The manufacturing of medical components must meet standards of accuracy, reliability, quality, and traceability that equal and sometimes exceed those required for aerospace and nuclear parts. In addition, global competition and efforts to restrain health care expense create great pressure to maximize productivity and reduce manufacturing costs. Tooling manufacturers are helping medical partmakers meet these challenges with a selection of milling tools custom-engineered for the machining of complex orthopedic replacement components.
Replacing Hips and Knees
Demand for replacement and reconstructive parts for the human body is growing rapidly. When considering components for knee and hip replacements, trauma reconstruction and orthobiologics, sales of the parts exceed $25.2 billion worldwide. More than 50 percent of the total consists of knee and hip components, with five major medical OEMs taking almost 90 percent of the business. Two main factors spur continuing growth.
First, the world’s population is staying alive longer, resulting in a gradual increase in the average age. The most rapid growth, about 3.5 percent a year, is in those 65 years and above. Coincidentally, the average age for knee surgery is 65. The other major trend contributing to a surge in orthopedic implants is the growing number of persons who are overweight or obese. Approximately 1.57 billion of the world’s 7.2 billion people are overweight, and 0.53 billion are classed as clinically obese (BMI > 30%). Excess weight increases the likelihood of the development of osteoarthritis, a major reason for joint replacement.
Parts of a Replacement Knee
Typically, a total knee replacement consists of three subcomponents: the femoral component, which replaces the rounded bottom end of the femur bone; the tibial tray, which replaces the top end of the tibia bone; and the tibial or bearing insert, which fits between and cushions the other two parts. The bearing insert usually is produced from UHMWPE (Ultra High Molecular Weight Polyethylene), an engineering polymer, whereas the femoral component and tibial tray are in most cases produced from cobalt chrome (Co-Cr) alloy or, in some cases, a titanium alloy. These alloys are strong and hard, biocompatible materials with high stiffness (Youngs modulus) and abrasiveness when being machined. (See Figure 1)
Machining the Femoral Component
Machining techniques for femoral components include both grinding and milling. The challenges are to achieve a burr-free profile with superior surface finish that minimizes the need for manual polishing, and at the same time maximize productivity and tool life. For tough milling operations, specially designed tapered ball nose cutters and high-performance cutters feature differential flute spacing to minimize vibration during operation. Among the machining methods employed are corner plunging, periphery machining, box roughing and finishing, cam finishing, and box blend machining. (See Figure 2)
The femoral component has rounded contours that mimic the condyle bone formation at the end of the femur. The shape has traditionally been produced via grinding, but that operation can generate high temperatures that may distort the part. Specialized tools have been created to replace the grinding process with milling. A large medical OEM performed trials with these tools, finishing a cast Co-Cr femoral component with a copy milling strategy that employed a special solid carbide ball end mill.
The result was cycle time reductions of up to 11 minutes per part, representing 50 percent less time compared to the grinding method used previously. Tool life exceeded 12 hours, enabling one cutter to machine more than 80 parts. Excellent control of radial depth of cut on a 5-axis milling machine contributed to the extended tool life. In 4-axis applications without such control, tool life reached 6 to 8 hours. The change from grinding to milling also eliminated the possibility of scrap parts due to distortion.
Machining the Tibial Tray
Machining the Co-Cr tibial tray also presents challenges in terms of surface finish and productivity requirements. In addition, the part has right-angle locking details that must be produced burr-free. Machining the part typically can take up to seven separate machining operations. (See Figure 3)
To achieve a superior finish on the base where the tibial insert is seated, a multi-flute cutter with special wiper geometry was applied. This tool produced Ra values of below 0.1 μm. In a later step, a combined wall finish/chamfer cutter was implemented. The combination of finish and chamfer tools provides a controlled way of mechanical edge profiling (MEP) and prevents secondary burrs while eliminating manual rework and reducing tool costs.
Machining the Bearing Insert
Bearing inserts for replacement knees typically are made of an engineered polymer (plastic) commonly known as UHMWPE. This material is relatively soft and, therefore, generates low cutting forces, but surface roughness requirements of 0.10 μm Ra demand that it be machined with sharp, top quality finishing tools. A solid end mill was designed to meet the specific requirements of a leading global medical OEM.
Overcoming Condyle Contour Machining Difficulties
The condyle shape of both the femoral component and the bearing insert can be difficult to machine. Previous to the development of the specialized end mills, condyle surfaces were machined using polished HSS form cutters or conventional solid carbide tools. Both methods have several disadvantages.
Form tools often create visible cusps on the part surface, especially when the machine tool control is not quick enough to generate a smooth cutting path. The zero rake angle and low helix angle of the High-Speed Steel (HSS) cutters make it harder to achieve appropriate surface results. Use of conventional carbide tools allows only product forms with a radius. In addition, not all radii can be generated due to design limits of the cutter body. When the shortcomings of the tooling made the required surface roughness unachievable, additional less-reliable operations such as manual polishing or soda blasting were necessary. Those operations were unpredictable in terms of time, costs, and quality.
To overcome these problems, a premier finisher form cutter was designed, based on concave and convex sections either tangent or connected with a straight line. Compared to mold and die tools the profile tolerances of the tools are quite generous. However, manufacturing these cutters requires special care regarding the cutting edge geometry and the overlap between the concave and convex shapes, areas where the contour starts or ends with a small contour radii, and considerations regarding the tools’ largest diameter. (See Figure 4)
Manufacturing must be controlled to avoid sudden changes in the pressure of the tool grinding wheel or generation of excessive heat, which may produce areas on the cutting edge that are not sharp enough for the required operation, resulting in a shearing instead of a cutting action. Clean cutting is essential in producing fine finishes in the UHMWPE workpiece.
Normal tool life for a specialized form cutter is between 1,000 and 2,000 parts. It is common practice to leave the cutter in the machine until it is worn. Removing and storing it may cause too much risk of damage. The tools should be handled with extreme care as the cutting edge is easily damaged. Metal to metal contact should also be avoided, taking into account that even fingernail contact could result in light wear on the cutting edge. Protection during shipment and transit is indispensable.
Within the medical industry, traceability is a very important requirement to ensure quality in sealed (validated) processes. Therefore, all tools should be marked with a unique laser marking. Reliability and quality standards are set by contactless measurement techniques and sealed grinding procedures. Results included in packaging tubes can prove quality assurance to the customer.
Operators should be instructed on how to remove a protective wax from the cutters and find correct diameters on the reports supplied. Tools can be reconditioned up to five times. When significant wear is present, safe packaging is needed to prevent uncontrolled wear or damage while in transit during the reconditioning process.
When the tools are applied on a milling machine that is also used for cutting metals common for orthopedic implants (titanium/cobalt chrome alloys, and stainless steels) attention should be paid to clean the machine thoroughly. Metal chips will harm the cutting edge of the tool when making direct contact. So, it is necessary to have machines equipped with coolant filtering systems sufficient to ensure clean coolant that is free from chips. The implant supplier should also take into account the quality of the UHMWPE material. Imperfections in the material could result in tool wear due to inclusions. Premature wear of the cutter could be a sign that the machined polyethylene is not clean. In this case, quality in procurement procedures should be point of attention.
To productively and profitably fulfill the increasing demand for high-precision orthopedic components and other medical parts, manufacturers of the parts must take advantage of every opportunity to enhance their production technology. A key contributor is tooling technology for medical component milling operations. Sophisticated tools, of course, command a higher price than the basic tools of the past. However, given that higher quality tools offer advanced capabilities in quality, productivity, and consistency, which costs per part by up to five times, investment in these tools is a worthwhile strategy.
This article was written by Teun van Asten, MSc., Engineer Marketing Services Solid Milling, Seco Tools, Fagersta, Sweden. In the US, Seco Tools is headquartered in Troy. MI. For more information, Click Here .