Why knee cartilage doesn't repair itself

Damaged knee cartilage is stubbornly slow to heal — and for most people, the body never fully restores what was lost. Understanding why helps explain both the range of treatments available and why some work better than others.

The culprit is the tissue's unusual biology. Articular cartilage — the smooth, pearl-white layer lining the joint surfaces — contains no blood vessels. Most injured tissues heal because blood carries clotting factors, oxygen, and repair cells to the site of damage. Without a blood supply, cartilage has no equivalent rescue mechanism. The specialist cells that maintain it (chondrocytes) are few in number, largely dormant, and have virtually no capacity to migrate to a defect and regenerate lost tissue.

When the body does attempt repair, it fills a cartilage defect with fibrocartilage — a scar-like substitute made primarily from type I collagen rather than the type II collagen that gives native hyaline cartilage its stiffness and resilience. Fibrocartilage can reduce symptoms in the short term, but it is mechanically inferior: it wears down faster under load and tends to deteriorate within two to three years in many patients.

This distinction — hyaline versus fibrocartilage — is the practical yardstick for every repair procedure discussed in this article. The closer a treatment comes to restoring true hyaline cartilage, the more durable the result is likely to be.

One further point shapes prognosis: the size and nature of the lesion matters enormously. A focal, contained defect — graded I–III on the ICRS or Outerbridge scale, in an otherwise healthy joint — is a realistic repair target. Widespread, high-grade (grade IV) or diffuse osteoarthritic change is a different clinical situation, where cartilage restoration is less reliable and the conversation shifts towards joint preservation or, ultimately, replacement.

Non-surgical care: managing symptoms and slowing damage

For many patients, non-surgical management is not a stepping stone to the operating theatre — it is the destination. A structured conservative programme can meaningfully reduce pain, improve function, and slow further joint deterioration, particularly in those with early or moderate cartilage damage.

Lifestyle and physiotherapy

Weight loss and low-impact aerobic activity — swimming, cycling, walking — are universally recommended as the starting point, both by NHS guidance and specialist clinics. Even modest weight reduction reduces compressive load across the joint surface with every step taken. Alongside this, physiotherapy focused on quadriceps strengthening improves the dynamic stability of the knee, distributing load more evenly and reducing direct stress on damaged cartilage areas.

For acute flare-ups or recent injuries, the RICE regimen (rest, ice, compression, elevation) and short-course NSAIDs such as ibuprofen, naproxen, or meloxicam help control pain and inflammation while the joint settles.

Intra-articular injections

When lifestyle measures and physiotherapy provide insufficient relief, intra-articular injections form the next tier. Each works by a different mechanism:

• Corticosteroids reduce intra-articular inflammation and swelling, offering short-to-medium-term pain relief.
• Hyaluronic acid (viscosupplementation) supplements the joint's natural lubricating fluid, reducing friction on the cartilage surface.
• Platelet-rich plasma (PRP) delivers a concentrated dose of the patient's own growth factors to the joint environment.

Arthrosamid — a non-degradable polyacrylamide hydrogel — is a newer option that works as a space-filling cushion within the joint, providing pain relief through mechanical support rather than biological activity. It is distinct from scaffold-based options such as ChondroFiller, which aim to recruit the body's own repair cells into a regenerative matrix.

What non-surgical care does not do

None of these approaches rebuilds cartilage structurally. Their value is symptom modification and joint preservation — buying time, improving quality of life, and in some cases reducing the urgency for surgical intervention. Patients whose damage is focal and contained may eventually need to consider restorative procedures if conservative measures plateau.

Single-stage cartilage repair: the main procedures

Defect size — measured in square centimetres at MRI or arthroscopy — is the main selection variable when comparing single-stage repair options. Each procedure has a characteristic fit range, moving roughly from smallest lesions to larger focal defects.

Microfracture

Historically the standard first-line choice for focal defects under approximately 2 cm², microfracture drills small holes into the subchondral bone to draw marrow stem cells to the surface. The repair tissue this produces is fibrocartilage — less durable under load than native hyaline cartilage, with studies showing significant deterioration at around two to three years. There is also evidence that the drilling damages the subchondral bone plate itself, which may limit the effectiveness of any subsequent repair procedure. For these reasons, microfracture is in declining use in specialist practice, though it remains part of some pathways.

OATS / Mosaicplasty

For focal defects in the 1–2 cm² range, osteochondral autograft transfer (OATS) — or mosaicplasty when multiple plugs extend coverage to roughly 4 cm² — transfers healthy cartilage and underlying bone from a non-weight-bearing area of the same knee. This delivers genuine hyaline cartilage to the defect, which is a meaningful tissue-quality advantage. The trade-off is donor-site morbidity: some patients experience discomfort or a residual defect at the harvest site, and this forms part of the pre-operative discussion.

AMIC

Autologous matrix-induced chondrogenesis (AMIC) combines microfracture with a collagen scaffold applied over the prepared defect in a single stage. The scaffold retains the marrow-derived cells at the site and can improve repair-tissue quality and stability compared with microfracture alone — a practical bridge between pure marrow stimulation and the two-stage cell-based approaches discussed in the next section.

ChondroFiller injection

For focal defects up to approximately 3 cm² — and in some cases extending to 6 cm² — a ChondroFiller injection offers a non-surgical, outpatient route within this category. An acellular collagen scaffold is placed via ultrasound-guided injection, gelling in situ to create a matrix that recruits the patient's own progenitor cells; no theatre booking or general anaesthetic is required. The device is CE-marked as a Class III medical device. Published outcome data report MOCART cartilage repair scores of 70–87 and IKDC improvements of approximately 30 points. Outcome data for OATS and the cell-based procedures are referenced in the adjoining sections — each option carries its own published evidence base, and no single approach is universally superior across all defect types.

ACI and MACI: the cell-based repair procedures

Cell-based repair represents the most biologically ambitious tier of cartilage surgery — the aim being to regenerate tissue that resembles native hyaline cartilage rather than the fibrocartilage produced by marrow stimulation.

Autologous chondrocyte implantation (ACI)

ACI was established as a clinical procedure by a landmark 1994 paper in the New England Journal of Medicine. The method is two-stage: a small sample of healthy cartilage cells is first harvested arthroscopically; those chondrocytes are then cultured in a laboratory for roughly six to twelve weeks before being re-implanted under a periosteal flap at the defect site. Long-term series from Peterson, Minas, and others document durable improvements at nine or more years in appropriately selected patients.

One clinically important consideration: patients who have previously undergone microfracture carry a higher risk of ACI failure, likely because prior drilling alters the subchondral bone architecture. This history is relevant to disclose during pre-operative assessment.

MACI — the matrix-assisted evolution

MACI seeds the cultured chondrocytes onto a Type I/III collagen membrane before implantation, reducing some of the technical drawbacks associated with first-generation ACI. In a prospective series of 65 patients, mean Lysholm scores rose from 28.5 at baseline to 76.6 at two years (p>0.0001), with the improvement sustained at 75.5 at five years and Tegner activity scores improving from grade II to IV. A 2024 study with a minimum ten-year follow-up confirmed that these patient-reported outcome gains remain durable over the longer term — the strongest long-term evidence currently available for any cartilage regeneration procedure.

Both ACI and MACI are best suited to active patients with focal defects broadly in the 2–10 cm² range within an otherwise healthy joint. Neither is indicated for widespread or diffuse osteoarthritis.

Emerging: single-stage ACI

A next-generation variant — sometimes referred to as STACI — aims to compress both operative steps into a single procedure. Early work is encouraging, but large-scale evidence remains limited, and this should be regarded as an emerging rather than an established pathway.

Availability and resource

Both procedures are resource-intensive: each requires two operative episodes and a period of laboratory cell culture in between. NHS provision varies significantly by region, and in many cases funding is accessed through individual funding requests or via the private sector. A consultant assessment is needed to confirm whether a cell-based approach is appropriate and which variant best fits the individual's defect, joint status, and activity goals.

Larger defects, alignment problems, and osteochondral allografts

Some patients arrive at a specialist consultation having been told their defect is too large for the procedures discussed in earlier sections, or that their leg alignment will undermine any repair that is attempted. Two distinct surgical approaches address these situations — osteochondral allograft transplantation and corrective osteotomy — and they are frequently used in combination.

Osteochondral allograft (OCA)

When a defect exceeds what autologous tissue can fill — whether because of its sheer size, its depth into the subchondral bone, or the extent of post-traumatic damage — osteochondral allograft (OCA) transplants fresh, matched cadaveric bone and cartilage into the affected site. Unlike OATS, which depends on the patient's own harvest, OCA draws on donor tissue and is therefore not limited by the patient's available supply. The procedure restores both the cartilage surface and the underlying bone architecture in a single stage — a meaningful advantage where structural integrity has been lost. Long-term follow-up data, including work by Gross (2008) and Levy (2013), document durable outcomes in appropriately selected patients.

Osteotomy and alignment correction

Where the mechanical axis of the leg directs load disproportionately onto a damaged compartment, high tibial osteotomy (HTO) or distal femoral osteotomy (DFO) reshapes the tibia or femur to redistribute that force. Osteotomy is not cartilage repair — the tissue itself is unchanged — but without correcting alignment first, any repair procedure placed into a continuously overloaded zone is at risk of early failure. A 2024 systematic review supports combining osteotomy with cartilage restoration, with the alignment correction typically preceding or accompanying the restorative procedure. For patients not yet at a surgical threshold, an unloader brace achieves a similar mechanical goal without an operation and may form part of conservative management.

Patients with widespread, diffuse osteoarthritis rather than a contained focal defect are generally outside the scope of these preservation and repair approaches; for those patients, the pathway leads to joint replacement.

How to choose and what to realistically expect

Choosing the right procedure comes down to several interacting variables that a cartilage specialist will weigh before making any recommendation.

What drives the decision

• Age and activity level — younger, active patients with focal, contained defects have the strongest evidence base for cartilage repair; older patients with diffuse or multi-compartment osteoarthritis are generally outside the repair window.
• Defect size and grade — lesions under 2 cm² may suit single-stage autograft or marrow-stimulation approaches; moderate-to-large focal defects in the 2–10 cm² range typically indicate MACI or ACI; very large or post-traumatic lesions favour osteochondral allograft.
• Limb alignment and meniscal status — malalignment or a compromised meniscus concentrates load on the repair site; correction may need to accompany any restorative procedure rather than follow it.
• Prior procedures — previous microfracture alters the subchondral bone architecture and raises the failure risk for subsequent cell-based repair.
• BMI — body mass is a routine part of surgical candidacy assessment because excess joint load compromises graft survival.

Recovery expectations

Cartilage repair requires protected loading while new tissue matures. Return to high-impact sport or physically demanding work typically takes 6–12 months from surgery; some procedures extend this to 18 months. Patients should plan for this before committing to an operative pathway.

On emerging cell therapies

BMAC, adipose-derived stem cells, and similar biologics can offer modest symptom relief in certain patients. A 2025 systematic review confirms that true regrowth of hyaline cartilage with these approaches remains elusive — they serve a symptom-management role, not a structural one, and should be understood in that context.

The range of procedures and the interplay of patient-specific factors make shared decision-making with a cartilage specialist the essential next step. A dedicated MSK directory — such as Search MSK — allows patients to search by region and clinical specialism to identify a consultant with the relevant expertise.