Recently, we have recognized that the meniscus is very important organ for knee joint function. Surgeons, who major in knee joint surgery, have proposed that damaged meniscus should not be resected, but should be maintained.
In the past, the total removal of the injured meniscus was demonstrated to be cause of knee joint failure. Surgeons realized the influence of total menisectomy on articular cartilage dysfunction. Therefore, partial menisectomy has been preferred to total menisectomy. Since arthroscopic procedure was introduced, the results of partial menisectomy have improved, in parallel with development of the technique. However, the meniscus does not regenerate once it has been removed because of trauma; therefore, secondary osteoarthritic changes have been occurred, due to the loss of meniscal function, including alterations in load distribution, impact absorption, and articular sliding and stabilization. The meniscus is fibrocartilage tissue, which is different from real articular cartilage. It includes the vascular area, which is in 3-5mm of the peripheral rim. The vascularized area can be classified into three zones; the red-red zone, red-white zone, and white-white zone.
To preserve the meniscus, many surgeons perform meniscal repairs for red-red tears and red-white tears, if possible. However, meniscal repair for degenerative tears and white-white tears is challenging. In our study, we discovered that acid mucopolysaccharides and collagen types 1, 2 and 3 exist in a good balance in the menisci, which results in the maintainance of meniscal function. In the ruptured fragments of the trauma-injured meniscus, collagen types 2 and 3 disappear first, followed by collagen type 1, resulting in the abrogation of fiber construction. It is considered that these functions are lost because insufficient nutrition is supplied to the meniscal cells in the ruptured fragments. If a tear occurs in the peripheral vascular area, collagen types 1, 2 and 3 are produced by the remaining meniscal cells or via the invasion of undifferentiated mesenchymal cells, possibly resulting in the full recovery of meniscal function. However, if a tear occurs is in the avascular portion, no collagen is produced via the invasion of undifferentiated mesenchymal cells, and so meniscal function of the rupture fragment cannot be maintained, then, most meniscal cells disappear or, even if any remain, only collagen types 2 and 3 are slightly produced around the meniscal cells. This is considered to be the cause of the poor results of meniscal suturing. Once a histological change occurs in the meniscus, it is not certain whether the repaired meniscus will regain good function or not. The meniscus does not regenerate once it has been removed, due to trauma and sports injury. Meniscal allograft transplantation is a viable candidate for patients who develop pain following total or near total menisectomy, in America and other countries. However, this procedure cannot be performed in Japan and some other countries. The ability of a meniscus allograft transplant provides no evidence of long-term chondroprotection. In synthetic nondegradable implant, finding the optimal combination of synthetic materials to allow for a wear-resistant functional substitute has been difficult. The clinical use of meniscal implants may be challenging and we must wait to see long-term results.
Recently, many studies have been attempted for the regeneration of the meniscus. This involves the meniscus being constructed or repaired using cultured cells. Differentiated cells have been used for meniscus regeneration as well. These attractive cell sources for regeneration of the meniscus are divided into two types; meniscal cells and stem cells. It is difficult to obtain enough volume of cells which are suitable for meniscal reconstruction. Stem cells have remarkably high proliferation and much potential. Embryonic Stem cells, induced Pluripotent Stem cells and mesenchymal stem cells have the greatest potential. However, Embryonic Stem cells and induced Pluripotent Stem cells have both advantages and disadvantages when used for regeneration of the meniscus. Mesenchymal stem cells are derived from bone marrow, fat tissue and synovium. They are most useful in that enough cells can be obtained easily. However, we have not still found the best cell resource for regeneration of the meniscus. In future, a meniscus which References has the same histological and biomechanical properties as the native meniscus might be made and used for meniscal reconstruction. Even if a meniscus is removed because of trauma and sports injury, secondary osteoarthritic changes will be prevented by meniscal reconstruction.