TrEndo Tracking System: Motion analysis in minimally invasive surgery

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Motion Analysis in Minimally Invasive Surgery

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 12 januari 2009 om 10:00 uur

door

Magdalena Karolina CHMARRA

magister in ˙zynier

Technische Universiteit Warschau, Polen geboren te Warschau, Polen.

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Copromotor: Dr. F.W. Jansen

Samenstelling promotiecommissie:

Rector Magnificus, Technische Universiteit Delft, voorzitter Prof. dr. J. Dankelman, Technische Universiteit Delft, promotor

Dr. F.W. Jansen, Leids Universitair Medisch Centrum, toegevoegd promotor Prof. dr. ir. C.A. Grimbergen, Technische Universiteit Delft

Prof. dr. P.J. French, Technische Universiteit Delft Prof. dr. I.A.M.J Broeders, Universiteit Twente

Prof. dr. ir. M. Steinbuch, Technische Universiteit Eindhoven

Dr. S.I. Brown, University of Dundee

Title TrEndo Tracking System.

Motion Analysis in Minimally Invasive Surgery.

Author Magdalena K. Chmarra

Cover design Dimirta Dodou

Copyright Magdalena K. Chmarra, Delft, The Netherlands, 2009 Print Proefschriftmaken.nl

ISBN/EAN 978-90-889108-4-5

All rights reserved. No part of this book may be reproduced by any means, or transmit-ted without the written permission of the author. Any use or application of data, methods and/or results etc., occurring in this report will be at the user’s own risk.

Financial support for the publication of this thesis was kindly provided by:

Medical Dynamics, Medtronic Bakken Research Center B.V., Biomet Nederland B.V., and Covidien Nederland.

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Summary

Minimally invasive surgery (MIS) has been introduced into surgery to the bene-fit of the patients. Since this kind of surgery is performed through small incisions, patients experience less trauma than after an equivalent conventional procedure. Moreover, MIS causes less postoperative pain and scarring, faster recovery, shorter hospitalisation, and reduced incidence of post-surgical complications (e.g., adhe-sions, infections). Therefore, MIS becomes more and more a common technique for major surgical procedures (e.g., gynaecological, gastrointestinal, urological, and vascular surgeries).

The advantages of MIS, however, are accompanied by special demands on the surgeon. The surgeon needs to possess certain non-intuitive psychomotor skills that are more complicated than in conventional open surgery. MIS is thus a difficult surgical technique and it takes a relatively long learning curve to master it fully. To guarantee the safe use of long MIS instruments with difficult handling, limited tactile perception, and working in a limited area, training of operative skills is very important.

To train psychomotor MIS skills, various facilities, such as box trainers and vir-tual reality (VR) trainers, have been developed. Studies have shown that training on those kind of simulators improves psychomotor MIS skills. Evaluation of MIS skills, however, is still a major impediment, since a standard and valid method to measure and certify competence of MIS skills is still missing.

The goal of this thesis was to develop a method that combines the advantages of the box trainer (natural force feedback) with the advantages of a VR trainer (ob-jective assessment) using a low-cost tracking system. To reach this goal, three aims were formulated for this thesis:

1. To develop a system that tracks and records movements of real MIS instru-ments in training models (e.g., box and VR trainers).

2. To determine factors that influence movements of instruments during train-ing MIS tasks.

3. To develop a method to objectively determine whether a resident can be called an “expert”, “intermediate”, or “novice” according to his/her psychomotor MIS skills in a box trainer.

Before developing a system that tracks and records movements of real MIS in-struments in training models, a review of the literature was performed on existing

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tracking systems for MIS (Chapter 2). This review showed that measuring the posi-tion and orientaposi-tion in an economic way is an important challenge when a scoring system based on motion analysis has to be developed. In case of MIS, various ap-proaches to this challenge have been studied. None of them, however, came out as clearly superior; each system used to measure movements of MIS instruments has inherent advantages and disadvantages. For example, nine out of sixteen tracking systems described in Chapter 2 can be used in one (out of three) environment only (in most of the cases it is a virtual environment). There are only a few systems that can be used to track motions of MIS instruments either during the surgery in the operating room (OR), or in a box trainer. There is no single system that could eas-ily be used in all environments for training MIS: OR, box trainer, and VR trainer. Moreover, tracking systems currently available on the market are complex and ex-pensive. Some of the systems limit the freedom of movement, and most of them are intended to be used in VR environments only. There was, therefore, a need to develop a simple and affordable tracking system that would record movements of a standard MIS instrument in real and virtual environments (e.g., in a box trainer and a VR trainer).

The TrEndo is a simple and affordable tracking system that has been developed at Delft University of Technology (Chapter 3). The TrEndo consists of three optical computer mouse sensors incorporated in a gimbal mechanism. The gimbal mecha-nism guides the MIS instrument while the optical sensors measure the movements of the instrument. The design of the TrEndo results in an inexpensive system, which allows realistic manipulation and measurement of the movements of the standard MIS instrument (∅ 5 mm) in four DOFs. Due to use of standard instruments, the TrEndo provides natural force feedback. Moreover, the TrEndo can be used in both box trainers and VR trainers. TrEndo’s compact size, portability, and plug-and-play installation make it an “easy to work with” tracking system. Because of its simplic-ity, TrEndo is also easy to produce.

To determine factors that influence movements of instruments during training MIS tasks, a number of tests has been performed. The study described in Chapter 4 showed that force feedback, although distorted and limited, influences psychomo-tor MIS skills, especially when pulling and pushing forces are applied (e.g., during grasping). It has also been found that for tasks in which pulling and pushing forces hardly play a role, there is no difference between groups performing tasks in the trainers with and without natural force feedback. During a task in which pulling and pushing forces play an important role, the switch from the trainer with nat-ural force feedback to the one without force feedback had a positive effect on the performance without force feedback. Switching the other way, however, did not have positive effect. This is an important finding from the clinical point of view, because it indicates that the training for advanced tasks in which pulling and push-ing forces play an important role should use trainers with natural force feedback. Training of eye-hand coordination, however, can be done in both trainers with or without natural force feedback.

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only a surgeon, but also at least one operating assistant. Both the surgeon and the assistant must be comfortable with the entire MIS setup. Depending on the pref-erence of the surgeon, the assistant can manipulate the camera and/or additional instruments during the procedure. It is, therefore, necessary to ensure that surgeons are able to perform MIS under two conditions: when the surgeon holds the camera him/herself, and when the assistant holds the camera. Studies described in Chap-ter 5 showed that experience has an influence on MIS task performance. Moreover, “camera holding” does influence performance of MIS tasks by novices. It has also been found that self-holding of the camera apparently improves novices’ eye-hand coordination. This finding is very important from a clinical point of view; residents who are hardly experienced in MIS should begin their active participation in the MIS by performing basic techniques while self handling the camera.

Goal-oriented movements (point-to-point movements) are very common in MIS (e.g., during grasping, placing a clip on a vessel, or while using diathermy). The study described in Chapter 6 showed that goal-oriented movements in MIS are not performed via the “shortest path”. The movements clearly distinguish two phases: a retracting, and a seeking phase. The normalised path length during the retracting phase is significantly shorter than during the seeking phase. This occurs for experts, intermediates, and novices. The experience in MIS influences the seeking phase; the shorter path length in that phase implies better performance. The seeking phase is, therefore, characteristic for the differences in performance. The retracting phase is also important from the clinical point of view; it improves safety of the patient by avoiding unpredicted contact with the tissue. The best strategy to perform the retracting phase is to pull back the instrument along its axis (in the direction of the incision point), and to avoid any movement in the plane perpendicular to the plane defined by the initial position of the instruments’ tip, the specified target position, and the pivoting point. Such performance of the retracting phase will minimise unpredicted contacts between the instrument and tissue. It would be beneficial to let novices learn how to perform a more-precise retracting movement.

From the clinical point of view, it is important to know which group (expert, intermediate, or novice) the surgeon belongs to. It has been recognised that one performance measure alone (e.g., time) does not adequately measure proficiency. Moreover, when measures are to be used as an examination (in order to determine whether a resident is an expert, intermediate or novice), a passing score needs to be established. There are several reasons that make it difficult to determine a suf-ficient score. Firstly, assessment is done using multiple assessment measures. Sec-ondly, there is no “gold standard” of surgical competency against which one can judge the validity of competency assessment. As a result, it is not clear whether a surgeon should be assessed on individual tasks or whether a composite assessment of all tasks should be used. Chapter 7 describes the study on objective classification of residents as experts, intermediates, and novices according to their psychomotor MIS skills. The classification has been done using two statistical methods: Princi-pal Component Analysis (PCA) and Linear Discriminant Analysis (LDA). Results of the study showed that it was possible to correctly classify 74% of the

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partici-pants based on their motor dexterity. This indicates that the proposed classifica-tion method has a good discriminant validity. Since the distribuclassifica-tions of experts’, intermediates’, and novices’ motion-analysis-based parameters were taken into ac-count, our classification me-thod was able to distinguish between residents with fine gradation in experience (e.g., between experts and intermediates). Moreover, the method allows classification of residents using a single training task as well as a combination of different tasks. Our study showed, however, that better clas-sification results are obtained for a combination of different tasks. Additionally, the proposed classification method is rather simple and general. Therefore, there should be no problem with applying it in present trainers that are equipped with a tracking system.

This thesis indicated that the comparison of the novices’, intermediates’ and experts’ motion-analysis-based parameters (e.g., path length) is an important and valid component of the overall criterion-based assessment of basic MIS skills. There-fore, motion analysis can be used as an objective tool to assess psychomotor skills in MIS. However, to properly evaluate performance, it is necessary to find correct parameters that measure the quality of actions. Based on the studies presented in this thesis, we can conclude that performance measures depend on the kind of exer-cise that is performed and its setting (e.g., position of the camera, camera operator, MIS instruments). Moreover, for different exercises different motion characteristics are optimal. It is, therefore, important to properly analyse instruments’ movements during training of MIS skills. The clinical impact of such extended analysis is that in this way is it possible to implement a correct objective score that can measure and certify the competence of surgeons’ psychomotor MIS skills (in addition to the existing criteria for the assessment of MIS performance, e.g., objective structured assessment of technical skill – OSATS). Such extended motion analysis can result in improvement of the training of basic MIS skills, since it can identify the difference between the experts’ and novices’ performance, as well as the areas that require more training.

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Contents in brief

1 Introduction 1

2 Systems for Tracking Minimally Invasive Surgical Instruments During

Training 11

3 TrEndo, a Device for Tracking Minimally Invasive Surgical Instruments

in Training Setups 33

4 Force Feedback and Basic Laparoscopic Skills 49 5 The Influence of Experience and Camera Holding on Laparoscopic

Instru-ment MoveInstru-ments Measured with the TrEndo Tracking System 65 6 Retracting and Seeking Movements During Laparoscopic Goal-Oriented

Movements. Is the Shortest Path Length Optimal? 81 7 How to objectively classify residents based on their psychomotor

laparo-scopic skills? 93

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4 Force Feedback and Basic Laparoscopic Skills 49 4.1 Introduction . . . 51 4.2 Methods . . . 52 4.2.1 Participants . . . 52 4.2.2 Tasks . . . 52 4.2.3 Experimental protocol . . . 52 4.2.4 Parameters . . . 53 4.2.5 Statistical analysis . . . 55 4.3 Results . . . 55 4.3.1 Participants . . . 55 4.3.2 Balls . . . 55 4.3.3 Ring . . . 56 4.3.4 Elastic band . . . 56 4.4 Discussion . . . 58 4.5 Conclusions . . . 63

5 The Influence of Experience and Camera Holding on Laparoscopic Instru-ment MoveInstru-ments Measured with the TrEndo Tracking System 65 5.1 Introduction . . . 67

5.2 Materials and Methods . . . 68

5.2.1 Participants . . . 68 5.2.2 Task . . . 68 5.2.3 Tracking system . . . 68 5.2.4 Experimental protocol . . . 69 5.2.5 Parameters . . . 71 5.2.6 Statistical analysis . . . 71 5.3 Results . . . 72 5.3.1 Participants . . . 72

5.3.2 Influence of the experience level . . . 72

5.3.3 Influence of camera holding . . . 72

6 Retracting and Seeking Movements During Laparoscopic Goal-Oriented Movements. Is the Shortest Path Length Optimal? 81 6.1 Introduction . . . 83 6.2 Methods . . . 83 6.2.1 Participants . . . 83 6.2.2 Task . . . 84 6.2.3 Data analysis . . . 84 6.2.4 Statistics . . . 86 6.3 Results . . . 86

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6.3.1 Participants . . . 86

6.3.2 Retracting and seeking phases . . . 87

6.3.3 Influence of the experience . . . 87

7 How to objectively classify residents based on their psychomotor laparo-scopic skills? 93 7.1 Introduction . . . 95

7.2 Materials and Methods . . . 96

7.2.1 Participants . . . 96 7.2.2 Tasks . . . 96 7.2.3 Motion analysis . . . 97 7.2.4 Statistical analysis . . . 99 7.2.5 Leave-one-out cross-validation . . . 102 7.3 Results . . . 102 7.3.1 Tasks . . . 102 7.3.2 Classification . . . 102 7.4 Discussion . . . 102

8 Conclusions, Discussion, and Future Directions 109 8.1 Recapitulation and general conclusions . . . 111

8.1.1 On development of a tracking system . . . 111

8.1.2 On factors that influence movements in MIS . . . 111

8.1.3 On a classification method . . . 112

8.2 General discussion . . . 112

8.3 Clinical implications . . . 117

8.3.1 On development of tracking system . . . 117

8.3.3 On a classification method . . . 118

8.4 Recommendations and future directions . . . 118

8.4.1 On development of tracking system . . . 118

8.4.3 On a classification method . . . 120 References 123 Samenvatting 137 Acknowledgments 141 Curriculum vitae 145 Publications 147

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Introduction

He who chooses the beginning of a road chooses the place it leads to. It is the means that determine the end.

Harry Emerson Fosdick, 1878-1969

In which minimally invasive surgery (MIS) is introduced, the aim of this study is stated, and the structure of this thesis is outlined.

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1.1 Essentials of Minimally Invasive Surgery

Minimally invasive surgery (MIS, e.g., laparoscopy) is nowadays widely used for therapeutic purposes. This surgical technique is used to perform operations through small incisions in the patient’s body [Cuschieri, 1992]. Through the incisions, can-nulas (trocars) are introduced permitting the insertion of long instruments (e.g., graspers, scissors) into the patient’s body. A workspace inside the patient and vi-sualisation of the abdominal cavity is possible due to the pneumoperitoneum es-tablished by insufflated CO2gas. Visual feedback of the operating area is obtained

by a small camera, which provides a two-dimensional (2D) image on a monitor. A view of the operating room (OR) during a MIS procedure performed by two gyne-cologists is presented in Fig. 1.1.

It is well known that MIS has many advantages for the patient, such as re-duced morbidity, shorter hospitalisation, better cosmetic results, and earlier return to normal activity. Performing MIS, however, requires unique psychomotor skills that are different from those needed to perform conventional open surgical proce-dures. These skills include a shift from a conventional 3D operating field to a 2D

Figure 1.1: The view of the operating room during MIS procedure. The patient is lying

on the operating table in the supine position. The gynaecologist leads the operation from the left side of the patient. The camera operator stands next to the gynaecologist. Both the gynaecologist and the camera operator watch the operating area on a monitor (not presented in this picture).

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monitor display, judgment of altered depth perception and spatial relationships, distorted eye-hand coordination, adaptation to the fulcrum effect, manipulation of long surgical instruments while adjusting for amplified tremor, diminished tactile feedback, and fewer degrees of freedom [Wentink, 2003; Breedveld, 2000; Bholat, 1999a; Den Boer, 1999; Hanna, 1999; Hanna, 1998a; Gallagher, 1998; Dion, 1997; Cuschieri, 1995]. Although a competent surgeon possesses knowledge, judgment, decisiveness, and professionalism, proficiency in technical skills seems to be fun-damental to perform surgery safely [Feldman, 2004]. To guarantee safe use of MIS instruments with difficult handling, in a limited working area, and with limited tactile perception, training of the operative skills is very important.

1.2 Training of Minimally Invasive Surgical Skills

Mastering MIS skills requires repeated practice. Traditionally, residents learn ba-sic (psychomotor) MIS skills while operating on patients under the supervision of an expert surgeon [Dankelman, 2005]. In the first stage of the training, they ob-serve experienced surgeons during several operations. After that, residents are al-lowed to participate more actively in the operation; they perform many of the basic techniques that have been observed in the first stage of the training. Finally, they take a more autonomous role as primary surgeon. This way of training is not stan-dardised, potentially unsafe for the patient, and results in very long learning curve [Moore, 2002]. Moreover, such training is expensive [Babineau, 2004; Bridges, 1999]; in the USA, for example, the annual cost of training of 1,014 general surgery resi-dents in the OR costs around €53 million [Villegas, 2003].

Training of laparoscopic skills is also done on animal models and human ca-davers [Giger, 2008; Nebot, 2004; Cundiff, 2001]. Human caca-davers offer accurate anatomy for learning MIS skills. However, problems with conserving the tissue and lack of bleeding in case of damaging vessels are the main reasons for using live animal models instead of human cadavers. Animal models offer physiological fea-tures and tissue characteristics similar to those of humans [Waseda, 2005; Olinger, 1999; Crist, 1994; Bohm, 1994; Wolfe, 1993; Bailey, 1991]. It is, however, difficult to find animals whose anatomy resemble that of human. Another disadvantages of using animal models and human cadavers are costs, and one-time usability. Ad-ditionally, training on animals is forbidden in several countries because of ethical aspects.

Above mentioned shortcomings of training on patients, animals, and human cadavers suggest the need for the acquisition of the standardised pre-OR training modules. It has already been proven that training outside the OR – in a skills lab – is efficient [Aggarwal, 2007b]. Therefore, various facilities, such as box trainers and virtual reality (VR) trainers, are being developed [Kolkman, 2007; Halvorsen, 2005; Jakimowicz, 2005; Youngblood, 2005; Katz, 2005; Hance, 2005; Schijven, 2003; Anastakis, 1999; Rosser, 1998; Shapiro, 1996]. Basically, a box trainer is a box that is used to mimic the surrounding and a part of a patient’s body (e.g., abdomen). The content of the box can vary between different synthetic inanimate models (e.g.,

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Figure 1.2: An example of a box trainer used to train basic skills in minimally invasive

surgery.

simple physical objects such as pegs), synthetically produced organs, and animal parts [Waseda, 2005; Scott, 2000]. Box trainers provide an environment with natu-ral force feedback, which is obtained due to the use of conventional laparoscopic instruments and equipment (Fig. 1.2).

VR trainers (Fig. 1.3) can be defined as a collection of technologies, which allow an interaction with a computer-simulated environment. VR trainers are used to simulate real or imagined environment that can be experienced in various ways (e.g., visually). Only few current VR trainers for laparoscopy are equipped with mimicked force feedback [Halvorsen, 2005; Schijven, 2003]. Force feedback in VR trainers, however, is costly and not very similar to the feedback obtained when using the real laparoscopic instruments during operation.

From the clinical point of view, it is important that a surgeon develops a high degree of operative skills. Without development of those basic psychomotor skills, the surgeon could put a patient at risk. Assessment methods to credential surgeons as technically competent and ready to operate on a patient are, therefore, needed. Current training facilities, however, do not provide standardised objective assess-ment of basic MIS skills, which can quantify and qualify competencies of the user.

1.3 Assessment of Minimally Invasive Surgical Skills

Surgical organisations (e.g., ACGME) are calling for methods to ensure the main-tenance of skills, advance surgical training, and to credential surgeons as techni-cally competent [Park, 2002; Roberts, 2006]. Simulators in their current form have demonstrated to improve the OR performance of surgical residents. Evaluation of MIS skills, however, is still a major impediment. “What seems to be missing are

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Figure 1.3: Simendo – a virtual reality trainer developed by DelltaTech. (Courtesy of

DelltaTech.)

standard and valid methods to measure and certify competence in these psychomo-tor skills” [Park, 2002].

Assessment of technical competency of surgeons can be done in two ways: sub-jectively, and objectively [Feldman, 2004]. In most of the current training programs, assessment of residents heavily relies on subjective assessment measures, which are influenced by personal traits and relationships [Darzi, 1999; Wanzel, 2002; Mar-tin, 1997; Moorthy, 2003b]. There is, therefore, urge for objective assessment, which is less likely to be biased by above mentioned factors. Moreover, most objective assessment is more reliable than the subjective one. In consequence, residents are more likely to accept objective feedback on their skills and incorporate it construc-tively in training.

An objective, reproducible, and universally accepted assessment tool is cur-rently lacking. Nonetheless, objective evaluation of skills is a very important fac-tor in developing an effective curriculum, since it motivates residents to actively engage with training of his/her skills and it can provide a valuable feedback. It has been demonstrated that motion analysis is a valuable objective assessment tool in training of basic MIS skills [Cotin, 2002; Darzi, 1999; Martin, 1997]. However, in order to use motion analysis, a system that tracks and records those motions is needed.

1.4 Problem Statement and Aim of this Thesis

Since the introduction of simulators into training of MIS skills, there is growing con-sensus about the need to assess surgical competence objectively. Although progress

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has been made over past years, “at this stage, the science of objective assessment of technical performance is in its infancy” [Darzi, 2001b]. It is indisputable that a sur-geon needs to acquire a certain level of manual dexterity to perform surgery safely. It is, therefore, necessary to implement objective assessment methods, which will measure, identify, and certify the competence of basic psychomotor MIS skills.

Ideally, objective assessment methods address at least two topics:

- determining whether a resident is competent to move to the next level of training (e.g., to operate on the patient),

- identifying potential areas for improvement.

To maximise the ability of MIS trainers to differentiate between surgically competent and surgically not (yet) competent, objective methods to determine whether a resident acquired an expert, intermediate, or novice level of surgical skills in MIS need to be established. Those methods should be used to certificate and monitor progress of surgeons. Assessment methods (e.g., based on motion analysis) should also be used to give feedback on the nature of possible limitations of the surgeon.

As mentioned in Section 1.3, objective assessment of basic MIS skills can be done using motion analysis. To use motion analysis as an assessment tool, a system that tracks and records those motions is needed. Motions of the MIS instruments can be tracked and recorded in three different environments: in the operating room (dur-ing an operation), in a box trainer, or a VR trainer (dur(dur-ing train(dur-ing). The operat(dur-ing room is the most realistic environment, where a surgeon uses real laparoscopic in-struments, which provide natural instrument-tissue interaction with realistic force feedback. The box trainer also presents a realistic environment where force feed-back is obtained due to the use of real laparoscopic instruments. VR trainers with a less realistic (virtual) environment and simulated MIS instruments are by definition equipped with interfaces between the user and computer (i.g., tracking systems). VR trainers are, therefore, able to provide an objective scoring system based on instruments motion analysis [Torkington, 2001; Gallagher, 2001; Gallagher, 2004; Schijven, 2003].

Designing a system that can track real MIS instruments in the OR seems to be difficult, since factors such as patient safety and ergonomics in the OR play a critical role. In box trainers, tracking systems have to track real MIS instruments. Ideally, a tracking system should be designed in such a way that it is possible to use it in all three above mentioned environments. Currently, there are only a few tracking sys-tems able to measure movements of real MIS instruments in box trainers [Chmarra, 2007]. Some of those systems are at a prototype stage, and the systems available on the market are very expensive. There is, therefore, a need for a low-cost tracking system that can measure movements of real MIS instruments in both box and VR trainers.

The general goal of this thesis is to develop a method that combines the ad-vantages of the box trainer with natural force feedback together with a low-cost tracking system to make objective assessment possible. To reach this goal, three aims are formulated for this thesis:

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1. To develop a system that tracks and records movements of real MIS instru-ments in training models (e.g., box and VR trainers).

2. To determine factors that influence movements of instruments during train-ing MIS tasks.

3. To develop a method to objectively determine whether a resident can be called an expert, intermediate, or novice according to his/her psychomotor MIS skills in a box trainer.

1.5 Thesis Outline

This thesis contains three parts: Part I describes the state of the art in research on tracking systems for MIS; Part II deals with objective assessment of psychomotor MIS skills based on analysis of instrument motion; and Part III introduces a new method to objectively classify residents based on their psychomotor MIS skills. Each chapter is a journal article, unmodified with respect to the form in which it has been published or submitted. This leads, therefore, to some repetitions in the

introductionand/or the methods sections of the chapters, but it gives the opportunity to read the chapters in random order. The three aims of this thesis are addressed in the following chapters:

I. To develop a system that tracks and records movements of real MIS instru-ments in both box and VR trainers.

- Chapter 2 presents state of the art in research on tracking systems for MIS.

- Chapter 3 introduces TrEndo – a new system for tracking surgical instru-ments during training of MIS skills.

II. To determine which external factors influence movements of instruments dur-ing traindur-ing MIS tasks.

- Chapter 4 studies the influence of force feedback on instrument move-ments during performing MIS tasks and answers the question whether force feedback is required in training basic MIS force applica-tion tasks.

- Chapter 5 deals with the influence of the camera holding (by the partic-ipant self or by an assistant) on instrument movements during perform-ing MIS tasks. It also investigates whether the level of basic skills of a resident in MIS is a significant factor in aforementioned issue.

- Chapter 6 decomposes and analyses movements during goal-oriented tasks and answers the question whether shortest path length is optimal in MIS.

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III. To develop a method to objectively determine whether a resident can be called an expert, intermediate, or novice according to his/her psychomotor MIS skills.

- Chapter 7 introduces a new method to objectively classify residents as experts, intermediates, and novices according to their psychomotor MIS skills.

Finally, Chapter 8 presents a discussion, general conclusions, and directions for future research.

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Systems for Tracking Minimally Invasive

Surgical Instruments During Training

Magdalena K. Chmarra, Cornelis A. Grimbergen, Jenny Dankelman

Minimally Invasive Therapy & Allied Technologies16:6; 328-340 (2007)

To measure is to know.

William Thomson (Lord Kelvin), 1824-1907

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Abstract

Minimally invasive surgery (e.g., laparoscopy) requires special surgical skills, which should be objectively assessed. Several studies have shown that motion analysis is a valuable assessment tool of basic surgical skills in laparoscopy. However, to use motion analysis as the assessment tool, it is necessary to track and record motions of laparoscopic instruments. This article describes the state of the art in research on tracking systems for laparoscopy. It gives an overview on existing systems, on how these systems work, their advantages, and shortcomings. Although various approaches have been used, none of the tracking systems to date comes out as clearly superior. A great number of systems can be used in training environment only, most systems do not allow the use of real laparoscopic instruments, and only a small number of systems provides force feedback.

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2.1 Introduction

Minimally invasive surgery (e.g., laparoscopy) is a technique that requires special surgical skills [Arregui, 1995]. Traditionally, resident surgeons start their surgical education observing experienced surgeons in the operating room (OR). Afterwards, they are allowed to contribute to the operation (e.g., they perform a number of basic techniques). Finally, they become primary surgeons; however, this generally occurs without an objective assessment of their skills. Since the skill and the level of experience of the surgeon are not exactly known, the current method of training is potentially unsafe for the patient [Dankelman, 2005].

Since the emphasis on medical safety and the complexity of laparoscopic tech-niques and equipment continuously increases, it is essential to develop new, safe training and assessment methods for laparoscopic skills. Training methods, such as box trainers and virtual reality (VR) trainers, have already been developed to learn basic laparoscopic skills outside the operating room [Aggarwal, 2004; Gal-lagher, 2004; McClusky, 2004; Muntz, 2004]. However, the objective assessment of residents’ skills still remains a challenge [Cotin, 2002].To date, there is no one widely used automatic assessment method of the basic minimally invasive surgical skills. Commonly, assessment relies heavily on the expert surgeons and, therefore, is not always objective [Darzi, 1999; Martin, 1997; Moorthy, 2003b]. An objective as-sessment of skills is a very important factor in developing an effective curriculum, since it motivates residents to actively engage with training of his/her skills and provides valuable feedback as to whether that engagement translates into gaining experience.

In the literature, it has been demonstrated that basic psychomotor laparoscopic skills can be assessed by analysing motions of the instrument [Cotin, 2002; Moor-thy, 2003b]. Several measures have been proposed (e.g., the length of the curve de-scribed by the tip of the instrument, the total distance travelled by the instrument along its axis, economy of the movement, changes in instrument velocity over time) [Gallagher, 2004; Cotin, 2002; van Sickle, 2005; Acosta, 2005; Cavallo, 2006]. In or-der to use motion analysis as the assessment tool, a system is needed to track and record these motions. In VR trainers such tracking systems are inherently present. Tracking systems that can track real laparoscopic instruments during surgery or in the box trainer are still in their infancy.

In general, one may consider the tracking system as an independent component of a training system, not directly related to the choice of exercises and scoring mea-sures. The sole task of the tracking system is to measure the position (x, y, and z coordinates) and the orientation (yaw, pitch, and roll) of the laparoscopic instru-ment with respect to a fixed reference frame.

Presently, there is a number of (commercially) available devices for tracking movements of laparoscopic instruments. These devices are specific for different en-vironments: Box trainers, virtual reality trainers, and operating rooms. Our study of the literature showed that no overview of the systems used for tracking motions of the laparoscopic instruments has been published. Since there is a number of such

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systems already available, and since new systems are being developed, it is diffi-cult for a potential customer to find appropriate information and to choose a proper tracking system. To produce a complete and structured overview of tracking sys-tems, a large number of journals and proceedings available via PubMed, Scholar Google, and patents have been studied. In this article, special care was taken to en-sure that presented information is correct and up-to-date; for each approach it was attempted to answer the following questions: of MIS instruments should meet the following requirements:

- How does it work? - Where can it be used?

- What are its chief advantages? - What are its shortcomings?

For this reason, nine features were used to evaluate the systems: the kind of system (active or passive), the mechanics, the number of degrees of freedom, possibility of using real laparoscopic instruments, environment (box trainer, VR trainer, and/or operating room), haptic feedback, portability, reported accuracy, and commercial availability.

2.2 Tracking Systems

2.2.1 Aspects of General Tracking Systems

Tracking systems are intended as an interface between humans and computers. In terms of hardware, three components required in such systems can be distin-guished: a source that generates a signal, a sensor that receives the signal, and a data acquisition system, which processes the signal and communicates with the computer [Baratoff, 1993]. Depending on the technology, either the source or the sensor is attached to the object, while the other serves as a reference point and is located at a fixed point in the environment. Many of the currently used tracking systems are active; the sensor, which measures the actual movement, is attached to the target to be tracked. Passive tracking systems localise (from a distance) mark-ers/transmitters that have been placed on instruments (or objects) to be tracked in the field.

Current tracking devices are based on mechanical, optical, acoustic, or electro-magnetic technologies. Mechanical tracking devices, typically taking the form of small volume mechanical arms, use a direct mechanical connection between a ref-erence point and a target (angles formed by each joint) to measure the position and orientation of the object in the environment. Optical position trackers work one of two ways:

- One or several cameras are connected to the object, and a set of light emitting diodes (LEDs) is placed at the fixed reference points, or

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- the cameras are mounted at fixed points, while a set of LEDs is placed on the object.

Acoustic tracking devices employ high frequency (20 kHz or greater) ultrasonic sound waves in the form of time-to-flight transducers/sensors or phase-referent systems. The latter class of systems relies on comparing the phase of a reference signal to that of a sensed emitted signal. In a time-to-flight system, the duration of the travel of an emitted signal is correlated with the distance travelled in the air at a given temperature. Electromagnetic tracking is based on the movement of a num-ber of small sensor units, each housing three small (orthogonally positioned) wire coils, within a low frequency electromagnetic field generated by a second three-coil source or transmitter.

2.2.2 Aspects of Tracking Systems for Minimally Invasive Surgery

Minimally invasive surgery requires unique psychomotor skills that are different from those needed to perform open surgery. The surgeon’s hand movements are transmitted through the incision point via a trocar (instrument shaft) to the tip of the instrument. This limits the range of motions from six to four degrees of

free-dom(DOFs): translation of the instrument along its axis (z coordinate – 1st DOF),

rotation of the instrument around its axis (roll – 2nd DOF), left-right and forward-backward rotations of the instrument around the incision point (yaw – 3rd DOF, and pitch – 4th DOF, respectively). Information about instrument motions in all four DOFs provides valuable information, which can be used during the assess-ment of basic laparoscopic skills. Therefore, tracking systems should track motions in all four DOFs.

A great number of active systems contain a mechanical part that mimics the incision (pivoting point). Many of these systems use a gimbal mechanism, which allows the control and measurement of the object’s rotations in three-dimensional Euclidean space. Usually, the gimbal consists of a set of two or three rings, mounted on axes at right angles [Wikipedia]. These rings provide a stable reference to the po-sition and attitude in all three dimensions. Since it is necessary to define a reference point (position, space) in the environment in order to measure the movements of the object, it seems reasonable to use the pivoting point as a reference point in active systems.

A wide variety of different instruments is available for use in minimally in-vasive surgery. Instruments are made by various companies and have different purposes, consequently, the length, the diameter, the handgrip, and the tip of real laparoscopic instruments can differ largely. The variation in instruments has an in-fluence on the design of a tracking system, which should ideally be suitable for any laparoscopic instrument.

Motions of the laparoscopic instruments can be tracked and recorded in three environments: In the operating room (during an operation), in a box trainer, or a virtual reality trainer (during training). The operating room is the most realistic environment, where a surgeon uses real laparoscopic instruments, which provide

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natural instrument-tissue interaction with realistic force feedback. The box trainer also presents a realistic environment where force feedback is obtained due to the use of real laparoscopic instruments. The virtual reality trainer presents a less real-istic (virtual) environment for the training of laparoscopic skills, where simulated laparoscopic instruments are often used. Force feedback in VR trainers (if present) is far from that provided by real laparoscopic instruments. The use of simulated laparoscopic instruments in a VR trainer, however, results in simplified tracking in a VR trainer. Designing a system that can track real laparoscopic instruments in the OR seems to be the most difficult, since factors such as patient safety and er-gonomics in the OR play a critical role. In box trainers, tracking systems have to track real laparoscopic instruments. Ideally, a tracking system should be designed in such a way that it is possible to use it in all three environments. Besides, the sys-tem should be small in order to be portable and easy to place at a desired position. Surgeons execute laparoscopy using hand-held instruments that provide little haptic information [Heijnsdijk, 2004; Bholat, 1999; Bholat, 1999a]. It is still unknown how important haptic feedback is during a laparoscopic task and, therefore, it is very difficult to say whether force feedback is needed during training of basic la-paroscopic skills.

Precision, a degree to which measurements show the same or similar results, is a very important characteristic of each tracking system. The precision with which instrument movements can be measured depends on the resolution and accuracy of the system [Wikipedia]. The resolution is defined by the smallest change detected by the sensor, and is fixed for a given system. The accuracy of the system is defined by the range within which a measured position is correct.

A number of tracking systems is provided only in combination with VR trainers, while other systems can be acquired separately. In the paragraphs that follow, we present both tracking systems that are currently commercially available and proto-types, i.e., systems that are still being developed. The practical use of these tracking systems will also be described.

Passive Tracking Systems

In the ProMIS simulator (Haptica Inc., Boston, USA, http://www.haptica.com), the measurement of the instrument movements is taken using a passive tracking system (Fig. 2.1) [Lacey, Patent]. Three separate cameras capture the video images of the internal movement of the laparoscopic instrument from three different an-gles [van Sickle, 2005]. This design allows for measurement of the motions in the x, y, and z directions. Standard laparoscopic instruments are covered with two strips of yellow tape - markers for the camera tracking system. The tracking system is sit-uated in a large mannequin, thus not easily portable. The system is commercially available as a combination of a real and a virtual environment. It cannot be used during operation (in the OR), however. The ProMIS simulator provides force feed-back.

Sokollik et al. used a 3-D ultrasound measurement system to record the mo-tion along the trajectories of the instruments (Zebris Medial GmbH, Isny, Germany,

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Figure 2.1: In the ProMIS surgical simulator from Haptica (courtesy of Haptica Inc.),

measurement of MIS instrument movements is tracked using a passive system. MIS in-struments are covered with two stripes of yellow tape (a marker). Internal movements of the instruments are captured with three separate cameras.

http://www.zebris.de) [Sokollik, 2004]. The system (Fig. 2.2) determines the spa-tial coordinates (x, y, and z together with rotation) of miniature ultrasound mitters placed on the instruments by means of the relative position of these trans-mitters to a fixed system of three microphones. The system is portable and com-mercially available. Natural haptic feedback is obtained due to the use of real la-paroscopic instruments. Since ultrasound transmitters can be sterilised, the 3-D ul-trasound measurement system can be used in the operating room as well as in a box trainer and VR trainers.

An ultrasound wireless positioning system is being developed at Delft Univer-sity of Technology (Delft UniverUniver-sity of Technology, Delft, the Netherlands, http:// www-etis.et.tudelft.nl). This system is intended to be used in the operating room to detect the exact 3-D location and orientation of the instrument in the pa-tient [Tatar, 2002]. An array of ultrasound receivers detects the positions of the two markers/transmit-ters placed on each of the instruments, outside the patient’s body (Fig. 2.3). This allows readings with an accuracy of 40 µm at a distance of about 1 m between transmitter and receiver. The resolution of the system is about

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Figure 2.2: The Zebris ultrasound system adapted for laparoscopic surgery application

(courtesy of C. Sokollik). In this passive tracking system, ultrasound transmitters are placed on the MIS instruments. Spatial coordinates are determined by means of relative position of the transmitters to a fixed system of three microphones.

5 µm [Tatar, 2005]. This prototype measures movements in one DOF. The size of the system is rather large and, therefore, the system is not easily portable. Natural haptic feedback is obtained due to the use of real laparoscopic instruments.

Active Tracking Systems

Laparoscopic Surgical Workstation, Virtual Laparoscopic Interface, and Laparo-scopic Impulse Engine are the best known hardware interfaces designed for la-paroscopic virtual simulations offered by Immersion Inc. (Immersion Inc. Gaithers-burg, USA, http://www.immersion.com). The Laparoscopic Surgical Workstation and the Virtual Laparoscopic Interface offer two fully instrumented tools (Fig. 2.4). The movements of the instrument in four DOFs are measured and recorded using four electromechanical transducers mounted in the gimbal mechanism [Rosenberg, Patent 3; Rosenberg, Patent 1; Jacobus, Patent2; Jacobus, Patent1; Martin, Patent]. The Laparoscopic Surgical Workstation provides force feedback. The sensor resolu-tion is 8 mm for translaresolu-tion, 0.03°

for roll (rotation), and 0.01°for pitch and yaw. The Laparoscopic Surgical Worksta-tion is rather big (300 mm X 340 mm) and, therefore, not easily portable.

The Virtual Laparoscopic Interface does not provide force feedback. The sen-sor resolution is 22 mm for translation, 0.26°for roll (rotation), and 0.064°for pitch and yaw. The Virtual Laparoscopic Interface is easily portable. Both the Laparo-scopic Surgical Workstation and the Virtual LaparoLaparo-scopic Interface are

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commer-Figure 2.3: An ultrasound wireless positioning system that is being developed at Delft

University of Technology (courtesy of F. Tatar). In this passive tracking system, an array of ultrasound receivers detects the positions of the two markers/transmitters placed on each of the instruments.

cially available. The Laparoscopic Impulse Engine is a tool-based force feedback device that uses servo-motor actuators (Fig. 2.5). Laparoscopic Impulse Engine al-lows movements in four DOFs. A variety of surgical tools (or tool handles) can be fitted in the device. The Immersion devices cannot be used in the operating room.

The CELTS (Computer Enhanced Laparoscopic Training System) is a prototype simulator (Fig. 2.6) developed at the Center for Integration of Medicine and In-novative Technology (CIMIT, Boston, USA, http://www.cimit.org). CELTS is a modified Virtual Laparoscopic Interface from Immersion Inc.; tool handles and main shafts from the Virtual Laparoscopic Interface were replaced with a system that allows for the use of real laparoscopic instruments to detect the trajectories of these instruments in a simulator [Cotin, 2002; Stylopoulos, 2003; Stylopoulos, 2004]. Pitch, yaw, roll, and translation are measured with the original Virtual La-paroscopic Interface sensors. CELTS can be used in a box trainer as well as in a virtual reality environment. It is not possible to use this system in the OR during operations. The system is easily transportable, just like the Virtual Laparoscopic In-terface. The possibility of using real laparoscopic instruments in the CELTS results in a natural haptic feedback.

The ADEPT (Advanced Dundee Endoscopic Psychomotor Tester) was devel-oped at the University of Dundee (University of Dundee, Dundee Tayside,

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Scot-Figure 2.4: The Laparoscopic Surgical Workstation (left) and Virtual Laparoscopic

Inter-face (right) from Immersion (http: // www. immersion. com). These active tracking

sys-tems measure movements of MIS instruments using four electromechanical transducers mounted in the gimbal mechanism.

Figure 2.5: The Laparoscopic Impulse Engine from Immersion (http: // www. immersion. com). This active tracking system is a tool-based force feedback device that uses servo-motor actuators.

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Figure 2.6: The CELTS interface device from Center for Integration of Medicine and

Inno-vative Technology (courtesy of N. Stylopoulos). This tracking system is a modified Virtual Laparoscopic Interface from Immersion Inc. Main shafts and tool handles are replaced with a system that allows use of real MIS instruments.

land). The ADEPT consists of a gimbal mechanism that accepts real endoscopic instruments (Fig. 2.7) [Hanna, 1996; Hanna, 1998b]. This active system tracks and records positions of the instrument in 3-D space, and detects rotational movements. The measurements are taken using potentiometers mounted in the gimbal mecha-nism; this allows readings to within a millimetre. The accuracy of the x, y, and z coordinates (at a distance of 300 mm from the pivoting point) is within 0.5 mm. The smallest angle recognised by the ADEPT is approximately 0.005°. Real laparo-scopic instruments can be used in the ADEPT; nevertheless, it is not possible to use this system to record instrument movements during an operation in the OR. The commercially used ADEPT provides natural haptic feedback.

The Simendo (Fig. 2.8) is a virtual reality simulator for minimally invasive sur-gery (DelltaTech, Delft, the Netherlands, http://www.delltatech.nl) [Demirtas, Patent]. DelltaTech manufactures the Simendo, which comprises both the instru-ment interface and the virtual reality training software. The tracking system in the instrument interface consists of a gimbal mechanism. Translation and rotation of the simulated laparoscopic instruments are measured by an optical sensor, while pitch and yaw are measured by optical encoders. This combination allows mea-suring the movements of the instrument in four DOFs. As the Simendo was es-pecially developed for application in virtual reality simulation, it does not accept real laparoscopic instruments and does not provide force feedback. The instrument interface of the Simendo is small and light and, therefore, easy to transport. The Simendo is commercially available.

The BlueDRAGON tracking system was developed at the University of Wash-ington (University of WashWash-ington, Seattle, USA, http://brl.ee.washWash-ington.edu). This system consists of two four-bar passive mechanisms that are connected to the instruments (Fig. 2.9) [Rosen, 2002b; Rosen, 2002a]. The measurements of the in-strument positions, orientations, and translation are taken with multi turn poten-tiometers integrated into four joints of the mechanism. The system is rather large,

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Figure 2.7: The ADEPT system from the University of Dundee (courtesy of M. Schijven).

This active tracking system measures the position of the MIS instrument using poten-tiometers mounted in the gimbal mechanism.

Figure 2.8: The Simendo from DelltaTech (courtesy of DelltaTech). In this active tracking

system, translation and rotation of MIS instrument is measured with an optical sensor, while pitch and yaw are measured with encoders.

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Figure 2.9: The BlueDRAGON from the University of Washington (courtesy of J. Rosen,

reprinted from [Rosen, 2002b] with permission of IEEE). This active tracking system con-sists of two four-bar passive mechanisms that are connected to the instrument. Multi turn potentiometers are integrated into four joints of the mechanism in order to measure move-ments of MIS instrument.

and thus not easily portable. The BlueDRAGON allows the use of real laparoscopic instruments and can be used in the OR during operation.

The Patriot is a dual sensor tracking system produced by Polhemus (Polhemus, Colchester, USA, http://www.polhemus.com/). The Patriot consists of an electro-magnetic transmitter and receiver. The transmitter serves as the system’s reference frame for receiver measurements; the receiver detects magnetic fields emitted by the transmitter (Fig. 2.10) [Polhemus, Brochure]. In laparoscopic training setups (e.g., SimSurgery), the receiver is placed on the laparoscopic instrument. The Pa-triot has a resolution of 0.038 mm for the x, y, and z positions, and 0.1°for the re-ceiver orientation. Static accuracy of the system is 2.54 mm, and 0.75°. Since the Patriot can be used to track real laparoscopic instruments, it is possible to use the Patriot in training setups (VR and box trainers). The Patriot system is not certified for medical or bio-medical use; therefore, it should not be used in the OR.

Xitact ITP (Instrument Tracking Port) and Xitact IHP (Instrument Haptic Port) are virtual reality simulation platforms produced by Xitact S.A. (Xitact S.A. Morges, Switzerland, http://www.xitact.com). Both systems (Fig. 2.11) measure move-ments of the instrumove-ments in four DOFs. Both Xitact systems consist of the Panto-Scope (Hybrid Parallel Serial drive), and the LinRot (Linear and Rotational drive). The longitudinal and angular positions of the instruments are measured using op-tical sensors placed in the LinRot [Vecerina, Patent]. The yaw and pitch are mea-sured using two optical encoders situated in the PantoScope. The sensor resolution is 0.057 mm for translation, 0.58°for roll, and 0.03°for both pitch and yaw. The

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foot-Figure 2.10: The Patriot, a dual sensor tracking system produced by Polhemus

(http: // www. polhemus. com). Patriot is an active tracking system, which uses elec-tromagnetic transmitter and receiver to measure the movements of MIS instrument.

print and the weight of the Xitact ITP are smaller than those of the Xitact IHP. Both systems are easy to transport. Contrary to the Xitect ITP, the Xitect IHP provides force feedback.

The IOMaster5D and the IOMaster7D are prototype hardware interfaces de-signed at the Research Centre Karlsruhe (Karlsruhe, Germany, http://wwwkismet. iai.fzk.de). The mechanical construction of these systems is based on cable drives and a pantograph mechanism (Fig. 2.12) [Maass, 2003; Kuhnapfel, 2004; Maass, 2005]. This combination allows movements in four DOFs in the IOMaster5D and six DOFs in the IOMaster7D. Additionally, the IOMaster7D captures the movements of the trocar. The yaw, pitch, and angular position of the simulated laparoscopic instrument are measured by optical encoders; the translation is measured with a Hall-sensor. The IOMaster5D and the IOMaster7D have been designed for laparo-scopic virtual simulation and are not able to track real laparolaparo-scopic instruments. Both systems provide force feedback [Maass, 2003; Kuhnapfel, 2004; Maass, 2005]. The IOMaster7D has a large working space (600 mm (W) x 6600 mm (H) x 6300 mm (D)) and, therefore, is not easily portable.

The TrEndo (Delft University of Technology, Delft, the Netherlands, http:// www.3me.tudelft.nl) is a prototype tracking system, which consists of a two-axis gimbal mechanism with three optical sensors (Fig. 2.13) [Chmarra, 2006a]. The gim-bal guides an instrument, while optical sensors measure the movements of the instrument in four DOFs. Natural haptic feedback is obtained due to the use of real laparoscopic instruments. The TrEndo is small and light and, therefore, easy to transport. The TrEndo can be mounted on a box or on a VR trainer. It is not pos-sible to use the TrEndo during an operation. The smallest movement that can be

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Figure 2.11: The Xitact ITP (left) and Xitact IHP (right) from Xitact S.A. (http: // www. xitact. com). In both Xitact interfaces, translation and angular position of the in-strument is measured with optical sensor placed in the LinRot (Linear and Rotational drive), while the yaw and pitch are measured using optical encoders placed in the Panto-Scope (Hybrid Parallel Serial drive).

Figure 2.12: Prototypes of the haptic devices for laparoscopic surgery applications from

Karlsruhe (courtesy of H. Maass). Mechanical construction of IOMaster7D (left) and IOMaster5D (right) is based on cable drives and pantograph mechanism. Yaw, pitch, and rotation of the MIS instrument around its axis are measured using optical encoders. The translation of the instrument is measured with Hall-sensor.

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Figure 2.13: The TrEndo tracking system from Delft University of Technology. In this active

tracking system, movements of the MIS instrument are measured by three optical sensors mounted on the gimbal mechanism.

recognised by the TrEndo is 0.06 mm for translation and 1.27°for rotation of the laparoscopic instrument around its axis. The smallest recognised angle for rotation around the incision point is 0.23°. The relative error of the TrEndo is smaller than 5%. The relative error was defined as the difference between the real and measured travelled distance divided by the real travelled distance. The accuracy of the TrEndo is, hence, higher than 95% [Chmarra, 2006a].

2.3 Application of Tracking Systems in Trainers

Tracking systems are provided either only in combination with trainers (ProMIS, CELTS, ADEPT, Simendo) or separately (ultrasound measurement system, ultra-sound wireless positioning system, Laparoscopic Surgical Workstation, Virtual La-paroscopic Interface, LaLa-paroscopic Impulse Engine, BlueDRAGON, Patriot, Xitact ITP, Xitact IHP, IOMaster5D, IOMaster7D, TrEndo). Tracking systems that track real laparoscopic instruments are mostly used in research environments to develop new scoring methods [Cotin, 2002; Sokollik, 2004; Strom, 2004; Smith, 2001; Rosen, 2006; Chmarra, 2006b]. Presently, most of the companies that produce VR trainers for la-paroscopy focus on the development of the software and they use commercially available tracking devices. For example, LapSim and MIST-VR trainers include the Virtual Laparoscopic Interface, the LAP Mentor trainer includes the Xitact IHP, and the SimSurgery includes the Patriot tracking system. A number of articles on train-ing systems for laparoscopy has already been written [Schijven, 2003; Halvorsen,

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2005; Cosman, 2002; Sutherland, 2006; Dunkin, 2007]. These articles provide infor-mation about the features, assessment, advantages and the shortcomings of these systems. Since this information about training systems has already been published, the focus was placed on the technical features of the tracking systems.

Presently, the most often used objective performance measures in trainers are those based on the kinematics analysis theory:

- time – total time taken to complete the task

- path length – the length of the curve described by the tip of the instrument - movement economy – “ideal path length” divided by the actual path length,

where the ideal path length is the straight-line distance between two targets - deviation from the ideal path – the sum of the differences between the actual

and ideal path

- depth perception – total distance travelled by the instrument along its axis - rotational orientation – the amount of rotation of the instrument around its

axis

- motion smoothness – parameter which represents a change of the accelera-tion, etc. [Cotin, 2002; Acosta, 2005; Cavallo, 2006; Strom, 2004; Smith, 2001] All these measures are based on the time-dependent three-dimensional represen-tation of the tip of the instrument and rorepresen-tation of the instrument around its axis (which represent four DOFs). Some of the measurement parameters, for example time and path length, do not require a very precise and accurate measurement of the movements. Other parameters, such as motion smoothness, do depend on the precision and the accuracy of the measurement.

Performance measures depend on the kind of exercise that is performed (e.g., clip application, suturing). For different exercises different motion characteristics are optimal. It is essential to realise that the tracking system is a device which can be seen as an independent component of the training system. It does not automatically assess the performance of the laparoscopic task and it is not related to the choice of exercises. Tracking systems consist of hardware and software that are only used to measure the position of the laparoscopic instruments at each time point. From these measured positions and their changes in time, additional software can derive performance measures.

Generally, tracking systems need a computer program that collects measured data and evaluates the performance of the user. Theoretically, it is possible to use one universal program to collect and analyse data from each particular tracking system mentioned above. Such a universal program would allow for easy applica-tion of a standard scoring system independent of the tracking system.

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2.4 Discussion

Sixteen tracking systems for minimally invasive surgery developed over the past decade have been presented (Table 2.1). The overview shows that various appro-aches have been used. These approappro-aches have their own advantages and disadvan-tages. Nine of the described tracking systems can be used in one environment only (in most cases in a virtual environment). There are only a few systems that can be used either in the operating room (however, still not very successfully) or in a box trainer.

Based on the technology used, the systems described in this paper have been di-vided into passive and active systems. Passive systems benefit from not requiring any type of cables wiring attached to the instrument. For that reason, this approach is to be favoured from the user’s perspective. However, there is a number of factors that limits the passive systems (for tracking hand-held objects). The most signifi-cant is the need for line-of-sight. Objects held in hand are, by definition, partially hidden behind a person’s hand, and totally hidden behind a person’s body from certain perspectives. A solution to this problem can be to add extra trackers (lo-calisers). However, this will introduce additional cost and computational complex-ity. Therefore, many of the currently used tracking systems for minimally invasive surgical instruments are active. Most of the active systems use gimbal mechanisms to guide and measure the movements of the instrument in four degrees of freedom. Such a mechanism benefits from its simplicity; a gimbal is an easy and inexpensive to produce mechanism. On the other hand, it would be very complex to use gim-bal mechanisms to guide the motions of the laparoscopic instrument during real surgery.

In laparoscopy, the surgeon’s hand movements are transmitted through the in-cision point to the tip of the instrument. This results in a reduced number of degrees of freedom from six to four. Since information of these four DOFs provides valu-able information that can be used to assess basic laparoscopic skills, the focus of this article was only on these DOFs. Nevertheless, laparoscopic instruments also provide a fifth DOF: The opening and closing of the instrument handles. Therefore, a number of tracking systems (mainly for VR trainers) tracks and records also this fifth DOF.

Precision and accuracy are important characteristics of each tracking device. Ideally, a measurement device (or system) is both precise and accurate. In laparosco-py, small movements in the incision point can result in large movements of the tip of the instrument. Therefore, sensors that record the instrument’s movements at the incision point should be able to recognise small movements. The literature survey showed that there is no study which defines how precise and accurate the mea-surements of the laparoscopic instrument motions should be. Therefore, it is diffi-cult to say whether the precision and accuracy of the present tracking systems are high enough to be used to track motions of instruments. Some systems have to be calibrated before use in order to make measurements more reliable and accurate. Often, such calibration is done by positioning the laparoscopic instrument in a

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pre-T a b le 2 .1 : O ve rv ie w o f tr a ck in g s y s te m s N ame Sy st em M ec ha ni cs D O F M IS in st r. E nv ir on me nt P or ta bi lit y Fe ed ba ck Ac cur ac y A va ila bi lit y P ro M IS P -3 + bo x, V R -+ + UM S P -4 + bo x, V R ,O R -+ + UW P S P -1 + O R -+ < 40 µ m -L SW A gi mb al 4 -V R -+ + V L I A gi mb al 4 -V R + -+ L IE A jo ys ti ck 4 -V R + + + C E LT S A gi mb al 4 + bo x, V R + + -AD E P T A gi mb al 4 + bo x + ± 0. 5 mm + Si me nd o A gi mb al 4 -V R + -+ B lue D R AG O N A ba r p as si ve 4 + bo x, O R -+ P at ri ot A -4 + bo x, V R + + 2. 54 mm, 0. 75 ° + X it ac tI T P A P an to Sc op e/ 4 -V R + -+ L in R ot X it ac tI H P P an to Sc op e/ 4 -V R + + + L in R ot IO M as te r5 D A ca bl e d ri ve / 4 -V R + + -p an to gr ap h IO M as te r7 D A ca bl e d ri ve / 4 -V R -+ -p an to gr ap h Tr E nd o A gi mb al 4 + bo x, V R + + > 95 % -D O F – d eg re es of fr ee d om; UM S – ul tr as oun d me as ur eme nt sy st em; UW P S – ul tr as oun d w ir el es s p os it io ni ng sy st em; L SW – la p ar os co p ic sur gi ca lw or ks ta ti on ;V L I– vi rt ua ll ap ar os co p ic in te rf ac e; L IE – la p ar os co p ic imp ul se en gi ne ;P – p as si ve ;A – ac ti ve ; O R – op er at in g ro om; V R – vi rt ua lr ea lit y tr ai ne r; bo x – bo x tr ai ne r.