Advanced additive manufacturing in orthopedics: a comprehensive review of biomaterials, structural design, biological functions and clinical technology applications

Large bone defects resulting from trauma, disease, or congenital aberrations present a significant clinical challenge because they often exceed the body's natural healing capacity. While conventional 3D printing has revolutionized orthopedics by providing patient-specific anatomical replicates, these constructs remain inherently static and fail to adapt to the dynamic physiological environment of a healing bone. This review provides a comprehensive analysis of the transition toward 4D additive manufacturing in orthopedics, a landmark shift that integrates the dimension of time into tissue regeneration. We evaluate the strategic landscape of stimuli-responsive smart materials such as shape memory polymers (SMPs) and functionalized hydrogels, which execute programmed morphological or functional changes in response to triggers like body heat, pH, and magnetic fields.