The reasons for using ASTM F136 titanium rods GR5 (Ti-6Al-4V ELI) in human bone implants

The reasons for using ASTM F136 titanium rods GR5 (Ti-6Al-4V ELI) in human bone implants

Baoji Dongint Metal Materials Co., Ltd. is rooted in Baoji, Shaanxi Province, which is known as the "China Titanium Valley". Leveraging the local complete titanium industrial chain advantages, the company focuses on the research, production and supply of high-end titanium and titanium alloy materials. Its core product is ASTM F136 Gr5 (Ti-6Al-4V ELI) medical titanium rods, providing stable and compliant medical titanium raw material solutions for domestic and foreign medical device manufacturers.

ASTM F136 GR5 titanium rods are the specific standard for surgical implants made of Ti-6Al-4V ELI titanium alloy, corresponding to China's TC4-ELI. They have ultra-low interstitial elements (oxygen, nitrogen, carbon, iron), and are the most mainstream metal implant materials for bone fixation plates, screws, intramedullary nails, artificial joints, and bone scaffolds. Compared to ordinary GR5, pure titanium, cobalt-chromium alloys, and stainless steel, they have comprehensive advantages.

Differences: Ordinary GR5 titanium rods follow the standard ASTM B348; medical titanium rods must be ASTM F136 GR5, with strict reduction of interstitial impurities, suitable for human body.

The elastic modulus is much lower than stainless steel and cobalt-chromium alloys

The elastic modulus of TC4-ELI is approximately 110 GPa; human cortical bone is approximately 10-30 GPa; 316L stainless steel ≈ 200 GPa, cobalt-chromium ≈ 230 GPa.

The lower the modulus, the smaller the stress shielding effect:

Implants with too high hardness will bear most of the load, and the bone will not receive the stimulation of force, resulting in bone resorption, bone atrophy, and loosening of the implant. ASTM F136 GR5 titanium rods have a modulus closer to the bone, significantly reducing stress shielding.

Sufficient strength, toughness, and fatigue resistance

Bone implants must withstand repeated human activity cyclic loads (walking, movement), with high tensile and yield strengths, good fracture toughness, excellent fatigue resistance, and long-term force-bearing screws and fixation plates are less likely to break.

ASTM F136 GR5 titanium rods have further improved toughness and reduced brittle fracture risks, especially suitable for trauma orthopedics.

2. Excellent biocompatibility

The titanium surface spontaneously forms a dense inert TiO₂ oxide film: corrosion-resistant, human body fluids (chloride ions, proteins) hardly corrode or dissolve, very few metal ions are released, and rarely cause inflammation, rejection, or allergy.

It can achieve osseointegration: bones can directly grow on the titanium alloy surface, achieving a firm connection between bone and implant, which stainless steel cannot achieve.

Note: Although aluminum and vanadium are present in the alloy, the dense oxide film blocks them, and the release amount is extremely low under normal implantation; ELI controls impurities further reducing risks.

3. The significance of the ASTM F136 GR5 titanium rod standard, why not use ordinary GR5

Ordinary GR5 titanium rods (ASTM B348) have an upper limit on oxygen content;

ASTM F136 GR5 titanium rods force the reduction of O, N, C, Fe interstitial elements:

Excess oxygen will make the alloy brittle and reduce toughness, and after implantation under force, it is prone to micro-cracks and fractures;

Strict composition, impurity, and flaw detection requirements, eliminating internal pores and inclusions, ensuring the safety of the implant.

Medical implant legal and clinical requirements must comply with ASTM F136, and cannot directly use industrial TC4 rod materials to process implants.

4. Processing and clinical compatibility

It can be forged, rolled, easily made into rods, plates, screws, and fixation plates; it can also be 3D printed for bone multi-hole prostheses.

It can undergo anodic oxidation, surface modification, further promoting bone ingrowth;

It is non-magnetic, does not heat up during MRI examination, does not shift, and postoperative re-examination is not restricted; stainless steel implants will interfere with MRI.

5. Comparison of the shortcomings of other implant materials (understanding why it was chosen)

316L stainless steel: cheap, but has a too high modulus, severe stress shielding, will release metal ions, interfere with MRI, and is now gradually being replaced by titanium;

Cobalt-chromium alloy: Extremely high strength, but with a higher modulus and greater weight. It is mostly used for joint ball heads and is not suitable for trauma bone plate screws;

Pure titanium (Gr2 ASTM F67 titanium rod): Has the best biocompatibility, but the strength is relatively low. It can only be used for small implants and cannot withstand heavy bone load;

ASTM F136 GR5 titanium rod: Balances strength, toughness, low modulus, bone integration, and corrosion resistance. It has the best overall balance and thus becomes the preferred titanium alloy for weight-bearing orthopedic applications.

Limitations of ASTM F136 GR5 titanium rod (objective supplement)

The modulus is still higher than that of human bone, and it can only reduce stress but not completely eliminate stress shielding; The 3D printed porous structure is designed to further reduce the equivalent modulus;

The alloy contains aluminum and vanadium, and it will release trace amounts under extreme wear, and the overall long-term safety in the body has been clinically verified to be reliable for several decades;

The hardness is not very high, and the friction and wear performance is poor, making it unsuitable for joint friction surfaces. Joint friction surfaces are mostly made of cobalt-chromium or ceramics.

Summary in one sentence

ASTM F136 GR5 titanium rod relies on ultra-low interstitial elements to deliver high toughness, a moderate modulus to reduce stress shielding, excellent corrosion resistance and osseointegration capability, MRI compatibility, and a balanced combination of mechanical and biological properties. It is the preferred titanium alloy for repairing human load-bearing bones. Industrial GR5 titanium rods do not have the same impurity control as ASTM F136 GR5 titanium rods and are strictly prohibited from being used directly for implantation.