Meniscal allograft transplantation

Once a meniscus is gone, the cartilage underneath is carrying a load it was never shaped to carry. Transplantation puts a size-matched donor meniscus in its place and anchors both horns to bone.

What the graft restores, and on what condition

Ten cadaver knees were tested in sequence — intact, then after total lateral meniscectomy, then after an allograft fixed with bone plugs, then after the horn attachments were released.

Meniscectomy cut total contact area by 45% to 50% and raised peak local contact pressure by 235% to 335%. The allograft then raised contact area by 42% to 65% and cut peak pressures by 55% to 65% at every flexion angle — a large recovery, and an incomplete one: pressures stayed significantly above the intact knee's.

Then the experiment did the thing that makes it useful. Releasing the anterior and posterior horn attachments returned contact area and pressure to the meniscectomised values exactly. A transplanted meniscus is a hoop, not a spacer. It carries load by resisting tension around its circumference, and a graft whose horns are not anchored is mechanically the same as no graft at all. That single result is why fixation dominates the surgical literature.

How patients do, and for how long

Across twenty studies — six at level III, fourteen at level IV — covering 1,096 patients at a mean follow-up of about twelve and a half years, patient-reported outcomes improved from before surgery to final follow-up and stayed there. Pooled objective graft failure was 31%, and pooled conversion to a total or unicompartmental knee replacement was 17%, though individual studies reported arthroplasty rates anywhere from 2% to 25%.

A separate long-term series of 42 transplants followed at least ten years found pain and function significantly improved in every group. The group that improved most was the one where a medial transplant had been done alongside a high tibial osteotomy — which is also the group that had its alignment corrected, so the graft is not the only thing that changed.

Whether adding other procedures helps or hurts has been examined across 24 studies and came back unresolved: no separation on Lysholm, Tegner, IKDC or visual analogue scores, and directly conflicting survivorship findings — four studies reporting no effect, three naming ligament surgery, realignment osteotomy and osteochondral autograft transfer as failure risks, and one finding better survival when a medial transplant was combined with an osteotomy.

Does it protect the cartilage?

This is the claim the operation is really sold on, and it is the weakest part of the evidence.

A systematic review asked it directly across 38 studies and 1,056 allografts. Weighted mean joint space loss came to 0.032 mm at four and a half years in the eleven studies that reported it. Healing rates were high. Graft size, shape and signal intensity commonly differed from a native meniscus. And meniscal extrusion was present in nearly every case while predicting neither clinical nor other radiological outcomes — a surrogate that looks alarming on a scan and appears not to matter.

The reviewers' own conclusion is the careful one: there is some evidence that transplantation reduces osteoarthritis progression, and it is unlikely to be as effective as the native meniscus. Their quality assessment rates the underlying studies low, with a high risk of bias, and none of them has a comparator arm — joint space loss is measured against the same knees' own baselines.

One study does carry a control group. Seventeen transplanted knees and 26 meniscectomised knees, followed a mean of 11.3 years, gave joint space narrowing of 0.58 mm (SD 0.66) and 1.26 mm (SD 1.13) respectively, which reached significance; a cartilage degeneration index derived from T2-mapping MRI rose 1132 and 2182, which did not. That is the whole controlled long-term evidence base, and it is 43 knees whose operations were chosen rather than assigned.

Reading this before an operation

The mechanical case is strong and specific: a well-fixed graft demonstrably restores much of the contact area a meniscectomy removed. The symptom case is consistent and long. The structural case — the one that justifies operating on a knee to protect it rather than to relieve it — is uncontrolled radiography that its own reviewers do not trust much, plus one 43-knee comparison.

Roughly a third of grafts fail on the pooled definition, and a sixth of patients go on to a replacement, at a mean follow-up beyond twelve years. Both numbers come with wide heterogeneity, because failure is defined differently from study to study.

What would move it

A controlled comparison against meniscectomy alone, with a cartilage-specific imaging endpoint and enough knees to detect a difference, is the study this field is missing — the existing one has 43. Prophylactic transplantation in a young meniscus-deficient knee is the idea the chondroprotection literature keeps gesturing at, and the review that raises it conditions it on the claim being proven first, which is the right order. And the extrusion finding deserves attention on its own terms: something happens to nearly every graft, is clearly visible, and appears to predict nothing, which usually means the surrogate is measuring the wrong thing.