Features

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

Fig. 1 – Shown is a complete knee replacement assembly: the femoral component, the tibial tray, and the bearing insert.
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

Fig. 2 – The femoral component has rounded contours that mimic the bone formation at the end of the femur.
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.

Fig. 3 – The tibial tray has locking details that must be burr-free.
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)

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