The one question that shapes everything: focal damage or widespread disease?

The most important question is not which procedure to choose — it is how much of the joint is affected in the first place. Everything else follows from that.

Think of it this way: a single pothole in an otherwise sound road can be filled and the surface restored. A road where the entire tarmac has crumbled needs resurfacing, not patching. Knee damage works the same way. An isolated, focal cartilage defect — a discrete area of damage in a joint that is otherwise healthy — can, in the right patient, be repaired using tissue-restoring techniques. Widespread, bone-on-bone osteoarthritis affecting the whole joint or multiple compartments is a different problem entirely; cartilage repair cannot address diffuse disease, and attempting it would leave the underlying cause untouched.

The difficulty is that patients often cannot tell which situation they are in. Localised pain does not always mean localised damage, and some people with large defects have surprisingly mild symptoms early on. Self-assessment is unreliable at this stage.

What confirms the picture is a specialist assessment combining clinical history and physical examination with MRI imaging, which maps the extent and depth of cartilage loss across the joint. That assessment is the clinical gate — it determines whether cartilage repair is even on the table before age, activity level, or technique choice become relevant.

The patient profile for cartilage repair

Age is the first filter, but not in the way people might assume. The headline figure — cartilage repair typically suits patients under 50 — is a starting point, not a hard cutoff. What matters alongside age is the combination of an active lifestyle, a focal injury rather than age-related joint-wide wear, and a joint that is otherwise in reasonable health.

Activity goals carry real clinical weight here. Patients who want to return to sport, physically demanding work, or pain-free movement without restriction have stronger grounds to pursue joint-preserving repair. The aim is to restore durable, functional tissue — not merely manage symptoms — and that goal makes most sense where the rest of the joint can benefit from it.

Timing is clinically meaningful. A focal injury addressed early keeps the full menu of repair techniques open; once surrounding cartilage degrades or the subchondral bone becomes involved, options narrow. The 45–55 age bracket is where the decision is most contested — patients in this range may still be candidates for restoration, but the threshold for what the joint can realistically support becomes central to any assessment.

Two further factors can change the picture significantly. Joint alignment matters because malalignment concentrates load on already-damaged cartilage, and correcting it — sometimes via osteotomy — may be needed alongside or before any restorative procedure. Prior marrow-stimulation surgery, including microfracture, is also relevant: it can alter the subchondral bone plate and may limit the options available for subsequent cell-based repair, which is one reason that a full surgical history forms part of any suitability review.

Cartilage repair techniques: how defect size guides the choice

'Cartilage repair' describes a family of quite different procedures — and understanding what separates them helps explain why specialist assessment is not optional.

Defect size, measured in square centimetres on MRI, is the primary clinical stratifier. Smaller focal lesions and larger osteochondral defects call for fundamentally different approaches.

Smaller defects (roughly 1–4 cm²)

For lesions in the 1–2 cm² range, osteochondral autograft transfer (OATS), also known as mosaicplasty, is a well-established single-stage surgical option. Healthy cartilage plugs are taken from a low-load area of the patient's own knee and transplanted into the defect. Mosaic configurations can extend coverage to around 4 cm², though donor-site morbidity — discomfort at the harvest site — is a meaningful consideration that forms part of the pre-operative discussion.

For focal defects where a minimally invasive, outpatient route is appropriate, ChondroFiller injection is an ultrasound-guided injectable collagen scaffold treatment. The acellular scaffold is placed directly into the defect under image guidance, gels in situ, and recruits the patient's own progenitor cells — a process called matrix-induced chondrogenesis (the scaffold prompts the body to generate repair tissue rather than delivering cells itself). It is a CE-marked Class III medical device, suitable for focal defects up to approximately 3 cm², with evidence extending to 6 cm² in some cases.

Medium to larger defects (2–10 cm²)

AMIC (autologous matrix-induced chondrogenesis) sits between basic marrow stimulation and cell-based repair in a single surgical stage: a collagen matrix is applied over a prepared defect to guide the formation of new tissue.

For defects in the 2–10 cm² range, MACI (matrix-induced autologous chondrocyte implantation) and first-generation ACI (autologous chondrocyte implantation) are two-stage, cell-based techniques. Both require an initial biopsy to harvest and culture the patient's own chondrocytes before a separate re-implantation procedure. The SUMMIT trial showed MACI produced improved KOOS pain and function scores compared with microfracture at two and five years in defects of 3 cm² or larger; ACI has long-term follow-up data extending beyond nine years.

A note on microfracture

Microfracture — once the default for smaller defects — stimulates bone marrow to fill the defect with fibrocartilage (a stiffer, less durable repair tissue). Current evidence indicates this fibrocartilage tends to break down within two to three years, and the procedure can alter the subchondral bone plate in ways that may restrict future repair options. It retains a role in selected cases but is no longer considered a first-line modern choice for most candidates.

Across all these techniques, selection is clinician-led, guided by defect size, depth, location, and the overall joint picture — not by any single factor alone.

When knee replacement is the right answer

Total knee arthroplasty (TKA) is, for the patients it is designed for, one of the most reliably effective operations in orthopaedic surgery — not a fallback, but the correct clinical destination.

The core indication is advanced, symptomatic osteoarthritis in which joint damage is extensive rather than focal: bone-on-bone disease affecting the whole or majority of the knee that has not responded adequately to physiotherapy, intra-articular injections, or other conservative measures. Patients in this position are typically over 60, though age alone is not the determining criterion — it is the extent and distribution of damage that makes cartilage-preserving approaches no longer realistic.

On durability, the evidence is reassuring: prostheses routinely function well beyond 15–20 years in appropriately selected patients, and the surgery carries a low complication rate. The procedure is well tolerated by the majority of people who undergo it.

The rehabilitation arc is significant and worth understanding in advance. Most patients return to daily activities — walking, light housework, driving — within approximately 3–6 weeks. Full functional improvement, including stair confidence and sustained walking distances, typically takes 6–12 months, and the improvement continues gradually across that period.

One realistic trade-off applies particularly to active patients: return to high-impact sport is limited after TKA. Running, jumping, and contact activities carry risks for the prosthesis over time, and consultants generally advise lower-impact alternatives. This is not a reason to avoid replacement where it is genuinely indicated — it is simply part of the honest, pre-operative conversation.

The irreversibility of TKA is also why younger patients with focal defects are assessed for cartilage restoration first: preserving the biological joint leaves this option available later, rather than foreclosing it.

How a specialist works through the decision with you

Most patients arrive at a consultation having already researched specific procedures — which is useful background, but the clinical assessment itself follows a structured sequence driven by the diagnosis, not by prior reading.

Step 1: Imaging to map the damage

MRI, usually alongside plain X-ray, is the starting point. It confirms whether damage is focal — confined to a discrete area — or diffuse across compartments, maps defect size and depth, and flags any bone involvement beneath the cartilage surface. The distribution of damage on imaging is the most powerful sorting variable: it determines which options remain clinically viable.

Step 2: Age, activity goals, and joint alignment

Two patients with similar MRI findings can have very different risk–benefit calculations depending on their age and what they want to return to. Cartilage restoration is specifically designed for patients who want to remain active at higher-impact levels; knee replacement limits those possibilities. Joint alignment is assessed separately — malalignment causing focal overload may require corrective surgery alongside, or ahead of, any cartilage repair.

Step 3: Has conservative treatment had a fair run?

Surgery is not first-line. Physiotherapy, intra-articular injections, and hydrogel bridging injections — which cushion the joint without restoring cartilage tissue — are all considered at this stage. These may delay a surgical decision where the clinical picture allows, but they do not substitute for structural repair when one is genuinely indicated.

Step 4: Matching the procedure to the findings

Once the full picture is established, the aim is to select the least invasive procedure that offers durable benefit commensurate with defect size and the patient's goals.

One honest limitation is worth naming: for patients aged roughly 45–60 where the choice between cartilage restoration and replacement is genuinely contested, current guidance rests largely on observational cohorts and expert consensus rather than head-to-head trials. At that particular decision boundary, specialist judgement and the patient's own priorities carry particular weight.

What to look for when choosing a specialist

Finding the right clinician matters as much as understanding the options themselves. The specialist best placed to navigate this decision is one who assesses both pathways — cartilage repair and knee replacement — and can explain why the evidence tips one way or the other for your specific imaging findings and goals. Relevant backgrounds include orthopaedic surgery with a knee sub-specialism, sports medicine, and MSK radiology for diagnostic staging.

Patients in the 40–60 age range, or those presenting with focal MRI findings, are particularly well served by a clinician experienced in joint preservation — someone who will not default to replacement simply because a patient is no longer young.

Search MSK lists specialists across the UK who offer cartilage repair, knee replacement, and joint preservation care; filter by region and specialty to find one whose experience fits your situation.

The core question to carry into any consultation is the one this article has kept returning to: is the damage focal or diffuse, and has it been identified early enough for a preserving approach to be realistic? Good imaging and experienced assessment answer that, and the rest of the decision follows from there.