ADDITIVE MANUFACTURING is the magazine devoted to industrial applications of 3D printing and digital layering technology. We cover the promise and the challenges of this technology for making functional tooling and end-use production parts.
Issue link: https://am.epubxp.com/i/894355
NOVEMBER 2017 Additive Manufacturing 48 TAKING SHAPE A surgical drill bit resembles a commercially available industrial drill. The operational environment, however, is very different. Bone is a complex anisotropic, porous and viscoelastic composite that is also nonhomogenous, both in material properties and geometry. Cortical bone is also a relatively poor conductor of heat, notably the heat caused by drilling. And it has been estimated that approximately 60 percent of the heat energy generated during drilling is dissipated by bone chips, which is substantially less than the 80 percent likely to be removed by chips during, say, the drilling of metals. As a result, temperatures during bone drilling can rise above 50°C (122°F), which can result in the necrosis (death) of the bone. To keep the temperature low, surgery is usually performed iteratively, with the drilling process repeatedly interrupted. Cooling the drill would be better. And while closed-loop and open internal cooling systems are available, these are primarily lim- ited to orthodontic and dental applications. Since cooling the tool during surgery may cause fluid to enter the wound, tools with a cooling system are usually avoided. In an effort to develop surgical drills capable of cutting bone without causing ther- mal-induced osteonecrosis, the Institute of Production Engineering and Machine Tools (IFW) at the Leibniz Universität of Hannover, Germany, turned to Toolcraft, a manufacturer of precision parts, assemblies, molds and injection-molded parts. Toolcraft suggested creating internal, conformal cooling channels through metal laser melting to allow water to flow as coolant inside the tool and near the cutting edg- es—along the helix and back to the toolholder—without entering the wound. Although chances of breakage are slim, the drill bit is reportedly the most frequently broken surgical instrument. Using water ensures no harm is caused in case this happens. Toolcraft also developed a nonrotating pre-spindle attachment with an inflow and outflow function for the coolant. A continuous supply of coolant is provided by the attached tank and pump. The internally cooled prototype was modelled on a conventional bone drill with a diameter of 6 mm. Toolcraft says diameters from 5 mm are currently possible to pro- duce using laser melting technology (through surgical drills are available in sizes much smaller than this). Toolcraft and the IFW decided to keep the drill's geometry to make it easier for users to adapt to the new tool. The internal cooling channels take the thermal energy away from the cutting edge, while horizontal drilled holes were added to link the cooling circuit to the drill for the coolant supply and removal. The drill is made of biocompatible stainless steel 316L. Using an M2 multi-laser machine from Concept Laser, the drill bit and the internal cooling channels were ad- ditively manufactured, followed by centerless grinding to achieve the required surface quality as well as sharp cutting edges. The IFW performed diverse tests with the drill and measured the process tempera- ture in artificial and bovine bone. Traditionally, high feed rates have helped decrease 3D-Printed Medical Drills Keep Bone Cool During Surgery By Barbara Schulz Researchers created an internally cooled bone drill that compensates for heat in drilling, protecting the bone from damage.