[Empirical studies of the reliability of surgical navigation systems]

A number of studies demonstrate the clinical relevance of systems for computer assisted surgery (CAS systems). As however studies on human error in medicine indicate, reliability of the results strongly depends on aspects of usability and error tolerance of the system. This paper presents studies which aim to assess these aspects of reliability of CAS systems. In a clinical study, interaction with a CAS system by 16 expert and novice surgeons was observed and assessed. From 133 recorded incidents 41% were rated to have significant impact on the clinical result or to inhibit successful completion of the task, which indicates a low degree of error tolerance of the system. These findings are supported by the results obtained from questionnaire, were learnability and error tolerance were judged to be not sufficient.

[Developing authoring tools for web-based multi-media orthopedics education modules]

In the framework of the EC-Project VOEU ("Virtual Orthopaedic European University"), authoring tools have been developed to support the implementation of online interactive courses for multimedia orthopaedic educational modules. Based on the pedagogical concept of case-based, problem-oriented learning, different user-interaction scenarios and Learning Objects (LOs) were analyzed and developed. Each LO acts as an interactive dialogue presenting one kind of question with certain rules and interactions. The multimedia course content entailing anamnesis, diagnosis and therapy is managed in a database and can be adaptively generated as certain LO with the help of Active Server Pages (ASPs). As an example, an interactive course on Developmental Dislocation of Hip has been implemented.

[CRANIO--development of a system for computer- and robot-assisted craniotomy]

In the framework of the DFG project CRANIO, we are dealing with the problems of computer-assisted planning and robot-assisted realization of craniotomy, with optional skull reconstruction. A CT dataset was obtained from a phantom skull model and basic operation planning as well as different registration methods were performed. As one option it is possible to mill a contour on the skull or even to remove the entire area with a robot-guided microsurgical milling tool. In this context, work space tests were performed with a hexapod parallel robot. The milling of any contour geometry could be realized with different tool angles using the existing CRIGOS robot system without additional axes.

[Visual user guidance for registration based on A-mode ultrasound]

In this study we investigate the use of an A-Mode ultrasound probe tracked by a mechanical localizer system (MicroScribe 3D) for non-invasive transcutaneous palpation and registration of bone surface points. A CT of a femur-model has been scanned and subsequently processed by segmentation and 3D-reconstruction. During the ultrasound registration, a computer based assistance tool helped guiding the alignment of the ultrasound probe. Three different modes of registration have been compared. Process times for registration have been recorded and compared. The results for using an A-Mode ultrasound system demonstrate a translation RMS accuracy of 0.58 mm. Mean ultrasound registration time has been measured to 108 sec. for palpation of 10 bone surface points of distal part of femur.

An automatic camera-holding system for gynecologic laparoscopy

Ergonomic analysis of conditions prevailing during laparoscopy show that static holding work causes considerable problems. The surgeon's assistant is particularly affected, for example, by holding the endoscope optical system. To minimize strain for the entire operating team and to increase the range of activities that can be performed by the assistant, an automatic surgical camera-holding system was developed. Its value in clinical practice was tested in gynecologic laparoscopy, where it enabled the surgical team to work more comfortably.

Computer-assisted orthopedic surgery with individual templates and comparison to conventional operation method

STUDY DESIGN

Comparison was made of the accuracy of a pedicle bore performed by conventional technique and by using an individual template in the lumbar spine of cadavers.

OBJECTIVES

The fixation of pedicle screws necessitates a high amount of surgical skill and experience to avoid lesions of nerves and vessels. By using individual templates in a cadaver study the goal was to prove the accuracy and efficiency of this less-invasive image-guided surgery in comparison with the conventional technique by fluoroscopy and computed tomographic (CT) scan.

SUMMARY OF BACKGROUND DATA

Based on three-dimensional models generated from CT scans of the lumbar spine, precise preoperative planning of the position and trajectory of pedicle screws is possible. In comparison with other means of computer-assisted spine surgery with navigation systems, in which a time-consuming intraoperative matching of the bone surface structure is necessary, the use of individual templates enables the surgeon to reduce the operation time considerably.

METHODS

Individual templates are customized on the basis of three-dimensional reconstructions of the bone structures extracted from CT image data and depending on the individual preoperative surgical planning, which uses the desktop image processing system for orthopedic surgery (DISOS). A desktop-computer-controlled milling device is used as a three-dimensional printer to automatically mold the shape of small reference areas of the bone surface into the body of the template. Postoperative CT scans were obtained and the accuracy of the pedicle bore rated by two independent observers.

RESULTS

The preparation time with the individual template lasted slightly longer than with the conventional operation technique (555 seconds and 482 seconds, respectively). Fluoroscopic study took a mean time of 31.5 seconds, with the conventional operation technique and 5.5 seconds with the individual template. The assessment of the postoperative CT scans demonstrated a higher accuracy of the pedicle bore with the individual template.

CONCLUSIONS

This cadaveric study has shown that overall operation time including the fluoroscopy time can be shortened by using the individual template for the pedicle bore. The individual template is an alternative to the computer-assisted navigation systems with a good cost-performance ratio without excessive technical workload on the physicians or the surgical personnel. Further investigations must be conducted to validate the clinical applicability of this system.

Critical evaluation of known bone material properties to realize anisotropic FE-simulation of the proximal femur

PURPOSE

In a meta-analysis of the literature we evaluated the present knowledge of the material properties of cortical and cancellous bone to answer the question whether the available data are sufficient to realize anisotropic finite element (FE)-models of the proximal femur.

MATERIAL AND METHOD

All studies that met the following criteria were analyzed: Young's modulus, tensile, compressive and torsional strengths, Poisson's ratio, the shear modulus and the viscoelastic properties had to be determined experimentally. The experiments had to be carried out in a moist environment and at room temperature with freshly removed and untreated human cadaverous femurs. All material properties had to be determined in defined load directions (axial, transverse) and should have been correlated to apparent density (g/cm(3)), reflecting the individually variable and age-dependent changes of bone material properties.

RESULTS

Differences in Young's modulus of cortical [cancellous] bone at a rate of between 33% (58%) (at low apparent density) and 62% (80%) (at high apparent density), are higher in the axial than in the transverse load direction. Similar results have been seen for the compressive strength of femoral bone. For the tensile and torsional strengths, Poisson's ratio and the shear modulus, only ultimate values have been found without a correlation to apparent density. For the viscoelastic behaviour of bone only data of cortical bone and in axial load direction have been described up to now.

CONCLUSIONS

Anisotropic FE-models of the femur could be realized for most part with the summarized material properties of bone if characterized by apparent density and load directions. Because several mechanical properties have not been correlated to these main criteria, further experimental investigations will be necessary in future.

[Planning and performance of orthopedic surgery with the help of individual templates]

Operational interventions can be planned and simulated accurately with the help of a three-dimensional reconstruction of the anatomical bone structures, created from the tomographic data of a patient. Computer-assisted navigation systems or robotic systems can assist the intraoperative realization of the planned intervention and may support the spatial orientation within the operational field. As an inexpensive alternative without the high excessive technical workload on the physicians or the surgical personnel, individual templates were developed in the Helmholtz Institute Aachen. In this approach the negative three-dimensional bone surface, known exactly from the 3D-reconstruction of the patient's bone by the planning system DISOS, is milled into a small cubic block of polycarbonate. During the intervention, the template serves as a reference for the spatial orientation and as a tool-guide for cutting or milling of the bone according to the previous planning. Clinical and experimental studies have shown that operation times as well as intraoperative X-ray times can be shortened by the use of individual templates, while preserving the task sequence of the conventional operation procedures.

[CRIGOS: development of a compact robot for image-guided orthopedic surgery]

In this paper, we present a medical robot system dedicated to support the surgeon during challenging tasks within orthopedic interventions. The main goal of this work is to develop a system as technically simple as the surgical requirements allow to keep its cost and complexity to a minimum. Therefore, we primarily focus on calibrated X-ray imaging for image acquisition, an easy registration procedure and robotic execution using a positioning device with simple parallel kinematics. Examples of different orthopedic interventions using the compact robot system for image-guided orthopedic surgery (CRIGOS) are presented, as well as various modes of execution of the device.

CRIGOS: a compact robot for image-guided orthopedic surgery

The CRIGOS (compact robot for image-guided orthopedic surgery) project was set up for the development of a compact surgical robot system for image-guided orthopedic surgery based on user requirements. The modular system comprises a compact parallel robot and a software system for planning of the surgical interventions and for supervision of the robotic device. Because it is not sufficient to consider only technical aspects in order to improve in clinical routine the therapeutic outcome of conventional interventions, a user-centered and task-oriented design process has been developed which also takes human factors into account. The design process for the CRIGOS system was started from requirement analysis of various orthopedic interventions using information gathered from literature, questionnaires, and workshops with domain experts. This resulted in identification of conventional interventions for which the robotic system would improve the medical and procedural quality. A system design concept has been elaborated which includes definitions of components, functionalities, and interfaces. Approaches to the acquisition of calibrated X-rays will be presented in the paper together with design and evaluation of a first human-computer interface. Finally, the first labtype parallel robot based on low-cost standard components is presented together with the first evaluation results concerning positioning accuracy.

Computer assisted orthopaedic surgery with image based individual templates

Recent developments in computer assisted surgery offer promising solutions for the translation of the high accuracy of the preoperative imaging and planning into precise intraoperative surgery. Broad clinical application is hindered by high costs, additional time during intervention, problems of intraoperative man and machine interaction, and the spatially constrained arrangement of additional equipment within the operating theater. An alternative technique for computerized tomographic image based preoperative three-dimensional planning and precise surgery on bone structures using individual templates has been developed. For the preoperative customization of these mechanical tool guides, a desktop computer controlled milling device is used as a three-dimensional printer to mold the shape of small reference areas of the bone surface automatically into the body of the template. Thus, the planned position and orientation of the tool guide in spatial relation to bone is stored in a structural way and can be reproduced intraoperatively by adjusting the position of the customized contact faces of the template until the location of exact fit to the bone is found. No additional computerized equipment or time is needed during surgery. The feasibility of this approach has been shown in spine, hip, and knee surgery, and it has been applied clinically for pelvic repositioning osteotomies in acetabular dysplasia therapy.

The Impact of 3-D Video Endoscopy on Binocular Perception and Visually Guided Manipulation

Monocular video endoscopic systems are established in the clinical routine of surgical endoscopy. The introduction of 3-D video systems could improve visualisation of the intracorporal operating site because of the stereoscopic depth information. The goal of our investigations has been to quantify the influence of this visualisation technology on visual perception, on visually controlled endoscopic manipulations, and on the intraoperative performance, including ergonomic and psychophysical aspects. These results are used to define guidelines for improvement and for the integration of such systems into clinical routine so as to achieve optimal support of the medical team.

The comparison of 2-D and 3-D video endoscopic systems showed a general improvement in the performance of endoscopic procedures. However, 30% – 50% of the users had perceptive problems with 3-D endoscopy. To study the problems quantitatively, we compared the case of stereoscopic visualisation with the real situation of direct view onto the specific objects. The users with problems had insufficient binocular depth perception of stereoscopic images for visual discrimination tasks, although their depth perception of real objects was good. Analysis of their eye movements showed significant differences compared with those of users with good binocular depth perception of stereo images. In particular, there were differences in the relation of vergence movements and accommodation. When we compared visually guided manipulations under stereoscopic video sight and direct view, we found the overall manipulative performance of all users to be the same, but the users with problems showed a lower performance in general.

The experimental design and the results are discussed in detail.

Three-dimensional versus two-dimensional video endoscopy. A clinical field study in laparoscopic application

Stereoscopic visualisation systems are available for the medical application in clinical routine, especially in the field of endoscopic or minimal invasive surgery. As we have shown in laboratory studies in 1991 the use of stereoscopic visualisation systems leads to a significant improvement of endoscopic manipulation. A broader diffusion of these systems in clinical routine will be based on quantitative evaluation of the influence of stereoscopic visualisation systems on the intervention. As a first approach we performed a clinical field study to compare 2D and 3D video endoscopy in laparoscopic interventions in urological surgery.

The influence of LCD shutter glasses on spatial perception in stereoscopic visualization

For a general establishment of stereoscopic visualisation systems in clinical routine a fundamental analysis of the influence of technical, optical and physiological parameters onto visual spatial perception seems to be necessary to achieve an optimisation of the existing devices. As one important component of these systems we perform studies on the influence of LCD Shutter glasses on the individual binocular perception. The developed shutter system, the initial experiments and their results are presented.

Using human factor analysis and VR simulation techniques for the optimization of the surgical worksystem

The introduction of minimal invasive operating techniques into the conventional surgical worksystem, requires a fundamental analysis of the resulting problems. Based on ergonomic investigations, concerning working postures, static holding work and task sequence analysis, bottlenecks of workplace design can be demonstrated. As a contribution to the optimization of workplace design, the development of a CAD-based VR simulation environment with an integrated anthropometric man-model, as a very useful tool for anthropometric system design is presented.

A desktop image processing system for computer-assisted orthopedic surgery (DISOS)

Within the framework of our research activities in orthopedic surgery a PC-based imageprocessing system for the processing of x-ray computer-tomographic data has been developed. The system is used as a work platform for the investigations concerning interaction modalities, strategies and techniques for an efficient planning and execution of surgical interventions in orthopedic surgery by means of individual templates. The method of individual templates has been presented in /1,2/ and will not be subject of this paper.