Which option usually fits which knee problem

For a knee cartilage problem, the usual starting point is not the procedure name but the defect pattern: cartilage only, or cartilage plus bone. In 2021 cartilage guidance, lesion area around the 2–4 cm² range is one of the main sorting points. Contained cartilage-only defects often lead towards surface-based repair such as MACI, which is a two-stage knee procedure with biopsy first and implantation after lab expansion. Some centres also use STACi, here meaning scaffold-based autologous chondrocyte implantation; terminology varies, so it is best treated as a selected scaffold-based cell option rather than a fixed category. Once the knee defect includes subchondral bone loss, surface repair alone may be less suitable.

For osteochondral defects in the knee, OATS/OAT usually fits smaller, well-contained focal lesions, while OCA more often fits larger defects, post-traumatic craters, or revision cases where both cartilage and bone need restoring. Alignment can change the plan again: when valgus is overloading the lateral compartment, distal femoral osteotomy may be needed to protect a cartilage procedure, or may be the main joint-preserving operation. A systematic review covering 372 DFOs reported meaningful correction but a 34% reoperation rate, which helps explain why some knees need a combined or staged preservation plan rather than one isolated cartilage operation.

STACi or MACI for a contained knee cartilage defect

The practical fork here is simple: some newer scaffold-based autologous chondrocyte approaches aim to treat a contained knee cartilage defect in one operation, whereas MACI follows a planned two-stage route. The name “STACi” needs defining before any comparison, because it is not used consistently. In company material, STACi refers to “Scaffold-based Tissue-engineered Autologous Chondrocyte Implantation” using a 3D scaffold; in published literature from 2024, near-identical wording has also been used for a newer single-stage autologous chondrocyte coimplantation concept. That makes STACi better understood as a newer scaffold-based idea in the knee rather than one universally standardised procedure.

MACI, by contrast, is more clearly described and better established. HSS outlines an initial arthroscopic cartilage biopsy, around 1 month of cell expansion, and a later implantation of cells on a collagen membrane. For a contained chondral lesion without major bone loss, that extra step may still be acceptable when the priority is a defined technique with a firmer knee evidence base, rather than reducing everything to a single theatre episode.

The attraction of a STACi-type pathway is mainly practical. A prospective multicentre study of a single-stage autologous chondrocyte-based scaffold treatment reported significant pain and function improvement through 24 months; the commonest adverse events were arthralgia and effusion, and 2 implants were removed for delamination or adhesions. A 2025 matched-pair study also found no significant 2-year patient-reported outcome difference between MACI and other one-stage alternatives. Even so, the retrieved evidence still offers limited independent head-to-head data specifically comparing STACi with MACI. For that reason, the choice usually comes down to evidence maturity, surgical logistics, and how important it is to avoid a second knee operation.

OATS or OCA when the defect includes bone

Once a knee lesion becomes an osteochondral defect rather than a cartilage-only defect, the decision changes with it. In a post-traumatic crater, or where MRI shows clear subchondral bone loss, a surface-based option such as MACI may be structurally insufficient because it restores the cartilage surface but not the damaged bony bed. OATS/OAT and OCA are different in that they move cartilage with attached bone, so they are considered when the base of the defect is part of the problem.

At the smaller end of the spectrum, OATS usually sits further forward. It transfers one or more osteochondral plugs from a lesser weight-bearing part of the same knee, and knee sources consistently place it in the small focal lesion range rather than the large-defect range. One textbook-style source describes suitability for well-contained defects under 4 cm², while one specialist rule of thumb uses under 2 cm². Those figures are guides rather than a universal cut-off, but they point in the same direction: autograft supply is limited, and taking plugs from the patient’s own knee brings a donor-site trade-off.

As the defect gets larger, deeper, or more obviously bone-involving, OCA becomes more attractive. Review-level knee literature describes osteochondral allograft transplantation as a single-stage option for large chondral or osteochondral defects, especially in post-traumatic lesions, defects with major osseous involvement, or after failed previous cartilage surgery. A modern review reports overall graft survivorship of roughly 78% to 91% at 10 years. The clearest way to frame the choice is therefore as a continuum, not a rigid threshold: smaller, contained osteochondral lesions tend towards OATS, while larger or salvage-type defects with bone loss tend towards OCA. A further practical issue, although not quantified in this evidence packet, is access to suitable donor graft tissue for OCA.

When valgus alignment matters more than the graft

A lateral compartment cartilage graft can struggle if the knee keeps loading the wrong side. Reviews in Annals of Joint and Musculoskeletal Key note that increasing valgus shifts load into the lateral compartment, and lateral cartilage defects become more common as valgus rises. In that setting, the main problem may be less a single surface defect and more a mechanical pattern that keeps driving damage across the same part of the knee.

That is why distal femoral osteotomy (DFO) can sit at the centre of the plan rather than at the edge of it. In recent knee-preservation literature, DFO is described as a joint-preserving option for young, active patients with symptomatic valgus knees and isolated lateral compartment disease. When the deformity originates in the distal femur, femoral correction is generally preferred, because a tibial correction can create joint-line obliquity and alter contact mechanics. In some knees, DFO supports a cartilage restoration procedure; in others, correcting the axis is the more important preservation step.

The published results are encouraging, but this is not a minor add-on. A systematic review of 16 studies covering 372 osteotomies reported correction towards a near-neutral mechanical axis, and midterm review-level evidence found improvement in knee health-related quality of life. The same review, however, reported 3.2% nonunion, 3.8% delayed union, a 9% complication rate, 34% reoperation, and 15% conversion to arthroplasty. So in the valgus knee with lateral cartilage damage, the key decision is often whether the knee is failing because of the defect alone, or because malalignment keeps overloading it.

Can these treatments be done together or in stages

Where both valgus overload and a focal cartilage defect sit in the same knee, the useful rule of thumb is simple: a combined plan is easiest when the cartilage procedure is already a one-operation treatment. Review literature describes OCA as a single-stage option for larger chondral or osteochondral defects, and DFO is the operation used to correct a valgus knee, so those problems may sometimes be addressed in the same hospital admission when the defect pattern allows it.

MACI changes that practical picture because its pathway is built in stages. HSS describes a biopsy first, then implantation of the lab-grown membrane about 1 month later. If a DFO is also needed, the plan may therefore be sequenced rather than bundled into one day in theatre. That is not automatically a disadvantage. In a knee with more than one problem, staging can be a deliberate way to correct alignment first, or to combine the biopsy with other arthroscopic work, before returning for the cell implant.

The memorable decision point is mechanical: when distal-femoral valgus is clearly overloading the lateral compartment, alignment cannot be an afterthought, because cartilage repair in the wrong load environment may struggle. When malalignment is modest and the defect itself is the main issue, a one-stage restorative plan is more plausible. In real life, that often means weighing one bigger operation against two separate interruptions to work, sport and day-to-day life.

When knee preservation is unlikely to be the right route

Not every painful knee is a preservation case. In the knee literature, the procedures discussed here are mainly framed around focal, "contained" or "isolated" problems: a defined cartilage lesion, an osteochondral defect, or a valgus knee with isolated lateral compartment overload. Reviews in Annals of Joint and recent OCA papers do not describe these operations as a fix for cartilage loss spread through the whole joint. When damage is widespread across more than 1 compartment, when there is major inflammatory joint disease, or when a compartment has effectively reached end-stage failure, repairing one area may be less realistic than symptom control or replacement planning.

The practical work-up is therefore about sorting the pattern before naming the operation. That means checking whether the MRI shows a true focal defect or subchondral bone loss, whether alignment is contributing to load, and whether the symptoms actually match the structural finding. A 2023 review on distal femoral osteotomy and a broader systematic review of cartilage procedures both place patient selection at the centre of decision-making. A specialist knee assessment is what separates a repairable focal problem from a knee better managed with conservative care, preservation surgery, or arthroplasty planning.

1. [1] Single-Stage Autologous Chondrocyte-Based Treatment for the Repair of Knee Cartilage Lesions: Two-Year Follow-up of a Prospective Single-Arm Multicenter Study. (2020). https://doi.org/10.1177/0363546520912444 https://doi.org/10.1177/0363546520912444