3D-printed gradient scaffolds for osteochondral defects: Current status and perspectives.

Articular osteochondral defects are quite common in clinical practice, and tissue engineering techniques can offer a promising therapeutic option to address this issue.The articular osteochondral unit comprises hyaline cartilage, calcified cartilage zone (CCZ), and subchondral bone.As the interface layer of articular cartilage and bone, the CCZ plays an essentialpart in stress transmission and microenvironmental regulation.Osteochondral scaffolds with the interface structure for defect repair are the future direction of tissue engineering. Three-dimensional (3D) printing has the advantages of speed, precision, and personalized customization, which can satisfy the requirements of irregular geometry, differentiated composition, and multilayered structure of articular osteochondral scaffolds with boundary layer structure. This paper summarizes the anatomy, physiology, pathology, and restoration mechanisms of the articular osteochondral unit, and reviews the necessity for a boundary layer structure in osteochondral tissue engineering scaffolds and the strategy for constructing the scaffolds using 3D printing. In the future, we should not only strengthen the basic research on osteochondral structural units, but also actively explore the application of 3D printing technology in osteochondral tissue engineering. This will enable better functional and structural bionics of the scaffold, which ultimately improve the repair of osteochondral defects caused by various diseases.