In the past, the only way to replace missing teeth was to have a removable appliance. However, these days, dental implants are commonly being used to replace missing teeth. The dental implants are improving as a result of new technological and scientific advances. Different materials have been used in the past for dental implants such as lead, stainless steel, and gold. Currently, the focus is on using Roxolid, surface-modified titanium implants, and zirconia. These materials have superior esthetic and functional characteristics for dental implants.
Key points
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While selecting a dental implant, the microstructure of the implant, its surface composition, characteristics, design, toughness, strength, corrosion, wear, and fracture resistance should be considered.
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Current dental implant used are Titanium- Zirconium Alloy, Zirconia, Titanium, Titanium Alloy, Aluminum, Titanium, Zirconium oxides, Ceramic, Metal alloys, Cobalt, chromium alloy, and Iron chromium nickel-based alloy.
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Features of dental implants are shape, threads, surface texture, length, and diameter.
Introduction
In the past, the only way to replace missing teeth was to have a removable appliance. However, these days, dental implants are commonly being used to replace missing teeth. The dental implants are improving as a result of new technological and scientific advances.
Different materials have been used in the past for dental implants such as lead, stainless steel, and gold. Currently, the focus is on using Roxolid, surface-modified titanium implants, and zirconia. These materials have superior esthetic and functional characteristics for dental implants.
Many factors come into play when selecting a dental implant: the microstructure of the implant, its surface composition, characteristics, design factor, toughness, strength, corrosion, wear, and fracture resistance.
History
Dental implants can be traced back to the 1800s. In the early 1800s, Maggiolo used gold in the shape of a tooth root. Then toward the turn of the century, Harris used teeth made out of porcelain. The post of these teeth was coated with lead-coated platinum. Zemenski then used porcelain, gutta-percha, and rubber as implantation techniques.
In the early 1900s, Lambotte implants were fabricated using aluminum, brass, copper, gold, magnesium, and soft steel plated with gold and nickel. Then in Pennsylvania, Sholl used porcelain teeth implant having a corrugated porcelain root. The concept of submerged implant and healing tissue and dental implant immobility was introduced by E.J. Greenfield. The modern era encompasses using synthetic polymers, metal alloys, and ceramic for dental implants.
Definition
There are few properties of material science that need a review to understand the material sciences of dental implants.
An implant should have a high compressive and tensile strength to improve its functional stability and to prevent fractures. When an implant is able to transfer stress from the implant to bone it is reported as interfacial shear strength. This causes low stresses on the implant. During cyclic loading, the implant can have brittle fractures. Therefore, there should be high yield strength and fatigue strength. The modulus of elasticity of bone is 18 GPa. Therefore, the goal is to have an implant material that has a comparable modulus of elasticity. This will cause a more uniform distribution of stress at implant and minimize the relative movement at the implant-bone interface. The ductility of the dental implant is necessary for contouring and shaping of the implant. The ADA recommends a minimum ductility of 8%. Increasing the toughness of an implant prevents fracture of the implant. Increasing the hardness of the implant decreases the incidence of wear of implant material.
Implant surfaces influence its response to tissue and cells. By increasing the surface area of an implant, it improves cell attachment to the bone. Implant surface has been divided into surface roughness as minimally rough (0.5–1 m), intermediately rough (1–2 m), and rough (2–3 m). The texture of implants can vary as well, such as concave or convex. The concave texture is by being treated with hydroxyapatite coating and titanium plasma spraying. The convex texture is treating the implant surface by subtractive treatment such as etching and blasting. Furthermore, the implant surface is classified based on the orientation of surface irregularities such as anisotropic and isotropic surface irregularity. Anisotropic surfaces have clear directionality and vary considerably in roughness. Isotropic surface irregularity has similar topography independent of measuring direction.
Any implant material should show favorable biocompatibility. Materials are defined by their corrosion property to be biocompatible. Corrosion can result in the weakening of the restoration, roughening its surface, or releasing elements from the alloy or metal. Corrosion can be attributed to electrochemical, galvanic, pitting, or crevice reaction. Electrochemical corrosion can be defined as anodic oxidation and cathodic reduction resulting in metal deterioration and charge transfer. This reaction occurs due to electrons, and it can be prevented by the presence of a passive oxide layer on the metal surface. Galvanic corrosion reaction is due to the difference in electrical gradients. If there is leakage of saliva between implant and superstructure such as abutment or crown, it can lead to nickel or chrome ions to pass to periimplant tissue. Galvanic corrosion can lead to bone reabsorption. This bone loss will eventually lead to loss of implant stability and will lead to a failure of the implant. Pitting corrosion is when the metal ions from the implant combine with chloride ions. Pitting corrosion is due to small surface pits of an implant. The formation of these surface pits leads to roughening of implant surfaces. Crevice corrosion is due to metallic ions creating a positive charge environment when they dissolve. This usually occurs at narrow interfaces such as implant screw and bone interfaces.
Discussion
This section focuses on introducing different implant materials used currently and then looking back on what was used in the past.
Titanium-Zirconium Alloy
Titanium-zirconium alloy has better fatigue strength and increased elongation compared with pure titanium. Monophasic structures of titanium-zirconium alloy are sandblasted and acid-etched for it to be topographically identical to the pure titanium implant. Because titanium-zirconium alloy has better mechanical properties and good biocompatibility, it can be used for thin implants and implant components that can be subjected to high strains. This leads to the growth of osteoblasts, which are essential for osseointegration. Titanium-zirconium alloy is used by Straumann Roxolid implant, and it has 50% stronger than pure titanium.
Zirconia
Ceramic implants were introduced as an alternative to titanium implants due to their properties of less plaque build-up, aesthetic consideration, and it being kind to soft tissue. Zirconia structures are characterized in 3 crystal forms: monoclinic, cubic, and tetragonal. At room temperature, Zirconia is in its monoclinic structure and changes into the tetragonal structure at 1170 C. This structure changes into a cubic phase at 2370 C. As these structures are called, they are unstable and break into pieces. The cubic phase of Zirconia is stabilized by adding CaO, MgO, and Y2O3 (Yttrium). This structure is called partially stabilized zirconia combining monoclinic, cubic, and tetragonal phases in order of importance. The tetragonal zirconia polycrystals only contain the tetragonal phase. This phase is obtained by adding Y2O3 (Yttrium) at room temperature. The properties that make tetragonal zirconia polycrystal a suitable biomedical material are low porosity, high bending, high density, and compression strength.
Titanium
Titanium is most commonly used for dental implants. Titanium is biocompatible due to the formation of a stable oxide layer on its surface. Pure commercial titanium is classified into 4 different grades based on their oxygen content. Grade 1 has the least oxygen content (0.18%) and grade 4 has the most (0.4%). Different materials are added to commercially pure titanium to further enhance its properties. Vanadium is added due to its ability to act as an aluminum scavenger to prevent corrosion. Aluminum is added to increase strength and decrease density. Iron is added for corrosion resistance. Titanium is a dimorphic metal. It exists as a hexagonal closed packed crystal lattice, alpha-phase less than 883 C. When higher than 883 C, it transforms into a body-centered cubic lattice, beta-phase. Titanium is the material of choice for dental implants due to its high passivity, rapid formation, controlled thickness, resistance to chemical attack, catalytic activity for several chemical reactions, and modulus of elasticity compatible with bone. The major disadvantage of titanium is its color. The gray color titanium is not esthetically appealing when soft tissue is not optimal or if there is thin mucosa.
Titanium Alloys
Titanium alloy exists in alpha, beta, and alpha-beta forms. When titanium is heated and combined with either element such as Al and Va and then cooled it results in titanium alloy. Alpha-phase titanium is combined with aluminum as a stabilizer. This results in increased strength and decreased weight of the titanium-aluminum alloy. The beta-phase is combined with vanadium as a stabilizer. The alpha to beta transformation occurs at a range of temperatures as Al or Va is added to titanium. The alloys most commonly used for dental implants are the 6% Al and 4% Va alpha-beta variety.
Aluminum, Titanium, Zirconium Oxides
High ceramics from aluminum, titanium, and zirconium oxides are used to form endosteal plate, root form, and pin type dental implants. These alloy oxides have a high modulus of elasticity, compressive, tensile, and bending strength greater than compact bone. This results in specialized design requirements for this class of biomaterial.
Ceramic
Ceramic is rarely used as a surgical implant due to low ductility and brittleness. However, it does have positive properties such as good strength, inert behavior, and minimum thermal and electrical conductivity. The use of ceramic is limited to implant dentistry.
Metal Alloys
Metals such as gold, stainless steel, cobalt-chromium biomechanical properties made them an implant material of choice. These metals have a good finish, they are easy to process, and are able to be sterilized. These metals are still used in prosthetic components of implants for superstructures, bars, and crowns. However, the success and advancement of titanium-based implants and alloy have made them obsolete.
Cobalt-Chromium Alloy
Cobalt, chromium, and molybdenum are the major elements in the composition of this alloy. Cobalt provides a continuous phase for basic properties. Chromium provides corrosion resistance. Molybdenum provides strength and bulk corrosion resistance. The ductility of this alloy is enhanced by controlling the carbon and nickel biocorrosion products. The alloy can be used to manufacture customized subperiosteal implant frames. ,
Iron-Chromium Nickel-Based Alloy
Iron-based alloys were used in ramus blade, stabilizer pin, and mucosal inserts. These implants are not commonly used anymore. If a patient is allergic to any of the material, the use of this implant is not advised. This alloy has a high galvanic potential and corrosion resistance. It is also prone to pitting corrosion. If titanium, cobalt, zirconium, or carbon implant are used in combination with iron chromium-nickel alloy, it can lead to galvanic coupling and biocorrosion.
Features of dental implants
Shape
Dental implants are commonly available in tapered or parallel types. The tapered type of dental implants is known to have better primary stability compared with a parallel type. The increase in primary stability of tapered implants is due to their lateral compression of the bone and increased stiffness of the interfacial bone. Tapered implants can be used in softer bones such as posterior maxilla for greater stability. They are helpful in avoiding damaging roots of adjacent teeth that may be in close proximity to a site of interest. Tapered implants require a higher insertion torque compared with parallel implants. Both parallel and tapered implants can be used for immediate or delayed implant placement; however, because of higher primary stability of tapered implants, they are preferred. ,
Threads of Implant
There are 3 different types of thread implants that are commonly used in implant dentistry. These are V-shaped, reverse buttress, and square-shaped. , Studies have shown that the square thread design of a dental implant shows more bone to implant contact and greater reverse torque measurement compared with reserve buttress and V-shaped thread.
The threads of dental implants can be further characterized by pretaping and self-tapping implants. Pretapping implants have lower primary stability compared with self-taping implants.
The osteotomy for pretaping implants needs to be prepared using a tapered drill. These threads prepared from the taping drills are used to accommodate the pretaper implants. Pretapping implants are recommended for dense bones such as anterior and posterior mandible. Self-tapping implants make their own threads into the osteotomy site as they are being inserted at desired locations. Self-tapping implants can be used for the anterior and posterior maxilla. ,
Surface Texture
The surface texture of dental implants can either be smooth or rough. Rough-surfaced implants have a larger surface area compared with smooth surface implants. As the surface area increases it encourages bone healing and periimplant soft tissue. The greater the surface area the greater distribution of forces to which implant is exposed. Rough surface implants also have higher primary stability compared with smooth surface implants. Research also shows that rough dental implants have greater bone apposition and higher removal torque values. The implant surface is roughened by either blasting or acid etching or adding biocompatible material such as hydroxyapatite. If rough dental implants are exposed to the oral cavity, they have a tendency to accumulate plaque and bacteria, leading to periimplantitis.
Implant Length
The length of a dental implant is governed by the bone available, adjacent anatomic structures, width, and quality of bone. Generally speaking, the longer the implant the greater the surface contact hence higher primary stability. This is not a linear relationship. For example, a 10 mm implant has 30% more surface area compared with a 7-mm implant, whereas, a 13-mm implant only has 20% more surface area compared with a 10 mm implant. There has been a movement to use shorter implants if there is a limit in available bony or proximity of vital structures such as maxillary sinus or neurovascular bundle. Research shows the survival rate after 2 years for 5-mm implants is 93.1% compared with 9.5 mm implants at 98.6%. Therefore, shorter implants may fail in 4 to 6 years compared with standard implants that may fail in 6 to 8 years.
Implant Diameter
When choosing a dental implant to replace a tooth, the diameter of implants plays a major role in its success and implants the ability to withstand the occlusal load. As an increase in length is associated with the increased surface, the same goes for implant diameter. Increasing the diameter in a 3-mm implant by 1 mm increases the surface area by 35% over the same length. As the surface area of the implant increases, it lessens the stress to the crestal bone area and reduces both crestal bone loss and early loading implant failure. In instances where there is not enough bone, and augmentation is not possible, short and wide implants can be used. They can also be used when the bone bed is not optimal. Wide implants are used to increase the stability of the implant and improve stress distribution. , Wide implants are also used in immediate implant placement after tooth extraction.
Summary
Implant dentistry is the current phase of dentistry. With advances in technology, biomedical science, surgical technique, and success rate implant, dentistry is a hot topic. However, implant material science is not talked about as often as it should. The study of material science along with biomechanical sciences will provide optimization of design and material concepts for surgical implants.
Clinics care points
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Dental Implants should have high compressive and tensile strength to improve its functional stability and prevent fractures.
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Increasing implant surface area, will improve cell attachment to the bone.
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Titanium-zirconium alloy has better mechanical properties and good biocompatibility. It is used for thin implants, and implant components.
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Diameter of Implant plays a role it its success. A wider implant will increase its ability to withstand occlusal load.
Disclosure
The authors have nothing to disclose.
References
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