General description
Commercially pure titanium (cpTi) and titanium-6Al-4V, both alloys are biocompatible and can undergo osseointegration when in contact with bone and gingival tissue, and are very satisfactory materials in dentistry.
Composition and properties of titanium alloys used as implants
Grade 5 titanium is widely used in orthopaedics. This is due to its superior strength and low Young's modulus. However, it can also be used in dentistry and the use of this alloy has been shown to be biologically acceptable. However, this alloy releases aluminium and vanadium, both of which can cause biological problems. Aluminium interferes with bone mineralisation, leading to structural defects, while vanadium is cytotoxic and can cause type IV (allergic) reactions. To have these adverse effects, both substances need to be present in reasonable concentrations in the tissue, and the concentrations released from this alloy are far below those required to produce toxic effects. The release is also below the average nutritional absorption of these ions. Studies have shown satisfactory bone integration with this alloy, especially after treatment to strengthen the surface oxide layer.
The design of modern implants typically includes a thread that holds a metal alloy component into the bone of the mandible or maxilla. The smooth metal portion passes through the soft tissue of the gums and supports an artificial tooth, which is usually made of a ceramic material. Partly because of this design, care must be taken when selecting patients to receive implants.
Titanium is a transition metal that is able to form solid solutions with elements of similar atomic size. In the solid state below 882.5°C, titanium has a hexagonal close-packed geometry known as the alpha structure. Above this temperature, solid titanium transforms to a body-centered cubic geometry known as the beta structure until it melts at 1688°C. In alloys, titanium occurs in a variety of forms and can be pure alpha or pure beta, or a combination of both. Titanium's alloying elements are either alpha stabilizers, such as aluminum, or beta stabilizers, such as vanadium, iron, nickel, and cobalt. Oxygen is an alpha stabilizer. There are also some metallic elements, such as zirconium, that have no effect on the stability of either phase.
When making implants, titanium alloys that are fully or predominantly alpha are preferred because of their excellent corrosion resistance. Processing conditions are chosen to favor the alpha microstructure, which also affects the mechanical properties (strength, ductility, fatigue resistance, and fracture toughness).
Surface Chemistry
The two main alloys used to make implantable devices, commercially pure titanium (cpTi) and titanium-6A1-4V, have surfaces composed primarily of the oxide TiO2. The oxide layer is 4-6 nanometers thick and contains hydroxyl groups in addition to the oxides. The exact composition of the surface is important in promoting osteoblast adhesion, and the oxide layer tends to have favorable biological properties.
Titanium implants are often surface modified after initial manufacturing to ensure uniform oxidation and to remove any contamination. The modified surface has better biological properties, promoting cell adhesion and proliferation processes, both of which aid in bone integration.
The surface of the alloy Ti-6A1-4V contains aluminum and vanadium. Surface finish and roughness are also important characteristics of titanium implants because they affect the quality of interaction with bone.
Corrosion behavior is one of the most important factors affecting the biocompatibility of metal implants. This is because the metal ions released by corrosion can cause various adverse effects. These can affect the tissue surrounding the implant, but also the entire body, where allergic reactions may occur.
The interface zone between the titanium implant and the living bone is critical for the development of osseointegration. This zone is very thin (20-50 nanometers) and is where the bone cells release growth factors, thus initiating the steps of bone formation. The first step is the deposition of proteins from the plasma onto the surface oxide layer. This is followed by the formation of a fibrin matrix, a structure that acts as a scaffold for osteoblasts (bone-forming cells). Supported in this way, the osteoblasts lay down bone, which expands to fill the interface zone, allowing it to grow against the implant surface, thus allowing the implant to osseointegrate. An important aspect of proper osseointegration is that, unlike in the case of a fibrous capsule, the implant is firmly fixed, which in dentistry provides a secure anchor for the restorative device.
The oxide layer on the surface plays an important role in the success of osseointegration. A thicker and rougher oxide layer promotes reliable and rapid osseointegration, at least in the short term. The oxide layer also has the effect of passivating the metal, thereby inhibiting corrosion and minimizing the release of titanium ions.
;
Titanium Alloy Ti-6Al-4V
The widely used alloy Ti-6Al-4V has excellent properties for use as dental implants. However, in vitro study results generally find Ti-6Al-4V to be superior. Both alloys can osseointegrate and are highly biocompatible with bone and oral tissues. They exhibit minimal corrosion and have few systemic effects in a small number of patients. From a biomechanical point of view, they are fit for purpose and have high clinical survival rates after many years of use.
Titanium Alloy Ti-6Al-4V-ELI
Ti-6Al-4V-ELI has been shown to be non-cytotoxic compared to pure titanium. Other alloying elements have also been shown to be acceptable and have not been found to significantly increase cytotoxicity.
Titanium Alloy Ti-6Al-7Nb
Ti-6Al-7Nb, has good mechanical properties. Their hardness, yield strength and tensile strength generally exceed those of cpTi. Human fibroblasts in Ti-Nb alloys grow more slowly and to a smaller extent.
Ti-6Al-7Nb is a titanium-niobium multinary alloy that has been extensively studied for bone contact applications. In particular, it is increasingly used in the manufacture of dental implants. It is an alpha-beta alloy that was originally developed for orthopedics and has superior mechanical properties compared to cpTi. It is also corrosion resistant and when corrosion occurs, its biological properties are acceptable, primarily due to the absence of vanadium.
Ti-6Al-7Nb is similar to cpTi, with human gingival fibroblasts adhering, spreading and proliferating to similar degrees on both alloys. Short-term implantation of Ti-6Al-7Nb has been shown to induce a transient inflammatory response similar to cpTi, but followed by very satisfactory biological results. High corrosion resistance and good stability. When Ti-6Al-7Nb is prepared as a casting, mechanical strength and wear resistance are also good, confirming the prospects of this alloy for use as a denture.
