Metal laser melting continues to pave the way for ground-breaking success in the medical technology sector. One example is the use of a 3D-printed bone drill, capable of preventing possible tissue damage during operations with traditionally manufactured drills.
The Institute of Production Engineering and Machine Tools (IFW) at the Leibniz University in Hannover, Germany, turned to Toolcraft while carrying out a research and development project for a machining tool capable of cutting bone without causing thermal-induced osteonecrosis – heat produced when cutting bone may cause tissue damage, which occurs at temperatures ~48°C and higher. Since cooling the tool may cause fluid to enter the wound, it is not possible to use conventional tools with a cooling system. Presently, surgery is performed iteratively, where the drilling process is repeatedly interrupted in order to keep the temperature as low as possible.
Consistently low temperatures
Today, the use of metal laser melting makes it possible to manufacture drills with integrated cooling ducts, allowing the coolant to flow inside the tool – along the helix and back to the tool holder – without coming into contact with the wound. Toolcraft also developed a non-rotating pre-spindle attachment with an inflow and outflow function for the coolant. A continuous supply of coolant is ensured by the attached coolant tank and pump. The internally cooled prototype was modelled on a conventional bone drill with a diameter of 6mm. The drill's shape had to be kept the same to make it easier for users to adapt to the new tool. It was also important that the material used is well tolerated by patients. A flow and return pipe ensures a continuous flow of coolant. The internal circular cooling ducts with a diameter of 1.2mm take the thermal energy away from the cutting edge. Horizontal drilled holes were added which link the cooling circuit to the drill so that the coolant can be supplied and taken away. A circlip groove is used to attach the manifold. Sealing the two chambers in the manifold posed a further challenge.
Step by step to success
At the outset of the project, the cooling capacity was determined in terms of the volumetric flow rate, temperature, and thermal capacity of the coolant. The project team then developed a method for bringing a closed cooling circuit into the tool substrate while maintaining tool stability and ensuring that the tool was suitable for performing the required processes. The biocompatible material 1.4404 was selected for the drill and its shape and the internal cooling ducts was designed by Schmidt WFT using CAD and simulation software. After the drill was produced using 3D printing and the tool was machined to its final size, practical tests were performed by IFW. Using water as a coolant, IFW drilled and measured the process temperature in artificial and bovine bone, measuring reference temperatures at higher and lower feed rates and when the tool cooling system was turned on and off.
Improving surgical procedures
The results of the drilling tests show that the innovative drill significantly reduces the temperature produced by up to around 70%. Thanks to the ability of the internal cooling system to compensate for increases in temperature, low feed rates no longer lead to higher temperatures. The choice of tool has a considerable impact on the success of bone surgery. Excessively high process temperatures put bones at risk of damage during almost all bone cutting procedures. This means that the technology could also be beneficial in a wide variety of other areas, such as the manufacture of saws.