Meniscal injury is the most common knee pathological condition encountered in orthopaedic clinics. Although only a few decades ago the menisci were considered a vestigial structure, today we understand their importance as key structures in providing optimal knee function. , The menisci not only assist in shock absorption and reduction of pressure loads across the knee but also have an important role in providing knee stability and nourishment to the articular cartilage. Loss of meniscal function can be devastating and result in rapid articular cartilage degeneration and early-onset osteoarthritis. Functional meniscus loss commonly occurs after subtotal or total meniscectomy. However, compromise of meniscal function also results from a root tear or a deep radial tear, with a consequential impairment in transmission of circumferential hoop stresses, equivalent to meniscus absence. , With our evolving understanding of the cardinal role the menisci play in optimal knee function, meniscal repair has become the preferred modality for these meniscal tears. However, some tears are not amenable for repair, leaving the patient with a nonfunctional or absent meniscus.
Meniscal allograft transplantation (MAT) was developed for patients with an absence or functional loss of the meniscus to attempt to reduce pain, restore function, and perhaps mitigate the rate of osteoarthritis progression. Since the first MAT performed by Milachowski et al., MAT has become an acceptable surgical treatment for symptomatic patients with a nonfunctional meniscus or who have undergone a total meniscectomy. , Although still considered a relatively rare procedure (estimated at 1 out of 1 million people), rates of MAT are increasing in patients younger than 45 years, specifically in high-volume hospitals. With the evolution and improvement in patient selection, graft preservation, graft sizing, surgical techniques and rehabilitation protocols, MAT has proven to be an effective midterm treatment modality for select patients.
Patient selection is a critical process that in many cases predetermines the fate of a procedure and patient. , The process of patient selection includes evaluating the patient’s medical history, a meticulous physical examination and review of imaging. Patient selection should also be dependent on an open discussion with the patient regarding the procedure, its risks and benefits, rehabilitation period and patient expectations.
For many years the indications and contraindications for MAT were not well defined, and consequently mostly relied on an individual surgeon’s own experience. In general, MAT should be considered for patients with meniscus-deficient knees only after exhausting conservative measures have failed. The patients’ age cut-off is still under debate, but most would agree MAT is indicated in patients younger than 40 and highly active patients younger than 55 with minimal arthritis.
In 2015 the International Meniscus Reconstruction Experts Forum (IMREF) produced a consensus statement providing some guidance regarding the optimal indications for MAT. The consensus statement recommended the following primary indications for MAT:
Unicompartmental pain in the presence of total or subtotal ‘functional’ meniscectomy
As a concomitant procedure to revision anterior cruciate ligament (ACL) reconstruction to aid in joint stability when meniscus deficiency is believed to be a contributing factor to failure
As a concomitant procedure with articular cartilage repair in a meniscus-deficient compartment
Contraindications for MAT are less defined and most are still under debate and considered relative. At this point, MAT should not be considered a routine procedure in a young asymptomatic patient with a meniscus-deficient knee. Although several preclinical studies have demonstrated potential chondroprotective effects of MAT, , there is a paucity of clinical evidence supporting the notion that MAT can prevent or delay cartilage degeneration and osteoarthritis progression. , Moderate to severe osteoarthritis with Kellgren-Lawrence grade III or IV is considered a relative contraindication, with several studies demonstrating conflicting results in this subset of patients. , More specifically, Fairbanks changes such as flattening and squaring of the femoral condyle change the bone architecture of the involved compartment, resulting in extrusion of the meniscus allograft. We suggest caution in patients with a body mass index (BMI) more than 35, not only because of the expected excess mechanical load but also because of the proinflammatory state associated with obesity. Jiménez-Garrido et al. performed a retrospective cohort study that reported higher MAT failure rates in patients with BMI more than 30. However, of note, Saltzman et al. found no difference in outcomes between patients with a BMI more than 25 and patients with BMI less than 25 in their 7-year follow-up prospective study. An inability to correct concurrent pathological changes, such as irreparable chondral damage, uncorrectable malalignment and instability, is considered a relative contraindication. Active infection and inflammatory arthropathy can be detrimental to the transplanted graft and are considered absolute contraindications.
Graft Preservation and Preparation
Graft preservation techniques include fresh-frozen, viable, cryopreserved and lyophilised allografts. The IMREF recommends the use of fresh-frozen or viable allografts and reported that 68% of surveyed surgeons use fresh-frozen allografts, whereas 14% use viable grafts. Fresh-frozen grafts can be stored for up to 5 years at –80°C, which is advantageous in terms of their high availability. Although fresh-frozen allografts harbour a negligible number of viable cells, studies have not found a lack of cell viability to adversely affect graft survival or clinical outcomes. , Moreover, fresh-frozen grafts are associated with lower risk for disease transmission and relatively reduced costs.
In similarity to other organ transplants, the use of viable grafts may be ideal. The advantage of using viable grafts is anchored in the supply of viable cells within the graft. The preservation of live meniscus cells and an intact extracellular matrix in viable grafts is presumably the cause for its improved biomechanical properties and relatively long-term survivorship, as demonstrated by Verdonk et al. , The challenge with viable allografts is the relatively short period allowed from harvest to implantation, which is estimated at 10 to 14 days. Another limitation is the potential higher risk for disease transmission because the short time window can preclude some serological testing and cell-preservation precludes sterilisation of the graft.
Cryopreserved grafts and lyophilised grafts have generally gone out of favour because of inferior long-term survival of grafts stored using these methods. ,
Regardless of the preservation method used, grafts should be provided on a bone block with the meniscotibial (coronal) ligament attached. This allows the surgeon the freedom to choose between bony and soft tissue fixation techniques and also whether to incorporate the meniscotibial attachments. Although preservation of the meniscotibial ligament has not yet been proven to positively affect outcomes, many surgeons elect to preserve this structure as an augment to peripheral fixation.
Donor allografts should be compartment specific and size matched to the recipient. Studies support the notion that sizing of the width of the allograft is more important than sizing of the length. , Undersizing the width of the meniscal allograft by more than 5 mm can lead to increased rates of clinical and mechanical failures, whereas an oversized allograft can translate to meniscal allograft extrusion. Graft extrusion is a common finding after MAT, which had received much attention as a possible predictor of failure after MAT. However, most studies testing this theory have not shown an association between graft extrusion and clinical failure.
Graft sizing is performed using radiographs or magnetic resonance imaging (MRI). Although several measurement methods have been described, it is unknown whether a specific method is superior to others. The most commonly used sizing method was described by Pollard et al. The technique entails obtaining magnification-corrected plain anteroposterior (AP) and lateral radiographs. Width is assessed on AP view by measuring the distance between two lines perpendicular to the joint line, a line tangential to the margin of the proximal tibia metaphysis and a line between the medial and lateral tibial eminence. Length is assessed on lateral view by measuring the distance between a line marking the anterior surface of the tibia (above the tibial tuberosity) and a line tangential to the posterior margin of the tibial plateau. The lengths of the medial and lateral menisci are estimated at 80% and 70% of the measured tibial plateau, respectively. Yoon et al. described a modification to the Pollard method in an attempt to increase accuracy, particularly addressing measurement mismatch of the lateral meniscus. However, Yoon’s method is still less commonly used.
Several methods for performing measurements using MRI have been developed to increase accuracy and appreciate the three-dimensional structural attributes of the meniscus. , More surgeons are transitioning to graft sizing using MRI, whether commonly applying the Pollard method to MRI coronal and sagittal sequences or obtaining an MRI of the contralateral knee and using the Yoon method for MRI-based graft sizing. When sizing according to the contralateral knee, the surgeon should acknowledge that there is variability in meniscal size between opposite knees. Although less commonly used, graft size can be estimated by a multivariate regression model using height, weight, and gender, as described by Van Thiel et al. Finally, although matching the meniscal allograft to the recipient’s knee is critical, the tolerance to size mismatch in MAT is mostly unknown.
MAT surgical techniques include several approaches (mini-open or arthroscopic), horns/root fixation techniques (soft tissue, bone plugs or bone bridge) and peripheral suture techniques (inside-out or all-inside) ( Fig. 19.1 ).
Advocates of the mini-open approach indicate the ease and accuracy associated with the technique and perhaps also the shorter operative time. However, no high-quality studies have tested these assumptions or pointed towards a superior approach. Nonetheless, positioning of the graft is vital to restore physiological knee function. Choi et al. have reported that lateral positioning of a lateral MAT can affect meniscal extrusion. Similarly, Kim et al. found that changes in the positioning in the coronal plane of the anterior and posterior horns of a medial meniscal graft were associated with extrusion.
Techniques for root fixation include bone bridge, bone plugs and soft tissue fixation ( Fig. 19.2 ). Most surgeons prefer the use of bony fixation, with a bone bridge for the lateral meniscus and bone plugs for the medial meniscus. Several earlier studies suggested soft tissue fixation techniques are associated with inferior load distribution, greater meniscal extrusion and perhaps a higher complication rate compared with bony fixation. However, more recent clinical evidence supports the notion that there is no surgical technique for MAT that had proven superior in terms of clinical outcomes. , , , A 2018 meta-analysis by Jauregui et al. included 38 studies reporting on 1673 MATs. They found no significant differences between soft tissue fixation and bone fixation in terms of graft tear rates (13.4% versus 14.9%, respectively), failure rates (17.6% versus 18.8%, respectively), Lysholm scores, visual analogue scale (VAS) scores and meniscal extrusion.
Peripheral fixation is typically performed with all-inside devices, inside-out sutures or both. There is no evidence to support either suture technique, but regardless of technique, one should keep in mind the proximity of the neurovascular bundle to the posterior horn of the lateral meniscus, particularly in young and female patients. Interest in mitigating the risk for meniscal graft extrusion has resulted in several new techniques for peripheral fixation, including capsulodesis and transosseous fixation.
Concomitant Pathological Conditions
Concomitant pathological conditions should be addressed before or at time of MAT. Preoperative planning of MAT should include screening for concurrent pathological conditions. The physical examination must detect malalignment and/or instability. Standing long-leg radiographs should always be performed to determine the mechanical axis and allow relevant measurements. MRI should be assessed for any additional cartilage, ligamentous or meniscal pathological changes. Instability usually is due to an ACL tear and warrants an ACL reconstruction at time of MAT to restore stability and protect the graft. Malalignment should be corrected with a high tibial osteotomy (HTO) or distal femoral osteotomy, as indicated. The exact cut-off above which corrective osteotomy is indicated is unknown; the IMREF recommends considering realignment osteotomy in combination with MAT when the weightbearing axis falls within the affected compartment. Cartilage lesions should also be addressed at time of MAT, with several case series showing satisfactory outcomes of concomitant articular cartilage repair procedures and MAT. , A couple of more recent studies also reported favourable outcomes of concomitant osteochondral allograft (OCA) transplantation with MAT, , with an 86% OCA graft survivorship at 5 years.
Several meta-analyses have reported no difference in outcomes, failures and complications between patients undergoing isolated MAT and patients undergoing MAT with a concomitant HTO, ACL reconstruction or cartilage repair procedure. , , , These should be interpreted carefully because there are no prospective randomised trials comparing isolated MAT and MAT with a concomitant procedure.
MAT outcomes have been acceptable, with the majority of studies reporting improved patient-reported outcomes (PROs) and supporting the role of MAT in reducing pain and improving function in symptomatic meniscal-deficient patients.
Most studies on functional outcomes after MAT are retrospective level IV studies. Several meta-analyses of outcomes of MAT have all reported an improvement in PROs compared with preoperative scores after MAT at short-, mid- and long-term follow-up. , , , Interestingly, Vundelinckx et al. also reported that 90% of patients were very satisfied or satisfied with the procedure and would undergo the procedure again (although calculated after excluding patients who converted to arthroplasty). A 2019 systematic review of long-term outcomes of MAT by Novaretti et al. included 11 studies of 688 MATs with a minimum 10-year follow-up. They found PROs were fair and improved compared with preoperative scores. They also reported long-term survival rates of 73.5% and 60.3% at 10 and 15 years, respectively.
Mean times to failure for a medial and lateral MAT are estimated at approximately 8.2 and 7.6 years, respectively. Bin et al. published a meta-analysis comparing midterm and long-term outcomes of medial and lateral MAT. They found patients undergoing lateral MAT had significantly greater improvement in PROs and pain relief but no difference in survival rates (5 to 10 years, 85.8% to 89.2% survival rates; >10 years, 52.6% to 56.6% survival rates).
Complications after MAT are generally not severe and are comparable to meniscal repair. McCormick et al. retrospectively reviewed 172 patients who underwent MAT by a single surgeon with a minimum 2-year follow-up. Sixty-four patients (32%) underwent reoperation; of these, arthroscopic debridement was the most common secondary procedure (59%). Seventy-three percent of reoperations occurred within 2 years. A secondary procedure was not detrimental because these patients had an 88% graft survival rate in 5 years (compared with 95% in patients not requiring a second procedure). However, the secondary procedure was associated with risk for revision MAT or arthroplasty with an odds ratio of 8.4 ( P = .007).
Several patient factors can be associated with outcomes after MAT and should be considered. A 2019 study by Zaffagnini et al. challenged the 50 years old age limitation for MAT. They performed a matched cohort analysis of patients older than age 50 ( N = 26) and patients younger than 30 ( N = 26) who underwent MAT with a minimum follow-up of 5 years. They found both groups improved in terms of clinical outcomes, with two-thirds of patients in the older group returning to recreational sports activity and only two patients converting to arthroplasty. However, older patients had a significantly lower increase in clinical scores and, although not significant, higher failure rates (31% versus 15%, P = .32) and lower graft survival time (11.6 years versus 12.3 years, P = .69). Garrett published a review of 43 patients with 2 to 7 years follow-up and found unfavourable results to be associated with grade IV articular changes of the femoral condyles. Similar results were found by Parkinson et al. in a cohort study of 125 patients. They found the presence of severe cartilage damage and medial allografts to be predictive of failure. As described earlier, severe osteoarthritis and a BMI greater than 35 are relative contraindications for MAT. Higher BMI and workers’ compensation–related injuries were also found to be associated with failure to achieve clinically significant patient-reported outcomes and should be considered and discussed during the patient selection process. Additionally, Waterman et al. found that smoking is associated with failure in military recruits undergoing MAT ( P = .028), whereas a procedure performed by a high-volume surgeon had a significant protective effect ( P = .046).