The osteochondral complex consists of both the articular cartilage and underlying subchondral bone. Biochemically, cartilage tissue is largely comprised of water, chondrocytes, type II collagen, and proteoglycan [50, 81, 122]. The cartilage can be differentiated into four distinct zones: the superficial, middle, deep, and calcified cartilage zones (Fig. 23.2) [65]. Each zone is defined by a particular composition and organization of cells and extracellular matrix (ECM) molecules. The differential proportions in ECM composition influence the mechanical properties of each zone of the cartilage. For example, the superficial zone is strong in tension along the alignment of its collagen fibrils, thereby assisting in the resistance of shear forces at the surface. By comparison, the deep zone has more compressive strain.

Subchondral bone is a complex tissue consisting of water, collagen type I, and hydroxyapatite, with the two latter components providing the tissue’s stiffness and compressive strength [9, 81, 122]. The compressive modulus of subchondral bone is higher than that of the cartilage. The different morphological compositions and mechanical properties of subchondral bone and cartilage indicate the complexity of the tissue interface.

The osteochondral interface is described by the interaction of calcified cartilage and the underlying subchondral bone [16]. Structurally, collagen fibers extend from the deep zone to calcified cartilage through a wavy tidemark, which enables the dispersal of force through the vertical orientation of collagen fibrils [83]. However, despite the fact that calcified cartilage is mineralized tissue, its mechanical strength is lower than that of the subchondral bone [72]. Calcified cartilage is interdigitated with subchondral bone, but fibers do not extend across the zone into the bone [23, 83]. The wavy tidemark and vertically oriented fibers at the tidemark, as well as interdigitations present at the interface, may allow for reducing stress concentrations, as well as better integration with the underlying subchondral bone [81, 83].

An osteoarthritic joint is characterized by degenerative changes, such as articular cartilage loss, subchondral bone thickening, and osteophyte formation [12, 39, 64, 77, 96]. The primary morphologic changes include thinning, fissuring, and fragmentation of articular cartilage. With the progression of the disease comes a continuous loss of articular cartilage, accompanied with the decrease of collagen type II and aggrecan [31, 76], leading to exposure of subchondral bone. Secondary changes are frequently seen in the underlying bone, such as sclerosis, cystic change, and new bone formation (Fig. 23.3a, b). These changes are considered to be triggered by a multitude of factors, including aging, trauma, obesity, mechanical overload, congenital disorder, and infection, which do not heal spontaneously once damaged.

For an ideal repair of osteochondral lesions, it is important to regenerate subchondral bone and to facilitate zonal restoration of the cartilage and subchondral bone, layer by layer, mimicking the natural articular structure [37, 46, 53, 74, 85, 86, 102]. As a strategy to regenerate these structures in a layer-by-layer fashion, biphasic or triphasic constructs have been developed due to both mechanical and biological reasons, including the acquisition of initial mechanical strength, mimicking a natural articulate structure, a uniform tidemark at the osteochondral junction, and an integration of the biphasic implant with host tissue to sustain biological function (Fig. 23.4a–d) [2, 4, 18, 36, 43, 47, 69, 82, 84, 107, 110]. For satisfying the biological requirements, an osteochondral implant should ideally have a rigid osseous layer to support the overlying cartilage and integrate with the native bone and a chondral layer to allow the seeding and proliferation of chondrocytes or mesenchymal stem cells (MSCs) and subsequent deposition of cartilaginous ECM. Also, for the successful osteochondral repair, the integration between the implant and host tissue should be one of crucial factors. Our previous study showed that the tissue integration to native surrounding osteochondral tissue after the implantation of biphasic construct could influence the quality and maturation of repair tissue [109]. In case of the failure of integration with the adjacent host cartilage, the repair tissue might evolve into a pathological condition such as OA due to mechanically unstable condition, which potentially raises concerns regarding the long-term durability of the repair tissue.

The most direct cell source may be the biopsy specimens taken from the patients, from which mature osteoblasts and chondrocytes may be obtained. However, as the number of cells obtained is usually limited, it is typically not enough to allow seeding onto the scaffolds. Also, expansion of primary cells may result in a loss of differentiation capacity; for example, the expansion of articular chondrocytes can lead to dedifferentiation into fibroblast [13, 104, 115]. To overcome such potential problems with respect to dedifferentiation, three-dimensional (3D) culture can be used to retain the cellular phenotype and avoid dedifferentiation [116]. The most common method is the use of various scaffolds to produce a 3D culture condition [68, 128] and may be combined with the supplementation of growth factors [22], the use of bioreactor [35], the mechanical stimulation of the cells [33, 49], and the use of low oxygen tension [57] during cultivation. Also, even if chondrocytes lose their differentiated phenotype, dedifferentiated chondrocytes can regain their differentiated phenotype through the redifferentiation process of cultivation in a 3D scaffold combined with growth factors [3, 60].

As an additional option, stem cells may represent promising alternatives [113]. Specifically, mesenchymal stem cells (MSCs) have the capability to differentiate into a variety of connective tissue cell types, including the bone, cartilage, tendon, muscle, and adipose tissue [27, 108]. These cells may be isolated from various tissues, such as bone marrow, skeletal muscle, synovial membrane, adipose tissue, and umbilical cord blood [6, 7, 27, 52, 71, 99]. Moreover, allogeneic MSCs [25, 108] or induced pluripotent stem (iPS) cells [114, 119] may also be considered. However, there have not been much evidence using these cells forthcoming in terms of preclinical and clinical safety, and thus, further studies with such cells are likely necessary.

In addition, the use of a growth factor or its cocktail (combination), including insulin-like growth factor 1 (IGF-1), transforming growth factor beta 1 (TGF-β1), fibroblast growth factor 2 (FGF-2), and bone morphogenetic proteins (BMP-2, BMP-7), may support tissue maturation for the cartilage [10, 41, 87, 111]. Similar to the cartilage, the bone also possesses a large variety of growth factors that are involved in the regenerative process, including TGF-β; BMP-2, 4, 6, and 7; IGF-1 and 2; and platelet-derived growth factor (PDGF) [91, 97, 100].

On the other hand, some researchers have tested an acellular approach using a scaffold alone [29, 53]. Considering the time and cost-effectiveness, as well as safety issues associated with cell culture, this approach could represent a reasonable strategy in tissue engineering. Scaffolds should be developed to meet requirements such as the recruitment of enough tissue progenitor cells from the host tissue.

Several methods have been proposed to develop biphasic scaffolds with the hybridization of two distinct biomaterials, each of which being adequate to integrate with the respective surrounding tissue [67]. Many specific material types have been developed for both cartilage and bone regeneration, which are typically made of biocompatible and biodegradable polymers. For the cartilage layer, natural or synthetic polymer-based scaffolds are commonly used. More recently, scaffold-free implants have been developed and the potential feasibility tested. On the other hand, for a scaffold of the subchondral bone layer, it is important to choose materials with initial mechanical strength, good bone ingrowth, and integration of native surrounding bone. Ceramics, glasses, and metallic materials are commonly used. Also, natural or synthetic polymers, similar to cartilage layer, could be used alone or combined with ceramics [4, 18, 28, 54, 123, 124].

There have been many therapeutic procedures investigated to biologically repair damaged cartilage, some of which are already at the stage of clinical application. On the contrary, considering the higher incidence of OA, which involves subchondral bone pathology, by comparison to isolated chondral injury [8, 24, 26, 40, 88, 126], there is an urgent need to develop novel therapeutic methods for osteochondral repair with clinical relevance. In this regard, the number of animal experiments and clinical trials to treat osteochondral lesions has been recently increased. We focus on the latest preclinical animal studies using large animal and clinical studies.

At first, in order to evaluate the feasibility and safety of new implants with clinical relevance, the selection of appropriate animal models is important. Due to the differences in matrix structure and composition, as well as in the natural osteochondral healing response and technical difficulty in creating the lesions of consistent size and location, the use of small animals such as rabbits, rats, and mice may not be appropriate [1, 21, 95]. Rather, in consideration of clinical relevance, it is preferable to utilize larger animal models, such as pigs, sheep, goats, and horses. Marquass et al. used an MSC-seeded combined implant with a collagen I hydrogel and β-TCP in an ovine osteochondral defect model and showed comparable repair quality to osteochondral autografts in terms of histology and biomechanical testing [69]. Miot et al. prepared engineered cartilage, which was generated from autologous chondrocytes cultured in hyaluronic acid scaffolds of different pre-culture periods, and implanted the engineered cartilage above the hydroxyapatite/hyaluronic acid sponges into goat osteochondral defects. They concluded that 2 weeks pre-culture of engineered cartilage achieved a suitable compromise between tissue maturity and structural/integrative properties of the repair tissue. These data demonstrate that the stage of development of engineered cartilage is an important parameter to be considered in designing cartilage repair strategies [75]. Kon et al. used an aragonite/hyaluronate biphasic scaffold for osteochondral defects in a goat model and showed that mechanical modification with drilled channels in the cartilage phase and impregnation of HA within the coral pores enhanced the scaffold’s cartilage regenerative potential [56]. Schleicher et al. compared two biphasic scaffolds of either hydroxylapatite/collagen or allogenous sterilized bone/collagen and tested their integration in a sheep model. They showed that the latter scaffold proved to be stable and sufficiently integrated in the short term [103]. Sosio et al. treated the osteochondral lesions with a biphasic scaffold made of collagen type I and hydroxyapatite in a pig model, in which they compared the scaffold seeded with autologous chondrocyte with the unseeded scaffold [112]. Surprisingly, they showed that the unseeded scaffold exhibited better macroscopic and histological results than the cell-seeded scaffold. Kon et al. developed an acellular three-gradient multilayer scaffold made of collagen type I and nanoparticles of hydroxyapatite and tested the scaffold with or without autologous chondrocytes in sheep osteochondral defect model. They concluded that the scaffold contributed to the process of the bone and hyaline-like cartilage regeneration, regardless of the use of chondrocytes [54]. Also, they treated 27 patients with chondral or osteochondral lesions using an acellular scaffold [34, 53, 55] and demonstrated the safety and potential clinical benefit of the graded biomimetic osteochondral scaffold in promoting the bone and cartilage tissue with good clinical and magnetic resonance imaging results until the 5-year follow-up. Dhollander et al. treated 27 for cartilage lesions with an acellular osteochondral plug, which is composed of polylactide-co-glycolide copolymer, calcium sulfate, polyglycolide fibers, and surfactant (TruFit plug; Smith and Nephew, Andover, MA) [29]. In this clinical pilot study, a modest clinical improvement became apparent at 12 months of follow-up. Also, MRI data showed no deterioration of the repair tissue. However, 20 % of the patients had persistent clinical symptoms after surgery and had an additional surgery such as removal of the osteochondral plug remnants. The two latter studies were Level IV study, and further studies, which would be compared with conventional treatment such as bone marrow stimulation and osteochondral transplantation, are necessary. Also, in contrast with cell-free scaffolds, no clinical trial using cell-seeded scaffold has been reported, and these studies should be expected in the near future.

The management of OA remains challenging and controversial. Considering the steady progression of tissue engineering and cell-based technologies over the past decade, we may have new therapeutic options for osteochondral repair in clinical practice. In this chapter, we have focused on biomaterial scaffold for osteochondral repair, including the concept, scaffold fabrication, in addition to the selection of cells and materials. There have been many promising scaffolds developed, some of which contribute to good osteochondral repair in vivo. Moreover, some of them are already at the stage of preclinical, large animal studies, as well as clinical trials. In addition, the recent work has been focused on not only investigating the effectiveness of materials or cells but also applying several new concepts and techniques such as mechanical [56], microstructural [30], and local microenvironment modification [94] for the design and fabrication of scaffolds. Therefore, the application of additional new implants to osteochondral lesions could be expected in the near future. On the other hand, the most suitable biomaterials for the cartilage or subchondral bone layers have not been fully investigated, while there are many biomaterials available for osteochondral repair. Therefore, the comparison of these materials should be performed to ultimately determine the ideal material. Further studies will be needed and should be conducted in a methodologically rigorous fashion.