This paper will discuss the key properties of three categories of implant alloys; stainless steels, cobalt-based alloys, and titanium-based alloys, focusing on those properties which make the implant alloys ideal for skeletal implants. An additional focus of the paper will be on any disadvantages possessed by each group of implant alloys.
Wood was probably the first bioimplant, a sturdy, inert, and readily available material in the older days. But as mankind aged newer materials were discovered, and often created, that were indeed superior. The search for a exceptional implant alloy is one which has laboratories researching and testing different types of alloys for the best combination of strength, durability, corrosion resistance, and other important traits these alloys must possess.
The first implant metal to be discussed is stainless steel. The one most common stainless steel in use is 316L, grade 2. This particular alloy is mostly iron, chromium, and nickel, though it also contains nitrogen, magnesium, molybdenum, phosphorous, silicon, and sulfur. Most implant quality 316L has at least 62.5% iron, 17.6% chromium, and 14.5% nickel.
The implant quality 316L has improved corrosion resistance, structure, and ductility over the commercial quality form of the alloy. An important property of the stainless steel alloy is its high chromium content which fights corrosion by forming an surface oxide. The nickel is added to insure no delta ferrite, or to combat the impact the chromium, molybdenum, and silicon have in forming ferrite. No delta ferrite is a condition where there is no metallic resonance, allowing for the implant to still be safe even when the patient is undergoing an MRI.
There is a drawback to the use of nickel in the implant, which in turn means there is a drawback to the implant itself. Somewhere in between 3% and 5% of the population is allergic to nickel. Nickel causes inflammation and discoloration of tissue, retards reparative growth, produces excessive scars and erosion of bone(2, pg.8) Physicians are instructed to ask the patient if they know of any allergies before they place the implant within a patient.
Cobalt-based alloys that are most common are F75, F799, F90, and F562. These cobalt-based alloys have distinct characteristics and each has a different component to its creation, whether it be how it was created or which elements are added. In F75 the bulk composition, as well as the surface oxide, allows for a high tolerance to corrosion particularly in chloride environments. One main problem is the large grain size which causes a lower yield strength. Another problem arises during the casting process when defects can occur. Powder metallurgical methods has been used in an attempt to improve the microstructure and mechanical properties as well as avoiding the possible casting defects.
In order to improve fatigue, yield and ultimate tensile strengths of F75, the alloy was mechanically processed through hot forging after casting, creating F799. F799 has nearly double the strengths of F75. Another cobalt-based alloy, F90, adds the elements of Tungsten and Nickel in order to achieve machinability and fabrication properties. When this alloy is cold-worked to 44%, its properties are twice that of F75s. The last of the cobalt-based alloys is MP35N, or F562. The MP refers to multiple phases within its microstructure allowing for it to be both processed by thermal treatments and cold-working. This creates a high strength alloy which is actually among the strongest available for implantation.
Perhaps one of the best known biomaterials today is titanium and its alloys. Commercially pure titanium, also known as F67, is non-magnetic, and there is no harmful additives or alloying. The most common alloy used is called F136, or Ti-6Al-4V. This alloy is an alpha-beta alloy, meaning the properties will vary depending on treatments. However usually this alloy is corrosion resistant but not ware-resistant and has a higher strength than when in its pure form. The major drawback of this alloy is in its long-term usage. The Vanadium is biocompatable only in the short term.(3,pg. 2)
There are four grades of titanium, 1-4 with four being the strongest but least ductile. The amount of oxygen in the CP titanium is a major force on how strong the yield and fatigue strengths will be, and also determines the grade of the alloy. The low density of titanium makes it significantly lighter when compared to the stainless steels and cobalt-alloys. Due to the difficulty in electropolishing titanium, it is anodized, this is an electrochemical process which increases the thickness of the oxide film that lies on titanium. Here is where the colors that are associated with titanium, most often gold, is produced.
Man has made tracks into helping the injured, the elderly, the unfortunate. Biomaterials are the tools leading the way in the battle to make life longer, healthier, and more complete for many individuals. As documented in this paper each biomaterial has strengths, as well as its weaknesses. So which is the most useful That is the beauty of the situation as it exists today, no one material is the one, each is suited for different circumstances. The pursuit to discover or engeneer the perfect alloy for all situations continues.