Computer-based simulation has been used for decades in aviation and other professional fields. However, the last 15 years have seen numerous attempts to introduce computer-based simulation into clinical medicine. Surgery, and specifically minimally invasive surgery (MIS), has led the way in the development and application of this technology in clinical practice. Recently, use of computer-based simulation for training has expanded into the multidisciplinary fields of catheter-based, image-guided intervention, enabling both surgeons and non-surgeons alike to train on new procedures. The widespread introduction and use of computer-based simulation is changing the way physicians are trained and positively affecting the treatments patients receive. We believe that this revolution represents a paradigm shift in the way procedural-based medicine will be learned and practiced.
The terms virtual reality and computer-based simulation are often used interchangeably. Virtual reality, or VR, commonly refers to "a computer-generated representation of an environment that allows sensory interaction, thus giving the impression of actually being present" . However, VR is probably best defined by Riva , who suggested that it was a communication interface based on interactive visualization that allows the user to interface, interact with, and integrate different types of sensory inputs that simulate important aspects of real world experience. It allows the user to interact and experience important aspects of the encounter rather than simply observing. This interaction has important learning implications, which is highlighted shortly. Although first proposed as a training strategy for surgery in 1991 by Satava , acceptance of the use of VR for training approach has been slow due to costs, skepticism within the medical community, and the lack robust scientific evidence to support the efficacy and effectiveness of this training strategy. However, this is rapidly changing.
The first VR surgical simulator in laparoscopic surgery was designed by Satava (1993) . He developed it primarily as a training tool to help counteract many of the difficulties he observed many of his colleagues were having in acquiring the skills for endoscopic sur gery. However, because of the limitations in computer processing capacity, the virtual abdomen was cartoonlike in appearance. Despite this, the simulation was realistic in its anatomical and technical accuracy, allowing trainees the ability to practice skills outside the operating theater in a computer-based environment.
There have been numerous developments in VR simulators since 1991, and these have been reviewed elsewhere . However, we believe that more can be learned from an in-depth analysis of our experience of one particular simulator, the Minimally Invasive Surgical Trainer - Virtual Reality (MIST-VR), over the last decade. Although this represents the experience on a single simulator that trains and assesses simple surgical skills, the principles are applicable to all types of simulators.
One of the things that our experience with this simulator has taught us is that most surgeons are very naive when evaluating the functionality of simulators. Surgeons tend to evaluate simulators on a very superficial level, i.e., does it look like "real surgery," rather than how the instruments or tissue behave, how appropriate the metrics are, or most importantly how appropriate is the simulation curriculum. In the past, surgeons believed that there were two important requisites of any surgical simulator, an accurate depiction of detail and a high level of interactivity. Many felt that organs must be anatomically correct and have appropriate natural properties when grasped, clamped, or cut. Many surgeons believed that grasping an object without weight, shape, or texture made training in a virtual environment insubstantial. However, the best validated VR simulator in medicine, the MIST-VR, has demonstrated these beliefs to be at least partly incorrect.
Another important advantage of computer-based (including VR) simulators is that objective criteria must be built into the simulator to support the assessment tools. The student then trains until they reach the criterion, at which time they are said to have achieved a proficiency level. The proficiency level is established by having an experienced (expert) surgeon perform on the simulator until the surgeon's learning curve is flat for two consecutive trials (frequently by the third or fourth trial). These values then define the benchmark criteria, the figure of merit to which the student must achieve before going to the next task or until completing training on the simulator and graduating to the operating room.
5.1 Simulation Development: Lessons Learned
The first important lesson to be learned about the MIST-VR is that it was developed by a collaborative group including an engineer (Chris Sutton, London), the end user, i.e., a surgeon (Dr. Rory McCloy, Manchester), and an expert in curriculum/metrics development, i.e., a psychologist (Dr. Bob Stone, Manchester). Many simulators are developed by an engineer who has consulted an end user rather than intimately involving them, and rarely are a curriculum development and metrics expert involved. Much like a scientific experiment, a simulator is much more difficult to fix at the end of development than at the beginning. For optimal development, these groups need to be intimately involved at the outset. The experts must also be cognizant of the cost implications of their suggestions weighed against what it truly adds to the simulation. Lastly, in the development of a simulator, surgeons must give very serious consideration to the fidelity, i.e., anatomical realism, haptic feedback, and metrics, they require for the accruement of clinical benefit. One common mistake is that the simulation must look ultrarealistic. In many circumstances, especially when dealing with basic skills and novices, it is preferable to have a lower-fidelity graphic representation that accurately trains and assesses simple skills. Of paramount importance is to perform a task deconstruction (divide the task into its simplest components) and task analysis to ensure that the skills (hand motions, etc.) are correctly presented to the student in the simplest manner. Once simple tasks are mastered, then more complex, higher resolution simulations can be performed.
Our experience with the MIST-VR bears directly on this point. The MIST-VR system was designed to develop and assess minimally invasive surgical skills, using advanced computer technology in a format that could be easily operated by both tutor and trainee. The system is composed of a frame equipped with two standard laparoscopic instruments. This hardware is interfaced with a PC running the MIST-VR software. The software creates a virtual environment on the display screen and is able to track and display the position and movement of the instruments in real time. An accurately scaled operating volume of 10 cm3 is represented by a three-dimensional cube on the computer screen. The overall image size and the sizes of the target object can be varied for different skill levels. Targets appear randomly within the operating volume according to the task and can be "grasped" and "manipulated" .
In training mode, the program guides the trainee through a series of six tasks that progressively become more complex, enabling the development of hand-eye motor coordination essential for the safe clinical practice of laparoscopic surgery. Each task is based on an essential surgical technique employed in MIS (see above). Performance is scored for time, error rate, and efficiency of movement for each task, for both hands. Every time a trainee logs onto the system a record of the trainee's performance is stored in a database, thus providing an objective record of the trainee's progress. The ability to review the database can help the trainer identify specific areas for further practice. Together these features of the MIST-VR may help to establish objective standards of accomplishment and help to identify when a trainee is ready to enter the operating theatre. In achieving the proficiency level to graduate to the operating room, the MIST-VR can be practiced as often as the student chooses, reviewing the student's performance after each trial, and without requiring the presence of a faculty or observer—the system automatically assesses and reports the performance to the student. With little time for faculty to devote to training, this aspect of simulators is of great value.
5.2 Simulation Training:
In 1997 both Prof. Sir Ara Darzi's group at St. Mary's, London, and Dr. Tony Gallagher's group working at Queen's University, Belfast, were asked by a large US
laparoscopic instrument manufacturer to conduct a preliminary evaluation of the MIST-VR [6, 7]. Preliminary results from both groups were positive. Despite these encouraging results, the initial response of the international laparoscopic surgical community was the MIST-VR simply did not look or feel like laparoscopic surgery. As a follow-up to this preliminary work, an extensive list of scientifically robust studies demonstrating that when training with the MIST-VR was objectively compared to the current standard of training for the development of laparoscopic skills, the MIST-VR produced skills that were at least as good as, or good but usually better than, the conventional training program. Despite these studies the surgical community remained unconvinced. Many skeptics pointed to the fact that all of these initial studies simply demonstrated that training on the simulator improved performance on tasks in the skills laboratory and did not demonstrate benefits in operative performance. This was a valid criticism which needed to be addressed. In 2001, a multidisciplinary team at Yale University conducted a prospective, randomized, double-blind clinical trial to test whether training on the MIST-VR translated into improved intraoperative performance. The trial compared the performance of a group of residents who received standard surgical residency training to a matched group who received proficiency-based training on the MIST-VR; that is, the residents trained as many trials as necessary to reach the criteria and achieve the proficiency level. Both groups then were objectively assessed on their ability to dissect the gallbladder from the liver bed during a laparoscopic cholecystectomy .
The results of this study showed training on the simulator significantly improved intraoperative performance. VR trained residents performed the procedure 30% faster and made six times fewer objectively assessed intraoperative errors when compared with the standard-trained residents. Although the number of subjects was small (n = 16), the statistical power of this effect was 0.9996. These results have been independently replicated in Denmark .
The response of the surgical community to the results of this study was mixed; for some this was enough to convince them that simulation was a powerful training tool. However, the majority clung to the criticism that while the study was well designed, the small number of subjects and the fact that only part of the procedure had been performed reduced its widespread acceptance. In October 2004 at the  Clinical Congress of the American College of Surgeons, another prospective, randomized, double-blind trial from Emory University was reported, which used the exact same experimental design as the Yale study. However, there were two important differences: in the Emory study subject's performance was assessed on the full laparoscopic cholecystectomy procedure, and the Emory study used only surgical residents in postgraduate years 1 and 2, whereas the Yale study used residents in years 1-4. Again, the VR-trained group significantly outperformed the standard trained groups. We believe these results demonstrate two very powerful things, the first being that simulation, when applied correctly to training, succeeds in improving performance, and the second is that even a low-fidelity VR simulator such as the MIST-VR can produce a very powerful training effect. Why does simulation training produce such a powerful training effect? The answers lie in the understanding of the importance of metrics and application of simulation adhering to sound principles of education and training.
Computer-based simulation has several advantages when compared with conventional methods for surgical training. One of the major advantages of computer-based simulation is that the same experience or sequence of events can be replicated repeatedly. This repetition allows the trainee to learn from mistakes in a safe environment. Another benefit that is probably equally if not more important is the objective feedback a trainee can receive from a computer-based simulator. Since everything a trainee "does" on a computer-based simulator is essentially data, all actions can be tracked by the computer. In addition to crude measures such as performance time, detailed data such as instrument path length, speed of instrument movement, and the exact location in space of any instrument at any point in time is recorded. While this data alone is meaningless, it can be used by subject matter experts to create a set of very robust and objective performance metrics. A simulator without metrics is really no better than an expensive video game. While the main function of metrics is to provide the trainee with objective and proximate feedback on performance, they also allow the trainer to objectively assess the progress of the trainee throughout the training process. This allows the trainer to provide formative feedback to aid the trainee in acquiring skill. While providing this formative feedback is currently the most valuable function of objective assessment with simulation, inevitably simulators will be used for summative performance assessment. This testing will then be used for processes such as selection and credentialing in the future, much like knowledge testing is used now. In order for simulators to be applied to such high-stakes assessment, a much more rigorous set of metrics is required, and is still in the experimental phase. When this does come to the fore it is certain the metrics for that simulator must be shown meet the same psychometric standards of validation as any other psychometric test .
The formulation of metrics requires breaking down a task into its essential components (see above: task deconstruction, task analysis) and then tightly defining what differentiates optimal from suboptimal performance. Unfortunately this aspect of simulation has been given all too little attention by the simulation industry. Drawing on the example from the MIS community, almost all of the VR simulators use execution time as a metric. Unfortunately time analyzed as an independent variable is at best crude and at worst a dangerous metric. If one thinks of performance as being a function of time and quality, the relationship can be represented by the following equation:
Performance ~ Quality Time
Thus, performance is directly proportional to quality and inversely proportional to time. With this relationship, if quality is held constant and time decreases, then performance is improved. Conversely if a large increase in quality is gained from a minimal increase in time, performance is still improved despite the longer execution time. While this is obviously an oversimplified relationship, it serves to illustrate the importance of the fact that if time is to be used as a metric, some metrics to assess quality must also be included.
For example, in the MIS environment, being able to tie an intracorporeal knot quickly gives no indication of the quality of the knot. A poorly tied knot can obviously lead to a multitude of complications. There are only a few reports in the literature that use objective quality analysis because of the difficulty in acquiring this type of information, but this type of information is greatly facilitated in the computer-based environment.
There is no magic solution to the issue of quality metrics, and it is almost certain that good metrics will have to be simulator and procedure specific. For example, as we have illustrated, while time alone is not a crucial metric for MIS procedure performance, time and the resultant radiation exposure is very critical in the assessment of performance in many image-guided, catheter-based procedures where extreme doses of radiation can lead to burns and other dire consequences.
Quality measures can be assessed both inside and outside of the computer-aided environment. The Imperial College laparoscopic group, led by Sir Ara Darzi, has been researching economy of hand movement for number of years by an electromagnetic tracking system they have developed (ICSAD) . What they have found is that experienced surgeons have a smoother instrument path trajectory in comparison with less experienced surgeons. The elegance of this approach is that the system can be used to assess open as well as MIS skills. Other groups [12-14] have been using different metrics such as performance variability and errors as a key indicator of skill level. Senior or experienced surgeons perform well and consistently—the reduction of variability is an extremely important aspect of a proficient surgeon, so training to be consistent is as important as training to be proficient.
The most valuable metrics that a simulation can provide is identification of errors. The whole point of training is to improve performance, make performance consistent, and reduce errors. Simulation designers must take great care to create error metrics that both train safe behavior as well as not allow unsafe behavior. As mentioned previously, one of the major benefits of simulation is that trainees are allowed to make mistakes in a consequence-free environment, before they ever perform that procedure on a patient. But if a simulator allows a trainee to perform an unsafe maneuver without identifying it as an error, dangerous behaviors can be trained, possibly becoming difficult to untrain later. Thus, omitting important error metrics and allowing unsafe behavior must be avoided, and this requires close collaboration with procedure content experts who are also familiar with simulation. The end result of a good simulator with well-designed metrics is a training system where trainees can learn both what to do and what not to do when operating on patients. In the didactic part of the curriculum, the student must be taught exactly what the error is, and then should be tested (written) to ensure that the student is able to identify when he or she make an error, before starting on the simulator. The errors must be quantified so as to be completely unambiguous. Without robust metrics the simulator is at best an expensive video game, and at worst an adverse outcome waiting to happen.
The current published evidence clearly demonstrates that VR simulation can improve intraoperative performance. There seems to be some confusion as to whether simulators educate or train individuals, and the two terms are often used interchangeably. Simulation is frequently referred to as education rather than training, or education and training. Although closely related, education and training are not the same. Education usually refers to the communication or acquisition of knowledge or information, whereas training refers to the acquisition of skills (cognitive or psychomotor). Individuals being prepared to perform a procedure need to know what to do, what not to do, how to do what they need to do, and how to identify when they have made mistakes. Most available VR simulators provide technical skills training. They primarily teach the trainee how to perform the procedure and do not concentrate on the didactic information that the physician should know to efficiently and safely deal with adverse events such as complications or unusual anatomy. This however is not always the case.
A VR-based training study for carotid angiography in which preliminary results were reported at Medicine Meets Virtual Reality 2005, lends support to the power of VR simulation as both an education and a training tool. This study compares in vivo hands-on-mentored catheterization training in comparison with VR-based training for carotid angiography. The subjects are senior attending interventional cardiologist and fellows. Preliminary results are very compelling in favor of the VR-trained group in terms of catheter and wire-handling skills; based on the results of other simulators, this outcome is what was expected. However, one of the preliminary findings that we were not expecting is that the VR-trained group outperformed the standard training group with respect to acquiring the appropriate cranial and vasculature fluoroscopic images during the assessment procedure. This is not really a technical skill but rather knowledge-based skill. On considering this finding, the most reasonable explanation is that the VR trainees were acquiring knowledge about important aspects of the procedure such as order and image orientation while they were as a priority acquiring the technical skills. So while the benefit of VR as a training tool has been well demonstrated, its power as an educational tool may currently be underestimated.
5.5 Simulation Fidelity: Are Graphics Enough?
While one of the advantages of training on a high-fidelity, full-procedural simulator may be additional knowledge accrual, this should not be interpreted as a mandate that all types of computer-based simulation must be high-fidelity. In reality, there are many other means of conveying this knowledge-based information that will be equally or more effective with considerably less cost. The main function of a simulator is, in fact, that the cognitive component of technical skills training should be acquired prior to the psychomotor skills training on the simulator. As simulator fidelity increases so does the price of some current high-fidelity simulators, costing anywhere from $100,000 to over $1 million. Thus end users of surgical simulation must assess how much fidelity is required to achieve the greatest return on investment. The data from the MIST-VR clinical trials clearly demonstrate that a low-fidelity simulator can consistently improve intraoperative performance. However, this does not mean that simulation fidelity is unimportant. Consider, a straightforward laparoscopic cholecystectomy performed by a surgical resident under the direct guidance of an attending/consultant surgeon in the operating room. This is not a particularly high-risk training situation, and the risk of a life-threatening or life-altering complication is very low . Conversely, an endovascular surgeon performing a carotid angioplasty and stenting procedure carries much more risk. Results from the only multispecialty prospective randomized trial on this procedure performed by experienced physicians showed that the risk of stroke or death at 30 days was as high as 4.6% . In a high-risk procedure such as carotid angioplasty and stenting, the fidelity of the simulator should be maximized in attempt to replicate the exact procedure as closely as possible to take every step possible to minimize patient risk.
Another important point to make about fidelity of a simulator is that fidelity goes beyond computer graphics and presentation. Unfortunately many surgeons are overawed by and place too much emphasis on the pure graphics aspect of the simulator. In a high-fidelity simulation, the tissue and instruments should behave as close as possible to how they would in a patient. The instruments must not behave as if there is a predefined path for them or automatically tie the knot, and tissue behavior should also be as realistic as possible. A high-fidelity simulator must allow the trainee to make mistakes (both cognitive and psychomotor skills) and learn from these mistakes and the trainee's performance must be meaningfully quantified, with well-thought out metrics that distinguish between those who are good at the procedure and those who are not. A robust but very simple toolkit of reports for the analysis of performance should be incorporated into the simulator to give clear and easily understood feedback when an error is made. If surgeons ignore or fail to appreciate this issue, we risk spending large amounts of resources for simulators that will not meet our needs.
Whether a high-fidelity, full-procedural or low-fidelity, basic training simulator is purchased, it should be remembered that it is only a tool that must be integrated into a well-developed curriculum to be effective. Inappropriate application of simulation will lead the user to the erroneous belief that simulation does not work. So how should simulators be appropriately applied to a training curriculum? The goal of current simulation-based training is to create a pretrained novice. This term describes an individual who may have little or no experience with performing the actual procedure, but who has trained to the point where many of the required fundamental skills have already been mastered. With this accomplished, the trainee can devote nearly all of his or her attentional resources to learning the details of performing the actual procedure, such as how to identify the correct dissection planes or how to gain exposure in the operative field instead of concentrating on what his or her hands are doing. This results in optimization of the operating room experience, reduces frustration of both the trainee and mentor, and it should result in accelerated learning.
To achieve this goal, a training curriculum must be structured to optimize the skills gained from the simulator. Any valid simulator will have the ability to distinguish between the performance of individuals who are already proficient at the skill being trained, and those who are not. Using the carefully developed metrics and setting the criteria by which the figure of merit for the proficiency level is determined as discussed previously, the simulator can then objectively assess and quantify the performance of the proficient individual. This objectively determined proficiency level can then be used as a goal for those training on the simulator and in fact, this is the key aspect of implementing a successful simulation training curriculum. Training on the simulator should not be complete until the trainee has reached an objectively established level of proficiency.
As a guide to curriculum development, the design of any curriculum should contain 6 sequential parts: (1) anatomy instruction, (2) steps of the procedure, (3) identification of errors, (4) a written test to insure cognitive knowledge, (5) skills training and assessment on the simulator, and (6) results reporting and feedback to student.
The traditional way that simulation has been applied to training is through a prescriptive approach. Typically the trainee is required to train for a prespecified number of trials or number of hours. However, all that this approach achieves is considerable variability in posttraining skills . Individuals start from different baseline skill levels, they learn at different rates, and some are more gifted than others. Simulation allows for leveling of the playing field and sets a skill benchmark, which as individual can reach at his or her own pace. Individuals should also not be allowed to progress to the next phase of training until they demonstrate they are performing proficiently and consistently. When setting the proficiency level, the surgeons used to set the standard do not need to be the best of the best; rather, they should reflect a representative sample of the proficient population. If the proficiency level is set too high, trainees will never reach it and if set too low, an inferior skills set will be produced. Ideally, proficiency levels should be set nationally or internationally. While national or international proficiency levels on VR simulators may be some way off, proficiency levels can be set locally in each training program or hospital. The Yale VR to OR study and the Emory VR to OR study has shown the power of this approach [8, 10]. The whole point of training is not simply to improve performance, but also to make it more consistent. Indeed performing well consistently is emerging as one of the key indicators of training success [8, 12].
Proficiency-based training as a new approach to the acquisition of procedural-based medical skills took a giant leap forward in April 2004. As part of the roll-out of a new device for carotid angioplasty and stenting, the US Food and Drug Administration (FDA) mandated, as part of the device approval package, metric-based training to proficiency on a VR simulator as the required training approach for physicians who will be using the new device . The company manufacturing the carotid stent system informed the FDA that they would educate physicians with a tiered training approach utilizing an online, multimedia, didactic package, and training of catheter and wire-handling skills with a high-fidelity VR simulator, using a curric ulum based on achieving a level of proficiency in both the didactic and technical areas. What this approach allows is for training of physicians who enter training with variable knowledge, skill, and experience, but leave with objectively assessed proficient knowledge and skills. This is particularly important for a procedure like carotid angioplasty and stenting, as it crosses multiple clinical specialties with each bringing a different skill set to the training table. For example, a vascular surgeon has a thorough cognitive understanding of vascular anatomy and management of carotid disease, but may lack some of the psychomotor technical skills of wire and catheter manipulation. Conversely, an in-terventional cardiologist may have all of the technical skill, but may not be as familiar with the anatomical and clinical management issues. A sound training strategy must ensure that all of these specialists are able to meet an objectively assessable minimum level of proficiency in all facets of the procedure. We believe that this development represents a paradigm shift in the way procedural-based medicine is trained and will result in a reduction in turf wars concerning future credentialing for new procedures. As long as a physician is able to demonstrate that he or she possesses the requisite knowledge and skills to perform a procedure, specialty affiliation will become irrelevant. Overall, we see this development as a good thing for surgery, procedural-based medicine, and for patient safety.
Computer-based simulation or VR simulation in surgery has been around for more than a decade and a half, but has only recently begun to gain momentum. Despite considerable early skepticism, there is now a growing body of level 1 objective evidence to show that properly applied computer-based simulation training strategies can improve performance of surgical trainees. Developing simulators to produce these results is not easy and must be done collaboratively with experts in computer science, engineering, medicine, and behavioral and educational science to produce a robust training tool. Graphics and good looks are not enough, and robust metrics must be in place to help trainees learn both what to and what not to do. Finally, simulation must be incorporated as a piece of an overall education and training curriculum designed to produce a pre-trained novice with consistently reproducible skills.
Ironically, a training solution  that was proposed more than a decade and a half ago to help solve skills problems in laparoscopic surgery is helping to change the training paradigm in all of procedural based medicine. It is an approach to training that his here to stay.
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Organizing Surgical Simulation Centers in Response to Disruptive Technologies
Mark W. Bowyer*
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