Titanium Alloys

Titanium anchors; it does not slide. Titanium is the orthopaedic metal of choice for anchoring to bone, and the most biocompatible metal in orthopaedic use, because bone deposits directly onto its titanium-dioxide surface layer. Its modulus is far closer to bone than that of stainless steel or cobalt-chrome, which is why it suits uncemented stems.

Its weakness is the other half of the rule. Titanium is soft, notch-sensitive and wears badly, so it belongs at the bone interface (stems, shells, plates, screws, cages) while cobalt-chrome or ceramic forms the articulation (heads, liners).

Ti-6Al-4V. The most common orthopaedic alloy, known as Grade 5, is about 90% titanium, 6% aluminium and 4% vanadium. Aluminium stabilises the alpha phase and increases strength and oxidation resistance; vanadium stabilises the beta phase and increases ductility.

The two phases. Ti-6Al-4V is an alpha-beta alloy, a dual-phase structure that balances strength against ductility. It is heat treatable to optimise its properties and is the gold standard for orthopaedic use.

• Alpha, hexagonal close-packed (HCP), stabilised by aluminium: stronger, more resistant to creep and less ductile (brittle), and provides high-temperature strength
• Beta, body-centred cubic (BCC), stabilised by vanadium: more ductile, with better fatigue resistance, and provides formability

Commercially pure titanium. CP titanium is essentially unalloyed and comes in four grades (1-4) that differ chiefly by their interstitial oxygen (and iron) content. More oxygen raises strength but lowers ductility, so grade 1 is the softest and most ductile and grade 4 the strongest.

Why CP-Ti is a surface material. It is even more biocompatible and more ductile than the alloy, but considerably weaker. It is therefore used where bone apposition and formability matter more than bulk strength: porous coatings, sintered beads, fibre-metal pads, mesh and dental implants. The original Brånemark/Albrektsson osseointegration work used unalloyed CP-Ti screws.

Why the alloy is structural. Adding aluminium and vanadium roughly doubles the strength and fatigue resistance, which makes Ti-6Al-4V the substrate for stems, plates, screws and cages. The price is some ductility, and the (largely theoretical) ion-release concern that drove vanadium-free variants. The implant specification is the ELI (Extra-Low-Interstitial) grade, a low-oxygen variant with better fracture toughness and fatigue.

CP for the surface, alloy for the structure. The two are often combined: a CP-Ti porous coating sintered or diffusion-bonded onto a Ti-6Al-4V substrate gives bone-friendly ingrowth on top of a load-bearing core.

Passivation. Titanium forms a titanium dioxide (TiO₂) layer within nanoseconds of exposure to oxygen, 2-10 nm thick. The layer is extremely stable, protects against saline corrosion and is self-healing if scratched.

Bone bonds to the oxide. The oxide layer is the surface bone deposits directly onto. Osteoblasts adhere through integrin-mediated focal adhesions and lay down mineralised matrix in direct apposition, with no intervening fibrous membrane. That is true osseointegration, and the basis of cementless biological fixation.

Not hemidesmosomes. Hemidesmosomes are the epithelial soft-tissue seal of transmucosal dental implants, not the bone-attachment mechanism. The cell biology is developed in the osseointegration topic.

The numbers. Titanium's Young's modulus is about 110 GPa, roughly half that of cobalt-chrome or stainless steel and far closer to cortical bone at about 15-20 GPa. The comparison table below gives the values for each metal.

Why it matters. A modulus much closer to bone means less stress shielding. In a femoral stem, load transfer to the proximal bone prevents resorption (Wolff's law), which is what makes titanium ideal for uncemented femoral stems.

Porous coatings. A porous surface lets bone grow in for osseointegration, and there are several ways to make one:

• Plasma spray, as a coating 50-200 μm thick
• Sintered beads
• Electron beam melting (EBM)
• 3D-printed trabecular structures

Pore size. The target is approximately 50-400 μm for bone ingrowth. The window comes from Bobyn's canine data (see the Evidence Base), is approximate, and was derived in cobalt-chrome, not titanium.

Hydroxyapatite. HA is a calcium-phosphate ceramic chemically similar to the mineral phase of bone, plasma-sprayed as a thin layer of about 50-75 μm onto the titanium substrate of cementless stems and cups. It is bioactive and osteoconductive: bone bonds to it chemically and grows along it. It therefore accelerates early bone apposition and can bridge small gaps (of the order of a millimetre or two) that a bare metal surface would not, giving faster, more forgiving early fixation.

An accelerant, not a substitute. HA is partly resorbable. Over time it is remodelled and replaced, with bone bonding to the underlying roughened metal, and it is usually applied on top of a roughened or porous surface rather than instead of one.

The catch. HA may delaminate long-term, and a delaminating coating sheds particles that act as third-body wear, accelerating polyethylene wear and osteolysis. Coating durability depends on crystallinity and dissolution rate. While HA improves early fixation, most long-term studies show little durable advantage over a well-made grit-blasted or porous titanium surface, so its routine use is debated and many modern stems rely on surface roughening or porous titanium alone.

Poor tribology. Titanium is soft, with poor wear resistance, and makes a poor bearing surface. Used as one, it sheds black abrasive debris and provokes an adverse local tissue reaction (metallosis).

Notch sensitivity. Cracks propagate from surface defects, and thread roots are stress risers. In theatre, contour plates carefully and avoid scratching the surface.

Cold welding (galling). When a titanium screw is tightened into a titanium plate, the oxide layers can scrape off and the two raw metal surfaces fuse under pressure. The screw then cannot be removed. The risk factors are:

• Screw and plate of the same metal
• Multiple insertions and removals
• High torque

Preventing it.

• Type II anodisation, an electrochemical thickening of the oxide layer that makes it harder and grey; use anodised screws with plates
• A different alloy for screw and plate (less common)
• A single insertion at the correct torque
• The correct screwdriver (Star/Torx preferred over hex), held with axial pressure, and no cross-threading of the soft metal

Removing a cold-welded screw. Prevention is always better. When a screw is stuck, the options are:

1. Extraction bolt (reverse thread)
2. Drill off the screw head, remove the plate, then trephine the shank
3. Metal-cutting bur (destructive)
4. Stacking technique (foil or rubber for grip)

In a biomaterials viva the "differential" is the choice between the three structural metals. Match the property to the clinical job.

Stem and head. In a hip, the femoral stem is titanium for its low modulus and for bone ongrowth. The femoral head is cobalt-chrome (Co-Cr-Mo) for its high hardness, which resists scratching, and its wear resistance as a smooth bearing.

Metal allergy. Titanium very rarely causes hypersensitivity, which makes it the implant of choice in patients with documented nickel or cobalt-chrome allergy. Pre-operative patch testing can be considered in suspected cases.

Imaging. Titanium is radiopaque but less dense than cobalt-chrome (thinner lines on radiographs). It produces far less MRI artefact than cobalt-chrome or stainless steel, is MRI-safe at 1.5T and 3T, and metal reduction sequences (MARS) are available. It is the preferred metal where surveillance imaging matters: spine, tumour and revision surgery.

Uncemented stems. Bone ingrowth takes 6-12 weeks. Weightbearing is protected initially, and radiographs are monitored for subsidence.

Judging osseointegration. Over the long term, distinguish stable fibrous ingrowth from bone ingrowth and look for the absence of reactive lines; subsidence of less than 2 mm is acceptable. Retrieval studies show titanium stems with excellent long-term fixation, surface roughness maintained and the oxide layer intact after decades in vivo.

Fracture fixation. Hardware removal is possible, but carries the cold-welding risk. Consider leaving asymptomatic hardware, and document implant details for future surgery.

Where titanium is used.

• Hip arthroplasty: uncemented femoral stems (the standard) and porous-coated acetabular shells, but not femoral heads, which are cobalt-chrome or ceramic
• Fracture fixation: locking plates (distal radius, proximal humerus), intramedullary nails, and screws (beware cold welding)
• Spine: pedicle screws, rods (may be preferred over cobalt-chrome for MRI) and interbody cages (titanium or PEEK)

Modular junctions. A titanium trunnion under a cobalt-chrome head carries a risk of galvanic corrosion, whereas a titanium trunnion with a ceramic head is a good option; match materials when possible. Other solutions to the junction problem:

• Titanium nitride coating
• Ceramic-on-ceramic bearings
• Monoblock designs

Testing. Implant titanium is tested for tensile strength, for fatigue (rotating beam) and for corrosion (electrochemically).

Standards. ASTM F136 covers Ti-6Al-4V (the ELI grade) for surgical implants and ISO 5832-3 covers wrought Ti-6Al-4V, alongside specifications for surface roughness.

OrthoVellum is a global resource. Titanium implant selection is governed by international material standards and supported by convergent registry evidence worldwide.

Related pages: Cobalt-Chrome Alloys is the direct comparator and the reason this page exists — titanium for the stem because of its modulus and bone response, cobalt-chrome for the head because of the shear and wear resistance titanium lacks, and note that the pore-size window quoted here was actually derived in cobalt-chrome; Stainless Steel completes the three-metal comparison and remains the trauma workhorse where cost and ductility matter more than osseointegration; Tantalum is the alternative high-porosity ingrowth surface where scratch-fit stability is poor; PMMA Bone Cement is the fixation mode titanium is chosen to avoid, and the comparison is the substance of the cemented-versus-cementless debate; PEEK and Composite Materials for the radiolucent low-modulus alternative that pushes the stress-shielding argument further; Osseointegration carries the biology of the bone-implant interface that Albrektsson's work defined; Corrosion Mechanisms for the passive oxide layer that gives titanium its corrosion resistance and for what happens when it is breached; Trunnionosis and Taper Corrosion and Metal-on-Metal Complications for the mechanically assisted crevice corrosion at modular junctions and the adverse local tissue reactions that follow — the clinical face of titanium's poor wear behaviour; Tribology and Wear, Wear Mechanisms and THA Wear and Osteolysis for why titanium is never a bearing surface; and MRI Metal Artefact Reduction because titanium's low magnetic susceptibility is a genuine clinical advantage when a patient later needs imaging.

Beta-titanium and modulus matching. Low-modulus beta alloys (Ti-Nb-Ta-Zr, ~55-80 GPa) promise less stress shielding, but reducing stiffness too far risks higher implant strain, fatigue and micromotion. The optimum stem stiffness remains undefined, and clinical superiority over Ti-6Al-4V is unproven.

Vanadium and aluminium toxicity. Concern over vanadium and aluminium ion release drove vanadium-free alternatives (Ti-6Al-7Nb, Ti-5Al-2.5Fe). Clinical evidence of harm from Ti-6Al-4V in vivo is weak, so Grade 5 remains the workhorse alloy and the issue is largely theoretical.

Titanium debris and metallosis. Abrasive titanium wear, from a titanium bearing, fretting at modular junctions or impingement, can cause black staining and an adverse local tissue reaction. The threshold for revising a stained but asymptomatic joint is debated.

Trunnionosis and mixed-metal junctions. Fretting-crevice corrosion at the titanium-stem/cobalt-chrome-head taper can release metal debris. Whether to avoid mixed-metal couples or accept them with good taper engineering is unresolved.

3D-printed porous titanium. Additively manufactured trabecular structures reproduce ideal pore geometry, but long-term registry data, the fatigue behaviour of as-printed lattices and powder-residue effects are still maturing.

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