Finding a supplier that has a solid understanding of materials has a strong influence on the success of a product.

Machining medical implants and instruments is a complex process. The properties of the raw materials and the production processes used to manufacture medical devices can present extreme machining challenges. For those and other reasons, medical device OEMs should invest appropriate resources in finding a partner with the expertise and capabilities to meet and surpass their needs.

A key element of a good supplier’s machining capability is a detailed understanding of the materials being used. Suppliers to the device industry should possess an intimate knowledge of the material properties and be able to provide the specific material and grade that is required. In this way, an OEM can be certain that the supplier can collaborate effectively at a strategic level, balancing the manufacturing and machining challenges of each material with the intrinsic benefits of its properties.
The overall demand on material properties has increased in all of the key measures relating to a material’s suitability for implantation in the body. These requirements include, but are not limited to, acceptable nonmetallic inclusion levels, basic chemical composition and elemental balance, corrosion resistance, mechanical properties, microstructure and grain size.

Implantable materials
Characterized by high strength, low weight, outstanding corrosion resistance, and total biocompatibility, titanium is one of the few materials that naturally matches requirements for implantation in the human body.

A variety of titanium alloys are used in medical applications. In particular, alpha-beta titanium alloys with high purity that confer improved ductility and fracture toughness are ideal for joint replacement and bone fixation devices.

Cobalt chromium alloys are used on a large scale in implants and prosthetics. Some of the main applications of CoCrMo alloys are in hip, knee, and shoulder joint replacements, and fixation devices such as screws, pins, and plates. The biocompatibility of the alloys is closely linked to their corrosion- and wear-resistance and fatigue strength. High- and low-carbon versions of the alloy fulfill medical application requirements.

Ultraclean stainless steel is commonly used to manufacture implants. The material is prized for its corrosion resistance and microcleanliness, its enhanced mechanical properties for safe load bearing and resistance to fatigue, ductility for improved formability, and dimensional stability for precise machining and finishing. The most commonly used grades are a vacuum-melted version of AISI 316L conforming to international standards ASTM F138/ISO 5832-1 and a high-nitrogen alloyed austenitic stainless steel conforming to ASTM 1586/ISO 5832-9 (see Table I).

Until now, stainless-steel grades used for medical devices have been martensitic chromium steels such as AISI 420 and austenitic stainless steels such as the AISI 300 series. However, these grades cannot fully satisfy the demanding requirements in terms of material properties for devices such as fine bone drills. Although the martensitic chromium steels in the as-delivered condition have good formability and, after hardening and tempering, typically attain a strength of 1800–2000 MPa, they exhibit poor toughness. On the other hand, austenitic stainless steels have to be delivered in the cold worked condition to attain the required strength, leading to poor formability when manufacturing the final product.

For example, Bioline 1RK91 is a precipitation-hardenable stainless steel with a basic chemical composition consisting of 12% chromium, 9% nickel, 4% molybdenum, and 2% copper with a carbon concentration less than 0.01%. In the aged condition, the material can exhibit an ultimate tensile strength in excess of 2000 MPa while retaining good ductility. It is resistant to corrosion, has ultrahigh tensile strength, good toughness, excellent formability, and dimensional stability. The aging response is exceptionally large and permits forming operations in a softer state, followed by age hardening to the final ultrahigh-tensile-strength state. Brittle fracture and flaking is prevented by the material’s toughness, ensuring safe use of critical or slender devices.

Bioline 7C27Mo2, a martensitic stainless chromium steel alloyed with molybdenum, is characterized by high corrosion resistance and toughness and excellent fatigue strength properties. It produces excellent results in bone saws and meets the requirements of ASTM F899, a standard specification for wrought stainless steels for surgical instruments.

Table I. Materials that meet the demanding requirements of orthopaedic applications. Click table for enlarged version.

Rapid Prototyping Benefits
Turning raw materials into innovative, effective products as quickly as possible is crucial in the competitive medtech market. As a result, medical device OEMs need to work alongside suppliers that have real materials expertise and are able to help them effectively manufacture quality products while shortening the time it takes to bring the products to market. To this end, the supplier should have rapid prototyping and rapid production capabilities for the development and manufacture of orthopaedic implants, instruments, and medical materials.

For such manufacturing, a direct metal laser-sintering machine (DMLS) is recommended. It uses an additive manufacturing process to produce metal components directly from a CAD model using a powerful 200-W Yb-fiber laser and layers of fine metal powder. Sliced into 0.02-mm layers, the CAD model is effectively reconstructed as the laser fuses or melts each layer together. DMLS technology can build any geometry, including voids, tunnels, and undercuts. To achieve the same results with a material removal process such as CNC machining would require the additional support of electrical discharge machining.
Currently available alloys that can be used in the rapid prototyping process include 17-4 and 15-5 stainless steel, precipitation-aged hardening steel, CoCrMo chromium, Inconel 625 and 718, and titanium Ti6Alv4.

Form and Function
It is important to note that the very properties that make raw materials ideal for orthopaedic applications—toughness, strength, and corrosion resistance—also make them difficult to machine. The complex geometries that are now possible can also present extreme challenges.

For example, an implant can be coupled with a plastic insert to form a bearing. It is critical to have a highly polished defect-free surface to minimize wear and smoothen the articulations. The required surface finish value typically is below 0.05 Ra, but it can be much lower, with a mirror finish and zero surface defects. The finish must be maintained through a series of processes from metal cutting (milling and turning) to grinding, followed by a succession of abrasive media and polishing steps, which gradually improve the surface finish to the required specification.

The form also must be maintained since it must follow the anatomical profile of the body part that it is replacing. The surface profile on the computer-generated model, therefore, must be accurately replicated by the 3-D machining process on the surface of the implant.

Having considered the implications of the manufactured forms, it is necessary to consider the challenges presented by the materials most suited to the production of implants and instruments. Cobalt chrome molybdenum is used for many common implants because it has extremely good resistance to wear and hence provides many years of service. It has poor machinability, however, resulting in extremely high wear rates on cutting tools, which could affect the dimensional accuracy of the final product. Continuous monitoring of the machines to compensate for tooling wear can prevent this problem. On-board tool probes measure the tool tip radius as it wears and automatically make adjustments to maintain the component’s dimensional accuracy. A limit is set on the tool wear; when the limit is reached, the machine automatically exchanges the tool with a new one.

Bearing these challenges in mind, it makes sense to partner with suppliers that are able to taken on an intrinsic role in the process. After extensive development and testing, many orthopaedic devices will enable patients to return to normal function for many years. The particular capabilities of the product that make this possible may be achieved via anatomical design, instrument technology, or materials or manufacturing technology. Suppliers that have the acumen to work alongside OEM designers at the product’s inception can provide valuable input at strategic moments, thereby producing cost-effective solutions that are designed for manufacture.

Tool Selection
Using the right tools for machining is crucial. Here is an example of how one company accomplished the task.

The spherical turning of hip joints with round inserts was the first concept designed by Sandvik Coromant for the medical industry. To capture the advantages offered by applying large radii, the company developed a range of tools that pioneered traditional processes, increasing both productivity and tool life. Now available for diameters as small as 20 mm, the inserts offer a wider application, allowing medical device OEMs to double productivity and reduce tooling costs by a third.
In roughing operations, round inserts impart a strong cutting edge and resist notch wear. They offer reliability, durability, and the opportunity to machine longer with the same tool. The preparation of small-diameter components is facilitated by the extension of the tool holder program.

Round CoroCut cubic boron nitride (CBN) inserts provide surface quality and productivity in challenging applications within the medical industry. CB7015 is a polycrystalline CBN grade of fine grain size with a unique ceramic binder that delivers performance and reliability in finishing cuts and long total cut lengths.

CBN high-performance-grade CB7015 is introduced in CoroCut one-edge round inserts (3–8 mm), which have razor-sharp ground cutting edges—to give optimal grade performance when profiling heat resistant superalloys (HRSA)—as well as hardened steels. When machining CoCr alloys into hip joint heads, a 0.1-m Ra surface finish can be achieved.

For applications such as bone and spinal screws, a complete thread whirling method has been developed. Thread whirling is a fast way to thread components in difficult materials. This approach offers a number of benefits over traditional single-point threading techniques. Because it is a single-pass milling operation, thread whirling offers significant productivity improvements.
Single-pass machining from stock diameter reduces the cycle time by minutes, and deeper thread forms can be achieved more easily. The thread whirling inserts have longer cutting edges than single-point tools so tool life is extended. Further finishing is not required after thread whirling, which reduces costs, and improved chip control enables more continuous and productive machining. Finally, downtime is reduced because there is no need to match rough and finish insert forms or to use special support devices.

Conclusion
When sourcing suppliers of machining services, medical device OEMs must evaluate a potential partner’s technical expertise, but they also need to be certain that operations are underpinned by a solid quality management system. Regulatory oversight on supplier control is on the rise, and the cost of securing and proving this control with multiple suppliers has escalated. Reducing the number of different suppliers and, especially, being able to treat one supplier with multiple sites as a single entity can be a significant competitive advantage for medical device manufacturers. For their part, suppliers must have a consistent quality management system based on best practices across all their sites.

Many elements affect the machining of medical implants and instruments. To ensure that products are designed with an eye toward simplifying the manufacturing process and accelerating time to market, medical device OEMs should seek suppliers that have in-depth materials and manufacturing expertise and comprehensive tooling capabilities. If the supplier can add sophisticated machining capabilities to this portfolio of services, the OEM will receive fully finished parts rather than a casting or forging, thereby reducing total lead time and eliminating the need to maintain an inventory of cast and forged parts.

Article originally published in EMDT.
Stephen Cowen is general manager at Sandvik (Sheffield, UK).