Chondral and osteochondral lesions due to injury or other pathology commonly result in the development of osteoarthritis, eventually leading to progressive total joint destruction. Although current progress suggests that biologic agents can delay the advancement of deterioration, such drugs are incapable of promoting tissue restoration. The limited ability of articular cartilage to regenerate renders joint arthroplasty an unavoidable surgical intervention. This Review describes current, widely used clinical repair techniques for resurfacing articular cartilage defects; short-term and long-term clinical outcomes of these techniques are discussed. Also reviewed is a developmental pipeline of regenerative biological products that over the next decade could revolutionize joint care by functionally healing articular cartilage. These products include cell-based and cell-free materials such as autologous and allogeneic cell-based approaches and multipotent and pluripotent stem-cell-based techniques. Central to these efforts is the prominent role that tissue engineering has in translating biological technology into clinical products; therefore, concomitant regulatory processes are also discussed.

Extensive scientific investigations in recent decades have established the anatomical, biomechanical, and functional importance that the meniscus holds within the knee joint. As a vital part of the joint, it acts to prevent the deterioration and degeneration of articular cartilage, and the onset and development of osteoarthritis. For this reason, research into meniscus repair has been the recipient of particular interest from the orthopedic and bioengineering communities. Current repair techniques are only effective in treating lesions located in the peripheral vascularized region of the meniscus. Healing lesions found in the inner avascular region, which functions under a highly demanding mechanical environment, is considered to be a significant challenge. An adequate treatment approach has yet to be established, though many attempts have been undertaken. The current primary method for treatment is partial meniscectomy, which commonly results in the progressive development of osteoarthritis. This drawback has shifted research interest towards the fields of biomaterials and bioengineering, where it is hoped that meniscal deterioration can be tackled with the help of tissue engineering. So far, different approaches and strategies have contributed to the in vitro generation of meniscus constructs, which are capable of restoring meniscal lesions to some extent, both functionally as well as anatomically. The selection of the appropriate cell source (autologous, allogeneic, or xenogeneic cells, or stem cells) is undoubtedly regarded as key to successful meniscal tissue engineering. Furthermore, a large variation of scaffolds for tissue engineering have been proposed and produced in experimental and clinical studies, although a few problems with these (e.g., byproducts of degradation, stress shielding) have shifted research interest towards new strategies (e.g., scaffoldless approaches, self-assembly). A large number of different chemical (e.g., TGF-β1, C-ABC) and mechanical stimuli (e.g., direct compression, hydrostatic pressure) have also been investigated, both in terms of encouraging functional tissue formation, as well as in differentiating stem cells. Even though the problems accompanying meniscus tissue engineering research are considerable, we are undoubtedly in the dawn of a new era, whereby recent advances in biology, engineering, and medicine are leading to the successful treatment of meniscal lesions.

The inability to recapitulate native tissue biomechanics, especially tensile properties, hinders progress in regenerative medicine. To address this problem, strategies have focused on enhancing collagen production. However, manipulating collagen cross-links, ubiquitous throughout all tissues and conferring mechanical integrity, has been underinvestigated. A series of studies examined the effects of lysyl oxidase (LOX), the enzyme responsible for the formation of collagen cross-links. Hypoxia-induced endogenous LOX was applied in multiple musculoskeletal tissues (i.e., cartilage, meniscus, tendons, ligaments). Results of these studies showed that both native and engineered tissues are enhanced by invoking a mechanism of hypoxia-induced pyridinoline (PYR) cross-links via intermediaries like LOX. Hypoxia was shown to enhance PYR cross-linking 1.4- to 6.4-fold and, concomitantly, to increase the tensile properties of collagen-rich tissues 1.3- to 2.2-fold. Direct administration of exogenous LOX was applied in native cartilage and neocartilage generated using a scaffold-free, self-assembling process of primary chondrocytes. Exogenous LOX was found to enhance native tissue tensile properties 1.9-fold. LOX concentration- and time-dependent increases in PYR content (∼ 16-fold compared with controls) and tensile properties (approximately fivefold compared with controls) of neocartilage were also detected, resulting in properties on par with native tissue. Finally, in vivo subcutaneous implantation of LOX-treated neocartilage in nude mice promoted further maturation of the neotissue, enhancing tensile and PYR content approximately threefold and 14-fold, respectively, compared with in vitro controls. Collectively, these results provide the first report, to our knowledge, of endogenous (hypoxia-induced) and exogenous LOX applications for promoting collagen cross-linking and improving the tensile properties of a spectrum of native and engineered tissues both in vitro and in vivo.

Patients suffering from damaged or diseased fibrocartilages currently have no effective long-term treatment options. Despite their potential, engineered tissues suffer from inferior biomechanical integrity and an inability to integrate in vivo. The present study identifies a treatment regimen (including the biophysical agent chondroitinase-ABC, the biochemical agent TGF-β1, and the collagen crosslinking agent lysyl oxidase) to prime highly cellularized, scaffold-free neofibrocartilage implants, effecting continued improvement in vivo. We show these agents drive in vitro neofibrocartilage matrix maturation toward synergistically enhanced Young’s modulus and ultimate tensile strength values, which were increased 245% and 186%, respectively, over controls. Furthermore, an in vitro fibrocartilage defect model found this treatment regimen to significantly increase the integration tensile properties between treated neofibrocartilage and native tissue. Through translating this technology to an in vivo fibrocartilage defect model, our results indicate, for the first time, that a pre-treatment can prime neofibrocartilage for significantly enhanced integration potential in vivo, with interfacial tensile stiffness and strength increasing by 730% and 745%, respectively, compared to integration values achieved in vitro. Our results suggest that specifically targeting collagen assembly and organization is a powerful means to augment overall neotissue mechanics and integration potential toward improved clinical feasibility.

The objective of this study was to improve the biomechanical properties of engineered neotissues through promoting the development of collagen cross-links. It was hypothesized that supplementing medium with copper sulfate and the amino acid hydroxylysine would enhance the activity of lysyl oxidase enzyme to form collagen cross-links, increasing the strength and integrity of the neotissue. Neocartilage constructs were generated using a scaffoldless, self-assembling process and treated with copper sulfate and hydroxylysine, either alone or in combination, following a 2-factor, full-factorial study design. Following a 6-wk culture period, the biomechanical and biochemical properties of the constructs were measured. Results found copper sulfate to significantly increase pyridinoline (PYR) cross-links in all copper sulfate-containing groups over controls. When copper sulfate and hydroxylysine were combined, the result was synergistic, with a 10-fold increase in PYR content over controls. This increase in PYR cross-links manifested in a 3.3-fold significant increase in the tensile properties of the copper sulfate + hydroxylysine group. In addition, an 123% increase over control values was detected in the copper sulfate group in terms of the aggregate modulus. These data elucidate the role of copper sulfate and hydroxylysine toward improving the biomechanical properties of neotissues through collagen cross-linking enhancement.