What is actually happening in the bone and cartilage

Receiving a scan report that mentions 'osteochondritis dissecans' can be alarming — the name sounds serious, and the temptation is to interpret it as a sign that the whole knee is wearing out. It is not. OCD is a focal, discrete lesion confined to a specific patch of subchondral bone, and understanding that distinction matters enormously for what comes next.

The process begins beneath the cartilage surface. A localised loss of blood supply to the subchondral bone causes that small segment of bone to weaken and, over time, begin separating from the surrounding tissue. Because cartilage depends on the bone beneath it for mechanical support, once that foundation is compromised the overlying articular surface starts to crack and may eventually lift away entirely. Repetitive mechanical loading — the kind accumulated through running, jumping, or change-of-direction sport — is implicated in triggering and worsening this sequence.

The medial femoral condyle is involved in the majority of cases. Patellar OCD is considerably rarer but tends to present in a more advanced, unstable state when it does occur. OCD can arise at any of these sites independently of any generalised degenerative process.

The single concept that governs every clinical decision that follows — imaging interpretation, treatment choice, and prognosis — is whether the lesion is stable or unstable: that is, whether the affected fragment remains securely attached or has begun to separate.

What the MRI report is actually describing

Radiology reports have a vocabulary of their own, and the terms used to describe an OCD lesion can read as more alarming than they need to. Translating them into clinical meaning makes the findings easier to discuss with your surgeon.

The key question MRI is trying to answer is the same one driving every management decision: is the lesion stable or not? The sequences radiologists use focus on the osteochondral interface — the junction between the affected bone fragment and the surrounding tissue. High T2 signal at that interface means fluid may be tracking under the fragment, which is one sign of possible instability. Bone marrow oedema reflects reactive changes in the surrounding bone. Articular breach describes a visible gap or irregularity in the overlying cartilage surface. Fragment displacement means the piece of bone and cartilage has shifted from its original position. Each of these features, taken together, moves a lesion toward the unstable end of the spectrum.

Three established grading systems — Hefti (five groups), Nelson (four groups), and Kocher (three groups) — exist to classify lesions systematically. All three have a practical limitation worth knowing: independent radiologists reading the same scan may not always reach the same grade, with inter-rater agreement in the moderate range across all systems (Krippendorff α approximately 0.49–0.51). The simpler Kocher three-group system performs as reliably as the more detailed Hefti classification and maps closely onto the treatment categories used at arthroscopy — making it a useful clinical shorthand. When MRI findings are borderline, arthroscopy remains the reference standard for confirming true stability.

Some centres are now using quantitative MRI sequences — IVIM perfusion fraction and diffusion-weighted ADC mapping — which can detect instability with considerably higher accuracy (area under the curve up to 0.91–0.95). These techniques are not yet standard clinical practice and have been studied mainly in younger cohorts, but patients undergoing advanced imaging may encounter them. MRI is a very useful guide; it is one input that your surgeon will interpret alongside your symptoms and examination findings.

Stable versus unstable: why this one distinction drives the whole decision

The stability classification is the fork in the road from which every management decision branches. Understanding where a lesion falls — and why age matters to that classification — gives active adults a realistic framework for the conversation ahead.

Stable lesions retain intact overlying cartilage with no fragment displacement. The subchondral bone is compromised, but the fragment remains in place. Conservative management — activity modification, load restriction, and structured rehabilitation — is a reasonable first approach.

Unstable lesions show at least one of: articular cartilage breach, fragment displacement, extensive bone marrow oedema, or high T2 signal at the osteochondral interface. In a skeletally mature adult, this pattern almost always warrants surgical intervention. The reason is biological: adolescents with open growth plates retain active bone remodelling capacity that drives natural repair. At 35, that mechanism is no longer available. The same 'stable' classification therefore carries a lower spontaneous-healing likelihood in an adult than in a teenager.

One scenario bypasses the stable/unstable question altogether: a loose body — a detached fragment floating freely in the joint. This is a clear surgical trigger at any age, because a migrating fragment can abrade cartilage across a much wider area of the joint surface.

Lesion location adds a final nuance. Lateral femoral condyle OCD carries roughly five times the failure risk of medial condyle lesions after internal fixation. For an apparently stable lateral lesion this is not a reason for alarm — it is a reason for closer monitoring and a lower threshold for specialist review if symptoms fail to settle.

What conservative care involves and how long to give it

Conservative care for a stable OCD lesion is an active commitment, not a period of passive rest and waiting. The four pillars are structured load management (stopping running, jumping, and court sports for an agreed period), formal physiotherapy targeting quadriceps control and lower-limb alignment, offloading aids — crutches or a functional brace — where pain or swelling is significant, and serial imaging to confirm the lesion is responding.

The spread in published outcomes across non-operative studies is driven largely by two factors: how rigorously patients restricted high-impact activity, and how strictly individual studies defined 'success'. A lesion that settles symptomatically but shows no MRI change is not the same as one that demonstrates measurable fragment healing; studies that conflate the two tend to inflate apparent success rates. Honest non-operative management requires both symptomatic improvement and imaging evidence of stability or reduction in lesion size.

Adults entering this pathway should calibrate expectations carefully. Spontaneous healing is possible, but the biological repair mechanisms that make conservative care reliably effective in adolescents are less robust after skeletal maturity. A frank discussion with the treating specialist about lesion size, location, and the realistic probability of healing without surgery is worth having before committing to a prolonged non-operative trial.

Failure has no fixed week-count. It is defined by persistent or worsening symptoms, absence of MRI healing at agreed review intervals, or progression from stable to unstable imaging features. When any of these occur, surgical options — internal fixation, or reconstruction if the lesion has deteriorated — move into the foreground.

When surgery or a regenerative procedure is the right path

Once conservative care has been exhausted, or when a lesion is unstable from the outset, the clinical pathway moves through a logical sequence: preserve the native fragment if possible; reconstruct the surface if not; and consider a biologic injectable approach where the defect is eligible and surgical reconstruction is not yet indicated.

Internal fixation — first line for unstable lesions

Stabilising the OCD fragment with screws or pins preserves the original articular surface and is the preferred approach for unstable lesions in skeletally mature adults. In a multicentre cohort followed for a mean of 11.3 years, 71.6% of patients healed after internal fixation — a rate comparable to adolescents, which provides some reassurance that skeletal maturity alone does not doom the attempt. Lesion location materially changes that picture: lateral femoral condyle OCD carries roughly five times the failure risk of medial condyle lesions, and this warrants frank discussion before surgery is booked.

Reconstruction when fixation is not viable

When the fragment is too damaged to reattach, fixation has already failed, or the defect is large, reconstructive techniques take over. Autologous chondrocyte implantation (ACI), matrix-induced ACI (MACI), and osteochondral allograft transplantation (OCA) each aim to resurface the joint with new tissue. Across 16 studies and 458 adult lesions, IKDC subjective scores improved from 42.4 to 78.6 at three years, with an overall complication rate of 8.9%. It is worth noting that adult-specific randomised trial data is absent; these figures come from mixed-age cohorts and should be understood as directional rather than definitive. In-situ mosaicplasty — a combined mechanical and biological technique — achieved MRI-confirmed healing in 92% of a small young-adult series (13 knees, mean age 22).

Injectable scaffold for eligible focal defects

For contained focal defects where the lesion is stable or early-stage and surgical reconstruction is not yet indicated, an injectable collagen scaffold such as a ChondroFiller injection offers a minimally invasive biologic option. Delivered as an ultrasound-guided outpatient procedure, the scaffold provides a matrix that supports the patient's own progenitor cells in forming new cartilage tissue. This is a biologic repair pathway, not a structural reconstruction, and sits in a distinct category from ACI, MACI, OCA, and mosaicplasty. Individual suitability — determined by defect size, lesion stability, and joint alignment — is assessed at specialist consultation.

Long-term outlook and what to do next

Poorly managed OCD carries a real long-term cost: fragment detachment, once it occurs, accelerates articular damage in ways that are difficult to reverse, and that trajectory is the clearest argument for acting on unambiguous imaging findings at the right time rather than waiting to see what happens.

Return to sport: milestones, not a date

Getting back to running, court sport, or contact sport after OCD treatment follows clinical milestones rather than a calendar. Restored quadriceps and hamstring symmetry on functional testing, the capacity to load the limb progressively without pain or swelling, full range of motion, and specialist clearance at each stage all inform when that return is safe. The appropriate pace depends on the treatment pathway and on how the joint actually responds to graded load reintroduction — not on a number of weeks agreed at diagnosis.

What the evidence does not yet tell us

Large randomised controlled trials in skeletally mature, active adults are absent. Most OCD literature is weighted toward juvenile and adolescent cohorts, and adult-specific return-to-sport data in particular remains sparse. The healing rates and functional score improvements discussed in this article come from mixed-age series where adult subgroups are often small — the figures are directional, not individual predictions.

For any active adult navigating a new OCD diagnosis or an ambiguous scan finding, a specialist assessment that integrates clinical history, examination, and imaging in full context is the appropriate next step. Both injectable scaffold options and surgical pathways for focal knee OCD are assessed at the London Cartilage Clinic on Harley Street; appointments are available at londoncartilage.com.

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