Titanium Alloys
TITANIUM ALLOYS
Ti-6Al-4V and Osseointegration
Alloy Types
Critical Must-Knows
- Definition: Titanium (Ti) and its alloys (most commonly Ti-6Al-4V) are biocompatible metals used for uncemented implants and fracture fixation
- Definition: They are known for excellent osseointegration and low modulus of elasticity
- Mechanism: Titanium (Base), Aluminium (Stabilises Alpha phase - Strength), Vanadium (Stabilises Beta phase - Ductility)
- Management: Surface treatments (Plasma spray, Acid etching, Grit blasting) enhance osseointegration
Examiner's Pearls
- "Young's Modulus: ~110 GPa (Closer to cortical bone at 20 GPa than SS/CoCr)
- "MRI Compatible (Low artifact)
- "Excellent biocompatibility (inert)
- "Poor wear resistance (Notch sensitivity - not a bearing surface)
Exam Warning
Titanium is the Most Biocompatible metal because osseointegration occurs directly onto the $TiO_2$ layer. However, it is soft and susceptible to Notch Sensitivity and Abrasive Wear (black debris). NEVER use a Titanium femoral head articulating with Polyethylene (creates massive wear/black synovitis). Ti is for anchoring to bone, not for sliding.
Composition & Structure
Ti-6Al-4V
The most common orthopaedic alloy ("Grade 5").
- Titanium (Ti): ~90%.
- Aluminium (Al): 6%. Alpha stabiliser. Increases strength and oxidation resistance.
- Vanadium (V): 4%. Beta stabiliser. Increases ductility.
Phases:
- Alpha: HCP (Hexagonal Close Packed). Stronger, brittle.
- Beta: BCC (Body Centred Cubic). Ductile.
- Ti-6Al-4V is an Alpha-Beta Alloy.
Passivation:
- Forms Titanium Dioxide ($TiO_2$) instantaneously upon exposure to oxygen.
- Extremely stable and protective against saline corrosion.
- The oxide layer is what bone cells attach to (Hemidesmosomes).
At a Glance
Titanium (Ti-6Al-4V) is the most biocompatible orthopaedic metal due to its spontaneous TiO₂ passivation layer that permits direct bone apposition—the basis of osseointegration. It has lower elastic modulus (~110 GPa) compared to stainless steel/CoCr (~200 GPa), reducing stress shielding and making it ideal for uncemented femoral stems. The alpha-beta alloy structure provides strength (aluminium) and ductility (vanadium). However, titanium is soft with poor wear resistance and notch sensitivity, making it unsuitable for bearing surfaces—never use titanium femoral heads against polyethylene. Cold welding between titanium screws and plates is prevented by Type II anodisation. Applications include uncemented arthroplasty components, locking plates, and spinal instrumentation.
B-O-N-E vs W-E-A-RTitanium Pros and Cons
Memory Hook:Ti is for BONE, not for WEAR
Properties
Modulus of Elasticity
- Titanium: ~110 GPa.
- Stainless Steel/CoCr: ~200-240 GPa.
- Cortical Bone: ~20 GPa.
Significance:
- Titanium has a modulus much closer to bone than Steel/CoCr.
- This creates Less Stress Shielding.
- Ideal for femoral stems where load transfer to proximal bone prevents resorption (Wolff's Law).
Cold Welding
Problem:
- When a Ti screw is tightened into a Ti plate, the oxide layers can scrape off.
- The two raw metal surfaces fuse under pressure ("Gall").
- Result: Screw cannot be removed.
Solution:
- Anodisation (Type II): Electrochemical thickening of the oxide layer (makes it harder/grey).
- Using different alloys for screw vs plate (less common).
Osseointegration
- Defined osseointegration as direct structural connection between living bone and implant surface without intervening soft tissue
- Surface Roughness is critical
- Rough surfaces (Ra over 1-2 microns) promote osteoblast differentiation and stronger pull-out strength than smooth surfaces
Overview
Why Titanium?
Clinical Applications:
- Uncemented femoral stems (THA)
- Acetabular shells and cups
- Locking plates (distal radius, proximal humerus)
- Spinal instrumentation (pedicle screws, rods)
Key Advantage: Direct bone apposition (osseointegration) via TiOâ‚‚ layer
Material Selection
Ti for Anchoring, NOT Articulation:
- Titanium = Bone interface (stems, shells, plates)
- CoCr/Ceramic = Bearing surface (heads, liners)
Never use Ti femoral head against polyethylene - catastrophic wear
Microstructure
Alpha Phase (HCP)
Hexagonal Close Packed Structure:
- Stronger, more resistant to creep
- Less ductile
- Aluminium (6%) stabilizes alpha phase
- Provides high-temperature strength
Beta Phase (BCC)
Body Centered Cubic Structure:
- More ductile
- Better fatigue resistance
- Vanadium (4%) stabilizes beta phase
- Provides formability
Alpha-Beta Alloy
Ti-6Al-4V Combination:
- Dual-phase structure
- Balance of strength AND ductility
- Heat treatable for optimized properties
- Gold standard for orthopaedic use
Surface Oxide
TiOâ‚‚ Passivation Layer:
- Forms within nanoseconds of air exposure
- 2-10nm thick
- Self-healing if scratched
- Bone cells attach via hemidesmosomes
Classification
Titanium Grades for Orthopaedics
Titanium Classifications
| Grade | Composition | Properties | Applications |
|---|---|---|---|
| CP-Ti (1-4) | Pure titanium | Softer, excellent biocompatibility | Dental, porous coatings |
| Ti-6Al-4V (Grade 5) | 6% Al, 4% V | High strength, osseointegration | Stems, shells, plates |
| Ti-6Al-7Nb | 6% Al, 7% Nb | No vanadium (less cytotoxic) | Alternative to Grade 5 |
| Beta-Ti (TNZT) | Ti-Nb-Ta-Zr | Ultra-low modulus (~55 GPa) | Research/newer stems |
Material Selection Considerations
When to Choose Titanium
Ideal for Titanium:
- Uncemented stems (low modulus = less stress shielding)
- Acetabular shells (bone ingrowth)
- Locking plates (MRI compatibility)
- Spinal instrumentation (low artifact)
Avoid Titanium:
- Bearing surfaces (poor wear)
- Modular junctions with high fretting (cold welding)
Clinical Relevance
Why Ti for Stems, CoCr for Heads?
Testing and Imaging
Imaging with Titanium Implants
Radiographs:
- Titanium is radiopaque
- Less dense than CoCr (thinner lines on XR)
MRI Compatibility:
- Low artifact compared to other metals
- Safe at 1.5T and 3T
- Metal reduction sequences (MARS) available
Clinical Applications
Orthopaedic Applications
Total Hip Arthroplasty:
- Uncemented femoral stems (standard)
- Acetabular shells (porous coated)
- NOT for femoral heads (use CoCr or ceramic)
Fracture Fixation:
- Locking plates (distal radius, proximal humerus)
- Intramedullary nails
- Screws (beware cold welding)
Spine:
- Pedicle screws
- Rods (may be preferred over CoCr for MRI)
- Interbody cages (Ti or PEEK)
Surgical Handling
Handling Titanium Implants
Screw Insertion:
- Use correct screwdriver (Star/Torx preferred over Hex)
- Maintain axial pressure
- Avoid cross-threading (soft metal)
- Single insertion preferred (reduces cold welding)
Plate Application:
- Contour carefully (notch sensitivity)
- Avoid scratching surface
- Use anodised screws with plates
Complications
Titanium-Specific Complications
Cold Welding (Galling):
- Screw fuses to plate, cannot remove
- Oxide layers disrupted, raw metal surfaces bond
- Prevention: Anodisation, correct technique
Notch Sensitivity:
- Cracks propagate from surface defects
- Avoid scratching during surgery
- Thread roots are stress risers
Poor Wear Resistance:
- Black debris if used as bearing surface
- Adverse tissue reaction (metallosis)
- NEVER use Ti head on polyethylene
Postoperative Considerations
Follow-up for Ti Implants
Uncemented THA Stems:
- Bone ingrowth: 6-12 weeks
- Protected weightbearing initially
- Monitor for subsidence on radiographs
Fracture Fixation:
- Hardware removal possible but cold welding risk
- Consider leaving asymptomatic hardware
- Document implant details for future surgery
Outcomes
Clinical Outcomes
Uncemented THA:
- Excellent survival at 15-20 years
- Lower stress shielding than CoCr stems
- AOANJRR data supports Ti stems
Fracture Fixation:
- Comparable healing to SS plates
- Lower MRI artifact (advantage)
- Cold welding is main concern at removal
Evidence Base
Key Evidence
Brånemark Osseointegration (1977):
- Defined direct bone-implant contact
- Demonstrated Ti biocompatibility
- Foundation for modern implant design
Surface Roughness Studies:
- Ra 1-2 μm optimal for bone ingrowth
- Grit-blasted, plasma sprayed surfaces
- Pore size 100-400 μm for ingrowth
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
MCQ Practice Points
Exam Pearl
Q: What is the composition and key mechanical property of Ti-6Al-4V alloy used in orthopaedic implants?
A: Ti-6Al-4V contains 90% titanium, 6% aluminium (alpha stabilizer), 4% vanadium (beta stabilizer). Key properties: Elastic modulus 110 GPa (closest to cortical bone at 18-20 GPa of any metal), excellent corrosion resistance, excellent biocompatibility. Lower modulus reduces stress shielding compared to CoCr (210 GPa) or stainless steel (200 GPa).
Exam Pearl
Q: Why is titanium NOT used for articulating bearing surfaces in joint replacement?
A: Titanium has poor wear resistance and high coefficient of friction. Titanium oxide layer (provides corrosion resistance) is easily disrupted by articulation, causing abrasive wear, metal debris, and adverse tissue reactions. Titanium is used for: stems, shells, plates, screws - NOT for femoral heads or tibial trays articulating with polyethylene. CoCr or ceramic used for bearing surfaces.
Exam Pearl
Q: What is the mechanism of titanium's corrosion resistance?
A: Spontaneous formation of a passive titanium oxide (TiO2) layer 2-10nm thick. This layer reforms within milliseconds if damaged. The oxide layer prevents further oxidation and ion release. Titanium is "bioinert" due to this stable oxide. Contrast with CoCr which releases metal ions (cobalt, chromium) and stainless steel which may corrode in vivo.
Exam Pearl
Q: What is "notch sensitivity" in the context of titanium implants?
A: Tendency for crack initiation and propagation from surface defects (scratches, notches, thread roots). Titanium is more notch-sensitive than stainless steel. Implications: careful handling during surgery (avoid scratching), smooth surface finish, avoidance of sharp corners in implant design. Screw threads are stress risers - titanium screws can fail at thread root.
Exam Pearl
Q: What is the advantage of porous titanium coatings on cementless implants?
A: Allows bone ingrowth for biological fixation. Pore size 100-400 μm optimal for bone ingrowth. Surface treatments include: plasma spray, sintered beads, electron beam melting (EBM), 3D printing (trabecular metal-like structures). Titanium's biocompatibility and ability to osseointegrate makes it ideal for cementless fixation. Hydroxyapatite coating may accelerate early osseointegration.
Australian Context
AOANJRR Data
Registry Evidence:
- Ti uncemented stems: 95% survival at 15 years
- Ceramic-on-ceramic with Ti: Excellent outcomes
- Lower revision rates than MoM combinations
Ti stems are standard of care in Australia for uncemented THA
TGA Requirements
Implant Regulation:
- ARTG listing mandatory for implants
- ASTM/ISO material standards
- Post-market surveillance via AOANJRR
- Manufacturer quality systems (ISO 13485)
Clinical Practice
Australian Trends:
- Uncemented Ti stems dominant for younger patients
- Ceramic bearings increasingly common
- Locking plates standard for fragility fractures
- Spine: Ti rods for MRI compatibility
Exam Relevance
Exam Points:
- Know Ti-6Al-4V composition (90% Ti, 6% Al, 4% V)
- Modulus 110 GPa (closest to bone)
- Cold welding mechanism and prevention
- Notch sensitivity and wear limitations
Clinical Pearl
Exam Viva Point - Australian Practice: AOANJRR data strongly supports uncemented Ti stems for THA in Australia. Know that ceramic-on-ceramic bearings with Ti stems show excellent survival. Understand why Ti is used for stems (osseointegration, low modulus) but NOT for bearing surfaces (poor wear resistance, black debris).
Management Algorithm
Titanium Quick Facts
High-Yield Exam Summary
Composition (Grade 5)
- •Titanium (Base)
- •Aluminium (Alpha)
- •Vanadium (Beta)
Key Features
- •Low Modulus (110 GPa)
- •Biocompatible (TiO2)
- •Cold Welding risk
References
- Branemark PI, et al. Osseointegrated implants in the treatment of the edentulous jaw. 1977.
- Long M, Rack HJ. Titanium alloys in total joint replacement--a materials science perspective. Biomaterials. 1998.