By Gelareh Eslamian
Metals and polyethylene (a type of plastic) are two types of materials commonly used in implants. These substances are mostly combined
together, wherein; one component is made of metals and the other from polyethylene. These two materials, when combined together, facilitates
smooth rubbing of the joints and hence avoids wear and tear.
Metals, mixed with a combination of different alloys, are mostly applied
as a common material in knee implant. The metal parts that are joined to the bone should be designed specifically so that it merges
easily with the body’s natural system. Some of the materials used in the implants are not safe for the human body, as they release
tiny particles. When some of them mixed with the chemicals released by the human body, they might be quite fatal since they can easily
travel into the circulation system and ultimately to other organs like heart, lungs and spleen. Thus special attention is paid while
choosing the suitable combination of metals to be used in the implants. Typically the bearing used in the knee joint is made of a
combination of several alloys such as cobalt chrome and polyethylene. There is occasional use of other materials such as ceramic to
make the metal materials soft enough to merge easily with the human body and be capable of wear and tear.(13).The following is the
list of the common materials applied in knee implant:
1) Stainless Steel:
Stainless steel is a very well-built and tough alloy, and it is one of the main metals being applied in implants. It is planned to
assist repair fractures, such as bone screws, bone plates, rods, and pins. Stainless steel is mainly used in knee implants, as it
is resistant to corrosion. Although it is mainly made of iron, it has various amounts of metals like chromium (more than 11%) and
molybdenum (about2-3%), which results in corrosion resistance property against the chemicals found in the body (20).
There are two
forms of stainless steel 316 and 316L, and the difference between them is the percentage of the present carbon in each of those materials.
The carbon in these alloys makes them corrosion resistant against chloride solution. Alloy 316 contains 0.08% carbon and 316L has
0.03% carbon. Stainless steel undergoes different procedures such as heat treatment and cold working to enhance its hardness and strength.
Yet, still it will not be a great choice for knee implant because even alloy 316L will corrode in oxygen reduced regions and under
high stress like fracture palates. Therefore, stainless steel is good to be used in temporary implants (20).
2) Cobalt-chromium Alloys:
Cobalt-chromium alloys are hard, tough, corrosion resistant, and biocompatible metals and they are not only utilized in a diversity
of joint replacement implants, but also in some fracture restore implants, that will apply for a long period of time. Adding various
metals like molybdenum increases the strength of this alloy. Titanium and cobalt chrome are the two metals that are widely used in
knee implants, however there is no common agreement as to which material is better and more suitable. The main problem of concern,
however, is the issue of tiny particles that release due to the rubbing of metal and plastic wear as a result of the knee movement.
Needless to say, sometimes these particles cause some harmful reactions in the human body, especially in case of those patients who
have allergy to special metals like nickel, although the percentage of patients having such kind of allergies is very low.
3) Titanium and Titanium Alloys:
Pure titanium is generally used in implants that high strength is not necessary. It is mostly used in making fiber metal, which helps
the bone to grow into the implant, or cement to pour into the implant, and fix it better at the right position. Titanium alloys are
biocompatible in nature, and they commonly contain different amount of vanadium and aluminum. The most used titanium alloy in knee
implants is Ti6Al4V.
Titanium and Titanium alloys have great corrosion resistance characteristic, and this point makes them inert biomaterial,
which means they will not change after implanting in the body. Most of the times, the surface of these materials will be modified
through plasma spraying process. In this procedure, the surface of the material will be coated by hydroxyapatite, and as a result
a bioactive surface will be formed that contributes in bone bonding. Therefore, mechanical fixation devices or special cements are
not necessary to fix the implant in the position. Another point that makes titanium and its alloys good biomaterials is their density.
As it can be seen table 1, they have a very lower density compare to other metals used in knee implants. Moreover, the elastic modulus
of titanium and titanium alloys is lower than the elastic modulus of the other applied metals in knee implants. Because of this property,
the implant acts more like the natural joint, and as a result, the risk of some complications like bone resorbs and atrophy will be
reduced. The elastic modulus of various metals used in knee implants has been represented in table1 (22) :
Material | Density | Elastic Modulus |
Cobalt-Chrome alloy | ~8.5 g.cm^-3 | 7-30 |
316L Stainless Steel | 8.0 g.cm^-3 | 230 |
CP
Titanium | 4.51 g.cm^-3 | 200 |
Ti6Al4V | 4.40 g.cm^-3 | 106 |
Table1.Density and Elastic Modulus of some biomaterials used in knee implants (22).
Tantalum is a type of pure metal, which has excellent biological and physical properties namely flexibility, corrosion resistant,
and biocompatibility. Recently, a new porous substance has been made of tantalum named Trabecular Metal. It contains some dodecahedron
pores with a diameter of 550 µm. The size of the pores makes this material very good for bone in growth. In addition, Trabecular Metal
has an elastic modulus of about 3 GPa, which is not close to elastic modulus of titanium (110 GPa) or cobalt-chromium alloys (220
GPa).The mentioned points cause bone remodeling, which can be considered as a good point for a metal being used in knee implant
(4).
5) Polyethylene:
The tibial and patellar parts in knee joint are usually made of polyethylene. Yet unfortunately, the polyethylene surfaces suffer
from wear damages so that in some cases the polyethylene parts will be destructed completely and results in replacing the knee implant.
During wear process, tiny particles of polyethylene surfaces will be released. These particles will participate in various biological
reactions that loose the fixation of the knee implant to the bone. Therefore, scientists have started using a new designed form of
polyethylene called UHMW polyethylene in knee implants. This material inhibits increased wear at different locations so the implants
will last for a longer time.
This kind of polyethylene can be produced using electron beam or ? rays radiation that changes chain structure
through a process known as cross linking. This process forms stronger bonds among the elements present in the polyethylene. This type
of polyethylene is most appropriate and beneficial in case of patients belonging to the younger age group, as it is highly resistant
to wear and tear caused by heavy and continuous movement of the joint (13).
6) Oxinium oxidized zirconium:
It is the name of a new material is being applied in knee implant since 2001.It is basically a transformed metal alloy that has a
ceramic bearing surface. It contains zirconium and niobium alloy (Zr-2.5Nb) that was oxidized to convert the surface of the material
into zirconia ceramic. The advantage of this metal is that just the surface has been changed, so the rest of the implant component
is metal, which has high strength. Although it is twice harder than cobalt chromium alloys, it provides half the friction of those
alloys. Thus performs with higher quality and last for a longer time. Scientists believe that using oxinium oxidized zirconium knee
implant helps patients to restart ordinary activities with less need of repeating knee replacement surgery (18).
7) Zirconium alloy and plastic tibial component:
It is used in the new ceramic knee which uses a combination of zirconium alloy and a plastic tibial component which has replaced the
cobalt chrome alloy used previously for knee implants .Researches have shown that the combination of metals like cobalt chromium alloy
for femoral part and plastic tibial part in the knee implant will be effective for about 15-20years.Yet, the new designed knee replacement
by applying zirconium alloy and plastic tibial component will last for 20-25 years (16). Life expectancy can be considered as the
key point about the effectiveness of a knee replacement. This factor depends upon the wear on the knee joint, which is related to
the femoral and tibial parts moving against each other. Using the recent designed combination decreases the wear on the joint; therefore,
it will be a better and more effective choice. In addition, the new combination has two other properties that even lower the wear
on the knee joint more. First of all, it is scratch resistant, so in contrast to cobalt chromium alloy, it will not develop tiny scratches
in the patient body. Secondly, it is “wettable”, which results in a smoother and easier articulation through plastic (16).another
important characteristic of these materials is that they are biocompatible. For instance, people who are allergic to nickel can not
have knee implant made of cobalt chromium alloy because nickel is a ingredient of this alloy .But this new material does not contain
nickel, thus it will be absolutely safe for those patients (16).
