Feature Article

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 into finding a partner with the expertise and capabilities to meet and, indeed, surpass their needs. 
 

A key step in the delivery of an effective 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 required. In this way, the OEM can be certain 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 the material’s suitability for human implantation. 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.

 

Characterised 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 are available to fulfil various medical application requirements.

 

Ultraclean stainless steels are commonly used to manufacture implants. The material is prized for its corrosion resistance and micro-cleanliness, 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. 
 

Stainless-steel grades used for medical devices have until now been essentially 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. Whilst the martensitic chromium steels in the as-delivered condition have good formability and, after hardening and tempering, attain a strength of typically 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. 
 

Sandvik Bioline 1RK91 is a precipitation hardenable stainless steel with a basic chemical composition consisting of 12% Cr, 9% Ni, 4% Mo and 2% Cu with a carbon concentration below 0.01%. In the aged condition, the material can exhibit an ultimate tensile strength well in excess of 2000 MPa, whilst retaining good ductility. It is resistant to corrosion, has ultrahigh tensile strength, good toughness, excellent formability and dimensional stability. The ageing 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. 
 

Sandvik Bioline 7C27Mo2, a martensitic stainless chromium steel alloyed with molybdenum, is characterised by high corrosion resistance and toughness and excellent fatigue strength properties. Sandvik Bioline 7C27Mo2 produces excellent results in bone saws and other medical edge applications and meets ASTM F899, a standard specification for wrought stainless steels for surgical instruments.

 

Turning these raw materials into innovative, effective products in as short a time as possible is crucial in the competitive medtech market. That is why medical device OEMs need to work alongside suppliers with real materials expertise who are able to help them effectively manufacture excellent 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 material.

 

A direct metal laser sintering machine (DMLS) is recommened. It uses an additive manufacturing process to produce metal components directly from a CAD model using a powerful 200-W Yb-fibre laser and layers of fine metal powder. Sliced into 0.020-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, cobalt chromium, Inconel 625 and 718, and titanium Ti6Alv4.

 

It is important to note that the very properties that make raw materials ideal for orthopaedic applications—toughness, strength, corrosion resistance and so forth—make them difficult to machine. The complex geometries that are now possible also can 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 minimise 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 has to 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, gradually improving the surface finish to the required specification. 
 

The form also must be maintained since it has to follow the anatomical profile of the body part it is replacing. The surface profile on the computer-generated model, therefore, has to 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 become an intrinsic part of the process. After extensive development and testing, a medical device 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, 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.

 

Using the right tools for machining is crucial. The company that I work for has the good fortune to draw on Sandvik Coromant’s tooling expertise. 
 

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 CBN (cubic boron nitride) inserts provide excellent 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 excellent performance and reliability in finishing cuts and long total cut lengths.

 

CBN high-performance grade CB7015 is introduced in CoroCut 1-edge round inserts (3 to 8 mm), which have razor-sharp ground cutting edges—to give best grade performance when profiling heat resistant superalloys (HRSA)—as well as hardened steels. When machining CoCr alloys into hip joint heads, a 0.1-μm Ra surface finish can be achieved. 
 

For applications such as bone and spinal screws, a complete thread whirling solution 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 while enhancing overall quality.
 

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. 
 

When sourcing suppliers of machining services, medical device OEMs must evaluate their potential partner’s technical expertise in these and other areas, 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 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. 

 

is responsible for Sales and Marketing, Medical Sector, Sandvik, Parkway Close, Parkway Ind. Est., Sheffield S9 4WH, UK

tel. +44 1142 942 300

e-mail: stephen.cowen@sandvik.com
www.smt.sandvik.com/medical