Choosing Alloys for Medical Implants

## Biocompatibility Requirements The human body is a warm (37 degrees C), chloride-rich saline environment. Implant materials must not release toxic ions, not provoke excessive inflammatory response, and form a stable passive oxide layer. ISO 10993 defines the biological evaluation framework. ## The Big Three Implant Alloys ### Ti-6Al-4V ELI (ASTM F136) The dominant orthopedic implant alloy. ELI specifies tighter limits on interstitials for improved fatigue life. | Property | Value | |----------|-------| | Tensile Strength | 860 MPa (min) | | Elastic Modulus | 110 GPa | | Density | 4.43 g/cm3 | Applications: Hip stems, knee tibial trays, spinal pedicle screws, dental implants. Stress shielding (modulus mismatch with bone at 17-20 GPa) is addressed by porous and lattice structures via additive manufacturing. ### CoCrMo (ASTM F75/F1537) Highest wear resistance among implant metals. Applications: Femoral heads, knee femoral components, dental frameworks. Metal ion release concerns have led to decline of metal-on-metal hip articulations. ### 316LVM Stainless Steel (ASTM F138) The oldest implant alloy still in use. LVM (Low inclusion, Vacuum Melted) ensures fatigue cleanliness. Used for temporary fracture fixation and surgical instruments. Not recommended for permanent implants in nickel-sensitive patients. ## Emerging Alloys **Ti-15Mo (ASTM F2066)**: Lower modulus (78 GPa) for reduced stress shielding. **NiTi (Nitinol, ASTM F2063)**: Shape-memory and superelastic behavior for self-expanding stents and orthodontic archwires. **Biodegradable Mg alloys**: Dissolve in body fluid over 12-24 months, eliminating removal surgery. ## Selection Matrix | Application | First Choice | Reason | |-------------|-------------|--------| | Hip/knee bearing | CoCrMo | Wear resistance | | Hip stem | Ti-6Al-4V ELI | Fatigue, modulus | | Fracture fixation | Ti-6Al-4V ELI | Fatigue, biocompatibility | | Stents | NiTi | Superelasticity | | Dental implant | CP Ti Grade 4 | Osseointegration |