Why the number of operations matters

For many patients considering knee cartilage repair, the first practical question is straightforward: will this mean one operation or two? The answer shapes how you plan time off work, how much the treatment costs, and how long you wait before rehabilitation can begin.

Some cartilage repair techniques are two-stage procedures. In the first operation, a small sample of healthy cartilage cells is taken from your knee. Those cells are then sent to a specialist laboratory and cultured — a process that typically takes three to six weeks. A second operation, under a separate general anaesthetic, is then needed to implant the expanded cells into the damaged area. The waiting period is not a delay in the usual sense; it is built into the biology of the technique.

Single-stage procedures complete the repair within one operation. How they achieve this varies: some avoid cell culture entirely by using a scaffold combined with the body's own bone marrow signals; others bring laboratory processing directly into the operating theatre. The route is different for each technique, but the result is one anaesthetic, one recovery, and no inter-stage waiting period.

It is worth noting that staging is primarily a logistical distinction. It does not, by itself, determine how durable or biologically sound the repair tissue will be — that depends on defect size, the technique chosen, and individual patient factors. Both approaches carry a meaningful evidence base, and the right choice depends on clinical assessment rather than a preference for fewer operations alone.

ACI and MACI: what two-stage repair involves

Traditional ACI begins with an arthroscopic biopsy: a 200–300 mg cartilage specimen is taken from a non-weight-bearing area of the femoral trochlea, sealed, and sent to an external laboratory. Over three to six weeks, the harvested chondrocytes are expanded in culture — at which point the patient returns for a second operation under a separate general anaesthetic.

At that second stage, the cultured cells are implanted into the prepared defect. In the original first-generation technique, they were secured beneath a periosteal flap harvested from the tibia. MACI (matrix-induced ACI) replaced that flap with a porcine collagen I/III membrane onto which the expanded chondrocytes are seeded beforehand, simplifying fixation and removing the need for a separate periosteal harvest. The fundamental two-stage biology — biopsy, laboratory expansion, reimplantation — remains unchanged, and two general anaesthetics separated by a laboratory waiting period remain an inherent feature of this pathway.

The SUMMIT randomised controlled trial provides the strongest published RCT evidence for cell-based repair: MACI produced meaningfully better KOOS pain and function scores than microfracture at both two and five years in patients with defects of 3 cm² or larger. MACI carries FDA approval for symptomatic full-thickness knee defects in adults. Longer-term data — from a cohort followed to a mean of 11.3 years after matrix-associated ACI — showed that male sex (OR 3.1) and a BMI of 20–29 (OR 3.9) were independently associated with achieving the Patient Acceptable Symptomatic State, a reminder that individual characteristics shape long-term trajectory alongside technique.

Standard MACI's cell density is relatively low, which may yield softer repair tissue than native hyaline cartilage. High-density ACI (HD-ACI) targets this by delivering 5 million chondrocytes per cm², with animal models demonstrating more hyaline-like tissue formation — a distinction that becomes relevant when weighing technique options for different defect sizes.

AMIC: single-stage scaffold over a marrow clot

AMIC takes a different route to single-stage repair entirely — one that requires no cells, no culture, and no laboratory involvement at all.

The procedure begins with microfracture: small perforations are made in the subchondral bone beneath the cartilage defect, releasing bone marrow progenitor cells that pool over the damaged area and form what is sometimes called a 'super-clot'. Microfracture alone is the older, simpler technique — but the repair tissue it produces tends to be fibrocartilage, which lacks the structural resilience of native hyaline cartilage and may break down with sustained loading. That is the limitation AMIC addresses.

The critical addition in AMIC is the application of a bi-layer collagen I/III membrane directly over the clot — in the same operation, without closing and reopening the joint. The membrane stabilises the marrow clot, protects it from being washed out, and provides a scaffold that guides the body's own progenitor cells towards more organised chondrogenesis. The patient's marrow does the repair work; the membrane provides the structure to make that possible.

A 2023 meta-analysis of 24 studies found that arthroscopic and mini-open AMIC produce equivalent IKDC and KOOS scores overall, though the arthroscopic approach was associated with lower post-operative VAS pain scores while the open approach achieved higher MOCART cartilage-fill scores — suggesting that technique selection should be guided by surgeon expertise and lesion location rather than by a blanket preference for either route.

Evidence from an adolescent cohort adds a useful clinical anchor. In 27 patients aged 12–19 with ICRS grade III–IV lesions (mean defect 2.3 cm²), AMIC produced statistically significant KOOS improvements across all domains at a mean of 2.6 years: pain scores rose from 58% to 87%, quality-of-life from 31% to 71%, and sports and recreation from 29% to 73% — though some patients remained symptomatic, reflecting the technique's real-world variability.

STACI: single-stage cell-based repair without waiting

The logical question STACI raises is direct: if the goal of ACI is to deliver healthy, culture-expanded chondrocytes into a defect, what exactly does the inter-stage wait achieve — and what is lost by removing it?

In traditional ACI and MACI, the waiting period exists because chondrocyte expansion happens in an external laboratory over several weeks. STACI (Single Treatment Autologous Chondrocyte Implantation) dismantles that dependency by bringing the laboratory team directly into the operating theatre. Everything that would otherwise happen off-site is performed within the same surgical episode.

The intraoperative sequence works as follows: cartilage is harvested from a non-weight-bearing area of the knee, cut into fragments, and enzymatically digested to free the chondrocytes — a process designed to liberate cells without altering their character. Simultaneously, bone marrow is aspirated from the same surgical site; a laboratory technician concentrates and isolates the mesenchymal stem cells (MSCs), checks them under the microscope, and combines them with the freshly released chondrocytes. That combined cell population is then seeded onto a collagen scaffold and implanted, all before the patient leaves theatre.

The biological rationale — repairing cartilage with the patient's own chondrocytes rather than relying on marrow progenitors alone — is preserved. What is removed is the second general anaesthetic, the external laboratory culture period, and the associated cost burden. One health economic model estimated two-stage ACI at approximately €29,741 versus around €11,797 for a single-stage cell-based approach.

Early matched-pair evidence is encouraging: single-stage minced cartilage implantation — a closely related technique — achieved comparable IKDC scores to two-stage ACI at 24 months (74.3 versus 71.3), with similar re-operation rates. Taylor and Lee (2019) described STACI as the next generation of ACI, reflecting its position as an evolution rather than a departure from established cell-based principles.

However, long-term follow-up data for STACI specifically remain limited, and two-year matched-pair results should not be read as confirmation of long-term equivalence to the established two-stage evidence base. STACI is best characterised as a promising emerging technique — biologically coherent, logistically leaner, but awaiting the depth of outcome data that supports two-stage ACI and MACI.

How surgeons decide: defect size, patient factors, and centre capability

Defect size sits at the top of the decision hierarchy. Lesions smaller than roughly 2 cm² can reasonably be managed with microfracture or OATS; once a defect exceeds 2–4 cm², the evidence consistently favours cell-based or scaffold-augmented repair. The mechanical reason is straightforward: marrow stimulation alone produces fibrocartilage, which lacks the compressive strength of native hyaline cartilage and tends to break down under the repetitive loading a knee generates — the biological rationale for escalating to AMIC, ACI, MACI, or STACI in larger defects, as the SUMMIT trial confirmed for MACI at 3 cm² and above.

Prior treatment history is the next lever. Patients in whom an earlier microfracture has failed carry a stronger indication for cell-based repair; chondrocytes harvested from previously stimulated bone behave differently, and a compromised subchondral plate narrows subsequent options. When bone is involved — a true osteochondral lesion rather than a purely chondral one — bone grafting may be needed alongside the cartilage procedure, adding complexity that can shift the choice of technique or require staged surgery.

Patient factors refine the decision further. At a mean 11-year follow-up after matrix-associated ACI, male sex (odds ratio 3.1) and BMI in the 20–29 range (odds ratio 3.9) emerged as significant positive predictors of reaching a patient-acceptable symptomatic state — illustrating how body composition and sex interact with long-term outcomes, not just the surgery itself. Younger, active patients with focal defects have the most to gain from hyaline-like tissue restoration; in older patients or those with more diffuse joint changes, the calculus shifts.

Finally, centre capability is a legitimate selection variable. STACI requires a laboratory team present in theatre; standard MACI and ACI require an accredited external cell-culture facility. Where those resources are unavailable, AMIC represents the practical single-stage alternative that avoids cell culture entirely. No direct head-to-head randomised trial yet compares all four approaches, which means surgeon expertise and institutional infrastructure remain meaningful — and honest — parts of the equation rather than shortcomings in the evidence.

Getting a cartilage assessment in Lincolnshire

Choosing between the approaches outlined above depends on imaging and biomechanical data that no consultation can reliably generate without them. Before any surgical planning, an MRI — ideally with cartilage-sensitive sequences such as T2 mapping or cartilage segmentation — establishes defect size, depth, and ICRS grade: the three variables that, as the preceding sections show, determine which technique is even on the table. Where alignment is suspected to play a role, a biomechanical assessment can identify load distribution patterns that would need correction alongside cartilage repair to give any procedure a reasonable chance of lasting.

Lincolnshire Knee is part of the MSK Doctors group and offers specialist knee consultation alongside Open MRI at its Sleaford (NG34) and Grantham (NG31) sites. Patients can self-refer — no GP letter is required — which removes one of the more common delays between a decision to seek a second opinion and actually getting the imaging and clinical assessment needed to act on it.

Appointments and further information are available at lincolnshireknee.co.uk.

1. [1] Autologous chondrocyte implantation. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150
2. [2] Single Stage Autologous Cartilage Repair Results in Positive Patient Reported Outcomes For Chondral Lesions of the Knee: A Systematic Review.. (2023). https://doi.org/10.1016/j.jisako.2023.05.003 https://doi.org/10.1016/j.jisako.2023.05.003
3. [3] Efficacy of one-stage cartilage repair using allogeneic MSCs and autologous chondron transplantation (IMPACT) vs nonsurgical treatment: RCT protocol. (2020). https://doi.org/10.1186/s13063-020-04771-8 https://doi.org/10.1186/s13063-020-04771-8
4. [4] Autologous matrix-induced chondrogenesis. https://en.wikipedia.org/?curid=29760859 https://en.wikipedia.org/?curid=29760859
5. [5] Cartilage Defect Treatment Using High-Density Autologous Chondrocyte Implantation (HD-ACI). (2023). https://doi.org/10.3390/bioengineering10091083 https://doi.org/10.3390/bioengineering10091083