What actually decides the next step

The useful first question is not whether the knee needs AMIC, MACI or a graft. It is whether this is still a focal repair problem, or whether the joint has moved into diffuse, end-stage osteoarthritis. A symptomatic contained defect on a femoral condyle may still sit within a joint-preservation pathway, whereas widespread cartilage loss across the knee usually makes biologic repair unrealistic and shifts the discussion towards replacement. A 2024 review of knee cartilage treatment reflects that broader pattern: the decision starts with the damage pattern, not the procedure name.

Once the knee still looks biologically preservable, the defect itself drives the next step: surface area, depth, whether the edges are contained, the exact site in the knee such as the patella or trochlea, and whether the subchondral bone is involved. Those details help separate defects that may suit cartilage repair from defects that need cartilage-and-bone restoration. In a meta-analysis of 47 studies, ACI, MACI, OAT and OCA all improved pain and function overall, so the more useful distinction was defect size, depth, location and prior surgery rather than a single “best” technique.

The rest of the knee can still make or break preservation. Meniscus loss, varus or valgus alignment, ligament instability, previous procedures, age and activity demands all affect whether a repair has a fair chance. In that map, an unloader brace is usually a temporising offloading measure for compartment overload, grafting becomes more relevant when bone is involved or a prior repair has failed, and replacement becomes the realistic next step when the whole knee is no longer salvageable. A 2025 randomised trial in younger patients with medial knee osteoarthritis found high tibial osteotomy outperformed valgus unloader bracing at 12 months, which helps show when “buying time” stops being enough.

When an unloader brace genuinely helps

The practical role is narrower than many patients hope: an unloader brace can shift force away from one overloaded side of the knee and ease symptoms, but it does not regrow cartilage. A 2018 review covering 14 studies supported brace use mainly for unicompartmental knee osteoarthritis, with reduced pain, better function and quality of life, and in some cases a delay to surgery. That makes most sense when symptoms are being driven by a single overloaded compartment and the knee is still functionally preservable, rather than by widespread joint damage.

There is also a useful adjunct role around cartilage treatment. A 2014 pilot study noted that unloading braces are commonly used after tibiofemoral cartilage repair, although clear biomechanical unloading in normally aligned knees was not established, and a later randomised microfracture study reported thicker repair tissue in braced patients at mid-term follow-up. In that setting, the brace sits alongside rehabilitation and activity modification as a bridge, not a definitive fix. When pain, swelling or mechanical limitation continue despite offloading, especially in the setting of diffuse or end-stage change, that usually signals the need to reassess whether alignment surgery, cartilage restoration or knee replacement is the more realistic next step.

When a single-stage repair is enough

Once the decision has narrowed to a one-operation biological repair, AMIC has the clearest published footing in the knee. AMIC — autologous matrix-induced chondrogenesis — combines marrow stimulation with a collagen membrane, so it sits between simple microfracture and the more resource-heavy cultured-cell routes. In a 2024 systematic review of 18 studies covering 490 patients, AMIC was associated with significant improvements in pain and function, with a mean defect size of 3.47 cm². That makes it a practical single-stage option for a focal knee defect when avoiding two separate operations matters and the main problem is not a major bone-restoration issue.

The support for AMIC is not limited to early follow-up. A 10-year randomised study reported more sustained outcomes with AMIC than with microfracture for focal knee chondral defects, and a multicentre series of 101 patients treating 2 to 8 cm² osteochondral lesions found significant improvement at a mean 30 months. A 2-year matched comparison of MACI, AMIC and arthroscopic minced cartilage also found no significant difference in the reported pain and function scores between groups, which fits the broader 2024 review trend towards one-stage cartilage procedures.

STACi sits in a different evidence category. Clinic and company material describe it as a scaffold-based, single-treatment ACI-style approach using the patient’s own cartilage cells within a three-dimensional scaffold, aiming to avoid the classic two-stage ACI or MACI pathway. However, the independent published literature specifically naming STACi remains more limited and less standardised than the evidence base for AMIC, MACI/ACI, OATS or osteochondral allograft. For now, it is more accurate to treat STACi as an emerging single-stage concept than as an equally established knee option.

When MACI or ACI earns the extra stage

In practice, MACI or ACI tends to earn the extra stage when the problem is a larger, contained cartilage-only defect in a knee that is still worth preserving, rather than a small area that could be managed with a few osteochondral plugs. ACI is the classic two-step pathway: an arthroscopy to biopsy cartilage first, then a later implantation after the cells have been expanded. MACI keeps that same cultured-cell principle but places the chondrocytes onto a collagen membrane before implantation, which makes the second-stage delivery more straightforward than older ACI techniques.

What that extra time buys is the chance to treat a broader chondral surface without borrowing multiple plugs from elsewhere in the same knee. That remains relevant even as a 2024 review described a wider shift towards one-stage cartilage procedures. Reviews of ACI report durable long-term outcomes, so the added logistics can still be justified when the aim is joint preservation and the lesion characteristics favour a cell-based resurfacing approach rather than a small autograft transfer or a bone-restoring graft.

That said, two stages are not the automatic answer. A 2025 matched-pair study found single-stage minced cartilage implantation produced similar 24-month improvement and similar re-operation rates to ACI, and a separate 2-year matched comparison found no patient-reported outcome advantage for MACI over AMIC or minced cartilage. Sequencing also matters: prior marrow-stimulation is commonly treated as a caution before later ACI-type repair, although the sources provided here support that point more as a clinical warning than as a fixed numerical rule for every knee.

OATS or osteochondral allograft

A useful dividing line here is not one more generic size rule, but whether the knee problem can be filled with a few plugs or needs the whole osteochondral unit restored. OATS transfers the patient’s own cartilage-and-bone plugs in a single operation, and the strongest knee literature places it in the small, focal, contained niche — especially femoral condyle lesions. Technical reviews commonly describe defects of 3 cm² or less as the usual territory, with 4–6 cm² often treated as the upper end because donor tissue is finite and donor-site morbidity becomes a real trade-off.

Osteochondral allograft moves into the discussion when that plug logic starts to break down. Fresh donor graft can replace both surface cartilage and the underlying subchondral bone without borrowing more tissue from the same knee. In a narrative review, osteochondral allograft was described as the current single-stage standard for larger full-thickness chondral or osteochondral defects, often quoted as above roughly 2–4 cm², and it is particularly relevant when there is bone loss or a failed previous cartilage procedure. In a 2024 systematic review of secondary OCA after failed index surgery, the mean defect size was 5.8 cm², and larger 9–10 cm² defects had higher failure and reoperation rates.

Location still changes the practical choice. OATS literature is centred on focal femoral condyle lesions, where round, contained plugs make mechanical sense, whereas irregular, uncontained or revision defects are often less tidy. That is one reason the patellofemoral joint and other complex surfaces are usually judged more individually rather than by a fixed cut-off alone.

When repair gives way to osteotomy or replacement

Sometimes the decisive procedure is neither a membrane nor a graft. In a knee with clear varus or valgus compartment overload, the cartilage lesion may sit inside a broader "mechanical problem", so correcting alignment can matter more than debating one repair technique over another. In that setting, osteotomy is not a defeat for cartilage preservation; it is often the joint-preserving step that makes the rest of the plan make sense.

The limit of temporary offloading is clearest in younger medial-compartment disease. In a 2025 randomised trial of patients aged 18–65 with symptomatic medial knee osteoarthritis, high tibial osteotomy outperformed a valgus unloader brace at 12 months, with a KOOS pain difference of -28 in favour of HTO. That does not make bracing irrelevant. It shows that when symptoms persist and overload is structural, symptom relief alone may no longer be enough.

There is also a point where focal repair logic no longer fits. In "end-stage" or diffuse knee osteoarthritis, sources caution that biological repair is unlikely to restore the whole joint, so the next discussion often shifts to knee replacement rather than another focal repair attempt. A sound work-up therefore checks alignment, meniscus and ligament status, MRI-defined defect pattern, subchondral bone involvement, and whether the whole knee is still biologically preservable. Lincolnshire Knee, part of the MSK Doctors group, offers consultant-led assessment without referral. When that full-joint answer is no, moving to osteotomy or replacement is often the more honest next step.

1. [1] Osteochondral Allograft Transplantation as a Salvage Procedure After Failed Index Cartilage Surgery of the Knee: A Systematic Review. (2025). https://doi.org/10.1177/03635465241238466 https://doi.org/10.1177/03635465241238466