Which is right for your knee — repair or replacement?

If you have been told you might need knee surgery, the first question is rarely about which operation to book — it is about what is actually wrong with your knee. Cartilage repair and knee replacement are not two points on the same scale of severity; they address fundamentally different problems.

Cartilage repair is designed for focal chondral defects — localised areas of damage in a knee that is otherwise structurally sound, with healthy bone, intact ligaments, and cartilage that is still viable in the surrounding joint surfaces. Knee replacement, by contrast, is indicated when osteoarthritis has spread across one or more compartments of the joint, destroying the articular surface broadly and leaving inadequate tissue for biological restoration.

The deciding factor is therefore the pattern of damage — focal versus diffuse — not pain intensity alone. A patient in considerable pain from a discrete traumatic lesion may be an excellent candidate for repair, while someone with milder but widespread bone-on-bone disease may require replacement far sooner.

Underpinning cartilage repair is the principle of joint preservation: using biological and surgical techniques to restore the native knee rather than replace it, particularly in active adults who are typically under 55. Crucially, choosing repair does not foreclose future options. If a cartilage procedure eventually loses ground to advancing arthritis, knee replacement remains available. The reverse is not true — once a replacement is fitted, the biological joint is gone permanently. For the right patient, repair is not a compromise; it is the more complete clinical strategy.

The cartilage repair options and what each one does

Five distinct techniques sit within the repair spectrum, each suited to a different defect size and clinical profile.

Microfracture drills small channels into the subchondral bone to draw a blood clot into the defect and stimulate fibrocartilage growth. Historically the most widely used approach for lesions under 2 cm², it is low-cost and straightforward — but the repair tissue is mechanically weaker than native hyaline cartilage, and outcomes frequently deteriorate after two to three years. Repeated microdrilling also damages the subchondral bone plate, which can compromise the success of any subsequent procedure. Its role in current practice is declining.

AMIC (autologous matrix-induced chondrogenesis) places a collagen scaffold over a standard microfracture site to stabilise the clot and support better-quality repair tissue. It is a single-stage procedure; registry data report mean defect sizes of around 3.4 cm² in patients averaging 37 years of age, with significant pain and function gains at one and two years postoperatively.

OATS / mosaicplasty transplants one or more osteochondral plugs — bone with its own intact hyaline cartilage — from a lower-load zone of the knee into the defect. Best suited to lesions of 1–2 cm² (with the mosaic technique extending to around 4 cm²), it offers genuine hyaline cartilage at the repair site. Donor-site morbidity at the harvest area is a meaningful consideration when planning this approach.

ACI / MACI are two-stage cell-based procedures designed for defects in the 2–10 cm² range. The patient's own chondrocytes are harvested, cultured, and reimplanted on a collagen membrane. The SUMMIT randomised trial found that defects of 3 cm² or larger treated with MACI achieved significantly better KOOS pain and function scores at both two and five years compared with microfracture, and published series report good clinical scores with high patient satisfaction at ten or more years.

ChondroFiller injection takes a different route: an acellular collagen scaffold delivered as an ultrasound-guided outpatient injection, with no theatre admission required. The scaffold gels in situ and recruits the patient's own progenitor cells to support matrix-induced chondrogenesis, while preserving all future treatment options.

Alignment correction (osteotomy) sits alongside these techniques rather than within them. Where malalignment is concentrating load on the damaged compartment, a tibial or femoral osteotomy may be planned in combination with cartilage repair to protect the new tissue from uneven stress — a joint-preservation adjunct whose role in patient selection is discussed further under the decision variables ahead.

When total knee replacement is the right pathway

Total knee replacement is not a fallback — for the right patient, it is the most effective treatment available. That patient typically has severe, widespread osteoarthritis affecting multiple compartments of the joint: bone-on-bone disease that has destroyed too much articular surface for biological restoration to be feasible. Most are in the 60–80 age group; over 70,000 TKRs are performed each year in England and Wales, the majority within that bracket.

The procedure resurfaces the damaged joint with metal and polyethylene components and reliably eliminates the diffuse pain that advanced OA produces. Most patients are walking within days and return to substantive daily function — cycling, swimming, unhurried walking — within three to six months.

Replacement knees last more than 20 years in many patients, but younger, more physically active recipients are more likely to wear the implant sooner, raising the probability of a complex revision procedure later. This revision risk is the principal clinical reason joint preservation is prioritised in patients under roughly 55 — not because replacement performs poorly for them, but because it may need to be repeated during their lifetime.

As established earlier, TKR permanently forecloses biological repair. In practical terms that also means high-impact activities — running, contact sport, repetitive heavy loading — are generally discouraged thereafter to protect implant longevity. For the patient with diffuse, multicompartmental disease or significant bone loss, those trade-offs are clinically appropriate. Cartilage repair techniques are not designed for that degree of joint involvement: they require intact surrounding bone, viable adjacent cartilage, and a focal rather than widespread pattern of disease.

If symptoms and imaging suggest advanced OA rather than an isolated lesion, a consultant assessment focused on replacement — not restoration — is the right conversation to have.

The five variables that guide the decision

No single factor determines which pathway is right. Clinicians weigh five variables together, and the balance between them is more important than any one criterion in isolation.

Age and biological potential

Joint preservation is most reliably indicated in patients under approximately 55, whose cartilage stock, bone quality, and healing biology support durable repair. The 60–80 age group forms the core TKR cohort — not because age rules out preservation automatically, but because diffuse OA is far more common by that point and biological repair becomes less predictable. Younger patients also have the most to lose from the activity restrictions that follow replacement.

How large is the damaged area?

Defect size is the most directly measurable variable and shapes technique selection within the repair pathway. Evidence supports microfracture and OATS for lesions under 2 cm², MACI or ACI for defects in the 2–10 cm² range — with the SUMMIT trial confirming MACI's superiority over microfracture at both two and five years for defects of 3 cm² or above. Defects larger than this, or those involving bone loss, may require osteochondral allograft. Diffuse disease across multiple compartments sits outside repair territory altogether.

OA grade: focal high-grade defect or widespread disease?

A focal, full-thickness cartilage lesion in an otherwise healthy joint is the target for restoration. Widespread OA — particularly where more than one compartment is affected — is replacement territory. Imaging and clinical grading are essential to distinguish the two.

What are your activity goals?

Active adults aiming to return to sport, manual work, or sustained physical loading have more to gain from preserving the native joint than patients whose priority is pain-free daily function. Replacement reliably achieves the latter; it permanently forecloses the former.

Recovery commitment

Cartilage repair demands 6–12 months of structured rehabilitation, including a non-weight-bearing phase of around six weeks for some procedures and a return to sport measured in months. TKR patients typically walk within days. Both timelines suit different lives — neither is objectively better, but patients must be realistic before choosing.

A note on prior surgery: previous microfracture can damage the subchondral bone plate in ways that narrow future repair options. This is clinically relevant history that any pre-operative assessment must capture. Imaging — including cartilage-sensitive MRI — and objective functional data help ensure the decision reflects the joint's actual condition rather than assumptions about age or lifestyle alone.

Recovery: what each pathway actually demands

The two pathways ask very different things of the patient — and that gap matters most during the first year after surgery.

Cartilage repair: a long runway before full function

After a procedure such as MACI, AMIC, or an osteochondral graft, repaired tissue needs time to mature before it can bear load. The first six weeks are typically non-weight-bearing or partial weight-bearing: crutches, strict load restrictions, and physiotherapy focused on range of motion rather than strength. Active adults accustomed to daily training should plan for this phase explicitly — it is a clinical requirement of the repair, not an optional precaution.

Low-impact activity — swimming, cycling on flat terrain — may resume from around three to six months once the graft has sufficient integrity. Return to running, court sports, or sustained impact typically waits until nine to twelve months post-operatively, and the timeline extends for larger defects or more complex reconstructions. Long-term MACI data published by Ebert and colleagues in 2024 reported good clinical scores and high patient satisfaction at ten or more years post-procedure, suggesting that the rehabilitation investment can translate into lasting function.

For scaffold-based approaches such as AMIC, five-year outcome data are available and encouraging; comparable ten-year series have not yet been published at scale, which is worth discussing openly at consultation when calibrating long-term expectations.

TKR: faster to daily life, but with a permanent ceiling

The TKR trajectory differs markedly. Patients are typically mobile on the ward within one to two days of surgery, and most manage stairs, driving, and ordinary daily tasks within three to six months. What is less often emphasised is the long tail: residual swelling, range-of-motion improvement, and strength recovery continue for up to twelve months post-operatively. Functional independence and full physiological recovery are not the same milestone.

The lasting trade-off is the activity ceiling. High-impact and contact sport — running, racket sports at intensity, heavy repetitive loading — are generally discouraged after replacement to protect implant longevity. For patients whose priority is pain-free daily function, that ceiling is rarely limiting. For active adults whose lives involve sustained physical loading, cartilage repair — where the joint is suitable — keeps those options open in a way that replacement does not.

Getting an accurate assessment before deciding

Deciding between cartilage repair and knee replacement cannot rest on symptom description alone. Pain scores tell a clinician what a patient is experiencing — not the size, grade, or precise location of the cartilage lesion, nor whether the surrounding bone and tissue can support a repair. That distinction requires structural imaging and, where load distribution is relevant, objective data on how the joint actually bears weight.

Cartilage-quality imaging

Standard knee MRI identifies structural damage, but cartilage-sensitive sequences — including T2 mapping, which reflects collagen organisation and water content within the tissue — characterise defect depth, grade, and repair-tissue quality far more precisely. The difference matters when choosing between, say, an injectable scaffold for a contained lesion and a two-stage cell-based procedure for a larger full-thickness defect. At the Sleaford (NG34) and Grantham (NG31) sites, onMRI™ AI-driven analysis provides cartilage and meniscus segmentation with T2 mapping as part of the pre-operative workup, giving the consulting clinician objective characterisation rather than a visual estimate alone.

Biomechanical assessment

Where varus or valgus malalignment may be contributing to uneven compartment loading — a factor when osteotomy might accompany cartilage repair — gait analysis adds context that imaging cannot supply. MAI Motion® biomechanical assessment captures objective loading patterns across the knee, informing that part of the decision.

Lincolnshire Knee accepts patients without a GP referral and offers consultant-led assessment at both Lincolnshire sites, typically without the waiting times of an NHS pathway. Book an assessment at lincolnshireknee.co.uk.