Alexander Korff

A novel concept for smart trepanation.

Abstract Trepanation of the skull is a common procedure in craniofacial and neurosurgical interventions, allowing access to the innermost cranial structures. Despite a careful advancement, injury of the dura mater represents a frequent complication during these cranial openings. The technology of computer-assisted surgery offers different support systems such as navigated tools and surgical robots. This article presents a novel technical approach toward an image- and sensor-based synergistic control of the cutting depth of a manually guided soft-tissue–preserving saw. Feasibility studies in a laboratory setup modeling relevant skull tissue parameters demonstrate that errors due to computed tomography or magnetic resonance image segmentation and registration, optical tracking, and mechanical tolerances of up to 2.5 mm, imminent to many computer-assisted surgery systems, can be compensated for by the cutting tool characteristics without damaging the dura. In conclusion, the feasibility of a computer-controlled trepanation system providing a safer and efficient trepanation has been demonstrated. Injuries of the dura mater can be avoided, and the bone cutting gap can be reduced to 0.5 mm with potential benefits for the reintegration of the bone flap.

User-Interaction of a Semiautomatic Trepanation System

During trepanation, a neurosurgical procedure for opening the skull, the protection of the underlying dura mater and a minimized loss of bone are major concerns. A concept for a novel trepanation system has been developed in order to open the skull less invasively and more safely. This trepanation system is based on a soft tissue preserving cutting tool and an autonomous control of the cutting depth.

Concept and evaluation of a synergistic controlled robotic instrument for trepanation in neurosurgery

A robotic instrument which synergistically cooperates with the surgeon for opening the skull in neurosurgery is proposed. To reduce frequent complications of this intervention, tear of the dura mater and bad reintegration of the skull bone a soft tissue preserving saw is combined with automatic depth regulation on the basis of a priori acquired medical imaging data (CT/MRI). By fusing the individual capabilities of the surgeon and a robotic device, it is possible to design an instrument which is significantly smaller than a fully autonomous system. The acceptance is enhanced by the integration of the surgeon into the process with direct control over the procedure. During the intervention, the instrument is manually guided by the surgeon on a freely defined trajectory. To be able to control this instrument, a method for real-time depth regulation, medical imaging data pre-processing and reduction as well as appropriate interfaces for the surgeon have been developed. In an experimental setup with phantom skull caps the system has been evaluated and has shown promising results, with a mean error of 0.62mm. Future work will include a detailed analysis of the persisting errors, integration of different sensors to control the instrument and preclinical trials.

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).

Real-time determination of skull thickness for a manually-navigated synergistic trepanation tool

Trepanation of the skull is a common procedure in neurosurgery with the problems of dural tears and wide cutting gaps. A hand-guided instrument containing a soft-tissue preserving saw whose cutting depth is automatically adapted on the basis of a-priori data (CT, MRI) is envisioned to reduce these problems.

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.

Smart Trepanation System: Preclinical Analysis of Safety, Efficiency, and User Satisfaction

BACKGROUND/OBJECTIVE To reduce the risk of dural tears during craniotomies and the associated complications, we developed the Smart Trepanation System (STS) that provides an image- and sensor-based automatic control of the cutting depth of a manually guided soft tissue preserving saw. This article presents the results of an initial user-centered evaluation. METHODS Interactive usability tests with six neurosurgeons were conducted. Resection time and accuracy were recorded in a standardized laboratory setting and compared with a standard craniotome. User satisfaction and subjective workload were assessed using the National Aeronautics and Space Administration Task Load Index scale and a questionnaire regarding intuitiveness, fault tolerance, learnability, and user satisfaction. RESULTS The mean resection time after getting used to the STS was 36.4 ± 9.2 second longer than with the conventional craniotome. All task load indexes except for the temporal demand were rated higher when using the STS, but all were rated smaller than 3 and thus classified as only a small extra task load. The questionnaire showed that the system is not only feasible but also accepted by surgeons and that the user interaction seems to be designed as intuitive, fault tolerant, and easy to learn. CONCLUSION Although the conventional craniotome seems to perform a trepanation faster and with less workload, the advantage of performing a dura-preserving trepanation with significantly smaller cutting gaps outweighs those disadvantages. For validation of those promising in vitro results, further studies have to be conducted in a fresh human cadaver model or in a clinical setting.

Ultrasound and optically controlled robotic instrument for resternotomy in cardiothoracic surgery

A surgical robotic instrument is introduced, which synergistically supports the surgeon in the application of resternotomy, an important part of many reoperations in heart- and thoracic surgery. A frequent complication occurring during this operation is injury of underlying soft tissue structures, which can even result in death of the patient. To improve safety, the proposed robotic instrument automatically adjusts the cutting depth on the basis of the related local bone thickness. To obtain anatomical information, most systems available are based on optical tracking and computed tomography. However, a combination of these modalities is not yet common in heart- and thoracic surgery. In this context, two different concepts for acquiring anatomical information and realizing real-time cutting depth adjustment are presented, using ultrasound and optical bone surface characteristics. Both approaches were evaluated in an experimental setup with phantom sternums demonstrating feasibility of the approach. Future work will include a combination of different sensors to improve robustness of the system and cadaver trials.

Using analytical redundancy to increase safety of a synergistic manually guided instrument for craniotomy

In this paper, two methods for bridging line of sight interruptions occurring during the use of the synergistically operated semiautomatic trepanation system (STS) are presented. In the STS, position information is acquired using an optical tracking system with the disadvantage of possible line-of-sight interruptions. Their compensation is crucial, as a real-time control system automatically adjusts the cutting depth of the instrument on the basis of position information and a-priori data. The surgeon is only responsible for guiding the instrument along the resection line. Hence, availability of position information is crucial for depth control, set point generation, and thus for patient safety. In favour of enhancing reliability of position and orientation acquisition, two approaches were developed which are intended to estimate the position during line of sight interruptions on the basis of a-priori system information and process parameters. To assure patients safety during this procedure, several parameters of the system (e.g. cutting radius, skull gradient) are used in order to estimate the possible cutting error while the redundant system is activated. These two algorithms and the online risk assessment were implemented, and afterwards evaluated. The evaluation was performed using a skull phantom, and yielded promising results.

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.