The knee joint meniscus: Can it be successfully repaired or replaced?
Authors: Hope N. G. A. 
Published: March 2003

This paper explores currently available and potential solutions to the common problem of meniscal injury and examines the question of repair or replacement. Knowledge of the structure of the meniscus is critical to these questions. The medial and lateral menisci are semicircular wedges of fibrocartilage lying within the knee joint. The peripheral 1/3 of each meniscus is vascularized and has the potential to heal. In contrast, the inner 2/3 has no blood supply and does not heal. The material and structural properties of each meniscus primarily serve to transfer load and absorb shock. Secondary functions include aiding joint stability, lubrication and proprioception.

A meniscal tear is the most common knee injury. Traumatic vertical lesions occur in younger people while degenerative horizontal tears are present in older patients. Symptomatic injured menisci are assessed arthroscopically to determine the stability and vascularity of the tear. Stable vascular tears are left alone and will heal spontaneously. An unstable avascular tear will not heal and is best resected to a stable rim. Suitable unstable vascular tears may be successfully repaired with an arthroscopic inside‐out suture technique. Success rates average 80% but may vary from 60 to 90%, primarily dependent upon selection of the correct tear to repair. Despite the success of meniscal repair, partial meniscectomy is commonly performed as tears that are unsuitable for repair are frequently found. Unfortunately, even partial meniscectomy results in chondral damage and subsequent osteoarthritis. Thus, replacement of damaged tissue is ideal for chondroprotection.

Meniscal allograft transplantation is an experimental procedure with little current clinical application. Animal trials revealed that transplanted tissue could attach and heal to the periphery as well as repopulate with host derived cells. However, human trials have been disappointing with 40–60% failure rates. Collagen based biosynthetic scaffolds have been used to successfully replace meniscal tissue in a canine model. In a subsequent human trial, newly formed tissue was seen at second look arthroscopy at 3 and 6 months. Unfortunately, no long‐term studies are available and once again, this is an experimental procedure only.

Adjuncts to meniscal replacement include growth factor and gene therapy. Studies to date indicate that appropriate growth factors may enhance cellular repair. Cellular migration of peripheral fibroblastic meniscal cells is stimulated by IL‐1 whilst PDGF increases cellular proliferation. Contrasting this, cellular migration is stimulated by IGF‐1 and BMP in the central chondrocytic zone. Recently, gene transfer has been performed into meniscal cells in vitro. Additionally, direct transfer into meniscal tissue has been shown in vivo. Therefore, it may be possible to transfect meniscal cells with genes to stimulate protein production, which in turn could stimulate proliferation, and matrix production.

Combining the technologies of biosynthetic scaffolds, cell culture, growth factor and gene therapy is the evolving science of meniscal tissue engineering (MTE). Biodegradable synthetic polymers may be fabricated into complex shapes. These polymer matrices act as a scaffold for cellular attachment and growth. Layered release of growth factors could enhance attachment, proliferation and matrix production. A polyglycolic acid scaffold was used to successfully repair meniscal defects in rabbits. Explant defects were also healed using fibrochondrocytes on a PGA scaffold in a canine model. A cell–polymer construct has been engineered to replace the sheep meniscus. Analysis showed gross, histological and biochemical similarities to the normal meniscus. This demonstrated the feasibility of regenerating meniscal tissue in vivo using adult cells in an immune‐competent model. The author is currently involved in MTE research.