The size threshold that drives the choice

When a surgeon mentions a need for donor cartilage, the question patients most often ask is whether their defect is simply too large to use tissue from their own knee. The honest answer is: usually, yes — and the size at which that changes is roughly 2 cm².

Below that threshold, a technique called osteochondral autograft transfer — OATS, or mosaicplasty — is typically feasible. A surgeon harvests one or more small plugs of bone and cartilage from a low-load area of the same knee, then presses them into the damaged zone. The procedure is single-stage and uses the patient's own tissue, which has obvious biological and practical advantages.

The constraint is supply. The knee has only a limited area of low-load surface from which plugs can safely be taken without causing problems at the harvest site. For defects approaching 2 cm², that supply is already being stretched; mosaicplasty — in which multiple smaller plugs are arranged like tiles — can extend the reach to around 4 cm², but donor-site morbidity risk rises accordingly. Beyond that, there simply is not enough suitable tissue available in the same joint.

In practice, patients who go on to receive a fresh osteochondral allograft (OCA) — a graft from a donor — tend to present with defects considerably larger than the autograft ceiling. In one series of 156 knees, the mean defect treated with OCA measured 6.4 cm².

The 2 cm² figure is a convergence point across surgical guidelines rather than a hard rule. Defect grade, location within the joint, and the surrounding cartilage quality all inform the final decision, but size is the single most consistent driver.

What fresh osteochondral allograft (OCA) actually involves

Fresh osteochondral allograft transplantation works by replacing the full osteochondral unit — bone and cartilage together — rather than the cartilage surface alone. The graft is a plug or shell taken from a screened deceased donor: a section of donor condyle with intact articular cartilage sitting on top of a layer of supporting bone. That plug is shaped to match the recipient's defect and pressed into the prepared site, restoring the joint surface in a single operation.

The word 'fresh' carries specific clinical weight. Tissue banks source grafts from donors ideally aged 13–35, with procurement completed within 24 hours of death. Chondrocyte viability — the biological currency that allows the transplanted cartilage to survive long-term — declines noticeably after day 14 in storage and drops below the accepted 70% threshold by around day 28. Frozen or cryopreserved grafts lose substantially more of that viability, which is why retrieval studies have confirmed viable chondrocytes and preserved cartilage matrix in fresh OCAs many years after transplantation. Coordinating procurement and implantation within this window requires close liaison with the tissue bank and is a routine part of planning an OCA case.

Because the graft comes entirely from the donor, there is no parallel harvest from the patient's own knee — the donor-site constraint that limits autograft does not apply here. For shell grafts in particular, precise size-matching between the donor condyle and the recipient anatomy is critical to restore normal contact mechanics across the joint surface.

As a single-stage procedure, OCA differs meaningfully from two-stage cell-based approaches such as MACI or ACI, which require a preliminary biopsy and a separate implantation operation months later.

Defect characteristics that point toward OCA

Size points the decision toward OCA; defect characteristics determine whether it is the right reconstructive tool for the specific damage pattern.

OCA is most appropriate for ICRS grade III and IV osteochondral lesions — damage that has worn through the full depth of the cartilage into the bone beneath. That subchondral involvement matters because techniques that rely on placing a scaffold or membrane over the defect require a well-defined rim of healthy cartilage for fixation. Where lesion edges are poorly defined or the margins are irregular, those approaches become technically difficult. OCA sidesteps this constraint by replacing the full osteochondral unit, so clean margins are less critical to the outcome.

Joints with concurrent early osteoarthritis, or knees where a previous marrow-stimulation procedure such as microfracture has already failed, are not automatically excluded from OCA consideration. Its reconstructive scope appears broader than some restorative alternatives precisely because it does not depend as heavily on the quality of surrounding tissue.

Location within the joint adds further nuance. The femoral condyle accounts for most published OCA data; patellofemoral involvement is a distinct situation — graft survival there has been reported at 87.9% at five years and 77.2% at ten years, a different curve from condylar figures and worth a specific conversation rather than an assumption.

Where underlying mechanics contribute to the damage — a malaligned limb or a deficient meniscus — OCA can be combined with an alignment-correction osteotomy or a meniscal allograft in the same or a staged procedure. Raising these possibilities at a first consultation is entirely reasonable: a surgeon experienced in large-defect reconstruction will address them if the joint warrants it, and knowing they exist helps patients ask the right questions from the outset.

Survivorship and functional outcomes: what the evidence shows

Durability data for OCA come from several independent cohorts, and the figures span a meaningful range — which is worth understanding rather than collapsing into a single number.

One series reported graft survivorship of 82.6% at five years and 69.6% at ten years, where 'survival' means the graft remained in place without conversion to arthroplasty. A separate long-term analysis found 95% survivorship at a mean of nearly 13 years, with 68% of 65 grafts still functioning at that point. The divergence between these figures likely reflects differences in cohort composition, lesion severity, and how failure was defined — not a contradiction, but a reminder that no single percentage captures the full picture.

Functional recovery in published series is generally strong. At a mean six-year follow-up, 75% of patients had returned to sport or recreational activity, and 71% recorded 'very good' or 'excellent' knee function on IKDC evaluation. Objective IKDC scores improved from roughly 42 before surgery to 68 afterwards — a clinically meaningful gain. Patellofemoral grafts, as discussed in the previous section, follow a distinct survivorship trajectory and warrant a separate conversation rather than an assumption based on condylar data alone.

Younger age at surgery is a consistent predictor of better results across cartilage repair procedures, and OCA is no exception. For older patients results remain meaningful, but age is a variable a surgeon will weigh carefully at initial assessment alongside lesion grade and prior treatment history.

The honest framing for OCA is that it functions as a joint-preservation bridge: durable by the standards of biological reconstruction, but not permanent for most patients. For many, it delays — often by a decade or more — a decision about joint replacement. That is a realistic and valuable outcome when the alternative is earlier arthroplasty.

Graft biology and the scheduling window

Patients who learn their surgeon plans to use donor tissue often ask the same question first: will the body reject it? The short answer, based on retrieval studies of failed grafts, is that frank immunological rejection appears rare. Histological analysis of retrieved OCAs has found little or no evidence of immune-mediated failure — the pattern seen in solid-organ transplant rejection is not what typically causes OCA to fail. Patients do not require long-term immunosuppression, which is a meaningful practical distinction from other forms of transplantation.

The complication that does shape the experience is logistical rather than immunological. Because the graft is a living biological product — cartilage cells must remain viable to integrate into the recipient site — there is a hard deadline between procurement and implantation. That time pressure is why scheduling an OCA differs from booking most other elective orthopaedic procedures: the surgical team must coordinate closely with the tissue bank, and surgery cannot always be planned weeks ahead at a time of the patient's choosing. The graft, in effect, sets part of the timetable.

This is worth flagging not to cause anxiety but to set realistic expectations. Patients who understand they may need to remain flexible around a date — rather than selecting one from a calendar — are better placed when the tissue bank confirms availability.

Rehabilitation and realistic return to activity

For most patients, the first practical question once OCA is on the table is how long they will be off their feet — not the surgery itself.

The standard answer is six to eight weeks of protected weight-bearing, usually with crutches, during which the graft begins to integrate with the underlying bone. Critically, range-of-motion exercises start immediately within this window, not after it. Keeping the joint moving early reduces stiffness and helps maintain the nutrition supply to cartilage tissue, which has no direct blood supply of its own.

Return to sport or high-demand activity is generally not before six months, and even that requires radiographic evidence of graft healing — not just the absence of pain. Feeling comfortable is not the same as being structurally ready to load the joint. In practice, many patients return to recreational sport somewhere between six and twelve months; those in physically demanding occupations or competitive sport should expect a timeline at the longer end of that range.

The rehabilitation window is longer than for OATS or smaller autograft procedures, reflecting the larger surface area being restored and the need to protect osseous integration before progressive loading begins.

Where concomitant procedures have been performed — osteotomy to correct alignment, or meniscal allograft — the combined rehabilitation pathway is longer still. The treating surgeon and physiotherapy team will set a timetable that accounts for all procedures together; individual timelines will always vary and are best discussed at that level.