Meniscus Deficiency and Meniscal Transplants

Introduction

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

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:

• 1.

  Unicompartmental pain in the presence of total or subtotal ‘functional’ meniscectomy

• 2.

  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

• 3.

  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.