Ergonomic Evaluation of a Prototype Console for Robotic Surgeries via Simulations with Digital Human Manikins
Work-related musculoskeletal disorders impact surgical performance, which increase risks for patient safety. A new console has been designed to reduce workload for robotic surgery surgeons. Due to high costs and long waiting time of the production process, a pre-production ergonomic evaluation of the new design is preferable. In this paper, we evaluate if the new console at the pre-production stage by using an US checklist, and the Swedish standard for visual display unit work. A 3D model of the new designed console was introduced to the virtual environment of a digital manikin (Intelligently Moving Manikin, IMMA). The work-ranges of the console were calculated. Various individual work distances of 12 manikins (3 men and 3 women per each of the US and the Swedish population) were “measured”. The data were integrated and used as an objective reference to compare with the Swedish standard, and the US checklist. The result shows that the criteria in the Swedish standard and the US checklist are fulfilled, except for those are related to the adjustable range of the screen view height, the height range of the armrest and the adjustable distance of the pedals. The new console fulfills most of the criteria in the checklist and the standard, but there is room for a few improvements. The DHM tool IMMA provides the possibility for a pre-production assessment. However, the limited virtual measurement tools of IMMA restrained the time efficiency of the ergonomic assessment.
KeywordsErgonomic evaluation Robotic surgery Digital human manikins
Work-related pain, fatigue, stiffness and numbness, especially related to the neck, back and shoulders, are frequently reported among surgeons in laparoscopic practice (74%) . Such symptoms can affect surgical performance  and likely the patient safety.
Recent years’ introduction of a robot-assisted laparoscopy approach provides ergonomics benefits for the surgeon [3, 4]. However, it still involves constrained and static working postures associated with risk factors for developing musculoskeletal disorders (MSDs) . A new surgical system has been designed to allow the user more flexible working postures by e.g., offering an open display. The operation system that is so far not yet produced will consist of two main parts, a cart with interactive robotic arms which are in contact with the patient, and a console including hand controls, foot controls, and two screens (one 3D monitor in front of the surgeon, and one side screen for displaying medical images and to control the panel when necessary) which provides the surgeon to control the robotic arms and monitor the surgical site. Moreover, the console is equipped with an armrest frame. For flexibility between various users, the foot controls are adjustable in forward and backward direction and the armrest and the 3D monitor are adjustable in height. Both screens are also adjustable in angle position. The hand controls have seven degree of freedoms that allow for flexible control of the robotic arms.
However, the prototype with its promising improvements has not been ergonomically evaluated. The promising improvements have not been yet validated. Since the console is a complex and expensive system, manufacturing process can be both time- and finance-consuming. Any defects from the prototype can multiply the total cost. Therefore, a pre-production ergonomic evaluation is preferable to identify potential risks of the prototype.
Digital human modeling (DHM) tools have been used in human activity simulation within multiple scenarios in industry . Most manikins require manually setting up every joint when using the model. To overcome the weakness of this method, a new IMMA (Intelligently moving manikins) was developed for simulation and visualization of human work and for ergonomic assessments . The IMMA tool simulates body postures and motions required by assigning joint angels to the internal model of a manikin´s skeleton [8, 9]. Within the IMMA tool, an anthropometric module allows the user to specify the requirements for anthropometric diversity in the assessment without repetitively setting up each individual manikin.
The aim of this project is to evaluate a prototype laparoscopy robotic console design at the pre-production stage, using a US checklist and the Swedish standard, that are relevant for visual display unit work, as references.
A 3D model of a prototype console was introduced to the virtual environment of IMMA. Two manikin families were implemented, which represented 90 per cent of the US and Swedish population, respectively. Each family contained three digital manikins that referred to a low, medium and upper border of the targeted population. The manikin was controlled in a seating position in the measurement, where the angle between the lower leg and thigh was 90 degree, as recommended in both ergonomic standards.
3.1 Measurements in the Virtual Environment
Several landmarks were set up to conduct the measurements. Reference points and reference planes were set up to facilitate the comparison between the manikin’s and the console’s work range. The prototype was designed in a condition that the user’s chair is equal or higher than 43 cm. Therefore, the manikin sitting height was regulated accordingly. All measurements were static. The possible work ranges of the manikins and of the console were calculated from the data and was normalized. The results were compared with each other.
3.2 Ergonomic Standards
Since ergonomic standards within surgery are lacking, and since the work posture in the console is similar to computer work, in this study ergonomic standards and checklists criteria for computer work were used for the ergonomic assessment as references. Swedish regulations for computer work places were used for comparisons with geometries from simulations using the manikins representing the Swedish population , which in some parts refer to the regulation for workload ergonomics . The American checklist for computer workstations  was used in comparisons with geometries from simulations with US population manikins.
The Swedish standard is only available in the Swedish language and was therefore first translated into English. The text was then shortened and transferred into a table checklist to facilitate the comparison and the presentation of the outcomes. Parts and paragraphs of the checklist and standards not relevant for the present assessment, e.g., details related to the chair, glare from the monitor or windows as well as placement of accessories, were not considered in this study.
3.3 Comparison of Outcome Measures to Checklist and Standard
The ergonomic assessment was guided by the checklist and the standard, and was verified by comparison of the parameters of the optimized working postures of manikins and parameters that were allowed by the adjustment ranges of the design of the console.
4.1 Work Ranges
4.2 Ergonomic Assessments
Original from evaluation were listed here.
Working Postures #1. Since the top height of the 3D monitor, when adjusted to lowest position, is in line with the eye height of the tallest female manikin, the shortest, and average female manikins, as well as the shortest male manikin, must bend their head backwards to be able to look at the top of the 3D monitor. For the average male manikin, the screen is within an adjustable range for the eye height, but for the tallest male manikin, the screen, when adjusted to the highest position, is too low, which means that the manikin always has to bend the neck forward when watching the screen.
Working Postures #6. None of the female manikins can achieve a relaxed sitting position for the forearm when using the armrest. When adjusted to the lowest height, the armrest is still too high (53 mm) for the tallest female manikin. For the shortest male manikin, the situation is the same, but both the average and tallest male manikins can achieve a relaxed sitting position.
Working Postures #8. While sitting with the thighs parallel to the floor, none of the manikins is able to reach the foot controls. The tallest manikin is within the closest range to reach, but still 28 mm in horizontal direction is missing. The missing distance is considerable longer for shortest female manikin, i.e., 141 mm. To be able to reach the foot controls, the knee angle between the thighs and the calves must increase. For interest, the knee angle was measured when the manikin was set to reach the foot control, and the knee angle, calculated as the angle between the calves and a line perpendicular to the floor, increased with 6° (tallest male manikin) to 30° (shortest female manikin) within the manikin family.
Working Postures #9. When placing the manikins in a seated position in front of the console, the feet can rest flat on the floor. But during surgery work, the user has to use the foot controls frequently, which means that only the heels may rest on the floor.
Monitor #1. See “Working postures #1”.
Monitor #4. The 3D monitor is positioned directly in front of the user. The side screen, positioned at the side of the console armrest, is generally used only during a few minutes of the working time and therefore not considered appropriate to assess further.
Paragraphs within the standard not fulfilling the requirements in the Swedish standard
3. All screens, armrest and handles are adjustable in height, which means that the work posture, to a certain extent, can be changed while seating. However, it is not possible to raise the console to a level suitable for standing work.
4a. Only the tallest female manikin and the average and the tallest male manikin are able to use the armrest properly, i.e., while sitting with the forearm in a relaxed position. For the rest of the manikins, the armrest was too high, i.e., was not adjustable to a lower level.
4b. The armrest is narrow and tilted inward and cannot support the entire forearms.
5a. When adjusted to lowest position, the top height of the 3D monitor is above the eye level of the shortest and average female manikins as well as for the shortest male manikin. When adjusting the screen to the highest position, it is still too low for the tallest male manikin. For the tallest female manikin and for the shortest and average male manikins, the 3D monitor is within the adjustable range for the eye level.
5c. The screens are able to tilt but are not sufficient adjustable in height, see #5a.
5d. See question 5a.
6b and 11b. See question 4a.
12c and 13d. The console’s limited adjustability in height, reduce the possibility to work in a standing posture.
13b and 13c. While having the manikins placed in front of the console, the feet can rest flat on the floor. However, during surgery work, the foot controls are used frequently.
This study used the U.S. checklist and the Swedish standard requirements for computer work as references to evaluate the ergonomics of the prototype console from the outcome measures, i.e., distances and angles, from two manikin families (one based on the US population anthropometric data and the other on Swedish population data) placed in working position in the prototype console. The measures were obtained from the digital human modeling tool IMMA which provides objective and detailed information.
5.1 Comparison Between Console-Manikin Outcome Measures and Checklist/Standards Requirements
The outcome of the comparisons showed that several of the requirements (relevant for the study) were fulfilled, but also that a number of requirements were not satisfied.
Both manikin families were within the required viewing distance range, i.e., the preferred viewing distance, measured from the eyes to the front surface of the screen, which according to the OSHA should be within a range between 50 and 100 cm while the Swedish standard recommend approximately 70 cm. Moreover, all manikins could take advantage of the flexibility of the control handles, which allows the user to work with reduced load, with the arms close to the body (within the inner work area) and with the wrists in a neutral position.
The armrest allowed for a relaxed arm working posture, but it was not enough adjustable in height to suit all the manikins. Also, the adjustability of the screen height and tilt allowed for individually adapted working postures, however, the limitation of adjustability in height reduced the ability to achieve a good working position for some of the manikins.
Requirement regarding armrest height was not fulfilled while placing the manikin in an ergonomically correct sitting posture. Several of the manikins were not within the range of the adjustable height, and in about half of the cases, the height of the armrest was too high for the user to be able to sit in a relaxed position. However, if working in a position with arms stretched forwardly (i.e., upper arms not parallel to the back) the elbow angle increases, which means that the height of the forearm (measured from the floor) also will increase. In such a working position, the lowest height of the armrest may be suitable for those manikins where the armrests were too high while sitting with the upper arms ergonomically correct.
Neither the adjustability range of the 3D monitor height was suitable for all manikin members. However, we have not considered the preferred viewing angle to the center of the screen, which according to OSHA normally should be located 15°–20° below the horizontal eye level and should not to be greater than 60°. Considering this in the presented assessment, the user could possibly achieve a suitable working posture of the neck despite the limitation of screen adjustability in height.
The limitation of the adjustability of the 3D monitor and the armrest height also affects the possibility to work in a standing position. Long time sitting work is a risk factor associated with cardiovascular disorder, lower back pain and diabetes, which speaks for further investigation of the ability to switch between sitting and standing work. The use of foot controls while standing may be problematic, especially if both feet should be used simultaneously, but could be solved by also have some of the foot control functions combined with the hand control system.
Stretching the legs forwardly to reach the foot controls means that an ergonomically correct sitting posture may be difficult to maintain. Long time stretch of the legs in combination with a forward bent sitting posture may negatively affect the nervous system (e.g., the sciatic nerve) and the lack of support from the feet tend to affect the back posture causing long-term nerve irritation or/and back pain.
While sitting with the knees at a 90° angle, there is no strain on the low back. But when stretching for the foot controls, the thighs will be forced into a downward direction, which in turn will increase the knee angle and the back curvature, pulling on the low back and creating muscle strain.
Even though the armrest will support the forearm, support for the wrists is lacking. In computer mouse work, support for the wrist is essential, since long-term work without support is associated with overload of the wrist, potentially causing MSDs. In the present work with handheld controls, such support can both affect the work in a negative and positive direction. A wrist support may limit the flexibility of the control and the ability to move the wrist easily and freely. At the same time, such support can contribute to a more relaxed work posture for the wrist with reduced muscle activity, reducing the risk of wrist related MSDs.
5.2 The Use of a DHM Approach for Pre-production Assessment
The DHM approach allows for a pre-production ergonomic assessment of a product in the design phase. Here, the manikin families represented the stature characteristics of the populations required, and the size of the console prototype imported to the IMMA environment was consistent with the real measures of the product. Hence, the manikin postures in interaction with the console can represent the reality, which contributes to a good reliability of the results.
Accuracy and approximation.
The population database that was applied was from 1989 (for US) and 2008 (for Sweden). It is known that anthropometry of a certain population evolves through year. Therefore, there could be a bias of the result due to the outdated data. Additionally, the operation in reality involves a series of complex movements and various possible postures. The assessment would be too complicated to perform if including all the factors and movements were taken into consideration. Therefore, certain approximations were applied before the measurement and assessment with two bottom lines. One is that a posture will be excluded if it is not maintained for significant long time during a typical operation. Since certain approximations have to be made due to the limitation of technical reasons, the quantity of the approximation was controlled to decrease effects on the outcome.
Limitation of the assessment.
In the present study, we only assessed the manikin from an “upright sitting” posture. However, there are other ergonomically correct sitting alternatives, e.g., the declined sitting posture (OSHA) where the buttock is higher than the knee and the hip angle, i.e., the angle between the thigh and the spine is greater than 90°. In such a sitting position, the armrest, which in the upright sitting posture is slightly too high, may be in a suitable position. However, in order to objectively assess the working condition, clear definitions are required, and “declined sitting posture” is a far too wide term to set up the case.
Moreover, IMMA offers different view modes for better visualization of 3D objects. However, there is a lack of consistency among those modes. For example, the thigh of the manikin is modeled as a cylinder in the skeleton mode while in the rendering mode the thigh has an uneven thickness of the knee end and the hip end. It may confuse user when setting up the measurement markers for the estimation of sitting height when switching different view mode. It would be beneficial if an embedded measurement system is implemented to minimize the confusion. It would be even more useful if the embedded system is designed based on ergonomic checklists.
In general, IMMA was designed for better performance in dynamic ergonomic assessment. However, static postures were the “key frames” for motions. The difficulty of setting static postures may influence the effectiveness and efficiency of the total process. An improvement in functions regarding to manipulations of static postures of manikins can not only facilitate static ergonomic assessment such as this study, but also increase precision and accuracy of dynamic ergonomic assessment.
The console with its flexibility, i.e., the adjustable 3D monitor, foot controls, controlling handles and the armrest frame, fulfilled most of the requirements in the checklist and standards for all members of the two manikin families. There were, however, a few of the requirements that were not fulfilled for all the population representing manikin families, because of too limited adjustable ranges that restricted, especially the shortest manikins, to work in recommended postures. The limited adjustability also reduces the possibility of working in a standing posture. Hence, considering the design revisions, there is still room for a few improvements. The DHM tool IMMA provided the possibility to compare static surgery work in the digital prototype phase to ergonomic checklists and standards for visual display unit work, considering human diversity. The limited virtual measurement tools of IMMA restrained the time efficiency of ergonomic assessment. Improvements may also be made to the IMMA tool for these types of evaluations.
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