The size threshold that changes the decision

When a hip surgeon says the cartilage damage is too large for a single plug, the immediate question is a practical one: how much surface area actually needs to be restored, and which technique can realistically fill it?

A single OATS plug — the cylindrical bone-and-cartilage core harvested from the patient's own joint — is typically 6 to 10 mm in diameter. On the curved surface of the femoral head or acetabulum, that translates to roughly 0.3–0.8 cm² of restored articular area. For a defect that approaches or exceeds approximately 1.5–2 cm², one plug will not achieve full coverage. At that point, the operative choice narrows to either a multi-plug mosaicplasty arrangement or osteochondral allograft (OCA) transplantation using fresh donor tissue.

The wider cartilage repair literature reinforces why the arithmetic matters. Evidence consistently shows that defects of 3 cm² or greater have significantly improved pain and function outcomes at both two and five years when treated with an advanced restoration strategy rather than microfracture alone — underlining that larger lesions demand a more substantial grafting solution.

In the hip, this size threshold is clinically sharper than in a flatter joint such as the knee. The tight ball-and-socket geometry leaves little tolerance for incomplete coverage: any gap between plugs, or any area of defect left unaddressed, is subject to altered joint contact pressure that can compromise the repair and the surrounding native cartilage. Lesion size, therefore, is not simply one factor among many — it is the primary variable that determines whether OATS remains a viable option or whether OCA becomes the more appropriate choice.

Why the hip joint makes graft-based repair harder than the knee

Fitting a graft onto the femoral head is closer to tiling a dome than patching a flat floor. The femoral head is a sphere — a tightly curved ball that must articulate smoothly with the concave acetabulum — and any osteochondral graft placed on its surface must follow that curvature precisely. In a flatter joint, minor contouring inaccuracies are better tolerated; in the hip, they matter considerably more.

The reason is biomechanical. Research by Koh and colleagues (AJSM, 2004) showed that even a fraction-of-a-millimetre height mismatch between a graft plug and the surrounding cartilage rim can significantly alter joint contact pressure at the repair site. A proud graft takes disproportionate load with every step; a recessed one leaves a stress-concentrating edge. Either scenario risks failure of the graft itself or progressive damage to the intact cartilage immediately around it.

For multi-plug mosaicplasty, this geometry creates an additional problem. The small gaps between individual autograft cylinders heal with fibrocartilage rather than hyaline cartilage. On a relatively flat tibial condyle, those gaps are a modest compromise; on the curved surface of the femoral head, the irregular patchwork sits less conformally, and the fibrocartilage islands are exposed to shear forces that the surrounding spherical geometry amplifies.

Surgical access adds a further layer of complexity. Reaching the femoral head to seat plugs or shape allograft tissue accurately requires either a formal surgical hip dislocation or an advanced arthroscopic approach — neither is routine in the way knee cartilage surgery is. This procedural overhead reinforces why careful technique selection, including graft type and size, carries particularly high stakes in hip cartilage repair.

OATS in the hip: what it offers and where it runs out

There is a genuine biological case for OATS when the anatomy allows it. Because the graft is the patient's own tissue, there is no risk of disease transmission and no immune response to manage. The chondrocytes within an autograft plug are alive at the moment of implantation — a meaningful advantage over preserved or processed alternatives, and one reason that single-stage autograft transfer remains a sound first choice for contained hip defects within the appropriate size range.

The constraint is supply, and in the hip the supply is tight. The knee offers accessible low-weight-bearing harvest zones — the far medial or lateral condylar margins — that can yield multiple plugs with acceptable donor-site consequences. The hip does not have an equivalent reservoir. The usable low-load articular surface within the hip itself is limited, and harvesting from the ipsilateral knee to repair a hip defect is not an established standard practice. This anatomical reality sets a firm ceiling on how much autograft a surgeon can safely obtain for a single hip procedure.

When a lesion requires only one plug — generally for defects below roughly 1.5 cm² that are well-contained — OATS remains a clinically reasonable option. Beyond that threshold, the arithmetic shifts. Each additional plug needed increases donor-site morbidity, reduces the biological surplus at the harvest site, and introduces more fibrocartilage-filled gaps between cylinders. At some point the technique is no longer constrained by surgical skill or planning; it is constrained by the patient's own articular tissue inventory.

What fresh osteochondral allograft (OCA) brings to larger hip defects

Fresh osteochondral allograft solves the supply problem directly. The donor tissue comes from a cadaveric source, meaning the graft footprint is not limited by the patient's own articular inventory — it can be sized and shaped to match the defect area and, crucially, contoured to follow the curvature of the recipient femoral head. For a large or irregularly shaped lesion that exhausts the autograft option, that design flexibility is the central clinical argument in OCA's favour.

The long-term outcome record for fresh OCA is largely drawn from knee series rather than hip-specific data. Gross and colleagues (Clin Orthop Relat Res, 2008) reported durable results in post-traumatic knee defects at extended follow-up — the biological principles that underpin graft survival are the same in the hip, but hip-specific OCA outcome series are sparser, and that gap in the evidence base should be factored into treatment discussions.

Two practical considerations shape how OCA is delivered. First, chondrocyte viability within a fresh allograft declines with storage time, which means surgical scheduling must align closely with tissue procurement. In both NHS and independent-sector pathways in the UK, this places real demands on coordination with a tissue bank and on theatre availability within a narrow window after graft release. Second, the graft must be size-matched to the recipient femoral head anatomy — a planning step that requires accurate pre-operative imaging and may affect lead times.

On immunology, an allograft does provoke a documented immune response, but the cartilage matrix provides a degree of immunological shelter for the embedded chondrocytes. Clinical graft rejection remains uncommon in practice, though it is not eliminated entirely.

Who is a suitable candidate for hip OCA over OATS

The clearest candidates for hip OCA over OATS are patients with a focal area of damage to the ball of the hip joint — the femoral head — or the socket lining that has grown too large for a single autograft plug to cover adequately.

Several underlying conditions bring patients to this decision point. Femoroacetabular impingement (FAI), where the ball and socket make abnormal contact during movement, can strip cartilage from the femoral head over time, sometimes producing lesions of this magnitude. Osteochondritis dissecans — a condition in which a fragment of bone and cartilage loses its blood supply and may separate — is another recognised cause when it affects the hip joint, presenting in advanced stages with activity-related pain and catching or locking. Post-traumatic lesions following a significant hip injury, and early avascular necrosis at the stage where cartilage disruption is already present, can similarly produce defects that exceed what an autograft procedure can restore.

The patient profile that benefits most is typically a younger or middle-aged active adult with a focal, contained defect in an otherwise well-preserved hip joint — someone who is not yet a candidate for hip replacement. OCA in this setting is a joint-preservation procedure: the aim is to restore the articular surface and extend the functional life of the native hip, deferring or avoiding replacement rather than competing with it.

Patients with advanced or diffuse hip osteoarthritis sit outside this preservation window. Widespread joint space loss and non-focal cartilage damage are not amenable to point-specific restoration; for those patients, a separate pathway discussion centred on hip replacement is more appropriate.

Pre-operative planning includes MRI to define defect size, depth, and the condition of the surrounding joint. Where FAI is the underlying mechanical cause, correcting the bony geometry at the same time as the graft is part of standard planning — leaving the impingement unaddressed risks early graft failure. Individual suitability depends on the full imaging picture, clinical history, and shared decision-making with a specialist experienced in hip joint preservation.

What the evidence shows — and where it is still developing

Hip-specific randomised trial data comparing OCA with OATS remain sparse. The procedural complexity of hip cartilage restoration, combined with the relative rarity of large focal defects in younger patients, makes recruiting comparable populations difficult — and that difficulty explains the evidence gap, rather than reflecting a pattern of poor procedural outcomes.

The strongest long-term outcome anchors come from knee series. Decades of fresh allograft experience in post-traumatic knee defects demonstrate that OCA can sustain functional results well beyond ten years; the biological principles governing graft survival apply equally in the hip, but hip-specific series remain smaller and less mature. For patients weighing a decision now, OCA in the hip is therefore supported by biological coherence and transferable outcome data, even where hip-specific evidence is still accumulating.

Surgical approach introduces a variable with no direct equivalent in knee repair. Open surgical dislocation — the traditional route for hip OCA — provides direct graft access and precise seating, but carries a more demanding recovery than hip arthroscopy. Arthroscopic delivery, where technically feasible, reduces soft-tissue disruption but requires specialist training and is not universally available. The chosen route influences both graft placement precision and the rehabilitation trajectory that follows.

Post-hip-OCA rehabilitation is less standardised than its knee equivalent, and weight-bearing timelines vary between centres. What the published series consistently show, however, is that younger, active patients with contained focal defects in an otherwise healthy hip joint achieve meaningful and sustained improvements in pain and function — a pattern that holds across different surgical approaches and follow-up periods.

1. [1] Articular cartilage stem cell paste grafting. https://en.wikipedia.org/?curid=36740925 https://en.wikipedia.org/?curid=36740925
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4. [4] Osteochondritis dissecans. https://en.wikipedia.org/?curid=3762029 https://en.wikipedia.org/?curid=3762029