Specialty Alloys and Titanium Shapes to Consider for Latest Medical Materials Requirements

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Carpenter Technology Corporation has developed a series of specialty alloys that can be considered to meet the latest requirements of the medical and dental industries for hand-held instruments, implanted fixation devices, replacement joints and orthodontic material.

Similarly, Dynamet, Inc., a Carpenter subsidiary based in Washington, PA, has expanded its line of titanium products to include special shapes as candidates for spinal implants and fixation devices, bone plate profiles and hexagon shaped bar for a diversity of medical, surgical and dental applications.

Alert to current trends, Carpenter reports that instrument manufacturers are looking for alloys (a) with more corrosion resistance during autoclaving (b) that will provide longer instrument life (c) that will give shaping and cutting instruments improved edge retention (d) with improved magnetic properties for use in MRI environments. The need also continues to grow for improvements in mechanical properties such as tensile strength, torsional strength, fracture toughness and ductility.

BioDur® TrimRite® stainless, which has been used for instrument applications, has been found superior to stainless Type 410 and Type 420 in terms of corrosion resistance, ease of fabrication and edge retention. Custom 465® stainless has been established as a candidate alloy for instruments because it has better corrosion resistance and fracture toughness than Custom 455® stainless at the same strength level.

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BioDur 108 alloy has gained acceptance and has been used for instruments in the MRI field because it is essentially a non-magnetic material with very high strength. It can achieve UTS greater than 351 ksi while possessing low magnetic permeability in a heavily cold worked condition.

Improved tensile strength and fatigue strength are major current requirements of alloys for implanted fixture devices. The medical/surgical industry in Europe, particularly, wants material that contains less than 0.05% nickel to avoid problems associated with nickel toxicity. An estimated 2% to 7% of the general population may be susceptible to complications caused by exposure to nickel.

BioDur 108 alloy, that is essentially nickel free, has satisfied this requirement. BioDur 108 alloy has been used in various implant applications because it offers higher fatigue strength and greater corrosion resistance than BioDur 316LS stainless, 22Cr-13Ni-5Mn nitrogen-strengthened stainless and Alloy 734. All of these grades have very low magnetic permeability, which keeps devices made of those materials from moving when a patient undergoes an MRI.

Quite recently, Carpenter has made its BioDur 108 alloy available in cannulated bar stock (pictured) with a hole from end to end and a heavy wall for implantable orthopedic applications. The alloy in this new form can be considered for bone screws, intramedullary nails for the femur, instruments and any other trauma devices requiring a hole from end to end.

The hole in the cannulated stock allows space for bone marrow to grow, and facilitates the insertion of guide pins for bone screws used in fracture fixation. Cannulated bar also can eliminate or minimize the need for costly drilling operations currently required when using solid bar stock. Despite its higher strength, BioDur 108 alloy has fabricability and ductility similar to that of 22Cr-13Ni-5Mn alloy.

BioDur 108 alloy is available in cannulated bar with outside diameters of 0.118-in. (3 mm) to 0.750-in. (19 mm), with inside diameters of 0.055-in. (1.4 mm) to 0.354-in. (9 mm). Typical OD dimensional tolerance is ±0.001-in. (±0.025 mm) and ID tolerance is ±0.002-in. (±0.05 mm).

The search continues for materials that will extend the life of replacement joints. Key to longer life is the degree of success experienced in reducing wear and wear debrision at the bearing interfaces.

BioDur CCM Plus® alloy, a premium high-carbon grade made by powder metallurgy, has combined several attributes that have helped to produce longer joint life. The alloy has homogeneous chemistry and microstructure with very fine, uniformly distributed carbides that give the material superior wear properties when mating with ultra-high molecular-weight polyethylene or when mating with itself in a metal-to-metal joint. It also has exhibited high strength and corrosion resistance in such applications.

Based on customer results, metal-to-metal joints appear to be a viable, long term solution to metal-to-polyethylene joints which, in time, tend to generate wear debris at the bearing surface.

Manufacturers of orthodontic braces and retainers are still seeking stainless steel wire with low nickel content, and more strength and corrosion resistance than that found in stainless Type 304.

BioDur 108 alloy wire has provided all of these properties, as well as several more of value to orthodontic manufacturers, ie: low friction properties, exceptional surface smoothness, improved ductility and bending characteristics, and the ability to be processed to a high-luster finish.

Titanium Shaped Bar 

Dynamet, responding to current needs of medical and surgical device manufacturers, now makes shaped titanium bar that can be considered for spinal implants and fixation devices in seven standard alloys – Ti 6Al-4V ELI (extra-low interstitial), Ti 6Al-4V, Ti 6Al-7Nb and Ti-CP (commercially pure) grades 1, 2, 3 and 4. Compositions and mechanical properties also can be tailored to meet custom requirements.

Shapes produced consist primarily of rectangles and squares, of 25/26mm (1-in.) cross section or smaller, for machining into implants. Bars are available in random 3.05 to 3.66m (10 to 12-ft.) lengths, or custom cut lengths.

Due to its proprietary process capability, Dynamet says it is able to achieve tighter dimensional tolerances than possible with typical hot rolled titanium rectangles and squares. Its shapes are described as very square, straight with perpendicular surfaces that are easy to fixture and clamp for machining.

Fabricators have found that these features allow them to save material, reduce machining time and improve yield. At the same time they are able to make devices from stock that is closer to the desired net shape.

According to Dynamet, the combination of good squareness, close tolerances and fine surface finish offered in these titanium bars permits the implant manufacturer to skip qualification machining of parts and move directly to final machining. Since the bars are processed by Dynamet to reduce residual stresses, the fabricator is able to hold closer tolerances during machining operations and minimize scrap caused by distortion or "springback."

Dynamet has produced bone plate profiles or net shapes, primarily from the titanium CP alloys, to user design specifications or prints. When using these profiles, the fabricator needs to do nothing more than drill screw holes.

The titanium producer can adjust its production process to make profiles with very specific mechanical properties. It can, for example, provide several different strength levels in the same profile while holding required profile geometry and tolerances. This material can be supplied in random length bars and custom cut lengths for the manufacture of parts generally under 16mm (5/8-in.) wide.

In addition, Dynamet now produces dimensionally true hexagon shaped bar in numerous English and metric sizes for consideration in a diversity of medical and dental applications. They are made from all seven of the titanium alloys mentioned previously.

The hexagon sides are reported to have surfaces precisely 60° apart, a dimensional accuracy that has been very difficult to achieve when machining hexagons from round bar. These hexagons have helped fabricators eliminate the cost and time of machining operations, save material and reduce scrapped parts that are the consequence of uneven sides machined from round bar.

All Standards Met 

All of the Dynamet titanium products mentioned meet ISO 5832 requirements and standard industry specifications such as ASTM F-1472 (for Ti 6Al-4V), ASTM F-136 (for Ti 6Al-4V ELI), ASTM F-1295 (for Ti 6Al-7Nb) and ASTM F-67 for all four of the Ti-CP alloys. The producer, in addition, is able to meet special customer specifications.



Published in European Medical Device Manufacturer

Carpenter Technology Corporation
Reading, PA