This article was inspired by conversations we had with our customers about the materials we use in manufacturing implants. We have frequently observed misunderstandings in defining a titanium implant.
For most people the answer seems obvious – titanium implants are made of titanium. That isn’t exactly true because most bone implants available on the market are made of titanium alloys. The further sections of this article are to explain why alloys are better for the purpose than pure titanium.
Titanium and Titanium alloys
To classify titanium and its alloys, each material is marked with „Grade X”, where X is number from 1 to 38.
Pure Titanium
Pure titanium is marked with numbers from 1 to 4. Below we provide the characteristics of each of the types:
Grade 1 – the softest and the most malleable type of titanium, therefore applicable where moulding ability is the most important.
Grade 2 – the most popular kind of pure titanium. It has the same features as Grade 1, but it stands out due to higher strength (caused by higher oxygen content). This variant is relatively widely used in implants.
Grade 3 i 4 – types of titanium with higher oxygen content than in Grade 2, ensuring higher strength . These types of titanium are widely used in industry; in medicine they are applied in surgical instruments and dental implants.
Titanium alloys
Titanium alloys are marked with numbers from 5 to 38. Due to their multiplicity , we will describe the most popular ones:
Grade 5 (Ti6Al4V) – the most commonly used titanium alloy in the world. It is estimated to constitute over 50% of total titanium use in the world. It owes its popularity to unique characteristics: high strength at low weight, high corrosion resistance and easier treatment than other types of titanium alloys.
The name of Ti6Al-4V means that the alloy contains titanium, 6% of aluminium and 4% of vanadium. Grade 5 is used in many areas of industry e.g. aviation, space industry, maritime industry and of course medical industry, where it is one of basic alloys used in implants.
Grade 7 (Ti0.15Pd) – mechanical equivalent of pure titanium Grade 2, characterised by easy treatment and the highest corrosion resistance of all types. For that reason, it is used in chemical industry.
Grade 11 (Ti0.15Pd) – equivalent of Grade 1, with higher corrosion resistance due to an addition of palladium. It is applied in chemical and maritime industry.
Grade 12 (Ti0.3Mo0.8Ni) – very durable alloy, characterised by really good weldability and strength at high temperature.
Grade 23 (Ti6AL4V ELI) – equivalent of Grade 5, but of higher purity, which means less oxygen, nitrogen and iron. It is the best choice when one needs high strength, lightness, and good corrosion resistance. It has higher malleability and higher fracture toughness in comparison to Ti6AL- 4V.
Titanium in medicine
The most relevant mechanical properties used in classifying implant materials are : Yield strength, modulus of elasticity, ultimate tensile strength, and hardness. In biomedical applications, the most important types of titanium are: Grade 2, Grade 5 and Grade 23.
The grade to be chosen depends on the way the implant is to be used, because each of them is characterised by different mechanical properties. Yield strength and ultimate tensile strength are the key parameters contributing to implant strength. Data from manufacturers shows that Grade 5 alloy is characterised by almost twice as good strength as pure titanium. Pure titanium has a lower value of Young’s modulus, but it is only lower by 12%.
If titanium alloys have got much better mechanical parameters than pure titanium, is it worth it to use pure titanium?
Titanium alloys applied in implantology contain aluminium and vanadium, elements that make their corrosion products toxic for the patient. This phenomenon isn’t frequent because titanium and its alloys are protected from corrosion through passivation process. Passivation means creating stable and sealed oxide layer TiO2 afloat (on contact with air). This layer protects the human body dangerous elements. In rare cases the oxide layer may be broken, resulting in toxic metal ions entering the body.
In other words, pure titanium as implant material is the most biocompatible metallic material, but due to its weaker mechanical parameters, it is only used to produce dental implants.
Ultimate Tensile Strength – the maximum tension a material withstands during a tensile test.
Modulus of elasticity – also called Young’s modulus, it determines the elasticity of a material. Lower value implies higher elasticity.
Yield strength – means tension beyond which object does not return to initial shape
after the tension is removed.
Ti6Al4V vs Ti6Al4V-ELI
In other applications, there are two alloys to choose from, namely Ti6Al4V (Grade 5) and Ti6Al4V-ELI (Grade 23). Ti6Al4V-ELI (Grade 23) is very similar to Ti6Al4V (Grade 5), but the shortcut ELI (Extra Low Interstitials) means that the alloy is characterised by higher purity i.e. contains less oxygen, nitrogen, carbon and iron. Higher purity ensures improved malleability and higher fracture toughness of material, which is particularly important in heavily laden implants.
Only medical standard implant
Raw titanium is produced in a couple of standards defined by ASTM International norms. For example, titanium Ti6Al4V Grade 5 is commercially available in several types, e.g. ASTM B265, B348 or F1472. If so, how to determine from which we can make an implant?
It would appear that if it is the same titanium alloy, additional standards and indications are unnecessary. However, each producer can pursue the process of producing the same material in various ways, thus it can be produced according to different standards. Base norms concerning titanium would be categorised per standards for titanium, which shall apply in industry and medicine.
Industrial titanium:
- ASTM B265
- ASTM B348
Medical titanium:
- ASTM F67 (ISO 5832-2) – Pure titanium (Grade 2)
- ASTM F1472 (ISO 5832-3) – Alloy 6AL4V (Grade 5)
- ASTM F136 (ISO 5832-3) – Alloy 6AL4V ELI (Grade 23)
Our implants are made from titanium Grade 2, Grade 5 and Grade 23, which are in accordance with norms for medical titanium (ASTM F67 / F1472 / F136). For every clinical application we provide advice in selecting titanium or titanium alloys, but the final choice should always be made by the physician to provide the patient with the implant.
