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International Journal of Bioprinting 3D-printed scaffolds for osteochondral defects

(OA). In addition, their pathological characteristics are However, there are also disadvantages, such as excessive
also different. Osteochondral lesions usually require tissue fibrosis, grafts sinking, abnormal bone formation,
surgical treatment. If fibrocartilage is formed with different excessive cartilage growth, and scaffold separation .
[6]
biomechanical properties from hyaline cartilage, it will Compared to articular osteochondral tissue, single- or
cause degeneration of the adjacent normal cartilage and double-layered scaffolds lack a “boundary structure”
subchondral bone, eventually leading to severe pain, between cartilage and bone, that is, calcified cartilage
joint deformity, and mobility loss . The different causes zone (CCZ) . This usually causes an imbalance in
[1]
[7]
of osteochondral defects contribute to the complexity of the microenvironmental homeostasis of the articular
their treatment. Significant progress has been made in the cartilage and subchondral bone, as well as altered stress
repair of articular cartilage defects in recent decades, but transmission patterns, ultimately leading to repair failure.
osteochondral defects deep into subchondral bone have In order to achieve complete biomimicry, researchers
not gained much attention. have designed a strategy for constructing a multilayered
The current treatment options for osteochondral defects osteochondral scaffold with a boundary layer structure.
include nonsurgical and surgical treatments, such as joint With the advent of additive manufacturing technology in
debridement, microfracture, autologous osteochondral recent years, 3D printing has developed rapidly, providing
grafting or mosaic inlay, matrix-associated autologous new tools and technical methods to solve this challenge.
chondrocyte implantation (MACI), and autologous As shown in Figure 1, this paper summarizes the anatomy,
chondrocyte implantation (ACI) . Although conventional physiology, pathology and restoration mechanisms of the
[2]
osteochondral repair strategies have their corresponding articular osteochondral unit, and reviews the necessity
advantages, their inherent disadvantages are also evident. for a boundary layer structure in osteochondral tissue
For example, arthroscopic debridement is not effective; engineering scaffolds and the strategy for constructing
microfracture repair tends to form fibrocartilage rather the scaffolds using 3D printing.
than normal hyaline cartilage ; autologous or allogeneic
[3]
osteochondral implantation have a limited source of graft 2. Osteochondral tissue: Anatomy and
tissue and high incidence in the area of the graft origin; the physiology, pathology, and restoration
[4]
defective area repair does not fit the surrounding articular mechanisms
cartilage; and among other problems. Therefore, their
clinical practical restorative results are not satisfactory. 2.1. Anatomy and physiology
Cell-based treatments, such as ACI and MACI, also involve The articular osteochondral structure can be roughly
the possibility of fibrocartilage production in the repair divided into five layers according to the biological
area, incomplete filling of the repair, and poor integration differences in fiber orientation, cell morphology and
with the surrounding tissues, and their actual results have density, content of glycosaminoglycans (GAGs), collagen
not been uniformly accepted. As a result, there is a lack of and water, and their corresponding mechanical gradients.
practical and effective treatment for osteochondral defects As shown in Figure 2A and C, the five layers are superficial
in clinical practice. layer, intermediate layer (transitional layer), deep layer
(radial layer), calcified cartilage layer, and subchondral
The development of tissue engineering techniques bone layer. The first three layers are generally referred to
offers a novel approach to the treatment of osteochondral as hyaline cartilage layer. There is a tidemark structure
defects. Long-term restorative results can be achieved that connects the relatively soft articular cartilage to hard
through the use of an integrated tissue-engineered calcified cartilage. The subchondral bone lies beneath the
osteochondral bionic scaffold, combined with the calcified cartilage layer, and the structure formed by the
relevant advantages of existing treatment methods interlocking of these two layers is called cement line .
[8]
and a systematic and personalized postoperative
rehabilitation program. Tissue engineering technology 2.1.1. Hyaline cartilage layer
aims to repair the structure and function of damaged The superficial layer comprises 10%–20% of the total
tissue by combining seed cells and growth factors with thickness of hyaline cartilage layer and is characterized
material scaffolds. The articular cartilage is relatively by thin and densely arranged collagen fibers that run
homogeneous in composition and simple in structure parallel to the cartilage surface. This layer has a high
with no complex vascular system . Due to its low density of chondrocyte distribution with a long, thin, and
[5]
difficulty of tissue engineering, it is considered the most flattened morphology . The chondrocytes in this area are
[9]
promising alternative for osteochondral defect treatment. primarily associated with the outward tissue growth and
Since the 1990s, research into articular osteochondral are referred to as persistent chondrocytes. The uppermost
tissue engineering has made significant progress. area of the superficial layer is overlaid with a thin layer

Volume 9 Issue 4 (2023) 130 https://doi.org/10.18063/ijb.724