Steel, particularly medical-grade alloys, has a long history in health care due to its favorable mechanical properties and cost. However, the complex, corrosive environment inside the body presents significant challenges, primarily related to the release of metal ions and subsequent biological responses. While once a primary material for permanent implants, steel is increasingly being replaced by advanced alternatives that offer superior long-term compatibility.
Steel in Medical Applications
Medical professionals rely on specific steel alloys, notably 316L stainless steel, due to their excellent balance of physical characteristics and cost-effectiveness. This material provides superior strength and rigidity for load-bearing devices. The addition of molybdenum enhances its resistance to pitting and crevice corrosion, a major concern in the high-saline environment of bodily fluids.
The properties of 316L steel make it suitable for orthopedic fixation devices, such as plates, screws, and pins, often intended for temporary use. These applications require a material that can withstand high mechanical stress without deforming or fracturing while the patient’s bone heals. Steel is also the preferred material for manufacturing reusable surgical instruments because it endures repeated, rigorous sterilization cycles without degrading.
Historically, steel was used for long-term implants, but its primary role today is shifting toward temporary scaffolding and instrumentation. The material’s ability to resist corrosion ensures reliability in the operating room. The “L” in 316L denotes a low carbon content, which prevents a loss of corrosion resistance in areas exposed to high heat during manufacturing.
Biological Reaction to Metal Ions
Despite its high corrosion resistance, 316L is not completely inert within the body, which is the core limitation of steel as a long-term solution. The physiological environment, which is aqueous and rich in chloride ions, can cause the material’s protective oxide layer to break down, leading to corrosion. This process causes the release of trace amounts of alloying elements, such as nickel (Ni) and chromium (Cr), into the surrounding tissue.
The biological response to these released metal ions and microscopic wear debris can be complex and varied. Nickel is a known allergen, and its release can trigger Type IV hypersensitivity reactions, commonly known as a metal allergy. These immune responses can manifest as chronic inflammation, which may lead to complications like pain, tissue damage, and ultimately, implant failure.
The continuous presence of metal debris stimulates macrophages and other immune cells, leading to a chronic inflammatory state. This reaction, referred to as adverse local tissue reaction, can contribute to the loosening of the implant from the surrounding bone, a process called osteolysis. Research indicates that certain metal ions, like chromium, may have genotoxic and cytotoxic effects on cells, potentially affecting tissue function, even at concentrations considered subtoxic.
Modern Material Alternatives
The limitations of steel have driven the development of alternative materials that offer superior long-term biocompatibility and stability. Titanium alloys, such as Ti-6Al-4V, are now considered the “gold standard” for permanent implants. They naturally form a stable, highly biocompatible oxide layer that is resistant to breakdown, resulting in significantly less metal ion release compared to steel.
Titanium alloys also possess an elastic modulus closer to that of natural bone than steel, which helps prevent stress shielding. Stress shielding occurs when a stiffer implant carries too much load, causing the surrounding bone to weaken over time. For high-load, articulating surfaces like knee replacements, cobalt-chromium alloys are preferred for their exceptional hardness and wear resistance.
Ceramics, such as alumina and zirconia, are utilized in joint replacements for their extreme hardness, smoothness, and wear resistance. Advanced polymers, including ultra-high-molecular-weight polyethylene, are also used in joint components for their flexibility and low friction. When steel must be used, its performance is enhanced with surface modifications, such as coatings of hydroxyapatite or ceramic layers, which promote bone integration and protect the alloy from the biological environment.