3D-printed knee implants improves quality and reliability, suggests study

Customized 3D-printed medical implants are becoming more common, and a new study has taken this technology to the next level. Researchers at Naton Biotechnology have developed the world’s first laser 3D-printed total knee implant, which has received official approval from China’s National Medical Products Administration as an innovative medical device.

The study focused on improving the strength and consistency of cobalt-chromium-molybdenum (CoCrMo) alloy implants made using laser powder bed fusion (LPBF), a 3D printing process. The team discovered and corrected inconsistencies in the structure of the material by optimizing heat treatment, ensuring the final implants are stronger, more reliable, and safer for patients.

This research provides key insights into how 3D printing affects metal implants and lays the foundation for better quality control in orthopedic manufacturing, helping to advance the future of customized medical implants.

This research was led by Professor Changhui Song from South China University of Technology and Professor Jia-Kuo Yu from Beijing Tsinghua Changgung Hospital as co-corresponding authors. The study was conducted in collaboration with Senior Engineer Renyao Li from Naton Biotechnology (Beijing) Co., Ltd and other members of the team.

The Problem: Uneven Strength in 3D-Printed Metal Implants

The layer-by-layer manufacturing process of CoCrMo, a widely used implant material, occurs at extremely high cooling rates (~10⁵–10⁶ K/s). This rapid solidification often leads to anisotropy, meaning the material's properties vary depending on the direction of force. The main causes include columnar grain structures, porosity, and residual stress, all of which are inherent to additive manufacturing.

While extensive research has been conducted on LPBF-fabricated CoCrMo alloys, most studies have only examined their performance in a single direction, overlooking how anisotropy affects overall durability. However, implants inside the human body must withstand forces from multiple directions. Then, if the material's strength is inconsistent, weak spots can develop, increasing the risk of breakage or failure.

In mechanical tests, CoCrMo samples stretched significantly more in one direction (19.1% elongation) than in another (9.3% elongation)-a disparity of over 100%. This inconsistency makes the material unreliable for long-term medical use, as implants must perform uniformly and safely under everyday stresses.

The Solution: A New Heat Treatment Process

The team found that a two-step heat treatment process significantly improved the uniformity of the metal’s structure and strength. The process included:

Solution Treatment – Heating the material to 1150°C, holding it for an hour, and then rapidly cooling it in water. This helped restructure the uneven metal grains.

Annealing – Reheating the material to 450°C for 30 minutes and then cooling it again. This step refined the grain structure and further balanced the material's properties.

As a result, the metal’s strength and flexibility became nearly identical in all directions. The ultimate tensile strength reached 906.1 MPa and 879.2 MPa, while elongation values balanced at 20.2% and 17.9%, making the material stronger and more reliable for medical use.

The Future: Improving Durability and Biocompatibility

With this breakthrough, scientists are now looking at surface treatments to further enhance the wear resistance and biocompatibility of implants. Methods like shot peening (where tiny metal beads are blasted onto the surface) and ultrasonic peening could improve the fatigue resistance of implants, helping them last longer under daily stress. These next-generation treatments could make 3D-printed joint implants even more durable and widely used in clinical settings.

The Impact: A Safer Future for Medical Implants

This research offers new insights into how to improve 3D-printed metal implants, making them safer and more durable for patients. By addressing uneven strength and material quality, this breakthrough lays the foundation for better orthopedic implants, particularly for joint replacements.

The findings were recently published in the international journal Materials Futures, further advancing research in medical-grade additive manufacturing.

Reference:

Lijin Dai, Changhui Song, Houxiong Fu, Hongyi Chen, Zhongwei Yan, Zibin Liu, Renyao Li, Anming Wang, Yongqiang Yang, Jia-Kuo Yu. Recrystallization induced by heat treatment regulates the anisotropic behavior of CoCrMo alloys fabricated by laser powder bed fusion[J]. Materials Futures. DOI: 10.1088/2752-5724/adb50a