Surgical plates and screws, known medically as osteosynthesis hardware, are specialized devices used to stabilize fractured bones and correct skeletal deformities. Their purpose is to hold bone fragments firmly in alignment while the natural healing process takes place. Because these devices often remain inside the human body permanently, material selection requires a complex balance of mechanical strength, resistance to the body’s corrosive environment, and biological acceptance.
The Primary Metals Used in Surgical Hardware
The majority of plates and screws are fabricated from two principal types of metal alloys: titanium and stainless steel, each meeting strict medical-grade specifications. Titanium alloys, particularly Ti-6Al-4V (6% aluminum and 4% vanadium), are considered the gold standard for many permanent implants. Titanium is prized for its exceptional biocompatibility, meaning the body rarely mounts an immune response, and its ability to promote osseointegration, where bone tissue grows directly onto the implant surface. Titanium is also non-ferromagnetic, which minimizes image distortion during magnetic resonance imaging (MRI) scans.
Medical-grade stainless steel, most commonly 316L, is often chosen for temporary fixation hardware. This alloy is a mixture of iron, chromium (17–19%), nickel (13–15%), and molybdenum (2–3%), with the “L” indicating a low carbon content for corrosion resistance. Stainless steel offers superior tensile strength and is generally more cost-effective than titanium. However, the nickel content can occasionally trigger allergic reactions, and its higher stiffness and magnetic properties limit its use in certain long-term applications.
Advanced Materials and Bioresorbable Options
A significant advancement is the introduction of bioresorbable, or biodegradable, options that safely dissolve within the body over time. These materials, primarily polymers such as Polylactic Acid (PLA) and Polyglycolic Acid (PGA), eliminate the need for a second surgical procedure to remove the hardware. The polymers slowly break down through hydrolysis into non-toxic compounds that are naturally metabolized or excreted by the body. The degradation rate can be precisely engineered by adjusting the ratio of PLA and PGA, allowing the implant to maintain mechanical support during initial bone healing. As the bone gains strength, the implant’s strength diminishes, transferring the load back to the healing bone. Research is also exploring degradable metal alloys, such as those based on magnesium, which offer mechanical properties closer to bone and are absorbed by the body.
Key Properties That Determine Material Selection
The choice between materials hinges on precise physical and biological properties required for successful integration. Biocompatibility is the most fundamental requirement, ensuring the material does not provoke an adverse reaction, inflammation, or the release of toxic substances. The material must be completely inert or break down into harmless components to maintain patient safety.
Corrosion resistance is an absolute necessity, as the body’s internal environment is highly saline and aggressive. Titanium forms a stable, passive oxide layer on its surface that provides exceptional protection. Stainless steel achieves resistance through the inclusion of chromium and molybdenum, which create a protective surface film.
Mechanical properties, including strength and fatigue resistance, must allow the hardware to withstand the forces of daily life without fracturing. The modulus of elasticity measures a material’s stiffness. Stainless steel has a high modulus, making it much stiffer than natural bone. This can lead to “stress shielding,” where the implant takes too much load, causing the bone underneath to weaken. Titanium’s modulus is significantly lower and closer to that of bone, making it the preferred choice in many applications to encourage the bone to continue bearing a healthy amount of stress.
What Happens to Plates and Screws After Healing
The long-term fate of surgical hardware depends on the material used and the patient’s clinical circumstances. If a traditional metal alloy like titanium or stainless steel is used, the hardware is often left in place permanently once the bone has fully healed. These implants are designed to be bio-inert and typically cause no issues. However, hardware may require removal if it causes local soft tissue irritation, pain, or infection. The decision to remove permanent hardware is a surgical one, generally made only when the implant causes symptoms, as removal requires another operation with associated risks. Conversely, bioresorbable implants fully absorb into the body over several months to a couple of years, ensuring no foreign material remains once the bone is structurally sound.