaged tissue with healthy cartilage at the margins. None is appropriate for diffuse osteoarthritis, where the damage is too widespread for any localised repair to hold.

Microfracture was, for many years, the standard first response to focal defects. It works by puncturing the subchondral bone to recruit marrow-derived cells, which fill the defect with fibrocartilage — a mechanically inferior substitute that tends to break down within two to three years. Network meta-analyses of randomised controlled trials now rank it last among cartilage repair techniques at longer follow-up, and there is evidence that microfracture damages the subchondral bone plate in ways that can make subsequent repair surgery harder.

MACI — the FDA-approved, two-stage cell-therapy procedure — provides the most mature comparator: at ten-year follow-up, it shows durable patient-reported improvements, a 9% all-cause reoperation rate, and a 7.4% rate of progression to total knee replacement. That benchmark helps contextualise the newer and older techniques around it.

Where STACI, OATS, and OCA diverge is principally a matter of defect size, whether subchondral bone loss is present, and whether the graft material comes from the patient or a donor — the three variables that determine which procedure fits which patient.

OATS: autograft transfer for small focal defects

OATS solves the repair problem in the most direct way available: it takes a cylindrical plug of healthy bone and cartilage from a low-load-bearing area of the same knee — typically the periphery of the femoral condyle — and press-fits it into the defect site. The transferred tissue is the patient's own native hyaline cartilage, not a substitute or a scaffold seeded with cultured cells. That biological authenticity is a genuine advantage: no risk of graft rejection, no disease-transmission concern, and no waiting for laboratory processing.

The technique is best suited to focal, traumatic, full-thickness defects of up to approximately 2 cm². For larger areas, mosaicplasty — using several smaller plugs — can extend coverage, though the gaps between plugs tend to fill with fibrocartilage rather than hyaline tissue, partially diluting the benefit. Published series report good-to-excellent outcomes in 86–90% of patients at three to ten years, with a failure rate of around 13% at a mean 3.6 years. Longer-term data place success at approximately 72% at ten years across studies. Among the three techniques covered in this article, OATS also offers the fastest return to sport in direct comparisons — a meaningful consideration for younger active patients.

Donor-site morbidity is the procedure's principal constraint. There is a finite amount of cartilage that can safely be harvested from the same knee; taking too much risks pain or deterioration at the harvest zone, which creates a second problem in trying to solve the first. Hard selection criteria reflect this: OATS is contraindicated in patients with a BMI above 40, age above 50, or osteoarthritis beyond Kellgren-Lawrence grade 2. Where any of those thresholds apply — or where the defect simply exceeds what one knee can donate — a different pathway is required.

OCA: fresh allograft when the defect is larger or the bone is involved

When defect size exceeds what one knee can safely donate, or when the damage has penetrated into the subchondral bone beneath the cartilage surface, fresh osteochondral allograft transplantation — OCA — becomes the more appropriate option. It draws on cadaveric donor tissue: a bone-and-cartilage graft matched to the defect by size and geometry, supplied fresh (not frozen) and used within a limited preservation window.

The cadaveric source has two practical consequences. There is no donor-site within the patient's own knee, so defect size imposes no ceiling — and crucially, OCA can restore the bone layer simultaneously, which surface-only techniques cannot. In return, it introduces a small risk of disease transmission and a minor immunologic response — managed carefully by tissue banking protocols but not eliminated — and scheduling depends on donor graft availability.

The evidence base is well established. Graft survivorship ranges from 78% to 91% at ten years; long-term series extending to 10–25 years show 59–91% survival, with outcomes lasting longer in younger patients with smaller lesions. A 2025 systematic review of 471 athletes reported that 72% returned to sport post-OCA, with 84% of those returning at an equal or higher competitive level at a weighted mean of approximately 11 months — roughly four months longer than comparable OATS cohorts.

A 2025 prospective randomised controlled trial refined the picture further. Augmenting OCA with bone marrow aspirate concentrate (BMAC) reduced the reoperation rate for graft debridement or revision from 35% to 5% (p=0.02), yet produced no significant improvement in patient-reported outcome scores at two years. That pattern points to graft integration — the process by which donor tissue incorporates into the host bone — as the primary driver of early failure, rather than any deficiency in the cartilage tissue itself.

OCA is indicated for medium-to-large defects, posttraumatic bone loss, and situations where the bone beneath the cartilage surface needs simultaneous restoration. Like the autograft techniques, it is not appropriate for diffuse osteoarthritis.

STACI: what single-stage ACI offers and where the evidence stands

The most technically advanced of the three techniques covered here, STACI compresses the conventional two-stage ACI/MACI pathway into a single operative session. In standard two-stage approaches, cartilage cells are harvested at a first operation, sent to an external laboratory for several weeks of culture, then implanted at a second surgery. STACI removes that interval entirely by bringing the laboratory into the theatre: chondrocytes are harvested from a non-weight-bearing area of the knee, enzymatically freed from their matrix, combined with mesenchymal stem cells concentrated from the patient's own bone marrow, and applied to a collagen scaffold cut to match the defect — all before the patient leaves the operating table.

Developed by Professor Paul Lee at the London Cartilage Clinic, an ICRS Teaching Centre of Excellence, STACI is currently available only in the UK and is priced at £28,000 all-inclusive. The practical benefit is real: one anaesthetic, one theatre booking, and no six-to-twelve-week wait between operations.

What the single-stage design does not change is the biology of cartilage maturation. The graft still requires the same period of protected weight-bearing — crutches for six to eight weeks — and full recovery typically takes a year or longer. Fewer operations is not the same as a shorter rehabilitation.

On the evidence spectrum, STACI sits at the more novel end. Published work, including Taylor and Lee (2019), supports the procedural rationale, but long-term comparative RCT data against OATS or OCA do not yet exist. The evidence base remains institutional and early-stage — a position that reflects where the technique sits in its development rather than a fundamental limitation of the approach. Patients weighing STACI against the decade-plus follow-up behind OATS and OCA should understand that distinction.

Which procedure fits which patient

Choosing between these three techniques is less a matter of preference than of anatomy: the defect's size, depth, and whether it has penetrated into the subchondral bone largely determines which procedure applies — they fill distinct clinical niches rather than compete for the same patient.

For small focal defects, typically no larger than 2 cm², in a younger active patient with intact subchondral bone and no history of prior marrow stimulation, OATS is generally the logical starting point. It uses native hyaline cartilage in a single operation; the practical ceiling is donor supply within the same knee, not a limitation of the technique itself.

When the defect is larger, or when bone loss is present beneath the cartilage surface, OCA is the more appropriate route. It imposes no size restriction, can restore bone and cartilage simultaneously, and is also well-suited to situations where a previous repair has failed or autograft supply is insufficient.

STACI occupies a different axis: a cell-based approach for focal, cartilage-only defects where a single-stage pathway is preferred over the conventional two-stage ACI/MACI route. Because its long-term comparative data are still accruing, the suitability decision warrants a detailed discussion with the treating consultant.

None of these procedures is appropriate for diffuse or advanced osteoarthritis — that conversation moves to joint-preservation strategies such as osteotomy, or to joint replacement.

An initial assessment typically involves clinical examination, gait and load evaluation, and MRI review of lesion depth and subchondral status. Lincolnshire Knee is part of the MSK Doctors group and accepts patients without a GP referral; assessments are available at Sleaford and Grantham — lincolnshireknee.co.uk.

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4. [4] Articular Cartilage Repair — Wikipedia. https://en.wikipedia.org/?curid=19042351 https://en.wikipedia.org/?curid=19042351
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