Meniscal Transplantation

Meniscal allograft transplantation (MAT) replaces a previously resected meniscus with a size-matched donor allograft. The goal is pain relief and delay of arthritis progression and eventual arthroplasty.

Ideal Candidate

• Young (typically under 50 years)
• Post-total or near-total meniscectomy
• Symptomatic compartment pain
• Minimal arthritis (Kellgren-Lawrence grade 2 or less)
• Normal alignment (or willing to correct with osteotomy)
• Stable knee (ACL intact or reconstructed)
• Reasonable BMI

Goals

• Pain relief (primary)
• Improved function
• Delay of arthritis progression
• Delay of arthroplasty

The meniscus transmits 50-70% of axial load in the compartment, deepens the tibial articular surface, and contributes to shock absorption, joint lubrication and secondary stability. Total meniscectomy removes this protection, and the rationale for MAT is to restore that biomechanical role.

Why Meniscectomy Damages the Joint

• Contact area falls and peak contact stress rises sharply — after total meniscectomy contact pressure can rise by roughly two-to-three-fold, concentrating load on a smaller cartilage footprint.
• Accelerated cartilage wear — the classic basis of the Fairbank changes (joint space narrowing, ridge/osteophyte formation, flattening of the femoral condyle) seen radiographically after meniscectomy.
• Lateral compartment is least forgiving — the lateral compartment is more conforming and more meniscus-dependent, so lateral meniscectomy is poorly tolerated and lateral compartment OA can progress rapidly.

Post-Meniscectomy Syndrome

This is the target clinical entity for MAT: activity-related pain localised to the meniscectomised compartment in a knee that is still relatively well preserved (cartilage largely intact, alignment neutral, ligaments competent). The graft restores load distribution; it does not regenerate cartilage that is already lost.

History

• Previous total or subtotal meniscectomy (often years earlier)
• Compartment-specific pain with activity, sport, prolonged standing or stairs
• Swelling after activity; sometimes mechanical symptoms
• Document prior ACL injury/reconstruction and any prior cartilage procedure

Examination

• Joint-line tenderness in the affected compartment
• Effusion; assess range of motion (stiffness is a relative contraindication)
• Ligament examination — Lachman/pivot-shift; instability must be corrected first
• Gait and alignment — varus or valgus thrust signals malalignment that must be addressed

Pre-operative Checklist (the four pillars)

1. Cartilage — must be acceptable (Outerbridge/ICRS low grade; KL grade 2 or less)
2. Alignment — neutral, or planned osteotomy to correct
3. Stability — ligaments competent or reconstructed
4. Sizing — accurate size-matched graft secured

Graft Types

Fresh-Frozen Allograft: Most common. Stored at -70°C. Maintains mechanical properties. No viable cells.

Cryopreserved: Some cell viability preserved. More expensive. Uncertain clinical advantage.

Fresh: Highest cell viability. Logistical challenges. Disease transmission risk.

Sizing

Critical for success. Undersized or oversized grafts have inferior outcomes.

Methods:

• Plain radiographs of recipient knee (AP and lateral). Pollard method calculates meniscal dimensions from tibial plateau size.
• MRI of contralateral knee (if meniscus intact).

Match within 5% of native dimensions (width, length, horn-to-horn).

The topic's graft-type list names "fresh-frozen", "cryopreserved" and "fresh" grafts and flags a "disease transmission risk", and the controversies note that irradiation may weaken the graft — but the why behind graft processing is a recurring viva theme in its own right. Processing determines cellularity, immunogenicity, mechanical strength and infection risk, and there is a genuine trade-off between sterility and graft integrity.

Processing Methods

• Fresh-frozen — deep-frozen (typically around minus 80°C). Donor cells are rendered non-viable, but the collagen ultrastructure and mechanical strength are preserved and immunogenicity is reduced. Long shelf life. This is the most widely used and best-supported graft.
• Cryopreserved — controlled-rate freezing with a cryoprotectant preserves a proportion of cell viability. More expensive, and a clear clinical advantage over fresh-frozen has not been shown.
• Fresh (viable) — highest cellularity but must be implanted within days, is logistically demanding, and carries greater immunogenicity and theoretical disease-transmission concern; rarely used.
• Lyophilised (freeze-dried) — historically used but associated with graft shrinkage and inferior outcomes, and is now largely abandoned.

The Sterilisation Trade-Off

Terminal gamma irradiation is dose-dependent: high doses (in the region of 25 kGy and above) damage collagen cross-links and reduce mechanical strength, which is why non-irradiated, aseptically-processed grafts are preferred. This is the mechanistic reason societies favour fresh-frozen non-irradiated tissue over heavily-sterilised grafts.

Disease Transmission and Regulation

Transmission risk is mitigated by donor selection, serological plus nucleic-acid testing for HIV, hepatitis B and hepatitis C, and bacterial culture; with modern screening the residual viral-transmission risk is extremely low. Historical clostridial infections from inadequately processed grafts drove stricter aseptic processing and regulation. Tissue banks are governed by bodies such as AATB accreditation and FDA oversight (US), the EU Tissue and Cells Directive, and the Human Tissue Authority (UK), covering procurement, screening, processing and traceability.

The topic repeatedly relies on graft "incorporation", "bone-to-bone healing" and "healing of horns/periphery", and its worst-case survivorship card attributes long-term failure to a "deleterious remodeling process" and "biological failure (poor graft integration)". Understanding how an essentially acellular graft heals explains both the rehabilitation restrictions and why the graft is not permanently normal tissue.

Two Healing Interfaces

• Peripheral meniscosynovial (soft-tissue) junction — heals to the capsule by host fibrovascular ingrowth from the vascular peripheral rim; this restores the peripheral attachment that generates hoop stress.
• Bony fixation — bone plug or bone bridge heals by bone-to-bone union within the tibial tunnels or slot, giving more secure and anatomic root fixation than suture-only.

Repopulation and Revascularisation

At implantation the fresh-frozen graft is essentially acellular. Over months the matrix is gradually repopulated by host-derived cells migrating from the synovium and peripheral vascular zone; DNA and immunohistochemical studies confirm the cellular population becomes predominantly of recipient origin. Repopulation and revascularisation are limited to the peripheral third (mirroring the native red zone), while the inner portion remains relatively avascular and hypocellular. The collagen matrix then remodels, with some loss of mechanical properties and MRI signal change over time — the basis of the graft degeneration seen on long-term imaging.

Immunology

Meniscal allografts are relatively immunoprivileged — avascular and low in cellularity — so HLA matching and systemic immunosuppression are not used clinically. A detectable immune response can occur but is rarely clinically significant, and fresh-freezing further reduces immunogenicity.

Imaging Work-up

Sizing Methods Compared

• Pollard radiographic method — widely used; estimates dimensions from tibial plateau width and height on AP/lateral films. Reproducible but prone to outliers.
• Contralateral MRI / 3D surface modelling — uses the intact opposite meniscus as a template; reduces sizing outliers compared with radiographs.
• Intentional slight downsizing — reducing the radiographic estimate by about 5% lowers post-operative extrusion without harming clinical scores.

Pain after meniscectomy has several causes; distinguishing them determines whether MAT is appropriate.

Rehabilitation

• Non-weight bearing or partial 4-6 weeks
• ROM early but protect flexion (limited to 90° initially)
• No deep squatting, pivoting for 4-6 months
• Progressive strengthening
• Return to sport 9-12 months (if permitted)

Outcomes

• Pain relief and functional improvement in the majority — reliable symptomatic benefit is the strongest evidence-based outcome
• Graft survival roughly 70-85% at 5 years, declining to ~45-75% at 10 years depending on definition of failure (clinical/reoperation versus strict MRI/radiographic criteria)
• Lateral MAT tends to fare better than medial in several series
• Chondroprotection is not proven — MAT may slow but does not reliably prevent OA progression
• Return to sport is variable and often not at the pre-injury level

Global Epidemiology

MAT remains a low-volume, specialist procedure performed in selected younger patients (typically third-to-fifth decade, male predominant in published cohorts) who have had total or subtotal meniscectomy. Absolute case numbers are small worldwide compared with primary meniscal surgery, and most evidence is Level III-IV observational data from high-volume centres.

Society and Consensus Positions

Across societies the message is consistent: strict patient selection (cartilage, alignment, stability, sizing) drives outcome, and chondroprotection should not be promised.

Registry and Outcome Tracking

• There is no large dedicated international MAT registry equivalent to arthroplasty registries; outcome data come from institutional series and systematic reviews.
• Several national systems (e.g. UK practice) encourage prospective audit/outcome registration of MAT given its specialised, evidence-limited nature.
• Tissue banks and national transplant/tissue authorities govern allograft procurement, screening and traceability.

High- vs Limited-Resource Practice Variation

• High-resource settings — access to screened fresh-frozen/cryopreserved allografts, MRI/3D sizing, combined osteotomy/ligament/cartilage procedures, and structured rehabilitation.
• Limited-resource settings — allograft availability and tissue-banking infrastructure are the main constraints; MAT is often unavailable, and management defaults to activity modification, unloader bracing, osteotomy where indicated, and eventual arthroplasty.
• Graft cost, regulatory tissue-banking requirements and surgeon experience are the principal global determinants of access.

• Chondroprotection — the central unresolved question. Despite biomechanical plausibility, high-level evidence that MAT prevents OA progression is lacking; benefit on imaging endpoints is inconsistent.
• Cost-effectiveness — symptomatic and quality-of-life gains make MAT likely cost-effective, but this cannot be proven on current data because there is essentially no randomised comparison with non-surgical care.
• Fixation technique — bone plug/bridge versus soft-tissue suture-only fixation: registry/systematic-review data show good outcomes with both, and a clear superiority of bone fixation has not been definitively established.
• Graft processing — fresh-frozen non-irradiated grafts are most widely supported; the clinical advantage of cryopreserved or fresh viable grafts is uncertain, and irradiation/high-dose sterilisation may weaken the graft.
• Extrusion significance — extrusion is common but its correlation with symptoms and chondroprotection is debated; some extrusion appears clinically tolerated.
• Indication creep — expansion to older patients, higher chondral grades, and concomitant cartilage/osteotomy procedures outpaces the supporting evidence.
• Sizing standard — Pollard radiographs remain common despite evidence that contralateral MRI/3D modelling is more accurate.