Stainless steel is one of the oldest and most widely used metallic materials in medicine, employed for surgical instruments and implanted devices. Biocompatibility—the ability of a material to perform with an appropriate host response—is required for any material placed inside the human body. While stainless steel offers durability and strength, its interaction with the biological environment requires careful evaluation. Understanding the specific composition and limitations of medical-grade stainless steel is necessary to appreciate its role in clinical applications.
Composition of Medical-Grade Stainless Steel
The term “stainless steel” refers to a family of iron alloys, but medical applications strictly require specific grades, most commonly 316L stainless steel. The ‘L’ signifies a low carbon content (below 0.03%), which minimizes the risk of sensitization during manufacturing. This low-carbon composition helps preserve the material’s inherent corrosion resistance.
The alloy’s performance in the human body is primarily dictated by Chromium and Molybdenum. Chromium (16–18%) forms an extremely thin, self-repairing passive oxide layer on the surface, which is the material’s main defense against corrosion. Molybdenum (2–3%) increases resistance to localized attacks, such as pitting and crevice corrosion, which are common failure modes in chloride-rich body fluids.
The remaining composition includes Nickel (10–14%), which stabilizes the austenitic microstructure, enhancing both strength and corrosion resistance. While standard industrial grades like 304 stainless steel are suitable for external equipment, the addition of Molybdenum and strict control of Carbon in 316L make it the standard for devices that contact internal tissues or fluids. These precise compositional controls ensure the material meets the stringent ASTM F138 and F139 standards required for safe use inside the body.
Mechanical Properties and Inertness
Stainless steel is selected for medical devices due to its mechanical performance, including high tensile strength, ductility, and fatigue resistance. These properties are valuable in load-bearing applications, such as bone fixation hardware, where the material must withstand repetitive stress until the body heals. High strength allows devices to be manufactured with thinner profiles, which can be beneficial for reducing the overall bulk of an implant.
The material’s inertness in the body is achieved through the spontaneous formation of the passive layer, a thin film of Chromium Oxide that forms instantly upon exposure to oxygen. This oxide layer acts as a chemical barrier, preventing the underlying metal atoms from reacting with the surrounding biological tissues and fluids. A stable passive layer ensures the material remains largely unreactive when exposed to the body’s aggressive, chloride-containing environment.
The high stiffness of bulk 316L stainless steel, with an elastic modulus around 193 GPa, presents a challenge in orthopedic applications. This modulus is significantly higher than that of natural bone, which can lead to a phenomenon known as “stress shielding.” Stress shielding occurs when the stiff implant carries too much of the mechanical load, causing the adjacent bone to resorb or weaken over time.
Risks of Corrosion and Metal Ion Release
The primary limitation of stainless steel as an implant material is the potential breakdown of the protective passive layer, leading to corrosion and the subsequent release of metal ions. The biological environment is highly conducive to localized corrosion, specifically pitting and crevice corrosion, which often occur at interfaces between plates and screws or in tight gaps where oxygen access is restricted. This localized attack is a major cause of implant failure.
When the passive layer is compromised, the alloy components—primarily Nickel, Chromium, Iron, and Molybdenum—can leach into the surrounding tissues. The release of these ions can trigger local inflammatory responses, potentially inhibiting bone formation, causing synovitis, or leading to the loosening of the implant. The quantity of ions released often increases under inflammatory conditions, such as those found around an infection or a fracture site.
Nickel release is a concern because it is a known sensitizer that can cause hypersensitivity reactions or contact dermatitis in susceptible patients. Although the Nickel is bound within the alloy, its release following corrosion affects patient selection, making stainless steel less desirable for individuals with known metal allergies. For long-term or permanent implants, the potential for continuous ion exposure and associated systemic toxicity risks has driven the search for alternative materials.
Applications in Temporary and Permanent Implants
Stainless steel’s balance of strength, formability, and cost-effectiveness ensures its use across a wide range of medical devices. Its most common application is in temporary internal fixation devices, such as bone plates, screws, rods, and wires used to stabilize fractures. These devices are intended to be removed after the bone has fully healed, limiting the duration of exposure to the corrosive environment.
In permanent applications, stainless steel is used for devices that do not bear high mechanical loads or are protected from direct tissue contact, such as certain cardiovascular stents and pacemaker casings. However, for high-load orthopedic devices like total hip or knee replacements, 316L stainless steel has largely been phased out. The long-term risk of mechanical failure from corrosion pits, combined with continuous metal ion release, led to the preference for more inert materials.
Modern permanent orthopedic implants are predominantly manufactured from titanium alloys or cobalt-chromium alloys, which offer better corrosion resistance and reduced ion release profiles. Stainless steel’s current role is defined by its suitability for instruments, external apparatus, and temporary internal devices where its mechanical strength and cost advantage can be safely utilized.