Engineering of human meniscus substitute

Injuries to the meniscus are often treated by partial or total meniscectomy, which is known to be associated with detrimental changes in joint function, ultimately increasing the risk of early degenerative joint diseases. Surgical approaches currently in use to substitute the damaged meniscus (e.g., the use of allografts or of a collagen-based material) can initially restore a stable and pain-free joint, but long-term clinical results, especially related to the protection of the articular surface, are still uncertain. Recently, tissue engineering strategies have been proposed for the generation of meniscus substitutes, based on the loading and culture of suitable cells into appropriate biodegradable porous scaffolds. With the intention to engineer a meniscus substitute based on autologous cells, the first aim of the thesis was to identify an appropriate human cell source. In particular, a comparative study was performed to assess the differentiation capacity toward a meniscus phenotype of different chondrogenic cells, readily available during arthroscopy, namely inner meniscus, fat pad, synovial membrane cells and articualr chondrocytes. Results indicate that only articular chondrocytes generated tissues containing relevant amounts of GAG and with cell phenotypes compatible with those of the inner and outer meniscus regions. The second aim of the thesis was to engineer tissues with a bi-zonal hyaline/fibrocartilaginous structure, which could resemble the inner and outer regions of native meniscus. Bi-zonal tissues were obtained by culturing articular chondrocytes seeded into hyaluronan non-woven meshes within a mixed flask or a Rotary Cell Culture System (RCCS). The engineered tissues consisted of an outer fibrous capsule rich in versicans and collagen type I, and an inner region containing more GAG and collagen type II. Within the bi-zonal tissue, the outer fibrocartilaginous capsule was stiffer in tension, and the hyaline-like inner region was stiffer in compression. Thus, the local biochemical and mechanical properties in the engineered bi-zonal constructs resembled in some way those of the different regions of native meniscus. These findings have finally been extended to a different scaffold with the size and shape of a native meniscus, and the resulting constructs are currently being tested in a sheep model of total meniscectomy. This study will determine whether the degree of tissue development reached in vitro is sufficient to support articular cartilage protection and further development of the graft towards a fully functional meniscus tissue, within the "in vivo bioreactor" of the joint.