Orthopaedic Stainless Steel vs Titanium: Strength, Springiness, and Stamina

I often hear from colleagues that stainless steel implants are stronger than titanium ones, and from others that the opposite is true. It turns out both are right, depending on what you mean by "strong."

I created a chart of the most common implant materials used in traumatology in Granda EduPack, based on the most up-to-date database (2023). The fatigue graphs are from the same source.

When we talk about a material being "strong" for an implant, we're often bundling several different ideas into one word. The graphs you have show some of these key properties, but not all of them.

Let's break down what these graphs show, and then look at the "hidden" properties that are even more important.

Chart 1: The "Bubble" Graph – Who is Strongest vs. Springiest?

This first graph is a snapshot that plots three main materials at once. Think of it as a chart comparing athletes on both their "brute strength" and their "agility."

1. Brute Strength (The Horizontal Axis)

• What it is: The X-axis is Yield Strength.

• Simple Analogy: This is how much you have to pull on a metal spring before it gets permanently stretched out and won't bounce back to its original shape. A higher number (further to the right) means it takes more force to bend it for good.

• What the Chart Shows: The Stainless Steel (orange bubble) is furthest to the right (at 737-975 MPa), meaning it has the highest "brute strength" in a single pull. The standard Titanium (purple bubble) is next (at 786-910 MPa), and the ELI Titanium (cyan bubble) is just behind it (at 760-900 MPa).

2. "Springiness" (The Vertical Axis)

• What it is: The Y-axis is Resilience.

• Simple Analogy: This is a measure of how much energy the material can soak up and still bounce back perfectly, like a high-quality rubber ball. A material with high resilience (higher on the graph) is very "springy."

• What the Chart Shows: The standard Titanium (purple bubble) is highest on the graph (at ~3.0 MJ/m³). The ELI Titanium (cyan bubble) is just below it (~2.8 MJ/m³), and both are significantly "springier" than the Stainless Steel (orange bubble, at ~2.4 MJ/m³).

However, "springiness" (elasticity) has its own drawbacks:

1. High Springback: Because titanium has a lower stiffness (Modulus), it is very springy. When you try to bend a plate to match a bone, it springs back towards its original shape much more than steel does.

   • The Consequence: To get a 20° bend, you might have to bend it to 25° or 30°. This forces you to push the material further.

   • The problem: Stainless steel, being more ductile, can handle this 30° bent, but Titanium alloys, being brittle, will not.

2. Notch Sensitivity & Low Ductility: Titanium hates being scratched. The bending tools often leave small surface marks. Because titanium is less ductile than steel, these marks become "stress risers."

   • The Deadly "Scratch and Harden" Effect: When you bend the plate, two things happen instantly:

     1. The metal between the benders "work hardens" (this is common for all metals), becoming stronger but more brittle.

     2. The benders themselves leave small scratches on the plate surface where they grip.

   The Snap: If you bend and try to bend it back (reverse contouring), the metal in the middle is now too hard to move. The force instead concentrates on the softer metal right next to it—exactly where your tools left those scratches. The scratch acts as a stress concentrator, causing the plate to snap cleanly at that specific point. Stainless steel is much more forgiving of this back-and-forth adjustment because it is more ductile. So, even with scratches, it provides more room for bending.

How does ductility enhance resistance to snapping?

When a surface is scratched, the defect due to scratching causes an abrupt change in the geometry of the structure at this cross-section. Abrupt changes in geometry are known as "stress risers". As we bent the metal at this cross-section, stainless steel being more ductile, deforms more in this area and consequently the defect boundaries become blunted. Imagine the edge of a defect transforming from a V shape to a U shape. The U shape is less of a "stress riser" as the geometry at its boundary changes less abruptly. Titanium alloys are less ductile. So, the shape of the stress riser will remain unchanged, and the material will snap at a lower load.

What is "ELI" Titanium and Why Use It?

You might wonder why the ELI version (cyan) shows slightly lower strength and springiness. This brings up an excellent point: the endurance graphs also show ELI is weaker, so why is it considered safer for long-term use?

This is a classic engineering trade-off between Strength and Toughness.

• Strength (what the fatigue graph shows): This test measures how long it takes for a microscopic crack to form on a perfectly polished, flawless material. In this test, the harder, stronger steel wins.

• Toughness (what the graph doesn't show): This measures how well a material resists a crack from growing after a flaw already exists (like a microscopic impurity or a scratch from surgery).

Think of it like glass vs. plastic. A pane of glass is very "strong" (it's hard to bend), but a single nick can cause it to shatter. It has low toughness. A plastic ruler is "weaker" (it bends easily), but you can scratch it and bend it, and it won't suddenly fail. It has high toughness.

"ELI" (Extra Low Interstitials) means the titanium is ultra-pure. This purity makes it less "strong" in a lab test but dramatically more "tough." For a 30-year implant, engineers would rather have the "weaker" material that resists shattering (ELI Titanium) than the "stronger" one that might fail suddenly if a tiny flaw appears (Steel).

The "Endurance" Graphs – Who Has Better Stamina?

The next graphs show long-term endurance, or Fatigue Strength.

Fatigue strength model for Ti-6Al-4V, annealed Titanium alloy

Fatigue strength model for Ti-6Al-4V ELI, annealed Titanium alloy

Fatigue strength model for AISI 316LVM cold-worked stainless steel

• Simple Analogy: Think of a paperclip. Bending it once is easy. But if you bend it back and forth in the same spot, it quickly breaks. That failure from repeated stress is called fatigue.

• What the Charts Show: These graphs show how much repeated stress a material can take before it's expected to fail, over millions of cycles.

The "Lab Test" Comparison

Let's compare their long-term stamina at a specific point: 100 million cycles (written as 10^8 on the far-right of the horizontal axis).

• Stainless Steel (316LVM, cold worked): This is the clear lab-test winner. Its strength band is the highest, ranging from 310 MPa to 510 MPa.

• Standard Titanium (Ti-6Al-4V, annealed): The strength band is lower, ranging from 260 MPa to 360 MPa.

• ELI Titanium (Ti-6Al-4V ELI, annealed): The new graph shows this band is the lowest of the three, from 230 MPa to 340 MPa.

Just like with the "brute strength" test, the higher-purity ELI grade shows slightly lower endurance in a lab test. This is the expected trade-off: it sacrifices this 'crack-initiation' strength (what the graph shows) to gain superior 'crack-growth' toughness and damage resistance (what the graph doesn't show), which are more critical for implant safety.

...But the Body is Not a Lab: The Corrosion Factor

This is a critical point. Those fatigue tests were done in clean air. The human body, however, is a warm, salty (saline) environment, which is highly corrosive.

This introduces a new problem called Corrosion Fatigue.

• Stainless Steel (316LVM): This steel is "stainless" because it has a protective layer. But in the body's high-chloride environment, it can still be vulnerable to microscopic corrosion (pitting). These tiny pits act as starting points for cracks, making the material fail from fatigue much, much faster.

• Titanium Alloys (Ti-6Al-4V and ELI): Both versions of titanium form an exceptionally stable and self-healing protective layer that is practically inert in the human body.

The takeaway: Because titanium resists corrosion so well, its "stamina" inside the body remains very high. The steel's "stamina," while higher in a lab, may drop significantly in the real world. Over decades, the titanium's actual endurance could be far better.

Hidden Properties Not on the Graphs

1. Biocompatibility (How the Body Reacts)

This is a simple question: "how well does the material get along with the body's tissues?"

• Stainless Steel: Is generally well-tolerated. However, it is an alloy that contains nickel (which is one of the reasons it is "stainless"). A small percentage of the population (10-15%) is sensitive or allergic to nickel. For these patients, the steel can release tiny amounts of nickel ions, causing inflammation, pain, or implant rejection.

• Titanium Alloys: Both versions are considered among the most biocompatible materials we have. They are exceptionally inert (non-reactive). Their stable oxide layer prevents any metal ions from "leaching" into the body. Allergic reactions to titanium are extremely rare, and the high purity of the ELI grade makes it an even safer choice.

2. Stiffness (or, "Working with Bone")

There's another property, stiffness (or elastic modulus), which is perhaps the most important of all.

• Simple Analogy: Think of stiffness as the difference between a stiff wooden ruler (high stiffness) and a flexible plastic ruler (low stiffness).

When an implant is placed in the body, it doesn't work alone—it works with the bone.

• Human Bone: Is relatively flexible. (Stiffness of ~10-30 GPa)

• Titanium Alloys: Are stiff. (Stiffness of ~110 GPa)

• Stainless Steel: Is very stiff. (Stiffness of ~190 GPa)

Why "Too Stiff" is a Problem: Stress Shielding

Your bone is a living tissue that follows a "use it or lose it" rule (known as Wolff's Law). It needs mechanical stress from daily activity (walking, lifting) to stay strong.

1. A very stiff stainless steel implant is like an internal splint; it carries all the load.

2. The bone around it is "shielded" from this stress.

3. The body senses the bone isn't being used, so it stops maintaining it and begins to resorb it. This leads to bone loss around the implant.

4. This bone loss can cause the implant to become loose over time, leading to pain and failure.

This is where titanium has a huge advantage. Because it is almost twice as flexible as steel, it's closer to the stiffness of bone. It shares the load with the bone, allowing the bone to stay stimulated, healthy, and strong.

Why Is Titanium So Much More Expensive?

This is a common question. It comes down to two main reasons:

1. Difficult Production: Titanium ore is common, but turning it into pure, usable metal is an extremely complex and energy-intensive process. It requires very high temperatures and special vacuum chambers. The ELI grade requires even more refining, which adds to the cost.

2. Difficult Manufacturing: Titanium is notoriously hard to work with. It's "gummy," meaning it wears out cutting tools very quickly. It also reacts with air at high temperatures, so it must be welded in a special, inert-gas environment. All of this specialised handling and tooling adds significantly to the final cost.

Stainless steel, by contrast, is one of the most common and easiest-to-manufacture materials in the world, making it much cheaper.

Final Summary: The Real Trade-Off

• Stainless Steel (316LVM, cold worked):

  • Pro: Excellent "brute strength" and "lab-test stamina." Cheaper and easier to make.

  • Pro: Easier for surgeons to contour (bend) to fit the bone without snapping.

  • Con: Very high stiffness can cause stress shielding, leading to bone loss.

  • Con: "Stamina" may drop inside the body due to corrosion fatigue.

  • Con: Contains nickel, which can cause allergic reactions in some patients.

• Titanium Alloys (Ti-6Al-4V and ELI):

  • Pro: Excellent "springiness" and fantastic in-body corrosion resistance.

  • Pro: Favourable, more flexible stiffness prevents stress shielding and keeps bone healthy.

  • Pro: Superior biocompatibility with almost no risk of allergic reaction.

  • Pro (ELI Grade): Superior toughness (resists a crack growing, making it safer and more damage-tolerant) due to high purity.

  • Con: Lower "brute strength" and "lab-test stamina" (resists a crack forming), which is the trade-off for its higher toughness.

  • Con: Harder to contour; prone to snapping if over-bent or reverse-bent.

  • Con: Much more expensive to produce and manufacture.

This is why titanium alloys are often the preferred choice for long-term implants. It's not just about simple strength, but about a combination of properties that work best with the human body for decades.