What separates cartilage repair from partial knee replacement

No — cartilage repair and partial knee replacement are not interchangeable. They are designed for different stages of joint deterioration, and a surgeon's choice between them is driven primarily by the pattern of damage rather than by patient preference or age alone.

Cartilage repair is indicated when damage is focal and isolated: a discrete patch of cartilage, typically caused by injury or early localised wear, with the surrounding cartilage and the underlying subchondral bone still intact. The joint as a whole remains structurally sound.

Partial knee replacement (PKR, also called unicompartmental arthroplasty) addresses a fundamentally different problem — bone-on-bone osteoarthritis that has consumed the cartilage across an entire knee compartment. Here there is no healthy tissue to repair; the joint surface must be resurfaced with an implant.

Both approaches preserve more of the native knee than a total replacement, but they intervene at opposite ends of a degeneration spectrum. Where on that spectrum a patient sits — established through imaging, clinical examination, and in some cases a biomechanical assessment — determines which pathway a consultant will recommend. The sections that follow map out exactly how that assessment works.

Signs that cartilage repair is the right first step

Three questions guide the initial assessment: how large is the damaged area, how healthy is the surrounding bone and cartilage, and what does the patient need from their knee in the years ahead?

Defect size is the primary filter. Lesions under roughly 2 cm² may suit marrow stimulation or, where the geometry allows, an osteochondral autograft transfer (OATS, typically 1–2 cm²). Between approximately 2 and 10 cm², focal defects favour cell-based techniques — MACI (matrix-induced autologous chondrocyte implantation) or ACI. The SUMMIT trial data demonstrate that MACI outperforms microfracture on KOOS pain and function scores at both 2 and 5 years for defects of 3 cm² or above. For suitable focal defects up to approximately 3 cm², a ChondroFiller injection — an ultrasound-guided outpatient injectable collagen scaffold — also sits within this size window as a single-stage, non-surgical option, recruiting the patient's own progenitor cells into the matrix. For very large or posttraumatic defects that exceed what autograft can reliably fill, a fresh osteochondral allograft (OCA) becomes the usual step.

The biology must be viable. Whichever technique is chosen, intact subchondral bone and healthy surrounding cartilage are prerequisites. Without a sound bone substrate, a repair has nothing to anchor to — and it is precisely when that substrate has been lost to bone-on-bone wear that the decision tilts away from restoration and toward replacement.

Patient profile matters too. Younger, active patients — particularly those aiming to return to high-impact sport — are the strongest candidates for a repair-first pathway. Ligament stability and overall leg alignment are assessed at the same time: a significantly unstable or malaligned knee will usually need those issues addressed before any cartilage work can succeed.

Microfracture deserves a direct note. Once a default first-line choice for smaller defects, its role has diminished considerably: the fibrocartilage it produces tends to break down within 2–3 years, and the procedure can damage the subchondral bone plate in ways that may limit future repair options.

When partial knee replacement is the stronger choice

Once osteoarthritis has worn a knee compartment down to bone-on-bone contact, the biology needed for repair no longer exists. At that point, resurfacing — rather than restoring — becomes the appropriate goal, and partial knee replacement (PKR) is typically the first arthroplasty step a consultant will recommend, provided the disease remains confined to a single compartment.

The operative word is confined. PKR is specifically indicated when OA is limited to the medial, lateral, or patellofemoral compartment, with the remaining compartments intact, the ACL functional, and the limb reasonably well aligned. If any of those conditions are absent — multi-compartment disease, ACL deficiency, significant fixed deformity, or inflammatory arthritis — PKR is not suitable, and total knee replacement becomes the appropriate escalation.

For patients who do meet the criteria, the evidence supporting PKR is robust. The TOPKAT randomised controlled trial — 528 patients across 27 NHS hospitals — found no statistically significant difference in Oxford Knee Score at five years between PKR and total knee replacement (difference 1.04, 95% CI −0.42 to 2.50), with area-under-curve analysis showing a modest advantage in favour of PKR. This confirms that PKR is a legitimate first-choice procedure, not a compromise or a temporary measure.

Part of its appeal lies in what it leaves untouched. PKR resurfaces only the affected compartment, preserving approximately 70–75% of the native knee structure, all major ligaments, and the natural kinematics of the joint — which patients frequently describe as feeling more like their own knee. Crucially, it keeps revision to total knee replacement as a straightforward future option if disease eventually progresses. For patients under roughly 50 with single-compartment disease in particular, that preserve-then-revise logic is a meaningful advantage over proceeding directly to total replacement.

Patients who fall in between: osteotomy and staged approaches

For some patients, the assessment lands in neither column cleanly — typically someone younger with early single-compartment disease but significant varus or valgus malalignment. That malalignment matters because it concentrates load on the damaged compartment: without correcting where force falls, a cartilage repair is likely to fail under continued overload, and PKR in a markedly malaligned limb carries a higher risk of early implant problems.

In these cases, the more appropriate first step is often a high tibial osteotomy (HTO) — for varus deformity — or a distal femoral osteotomy (DFO) for valgus deformity. Neither procedure touches the cartilage or introduces an implant. Instead, a carefully planned bone cut shifts the mechanical axis of the leg so that load is redistributed away from the worn area. For younger patients with unicompartmental disease and meaningful deformity, osteotomy can delay or, in some cases, avoid arthroplasty entirely. It is also combinable with cartilage restoration procedures, either in a single operative setting or as a staged sequence, depending on defect severity and patient factors.

A second staged scenario arises when ACL deficiency coexists with a repairable chondral defect. Restoring ligament stability is generally addressed first — or concurrently — because a lax knee subjects any cartilage repair to abnormal shear forces that compromise healing.

There is no RCT-defined threshold for when osteotomy transitions to arthroplasty in these borderline cases; the decision rests on clinical judgement, imaging, and what the patient wants from their knee long term. For those in this grey zone, the range of available options is often wider than it appears at first assessment — the conversation is about sequencing, not foreclosure.

Recovery timelines and how long each option lasts

The practical consequences of each pathway diverge from the day of surgery onward.

Cartilage repair involves restricted weight-bearing for several weeks while the repair site consolidates, followed by a longer functional recovery before return to full activity. Rehabilitation adherence is a genuine determinant of outcome: a biologically sound repair can underperform if loading is introduced too early or inconsistently. The technique chosen also shapes the timeline — cell-based procedures such as MACI carry a more demanding recovery than marrow stimulation. On durability, MACI demonstrates solid results at five years for defects of 3 cm² or more, and ten-year follow-up data on osteochondral autograft transfer show sustained superiority over microfracture in terms of clinical scores and tissue quality.

PKR follows a faster early course. Most patients walk normally by around six weeks. The implant typically has a lifespan of approximately ten years, after which revision to total knee replacement — a predictable, well-established operation whose reliability is supported by the TOPKAT trial — remains available.

No head-to-head randomised trial has directly compared cartilage repair with PKR, so the two durability records come from separate evidence bases. Both pathways carry a roughly ten-year planning horizon; what differs is its character. Cartilage repair offers a biological result whose longevity is shaped by defect size, technique, and rehabilitation discipline. PKR delivers an implant-based result with a clearly mapped revision route. Understanding that distinction helps patients think forwards — not just about the operation, but about the decade of knee function it is designed to support.

How the MSK Doctors team works through this decision

Reaching the right decision between these two pathways requires more than a scan and a consultation. A thorough assessment covers defect size and morphology on MRI, subchondral bone integrity, compartment involvement, limb alignment, ligament status, and the patient's functional goals — each factor feeding into the clinical picture before any recommendation is made.

Objective biomechanical data add a further layer. At the Sleaford and Grantham centres, MAI Motion® — a UKCA-registered markerless motion-capture system — provides quantified load-distribution analysis that complements imaging findings when alignment or candidacy is in question. MRI interpretation is supported by onMRI™ AI-driven analysis, which assists in characterising lesion boundaries and subchondral involvement.

Patients are seen without an NHS-style referral or waiting list. London-based patients can access the same clinical expertise through the London Cartilage Clinic.

To arrange an assessment, visit mskdoctors.com.