How defect size drives your treatment choice

The single biggest factor in deciding which cartilage repair is right for you is the size and depth of the defect — measured in square centimetres and graded on the ICRS scale from superficial thinning (Grade 1) to full-thickness loss down to bone (Grade 4).

Defect area creates a natural size ladder across the main repair options:

• Small focal defects (~1–4 cm²): OATS (osteochondral autograft transfer) transplants cartilage-and-bone plugs harvested from a low-load area of your own knee into the damaged site in a single operation.
• Small-to-medium defects (up to ~3 cm², extendable to 6 cm²): ChondroFiller injection — an ultrasound-guided outpatient procedure using an injectable collagen scaffold — recruits the body's own progenitor cells to regenerate tissue without a second surgery or cell harvest.
• Medium-to-large defects (~2–10 cm²): ACI and MACI use cells taken from your own cartilage, expanded in a laboratory, and then reimplanted in a second operation on a collagen membrane.

A meaningful distinction runs through all three modern options: each aims to restore hyaline or hyaline-like cartilage — the durable, low-friction tissue the joint was built with. Microfracture, which has largely been superseded, instead triggers a blood-clot repair that produces fibrocartilage (a scar-like substitute), with reoperation rates reaching 41% and results that frequently deteriorate within two to three years.

Everything that follows covers focal, contained defects only. Diffuse, multi-compartment arthritis places a patient outside this scope entirely, and the appropriate pathway for that situation is discussed separately.

OATS: autograft repair in a single procedure

Taken from a quieter, low-load area of the patient's own knee, OATS plugs consist of bone and its overlying cartilage, harvested in cylinders and press-fitted directly into the defect — completing the repair in a single surgical procedure. Because the tissue comes from the patient, there is no rejection risk and no laboratory interval; the operation begins and ends in the same session.

The technique is best matched to focal defects in the 1–4 cm² range. Where a single plug is insufficient, mosaicplasty — placing an array of smaller cylinders side by side — can extend coverage modestly beyond that ceiling, though the size of the repair remains constrained by the volume of graft the knee can safely yield.

The durability case for OATS rests on tissue quality. Where microfracture lays down fibrocartilage that tends to break down within a few years, OATS delivers genuine hyaline cartilage — structurally closer to what was there before. Gudas et al. (2012) followed patients for ten years and reported sustained benefit, lending support to OATS as a longer-lasting option for defects within its size range.

The primary trade-off is donor-site morbidity. Harvesting plugs creates a secondary wound in the same knee, and persistent discomfort at the harvest zone is a documented risk. How frequently this occurs and how long symptoms last varies between patients and depends partly on how many plugs are taken — it is a real factor to weigh in shared decision-making, particularly when a larger mosaic is needed.

ACI and MACI: cell-based repair for larger defects

For defects where OATS runs out of donor material — typically from around 2 cm² up to approximately 10 cm² — cell-based therapies move into range. Both ACI and MACI work on the same biological principle: a small sample of the patient's own cartilage cells (chondrocytes) is harvested in a first procedure, expanded in a laboratory over several weeks, and then reimplanted into the defect during a second operation. The interval between procedures is built into the biology; there is no single-stage shortcut with either technique.

Where the two differ is in how the cells are anchored at the repair site. ACI — used clinically for over 20 years and FDA-approved for full-thickness defects of the femoral condyle and patella — covers the implanted cells with a periosteal flap, a strip of tissue taken from the shin. MACI seeds the same expanded cells directly onto a Type I/III collagen scaffold, which is then fixed into the defect. Seeding cells onto the scaffold reduces the technical variability and periosteal-related complications associated with first-generation ACI, while retaining the same underlying cell-based mechanism.

The strength of the MACI evidence is worth stating plainly. The SUMMIT Phase 3 RCT — the highest-quality trial currently available in focal cartilage repair — demonstrated that MACI produced superior KOOS pain and function scores compared with microfracture at both two and five years, specifically for defects of 3 cm² or larger. No equivalent Phase 3 RCT exists for first-generation ACI, though long-term cohort data support durable outcomes over nine years or more.

Both procedures carry complication rates of up to 17% and reoperation rates of up to 37% — figures that require transparent discussion at the point of consent, not after. In August 2024, MACI Arthro received FDA approval for arthroscopic delivery in patients aged 18–55, representing an evolving delivery option rather than a change in the underlying two-stage pathway.

ChondroFiller injection as an outpatient scaffold pathway

Unlike the surgical options described above, ChondroFiller injection is delivered as an ultrasound-guided outpatient procedure — no general anaesthesia, no hospital admission, and no return to theatre for a second operation.

The mechanism is acellular. The injectable scaffold, composed of murine-derived Type I collagen, gels in place within the defect and provides a structural matrix that attracts the patient's own progenitor cells. Because no cell harvest is required, there is no biopsy stage and no laboratory interval — the body supplies the biology. This process, matrix-induced chondrogenesis, aims to produce hyaline-like tissue rather than fibrocartilage.

Published cohort data show approximately 70–85% of patients achieve significant symptom relief, with IKDC scores improving by approximately 30 points — broadly comparable to MACI cohort data on the same measurement scale. One honest caveat applies: these figures derive from observational cohorts, not a Phase 3 RCT of the kind underpinning the MACI evidence base. No head-to-head randomised trial comparing the ChondroFiller injection pathway with MACI has been published.

CE-marked as a Class III medical device, the treatment is suited to focal defects up to approximately 3 cm², with published use extending to 6 cm². There is no strict upper age limit and no cell-harvest requirement — characteristics that distinguish it from ACI and MACI, where fitness for surgery and a multi-week laboratory interval both factor into suitability.

The pathway is particularly relevant for patients in whom general anaesthesia or a two-stage surgical journey is not appropriate, and for those whose defect dimensions fall within the scaffold's range and who prefer a minimally invasive image-guided approach.

When focal repair is not the right path

Not every damaged knee is a candidate for focal repair — and identifying that boundary early is as important as choosing between the techniques described above.

All four approaches share the same prerequisite: a contained, focal lesion. When osteoarthritis has spread across multiple compartments, the defect is no longer focal by definition. In that setting, the conversation moves to joint-preservation strategies — unloader bracing, staged biologic support, or, where damage is advanced, joint replacement planning.

Pre-existing malalignment is a separate concern that applies regardless of which repair technique is selected. Varus or valgus deformity concentrates load on the repaired surface and sharply increases the risk of failure. High tibial osteotomy (HTO) or distal femoral osteotomy (DFO) corrects that load distribution so the cartilage repair has a viable mechanical environment in which to survive. Osteotomy is a joint-preservation adjunct — it does not restore cartilage itself, and patients should expect a frank explanation of both procedures when malalignment is present.

For defects that extend through bone as well as cartilage, or where posttraumatic damage exceeds what autograft or scaffold can address, fresh osteochondral allograft (OCA) or a joint replacement assessment becomes the relevant next step. MRI-based cartilage grading — reviewed at consultant level — is the gateway to knowing which category applies.

The four options compared: a decision-aid summary

Each technique occupies a distinct position across six practical dimensions. The table below is a reference, not a ranking — no single column determines the right choice, and suitability depends on how the full profile matches a specific patient and defect.

| | OATS | ACI | MACI | ChondroFiller injection | |---|---|---|---|---| | Stages | Single (surgical) | Two-stage (surgical) | Two-stage (surgical) | Single (outpatient) | | Defect size | 1–4 cm² | ≥2 cm² | ≥2 cm² | Up to 6 cm² | | Tissue produced | True hyaline | Hyaline-like | Hyaline-like | Hyaline-like scaffold | | Highest evidence level | 10-year follow-up cohorts | 20+ years clinical use | SUMMIT Phase 3 RCT | Observational cohorts; no head-to-head RCT versus MACI | | Complication / reoperation | Donor-site morbidity | Up to 17% / up to 37% | Up to 17% / up to 37% | ~0% / 3–8% | | Recovery demand | Moderate; one procedure | High; lab interval between two operations | High; lab interval between two operations | Low; ultrasound-guided injection, no admission |

What the table cannot capture is the interaction between these dimensions and individual circumstances — age, surgical fitness, defect location, and whether a two-stage pathway is realistic all shift the balance considerably. That judgement requires imaging and a clinical assessment rather than any published decision matrix.

Consultants at MSK Doctors, with clinics in Sleaford and Grantham, assess cartilage cases without a GP referral; appointments are bookable at mskdoctors.com.

1. [1] Autologous chondrocyte implantation – Wikipedia. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150