What a post-traumatic chondral defect actually means for your knee

Cartilage damage after a knee injury is not like a muscle tear or a broken bone — it does not grow back. The articular cartilage covering the ends of the femur and tibia has no blood supply of its own, which means the chondrocytes (cartilage cells) that sustain it cannot mount a meaningful repair response after trauma. When an impact load, twisting injury, or intra-articular fracture kills those cells, the loss is permanent without clinical intervention.

The resulting injury is called a post-traumatic chondral defect: a localised crater in the cartilage surface, sometimes extending into the subchondral bone beneath it. Left untreated, the defect does not stay static. Abnormal load distribution around the damaged area drives progressive cartilage breakdown, inflammatory changes, and — over months to years — post-traumatic osteoarthritis (PTOA) affecting the wider knee compartment.

Why the time between injury and assessment matters

Onset timing is the first branch point in any treatment plan. In an acute presentation — typically within days to weeks of the original injury — a large or displaced osteochondral fragment from a weight-bearing surface may still be biologically viable and suitable for primary fixation. Miss that window, and the fragment cannot be salvaged; the pathway then shifts to the same structured approach used for chronic defects, where the injury has already settled and a stepwise assessment is needed.

In that chronic presentation, surgeons use the ICRS (International Cartilage Repair Society) grading system to quantify depth and extent of damage. Grade I–II lesions involve superficial softening or partial-thickness loss; surgical restoration becomes most relevant at Grade III (deep, greater than 50% cartilage depth) and Grade IV (full-thickness loss through to subchondral bone). Defect area — measured in cm² at arthroscopy or on MRI — then determines which restorative technique is most appropriate, with the threshold between smaller procedures (such as OATS or AMIC) and larger-defect strategies (such as MACI or osteochondral allograft) typically falling in the 2–4 cm² range.

Acute injury — why the first surgical decision is time-critical

The time between a knee injury and specialist assessment can affect which treatments remain available. When a high-energy impact or twisting force shears a fragment of cartilage — or an osteochondral piece carrying cartilage and underlying bone — from a weight-bearing surface such as the femoral condyle, that fragment may initially retain enough cell viability to be refixed in place. Fragments larger than approximately 1 cm² on a weight-bearing surface are the most clinically significant; at that size, a displaced piece will alter joint mechanics and accelerate surface breakdown if left unaddressed.

The critical variable is biological viability. Chondrocytes within a displaced fragment begin to deteriorate once separated from their normal nutritional environment. Primary fixation — reattaching the fragment at acute surgery — can preserve native hyaline cartilage only while that window remains open. If the fragment has broken further, has lost its subchondral bone support, or is found to be non-viable at surgery, fixation is no longer possible. Management then moves into the chronic focal-defect pathway, where restoration rather than rescue becomes the objective.

Why most patients arrive after that window

In emergency settings the immediate priority is ruling out fracture, ligament disruption, or vascular injury. Cartilage damage — invisible on plain X-ray — is commonly identified only when symptoms persist: pain on weight-bearing, intermittent locking, or an effusion that fails to settle after several weeks. By that point the acute fixation window has typically closed, and the clinical question shifts to characterising what remains.

MRI with cartilage-sensitive sequences is the most practical tool for this: it can establish defect size and depth, subchondral bone integrity, and whether any adjacent compartment involvement would alter the surgical plan. Persistent mechanical symptoms after knee trauma — catching, giving way, or load-specific pain — warrant imaging and a specialist review rather than an extended period of observation.

OCA for large post-traumatic defects — when restoration is still possible

Once the acute window has passed and a defect has been characterised by size, the selection ladder for chronic focal lesions becomes the operative guide.

For defects below roughly 2–4 cm², single-stage procedures remain appropriate first options. OATS or mosaicplasty uses the patient's own cartilage-and-bone plugs for contained lesions up to about 2 cm²; AMIC (autologous matrix-induced chondrogenesis) augments marrow stimulation with a resorbable scaffold, offering a meaningful step up from marrow stimulation alone. Historically, straightforward marrow stimulation — microfracture — was the surgical first line for focal chondral defects of almost any size. Current evidence, however, shows the fibrocartilage it produces tends to deteriorate at around two to three years and may damage the subchondral bone plate, narrowing future options. For post-traumatic defects specifically, microfracture is no longer the preferred approach where better-matched procedures exist.

For medium to large defects amenable to membrane-based repair, MACI is supported by the SUMMIT phase-3 trial, which enrolled patients with a mean lesion size of 4.8 cm² and demonstrated superior KOOS pain and function scores against microfracture at both two and five years.

When OCA becomes the principal option

Fresh osteochondral allograft (OCA) transplantation is most directly suited to the defect profile that post-traumatic injury typically produces: large, geometrically irregular, sometimes multi-focal, and often involving subchondral bone loss. Autograft plugs cannot replicate that geometry without excessive donor-site morbidity; cell-based membranes alone cannot restore lost bone stock. OCA replaces the entire osteochondral unit with mature, viable donor hyaline cartilage in a single procedure.

Published series report OCA survivorship of approximately 82.6% at five years. Long-term data from Gross et al. (Clin Orthop Relat Res, 2008) show allograft survival of 59–91% at 10–25 years — a range reflecting variation in defect characteristics and patient selection rather than a single expected figure. Mean IKDC scores improved from roughly 41.8 before surgery to 68.1 afterwards, and approximately 75% of patients returned to sport at mean six-year follow-up. OCA does not guarantee permanent joint preservation; when grafts eventually fail, conversion to total knee arthroplasty is the most common next step, though in well-selected younger patients OCA may substantively delay that outcome.

The 28-day procurement window

One constraint patients consistently find unexpected: fresh OCA grafts must be transplanted within approximately 28 days of donor procurement, as chondrocyte viability falls below roughly 70% beyond that point. Surgery scheduling is therefore partly determined by tissue bank availability, not the patient's calendar alone — a logistical reality both patients and referrers benefit from understanding before the planning process begins.

Varus malalignment and HTO — when the bone angle is part of the problem

Leg alignment is rarely the first thing patients associate with cartilage wear — yet in many post-traumatic knees, varus (bow-legged) geometry quietly accelerates medial compartment damage. When the mechanical axis passes medial to the centre of the joint, compressive load concentrates disproportionately on that side. After a cartilage injury to the medial compartment, this added mechanical pressure can substantially hasten deterioration that might otherwise progress more slowly.

High tibial osteotomy (HTO) addresses this mechanically: by cutting and repositioning the upper tibia, it shifts the weight-bearing axis laterally, offloading the medial side onto the healthier lateral compartment. HTO does not repair cartilage directly. In younger, active patients with varus deformity and preserved lateral cartilage, however, it can meaningfully delay arthroplasty. The preferred contemporary technique is the medial open-wedge approach; early weight-bearing is generally possible within a few weeks with modern fixation.

When HTO alone is insufficient

If a focal cartilage defect accompanies the malalignment, correcting the axis without addressing the defect leaves a damaged surface in a better mechanical environment — but still damaged. The combined HTO and OCA transplantation procedure, supported by emerging evidence including Jildeh et al. (2022), addresses both problems simultaneously: the osteotomy normalises joint forces while the allograft restores the osteochondral surface, each reinforcing the durability of the other.

Mandatory arthroscopy immediately before the osteotomy is essential to confirm that the compartment bearing post-realignment load is sufficiently intact. Severe medial cartilage loss is a relative contraindication to HTO without concomitant restoration — realignment alone would increase load through an already degraded surface.

Staging depends on the repair technique chosen alongside the osteotomy. OATS and matrix-augmented marrow-stimulation procedures can typically be completed in a single operative sitting. ACI and MACI — which require laboratory cultivation of chondrocytes between a biopsy and subsequent implantation — generally necessitate a staged approach, affecting the overall treatment timeline.

Objective biomechanical alignment assessment, including gait analysis where available, forms part of the pre-operative workup and informs whether osteotomy should enter the surgical plan before cartilage restoration is attempted.

When joint replacement becomes the appropriate endpoint

The question that eventually arises in many post-traumatic knees is not whether joint replacement should be considered — it is whether that conversation has genuinely arrived.

Preservation strategies such as OCA, HTO, and combined procedures are designed for focal-defect anatomy: a contained injury within an otherwise viable joint. Once damage extends beyond a single focal site to involve multiple compartments, or once the articular surface across a compartment is diffusely worn, the biological basis for cartilage restoration no longer holds. That shift from focal to diffuse wear is the clearest clinical signal that arthroplasty must move from background consideration to active planning.

For carefully selected patients whose post-traumatic damage remains genuinely confined to one compartment, unicompartmental knee arthroplasty (UKA) may serve as an intermediate step — replacing only the diseased compartment rather than the full joint, and typically associated with less post-operative pain and shorter initial recovery than total knee replacement (TKR). UKA is not a routine progression; it suits a specific subset where single-compartment anatomy can be confirmed.

When prior preservation has been exhausted, or when the knee presents with advanced multi-compartment post-traumatic osteoarthritis from the outset, TKR is the well-evidenced endpoint. Younger post-traumatic patients, however, carry a higher revision risk, and national projections anticipate rising demand for both primary and revision knee replacement through 2030 — reinforcing why preservation pathways retain individual and systemic value wherever the joint still has focal-defect anatomy.

Arthroplasty reliably reduces pain and restores functional mobility, but it does not replicate the demands of a preserved native knee for higher-level activity. In practical terms, once diffuse compartmental wear is confirmed on imaging, the surgical question shifts from whether to replace to which compartment — and that distinction determines how soon the conversation needs to happen.

The borderline decision — middle-aged patients with moderate post-traumatic OA

For patients in their forties and fifties with moderate post-traumatic OA and borderline alignment, the evidence offers less certainty than the decision demands. No randomised trial compares OCA, HTO, and knee replacement stratified by injury onset timing; the available guidance comes from observational cohort data rather than head-to-head comparison.

The variables that most consistently inform this choice are defect focality (focal damage favours preservation; spread across compartments shifts the balance toward replacement), correctability of varus malalignment, patient age and activity demands, and how long symptoms preceded specialist assessment. That last factor — symptom duration before OCA surgery — remains under-researched as an outcome predictor and represents an acknowledged evidence gap.

Objective assessment reduces reliance on symptom reports. Biomechanical gait analysis, including approaches formalised in platforms such as MAI Motion®, quantifies load distribution across compartments; cartilage-sensitive MRI sequences including T2 mapping and segmentation, as offered through onMRI™ analysis, characterise cartilage volume more reproducibly than standard imaging. Neither replaces consultant judgement, but together they narrow the margin of uncertainty before a pathway is chosen.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral; consultations at Sleaford NG34 and Grantham NG31 combine imaging review, alignment assessment, and activity profiling in a single appointment. Book an assessment at lincolnshireknee.co.uk.

The core difficulty in this patient group is not a shortage of treatment options — it is establishing which side of the focal-to-diffuse boundary the joint has crossed. That determination, made through structured assessment rather than symptom history alone, is what converts a genuinely contested clinical picture into a clear plan.