Why size and location are the two main decision gates

The size of a cartilage lesion and where it sits inside the knee are the two questions a specialist asks before anything else — before age, before fitness level, before the sport someone plays. Understanding why helps patients make sense of the options they are offered.

Articular cartilage has no blood supply of its own, so a full-thickness hole in it cannot close by itself. These focal chondral defects are more common than many people realise: they turn up in between 11% and 34.6% of routine knee arthroscopies, often as an incidental finding alongside a ligament or meniscus injury. Left unaddressed, the damaged area tends to enlarge and can accelerate the joint wear that leads to early osteoarthritis.

Clinicians organise treatment choices around two primary axes. The first is lesion area, measured in square centimetres. A pivotal boundary sits at roughly 2–4 cm² — approximately the size of a small fingernail. Below that threshold, less invasive options are typically sufficient; above it, more substantial restoration is usually needed. The second axis is anatomic location — specifically, which compartment of the knee is involved. A 2 cm² lesion on the medial femoral condyle (the inner weight-bearing knuckle of the thigh bone) carries a better prognosis with simpler treatment than the same-sized lesion on the patella or in the trochlear groove, where joint mechanics and loading patterns are more complex.

Secondary factors — a patient's age, activity demands, leg alignment, or whether there is also a meniscus problem — refine the choice within a size-location band rather than overriding it. The practical effect is a tiered pathway: at the lower end, options include outpatient injectable scaffold treatments; at the upper end, surgical grafting procedures. Later sections map each tier in detail.

Small defects (under 2 cm²): the case for single-stage approaches

Roughly the area of a 5p coin — that is a practical way to picture a 2 cm² defect when reading an MRI report. Lesions at or below this size on the femoral condyle have the widest range of restorative options, because the surrounding cartilage and subchondral bone are usually still intact enough to support a repair.

ChondroFiller injection — an ultrasound-guided, outpatient injectable collagen scaffold — is the least invasive of those options. The scaffold is placed directly into the defect under image guidance, without cell harvest or theatre admission. Once in place, it acts as a matrix that recruits the patient's own progenitor cells to support chondrogenesis. The CE-marked device is indicated for focal defects up to approximately 3 cm², with some protocols extending to 6 cm², making it relevant across most of this size band.

AMIC (autologous matrix-induced chondrogenesis) offers a single-stage surgical bridge for patients where theatre access is appropriate. A collagen scaffold is combined with marrow-stimulation at the defect site in one procedure, and published follow-up data at a mean of 6.8 years show durable results with no meaningful age-related disadvantage for patients up to 69 years old.

OATS/mosaicplasty transfers one or more osteochondral plugs from a non-weight-bearing zone of the knee to the defect. Studies report an average treated area of around 2.3 cm², with IKDC scores improving from 46 to 70 over a 10-year period and very few patients requiring conversion to arthroplasty.

Microfracture dominated this size category for decades and remains a historical reference point. Its limitation is well-documented: the fibrocartilage it produces tends to degrade within two to three years, and repeated perforations can damage the subchondral bone plate in ways that narrow future repair options. It is no longer a preferred first-line choice for patients seeking durable restoration, though its prior use does not preclude later treatment.

The 2–4 cm² crossroads: injectable scaffold or cell-based repair?

Between 2 and 4 cm², the treatment decision branches in a way it does not at either extreme. This is the size band where an outpatient injectable scaffold, a single-stage surgical approach, and a two-stage cell-based programme all have reasonable evidence — and where treatment history and lesion location tip the balance as much as area alone.

ChondroFiller injection remains applicable across much of this band. Its licensed indication extends to 3 cm², with some clinical protocols reaching 6 cm², and its single-visit, no-cell-harvest pathway continues to distinguish it from surgical alternatives. For defects in the lower half of this range on the femoral condyle, that is a meaningful practical advantage.

MACI changes the calculus at or above 3 cm². The SUMMIT randomised trial confirmed that MACI produced significantly better KOOS pain and function scores than microfracture at two and five years for lesions of this size. That superiority comes at a cost: MACI requires two procedures — a cartilage biopsy, followed by cell expansion and reimplantation. Prior marrow-stimulation treatment, including microfracture, has been associated with lower success rates in subsequent cell-based repair, so a patient's treatment history is not a secondary concern but a primary one when this route is considered.

AMIC occupies the middle ground: single-stage and less resource-intensive than MACI, with durable mid-term follow-up data already established. The evidence base thins at the larger end of this band, and direct head-to-head trial data comparing injectable scaffold approaches against formal cell-based repair across the 2–4 cm² range are currently limited.

Next-generation single-stage ACI (sometimes called STACI) is an emerging option designed to reduce the two-visit burden of conventional cell-based repair. Early evidence is encouraging; the published dataset remains small compared with established techniques.

Location continues to modify these choices — a 3 cm² trochlear or patellar defect does not map cleanly onto the same pathway as a 3 cm² medial condyle lesion, for reasons covered in later sections.

Large defects (above 4 cm²): when surgical grafting is needed

Once a defect exceeds approximately 4 cm², the options narrow for practical reasons. The non-weight-bearing donor zones that supply plugs for mosaicplasty cannot yield enough tissue to fill a lesion of this size, so the repair toolkit shifts to techniques with greater coverage capacity.

MACI has the strongest current evidence base for defects in this range, with published data supporting use up to around 10 cm². ACI — the two-stage predecessor from which MACI derives — extends that evidence further still, with follow-up data reaching nine years and beyond. At this defect size, the case for cell-based repair is well-established in the literature.

Osteochondral allograft (OCA) transplantation is the principal surgical route when autograft volume is insufficient — for very large lesions, posttraumatic defects, or revision cases where previous procedures have compromised the subchondral bone. Unlike mosaicplasty, OCA uses fresh cartilage-and-bone tissue sourced from a donor tissue bank rather than the patient's own knee. Survival rates in published series reach 87.9% at five years and 77.2% at ten years, with aggregate IKDC scores improving by approximately 26 points — meaningful restoration, though the data also show that outcomes are not permanent for all patients, and that each compartment behaves differently so figures cannot be applied uniformly across the knee.

Mechanical alignment is a significant modifier at this defect size. In a varus (bowed) leg with medial compartment involvement, an osteotomy to offload the repair site may need to accompany the cartilage procedure rather than be deferred.

Emerging injectable scaffold data for large defects are also available — a 14-year follow-up series using a hyaluronic acid scaffold embedded with bone marrow concentrate reported a median lesion area of 6.6 cm² and durable functional scores. These results are encouraging, but direct comparison against MACI at this size range is not yet available from randomised trials, and the evidence base remains early relative to established surgical options.

Location specifics: condyle, trochlea, and patella

Where in the knee a defect sits changes the prognosis and the surgical plan, often as much as the size does. Two lesions of identical area in different compartments can point toward entirely different treatment pathways.

The medial femoral condyle is the most studied and most commonly treated site, accounting for roughly 75% of osteochondral autograft procedures in published series. The size-based benchmarks described in earlier sections — for microfracture, OATS, and MACI — were built predominantly on condyle data, so the evidence is at its most reliable here.

The trochlea and patellofemoral compartment are a different proposition. A systematic review of microfracture prognostic factors identified trochlear and patellofemoral location as an independent predictor of poorer outcomes compared with the medial condyle. A 2 cm² patellar defect does not carry the same prognosis as a 2 cm² condylar defect, even when every other variable matches.

Patellofemoral lesions carry additional mechanical complexity. If patellar tracking is abnormal, restoring the cartilage surface without correcting the mechanics is unlikely to protect the repair. Anteromedial tibial tubercle transfer or another realignment procedure may need to accompany cartilage restoration — making this potentially a two-part plan rather than a single intervention. The exact facet position of a patellar lesion (medial, central, lateral, or distal) influences which realignment approach is appropriate and correlates with the outcome of that correction.

For OCA transplantation, the patellofemoral compartment behaves differently from the condyle; each knee location has its own biomechanical characteristics, and survival figures for condylar OCA cannot be read across directly.

Tibial plateau and posterior femoral condyle lesions are underrepresented in the published literature. Most large series focus on the anterior condyle and trochlea, so treatment at these sites draws more on individual surgical experience than on robust comparative data.

Getting assessed at Lincolnshire Knee

Applying the size-and-location framework described in this article requires accurate data — specifically, MRI with cartilage-sensitive sequences that can resolve lesion depth, area, and exact facet position. Clinical examination alone cannot reliably size a defect or distinguish, for example, a central medial condyle lesion from a trochlear one.

At Lincolnshire Knee's Sleaford site, cartilage MRI analysis is supported by onMRI™, an AI-driven system that includes T2 mapping and cartilage segmentation — reducing the reporting subjectivity that can affect conventional radiological reads. A biomechanical assessment using MAI Motion® can also identify loading patterns that place particular compartments under greater stress, informing both the repair decision and any rehabilitation or alignment considerations that follow.

After imaging and biomechanical review, a consultant appointment brings the findings together to determine which pathway — if any — is appropriate for that patient's specific defect size, location, and clinical picture.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. Consultations and diagnostics are available at Sleaford NG34 and Grantham NG31, with no NHS-style waiting list. Book an assessment at lincolnshireknee.co.uk.

1. [1] A decision tree model for chondral lesion management in the knee based on clinical and mathematical integration. (2026). https://doi.org/10.1016/j.jisako.2026.101131 https://doi.org/10.1016/j.jisako.2026.101131
2. [2] The Large Focal Isolated Chondral Lesion. (2021). https://doi.org/10.1055/s-0041-1735278 https://doi.org/10.1055/s-0041-1735278
3. [3] Mosaicplasty/Osteochondral Autograft Transfer Remains a Durable Solution for Symptomatic Chondral Defects of the Knee: Two to Ten-Year Follow-up Analysis. (2024). https://doi.org/10.1177/2325967124s00003 https://doi.org/10.1177/2325967124s00003
4. [4] Arthroscopic versus Open Osteochondral Autograft Transplantation (Mosaicplasty) for Cartilage Damage of the Knee: A Systematic Review. (2019). https://doi.org/10.1055/s-0039-1692999 https://doi.org/10.1055/s-0039-1692999
5. [5] One-Step Cartilage Repair of Full-Thickness Knee Chondral Lesions Using a Hyaluronic Acid–Based Scaffold Embedded With Bone Marrow Aspirate Concentrate: Long-term Outcomes After Mean Follow-up Duration of 14 Years. (2024). https://doi.org/10.1177/03635465241287524 https://doi.org/10.1177/03635465241287524
6. [6] Regenerative Cartilage Treatment for Focal Chondral Defects in the Knee: Focus on Marrow-Stimulating and Cell-Based Scaffold Approaches. (2025). https://doi.org/10.3390/cells14151217 https://doi.org/10.3390/cells14151217
7. [7] Arthroscopic Treatment of Femoral Condyle Chondral Lesions: Microfracture Versus Liquid Bioscaffold. (2023). https://doi.org/10.1007/s43465-023-00878-7 https://doi.org/10.1007/s43465-023-00878-7
8. [8] Prognostic Factors for the Clinical Outcome after Microfracture Treatment of Chondral and Osteochondral Defects in the Knee Joint: A Systematic Review. (2023). https://doi.org/10.1177/19476035221147680 https://doi.org/10.1177/19476035221147680
9. [9] Microfracture with or without collagen scaffold insertion for adults with chondral or osteochondral defects of the knee: the SISMIC RCT. (2025). https://doi.org/10.3310/BRTS2415 https://doi.org/10.3310/BRTS2415