Focal defect or whole-joint wear — the distinction that decides treatment

When a consultant tells a patient their knee cartilage is damaged, the next question that matters most is not how bad it is, but how it started. Two patients can have identical pain scores yet sit at opposite ends of the treatment spectrum — because the nature of the damage, not just its severity, determines what repair is biologically achievable.

A post-traumatic cartilage defect is a focal, contained injury — a discrete lesion created by a specific event such as an ACL rupture, a patellar dislocation, or a direct impact. The surrounding cartilage remains intact, the subchondral bone is usually sound, and the joint environment is biologically receptive. The rest of the road is in good condition; only one pothole needs filling.

Early knee osteoarthritis is an entirely different problem. Here the damage is diffuse, affecting an entire compartment. Chronic loading, synovial inflammation, and subchondral bone change are all active simultaneously — the whole road surface is deteriorating. Placing a localised repair into that environment is far less likely to succeed, because the biological conditions that regenerative techniques depend on are no longer reliably present.

This distinction — focal versus diffuse, post-traumatic versus degenerative — is the primary clinical filter, and it shapes every decision that follows.

What a knee injury does to cartilage over time

An acute injury to the knee does not simply damage cartilage at the moment of impact — it sets in motion a biological sequence that, if left uninterrupted, can progress over years. When an ACL ruptures, the joint fluid changes almost immediately: research by King et al. (Cartilage, 2020) shows the post-rupture joint fluid proteome shifts into a pro-inflammatory, chondrodegenerative state. Urinary collagen biomarkers — a proxy for ongoing cartilage breakdown — can remain elevated for up to five years after the initial event, indicating that tissue turnover continues long after acute pain has settled.

The long-term consequences are well documented. Lohmander et al. found that more than 50% of female soccer players had developed knee osteoarthritis within 12 years of an ACL injury — a figure that illustrates how consistently an untreated post-traumatic defect progresses towards compartmental wear.

Surgeons assess the degree of cartilage involvement using the ICRS grading system: Grade 1 represents surface softening or minor fissuring; Grade 2 extends to less than half the cartilage depth; Grade 3 affects more than half the depth; Grade 4 means exposed subchondral bone. These grades, combined with defect area in cm², give the framework surgeons use to gauge severity and begin planning intervention.

What matters most at earlier grades is what the grading system does not capture: the joint around a post-traumatic lesion typically remains biologically intact. Surrounding cartilage is healthy, the subchondral bone is sound, and the synovial environment is receptive. That combination creates a repair window — one that narrows if the inflammatory cascade is allowed to run unchecked towards diffuse OA.

Repair technique options for a post-traumatic knee defect

Consider a 28-year-old footballer found at arthroscopy to have a 1.5 cm² full-thickness lesion on the medial femoral condyle, with an otherwise healthy joint. This is the scenario where the repair ladder is most clearly defined — surrounding biology is working in the patient's favour, and defect size becomes the primary selector.

Smaller focal defects (up to approximately 3 cm²)

For suitable focal lesions without significant bone loss, ChondroFiller injection offers a minimally invasive starting point: an ultrasound-guided outpatient procedure delivering an injectable collagen scaffold directly into the defect, which then recruits the patient's own progenitor cells to support tissue regeneration. No theatre visit is required.

For surgically managed smaller defects, OATS/mosaicplasty transfers cylindrical osteochondral plugs from a low-load donor site to fill the lesion in a single operation; ten-year follow-up data (Gudas et al., 2012) show superior outcomes versus microfracture. AMIC — autologous matrix-induced chondrogenesis — augments marrow stimulation with a protective collagen membrane, bridging towards cell-based repair in one stage without the complexity of a two-stage procedure.

Defects ≥3 cm²

MACI (matrix-induced autologous chondrocyte implantation) carries the strongest trial evidence at this size. The SUMMIT trial demonstrated superior KOOS pain and function scores at two and five years against microfracture for lesions of this size, and long-term ACI data reports successful outcomes in approximately 82% of patients at ten years or more — though higher patient age and defects exceeding 4.5 cm² are associated with greater failure risk.

Large defects with bone involvement

Where post-traumatic osteochondritis dissecans, cystic collapse, or osteonecrosis has compromised the subchondral foundation, fresh osteochondral allograft (OCA) is the most appropriate option. OCA transfers viable hyaline cartilage with its intact bone base from a donor, supported by long-term follow-up data from Gross et al. (Clin Orthop Relat Res, 2008) specifically in post-traumatic knee defects.

A note on microfracture

Microfracture has an established historical role but is no longer a modern first-line choice. The fibrocartilage it generates is biochemically inferior to native hyaline cartilage and deteriorates within two to three years in published series. Equally significant, the procedure can damage the subchondral bone plate — complicating any subsequent cartilage repair if the initial attempt fails.

Why early OA makes knee cartilage repair harder

Every cartilage repair technique in the previous section rests on the same biological assumption: that the tissue surrounding the defect is healthy enough to support and integrate new material. Early OA removes that assumption before the surgeon begins.

The subchondral bone is an early and independent casualty. Radin and Rose (1986) established that altered bone stiffness initiates and propagates cartilage damage — meaning the osseous foundation on which any repair or scaffold must sit is already compromised. Marrow stimulation in particular depends on a viable, well-vascularised subchondral bed; when that bed has undergone the remodelling characteristic of OA, the progenitor cell environment it is supposed to supply is degraded.

In an inflamed OA joint, the fibrocartilage that marrow stimulation generates deteriorates more rapidly than it would in a post-traumatic knee — making a technique already deprioritised in the focal-defect setting even less appropriate here.

Meniscal damage compounds the problem. Fairbank (1948) and Verdonk et al. (2016) showed that meniscal loss concentrates load across the joint surface and accelerates compartmental wear. Where early OA is accompanied by meniscal deficiency, the mechanical conditions required for sustained repair deteriorate further.

The evidence base adds a further caution: the major cartilage repair trials — including the SUMMIT trial that established MACI's advantage at ≥3 cm² — recruited patients with focal, post-traumatic lesions in otherwise intact joints. Reliable long-term data for the same techniques applied in true early OA settings remain limited and largely early-stage.

For patients in this category, the appropriate path forward is not the same repair ladder shifted one step — it is a different strategy altogether, one aimed at altering load rather than filling a lesion.

Joint-preservation strategies when OA is driving the damage

Altering where load lands in the knee — rather than patching its surface — is the defining principle of osteotomy, and the logical continuation of what the previous section identified as the appropriate change of strategy.

High tibial osteotomy (HTO) corrects varus (bow-legged) malalignment by redirecting the mechanical axis so that loading shifts away from a worn medial compartment toward healthier lateral cartilage. Distal femoral osteotomy (DFO) performs the equivalent correction for valgus deformity and lateral compartment disease. Neither procedure repairs cartilage directly; both change the mechanical environment so that surviving cartilage is no longer absorbing a disproportionate share of joint load — addressing the cause of wear rather than its consequence.

For patients under approximately 50–55 with unicompartmental early OA and measurable malalignment, osteotomy is the cornerstone joint-preservation option and is well-evidenced to delay or avoid knee replacement in appropriately selected individuals. Where a focal defect coexists with compartmental overload, osteotomy and a cartilage restoration procedure can be planned together; a 2024 systematic review by Han et al. supports this combined approach, though precise patient selection remains essential — the two problems must each be clearly defined and genuinely addressable.

Biologics such as PRP or BMAC may be used as adjuncts to support the joint environment alongside these strategies, but they serve a supportive rather than a restorative role at this stage of disease.

When OA has spread across more than one compartment or subchondral bone is significantly compromised, joint preservation is no longer a realistic goal. At that point the conversation appropriately shifts toward partial or total knee replacement — a separate pathway, and one that falls outside the scope of the strategies covered here.

Assessment and the overlap zone between trauma and early OA

Determining which category a patient's knee falls into — focal post-traumatic defect, early OA, or a mixture of both — is the question that shapes the entire treatment conversation, and it cannot be answered from symptoms alone.

Advanced MRI provides the primary diagnostic anchor. Cartilage T2 mapping and segmentation can distinguish a focal full-thickness lesion from diffuse thinning across a compartment — a difference that matters because, as earlier sections showed, those two findings point toward entirely different management strategies. AI-assisted analysis of MRI sequences, such as that applied through onMRI™, can further characterise subchondral bone signal changes alongside cartilage volume — relevant because subchondral compromise is an independent marker of how receptive a joint will be to any repair procedure.

Where imaging leaves load distribution uncertain, objective biomechanical assessment adds measurable context. Gait analysis through MAI Motion® can quantify how force is shared between compartments during walking: a patient may present with an apparently focal MRI finding yet show concentrated medial loading that suggests alignment needs addressing before, or alongside, any cartilage repair.

Patient-specific filters carry equal weight. Age, lesion multiplicity, body weight, and prior surgical history — particularly whether marrow stimulation has already been performed, which compromises the subchondral bed for subsequent procedures — all determine whether restoration is realistic regardless of aetiology.

The distinction between cause and consequence traced throughout this article ultimately converges at assessment: understanding what a knee is now, and what it has been through, is the prerequisite for any pathway with a realistic prospect of holding.