Compared to stainless steel used in industrial structures, medical-grade stainless steel is required to maintain excellent corrosion resistance within the human body. This requirement aims to minimize the release of metal ions, prevent occurrences of localized corrosion such as intergranular corrosion and stress corrosion, and avoid potential failures and fractures of implanted devices. Ensuring the safety of implanted instruments necessitates stricter chemical composition standards for medical-grade stainless steel.
Medical stainless steel, especially implant-grade stainless steel, features alloying elements like Ni and Cr at levels higher than those found in regular stainless steel (typically reaching the upper limits of regular stainless steel requirements). Impurity elements such as S and P are required to have lower levels compared to regular stainless steel. Moreover, explicit specifications dictate that non-metallic inclusions in the steel must have sizes smaller than grade 115 (fine series) and grade 1 (coarse series). In contrast, standard industrial stainless steel norms do not impose specific requirements regarding inclusions.
To prevent intergranular corrosion in medical stainless steel, it is additionally required to have lower carbon (C) content. In earlier specifications, two levels were set for carbon content, namely not exceeding 0.108% and 0.103% (by mass fraction). With advancements in metallurgical techniques and increasing application demands, recent revisions of both domestic and international standards for medical stainless steel have universally mandated that the steel’s carbon content must not exceed 0.103% (as seen in standards like ASTM F138-03, ASTM F139-03, ISO 5832-1:2007, and GB/T 4234-2003).
Commonly used austenitic stainless steels in medical applications, such as 316L or 317L, exhibit relatively low strength and hardness in their solution-annealed state. However, these properties can be enhanced through cold working deformation. Consequently, surgical implant stainless steel utilized in clinical settings is typically maintained in a cold-worked state (with a deformation level of approximately 20%) to meet the demands for high strength and hardness required by implant instruments. Nevertheless, this cold-worked state increases the susceptibility of medical stainless steel to stress corrosion and corrosive fatigue failure.
Due to its excellent biocompatibility, favorable mechanical properties, outstanding resistance to bodily fluid corrosion, and good formability, medical stainless steel has become widely used as both implant materials and medical tool materials in clinical applications. Medical stainless steel is extensively employed in the fabrication of various artificial joints and fracture fixation devices. These include a range of artificial hip joints, knee joints, shoulder joints, elbow joints, wrist joints, ankle joints, and finger joints. Additionally, medical stainless steel is used to manufacture various specifications of bone connectors, compression plates, cancellous screws, spinal screws, bone traction wires, artificial vertebral bodies, as well as skull plates and artificial vertebral bodies.
In the field of dentistry, medical stainless steel finds wide application in tooth restoration, orthodontics, dental implantation, and auxiliary devices. It is utilized for various dental crowns, bridges, fixed brackets, clasps, retainers, and different specifications of inlays. Moreover, it is used for dental archwires, dentures, and jawbone defect repairs, among other uses.
In recent years, the Chinese medical device industry has witnessed a significant increase in demand for high-quality and cost-effective semi-finished products made from medical stainless steel wire, rods, specialized orthopedic plates, screws, and more. This demand has reached several hundred tons annually. Currently, there are only two standards for medical implant stainless steel in China: GB 4234 (which is essentially equivalent to ISO 5832-1 and ASTM F138) and YY 0605-2007 (equivalent to ISO 5832-9 and corresponding to ASTM F1586). The standardization efforts for medical stainless steel in China lag far behind the United States, which has already revised standards for six types of surgical implant stainless steel. As a result, the revision of standards for new types of medical stainless steel will be an essential component of China’s research and development in the field of medical stainless steel in the future.