When other cartilage repairs are no longer enough

At some point in the cartilage repair conversation, a surgeon will say something like: the damage is too large, or too deep, for the usual options. Understanding what that means — and why it matters — is the first step in deciding what comes next.

The practical threshold sits at around 3–4 cm² of full-thickness cartilage loss. Below that, cell-based approaches such as MACI can perform well, provided the bone beneath the cartilage is largely intact. MACI and similar platforms are designed for lesions in the 2–10 cm² range, but they depend on a sound subchondral foundation — they restore the cartilage layer, not the bone. Once that bone stock is compromised, a cell-based scaffold has nothing structurally solid to bond to, and the repair is unlikely to hold.

Defect size alone does not always tell the full story. A smaller lesion that extends deep into the subchondral bone can cross the same threshold as a wider but shallower one. It is the combination of depth and bone involvement that tips the clinical decision.

Failed prior surgery is also a common route into this discussion. Microfracture — a widely used first-line technique — stimulates fibrocartilage rather than true hyaline cartilage, and that fibrocartilage typically breaks down within two to three years. It also leaves residual damage to the subchondral bone plate that can make any subsequent repair, whether cell-based or allograft, technically more difficult. When microfracture, mosaicplasty, or ACI has already been attempted without lasting success, the structural options that remain are narrower.

What the graft actually replaces

The graft used in OCA transplantation is a plug of donor tissue cut to match the shape and depth of the damaged area, comprising the overlying hyaline cartilage and the subchondral bone beneath — both transferred together in a single operation. This is the structural logic that sets it apart from cell-based approaches such as ACI or MACI, which restore only the cartilage surface and depend on the patient's underlying bone remaining sound enough to anchor the repair.

Why fresh tissue is preferred comes down to cell survival. Chondrocytes — the cells that produce and maintain cartilage — must be alive at the point of implantation for the graft to integrate properly. Freezing kills them; a frozen graft provides architectural scaffolding but not biological activity. Fresh donor tissue, sourced and used within a defined holding window, is therefore the clinical standard for osteochondral allograft.

Storage temperature and chondrogenic supplementation during that holding period both affect how many viable cells reach the operating table, and not all grafts arrive in equivalent condition. The usable life of fresh tissue is measured in days rather than months, meaning the surgeon, the tissue bank, and the patient's calendar must all align within a narrow timeframe. When a size-matched graft cannot be secured in time, surgery is deferred — a practical constraint on access and cost that does not apply to procedures using the patient's own tissue.

How the operation is performed

Inside the operating theatre, the surgeon works with two fundamental tools: a prepared recipient site and a precisely shaped piece of donor tissue. In the majority of cases, a circular coring instrument is used to remove the damaged cartilage and underlying bone, leaving a clean cylindrical socket. A matching cylindrical plug — the dowel — is then shaped from the donor graft, sized to fit, and press-fitted into position. No screws or fixatives are typically required; the interference fit holds the graft in place while osseous integration begins.

Where the defect is large or geometrically irregular — contoured across a curved surface in a way that circular instrumentation cannot adequately cover — a shell technique is used instead. Here the graft is cut as a flatter, contoured piece shaped to match the topography of the joint surface rather than a simple cylinder. The shell approach demands a higher degree of intraoperative judgement about curvature and surface contact.

In both cases, accurate sizing is not optional. A graft that stands proud or sits too deep creates abnormal mechanical loading at its margins and risks early failure at the repair interface.

The procedure requires general or regional anaesthesia and is not a day-case in the way that a minor knee arthroscopy might be — patients should plan for a hospital stay. In the weeks immediately afterwards, weight-bearing through the operated limb is restricted to protect the bone-to-bone healing interface while the osseous component of the graft incorporates into the recipient site.

Long-term outcomes and prognostic factors

The numbers are worth stating plainly. For isolated, single-surface (unipolar) lesions — the most common presentation — graft survivorship in published series runs between 78% and 91% at ten years, with clinically meaningful improvements in pain and function and high patient satisfaction scores reported across those studies. These figures represent the benchmark outcome for OCA in the setting it was designed for.

Four variables predict where an individual patient is likely to sit within that range: age below 30, a traumatic rather than degenerative cause, a unipolar lesion pattern, and surgery performed within twelve months of symptom onset. Patients who meet all four criteria tend to track toward the upper end of survivorship data; those presenting later, older, or after degenerative deterioration tend toward the lower end.

The picture changes substantially when damage involves both sides of the joint — the femoral and tibial surface (or patella), rather than one. These bipolar cases are categorically different in complexity: mean defect areas around 16.7 cm², with most patients arriving after a median of three prior procedures. Graft survivorship in this group falls to approximately 73.8% at five years and 58.9% at fifteen years, with a 34.8% graft failure rate. For these patients, OCA is better understood as a joint-salvage operation than a primary repair — a meaningful distinction in setting expectations before surgery.

The three variables most worth discussing in a pre-operative conversation are age, lesion pattern (unipolar versus bipolar), and prior procedure burden. Together they shape both the probability of success and the realistic goals of the operation.

Return to sport and the role of BMAC augmentation

Active patients tend to ask a specific question before anything else: will I be able to get back to my sport? The honest answer, drawn from a systematic review of 14 studies covering 471 athletes, is that 72% do return to competitive or recreational sport after OCA transplantation of the knee. Of those who return, 84% do so at the same level or higher than before injury, at a weighted mean of 11.1 months post-surgery. These are clinically meaningful figures — but the obverse matters equally. Roughly one in four athletes does not return to sport, and framing the probability plainly tends to build more trust in the decision-making process than leading with the positive number alone.

The rehabilitation pathway is structured rather than linear. Crutches and restricted weight-bearing typically continue for six to eight weeks to protect osseous healing, followed by supervised physiotherapy into the third month. Full return to high-level activity generally falls within the six-to-twelve-month window, though individual timelines vary by lesion size, concurrent procedures, and patient compliance. Planning around a phased return — rather than a fixed discharge date — is more realistic and more useful for athletes preparing training schedules.

BMAC augmentation: reducing the risk of reoperation

At the time of transplantation, some surgeons now add bone marrow aspirate concentrate (BMAC) to the graft interface. In a prospective blinded RCT (n=36), BMAC augmentation reduced subsequent reoperation rates from 35.3% in the sham group to 5.3% in the BMAC group — a statistically significant fivefold reduction (p=0.02). The mechanism is thought to relate to improved osseous integration rather than cartilage regeneration.

What the trial does not show is equally worth noting. Patient-reported outcome measures at six months, one year, and two years did not differ significantly between groups, though the BMAC group trended toward higher rates of achieving the minimum clinically important difference on the KOOS JR (88% vs 55%; p=0.076). The practical implication: BMAC, where used, appears to reduce the likelihood of a second operation rather than accelerate symptomatic recovery. It is an adjunct added during the primary procedure, not a separate treatment episode.

OCA's place in the repair pathway — and when to get assessed

OCA sits at the top of the cartilage repair hierarchy — above microfracture, AMIC, OATS, and MACI — for cases where defect size or bone loss makes simpler options structurally inadequate, or where those options have already been attempted and failed. The case for escalating to OCA rather than repeating marrow stimulation rests on the fibrocartilage breakdown evidence introduced in the opening section; the more direct argument is the long-term survivorship data, which hold up over a decade for patients presenting with the right combination of prognostic factors. Outcomes in appropriate cases are durable — the preceding sections set out the variables that determine where an individual sits within that range.

The evidence base, however, warrants honest framing. Most published series are level 3–4 — case series and registries — rather than prospective randomised trials. For the borderline zone of 3–6 cm² with mild bone involvement, no head-to-head RCT has compared OCA directly with MACI; decision-making in this grey area depends on clinical judgement, imaging, and institutional experience rather than guideline-grade evidence.

Access is a practical constraint as well. Fresh graft availability varies regionally, costs are considerable, and reliable outcomes depend on centres with established procurement relationships and adequate surgical volume. Specialist assessment is therefore a prerequisite rather than a formality. For patients in London presenting with complex defects or previously failed repair, Professor Paul Y. F. Lee's practice at the Harley Street clinic provides that evaluation — a consultation can be arranged via londoncartilage.com.

1. [1] Midterm Survivorship and Clinical Outcomes in Fresh OCA Transplantation for Large Bipolar Lesions of the Knee. (2025). https://doi.org/10.1177/03635465241313139 https://doi.org/10.1177/03635465241313139
2. [2] Optimum storage conditions for osteochondral allograft plugs: An ex vivo comparative study of 12 storage protocols. (2025). https://doi.org/10.1002/jeo2.70206 https://doi.org/10.1002/jeo2.70206
3. [3] Osteochondral Allograft Transplantation (Clinics in Sports Medicine, 2025). (2025). https://doi.org/10.1016/j.csm.2024.12.002 https://doi.org/10.1016/j.csm.2024.12.002
4. [4] Return to Sport in Athletes After Osteochondral Allograft Transplantation: A Systematic Review. (2025). https://doi.org/10.1177/03635465251315492 https://doi.org/10.1177/03635465251315492