The cartilage defects ACI is designed to treat

Articular cartilage — the smooth, load-bearing tissue lining the ends of bones inside a joint — has no direct blood supply. When it is damaged, the body has almost no capacity to repair it on its own. A superficial graze may stay stable for years; a deeper defect, left untreated, tends to widen and eventually exposes the underlying bone.

ACI is designed for a specific category of that damage: full-thickness defects graded ICRS 3 or 4, where cartilage is worn through to, or nearly to, the subchondral bone. The clearest clinical driver is defect size. For lesions smaller than roughly 2 cm², other techniques — marrow stimulation or osteochondral grafting — are generally tried first, in line with NICE guidance (TA477, 2017). Once a defect grows beyond that threshold, cell-based repair becomes the preferred approach; platform data confirm there is no upper size limit on what ACI can address, in contrast to the constraints that make simpler procedures inadequate for larger injuries.

Equally important is the nature of the joint as a whole. The technique is suited to focal damage — a discrete area of injury within a joint whose remaining cartilage is largely intact. It sits at the cartilage-restoration stage of the treatment pathway, after symptom management and biologic support have proved insufficient. Where cartilage loss has become diffuse, affecting an entire compartment, a different clinical conversation — about joint preservation or replacement — is appropriate instead.

Who is likely to be suitable for ACI

Most patients who go on to benefit from ACI share a handful of characteristics that a consultant will look for — and that can help you judge, before any appointment, whether a specialist assessment makes sense.

Age and activity level. The typical candidacy range is 13 to 55 years, reflecting the treatment's focus on joint preservation in people for whom staying active is a meaningful goal. That does not require elite-sport ambitions; anyone who wants to remain mobile and comfortable into later life falls within scope.

Body weight. A BMI below 35 kg/m² is a standard requirement. During the months when new cartilage tissue is consolidating, excessive joint load disrupts the repair. This is a mechanical argument, not a value judgement.

How long the joint has been symptomatic. Krishnan et al. (2006) identified symptom duration under two years as a positive prognostic factor. The longer a defect goes unaddressed, the more the surrounding joint environment — synovial fluid, adjacent cartilage, underlying bone — can deteriorate, narrowing the window for the best possible results.

Pre-operative function. Higher scores on validated function assessments before surgery are associated with better outcomes after ACI. In practice, earlier referral from a GP or physiotherapist tends to produce better starting conditions, and the evidence suggests that translates directly into better results.

Commitment to rehabilitation. ACI requires 6 to 12 months of supervised physiotherapy. The biology of the repair depends on protected, graduated loading during that period; the programme is part of the treatment, not a footnote to it.

When ACI is not the right option

Not every patient with a significant cartilage defect is a candidate, and the clinical reasons behind each exclusion are more instructive than the exclusion itself.

Established osteoarthritis affecting the wider compartment. ACI restores focal defects within a joint that is otherwise in reasonable condition. Where degeneration is diffuse — significant joint space narrowing, cartilage loss across a full compartment — there is no sound environment for the repair to integrate into. This is the clearest boundary between the cartilage-restoration and joint-replacement conversations.

Inflammatory joint disease. Rheumatoid arthritis is a contraindication because the immune process driving it does not spare implanted cells. Without controlling the source of inflammation, newly formed tissue faces the same attack as the original cartilage.

Uncorrected malalignment or ligament instability. A freshly implanted repair cannot survive in a mechanically unstable joint; abnormal load concentrations undermine the tissue before it matures. Where malalignment is the underlying problem, a corrective osteotomy can sometimes address it first — and in doing so, restore eligibility for ACI.

BMI ≥35 kg/m² and active smoking or chronic narcotic use are linked to impaired tissue healing and reduced capacity to complete the rehabilitation programme. The six-to-twelve month physiotherapy course is a functional part of the treatment, not an add-on; anything that undermines it puts the repair at risk.

Previous cartilage surgery. The most clinically significant variable here is prior microfracture. That technique creates channels into the subchondral bone to stimulate bleeding, and repeated or extensive marrow stimulation can disrupt the bone plate that ACI depends on as a structural foundation. A single prior microfracture in a joint whose underlying bone architecture remains intact is a meaningfully different starting point from one where the subchondral layer is already compromised — a distinction resolved by imaging at specialist assessment rather than by patient history alone.

How the two-stage ACI procedure works

The procedure unfolds in two planned surgical stages, separated by a six-to-eight-week laboratory interval that is as important to the outcome as either operation.

Stage 1 — harvesting the cells

A short arthroscopic procedure (keyhole surgery) takes a small sample of healthy cartilage — typically 200 to 300 mg — from a low-load area of the knee, most often the superomedial femoral trochlea or the lateral intercondylar notch. These are sites that carry little bodyweight during normal activity, so removing a small piece causes minimal functional disturbance. The sample is preserved at 4°C and dispatched to a specialist tissue-engineering laboratory the same day.

The laboratory phase

Over the following six to eight weeks, laboratory scientists enzymatically separate the chondrocytes from the surrounding matrix and culture them, growing a far larger population of the patient's own cells from the original small harvest. This expanded population is what makes it possible to fill a defect far larger than the donor site.

Stage 2 — implanting the repair

The second procedure is an open operation (arthrotomy). Surgeons prepare the defect, removing any damaged or sclerotic tissue to create a stable, clean boundary. In first-generation ACI, the cultured cell suspension is injected beneath a periosteal membrane stitched over the lesion and sealed with fibrin glue. In Matrix-Induced ACI (MACI) — the second- and third-generation approach — cells arrive pre-seeded onto a Type I/III collagen scaffold, which simplifies implantation and removes the need for a periosteal harvest from the shin. The SUMMIT trial provides mid-term RCT evidence supporting MACI outcomes at both two and five years.

STACi — the single-session evolution

STACi (Single-Treatment Autologous Chondrocyte Implantation) collapses both stages into one intraoperative session. The laboratory comes to theatre: chondrocytes are harvested and enzymatically processed during the same procedure, combined with bone-marrow-derived mesenchymal stem cells, and placed immediately onto a three-dimensional collagen scaffold fitted to the defect. As a next-generation technique, the evidence base is still accumulating — it is currently available at a small number of specialist centres.

Recovery timeline and realistic outcomes

Planning life around ACI starts with a single concrete milestone: protected weight-bearing begins almost immediately, but full, unrestricted loading of the repaired joint typically returns at around six to eight weeks after implantation. The weeks in between involve a careful progression — partial loading with crutches — designed to protect the repair while it begins to integrate.

The longer horizon matters more. Return to running and higher-impact sport is generally not before nine to twelve months, and some patients take longer. That is not a failure of the technique; it reflects the biological reality that the repair tissue needs time to mature and develop the mechanical properties of healthy cartilage. Rushing that process risks undermining an otherwise well-placed implant.

In published series, overall success rates sit at approximately 85%, rising to around 92% for isolated single-lesion repairs. These are aggregate figures drawn from clinical literature — no individual result can be predicted in advance, and a consultant assessment is needed to contextualise what they mean for any specific patient.

For patients in their thirties and forties, the most meaningful measure is often long-term: evidence supports joint preservation and a delayed, sometimes avoided, progression to replacement surgery. That sustained outcome is what distinguishes ACI from shorter-acting options at the same pathway stage.

Because ACI and its variants — particularly MACI and STACi — require specialist cell-culture infrastructure or intraoperative laboratory support, they are available at only a small number of UK centres. Early assessment at an experienced unit allows the logistics to be planned alongside the clinical decision.

Getting assessed for ACI

Whether ACI is appropriate for a specific joint cannot be determined from a checklist. Defect size, depth, surrounding cartilage quality, joint alignment, and the condition of the underlying bone all interact — and they can only be assessed through a specialist consultation supported by imaging, typically MRI.

For patients outside London, that assessment does not require a GP referral or a place on an NHS waiting list. MSK Doctors — a CQC-registered, consultant-led group — sees patients directly at two Midlands locations: Sleaford (NG34), where an Open MRI scanner and the Regeneration Hub are on site, and Grantham (NG31), which houses the MFO Life Sciences Lab alongside consultation and diagnostic services. London-based patients can be seen through the London Cartilage Clinic, the group's London arm.

A first consultation typically covers clinical history, a physical examination, and a review of any existing imaging — enough to clarify whether ACI, a related technique, or a different pathway stage is the appropriate next step.

Appointments can be booked directly, without referral, at mskdoctors.com.

1. [1] Autologous chondrocyte implantation – Wikipedia. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150