The International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine (ISAKOS) Knee Committee developed a standardized international meniscal documentation system, based on morphological tear characteristics such as tear length, depth, pattern and location observed at arthroscopy [43]. Moreover, this classification system provides sufficient inter-observer reliability for pooling of data from international clinical trials designed to evaluate the outcomes of treatment for meniscal tears [45].

By definition, traumatic tears arise as a result of a specific knee injury and may be isolated or associated with ligament or cartilage lesions. Ligament repair provides meniscal as well as cartilage integrity over time [51]. Traumatic tears generally occur in younger active patients following twisting movements with the knee in leger flexion. Two pathological conditions could be described, traumatic tears in stable or in ACL-deficient knee.

On the contrary, degenerative meniscal tears are slow developing lesions in older degenerative menisci and they have a more complex multifactorial pathogenesis than the traumatic meniscal tears. The prevalence of such tears in the general population increases with increasing age, ranging from 16% in knees of 50- to 59-year-old women to over 50% in the knees of men aged 70–90 years [37]. In knees with osteoarthritis, a prevalence of meniscus tear of over 90% has been reported [52–54].

Traumatic meniscal injuries in children are uncommon despite the constantly increasing incidence due to the higher number of sports activities. They could be either isolated or combined with ligament lesions. Most frequently, they are associated with congenital abnormalities such as the discoid or the hypermobile meniscus. Meniscal repair is recommended in children and should be considered first because of the higher potential healing compared to adults.

Platelet-rich plasma (PRP) is a volume of plasma fraction of autologous blood having platelet concentrations above baseline [70], generally obtained after centrifugation of peripheral blood. Its use is gaining increased attention in many clinical applications as an autologous regenerative technology. It is based on the delivery of a pool of growth factors and cytokines with tissue healing potential [71–73]. The growth factors also promote chondrogenesis and the maintenance of the phenotype of the chondrocyte could be useful in the treatment of injured cartilage or meniscus [74]. Among them, the following interesting factors have recently been identified: transforming growth factor-β₁ (TGFβ₁), platelet-derived growth factor bb (PDGF bb), insulin growth factor-I (IGF-I), fibroblast growth factor 2 (FGF2) and bone morphogenic protein-6. Nevertheless, the use of PRP is still highly controversial and faces regulatory constraints [75]. Although it has been widely used in a sports medicine setting aiming to enhance tissue healing [71, 76], in the case of meniscus, only a few clinical studies investigate its effect. Griffin et al. [77] analysed whether PRP augmentation at the time of meniscal repair decreases the likelihood of subsequent meniscectomy and also if PRP influences functional, clinical and patient-reported outcomes. At a minimum follow-up of 2 years, they reported similar results for both groups regarding all outcomes. On the other hand, Pujol et al. [78] reported slightly improved clinical outcomes and higher rates of meniscal healing after biological augmentation with PRP compared to a standard open meniscal repair.

Mesenchymal stem cells (MSCs) hold great promise in regenerative medicine. They are defined as multipotent cells derived from various human tissues, including bone marrow, adipose tissue, peripheral blood and synovium. By definition, stem cells have been characterized by their ability to self-renew and due to their developmental plasticity are able to differentiate into specific therapeutic cell types. To date many experimental, preclinical and clinical studies have been established in various clinical fields such as cartilage, tendon and meniscus [79, 80]. Regarding the latter, it is also known that the molecules secreted by MSCs form a regenerative micro-environment which promotes healing [81, 82]. Matsukura et al. [83] found high levels of mesenchymal stem cells in the synovium fluid after meniscus injury compared to normal knee joints, suggesting that they may play a role in endogenous meniscal regeneration, either as direct repair cells or as a source for secretion of bioactive modulators. Regarding this potential, the exogenous application of mesenchymal stem cells could be an intriguing novel strategy in meniscal intrinsic repair. Over the last decade, there have been several animal studies investigating the effect of MSCs with encouraging results. Zellner et al. [81] reported the efficacy of mesenchymal stem cells in the repair of meniscal defects in the avascular zone. Similar results were detected by Horie et al. [84] that reported a significantly higher meniscal regeneration in a rabbit model of partial meniscectomy after implantation of synovial tissue-derived mesenchymal stem cells. They also reported that synovial mesenchymal stem cells differentiated into cells resembling meniscal fibrochondrocytes. Desando et al. [85] investigated the intra-articular adipose-derived stromal cell injection in the healing process of the menisci in an experimental rabbit model. They concluded that these cells promoted cartilage and meniscal repair and were able to attenuate inflammatory events in synovial membrane and to inhibit OA progression. However, clinical studies that have focused on using MSCs for meniscal repair are currently limited. Centeno et al. [86], in a case study, determined if isolated and expanded human autologous mesenchymal stem cells could effectively regenerate cartilage and meniscal tissue when percutaneously injected into knees. At 24 weeks post-injection, they found statistically significant cartilage and meniscus growth on MRI, as well as increased range of motion and decreased pain. Equally, Pak et al. [87], in another case study, reported the repair of a grade II meniscal tear following a percutaneous injection of autologous ASCs along with PRP, hyaluronic acid, and CaCl₂. Vangsness et al. [88], in a randomized, double-blind, controlled study, investigated the safety of the intra-articular injection of human mesenchymal stem cells into the knee, the ability of mesenchymal stem cells to promote meniscus regeneration following partial meniscectomy, and the effects of mesenchymal stem cells on osteoarthritic changes in the knee. They reported evidence of meniscus regeneration and an improvement in knee pain. These results support the study of human mesenchymal stem cells for knee-tissue regeneration and protective effects. In conclusion, the use of mesenchymal stem cells seems to stimulate the regeneration of meniscal tissue and appears to be a promising approach to treat meniscal tears and defects in order to restore as much native meniscal tissue as possible. However, these regenerative technologies still need to be optimized [89].

The fibrin clot has also been evaluated during the last 20 years. It seems to act as a chemotactic and mitogenic stimulus for reparative cells and to provide a scaffolding for the reparative process [32]. Several clinical studies reported good results especially for meniscal repair in the avascular zone. Henning et al. [90] described that in isolated meniscal tears the injection of an exogenous fibrin clot decreases the failure rate of meniscal repair from 41% to 8%. Van Trommel et al. [91], in a small series of five patients, used fibrin clot to enhance meniscal repair for complete radial tears in the avascular popliteal zone of the lateral meniscus. They noted that all menisci were fully healed without further signs of degeneration in second look arthroscopy. Similarly, Ra et al. [92] reported successful meniscal repair using fibrin clots in complete radial tears evaluated with MRI and second look arthroscopy. Equally, Kamimura et al. [93] found that meniscal repair of degenerative horizontal cleavage tears using fibrin clots resulted in improved clinical subjective scores, and reached a healing rate of 70% in follow-up arthroscopy. Some disadvantages of this technique are the demanding placement and the difficult preservation of the clot in the meniscal tear.

Gene therapy is an interesting approach that aims to provide meniscal healing through local growth factor delivery. Using ex vivo and in vivo strategies, genes have been transferred successfully to, and expressed within, numerous tissues of the musculoskeletal system, including the meniscus [94]. Various vectors have been used such as retroviral, adenoviral and adeno-associated and each of them presents peculiar characteristics [95, 96]. To date, despite several in vitro studies [97–99], only a few preclinical studies have been performed on gene therapy as a treatment in meniscal injuries. Goto et al. [100] infected a monolayer culture of human and canine meniscal cells with retroviruses carrying either a human TGFβ₁ cDNA or marker genes. They reported an increased synthesis of collagen and proteoglycans to the addition of TGFβ₁ compared with the control group. Zhang et al. [101] investigated whether the introduction of human insulin-like growth factor 1 (hIGF-1) gene could improve the repair of full-thickness meniscal defects in the avascular zone of the anterior horn. They reported that repaired meniscal defects were filled with white tissue similar to that in normal meniscal fibrocartilage. To our knowledge, clinical trials using therapeutic gene transfer have not yet been performed. Nevertheless, we believe that the rapid evolution in this particular research area will soon provide surgeons with new treatment options.

Meniscal scaffolds (Fig. 26.10) are natural or synthetic structures composed of biomaterials designed to hold physical and/or mechanical properties comparable to the native meniscus. Hence, they should provide the following important characteristics: optimal mechanical strength, biocompatibility, porosity, safe degradation and ease of use in surgical practice. Recently, meniscal replacement involving synthetic scaffolds has been proposed as an option for symptomatic knees following partial meniscectomy with the aim of improving symptoms while also potentially reducing degradation. On the one hand, the cell-free meniscal scaffolds are only used in a very selective group of patients and despite promising results, none of them has currently demonstrated regeneration of a functional, long-lasting meniscal tissue. On the other hand, biological augmentation with cells or autologous growth factors might represent a viable and effective option to improve the overall regenerative potential of meniscal scaffolds as tissue engineering, which applies the principles of biology and engineering to the development of functional substitutes for damaged tissue [102]. Although the term has been applied to a multitude of biological strategies in previous literature, it correctly refers to the addition of cells or growth factors to a scaffold with the aim of host target tissue regeneration [103].