FREE ESSAY ON BIO IMPLANT MATERIALS

BIO IMPLANT MATERIALS

Homework #1:Implant Metals
Abstract:
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.
Introduction: 
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. 
Discussion: 
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) Physician's 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
F75's. 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. 
Figure 1 (1,pg.50)
Figure 2 (5,pg.43)
Conclusion: 
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. 
Bibliography
1) Vincenzini, P. Ceramics in Surgery
c. 1983 Elsevier Science Publishers
-Please note that many of the references in the library were taken out as early as the
Friday following the Monday the homework was assigned. I would attribute this to the
large number of people in the class. However, I was able to examine many of the remaining
books and was pleased to find some tables and other materials that were related to our
topic in books such as this one and the next one. This book focused on many topics by a
number of authors and researchers. The relevant material was found in an article by H.
Kawahara, titled Designing criteria of bioceramics for bone and tooth replacement. 
2) Hubert/Young Use of Ceramics in Surgical Implants
c. 1969 Gordon and Breach, New York
-Relevant material here was found in the section titled Prosthetic Metals which describes
the metals compared to bioceramics. Focuses on drawbacks and history of metals. 
3) Lecture on Metallic Biomaterials by John Disegi
Materials Development Director for Synthes, USA
-Lecture given in class on the fourteenth of February, 2000. Focuses on the three metal
implant groups, discussing the properties each possesses as well as insight into the
production process.
4) Profio, Edward A., Biomedical Engineering 
c. 1993 John Wiley & Sons, Inc. New York
-The relevant material here was found in the Biomaterials section. This section discusses
hip joints as well as the body's effects upon the materials and those materials effects
upon the body. 
5) Ratner, Buddy D., Hoffman, Allan S., Schoen, Fredrick J., Lemons, Jack E. BioMaterials
Science 
c. 1996 Academic Press San Diego, California
-The textbook for this course was helpful and included a table included in this paper.
The chapter focused on was chapter 2 with an emphasis on section 2.
Bibliography
included with document