An implant is a device used to help replace a biological structure that has been damaged due to trauma or bone and cartilage disorders (e.g., osteoporosis). Medical implants can also be used for cosmetic purposes such as breast implants. The main focus for this article is surgical metal implants and their application.
The most common metals used for surgical implants include:
The application of stainless steel has continued since its early use in the 1900s for surgical practices. There are multiple forms of stainless steel starting with the introduction of type 302 for its application in orthopedic surgery. Type 316L stainless steel is commonly used in surgical procedures to replace biological tissue or to help stabilize a biological structure, such as bone tissue to aid the healing process.
Type 316L stainless steel is popular for surgical practices as it is the most corrosion resistant when in direct contact with biological fluid. It is important that a surgical implant is not susceptible to corrosion when placed inside the human body to prevent the chances of infection occurring. In the event of an infected implant, the device is removed to prevent any further trauma to the surrounding biological tissue. This type of stainless steel is particularly effective as a surgical implant when in cold-worked condition. What makes the Type 316L ideal as an implant device is the lack of inclusion in this material. Material with inclusion will also contain sulfur and this is a key component to encouraging corrosion of metals.
Stainless steel is a metal alloy metal. By adding the chromium (16%) element to stainless steel, this metal becomes corrosion resistant. The addition of carbon and nickel (7%) to stainless steel helps stabilize the austenite to stainless steel. Type 316L stainless steel selected for the purpose of surgical implants contains approximately 17 to 19% of chromium and 14% nickel. As mentioned, it is fundamental that metal implants are not susceptible to corrosion. With surgical implants, molybdenum is added to the stainless steel alloy that forms a protective layer sheltering the metal from exposure to an acidic environment. Corrosion resistance can also be achieved with the carbon element but only when the carbon is in a solid solution state.
It has to be stressed that the ferrite element should not be incorporated into stainless steel as this gives the metal a magnetic property, which is never used for surgical implants as it could interfere with Magnetic Resonance Imaging (MRI) equipment. One of the most apparent problems with using magnetic implants is their susceptibility to heating which could change the shape or structural position of this metal implant.
Compared to stainless steel alloys that have been in medical practice since the early 1900s, Titanium is relatively new in its application as a medical implant for the replacement of a biological tissue. One of the biggest advantages to titanium is its strength – titanium retains as much strength as steel and is exceptionally lighter in weight (approximately 50% lighter), making this material ideal for its use as surgical implants.
Unfortunately, titanium is easily contaminated if exposed to hydrogen, nitrogen and oxygen, which can influence the corrosion process in this metal and may compromise its use in certain medical procedures. The 1960s experienced a shift in the selection of best-fit metals for surgical implants with titanium being a popular choice. Medical implants can also be used for cosmetic purposes such as breast implants. The main focus for this article is on surgical metal implants and their application.
The structural composition of titanium alloys falls into three distinct categories: á alloys, á-â alloys, and â alloys. Elements including aluminium, oxygen, tin, and zirconium all become vital for the stabilization of the á-based alloys. Magnesium, molybdenum, iron, and chromium become favourable as stabilizers for the â-based alloys. The most common forms of titanium are made of aluminium and vanidium or the aluminium and niobium combination that are typically applied for the manufacturing of rods and spinal clamps. The high tensile strength and lightweight characteristic to titanium makes this metal ideal for reconstructive surgery.
Stainless Steel vs. Titanium
- Titanium is stronger and lighter in weight compared to stainless steel.
- Titanium has a large resistance to repeated loads making it ideal for its application as an implant.
- Titanium has greater superior strength under repeated load stresses, making this metal capable of withstanding strain during internal fixation.
- With a lower modulus of elasticity compared to stainless steel, titanium is less rigid which limits the amount of stress on bone structures.
- Titanium is less prone generating an immune reaction based on the fact that this material is corrosion resistant compared to stainless steel implants.
- Davis J.R. 2003. Handbook of Materials for Medical devices. ASM International.
- Shrivastava S. 2004. Medical Device Materials. Proceedings of the Materials & Processes for Medical Devices Conference. ASM International.