| The Surgical Technologist | APRIL 2022 172 of a 3D-printed custom implant. In addition, 3D-printed custom talar prostheses have been increasingly used in the treatment of talar osteonecrosis. Total talus replacement with a 3D implant restores talar height and talar tilt while preserving the range of motion and normal alignment in unaffected joints.48,49 In addition, custom 3D-printed implants have been commonly used after the excision of primary and metastatic bone lesions.50-56 Patient-specific custom implants have become desirable alternatives to standard implants in TKA and total hip arthroplasty procedures. Custom implants have been shown to provide improved rotational alignment and tibial fit.57 Furthermore, compared with those treated with off-the-shelf implants, patients with custom implants have lost less blood, reported fewer adverse events, and were less likely to be discharged to a rehabilitation or acute care facility.58,59 Spine surgery has implemented 3D-printed patientspecific implants for complex spinal pathology with significant structural deformities, as in cases of neoplasia, degenerative disease, infection, trauma, and congenital anomalies. A systematic review evaluating the efficacy and safety of 3D-printed implants for spine surgery compared with off-the-shelf implants found that all included studies that reported clinical outcomes showed significant postoperative improvements.60 Several authors of articles included in this review commented on the significant commitment that 3D-printed spine implants require—there exist a large amount of preoperative work and requirements for specialized design, manufacturing equipment, and personnel that should be recognized before use.60 Surgeon involvement in the process is paramount, and they must work closely with the 3D printing company to design the implant. In addition to these intensive time and labor requirements, customized implants can accrue significant financial costs. A careful discussion with the hospital, patient, and insurance company regarding the financial burden of using custom implants is critical. Bioprinting/Four-dimensional Printing Three-dimensional printing technology has advanced rapidly, and several researchers are working on technology to print customized human tissue and organs. Known officially as 3D bioprinting, this process distributes cells, biomaterials, and supporting biological factors in a layer-by-layer fashion to form living tissues and organ analogs.61,62 To make this possible, the medium for printing is composed of inert material that can support live cells. Examples include hydrogels, microcarriers, tissue spheroids, cell pellet, tissue strands, and decellularized matrix components. The optimal medium must be stable, nontoxic, nonimmunogenic, biocompatible, and allow for cellular survival and proliferation.62,63 The metamorphosis to human tissue or organ analog is accomplished via droplet, extrusion, or laserbased methods. This process facilitates precise control of the microarchitecture and macroarchitecture of the final product, both of which are essential to the function of biologic tissues. These 3D products still face many challenges: growing the correct number of functioning cells, reaching the appropriate cell density, and retaining viability throughout the printing process, but its future potential could revolutionize regenerative medicine.64 Cartilage Bioprinting Surgical management for articular cartilage injuries vary depending on the location, size of the lesions, and patient factors.65-67 Although appropriately selected and performed surgical options can have good clinical results, they fail to fully restore the damaged cartilage tissue. Most restorative techniques create a form of functional cartilage; however, it is not the same as healthy articular cartilage at a molecular level.66,68 Three-dimensional bioprinting presents an alternative solution as the ability to print native cartilage would be groundbreaking in the management of cartilage defects and arthritis. Although most works on 3D bioprinting cartilage have been performed in vitro, in vivo animal studies have shown promise. Three-dimensional cartilage cells were implanted into rabbit models of cartilage defects and were found to demonstrate early cartilage formation and osteochondral integration.69,70 Moreover, a recent systematic review evaluating the published data surrounding bioprinted articular cartilage endorsed the potential of this technology for use in humans.71 Bone Bioprinting Bone possesses a unique set of mechanical and structural properties that is challenging to recreate artificially, and advances in 3D bioprinting could aid in bone formation and growth. Scaffolds are an essential technology for both bone tissue engineering and regenerative medicine as they provide the substrate where cells can attach, proliferate, and differentiate into bone. Important characteristics to consider are biocompatibility, biodegradability, microstructure, and osteoconductivity. With the advent of 3D printing, it has become easier to control the microstructure, which is critical to cell viability and 8 Journal of the AAOS Global Research & Reviews® ----- April 2021, Vol 5, No 4 ----- © American Academy of Orthopaedic Surgeons Three-dimensional Printing in Orthopaedic Surgery
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