Purity is not the only aspect to be taken into consideration, and in fact, one should be cautious in attributing higher purity as being 'better' in all instances. The context of the application of the titanium is of great importance. Yes, titanium purity is directly correlated to some other attributes, including mechanical strength and hardness/ toughness, but in industries like medicine (and a few others), it's the consideration of the 'fit' between the attributes and the requirements that is more important (not just the 'simplicity' of higher titanium purity).
Characteristics of Pure Titanium (Grade 1-4)
Pure titanium being purer' is correlated with it being softer and also poorer when it comes to strength/ hardness, and as a result, it becomes a better option in terms of corrosion resistance, and the property of biocompatibleness is enhanced as well.
Grade 1 has the highest purity (>99.5%) and is the softest, with the greatest elasticity (the least tissue irritation). This grade is preferable for people with thin alveolar bones and metal sensitivity, so it is often used in anterior dental implants.
Grade 2 is the highest used grade (in titanium implants). This grade has a little lower purity and is the best in terms of the balance between strength and toughness, so it is the most commonly used 'basic' grade in dental implants, as it is able to withstand the forces of everyday chewing.
Grade 3-4 has more oxygen in it and thus has greater hardness and strength/ toughness, to the point where it is comparable to ordinary steel. This grade is suitable for use in the posterior teeth and other regions with high occlusal forces.
Advantages of Titanium Alloys (such as Grade 5)
When even more strength, fatigue resistance, or load-bearing capacity is required, titanium alloys, rather than high-purity titanium, are utilized.
Grade 5 (Ti-6Al-4V) is 1.5 times stronger than Grade 4 pure titanium. Thus, it is commonly used in orthopedic plates and other load-bearing implants, including artificial joints.
Alloying components (aluminum, for example) could cause allergic reactions in an extremely small percentage of the population (around 2%). Thus, a history of metal allergies should be considered before using titanium alloys.
Best Choices for Different Scenarios
Dental implants: titanium alloys of Grade 2 are designated for the anterior teeth, whereas those of Grade 4 or 5 are for the posterior teeth due to the presence of more occlusal forces.
Orthopedic implants (e.g., plates, synthetic joints): titanium alloy of Grade 5 is suggested for these devices due to the best long-term stability in relation to the expected loads.
General applications (titanium mugs, eyeglasses frames, etc.): titanium alloys (e.g., β-titanium) are the best choice if lightweight and durable are the desired characteristics, although high-purity titanium (99% or more) is safer and has better corrosion resistance.
Therefore, the idea that "the higher the purity of titanium, the better" is a misconception. Just as tires shouldn't be too soft, choosing the "right" titanium material is crucial to achieving the optimal balance between safety, durability, and comfort.