Why there is no single best technique

Most patients arrive at a cartilage consultation having heard two or three technique names — MACI, OATS, OCA — without a clear sense of how surgeons choose between them. The honest answer is that no single technique is universally superior. Each has a defined role, and the selection follows a structured, evidence-based framework rather than individual surgeon preference.

Four variables drive that framework: the size of the defect, the condition of the underlying subchondral bone, the location of the lesion within the joint, and the patient's overall profile — age, activity level, and surrounding tissue health. These variables interact, and the decision branches accordingly: size is evaluated first, bone integrity next, then location and patient factors together.

One boundary applies to all four techniques equally. MACI, AMIC, OATS, and OCA are focal repair procedures. Each depends on healthy cartilage borders surrounding the damaged area — what clinicians sometimes describe as a contained 'pothole' in an otherwise sound road. Where damage has become diffuse and no healthy margins remain, none of these approaches can be applied; a different conversation about joint preservation or replacement becomes necessary.

The sections that follow map each branch of the decision in turn, so readers with different defect profiles can identify which considerations are most relevant to their situation.

Defect size: the first decision filter

The defect measurement that appears on an MRI report — typically expressed in square centimetres — is the first number a surgeon uses to narrow the options. The evidence base divides focal cartilage lesions into two broad categories: smaller lesions, broadly up to around 2 cm², and larger focal defects, generally from 3 cm² upwards. The zone between those figures is one of clinical judgement rather than a hard boundary.

For smaller defects, OATS and AMIC are the established approaches. OATS transfers osteochondral plugs harvested from a low-load area of the patient's own joint; it works well for lesions in the 1–3 cm² range, but beyond that the volume of donor tissue required becomes an unacceptable sacrifice from a healthy site. AMIC — which combines microfracture with a collagen membrane scaffold — is similarly suited to this smaller range, offering a single-stage option without the donor-site constraints of a plug harvest.

For larger focal defects, at or above approximately 3 cm², MACI is currently the best-supported cell-based choice. The SUMMIT randomised controlled trial showed that patients with lesions of 3 cm² or greater achieved meaningfully better KOOS pain and function scores with MACI than with microfracture, at both 2 and 5 years. That trial compared MACI against microfracture directly — not against OATS or AMIC — so cross-technique comparisons at this size remain limited by the available evidence.

OCA becomes relevant when defects are very large, when significant bone involvement is present, or when prior treatment failure places the case beyond what cell-based techniques can reliably address.

Subchondral bone: the binary branch that changes everything

Beneath the cartilage surface sits the subchondral bone plate — and its condition creates a binary branch in the decision tree, not a sliding scale.

When the bone bed is intact, the full range of resurfacing options remains available. MACI and AMIC can lay new tissue directly onto a sound foundation; OATS can anchor autograft plugs securely into healthy bone. The biology works because the repair tissue has somewhere structurally solid to adhere and integrate.

When significant subchondral bone loss is present — whether through cystic change, deep oedema, or 'kissing lesion' damage where opposing surfaces meet — that foundation no longer exists. Resurfacing cartilage on compromised bone is like tiling over a crumbling floor: no amount of surface repair compensates for structural failure beneath. In these cases the technique must restore both layers together. OATS with a full osteochondral plug addresses this for smaller affected areas; OCA — which uses a cadaveric bone-and-cartilage unit — is the appropriate choice when the zone is large.

Prior microfracture adds a specific complication here. Marrow-stimulation that has disrupted the subchondral bone plate can reduce eligibility for cell-based resurfacing later, including MACI. This is one clinical reason why microfracture is now viewed with increasing caution as a first-line choice rather than a reversible stepping stone.

Critically, bone integrity is established pre-operatively, not at the point of surgery. MRI sequences that capture subchondral oedema, cyst depth, and bone plate continuity form a core part of the workup — the technique decision is reached before the patient reaches theatre.

What each technique actually does — and where it excels

OATS

A plug of native hyaline cartilage and its underlying bone is harvested from a low-load area of the patient's own knee and press-fitted into the defect in a single operation. Validated for lesions in the 1–3 cm² range with an intact bone bed, published series report 79–100% good-to-excellent results across two to ten years. Donor-site morbidity — residual discomfort or deficit at the harvest zone — is a meaningful consideration discussed at the pre-operative planning stage.

AMIC

AMIC pairs microfracture with a bi-layer collagen I/III membrane that stabilises the regenerative blood clot and provides a scaffold for new tissue. Single-stage, no second anaesthetic. The 2017 evidence review found tentative short-to-medium-term benefit; however, long-term comparative data against MACI or OATS remain limited, and direct head-to-head trials at adequate scale have not yet been completed. It sits between marrow stimulation and full cell-based repair in the evidence hierarchy.

MACI

Chondrocytes are harvested at a first operation, cultured in a laboratory over several weeks, then implanted on a collagen membrane at a second — two anaesthetics and significant laboratory cost. For larger focal defects, MACI carries the most robust cell-based evidence of the four techniques. The two-stage burden is a meaningful practical factor whenever single-stage alternatives are also applicable for the defect in question.

OCA

Cadaveric donor bone and cartilage replace a large or structurally compromised defect in one surgical stage. It has the widest size reach of the four techniques and can succeed where prior procedures have failed. Published data show survivorship of 82.6% at five years and 69.6% at ten years — substantial durability for a salvage-range technique, though the measurable attrition over a decade is worth stating plainly when counselling patients.

One caveat applies across all four profiles: direct head-to-head trials comparing these techniques against one another remain sparse. Most comparative data — including the SUMMIT trial discussed in the previous section — use microfracture as the control, not the other three candidates here. Cross-technique ranking therefore rests on informed clinical judgement as much as on trial-level evidence.

Patient profile: who qualifies and what rules certain options out

Surgeons assessing a candidate for focal cartilage repair are essentially asking one question: is the biological environment capable of supporting a durable result?

The clearest yes comes from a younger adult with a single, contained lesion — a well-defined defect surrounded by healthy cartilage on all sides, with no advanced degeneration elsewhere in the joint. AAOS guidance consistently identifies this profile as the strongest indication for any of the four techniques. Healthy borders matter because they keep repair tissue localised and protected while it matures and integrates.

When that containment is lost — when damage has spread to multiple areas or the surrounding cartilage is too worn to provide a sound rim — none of the four techniques applies reliably. The clinical conversation shifts toward joint preservation or, where damage is truly diffuse, toward replacement.

Mechanical factors require specific attention. Malalignment, ligamentous instability, and obesity are not automatic disqualifiers, but each must be addressed before or alongside repair. A resurfaced knee that continues to load unevenly, or that buckles under instability, is unlikely to hold. Correcting alignment — via osteotomy where needed — is frequently planned as part of the same treatment episode rather than deferred as a separate intervention.

Treatment history also feeds into the assessment. Where a patient has had a prior marrow-stimulation procedure, the success rate of subsequent cell-based techniques such as MACI may be reduced; surgeons weigh this when choosing between two-stage and single-stage options at consultation.

Age is a guide, not a ceiling. Biological viability of the lesion and the quality of surrounding tissue matter more than calendar age alone.

What this means for your consultation

Armed with what the previous sections cover, the consultation can shift quickly from information-gathering to shared decision-making. A few practical steps help that happen.

Bring your imaging. Defect size in cm² and subchondral bone status — the two variables that immediately narrow the viable pathways — are both legible on MRI. If prior procedures have been performed (microfracture, drilling, or debridement), bring the operative record too: treatment history directly affects whether cell-based options such as MACI carry the same likelihood of success as they would in a treatment-naïve joint.

Questions worth raising at the first appointment. Is the subchondral bone intact? What is the measured defect size? Have prior marrow-stimulation procedures affected which techniques remain appropriate? Does alignment or ligamentous instability need addressing before or alongside repair?

Staging and cost belong in the shared decision. OATS, AMIC, and OCA each require a single surgical admission. MACI's two-stage structure — biopsy, weeks of cell culture, reimplantation — means two anaesthetics and meaningful laboratory cost on top of the second procedure. UK 2026 benchmarks place OATS at approximately £14,000 and OCA at approximately £28,000, with MACI running higher still. These figures are legitimate clinical variables, not secondary concerns to be raised after a technique has already been recommended.

Specialist assessment at MSK Doctors — available in Sleaford or Grantham without an NHS referral — is where defect size, bone status, treatment history, and personal priorities are weighed in combination. The more specific the questions a patient brings, the more that appointment can focus on what the imaging actually shows rather than on first principles.