The Institute of Production Engineering and Machine Tools (IFW) at the Leibniz University in Hannover, Germany, focuses on all aspects of machining, from the machining process and the development of machinery to production planning and organisation. In doing so, the institute combines experimental, theoretical and simulation-based methods, undertaking both basic and applied research and development work. The close link between the university and industry is crucial to the institute's activities. IFW turned to toolcraft while carrying out a research and development project on the development of a machining tool capable of cutting bone without causing thermal-induced osteonecrosis. The project was funded by the German Federal Ministry for Economic Affairs and Energy as part of the Central Innovation Programme for SMEs (ZIM).
The Institute of Production Engineering and Machine Tools (IFW) at the Leibniz University in Hannover
The Starting Point:
Innovative method for innovative products
The heat produced when cutting bone may cause tissue damage. This occurs at temperatures of around 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. To date, surgery has been performed iteratively, i.e. the drilling process is repeatedly interrupted in order to keep the temperature as low as possible. However, thanks to metal laser melting, it is possible to manufacture drills with integrated cooling ducts. This allows 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.
Consistently low temperatures
The internally cooled prototype was modelled on a conventional bone drill with a diameter of 6 mm. 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 1.2 mm 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
- Creation of a project team between IFW, toolcraft and Schmidt WFT
- Determination of the cooling capacity and design in terms of the volumetric flow rate, temperature and thermal capacity of the coolant
- Development of a method for bringing a closed cooling circuit into the tool substrate while maintaining tool stability and ensuring that the tool is suitable for performing the required processes
- Selection of the material (biocompatible material 1.4404)
- Design of the shape of the drill and the internal cooling ducts using CAD and simulation software (Schmidt WFT)
- Production of the drill using 3D printing
- Cutting of the tool to its final size
- Practical tests performed by IFW – drilling and measuring the process temperature in bone (artificial and bovine bone)
- Use of water as a coolant
- Reference temperature measurements when the tool cooling system is turned on and off and at higher and lower feed rates
Metal laser melting finds its way into the operating theatre
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.
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