Metallic Implants – A Revolutionary Made In Medicine
Metal is being used as potent implants in the biomedical industry. There are some key metals used as biomaterials. This blog is all about these metallic implants. Some of the important properties of metals are given by good conductance of heat and electricity, high density, high melting point, ductility, malleability, high reactive surface, corrosion, elasticity, plasticity, etc. The following are causes for metallic implants to fail.
Alloys – the implant material
Alloys are a homogeneous mixture of two or more elements in which at least one of them is metallic. Alloy is a combination of metals to observe the improvement of metal properties like strength, ductility, hardness, wear and tear, and corrosion resistance. The most commonly used metals/alloys in the fabrication of implants are.
- Stainless Steel
- Cobalt-Chromium alloys
- Magnesium Alloys
Some of the notable properties of magnesium alloys are,
- Possess same mechanical properties as that of bone
- Biodegradability in the physical environment
- Excellent biocompatibility
- Lower modulus of elasticity
- Lighter than Aluminium and titanium
These magnesium-based alloys are yet to be used as prosthetic and orthopedic implants.
Using stainless steel as a surgical implant has been in practice since 1900. Type 316 and 316L are widely used for the fabrication of implants. Composition of Stainless Steel 316L is given by, 19% of chromium, 13% of nickel, 2.5 % of molybdenum, 2% of molybdenum, 0.03% of carbon, and the rest is iron. The only difference between 316 and 316Ltypes is the amount of carbon contained. Under highly stressing and oxygen-depleted conditions, these implants tend to get corroded. Thus, Stainless steel-based implants are often used as temporary implants.
It is widely used as a biomaterial. The composition of the cobalt-chromium alloy is given by, 53 – 67% of cobalt, 25 – 32% of chromium, and 2 – 6% of molybdenum. It is most commonly used as dental implants and orthopedic implants. Cobalt chromium has a very high specific strength and good resistance against corrosion. This is achieved by the formation of a Cr2O3 layer on the surface. The following table compares the properties of metal alloys.
Titanium is the ultimate choice of biomaterials – Why?
- Lightness. Titanium is 60% less dense than stainless steel. This increases the comfort of the patient and also improves the activity.
- Strength. Titanium is stronger than that aluminum, magnesium, and stainless steel. Ideal for weight-bearing implants like knee and hip.
- Corrosion-resistant. It forms a layer of titanium oxide on its surface to prevent corrosion.
- Wear and tear-resistant. Ideal for knee implants
- Can be easily milled into any shape.
- Biocompatible. The titanium oxide layer helps in interfacing with plasma proteins.
- Low modulus. This means that the material won’t deform that much even if more stress is applied.
- Osseointegration. The ability of titanium to integrate in absence of connective tissue. This is essential for developing dental implants.
ASTM F67 is the commercially used titanium for biomaterials. The only problem associated with titanium is that it’s expensive and it’s tough to engineer it into an implant. The following table compares the mechanical and biocompatible parameters of stainless steel and titanium.
Stainless steel vs Titanium
Titanium is commonly used in permanent medical implants like hip, knee, tooth, screws, and even pacemakers. Three cases where titanium is being deployed as an implant are explained below.
This is also called Total Knee Replacement (TKR). This is a surgical procedure for end-stage knee diseases, mainly arthritis where the cartilage wears down. There are four components in a knee implant as listed below.
- Plastic insert
The materials used for femoral, tibial, and patellar components are cobalt-chromium-molybdenum cast and titanium alloys. These are interfaced using press-fit, bio fixation, or using PMMA (Poly Methoxy Meth Acrylate). The material used for the plastic insert is polyethylene interfaced using press fit. The knee joint bears most of the body weight.
Hence it is essential to use a material that has good mechanical properties. Also, the material has to interact with synovial fluid, so it must not corrode (corrosion resistance). The material must be light so that the patient won’t feel uncomfortable. Finally, it must be biocompatible and must not trigger host immune reactions.
This is for replacing the hip joint partially or totally. There are two parts to the hip joint. The articulating part (acetabulum) and the attaching point (femoral head). There are four components in hip implant along with the material used as mentioned below
- Acetabulum socket- strong metal (titanium/stainless steel)
- Liner- Mostly plastic or ceramic (for free locomotion)
- Femoral head- metal alloy
- Femoral stem- metal alloy
Mostly stainless steel and titanium are used for the hip prosthesis. The reason is that these materials are strong and have good mechanical properties like weight-bearing, tensile strength, and so on. The problem with stainless steel is that it can corrode in fluidic medium and oxygen depletion. One problem with using cobalt-chromium alloy is the fretting corrosion in the femoral head.
Titanium screws are used for dental implants. They are drilled into the cavity and are made to integrate with the bone. Titanium has good osseointegration and in some cases, HA (hyaluronic acid) is used to quicken it. Then they take an imprint and fix an artificial crown over it.
The applications of stainless steel are screws, fracture plates, bone plates where the implant is temporary.
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