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European Journal of Cardio-thoracic Surgery 16 (Suppl. 2) (1999) S97±S105 www.elsevier.com/locate/ejcts

Robotics and systems technology for advanced endoscopic procedures: experiences in general surgery q Marc O. Schurr*, Alberto Arezzo, Gerhard F. Buess Section for Minimally Invasive Surgery, Eberhard-Karls University, Tuebingen, Germany

Abstract The advent of endoscopic techniques changed surgery in many regards. This paper intends to describe an overview about technologies to facilitate endoscopic surgery. The systems described have been developed for the use in general surgery, but an easy application also in the ®eld of cardiac surgery seems realistic. The introduction of system technology and robotic technology enables today to design a highly ergonomic solo-surgery platform. To relief the surgeon from fatigue we developed a new chair dedicated to the functional needs of endoscopic surgery. The foot pedals for high frequency, suction and irrigation are integrated into the basis of the chair. The chair is driven by electric motors controlled with an additional foot pedal joystick to achieve the desired position in the OR. A major enhancement for endoscopic technology is the introduction of robotic technology to design assisting devices for solo-surgery and manipulators for microsurgical instrumentation. A further step in the employment of robotic technology is the design of `master-slave manipulators' to provide the surgeon with additional degrees of freedom of instrumentation. In 1996 a ®rst prototype of an endoscopic manipulator system, named ARTEMIS, could be used in experimental applications. The system consists of a user station (master) and an instrument station (slave). The surgeon sits at a console which integrates endoscopic monitors, communication facilities and two master devices to control the two slave arms which are mounted to the operating table. Clinical use of the system, however, will require further development in the area of slave mechanics and the control system. Finally the implementation of telecommunication technology in combination with robotic instruments will open new frontiers, such as teleconsulting, teleassistance and telemanipulation. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Endoscopic surgery; Robotics; Solo-surgery; Systems; Telemedicine

1. Introduction 1.1. The role of technology in minimally invasive surgery The advent of endoscopic techniques changed surgery in many regards. Besides the adaptation to new kinds of instrumentation and operative maneuvers, the surgeon had to cope with a full range of new devices resulting in a signi®cant change of his work-place environment. Starting out from the early 1990s, the design of dedicated surgical work-place systems for minimally invasive surgery still is a major topic for research and industry. With the increasing complexity of endoscopic surgery in the various clinical specialties, such as general, cardiac or gynaecologic surgery, came the demand for improved instrumentation. This paper intends to describe an overview q Presented at the International Symposium `Present State of Minimally Invasive Cardiac Surgery ± Meet the Experts', Dresden, Germany, December 3±5, 1998. * Corresponding author. Section for Minimally Invasive Surgery, Eberhard-Karls University, Waldhoernlestrasse 22, D-72072 Tuebingen, Germany. Fax: 149-7071-295-569.

about technologies to facilitate endoscopic surgery. The systems described have been developed for the use in general surgery. However, due to the technical similarities in all surgical disciplines using minimally invasive techniques, devices and experiences collected with them should be transferable to cardiac surgery as well. Besides improvements in the ®eld of endoscopic vision systems [1±3] leading research groups world wide focus their scienti®c interest especially on the increase of instrument functionality. Robotics was soon recognized as a major pacemaker in the drive towards the technological future of endoscopic surgery. The initial steps in the use of robotics for increasing instrument functions were in the ®eld of endoscope guidance, where robotic instrument holders were employed to direct the endoscope during surgery [3±5]. Robotic endoscope manipulators proved to be safe and ef®cient in various ®elds of use and are now accepted as assisting devices among endoscopic surgeons [6]. The ®eld of robotics for enhancing surgical instrumentation did not emerge as quickly as the former due to its higher technical complexity and safety questions linked to the use of robotic devices for surgical tissue manipulation. It is,

1010-7940/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 1010-794 0(99)00281-X

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M.O. Schurr et al. / European Journal of Cardio-thoracic Surgery 16 (Suppl. 2) (1999) S97±S105

however, of key scienti®c and clinical interest for endoscopic suturing. Besides robotics, systems technology will have signi®cant impact on work processes in the OR. The integration of the different devices used for endoscopic operations into systems structures, which are easy to control and to maintain, is an important prerequisite for optimizing processes and resource allocation in surgery. Finally, telecommunication technologies applied to surgery will help to leverage surgical expertise among centers, facilitate information transfer and accelerate the diffusion of surgical techniques among leading centers. 2. Enabling technologies in the endoscopic OR New technologies are of great help in the design of instruments for endoscopic OR. The introduction of system technology and robotic technology enables today to design a highly ergonomic solo-surgery platform. With the addition of telecommunication technology assistance, consulting and manipulation from a remote distance become possible. 3. Systems technology to create an ergonomic work place in the OR Minimally invasive interventions require a multitude of technical devices, such as cameras, light-sources, highfrequency and isuf¯ation. The devices used today often represent stand-alone units. They need to be put into the OR and set-up right before each speci®c surgery. From each of the single devices, cables, hoses and other supplies lead into the operative ®eld. They have to be connected on both sides. This does not only cause long set-up times in the OR but is also a source for infringement of the sterility of the operating ®eld. The major drawback of the standard solution lies in the lack of direct control of the devices and the confusing display of parameters and technical status of the single devices. The ®rst approach to solve these problems has been made with the Dornier OREST system in the early 1990s [7]. OREST (Fig. 1) integrates all devices into a mobile cabinet. The single devices are connected to a central computer and can be controlled remotely by the surgeon via a multifunctional monitor and input panel. This panel also informs about all function parameters on a graphical display. All supplies are guided into the sterile ®eld through an articulated arm. Up to four multi-plugs are used to connect all lines at a central terminal within the sterile area. Based on the same principle, other system solutions are now available on the market from different companies, such as the ENDOSURG system (Olympus, Tokyo) and the HERMES system (Computer Motion, Goleta). The posture of the surgeon during endoscopic interventions differs signi®cantly from regular open procedures. The long shafts of the instruments and the ®xation of the line of

Fig. 1. OREST II system (Dornier, Germany).

sight to the video monitor decrease the freedom of motion of the surgeon. Compared to open surgery the endoscopic surgeon remains ®xed to his position during the operation with little opportunity to move his body and change his posture. This ®xation in a relatively unergonomic position can cause fatigue especially during longer interventions. We developed a new surgeon's chair dedicated to the functional needs of endoscopic surgery (Fig. 2). The foot pedals for high frequency, suction and irrigation are integrated into the basis of the chair. This provides for an intuitive alignment of the foot pedals to the feet of the surgeon. The chair is driven by electric motors controlled with an additional foot pedal joystick. The seat offers a special ergonomic shape, which allows both comfortable sitting in a semi-standing position and inclination towards the OR table without slipping off. The chair is applicable for various kinds of endoscopic surgery. The device can be used in conventional team surgery with one surgeon and one or two assistants. However the combination with endoscopic solo-surgery techniques seems particularly attractive.

4. Robotics and solo-surgery Currently robotics technology has two main applications in minimally invasive surgery: assisting devices for solo-


Fig. 2. Ergonomic chair for endoscopic surgery.

surgery and robotic manipulators for enhanced microsurgical instrumentation. 4.1. Solo-surgery Since minimally invasive surgery appeared, the vision of the operating surgeon has depended by an assistant surgeon responsible for positioning the endoscope. This task requires to keep the surgical point of interest in the center of the video frame, providing an appropriate magni®cation and maintaining a horizontal image. The use of positioning devices returns direct control of the whole procedure to the operating surgeon. This increases precision of action and reduces costs. In the past, mechanical arms and pneumatically assisted devices derived from open surgery were used. The lack in ergonomy resulted in a scarce diffusion of use of these systems. The introduction of manipulator technology, has opened new frontiers in the development of MIS systems. The ®rst endoscopic positioning system appeared on the market was the AESOP arm (Computer Motion, Goleta) in 1995 [5]. It moves the endoscope around a pivoting point. Up to now more than 50 000 procedures have been performed in the United States. The experience in clinical settings was used to upgrade the system substantially, ®rst in the version named 2000, with the implementation of a voice control system, then, in the least version, named 3000, with a second joint that allows to reduce space requirements. Another system also available on the market is the ENDOASSIST (Armstrong Helthcare, UK). The system moves the endoscope around an invariant point of constraint motion, that has to be pre-adjusted. A pointer is placed in the front of a helmet held by the surgeon and a visual detector

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over the monitor. This way each movement of the head is detected and the information transferred to the computer that moves the optic in a correspondent way. The system is ®xed on a trolley and its architecture design has minimum space requirements. In cooperation with the Research Center, Karlsruhe we designed a passive positioning arm, named TISKA Endoarm [8] (Fig. 3) and a remote controlled optic positioning arm, named FIPS Endoarm, for endoscopic surgery. Both prototypes are based on an architecture that ®xes an invariant point of constraint motion. TISKA Endoarm maintains a stable position by means of an electromagnetic friction that is released by footpedal. The operation of the system is possible by using only one hand. The system is highly appreciated when used as instrument retractor. FIPS Endoarm (Fig. 4) is an optic positioning system driven by voice control or by a ®nger ring joystick which is clipped on the handle of the operating instrument. As the tip of the second ®nger is introduced in the controller, its movements correspond to the movement of the optic. In different trials conducted on phantoms at our institution more than 300 procedures were performed. Endoscopic solo-surgery proved feasibility and safety. A number of surgeons involved in the study declared positive judgment in terms of comfort for each of the different interfaces tested. All endoscope positioning systems combined with an instrument retractor allowed a precise and comfortable control of the optic. With all combination of systems operative time requirement was shorter than human assistance. Nevertheless, the shorter operative time was registered when the endoscope was driven by a passive system TISKA Endoarm. This demonstrates the important role of the man-machine interface, that in none of the solutions tested showed an intuitiveness and reliability competitive with hand positioning. The trials conducted allowed us also to focus on the crucial aspect concerning the position of the devices around the operating table. We were able to de®ne criteria, how to perform endoscopic solo-surgery ergonomically (Fig. 5). To limit possible interferences between surgeon and the assist devices, these have to be placed all opposite to the surgeon. The surgeon works in a comfortable position having access to the surgical ®eld by means of a conventional straight instrument and a curved instrument as designed by Cuschieri. 4.2. Robotic microsurgical manipulators In endoscopic operations the mobility of the instrument is signi®cantly decreased due to the invariant point of insertion through the patient's abdominal or thoracic wall [9±11]. Only four of the seven `natural' degrees of freedom (DOF) of the human arm remain [11]. This reduction of instrument mobility is not an issue in simple manipulations dominating laparoscopic cholecystectomy or hernia surgery, but it can lead to signi®cant handling restrictions

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Fig. 3. TISKA endoarm.

for the surgeon in interventions where complex dissection or tissue connection techniques are required. Complex, user driven robotic systems, called `masterslave manipulators' have been developed to provide the surgeon with additional DOF of instrumentation. The master-slave mode of operation is a control principle in which all movements done with a master input device are transformed in real-time to the slave output device [12,13]. The entire manipulator system can be guided by the user, no pre-programmed `robotic' motion is happening. Our own development in the area of surgical robotics started in 1991, together with the Karlsruhe Research Center, Karlsruhe, Germany. In 1996 a ®rst prototype of an endoscopic manipulator system could be used in experimental applications. The ARTEMIS (Advanced Robotic TElemanipulator for Minimally Invasive Surgery) manipulator system has two basic components, the user station (master) and the instrument station (slave). The surgeon sits at a console which integrates endoscopic monitors, communication facilities and two master devices to control the two slave arms which are mounted to the operating table. The slave arm is an external kinematic unit to guide a steerable instrument around the invariant point of insertion into the body of the patient. The arm has two segments and four joints, which are driven by integrated electromotors. The steerable instruments have a bending section, which allows to incline the tip around 908. The functional unit of both, steerable instrument and

guiding arm, restores full spatial mobility of the instrument tip with 6 DOF of motion. Two slave units (Fig. 6) can be attached to the operating table. The surgeon's work place comprises an endoscopic 3D monitor for visualization of the operative ®eld, as well as two additional monitors for display of a graphical model of the slave arms and various system data. After appropriate system function and safety could be proven in phantom models, animal experiments were performed in domestic pigs (female, weight approx. 50 kg) under general anesthesia. ARTEMIS arm was employed for mobilization of the sigmoid colon and ligation of sigmoid vessels for laparoscopic sigmoidectomy. The master arm was positioned aside the operating table, the surgeon was in a sitting position (Fig. 7). After dissection of the sigmoid colon and fenestration of the mesentery the blood vessels were encircled and ligated with the ¯exible section of the steerable instrument. Several ligatures were placed at different heights of the colon. The maneuver was easy to perform. It was found, however, that geometric changes in the ¯exible tip section are required to improve practicality of the device. Further steps of surgical evaluation are planned after modi®cation of the prototype. 5. Telecommunication technology Telematics applications are of interest in various ®elds of


Fig. 4. FIPS endoscope guiding system.

medicine. In therapeutic areas, such as all surgical disciplines, telemedicine has future relevance, but is not as widespread as on the diagnostic side. Three levels of telematics applications in surgery can be distinguished: teleconsulting, teleassistance and telemanipulation. Whereas the ®rst is based on intraoperative videoconferencing and bilateral audio-visual communications, the two other levels include remote guidance of an endoscope or even surgical instruments by a remote expert. We estimate, that teleconsulting techniques will enter the clinical routine of centers relatively soon, whereas teleassistance and telemanipualtion technologies will not result in clinical breakthroughs within the next few years. Clinical and ambulatory patient care are processes with a great deal of information to be handled. Therefore advanced information management and telecommunications services are of particular interest for the various medical disciplines [13,14]. The technological basis of teleconsulting applications usually is ISDN teleconferencing using two ISDN B-channels totaling to a transmission rate of 128 kbit/s (Europe) or 112 kbit/s (USA). These transmission rates are not suitable for real-time applications. Image resolution and image rates per second are visibly reduced compared to PAL or NTSC TV standards.

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applications with several external partners in a six B-channel setting providing a bandwidth of 384 kbit/s. A four site teleconsulting trial was performed including Tuebingen University Hospital, the Karlsruhe Research Center and the Institute of Research on Digestive Tract Cancers (IRCAD), Strasbourg, and a moderator located at Kiel. The clinical case was an endoscopic removal of a rectal tumour through the transanal TEM approach. The scienti®c question to answer through the experiments was whether the given bandwidth is suf®cient for appropriate image quality. In different phases of the intervention (tumour dissection, closure of the rectal wall defect) external `teleconsultants' from both the technical side (Karlsruhe) and the clinical side (Strasbourg) were confronted with questions on the case by the operating surgeon. The image resolution was judged by all teleconsultants to be suf®cient but at the lower end of acceptability for on-line investigations. Usually video transmission is perceived realtime with a bandwidth not less than 2 mbit/s. This requires ATM connections which are costly and hardly available for most hospitals. The overall judgment of the participants in the experiments was that six B-channel teleconferencing techniques are feasible for intraoperative surgical consulting, provided that the operative ®eld, the instruments or the endoscope itself are not moving signi®cantly. Considering the simple and standardized technological basis of ISDN telephone systems surgical intraoperative

5.1. Surgical teleconsulting In the clinical ®eld we were able to study teleconsulting

Fig. 5. Placement of positioning systems for endoscopic solo-surgery.

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Fig. 6. The ARTEMIS manipulator system. The steerable instrument is introduced into a phantom.

teleconsulting could soon become clinically feasible for surgical centers. 5.2. Surgical teleassistance During intraoperative teleassistance the remote consultant has access to move the endoscope to adjust the endoscopic image according to his own preferences. This requires the combination of telecommunications with robotics technology. Several trials have been made by our group as well as by several other researchers worldwide. In all applications a robotic endoscope guidance system is connected to any kinds of communication lines or networks. Control data are transmitted along appropriate lines (ISDN or B-ISDN) together with video and voice data. Usually robotic control data require a lower bandwidth compared to mass data such as video or audio so that their transmission does not add too much complexity to the set-up in terms of telecom capacities. Our current research is focused on using bundled ISDN lines for remote guidance of the FIPS endoscope guiding system. 5.3. Surgical tele-manipulation The ®eld of surgical tele-manipulation is subject to intensive research worldwide. Surgical tele-manipulation is characterized by performing an operation or phases of an operation without being physically present at the operating table. The ®eld of surgi-

cal tele-manipulation is always linked to both advanced communication technologies and robotics. A number of research groups worldwide have assessed the potential of tele-manipulation systems in experimental settings for several years. Especially the medical engineering group at SRI, Menlo Park, put particular emphasis on this topic [15]. Our own involvement in this ®eld started in 1994 with our initial evaluations of the DISTEL manipulator system [16]. The Karlsruhe DISTEL manipulator system was modi®ed for this purpose. The original system is a one arm master slave manipulator with six motion axes. Its purpose of use is remote handling of dangerous substances in the technical ®eld. For surgical application a conventional rigid instrument was attached to the manipulator arm. The distance between both sides of the system was 1.3 km. Communication and data transfer between both sites are possible along a bundle of 12 ®beroptic cables. With the appropriate transmission bandwidth the distance between the two sites is not relevant. Through broad band ATM connections there would be no major functional difference between the present 1.3 km or several thousands of kilometers. With this system several tests were carried out for the assessment of telesurgical working conditions for the user. A standardized series of repetitive task-board experiments was carried out by two groups, surgeons and engineers [16]. It was found that persons with experience in endoscopy adapted much faster to the remote handling situation than those without appropriate experience.


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Fig. 7. The manipulator applied in an animal experiment (one arm version).

Telecommunications technology will have great impact on future scienti®c and clinical work in most medical areas

[13,14,17]. Surgery, being a therapeutic specialty, has not been very receptive for telematics applications so far.

Fig. 8. New endoscope (MGB, Berlin) with additional illumination source for enriching image contrast. In addition, the scope has nozzles for rinsing the front lens.

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Teleconsulting techniques based on intraoperative teleconferencing are becoming clinically feasible and are already used in specialized centers. Teleassistance techniques with remote control of the endoscope is getting technically feasible for routine use, too, after it has been widely demonstrated by different research groups. From our point of view these applications will ®nd entrance into clinical surgery within the next few years. However, If the complex technologies merging robotics and telematics for telemanipulation will be of practical clinical value in the near and mid-term future, is still doubtful. 6. Discussion The restoration of functional qualities, which are available in open surgery, is an important demand for furthering endoscopic surgery. This is of particular interest for the restoration of basic functions of man-environment interactions, such as spatial vision [1,2], tactile sense and instrument mobility in the operative ®eld [18]. Developments of the last few years in the ®eld of endoscopic vision systems [3] have led to more natural endoscopic visualization in terms of improved image resolution, illumination, and clear vision maintenance techniques (Fig. 8). The advantages of these new vision systems are improved handling accuracy and time savings in endoscopic manipulations [19±21]. In recent years endoscopic surgery has become increasingly widespread, supported by new developments in the instrumentation available. Less attention has been paid to the comfort of the ®rst surgeon and his assistant, who were often forced into tiring standing positions and monotonous tasks. Moreover, high costs of the operating theater, even for standard laparoscopic procedures, require the involvement of less experienced fellows, such as residents, which leads to a further increase in the operation time. Especially in community hospitals and private institutions, where the role of the surgical assistant is assumed either by assistant physicians or trained nurses, the introduction of positioning systems for laparoscopic procedures may alleviate some of the pressure due to limited resources. Improved architecture design and interface of systems will lead to improved ergonomy and intuitive mode of operation of systems, in order to offer the surgeons better control. Restoration of full instrument mobility is a further technological challenge in endoscopic surgery. First prototypes of steerable endoscopic instruments with two additional DOF were introduced by our group in 1992 [22]. A ®rst functional master slave manipulator for surgery was introduced by Hill et al., from SRI International, Menlo Park, CA [15]. The SRI telemanipulator was not designed for endoscopic use and had only four DOF in its ®rst alignment of the grasper according to the operative situation. The restoration of full spatial mobility of the instrument in laparoscopic or thoracoscopic surgery is a complex

research task. The ARTEMIS system was the ®rst 6 DOF master-slave manipulator for endoscopic surgery found in the literature [23]. Clinical use of the system, however, will require further development in the area of slave mechanics and the control system. Intensive research and development efforts are currently put on manipulators for endoscopic microsurgery by various work groups at scienti®c institutions and industry. Several devices have already entered preclinical testing, and it appears to be highly probable that master slave manipulators will be in practical surgical use before the next millennium.

References [1] Becker H, Melzer A, Schurr MO, Buess G. 3D video techniques in endoscopic surgery. Endosc Surg Allied Technol 1993;1:36±39. [2] Crosthwaite G, Chung T, Dunkley P, Shimi S, Cuschieri A. Comparison of direct vision and electronic two- and three-dimensional display systems on surgical task ef®ciency in endoscopic surgery. Br J Surg 1995;82:846±851. [3] Schurr MO, Buess G, Kunert W, Flemming E, Hermeking H, Gumb L. Human sense of vision: a guide to future endoscopic imaging systems. Min Invas Ther Allied Technol 1996;5:410±441. [4] Begin E, Gagner M, Hurteau R, Santis S, Pomp A. A robotic camera for laparoscopic surgery: conception and experimental results. Surg Laparosc Endosc 1995;5:6±11. [5] Sackier JM, Wang Y. Robotically assisted laparoscopic surgery: from concept to development. Surg Endosc 1994;8:63±66. [6] Jakobs LK, Shayani V, Sackier JM. Determination of the learning curve of the AESOP robot. Surg Endosc 1997;11:54±55. [7] Schurr MO, Buess G, Weigelhofer G, Senft R, Groezinger R, Brandmaier R. The operating room system for endoscopic surgery: OREST I. Min Invas Ther 1995;4:57±62. [8] Schurr MO, Arezzo A, Neisius B, Rininsland HH, Hilzinger HU, Dorn J, Roth K, Buess GF. Trocar and instrument positioning system. Surg Endosc 1999;in press. [9] Melzer A, Schurr MO, Kunert W, Buess G, Voges U, Meyer JU. Intelligent surgical instrument system ISIS Concept and preliminary experimental application of components and prototypes. Endosc Surg Allied Technol 1993;1:165±170. [10] Mueglitz J, Kunad G, Dautzenberg P, Neisius B, Trapp R. Kinematic problems of manipulators for minimal invasive surgery. Endosc Surg Allied Technol 1993;1:160±165. [11] Schurr MO, Melzer A, Dautzenberg P, Neisius B, Trapp R, Buess G. Development of steerable instruments for minimal invasive surgery in modular conception. Acta Chir Belg 1993;93:73±77. [12] Breitwieser H. DISTEL: digitales steuerungssystem fuer telemanipulatoren. Robotersysteme 1991;1:160±164. [13] Huang HK. Telemedicine and teleradiology technologies and applications. Min Invas Ther Allied Technol 1997;6:387±392. [14] Wootton R. Telemedicine: the current state of the art. Min Invas Ther Allied Technol 1997;6:393±403. [15] Bowersox JC, Shah A, Jensen J, Hill J, Cordts PR, Green PS. Vascular applications of telepresence surgery: initial feasibility study in swine. J Vasc Surg 1996;23:281±287. [16] Schurr MO, Breitwiser H, Melzer A, Kunert W, Schmitt M, Voges U, Buess G. Experimental telemanipulation in endoscopic surgery. Surg Laparosc Endosc 1996;6:167±175. [17] Ecken BK, Harbick RF, Pease AL. Uses and bene®ts of telemedicine. Min Invas Ther Allied Technol 1997;6:444±447. [18] Buess G. Why this journal? (Editorial). Endosc Surg Allied Technol 1993;1:1±2.

M.O. Schurr et al. / European Journal of Cardio-thoracic Surgery 16 (Suppl. 2) (1999) S97±S105 [19] Buess G, Bergen P, Kunert W, Schurr MO. 2D und 3D sichtsysteme in der minimal invasiven Chirurgie. Chirurg 1996;67:1041±1046. [20] Kunert W, Flemming E, Schurr MO, Buess G. Optik mit natuerlich wirkender zusatzbeleuchtung. Langenbecks Arch Chir 1997;II:1232± 1234. [21] Peitgen K, Walz MK, Eigler FW. Praktische und experimentelle erfahrungen mit dem einsatz der 3D technologie in der laparoskopie. Endoskopie Heute 1996;9:64.

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[22] Schurr MO, Buess G, Rininsland H, Holler E, Neisius B, Voges U. ARTEMIS-manipulatorsystem fuer die endoskopische chirurgie. Endoskopie heute 1996;9:245±251. [23] Melzer A, Schurr MO, Dautzenberg P, Trapp R, Buess G. Erhoehung der freiheitsgrade bei instrumenten fuer die laparoskopische chirurgie. Langenbecks Arch Chir 1992:279±285.