Knee meniscus injury is frequent, resulting in over 1 million surgeries annually in the United States and Europe. Because of the near-avascularity of this fibrocartilaginous tissue and its intrinsic lack of healing, tissue engineering has been proposed as a solution for meniscus repair and replacement. This study describes an approach employing bioactive stimuli to enhance both extracellular matrix content and organization of neomenisci toward augmenting their mechanical properties. Self-assembled fibrocartilages were treated with TGF-β1, chondroitinase ABC, and lysyl oxidase-like 2 (collectively termed TCL) in addition to lysophosphatidic acid (LPA). TCL + LPA treatment synergistically improved circumferential tensile stiffness and strength, significantly enhanced collagen and pyridinoline crosslink content per dry weight, and achieved tensile anisotropy (circumferential/radial) values of neomenisci close to 4. This study utilizes a combination of bioactive stimuli for use in tissue engineering studies, providing a promising path toward deploying these neomenisci as functional repair and replacement tissues. Statement of SignificanceThis study utilizes a scaffold-free approach, which strays from the tissue engineering paradigm of using scaffolds with cells and bioactive factors to engineer neotissue. While self-assembled neomenisci have attained compressive properties akin to native tissue, tensile properties still require improvement before being able to deploy engineered neomenisci as functional tissue repair or replacement options. In order to augment tensile properties, this study utilized bioactive factors known to augment matrix content in combination with a soluble factor that enhances matrix organization and anisotropy via cell traction forces. Using a bioactive factor to enhance matrix organization mitigates the need for bioreactors used to apply mechanical stimuli or scaffolds to induce proper fiber alignment.

Fibrocartilage is found in the knee meniscus, the temporomandibular joint (TMJ) disc, the pubic symphysis, the annulus fibrosus of intervertebral disc, tendons, and ligaments. These tissues are notoriously difficult to repair due to their avascularity, and limited clinical repair and replacement options exist. Tissue engineering has been proposed as a route to repair and replace fibrocartilages. Using the knee meniscus and TMJ disc as examples, this review describes how fibrocartilages can be engineered toward translation to clinical use. Presented are fibrocartilage anatomy, function, epidemiology, pathology, and current clinical treatments because they inform design criteria for tissue engineered fibrocartilages. Methods for how native tissues are characterized histomorphologically, biochemically, and mechanically to set gold standards are described. Then provided is a review of fibrocartilage-specific tissue engineering strategies, including the selection of cell sources, scaffold or scaffold-free methods, and biochemical and mechanical stimuli. In closing, the Food and Drug Administration (FDA) paradigm is discussed to inform researchers of both the guidance that exists and the questions that remain to be answered with regard to bringing a tissue engineered fibrocartilage product to the clinic.

Knee meniscus injuries are the most frequent causes of orthopedic surgical procedures in the U.S., motivating tissue engineering attempts and the need for suitable animal models. Despite extensive use in cardiovascular research and the existence of characterization data for the menisci of farm pigs, the farm pig may not be a desirable preclinical model for the meniscus due to rapid weight gain. Minipigs are conducive to in vivo experiments due to their slower growth rate than farm pigs and similarity in weight to humans. However, characterization of minipig knee menisci is lacking. The objective of this study was to extensively characterize structural and functional properties within different regions of both medial and lateral Yucatan minipig knee menisci to inform this model’s suitability as a preclinical model for meniscal therapies. Menisci measured 23.2–24.8 mm in anteroposterior length (33–40 mm for human), 7.7–11.4 mm in width (8.3–14.8 mm for human), and 6.4–8.4 mm in peripheral height (5–7 mm for human). Per wet weight, biochemical evaluation revealed 23.9–31.3% collagen (COL; 22% for human) and 1.20–2.57% glycosaminoglycans (GAG; 0.8% for human). Also, per dry weight, pyridinoline crosslinks (PYR) were 0.12–0.16% (0.12% for human) and, when normalized to collagen content, reached as high as 1.45–1.96 ng/µg. Biomechanical testing revealed circumferential Young’s modulus of 78.4–116.2 MPa (100–300 MPa for human), circumferential ultimate tensile strength (UTS) of 18.2–25.9 MPa (12–18 MPa for human), radial Young’s modulus of 2.5–10.9 MPa (10–30 MPa for human), radial UTS of 2.5–4.2 MPa (1–4 MPa for human), aggregate modulus of 157–287 kPa (100–150 kPa for human), and shear modulus of 91–147 kPa (120 kPa for human). Anisotropy indices ranged from 11.2–49.4 and 6.3–11.2 for tensile stiffness and strength (approximately 10 for human), respectively. Regional differences in mechanical and biochemical properties within the minipig medial meniscus were observed; specifically, GAG, PYR, PYR/COL, radial stiffness, and Young’s modulus anisotropy varied by region. The posterior region of the medial meniscus exhibited the lowest radial stiffness, which is also seen in humans and corresponds to the most prevalent location for meniscal lesions. Overall, similarities between minipig and human menisci support the use of minipigs for meniscus translational research.

Meniscus tissue deficiency resulting from primary meniscectomy or meniscectomy after failed repair is a clinical challenge because the meniscus has little to no capacity for regeneration. Loss of meniscus tissue has been associated with early-onset knee osteoarthritis due to an increase in joint contact pressures in meniscectomized knees. Clinically available replacement strategies range from allograft transplantation to synthetic implants, including the collagen meniscus implant, ACTIfit, and NUSurface. Although short-term efficacy has been demonstrated with some of these treatments, factors such as long-term durability, chondroprotective efficacy, and return to sport activities in young patients remain unpredictable. Investigations of cell-based and tissue-engineered strategies to treat meniscus tissue deficiency are ongoing.