Milling tooling developments increase quality and productivity in the machining of orthopedic replacement components | By Teun van Asten |
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 maximise 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.
Demand for replacement and reconstructive parts for the human body are 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% of the total consists of knee and hip components, with five major medical OEMs taking almost 90% 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% a year, is in those who are 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.
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 Ultra High Molecular Weight Polyethylene, an engineering polymer (UHMWPE), 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.
Machining techniques for femoral components include both grinding and milling. The challenges are to achieve a burrfree profile with superior surface finish that minimises the need for manual polishing, and at the same time maximises productivity and tool life. For these tough milling operations, Seco has developed specially designed tapered ball nose and modified Jabro JHP770 high-performance cutters that feature differential flute spacing to minimise vibration during operation. Among the machining methods employed are corner plunging, periphery machining, box roughing and finishing, cam finishing and box blend machining.
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. The company has developed tools and performed tests to replace the grinding process with milling. A large medical OEM performed trials with the tools, finishing a cast Co-Cr femoral component with a copy milling strategy that employed a special solid carbide Jabro ball end mill. The result was cycle time reductions of up to 11 minutes per part, representing 50% less time compared to the grinding method used previously.
The tool life exceeded 12 hours, enabling one cutter to machine more than 80 parts. Excellent control of radial depth of cut on a five axis milling machine contributed to the extended tool life. In four axis applications without such control, tool life reached six to eight hours. The change from grinding to milling also eliminated the possibility of scrap parts due to distortion.
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.
Bearing inserts for replacement knees typically are made of 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. Under its Jabro brand, a ‘Premier Finish’ solid end mill was designed to meet the specific requirements of a leading global medical OEM. The condyle shape of both the femoral component and the bearing insert can be difficult to machine. Previous to the development of the Premier Finish endmills, 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 HSS cutters make it harder to achieve appropriate surface results and 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, the product design is based on concave and convex sections either tangent or connected with a straight line. Compared to mould and die tools the profile tolerances of the tools are quite generous. However, the manufacturing of 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.
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.
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 the features of the cutters and their capabilities in regard to quality, productivity and consistency, as well as the fact that they can reduce cost per part by up to five times, investment in these cutters is a truly worthwhile strategy.