Metal medical miracles

Published August 15, 2016

Part 3

The final edition of this three-part medical machining series from Sandvik Coromant looks at the metals used in medical devices, their properties, and why each is selected for certain medical applications.

Most metals used to create life- saving objects fall into a group known as heat resistant super alloys (HRSA). These tough materials are widely used in the medical industry due to their high strength, hardness, and corrosion resistance, as well as their ability to retain these properties even at elevated temperatures.

These biocompatible metals must not provoke immune responses from the human body or leach harmful minerals into the patient across time. For permanent implants, metals should also show a tendency toward osseointegration, meaning that they will gradually meld with the surrounding bone tissue and connective tissue and not come loose requiring surgical replacement after a few years.

Staying noble

Gold plays a critical role in many pacemakers. Guiding a stent through a blood vessel would be far more difficult without platinum. Radiopaque palladium marker bands help technicians navigate during X-ray fluoroscopy, and electrodes made of platinum- iridium alloys are used to stimulate brain tissue in Parkinson’s and epilepsy patients.

Because of their high cost, the parts they make up are typically small, often measuring just a few thousandths of an inch in diameter. Many of these tiny components are produced on Swiss-style screw machines, while others are milled, cold-formed, stamped, or EDMed.

All of these materials fall into the class of elements known as noble metals. Unlike most metals, they are resistant to oxidation and corrosion in moist atmospheres, and can even withstand mild acids, making them an ideal solution for many in vitro medical applications. Platinum and gold are also very conductive and are widely used in defibrillators and other electrical stimulation devices.

Noble metals are often soft and gummy, and attempting to feed a bar of gold through a lathe spindle can be difficult. To provide enough rigidity for machining, medical component manufacturers often start out with bar stock much larger than necessary, resulting in scrap rates of 90% or greater in some cases. However, the chips from machining operations are easily recoverable – melted down and reprocessed into new materials.

Medical components’ small size only makes matters worse. On a lathe operation, for example, very high spindle rpm is needed in order to maintain recommended cutting speeds, something that is often difficult to achieve given the delicate nature of the materials and the rpm limitations of most machine tools.

Because these metals are gummy, polycrystalline diamond (PCD) cutting tools are ideal, as these resist workpiece material sticking to the face of the cutting tool. Unfortunately, the high cost of PCD tools, often 6x to 10x that of carbide, does nothing to help an already expensive machining proposition.

Cobalt chrome

The heavy hitters for implant use are cobalt chrome (CoCr) alloys, which can be made hard through heat treating (45HRC to 50HRC). They are corrosion and wear resistant and extremely strong, making them an ideal solution for knee and hip replacements, dental implants, bone plates, and other prostheses that must bear heavy loads.

Cobalt chrome molybdenum (CoCrMo) can be cast into a variety of complex shapes, which are then finish-machined to size as necessary – with a hip replacement, the section of the femoral stem that is cemented into the thigh bone might be left in an as-cast state, and the opposite end would be machined to accept a spherical ball made of cobalt chrome or possibly ceramic.

The outer surface of this articulating ball is made perfectly round and highly polished to fit with a mating acetabular cup, which is typically lined with high-density polyethylene (HDPE) to provide a sliding surface, although ceramic or even all-metal cups may also be used.

Some medical manufacturers opt for a different type of CoCr alloy, cobalt nickel chrome molybdenum (CoNiCrMo). CoNiCrMo is readily hot-forged into similarly complex shapes, giving it a fine-grain structure that makes it suitable for heavy load bearing components such as hips and knees.

The metallurgical compositions of both alloys are clearly defined by ASTM International standards ASTM F75-12 and ASTM F562-13, respectively. Manufacturers have developed proprietary versions of these important metals, fine tuning them for improved mechanical properties and workability. For example, United Titanium produces UT35N, a type of CoNiCrMo that provides high strength and corrosion resistance. Carpenter Metals offers its BioDur CCM Plus product, a CoCrMo alloy that enjoys forgeability and strength characteristics similar to CoNiCrMo but with far less nickel content, a metal to which a small number of implant recipients are allergic.

Titanium

Some patients may be sensitive to cobalt or chromium as well, and for these people, only a titanium implant will do. Pure titanium is hypoallergenic, and when alloyed with small amounts of aluminum and vanadium (Ti-6Al-4V), it approaches the yield and tensile strength of CoCr. However, titanium is less shear resistant, so may be unsuitable for very heavy or athletic patients. However, due to its greater biocompatibility, many surgeons consider medical grade Ti-6Al-4V and the higher purity Ti-6Al-4V ELI (extra-low interstitial) an ideal choice for knee and shoulder replacements, spinal cages, bone screws, cardiovascular devices, and a wide range of other implantable components.

Commercially pure (CP) titanium is available in Grades 1 to 4. It is considered the most human-friendly of all titanium but is not as strong as alloyed titanium and sees limited use for load-bearing medical implants. CP titanium is often used to make dental prostheses such as crowns and bridges, which are then coated with porcelain for aesthetic reasons. It is also used in fiber metal, a porous material that is bonded to the surface of an implant to improve osseointegration.

As with CoCr alloys, a number of alloys have been developed to improve titanium’s mechanical properties or to alleviate concerns in the medical industry over potential cytotoxicity of the various titanium alloying elements. For example, Ti-6Al-7Nb contains 7% niobium. This gives it greater tensile strength than Ti-6Al-4V and eliminates the worries shared by some physicians over the possibility of vanadium ions leaching into the human body during prolonged implant use.

The beta alloy Ti-12Mo-6Zr-2Fe (TMZF) contains molybdenum, zirconium, and iron, and was designed specifically for orthopedic use. It contains neither aluminum nor vanadium, which proponents say gives it a biocompatible edge over Ti-6Al-4V. Two other biomedical beta alloys, Ti–29Nb–13Ta–4.6Zr (TNTZ) and Ti-35Nb-7Zr–5Ta (TNZT), are both high strength and non-toxic, and have a modulus of elasticity similar to human bone, which reduces stress between the implant and surrounding bone.

Stainless steel

Though less widely used for long-term implant use due to problems with corrosion, 304 and especially 316L stainless steel are widely used in the medical field, along with newer materials such as 18Cr-2Ni-12Mn and 21Cr-6Ni-9Mn. Surgical instruments, staples, catheters, syringes, nozzles, and wires require materials that are dependable, cost-effective, and easily sanitized; and low carbon, high nickel-chrome content stainless steels are well-suited. Machining and fabricating stainless steel is easier than cobalt chrome and titanium.

Though patient safety must always be first and foremost with any medical device or implant, manufacturing costs come in a close second. Each segment of the manufacturing industry has its own challenges, but medical machining is one of the most difficult. The parts are complex, the requirements stringent, and the materials challenging. Success is only achieved by understanding those materials and knowing the limitations of one’s machine tools and cutters.

Multiple cutting tool grades and geometries are needed for productive part making no matter which material is used, along with plenty of fine-tuning of the machining process. Luckily, there are knowledgeable cutting tool representatives and application specialists out there, ready to share some friendly advice.

Sandvik Coromant

www.sandvik.coromant.com

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