SICOSI – Smart Impedance Controlled Osteotomy Instrumentation

Within the framework of the SICOSI project, it was examined, whether the proposed concept of a cutting depth setting based on bioimpedance measurements for osteotomy saws is technically feasible and whether a relevant improvement in patient safety can be achieved. These considerations were carried out for the example application of craniotomy. It was investigated whether the depth of cut at the skull bone during sawing can be reliably determined by bioimpedance spectroscopy. Initial results suggest that the saw depth can be controlled based on both a bipolar and a monopolar measuring principle with a maximum overcut of 2 mm (monopolar) or 1 mm (bipolar).

For a more precise analysis of the measurement concept in the example application of the craniotomy, a FEM model was created and its validity successfully tested in a laboratory experiment on a cattle shoulder blade. Based on this model, FEM simulation was used to demonstrate the feasibility of impedance-based bone layer detection, to determine the expected ideal impedance curve depending on the sawing depth, and to analyze the influence of different electrode arrangements on measurement sensitivity. A bipolar arrangement with the smallest possible electrode area in the lowest area of the saw blade was determined to be the best arrangement with regard to measurement sensitivity. Apart from the quantitative magnitude of the measurement amplitudes, there was no qualitative frequency dependence of the impedance curve, so that a measurement at a fixed frequency can be regarded as sufficient, which basically makes faster measurements possible.

In order to integrate impedance measurement into a surgical saw instrument, a ceramic saw blade was designed that allowed the application of two electrode surfaces insulated from each other. Laboratory experiments have shown that the ceramic saw blade and the correspondingly adapted saw instrument are suitable for impedance measurement during the sawing process. After the first variants of the saw blade still had a thickness of 3 mm, this could be reduced to 1 mm.

Based on the impedance measurement data, a control system for the cutting depth was developed and integrated into a real-time control environment. The control was calibrated with the impedances of the individual layers of bicortical bone and air and simulated on different cutting paths. For different rotational speeds, the optimum point was determined during saw blade oscillation at which the measurement is to be triggered in order to measure the bioimpedance at the lowest point of oscillation. In a first experiment, it was shown that the control functions on the real controlled system with integrated measuring system works. The determined overcut deviated only minimally (0.2 mm) from the simulation value.

In order to enable the correct and self-explanatory operation of the instrument independently of the operator, the human-machine interface of the instrument was modelled. For this purpose, possible errors during commissioning and intraoperative operation of the system were analyzed and included in the design of the interface. The designed interface supports the surgeon with proactive hints to achieve optimal guidance of the instrument. The instrument uses multimodal information channels to provide feedback on all relevant conditions, deviations from the optimum sawing process and the approach to critical conditions even before they occur. In addition, the interface provides redundant confirmations of critical process steps in order to avoid errors during operation. Furthermore, the interface also allows the instrument to be operated in states in which the automatic cutting depth control can only work to a limited extent. This could be the case at transitions between the bone plates of the skull as it is difficult to follow a clean cutting path even with conventional craniotomes.

In series of experiments with the revised entire system, promising results were achieved in experiments on skull models. In two out of four trajectories, the saw blade was able to follow the lower edge of the bone well; in three of the four trajectories, the permitted overcut of 1.5 mm was not exceeded. In view of the very early stage of development, this shows that the measuring principle used with the control system designed for this purpose is in principle suitable for carrying out craniotomies and similar operations.