Annegret Niesche

Electrical Bioimpedance-Controlled Surgical Instrumentation

A bioimpedance-controlled concept for bone cement milling during revision total hip replacement is presented. Normally, the surgeon manually removes bone cement using a hammer and chisel. However, this procedure is relatively rough and unintended harm may occur to tissue at any time. The proposed bioimpedance-controlled surgical instrumentation improves this process because, for example, most risks associated with bone cement removal are avoided. The electrical bioimpedance measurements enable online process-control by using the milling head as both a cutting tool and measurement electrode at the same time. Furthermore, a novel integrated surgical milling tool is introduced, which allows acquisition of electrical bioimpedance data for online control; these data are used as a process variable. Process identification is based on finite element method simulation and on experimental studies with a rapid control prototyping system. The control loop design includes the identified process model, the characterization of noise as being normally distributed and the filtering, which is necessary for sufficient accuracy ( ±0.5 mm). Also, in a comparative study, noise suppression is investigated in silico with a moving average filter and a Kalman filter. Finally, performance analysis shows that the bioimpedance-controlled surgical instrumentation may also performs effectively at a higher feed rate (e.g., 5 mm/s).

Modular design of a miniaturized surgical robot system.

Abstract Currently, there are only a small number of robotic systems used in various surgical fields. As modified industrial robot systems have shown significant limitations in the past, specialized kinematic solutions have been proposed for specific surgical applications. The majority of these systems are designed for specific applications in only a limited number of cases. The acquisition and operating costs are high, hindering the dissemination and broad clinical application of such systems. To address this problem, a modular mini-robot system is proposed, which can be easily adapted to different application-specific requirements. Therefore, the requirements of different applications have been categorized and clustered to a standardized requirement profile. Next, a modular robot based on a hybrid kinematic module structure has been developed. This concept has been implemented and tested in in vitro studies for different applications, such as revision total hip replacement and unicondylar knee arthroplasty. User-orientated tests of the intraoperative handling, as well as accuracy tests, proved the feasibility of the concept.

Estimation of Penetrated Bone Layers During Craniotomy via Bioimpedance Measurement

Objective: Craniotomy is the removal of a bone flap from the skull and is a first step in many neurosurgical interventions. During craniotomy, an efficient cut of the bone without injuring adjoining soft tissues is very critical. The aim of this study is to investigate the feasibility of estimating the currently penetrated cranial bone layer by means of bioimpedance measurement. Methods: A finite-element model was developed and a simulation study conducted. Simulations were performed at different positions along an elliptical cutting path and at three different operation areas. Finally, the validity of the simulation was demonstrated by an ex vivo experiment based on use of a bovine shoulder blade bone and a commercially available impedance meter. Results: The curve of the absolute impedance and phase exhibits characteristic changes at the transition from one bone layer to the next, which can be used to determine the bone layer last penetrated by the cutting tool. The bipolar electrode configuration is superior to the monopolar measurement. A horizontal electrode arrangement at the tip of the cutting tool produces the best results. Conclusion: This study successfully demonstrates the feasibility to detect the transition between cranial bone layers during craniotomy by bioimpedance measurements using electrodes located on the cutting tool. Significance: Based on the results of this study, bioimpedance measurement seems to be a promising option for intra operative ad hoc information about the bone layer currently penetrated and could contribute to patient safety during neurosurgery.

Modeling of Bioimpedance Spectroscopy Measurements for the Process Control of an Orthopedic Surgical Milling Tool

Surgical standard procedures such as the Revision Total Hip Replacement (RTHR) are essential to maintain individual mobility and life quality in aging industrial societies. As a consequence the number of these surgeries increases, but the used instruments do not guarantee the desired precision and application security in all cases. Due to this fact the integration of medical measurement methods like the Bioimpedance Spectroscopy (BIS), which is cheap, fast, accurate and unobtrusive, into surgical instruments for online process control and fault detection is suggested to solve this drawback. To improve this we developed an Impendence Controlled Surgical Instrument (ICOS), which enables BIS measurements during the Bone Cement (BC) removal by milling over the fast rotating milling head as active electrode. To establish BIS as a measured variable for the control setup we predict the BIS values for our ICOS in Finite Elements Method (FEM) simulations of the physiological operation scenario. Based on this an experimental in vitro setup is designed and validated with FEM simulations to enable the reproducible experiments and analysis of additional influencing variables. Beyond that we model the BC removal with a variable capacitive impedance in dependence of the residual BC thickness and a constant serial impedance offset. Finally we experimentally parameterize the model and a sensitivity analysis and first results of the feedback control will be given.

Femoral Test Bed for Impedance Controlled Surgical Instrumentation

The risk for patients during the standard procedure of revision of cemented artificial hip joints is unsatisfactorily highdue to its high level of invasiveness and limited access to the operative field. To reduce this risk we are developing anImpedance Controlled Surgical Instrumentation (ICOS) system, which aims to establish real-time control during a BoneCement (BC) milling process. For this, the relationship between the thickness of the BC and its frequency-dependentelectrical impedance is used to estimate the residual BC thickness. The aim is to avoid unintended cutting of boneby detecting the passage of the BC/bone boundary layer by the milling head. In a second step, an estimation of theresidual BC thickness will be used to improve process control. As a first step towards demonstrating the feasibility ofour approach, presented here are experimental studies to characterize the BC permittivity and to describe the process indetail. The results show that the permittivity properties of BC are dominated by its polymethyl methacrylate (PMMA)fraction. Thus, PMMA can be used as a substitute for future experiments. Furthermore, a Femoral Test Bed (FTB) wasdesigned. Using this setup we show it is feasible to accurately distinguish between slightly different thicknesses of BC.

Smart bioimpedance-controlled craniotomy: Concept and first experiments

Craniotomy is part of many neurosurgical interventions to create surgical access to intracranial structures. The procedure conventionally bears a high risk of unintended dural tears or damage of the soft tissue underneath the bone. A new synergistically controlled instrument has recently been introduced to address this problem by combining a soft tissue preserving saw with an automatic cutting depth control. Many approaches are known to obtain the information required on the local bone thickness. However, they suffer from unsatisfactory robustness against disturbances occurring during surgery and many approaches require additional intra- or preoperative steps in the workflow. This article presents first concepts for real-time cutting depth control based on in-process bioimpedance measurements. Furthermore, sensor integration into a synergistic surgical device incorporating a bidirectional oscillating saw is demonstrated and evaluated in first feasibility tests on a fresh bovine bone specimen. Results of bipolar measurements show that the transition of different layers of bicortical bone and bone breakthrough lead to characteristic impedance patterns that can be used for process control.