What usually fits which problem

In practical terms, defect size and complexity often narrow the options quickly. For a small, isolated osteochondral defect, OATS or mosaicplasty is often the one-stage autograft choice because it moves cartilage and bone from the same knee; published references commonly place it in roughly the 1–4 cm² range, with donor-site morbidity limiting how much graft can be taken. When the defect is larger, post-traumatic, or includes meaningful bone loss, OCA is more often considered because cadaveric donor graft avoids that supply problem and can be matched to a bigger defect.

The single-stage versus two-stage decision is less clear-cut. AMIC combines marrow stimulation with a collagen membrane in one operation, whereas MACI requires a biopsy first and implantation later. Direct comparison data are mixed rather than decisive: a 2022 systematic review suggested AMIC may have better mid-term results at about 40 months, but a 2025 matched-pair study found no significant 2-year difference between AMIC and MACI. MACI does, however, have longer durability data, with case-series follow-up reported at about 15–17 years.

That is why lesion size, location, bone involvement and overall candidacy usually matter as much as the procedure name. Because cartilage damage may not show on X-ray and MRI is central to defining a focal lesion, cartilage-focused specialist assessment is often worthwhile before restorative surgery is chosen.

OATS or OCA for a cartilage defect

The key fork in the road is where the replacement plug comes from. In knee cartilage repair, OATS or mosaicplasty uses osteochondral plugs from the same knee, whereas OCA uses donor osteochondral tissue. That source difference usually drives the indication more than any abstract idea of one operation being the "best". Published thresholds are not identical, but they point in the same direction: one review places smaller lesions at under 2 cm² in the OAT range and larger defects at over 4 cm² in the OCA or ACI range, while a 2024 review snippet suggested OATS may be preferable for isolated full-thickness lesions of 1.44 cm² or less.

With OATS, the attraction is obvious: it is a one-stage procedure using the patient’s own cartilage and bone, so there is no donor graft to source. The limiting factor is supply. Only a modest amount can be taken from non-weight-bearing areas, and donor-site morbidity remains a real concern, which is why references commonly keep mosaicplasty/OATS within about 1–4 cm². In practice, that makes it better suited to a small, focal defect than to a broad area of damage.

OCA changes that trade-off. Because the graft comes from cadaveric donor tissue, there is no autograft harvest penalty, and the graft can be shaped or size-matched for a defect that would simply exhaust an autograft supply. That makes OCA more practical when the problem is not just cartilage wear on the surface, but a larger osteochondral lesion, post-traumatic damage, or a defect with meaningful bone involvement. The compromise is different rather than worse: OCA expands what can be reconstructed, but it depends on suitable donor graft availability and matching.

Single stage or two stages

One operation versus two is often the most practical dividing line. AMIC keeps everything in a single sitting by combining marrow stimulation with a collagen membrane, so it is commonly positioned between plain microfracture and more resource-intensive cell-based repair. MACI takes the longer route: cartilage is biopsied first, the cells are expanded, and implantation happens later. The FDA prescribing information for MACI notes that the interval between biopsy and implantation depends on the number and quality of cells obtained, which underlines why it is a staged pathway rather than a same-day repair.

That added complexity does not automatically translate into better early results. A 2022 systematic review found that AMIC may produce better mid-term outcomes than mACI at about 40 months, but it also said the optimal treatment remained controversial and that further studies were needed. More recently, a 2025 matched-pair study comparing 16 MACI, 16 AMIC and 16 minced-cartilage cases reported significant improvement in both the AMIC and MACI groups, with no significant 2-year difference in pain or functional scores between them. Taken together, the head-to-head evidence looks mixed rather than decisive.

MACI’s main strength is the longer track record. In a case series with roughly 15- to 17-year follow-up, outcome scores remained improved and 12 of 14 patients rated knee function as better or much better than before surgery. Single-stage approaches still have obvious practical appeal because they avoid cell expansion and a return to theatre, and economic modelling suggests that can improve cost-effectiveness if results are comparable. STACI belongs in that same single-treatment ambition, but at present it is better viewed as an emerging option than a settled alternative, so the live choice in most knee cases remains AMIC-style simplicity versus MACI’s deeper long-term evidence base.

Who is a good candidate for cartilage repair

Not every painful knee sits in the cartilage-repair lane. AAOS describes the clearest candidates as younger adults with a single, symptomatic focal lesion and otherwise fairly preserved joint surfaces, not people with widespread wear across the whole knee. In practice, the pathway usually starts with symptom management, may include injection-based or biologic support as an adjunct, then moves into cartilage restoration when there is a repairable focal defect; once damage is diffuse or end-stage, off-loading strategies or joint replacement may be more realistic than a focal repair. That distinction matters because a graft, membrane or cell-based implant is trying to solve a local problem, not reverse established osteoarthritis throughout the joint.

The next filter is the imaging and the broader mechanics. AAOS notes that cartilage damage may be hard to see on a plain X-ray, while MRI is central for showing the defect itself; X-rays still help show arthritis, alignment and the wider joint picture. Published reviews also suggest that candidacy depends on more than symptoms alone: lesion size, location, whether there is subchondral bone involvement, and the pattern of damage all change the sensible options. One literature review, for example, placed smaller lesions under about 2 cm² in a different treatment bracket from larger defects over 4 cm², with OCA particularly useful for large post-traumatic or osteochondral defects. That is why focal repair tends to suit a specific type of knee problem rather than every degenerative knee.

When specialist input matters most

An MRI report that simply says “cartilage defect” is often where specialist input starts to add the most value. This matters most when pain, swelling, catching or locking continue after an injury, or when the first assessment has not explained why sport, stairs or daily activity are still limited. AAOS also advises prompt medical review after a “pop”, marked swelling, severe pain, inability to move the knee properly, or the knee giving way. In that setting, a cartilage-focused opinion is not mainly about fast-tracking surgery; it is about confirming whether the problem is truly a focal cartilage injury and whether restoration is even the right lane.

What changes at that appointment is the match between the defect and the treatment. A lesion around 1–2 cm² in an otherwise healthy knee can sit in a very different category from a defect above 4 cm², a post-traumatic osteochondral injury, or damage with subchondral bone involvement. Reviews have placed smaller lesions closer to OATS/OAT-type solutions, while larger or post-traumatic defects often push consideration towards OCA or, in some knees, away from focal restoration altogether. The practical value of specialist review is that the same scan finding may lead to repair, a combined plan, or no restorative procedure once the wider knee picture is taken into account.

That is also why some patients seek a clinician or service that deals with cartilage restoration regularly, rather than stopping at a generic “knee pain” label. The aim is usually clarification: defect size, likely grade, bone involvement, candidacy, and whether realistic options are symptom management, cartilage preservation, or a different pathway entirely. Seeing a specialist does not commit a patient to OATS, OCA, AMIC or MACI; it narrows the choices. Search MSK lists specialists across the UK who assess knee cartilage problems, with filters by region and specialty.

Questions to ask before choosing a surgeon

The most useful consultation leaves a clear reason behind the recommendation, not just the name of an operation. Published evidence suggests that choice can shift with defect size, subchondral bone involvement and the practical differences between OATS/OCA and cell-based options such as AMIC or MACI; the direct AMIC vs MACI evidence is still mixed at roughly 40 months and again at 2 years.

• “Which cartilage procedures do you actually offer — OATS, OCA, AMIC, MACI or others — and how do you decide between autograft, allograft and cell-based repair?”
• “How do lesion size, bone loss, previous surgery, arthritis, meniscal damage or alignment problems change the plan, and what findings would make cartilage restoration the wrong option?”
• “Is the current MRI enough, or is further imaging needed, and might this need a combined procedure rather than cartilage repair alone?”
• “What outcomes are realistic in cases like this, what does rehabilitation usually involve, and if the first procedure fails, what are the revision options?”

Used as a comparison tool rather than a sales pitch, that checklist makes it easier to judge cartilage-focused practice across the UK; Search MSK lists specialists who assess knee cartilage damage and offer these treatments, with filters by region and specialty.