The purpose of this report discusses the state-of-the-art metal, titanium for dental implants. The mechanical properties relating to biomedical applications of titanium exceeds other metals. Titanium is a strong, light, and non-corrosive metal, with a density of 4.40 g/cm^3. The yield strength, young modulus, and fatigue limit allows the metal to withstand high loads without fracturing, which makes it ideal for dental implants.
The two grades of metals applied for dental implants are 316L stainless steel, and grade five titanium, Ti-6Al-4V. These metals can be placed for dental implants because of their mechanical properties of strength, corrosion resistance, and fatigue limit.
In surgical practice, stainless steel 316L was introduced in the 1900s. Titanium was then introduced 34 years after steel. Titanium metal was introduced into surgical practice because 316L is subject to fretting. Fretting occurs when a metal starts to wear away, which could negatively affect the success of the implant. The titanium implant was invented to improve problems of fretting and other issues that stainless steel implants cause. The new metal introduced into dentistry has more strength, fatigue limit, modulus, corrosion rate, osteointegration, and overall biocompatibility than stainless steel. Titanium metal implants increased the success rate to 95%, compared to steels success rate of 85%.
There are 6 grades of pure titanium metal compositions that dental implants are comprised of. This report will focus solely on the Ti-6Al-4V form of pure titanium, because it is the most commonly used between dentists. Also, Ti-6Al-4V will be the focus so quantifiable values can be added to provide theory to the claim. Additionally, stainless steel 316L will be used to compare to the titanium, to provide quantifiable comparisons. 316L is molybdenum-bearing stainless steel, one of five types of existing stainless steels
Titanium is classified as a metal, and is the 9th most abundant element on earth. The metal being the 22nd element on the period table has the atomic weight of 47.867. The composition of titanium is 6% aluminum, 4% vanadium, 0.2% oxygen, the rest titanium, with a melting point of 1941 K and a boiling point of 3560 K. The metal can be alloyed with other metals to make it stronger and more corrosive resistant. Titanium has a density of 4.40 g/cm^3, and has an outer layer that bonds with oxygen. These chemical properties of the metal directly relate to reasons titanium implants are applied for biomedical applications, because they cause high biocompatibility with human tissue. Biocompatibility occurs when the properties of metal are compatible to living tissue, and do not create an allergic or toxic reaction. Titanium is the most biocompatible metal because of its strength, resistance to corrosion, a rough outer surface, and a high fatigue limit. Additionally, having a high biocompatible level will directly increase the osteointegration of a metal, which determines the success of an implant. Osteointegration occurs when a dental implant is placed into the jawbone of a patient, and then the surface of the metal implant will make a connection to the bone by attaching itself directly to the bone. The rough outer surface of titanium provides more surface area for bone to attach to, therefore increasing osteointegration (Brunette 2-9). Titanium has an outer surface that provides the ability to be roughened mechanically to increase osteointegration. The process of hardening titanium is called plasma spray coating. The rough outer surface of titanium relates directly to the yield strength of the metal. Yield strength is the point of maximum stress a metal can take before deformation occurs.
?=F/A (Equation 1.1)
? is yield strength, strength of a metal, in Newton per meters squared
F is force, the force is applied onto the metal, in newtons
A is cross sectional area, the inside area of a metal, in meters (Dinkgreve 233-241)
High yield strength will increase osteointegration, because it will prevent the implant from becoming brittle when stress gets placed on the implant. If the implant becomes brittle, metal will fracture, and pieces of titanium may break off into the gum, causing pain and allergic reactions of the patient’s tissue in the gum. After calculating the yield strength of titanium, the young modulus of titanium can be calculated. Young modulus is defined as the stiffness of a material, and a mechanical property that increases osteointegration. The young modulus number of the metal number should be relative to the jaw bone’s young modulus number. The more parallel the number of modulus of bone to titanium, the more successful the implant will be. Young modulus can be calculated as stress divided by strain (Salvadori 56-70). Lastly, the fatigue limit of titanium is 427 MPa, therefore making it suitable for dental implants. The fatigue limit of an implant must be high so prevent loss of metal ions, which will then in return start breaking down the surface of the dental implant (Hermawan chpt 17). These mechanical properties of titanium that produces biocompatibility, will be compared to the mechanical properties of stainless steel to prove that titanium is currently the best metal for dental implants.
Titanium makes up 0.57% of the Earth’s crust, and can be found in igneous rocks. The element can be found in the minerals rutile, and ilmenite. The metal titanium was first discovered by William Gregor, a pastor in England, in 1791. Although the element was discovered, the process of extracting the metal from ore was not discovered until 1910 by M.A Hunter. After the discovery of extracting the metal from ore, it took until 1948 for Titanium to be applied to the medical field. The metal was then introduced into dentistry in 1965 by Per-Ingvar Branemark (Royal Society of Chemistry 1). Before introducing Titanium into dentistry, stainless steel dental implants were the most common implants, but stainless-steel implants were not as strong and were subjected to corrosion and failures. After studying osteointegration, Branemark discovered that Titanium would be a biocompatible metal for dental implants. In the 1950’s, he discovered this by computing an experiment on rabbits. Branemark inserted titanium bits into the tibia and fibula bones of rabbits and observed that the metal bits grew into the bone and could not be removed. Branemark found that Titanium had an increased rate of osteointegration than stainless steel, therefore making it more biocompatible to human bone.
In material science, yield strength is a materials property of maximum stress that can be applied before it permanently deforms. Stress defines the physical quantity of internal resistance that the particles within the material exert on each other. For a metal to be placed for a dental implant, it must have a minimum of 345 megapascals of yield strength. If the yield strength measures below the accepted level, the implant can fracture, causing a crack in the metal of the implant. If the implant cracks it becomes deformed, and will fail because the metal cannot withhold cyclic loading placed onto it. Stainless steel, 316L has a yield strength of 332 MPa, which reaches the requirement needed for dental implants, but will fail if that pressure is exceed (Amanov 176-185). The yield strength of Ti-6Al-4V is 940 MPa, this value exceeds the minimum needed pascals and the yield strength of steel. Therefore, proves why titanium implants are replacing stainless steel because titanium dental implants can withstand more cyclic loads than stainless steel (Castolo 1-2). The most common cyclic load placed on the implant occurs during mastication. Mastication is the process in which teeth grind up food. According to the journal of prosthetic dentistry, the average time it takes for a person to eat a meal is between 8 and 15 minutes, which equals 500,000 to 1 million cycles of mastication a year (Castolo 1-2). Each tooth has a different quantity of force exerted on it, the force exerted on each tooth ranges from 150 newtons to 500 newtons. Yield strength of titanium is directly affected by the cross-sectional area and force exerted on the metal, which is proved by the formula given in the theory section. Therefore, a dental implant must be able to withstand the force applied by maceration. Additionally, a larger cross-sectional area will decrease the yield strength of an implant, since yield strength and area have an inverse relationship. The relationship between yield strength, force and cross-sectional area was tested in Guillermo de la Rosa Castolo’s experiment. The purpose of the experiment was to study the influence on mechanical performance of titanium dental implants under conditions of ISO 14801. ISO 14801 is a method of testing for the strength of dental implants in relation to different designs and sizes. The results of this experiment were that the Ti-6Al-4V dental implant screw with the area of 3.4mm would rupture at a force of 559 newtons, but when the area of the implant increased to 3.5mm the implant ruptured at 555 newtons (Castolo 1-4). This experiment proves that the larger the cross-sectional area of a dental implant, the less yield strength the titanium metal will have. Lastly, to increase the yield strength of titanium the process plasma spray coating which thickens the outside layer of the metal, which causes an increase in corrosion resistance. A high yield strength will increase osteointegration because the metal will not break in the gum causing the tissue to reject the metal. The strength of titanium will allow the bone to latch onto the metal and become imbedded in the gum. Increasing osteointegration and directly increases biocompatibility.
In physics, young modulus is the ability of a metal to resist changes in length when under tension. Tension defines something being stretched. Dental implants must have a young modulus comparable to bone’s young modulus. The young modulus can be calculate knowing the yield strength of a metal.
E=?/? (Equation 1.2)
E is young modulus, the elasticity of a metal, in pascals
? is the stress placed on a metal, in pascals
? is the strain placed on a metal (Salvadori 56-70)
The young modulus of bone is 30GPa, Ti-6Al-4V has 110 GPa, and 316L stainless steel has a value of 193 GPa (Hermawan chpt 17). Titanium dental implants have a closer young modulus to bone than stainless steel. The more comparative young modulus numbers give titanium a biomedical advantage of stainless steel because the implant will change its shape to fit changes in load more compatible to bone. Since steel has a high young modulus it is not as flexible as bone and therefore has risks of fracture. Young modulus depends upon stress and strain, therefore the more stress or strain exerted onto the implant the less likely the metal will conform to the shape. The low young modulus of titanium implants increases biocompatibility and osteointegration making the implants success rate increase.
Fatigue limit is the highest amount of stress that can be applied to a metal before it begins to corrode. Similarly, to yield strength, the fatigue limit depends on the number of cycles placed on the titanium metal. The fatigue limit of Ti-6Al-4V is 427 MPa and the fatigue limit of stainless steel 316L measures 270 MPa. These values show that titanium can withstand larger loads than stainless steel before beginning to corrode. When the value of fatigue strength becomes succeeded, fretting can occur. When fretting occurs, the metal breaks down and decreases in strength. When the metal decreases in strength it will directly affect the dental implant because the yield strength will decrease causing the implant to fail when large loads are placed on them. Therefore, having a higher yield strength decrease the rate of corrosion. Since titanium has a higher yield strength than steel, titanium will be more resistant to corrosion than steel. The young modulus also effects the rate of corrosion. The lower the young modulus the less fatigue and corrosion will occur because the implant will be more elastic and less likely to experience wear. The experiment tested by Emily Brooks on the effects of simulated corrosion of 316L stainless steel proves that titanium is a more efficient metal for dental implants than steel. In the experiment, 316L was exposed to simulated inflammation by electrolytes and was placed in a humidified incubator. The results of this experiment were that the steel showed severe local corrosion and concluded that the screw is the location where corrosion occurs most (Brooks 200-205). Fatigue limit can affect the success of a dental implant, the more corrosion the higher rate of failure.
The only problem patient’s experience with titanium dental implants is an allergic reaction caused by the screw of the implant. Titanium is considered a non-allergenic metal; however, some patients have had allergic reactions to the metal. For example, after a 69-year-old man received a titanium dental implant, he had symptoms of eczema. Once the implant was removed the eczema went away completely (Hosoki 213-219). Allergic reaction to titanium are not common but do occur. Currently there shows no evolution to remedy the risk of an allergic reaction to titanium.
For a metal to be used for a biomedical application, it must be biocompatible to human bone. The chemical and physical properties of titanium metal increase the rate of osteointegration which also increases the biocompatibility of the element. Titanium’s mechanical properties of strength, modulus, and fatigue strength prove that titanium is the most successful implant in dentistry. Titanium has a yield strength of 940MPa, a young modulus of 110 GPa, and a fatigue strength of 332 MPa. These properties are greater than steels, making titanium the more preferred dental implant because it can withstand higher loads without fracturing or corroding.