Is ACI the right procedure for your knee?

ACI works best in a specific and relatively narrow set of circumstances — understanding those boundaries is the first practical step in deciding whether it belongs in your treatment plan.

The procedure is designed for focal, full-thickness cartilage defects: areas where the cartilage has worn completely through to bone, graded ICRS III or IV, and measuring more than 2 cm² in area. Crucially, the surrounding knee must be in good condition. Patients who are most likely to benefit are typically under 50, carry no significant osteoarthritic damage elsewhere in the joint, and have not previously had cartilage repair surgery on the same knee. NICE technology appraisal TA477 enshrines these criteria as the threshold for NHS funding.

ACI is not appropriate for diffuse or advanced osteoarthritis. Where cartilage loss is widespread rather than focal, the relevant conversation shifts to joint preservation or, in some cases, replacement — a different pathway entirely.

Conservative management also has to have been genuinely tried and found insufficient before ACI is considered. Physiotherapy, structured exercise, weight management, and analgesia are the expected first steps; a detailed MRI scan and specialist assessment then confirm whether the defect size and joint condition meet the threshold.

Finally, ACI is a two-stage procedure with a recovery measured in months, not weeks — patient motivation and the ability to commit to a structured rehabilitation programme are themselves clinical selection factors.

How the two-stage procedure works

The two-stage arc follows a logical sequence: first collect the raw material, then cultivate it, then implant it where it is needed.

Stage 1 — harvesting the cells

The first operation is arthroscopic and brief, typically lasting under 30 minutes. Under general or spinal anaesthetic, the surgeon uses a small camera-guided instrument to remove a thin sliver of healthy cartilage from a part of the knee that bears minimal load — usually the inner edge of the femoral trochlea. The sample taken is small, roughly the size of a pea, and the donor site heals without meaningful consequence. Before the procedure, patients undergo blood screening (including HIV, Hepatitis B and C, and HTLV) to satisfy laboratory safety requirements; NHS patients on the standard pathway have their harvested tissue sent to a specialist cell-processing facility in Germany.

The culture interval — growing the cell crop

In the laboratory, technicians use an enzyme to free the individual chondrocytes from the cartilage matrix without altering their biological character. Those cells are then placed into a controlled culture environment and allowed to multiply — at least 20-fold — over approximately four to six weeks. The delay is not a waiting-list issue; it is an intrinsic requirement of the biology.

Stage 2 — implanting the cultured cells

The second operation is open surgery. The surgeon prepares the defect by removing any damaged tissue back to a clean, stable cartilage rim and a sound bone base. A precisely shaped collagen membrane is sutured over the prepared cavity, forming a sealed pocket. The expanded cell population is then injected beneath that membrane to fill the lesion. The implanted chondrocytes anchor to the underlying bone within approximately 24 hours and, over the following months, progressively mature into new hyaline-like cartilage.

An emerging single-stage variant — STACi (Single Treatment ACI) — brings the cell-processing laboratory into the operating theatre, removing the culture wait entirely. It is not yet the standard NHS pathway, but represents the direction of travel in cartilage cell therapy.

Getting ACI on the NHS: what NICE TA477 means for you

Satisfying the clinical criteria is only part of the equation. The other part is navigating a tightly controlled NHS funding pathway — and that journey is not straightforward.

NICE issued technology appraisal TA477 in October 2017, formally endorsing ACI for NHS funding after roughly two decades of clinical trials largely led by the Robert Jones and Agnes Hunt Orthopaedic Hospital (RJAH) in Shropshire. The appraisal does not open the door broadly; it narrows it considerably. At an NHS list price of approximately £16,000 per patient, NICE concluded that the procedure reaches cost-effectiveness — below £20,000 per quality-adjusted life year — only in the well-selected subgroup who meet each criterion in the guidance. For many people referred with knee cartilage problems, the threshold will not be met.

The funding gate also specifies where treatment must take place. ACI is reimbursed only at a designated tertiary referral centre; RJAH is one of very few NHS hospitals in the UK currently approved to deliver it. That geographical constraint has practical consequences: patients may face considerable travel and, between Stage 1 and Stage 2, waits of several months while cells are cultured at an overseas processing facility — the NHS pathway currently routes harvested tissue to a laboratory in Germany.

Referral itself involves specialist assessment, MRI confirmation, and typically multidisciplinary review before funding is approved, adding time at every step. Patients and referring clinicians should ask the treating centre for stage-by-stage timelines, as these vary by site and are not published in aggregate NHS data.

For patients who do not meet TA477 criteria, or who prefer not to navigate NHS waiting times, the same procedure is available privately and alternative cartilage restoration techniques exist for different defect profiles. NHS eligibility does not define the full clinical picture.

What recovery from ACI actually looks like

Recovery from ACI is measured in months, not weeks — and the demands are consistent throughout. Understanding each phase before surgery helps patients commit fully rather than be caught off-guard.

Weeks 1–8: protecting the new matrix

For the first six to eight weeks, the knee carries no body weight. Patients move on crutches, wearing a hinged brace that limits load on the implanted cells while they anchor to the bone and begin to lay down new tissue. This period is non-negotiable: the forming cartilage matrix is mechanically fragile, and premature loading at this stage risks disrupting graft integration before it has taken hold.

Months 3–6: rebuilding movement

From around month three, the focus shifts to restoring full range of motion alongside a gradual return to daily activities and low-impact exercise such as cycling or swimming. Swelling, stiffness, and residual fatigue are common during this phase — they are a normal part of tissue maturation rather than signs of failure.

9–12 months: return to sport

High-impact activity — running, pivoting, sport-specific training — is typically cleared between nine and twelve months. Full athletic return is expected at around twelve months, once the new cartilage has matured sufficiently to tolerate repetitive loading.

Why the later phases matter most

Rehabilitation follows a recognised progression: restoration of daily function → metabolic conditioning → strength → neuromuscular control → sport-specific on-field work → return to play. The neuromuscular and on-field phases are where under-supervision is a well-documented clinical risk. Without appropriate guidance, patients may return to high-impact activity on immature cartilage that cannot yet distribute load correctly — raising the risk of graft stress, re-injury, and suboptimal long-term outcomes. Specialist physiotherapy across the full arc is not optional; it is part of the procedure.

How ACI performs over time — and how it compares to alternatives

Durability is the central question for any patient who is too young for joint replacement. The evidence on ACI — and on its third-generation scaffold-based form, MACI — is encouraging on this point.

A 2024 systematic review tracking 168 patients (mean age 37) with MACI at minimum ten-year follow-up found significant and durable improvements in patient-reported outcomes, satisfactory MRI defect fill in the majority of cases, an all-cause reoperation rate of 9%, and progression to total knee arthroplasty in only 7.4% at 10–17 years. For younger adults in whom early arthroplasty is clinically undesirable, those figures underline the procedure's role as a joint-preservation strategy — keeping a functioning knee in place for a decade or more.

How ACI compares to microfracture

A systematic review of six randomised controlled trials — covering 238 microfracture and 274 third-generation ACI patients, with defect sizes of 1.8–5.0 cm² and follow-up of two to six years — found treatment failure rates of 0–1.8% for ACI versus 2.5–8.3% for microfracture. KOOS scores favoured ACI significantly in four of the six studies.

The difference is partly biological. Microfracture stimulates fibrocartilage rather than hyaline-like cartilage; that tissue typically begins to break down at two to three years, and the procedure also disrupts the subchondral bone plate — compromising the environment for any future repair attempt. This is one clinical reason the NICE eligibility criteria exclude patients who have had prior cartilage surgery in the target knee: ACI failure rates are meaningfully higher when performed after prior marrow-stimulation procedures.

Emerging variants

Single-stage STACi and fourth-generation approaches such as HD-ACI and gel-type GACI are in active development and show early promise, but none has yet gained NHS adoption or the long-term outcome data available for MACI. They remain a next-generation conversation rather than a current NHS treatment choice.

The overarching goal of ACI — whichever generation — is to delay or avoid knee replacement for as long as possible in patients who have the most to gain from preserving their native joint.

Your next steps if ACI might be right for you

Establishing an accurate diagnosis is the logical first move. A specialist assessment — combining a detailed MRI to confirm defect size and cartilage-loss pattern with clinical examination of the surrounding joint — is what determines whether ACI sits within its evidence-supported criteria or whether a different cartilage repair pathway is the better fit. That distinction matters: a defect below 2 cm², meaningful osteoarthritic change, or a prior repair in the target knee each shift the clinical picture toward other options, and no amount of symptom history alone resolves those questions.

The same assessment that rules ACI in can also rule alternatives in, if needed — so attending with an open diagnostic question rather than a fixed procedure in mind generally serves patients better. A clear picture of the joint guides the conversation; it does not foreclose it.

For patients outside London, MSK Doctors offers consultant-led cartilage assessment at its Sleaford, Lincolnshire and Grantham sites without the need for a GP referral. Where clinically relevant, onMRI™ AI-driven MRI analysis and MAI Motion® objective biomechanical assessment can support that evaluation at initial consultation. London-based patients can access the same specialist-led pathway through the London Cartilage Clinic.

To arrange an assessment without a referral, visit mskdoctors.com.

1. [1] Third-Generation ACI versus Microfracture for Focal Chondral Defects of the Knee – Systematic Review of RCTs. (2022). https://doi.org/10.1016/j.arthro.2022.02.011 https://doi.org/10.1016/j.arthro.2022.02.011
2. [2] Minimum 10-Year Outcomes of Matrix-Induced Autologous Chondrocyte Implantation in the Knee. (2024). https://doi.org/10.1177/03635465231205309 https://doi.org/10.1177/03635465231205309