Articular cartilage covers the ends of bones in diarthrodial joints. Histologically, it is a hyaline tissue with no blood, lymphatic or nerve supply [1].

Hyaline articular cartilage is a highly specialized tissue derived from mesenchyme during embryonic development. It has the main function to protect the joint by distributing loads and thus preventing potentially damaging stress on the subchondral bone. At the same time, it provides a low-friction bearing surface to enable free movements. The tissue is composed of two phases: liquid and solid. Generally, 60–80% of total wet weight of articular cartilage is fluid (e.g., interstitial water and electrolytes). The remaining 40–20% of the tissue is composed by a solid extra cellular matrix (ECM) and chondrocytes [2].

Chondrocytes, the only cell type of mature cartilage, secrete, and deposit around themselves a characteristic ECM. Moreover, they maintain and remodel the ECM by a careful balance of catabolic and anabolic processes, involving matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) [3]. Mature chondrocytes are completely encapsulated in the dense cartilage ECM and are not able to migrate or proliferate, unlike cells in bone.

The ECM is composed primarily of water, collagens, proteoglycans, and other non-collagenous proteins. Besides collagen type a variety of collagens such as types VI, IX, X, and XI are present and contribute to the tensile properties of articular cartilage. In adult tissue, collagen type II represents 90% of total collagens. Conversely, the proportion of collagen XI is about 3%. All collagen fibers display oriented distribution and nonuniform size. Proteoglycans are mainly represented by aggrecan that provides compressive properties to the tissue. Aggrecans exhibit a bottlebrush structure, in which glycosaminoglycan (GAG) chains (chondroitin sulfate and keratan sulfate) are attached to a high molecular weight protein core. Such a structure aggregates with hyaluronic acid, known also as hyaluronan (HA), via a link protein. Aside from aggrecan, smaller proteoglycans like biglycan, fibromodulin, and decorin also occur in minute amounts [2]. ECM can be classified into three regions: pericellular, interterritorial, and territorial, based on their distance from chondrocytes. The thin pericellular matrix closely surrounds individual or a column of chondrocytes; the interterritorial matrix is farther, the territorial matrix is the farthest.

Mature articular cartilage, despite its thinness, presents a non-homogeneous distribution of cells and ECM among different zones, termed the superficial, middle (or transitional), deep (or radial), and calcified zones. In particular, when examined from the superficial to the deep zone: collagen and water contents gradually decrease; collagen fiber size increases; proteoglycans augment, with the middle zone having highest content. The superficial zone contains fibroblast-like chondrocytes that are flattened and oriented in parallel with the surface. Collagen fibers display a tangential orientation. In the middle zone, chondrocytes exhibit a round morphology; collagen fibrils have a random orientation. The deep zone displays chondrocytes packed in columns which remain parallel to the collagen fibers organized in radial directions. The calcified zone contains few inert chondrocytes embedded in a calcified ECM, having collagen type X, which helps cartilage mineralization and provides structure integrity.

Articular cartilage possesses significant mechanical properties with a compressive modulus of 0.7–0.8 MPa, a shear modulus of similar magnitude (0.69 MPa), and a tensile modulus of 0.3–10 MPa. Such mechanical properties are reminiscent of a hydrogel [4].

Despite its durability and ability to maintain itself, articular cartilage’s is vulnerable to injuries that lead to irreparable tissue damage, causing, over time, inflammation, disability, and pain. This originates from articular cartilage’s weak capacity for self-repair, which is due to its avascularity. Wound healing is further prevented due to the dense ECM, impairing chondrocyte migration capacity.

Articular cartilage lesions are areas of damaged or missing tissues that can be of traumatic or rheumatic origins [5].

Traumatic alterations are common in the general population and more often anticipated in young and physically active people. They can be divided into microtraumas, chondral, and osteochondral defects. Microtrauma or light repeated trauma could provide degeneration or cell death, collagen ultrastructure destruction, hydration increase, and superficial subchondral bone fissuration. These phenomena are similar to those of the early phase of osteoarthritis (OA). Chondrocyte ability to synthesize the ECM is not sufficient to counteract the progress of the degenerative process that results irreversible. Common chondral defects enclose chondromalacia/degenerative chondrosis, chondral flap, and chondral fracture. In chondromalacia, cartilage tears away unevenly, with shallow walls; in chondral flap, the tissue separates from the bone and moves like a door with a hinge at one end and in chondral fracture cartilage is removed from the bone and floats free. Chondral lesions may be degenerative, due to a “wear and tear” problem or traumatic, caused by an injury such as falling on the knee, jumping down, or rapidly changing direction while playing a sport. Such problems do not always produce symptoms at first because there are no nerves in the cartilage. Moreover, in general, no repair process occurs. Osteochondral defects involve both the articular surface and the subchondral bone. The degree of injury ranges from a small crack to a piece of bone breaking off inside the joint. These fragments, differing for size and depth, can stay attached (stable) to the area that was injured or become loose (unstable) within the joint. Those kind of injuries are more common in adolescents and young adults and typically occurs at the knee, ankle, or elbow. In osteochondral defects, a repair process is initiated by undifferentiated mesenchymal stem cells (MSCs) from the bone marrow tissue of the subchondral bone. Repair of full-thickness cartilage lesions depends mainly on the patient age, defect size, and location. Small full-thickness defects are repaired by the formation of hyaline cartilage, whereas large osteochondral defects are only repaired by the formation of a scar, fibrous tissue, or fibrocartilage in which the predominant component is collagen type I. From a clinical point of view, this kind of cartilage could be functionally active for a short period, but the tissue does not present the mechanical and strength characteristics of normal cartilage thus ensuring neither the healing of the defect nor the symptom remission. This situation could favor over time the appearance of OA [6].

Articular cartilage is prone to rheumatic pathologies, such as OA and rheumatoid arthritis (RA) that compromise not only the structural and functional characteristics of the tissue, but also of the whole joint [5].

OA is a group of progressive joint disorders involving articular cartilage, subchondral bone, ligaments, and synovial membrane and resulting in patient disability. Both systemic and local factors could contribute to its development. Systemic factors include: age, sex, racial, and genetic characteristics and still unidentified factors. Local factors comprise articular traumatic injuries and deformities (dysplasia), obesity, and muscle weakness. OA highest prevalence before 50 years is dependent from sex, while after 50 years females are more affected. Advanced stages of the pathology lead to the need of prosthesis surgical interventions that, in these cases, represent the only therapeutic approach to restore the articular functions [5].

RA is an autoimmune, chronic, systemic, and inflammatory disorder. It principally attacks synovial joints, but it may affect also many other tissues and organs. Such a disease is associated with progressive joint destruction that leads to pain and disability. It can affect people of all ages and races and is more common in women. The cause of rheumatoid arthritis is not known, but in some families, multiple members can be affected, suggesting a genetic basis for the disorder [7].

Despite being a known problem for a long time, cartilage restoration is a relatively new field in orthopedics. When developing such an approach, multiple factors should be considered: patient profile, age, type of lesion, concomitant factors, activity level, and expectations. The situation is further complicated by the lack of ability to self-repair of the tissue. For these reasons, there is no uniform approach to managing cartilage defects and a variety of surgical options have been developed [8]. These procedures can be classified as palliative, reparative, restorative, and regenerative, these last including the tissue-engineering-based strategies.

Palliative strategies are usually the primary approach to treat symptoms, but they do not lead to cartilage healing. Conservative approaches, such as physiotherapy, weight loss, intra-articular injections (analgesics, and viscosupplementation with glucosamine and chondroitin sulfate), and orthotic interventions are widely diffused. Debridement is utilized to remove all the debris deriving from the arthritis-mediated joint damage: inflammatory cells, unstable chondral flaps, osteophytes, superfluous synovia, degenerated menisci, and torn ligaments. Chondral shaving consists in the excision of damaged cartilage due to trauma. Knee joint lavage, a simply rinsing of the joint with physiological fluids to wash out degradation products, is usually performed when conservative treatment of knee OA is inadequate and replacement is not yet indicated [9].

Reparative approaches, known as abrasion, drilling, and microfracture, are aimed at the reconstruction of a new tissue that fills the defect site. In particular, they all enclose a phase consisting in the subchondral bone penetration, thus inducing bleeding. For this reason, these surgical strategies are commonly referred to as “marrow stimulation techniques.” As a consequence of drilling, there is a migration of bone marrow stem cells to the site of injury, along with blood clot formation. The resulting repair tissue is mainly composed of fibrocartilage. Although excellent short-term clinical outcomes have been demonstrated [10], the clinical durability of marrow stimulated repair tissue has shown a functional decline with further follow-up.

Restorative techniques’ purpose is the reconstitution of cartilage structure and function. Knee arthroplasty is widely diffused technique that is currently recommended for elderly patients. It has been shown to relieve pain and improve mobility in people suffering from end-stage lesions. However, it is an invasive procedure involving articular surface replacement with prosthetic implants. Complications may be stiffness, instability, aseptic loosening, infection, prosthesis failure, and malalignment [8].

Biological, regenerative approaches include osteochondral grafting and autologous chondrocyte transplantation (ACT), aimed at the formation of a new tissue that is indistinguishable from native cartilage. Osteochondral transplantation can be autologous (mosaicplasty) or allogeneic (allograft transplantation). Main mosaicplasty drawbacks are donor-site morbidity and graft failure due to marginal chondrocyte death. Limitations of allograft transplantation include graft availability, possible disease transmission, and a short cell viability and biomechanical integrity of the donor graft. For this last reason, fresh osteochondral allografts are recommended [11–14].

ACT was first used by a Swedish Group [15] to treat full-thickness chondral defects of the knee. In the original procedure (first-generation technique), small grafts of normal cartilage removed from non-weight bearing areas of the knee were treated in a proper laboratory. Individual chondrocytes were isolated, cultured in specific conditions, and, following a period of cellular division, retrieved for re-implantation. Cell suspension was then injected beneath a periosteal patch harvested from the proximal medial tibia and sutured to the defect. First-generation ACT revealed, however, several disadvantages [6]. Therefore, variations of the technique and new strategies have been developed, converging in the tissue engineering approach, which associates to the cells biomimetic scaffolds that can also be treated with soluble or mechanical stimuli. Moreover, researches have also been addresses towards the evaluation and development of cell sources alternative to chondrocytes.

Scaffolds play a critical role in providing a 3D environment to support cell growth, matrix deposition, and tissue regeneration. An ideal scaffold should satisfy several essential criteria that concern its biological, structural, composition, and fabrication properties that make it available for tissue engineering applications:

Currently, scaffolds suitable for cartilage tissue engineering can be divided into native biological and synthetic polymeric materials. Each type of scaffold presents specific advantages and drawbacks. Therefore, future research priorities need both to improve existing materials and fabrication techniques and to further develop new ones. In particular, recent developments in the manufacturing techniques allowed the creation of new types of scaffolds, such as nanostructured materials that are now in the spotlight for their potentiality in cartilage tissue engineering [2, 3, 16].

The choice of the cell population that best allows reaching cartilage regeneration is still a challenge and nothing has been identified yet as the best. In particular, an ideal cell component should be:

The cell source can be autologous, avoiding immune response issues and disease transmission or allogenic to eliminate donor-site morbidity and to maximize availability. Cells can be utilized alone or in combination with a scaffold and/or biological or mechanical stimuli. They can be ex vivo expanded prior to implantation or not [21].

Chondrocytes and stem cells are the most widely used cell population for tissue articular cartilage engineering [9].