Material design to enhance bioactivity of porous tantalum implants - new generation of orthopedic implants

Bones are the hard organs that make up the endoskeleton of vertebrates, featuring a complex inner and outer structure that allows them to remain hard while reducing weight. Their functions include enabling movement, providing support, protecting the body, producing red and white blood cells, and storing minerals. However, fractures, bone defects, and joint degeneration--due to accidents, diseases, and aging--necessitate the use of artificial metallic orthopedic implants. Therefore, the interest in fabricating effective metallic orthopedic implants through material design is growing.

Published in the International Journal of Extreme Manufacturing, Prof. Ying Cai and Prof. Mingchun Zhao's team from Central South University provides a comprehensive overview of 3D-printed porous tantalum (pTa), an emerging material for bone defect replacement as an orthopedic implant.

This review, using material design to enhance the bioactivity of pTa implants as an entry point, offers readers insight into how material design can transform an emerging orthopedic implant. Specifically, it provides a comprehensive discussion of recent advances in using pTa materials for implant development, detailing various designs based on surface morphology, pore structure, alloying, and functionalization modifications. The review also discusses the potential limitations of these designs and methods for in vitro cellular bioactivity, in vivo animal osseointegration, and clinical translation of pTa implants.

Porous tantalum (pTa) has an elastic modulus comparable to that of bone, effectively reducing the shielding effect while providing sufficient physiological support for new bone tissue with high strength. Additionally, its friction-resistant properties enhance the initial stability. Its porous structure, resembling that of natural bone, facilitates cell adhesion, vascularization, and nutrient exchange, as well as inward bone growth and osseointegration.

However, to manufacture functional products with better bioactivity, pTa orthopedic implants must meet basic standards, including mechanical properties, biocompatibility, and biosafety. These features can be achieved through specialized material design. Additionally, the osseointegration of pTa orthopedic implants for bone defect repair with surrounding tissues needs to be addressed, especially in more complex human environments with diseases. The pTa orthopedic implants should provide support while allowing bone to grow into the material.

How the material is designed depends on the complexity of the bone defect site, the anatomic location, and the desired clinical outcome. Prof. Mingchun Zhao highlighted, “The initial morphology and structure of orthopedic implants are crucial for the success of implant placement. Alloying and modification can be adapted to different application environments to improve bioactivity, achieving faster bone growth and more stable osseointegration. Clinical surgeons should carefully evaluate and determine the application scenarios of the implants to meet specific clinical indications, considering the variability of different human bodies (diabetes, tumors, etc.) and the stochastic nature of bone defect sites (osteoporosis, bone deformities, etc.).”

Despite significant advancements in pTa orthopedic implants, manufacturing complex pTa bone implants that achieve an optimal balance among morphological structure (such as micro-nano surfaces, pore shape, pore size, and porosity), mechanical properties (such as elastic modulus, fatigue performance, and wear resistance), and biological characteristics (such as bioactivity and biocompatibility) remains challenging. Prof. Cai Ying, a specialist in rehabilitation medicine from the team, pointed out, “In the postoperative rehabilitation process, personalized customization of bone defect sites is still a major challenge. Currently, there are no commercially available pTa orthopedic implants, with the fastest only in the clinical validation stage. A key factor is the precise anatomical positioning required for different bone defect sites (such as the medial and lateral areas of the tibial plateau or femoral condyles), along with the limitations imposed by surrounding tissues on the defect site.”

In postoperative rehabilitation, correct implant positioning and design can significantly improve patient recovery outcomes and quality of life. To achieve optimal clinical results, rehabilitation physicians and surgeons must collaborate closely to evaluate each patient's specific situation, ensuring that the implants meet not only mechanical and biological requirements but also adapt to individual anatomical features and rehabilitation needs.

“In order to overcome this problem, the most attractive and innovative approach is the simultaneous improvement of 3D printing technology and the combination of multiple strategies to design novel Ta-based implants. At the same time, various advanced technologies in healthcare such as control and feedback systems, surgical tools, and visualization processes can be used for personalized replacement of bone defects.”

In addition, when enhancing the performance of Ta-based implants, researchers usually concentrate on the biocompatibility and osseointegration properties of the materials, while the stability and durability under the biomechanics and force changes in the implanted body are neglected, which make the biological evaluation in animal experiments affected and leads to the delay of clinical application. Therefore, accelerating the evaluation of the stability and durability of Ta-based implants under dynamic mechanics in animal experiments may be another key point to advance clinical application.

Although there is still a long way to go, the team has mapped the route forward for orthopedic implants. “The preparation and application of future orthopedic implants will not just be a matter of biomedical engineering, but a highly interdisciplinary approach involving advanced technologies from various disciplines (e.g., Artificial Intelligence (AI), Virtual Reality (VR), Augmented Reality (AR), and 5G technologies) interacting with the clinical team.”

About IJEM:

International Journal of Extreme Manufacturing (IF: 16.1, consecutive 1st in the Engineering, Manufacturing category) is a multidisciplinary, open-access, and double-anonymous peer-reviewed journal uniquely covering the full spectrum of extreme manufacturing.

The journal is devoted to publishing original articles and reviews of the highest quality and impact in the areas related to extreme manufacturing, ranging from fundamentals to process, measurement, and systems, as well as materials, structures, and devices with extreme functionalities.

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