Which option fits which problem

The quickest way to sort these options is to ask what the defect involves. If the problem is a focal cartilage-only lesion, AMIC and MACI usually sit higher on the list; if there is cartilage plus underlying bone loss, especially after a post-traumatic injury, fresh OCA is often the more fitting route.

• AMIC is a single-stage repair that adds a collagen membrane to microfracture. Published knee series cluster around defects of roughly 2 to 8 cm², and a 10-year randomised study found AMIC remained more stable while microfracture deteriorated over time. That makes AMIC a reasonable option for medium-to-larger focal defects when surgeons want something potentially more durable than microfracture alone.
• MACI is a two-stage cell-based procedure: biopsy first, implantation later. It is generally considered for larger symptomatic focal cartilage defects when the subchondral bone is not the main problem and the rest of the joint is suitable. What changed from older ACI techniques was mainly the implantation step, with a simplified second operation rather than the older, more technically demanding approach.
• Fresh OCA is the main option when both surface cartilage and the bone beneath it need restoring, including salvage after failed cartilage surgery. Long-term series report about 79% survivorship at 10 years in a major post-traumatic tibial plateau cohort, and recent reviews cite 10-year survivorship in the 78% to 91% range.

No single technique is "best" in every knee. These procedures are aimed at focal defects in a repairable joint, not established diffuse osteoarthritis. Size, bone involvement, previous surgery, alignment, meniscal status, and age or activity demands all matter.

Why MACI changed ACI without making it one stage

MACI’s real advance was in how the cells are implanted, not in abolishing the classic ACI pathway. Earlier ACI generations made the second operation more technically demanding. MACI changed that second stage with a simpler implantation method.

Why surgeons moved in that direction is fairly straightforward. A 2020 review describes MACI as a two-stage procedure with a simplified surgical technique and quicker rehabilitation than earlier ACI generations, which fits the practical appeal of a cleaner second operation with less local tissue trauma. What it did not do was turn ACI into a one-stage treatment: there is still a cartilage biopsy first, lab expansion of the cells, and a later implantation. The safest takeaway is that MACI refined the implantation step within the same staged pathway, rather than removing the need for cell harvest and culture.

When AMIC is used instead of microfracture

In current practice, AMIC is usually considered when a focal knee defect looks too substantial for marrow stimulation alone, but a full two-stage cell-based pathway may be more treatment than the situation calls for. The published case mix helps show that distinction. In a 2023 multicentre series of 101 patients, AMIC was used for defects measuring 2 to 8 cm², with a mean size of 3.44 cm²; a 2024 systematic review across 18 studies reported a very similar mean defect size of 3.47 cm². That pattern places AMIC in a middle position: still a single-stage option, but one that uses a collagen scaffold to support the repair environment rather than relying on microfracture alone.

The main reason surgeons choose it over isolated microfracture is the longer-term pattern, not a dramatic difference in the first few months. In the 2024 randomised 10-year follow-up, both strategies improved during the first 2 years, but the microfracture group then showed progressive deterioration while the AMIC groups remained stable. A 2024 systematic review across 18 studies also reported clinically significant improvements in pain and function after AMIC. Even so, the literature is heterogeneous in scaffold type, lesion site and surgical approach, so the exact cut-off is still somewhat centre-specific. In other words, AMIC is best viewed as a bridge between simple marrow stimulation and more resource-intensive cell-based repair, not as a universal replacement for every cartilage lesion.

Who fresh OCA is most likely to suit

Fresh osteochondral allograft is usually reserved for the bigger structural problems: a larger focal defect, often post-traumatic, where the surgeon needs to replace subchondral bone as well as cartilage. That makes it a different proposition from cartilage-only repairs. Recent reviews place OCA in symptomatic focal chondral or osteochondral knee defects, and also in the salvage setting after an earlier cartilage procedure has failed; in a 2025 systematic review of secondary OCA, the mean defect size was 5.8 cm², which gives a sense of the scale at which autograft tissue may simply be insufficient. It is especially relevant when the lesion is deep, bone loss is part of the problem, or previous surgery has already narrowed the remaining options.

The long-term picture is encouraging, but it is not a lifetime guarantee. In a JBJS series of young, high-demand patients with large post-traumatic tibial plateau lesions measuring more than 3 cm in diameter and 1 cm in depth, graft survivorship was 79% at 10 years, falling to 64% at 15 years and 47% at 20 years. A recent review of knee OCA also cited overall 10-year survivorship in the 78% to 91% range. Taken together, those reports suggest that many grafts hold up well into the first decade, with durability then varying by lesion site and case mix.

Whether OCA is practical depends on more than defect size alone. Graft availability, lesion location, the amount of bone loss, and the burden of prior surgery all matter. That is why fresh OCA tends to suit carefully selected focal defects in a repairable joint environment, rather than being a like-for-like substitute for every cartilage lesion.

Why alignment and joint mechanics can change the plan

A scan can show the same 4 cm² defect in two different knees and still not produce the same surgical plan. The reason is that cartilage restoration is aimed at a focal problem in a joint that is otherwise worth preserving. Published AMIC series cluster around focal defects of roughly 2 to 8 cm², MACI is described for chondral lesions with minimal subchondral bone involvement, and fresh OCA is used for larger focal chondral or osteochondral defects when structural restoration is needed. Once the damage is more diffuse across a compartment, an isolated cartilage procedure is less likely to match the real problem.

Joint mechanics can shift the plan just as much as lesion size. If varus or valgus overload, meniscal deficiency, or instability means one compartment is still taking abnormal force, a graft or scaffold repair may underperform because the knee environment has not been normalised.

So two patients with a similar MRI report may still be offered different operations. A contained 4 cm² condylar lesion in a stable, well-aligned knee may be suitable for a focal repair alone; the same-sized lesion in an overloaded compartment may lead to a combined joint-preservation plan, or sometimes away from cartilage repair altogether.

What to ask before choosing a repair

Before consent is discussed, the most useful consultation questions tend to be these.

• Is this a cartilage-only defect, or an osteochondral problem with subchondral bone loss from trauma or an earlier operation?
• What is the measured size on MRI or at arthroscopy — for example in the 2 to 8 cm² range, or larger — and how much do location, containment, alignment and meniscus status change the plan?
• Has any previous microfracture or cartilage procedure changed the remaining options, particularly if a salvage route such as fresh OCA is being considered?
• Why does this knee suit a single-stage option such as AMIC, a two-stage option such as MACI, or a grafting option such as OCA?

The evidence is not equally precise at every threshold. MACI is best supported as a way of simplifying the implantation step within a two-stage ACI pathway; fresh OCA has useful 10-year survivorship data for larger structural defects; and AMIC appears more durable than microfracture over longer follow-up, although exact cut-offs still vary between published series.

The next step is a specialist assessment that matches symptoms, scans, joint mechanics and goals, with any consultation booking kept separate at londoncartilage.com.

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