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Design and evaluation of a new ergonomic handle for instruments in minimally invasive surgery

Published:December 30, 2013DOI:https://doi.org/10.1016/j.jss.2013.12.021

      Abstract

      Background

      Laparoscopic surgery techniques have been demonstrated to provide massive benefits to patients. However, surgeons are subjected to hardworking conditions because of the poor ergonomic design of the instruments. In this article, a new ergonomic handle design is presented. This handle is designed using ergonomic principles, trying to provide both more intuitive manipulation of the instrument and a shape that reduces the high-pressure zones in the contact with the surgeon's hand.

      Materials and methods

      The ergonomic characteristics of the new handle were evaluated using objective and subjective studies. The experimental evaluation was performed using 28 volunteers by means of the comparison of the new handle with the ring-handle (RH) concept in an instrument available on the market. The volunteers' muscle activation and motions of the hand, wrist, and arm were studied while they performed different tasks. The data measured in the experiment include electromyography and goniometry values.

      Results

      The results obtained from the subjective analysis reveal that most volunteers (64%) preferred the new prototype to the RH, reporting less pain and less difficulty to complete the tasks. The results from the objective study reveal that the hyperflexion of the wrist required for the manipulation of the instrument is strongly reduced.

      Conclusions

      The new ergonomic handle not only provides important ergonomic advantages but also improves the efficiency when completing the tasks. Compared with RH instruments, the new prototype reduced the high-pressure areas and the extreme motions of the wrist.

      Keywords

      1. Introduction

      The expansion of minimally invasive surgery (MIS) during the last 20 y has been because of the proven benefits for the patients and the substantial improvement in the cost-effectiveness ratio for health care systems. However, MIS instruments have many ergonomic deficiencies affecting surgeons during their interventions. Since the emergence of these techniques, these deficiencies have been widely addressed in the literature. In [
      • Matern U.
      • Waller P.
      Instruments for minimally invasive surgery: principles of ergonomic handles.
      ], the authors describe the main drawbacks caused by the lack of ergonomic design of the instruments. One of the main reasons for these drawbacks lies in the inefficient handle design of this kind of instrument. Indeed, laparoscopic instruments are different from those used in open surgery and usually incorporate ring handles (RHs) or pistol grip handles in their designs. These handle configurations are not only uncomfortable and cause fatigue but also produce different types of injuries in the short, medium, and long-term. These injuries are mainly associated with two causes. The first one is because they produce compression in reduced areas of hands and fingers, and the second one is because of the nonneutral posture of the hand during surgery.
      In the early stages of laparoscopic surgery, Horgan et al. [
      • Horgan L.F.
      • O'Brian D.C.
      • Doctor N.
      Neuropraxia following laparoscopic procedures: an occupational injury.
      ] investigated the influence of RH instruments on the nerves of the hands, arguing for new ergonomic designs. Park et al. [
      • Park A.
      • Lee G.
      • Seagull F.J.
      • Meenaghan N.
      • Dexter D.
      Patients benefit while surgeons suffer: an impending epidemic.
      ] stated in their article that 87% of surgeons, who regularly perform laparoscopic surgery, suffer symptoms and injuries that can be associated with the instruments. The influence of the wrist flexion, ulnar deviation, and forearm supination is highlighted in [
      • Szeto G.P.
      • Cheng S.W.
      • Poon J.T.
      • Ting A.C.
      • Tsang R.C.
      • Ho P.
      Surgeons' static posture and movement repetitions in open and laparoscopic surgery.
      ]. These works mention a fact that itself proves the lack of ergonomic design. This fact is that experienced surgeons do not use the ring for the thumb finger. Instead of this, they alter the hand grip position with the thumb out of the ring, with a position of the hand to which the handle is not designed for. The influence of the size of the hand is another issue, which is frequently treated in the literature. For instance, Berguer and Hreljac [
      • Berguer R.
      • Hreljac A.
      The relationship between hand size and difficulty using surgical instruments: a survey of 726 laparoscopic surgeons.
      ] presented a survey about the influence of the hand size on the difficulty of using laparoscopic instruments. Furthermore, it is important to point out that the lack of ergonomic designs is magnified by other kinds of problems inherent to this type of surgery, such as the reduced degrees of freedom or the indirect visualization of the surgical field. To reduce the effect of these problems, the authors in [
      • Matern U.
      • Waller P.
      Instruments for minimally invasive surgery: principles of ergonomic handles.
      ] establish 14 criteria that they recommend should be considered in the designs of new MIS instruments. These recommendations are expanded to 22 in [
      • Matern U.
      Principles of ergonomic instrument handles.
      ], which provides a good reference to evaluate the ergonomic characteristics of a handle design.
      Several researchers have proposed different solutions for the ergonomic design of handles. Emam et al. [
      • Emam T.A.
      • Frank T.G.
      • Hanna G.B.
      • Cuschieri A.
      Influence of handle design on the surgeon's upper limb movements, muscle recruitment, and fatigue during endoscopic suturing.
      ] propose a new rocker handle for needle drivers. This handle improves the quality of task performance by reducing the velocity of elbow and shoulder during its use. To improve the ergonomic design in [
      • Veelen M.A.
      • Meijer D.W.
      • Goossens R.H.
      • Snijders C.J.
      • Jakimowicz J.J.
      Improved usability of a new handle design for laparoscopic dissection forceps.
      ], a new handle with a rotating ring for the thumb is presented. This handle attempts to avoid the hyperflexion of the wrist, adapting the position of the hand, and forearm to the task being performed. Nevertheless, in this solution the rings remain as the actuation system of the handle, and hence cause high pressure in the hands. A new ergonomic pistol handle was presented by Büchel et al. [
      • Büchel D.
      • Mårvik R.
      • Hallabrin B.
      • Matern U.
      Ergonomics of disposable handles for minimally invasive surgery.
      ], which provides better contact between the hand and the handle. However, a ring is still used to activate the open–close motion of the grasper with the index finger.
      Despite the widespread use of MIS and the acknowledgment of the problems caused in surgeons, there is not a standard procedure for the ergonomic assessment of the instruments. In the literature, the procedure used in postural assessments in operation rooms is the ISO 11226 [
      ], the Rapid Entire Body Assessment (REBA) [
      • Hignett S.
      • McAtamney L.
      Rapid entire body assessment (REBA).
      ] and the Rapid Upper Limb Assessment [
      • McAtamney L.
      • Corlett E.N.
      RULA: a survey method for the investigation of work-related upper limb disorders.
      ]. These are general methods for light manual work and can be considered adequate for surgical tasks. However, they are clearly insufficient for the ergonomic assessment of specific hand motions in laparoscopic surgery [
      • Burns L.R.
      • Lee J.A.
      • Bradlow E.T.
      • Antonacc A.
      Surgeon evaluation of suture and endo-mechanical products.
      ]. The specific motion of the hand, wrist, and forearms during laparoscopic suturing was studied by Hansen and Schlinkert [
      • Hansen A.J.
      • Schilnkert R.T.
      Hand movements in laparoscopic suturing: a simple vector analysis.
      ]. Gonzalez et al. [
      • Gonzalez D.
      • Carnahan H.
      • Praamsma M.
      • Dubrowski A.
      Control of laparoscopic instrument motion in an inanimate bench model: implications for the training and evaluation of technical skills.
      ] establish a procedure for the study of hand motions using an inanimate bench model. The effect of muscle activation using laparoscopic instruments is studied in [
      • Quick N.E.
      • Gillette J.C.
      • Shapiro R.
      • Adrales G.L.
      • Gerlach D.
      • Park A.E.
      The effect of using laparoscopic instruments on muscle activation patterns during minimally invasive surgical training procedures.
      ]. In this work surface electromyography (EMG) electrodes are used for the assessment of muscle fatigue and injury.
      One important aspect in ergonomic assessment is to establish the ideal conditions in which the surgeons must perform their tasks. These conditions permit the necessary information about the instruments to be obtained during the experimental evaluation. A number of options are currently available, including the real situation in operating rooms, human cadavers, live animals, inanimate models, and virtual reality. Although real situations, cadavers, or live animals are more realistic, they cannot maintain exactly the same ergonomic conditions in the assessment of different volunteers. For this reason, inanimate models within a training box are normally used with this aim [
      • Berguer R.
      • Smith W.
      An ergonomic comparison of robotic and laparoscopic technique: the influence of surgeon experience and task complexity.
      ]. Different inanimate models can be found in the literature. For instance, in [
      • Trejo A.
      • Jung M.C.
      • Oleynikov D.
      • Hallbeck M.S.
      Effect of handle design and target location on insertion and aim with a laparoscopic surgical tool.
      ] a simulated abdomen is used for evaluation or in [
      • Büchel D.
      • Mårvik R.
      • Hallabrin B.
      • Matern U.
      Ergonomics of disposable handles for minimally invasive surgery.
      ] seven tasks are proposed to evaluate different properties of a new handle. Matern et al. [
      • Matern U.
      • Kuttler G.
      • Giebmeyer C.
      • Waller P.
      • Faist M.
      Ergonomic aspects of five different types of laparoscopic instrument handles under dynamic conditions with respect to specific laparoscopic tasks: an electromyographic-based study.
      ] established three test courses to analyze the precise movements of the instruments and their relationship with hand and elbow motions. In general, the ideal inanimate model should provide in a single task a wide range of hand, wrist, and forearm motions.
      The main goal of this article was to present a new handle for laparoscopic surgical instruments. The new design improves the ergonomic characteristics of instruments reducing the high-pressure zones in the hand and awkward positions of the wrist. The design of the experiments and the experimental evaluation of the handle are also presented, comparing the new prototype handle (PH) with other handle common on the market.

      2. Materials and methods

      2.1 Description of the new handle

      The new PH is based on the patented pistol configuration shown in Figure 1, and its use is depicted in Figure 2. The instrument is symmetrical; therefore, it can be used with either right or left hand. The open and close motions of the end-effector is manipulated by mean of the index finger and thumb, which are placed on two levers located on each side of the instrument (see Fig. 2A). As the finger and thumb get closer, the motion of the levers closes the end-effector, whereas moving the finger and thumb apart opens the end-effector. This motion is similar to using tweezers and very intuitive for the surgeon. The levers have smooth, curved surfaces, and adapted to the shape of the fingers. Each lever has an external surface finish to help fingers during the opening motion of the end-effector. In Figure 2C, the normal position of the hand during surgery is shown. Figure 2D shows how the manipulation for the rotation of the end-effector is done. In this case, a rotation wheel is located behind the levers and can be manipulated by the middle finger.
      Figure thumbnail gr2
      Fig. 2Activation of the instrument motions.
      This handle allows the surgeon to grasp the instrument in the so-called power grip mode in combination with precision grip. Power grip mode permits the handle to be held tightly and a great force to be applied if it is necessary, whereas precision grip enables accurate control in manipulation of the levers and the orientation wheel.
      The pistol handle configuration has been selected because it provides two main characteristics. The first one is that the whole surface of the hand is in contact with the handle avoiding high-pressure zones. The second one is that it keeps the hand and wrist close to neutral position during surgery. The size of the grip diameter of the handle has been selected carefully following ergonomic criteria to provide sufficient support and to be adapted to different hand sizes.

      2.2 Testing the instruments: volunteers and tasks

      To assess the ergonomic characteristics, the new handle has been compared with a common RH instrument from the company Covidien (Dublin, Ireland) (see Fig. 3). The instruments selected for the experiments are 5-mm dissectors. Twenty-eight volunteers were selected from novices, medical students, and surgeons. Surgeons were introduced in the experiment as volunteers because they are the targets of the study and their comments about the instrument provided important ideas for discussion and future work. However, they are influenced by their previous use of the RH instruments, and this can be considered a negative aspect. To limit the influence of the previous use of the RH, surgeons with >5 y of experience were excluded as volunteers, and only surgeons with between 3 and 5 y of experience were considered. All volunteers were selected with different age, sex, hand size, muscular strength, weight, and height, but all of them were right-handed to avoid influence of the hand preference on the survey. All participants were healthy, and they did not report any injuries or musculoskeletal disorders.
      After informed, consent was signed by each volunteer, a first questionnaire was completed about the anthropometric measurements of the individuals and their specific variables. Furthermore, they were informed about the practical aspects of the experiments. Before starting the test, they were also informed about the specific objectives of the experiment, paying special attention to the comfort required from the handle. To avoid preferences in one sense or another, they were not informed that one of the instruments was a PH.
      Each volunteer had to do one test, which comprises three different tasks, each one with both handles, the PH and RH. The order of use of the instrument during the test performance was randomly changed to avoid the learning effect. In other words, when the volunteer uses one handle first, he or she improves his or her skill in accomplishing the test. During the test, the order of use was changed to compensate for this effect.
      The tasks were developed based on the work presented by Valentine et al. [
      • Valentine R.J.
      • Rege R.V.
      Integrating technical competency into the surgical curriculum: doing more with less.
      ] using training boxes, which have proved to be adequate for this kind of experiments. Real operations were avoided for the experiments because of the lack of reproducibility and the risk for the patients that could appear during the performance. Thus, the objective of these tasks is to reproduce the motions of hand, wrist, and forearm done in real performance of surgery. In all tasks, the laparoscope remained fixed, so the volunteers were only concerned with the motion of the instruments that they were handling. The use of the dominant hand was preferred, whereas the use of the nondominant hand is recommended as support. The volunteers were located on an adjustable platform in front of the training box to modify the height of the subject, so their body and arms were in a neutral position. The first task (T1) consists of three metal rings located at the top of the three blocks (see Fig. 4A). The three blocks have different orientation, and the volunteers had to pass a curved needle with the thread through the metal rings. The actions necessary to complete this task include grasping the needle with the instrument in the dominant hand, rotating the needle to orientate it towards the ring, and helping the instrument with the nondominant hand to pass the needle through the ring. This task requires exerting a moderate force while grasping the needle and requires a significant rotation of the wrist to rotate the instrument.
      Figure thumbnail gr4
      Fig. 4The three different tasks used in the experiment.
      In the second task (T2) a string with a length of 700 mm must be rolled around a cylinder (see Fig. 4B). The volunteers are allowed to grasp only in the colored parts of the string, which are separated from each other by 100 mm. The actions necessary to complete this task are to grasp the string within the marks with both instruments and to tighten the string and roll it around the cylinder. This task requires a moderate force and a wide motion of the forearm and elbow.
      The third task (T3) consists of eight numbers and eight letters drilled onto a flat board (see Fig. 4C). The volunteers have to insert the numbers and letters in the corresponding groove. At the beginning of the task, the figures are placed with the bottom up, so the volunteers have to rotate it before inserting it in the correct location. This task was carried out mainly with the dominant hand, whereas the nondominant hand is used only to help drag the figures. An intense effort is necessary for grasping the figures together with a wide rotation of the wrist.

      3. Ergonomic assessment

      The ergonomic assessment of each volunteer was done using two different types of surveys. The first one is based on a subjective questionnaire of the opinion of the volunteers. The second one is based on the measurement of objective parameters such as angles, time, errors, and so forth. For the subjective part, a questionnaire was developed to establish the opinion of the volunteers about the instruments. The objective measurements were split into four different methods. They consist of the muscular effort required with each instrument, the goniometric study of motion, and the efficiency and effectiveness in completing the tasks.

      3.1 Subjective survey: design of the questionnaires

      The subjective part of the experiment was related to the satisfaction of the volunteer after using the instrument. Thus, after each task, the volunteers completed a questionnaire with their opinion about the two handles. They were asked about the difficulty of completing the task and had to tick a value on a scale from 0 to 10 as is shown in Table 1. After each test a second questionnaire was completed. Thus, the volunteers were asked the following questions:
      • Q1: Using the scale in Table 1 indicates the degree of difficulty necessary to perform the task.
      • Q2: Which one of the two handles has caused you more pain?
      • Q3: In a scale from 0 to 10 indicate the pain that each instrument has caused you.
      • Q4: If you had to repeat the whole test with only one of the two instruments which one would you choose?
      Table 1Rating scale for the description of the difficulty in completing the task .
      Index012345678910
      DescriptionRestedVery easyEasyFairly easyA little hardRather hardHardVery hardExtremely hard
      After the volunteer answered the questionnaire, a physical examination of the hand was made. The objective of the examination was to find physical evidence of the pain caused by the handles. This evidence was recorded in a photographic register.

      3.2 Objective survey: design of the experiments

      Four EMG electrodes were used to assess the activity of different muscles during the test. They were placed on the following muscles:
      • -
        EMG1: Thenar muscle.
      • -
        EMG2: Flexor digitorum superficialis.
      • -
        EMG3: Extensor digitorum communis.
      • -
        EMG4: Trapezius pars descendens.
      The root means square values have been used to quantify the intensity of the muscular activity and compare the effort necessary to use the two instruments. The maximum voluntary contraction (MVC) was evaluated at the beginning of the test for each one of the volunteers in the four muscles involved. The root means square values obtained from the EMG are used as a percentage of the MVC.
      The tests were observed live and recorded with four cameras simultaneously; three external cameras focusing on the motions of the volunteer and the another one focusing on the task inside the training box (see Fig. 5). One of the external cameras was positioned above the volunteer, the other in front of the subject, and the last one at an angle of 90° between the two other cameras. The angles for the goniometric study measured were the following:
      • -
        G1: Flexion and extension of the wrist.
      • -
        G2: Radial and ulnar deviation.
      • -
        G3: Pronation and supination of the forearm.
      • -
        G4: Shoulder abduction and adduction.
      The measurement process starts by marking the points used as a reference in the arm-hand system. Figure 6A and Figure 6D show the neutral position of the wrist when the forearm is aligned with the hand. In the same figure, the markers A, B, and C establish the references for the measurement of the angles in the wrist–hand system. Marker A is located at the styloid process of the ulna, markers B and C are located on the knuckles of the little and index fingers, respectively. To obtain the angles, a reference line is used passing through the humeroradial joint in the elbow and marker A. Figure 6B shows the angle used for the measurement of the flexion of the wrist, which is considered a positive angle, and Figure 6C shows the angle used for the extension, which is considered a negative angle. Figure 6E shows the angle used for the measurement of the radial deviation, which is considered a positive angle, and Figure 6F shows the angle for the measurement of the ulnar deviation. Figure 6D shows the markers B and C, which are used as the reference for the measurement of the pronation and supination angles. The value of this angle is considered 0° when a line vertical to the floor passes through both markers, and marker B is below marker C. Pronation occurs when the rotation of the wrist is counterclockwise from the point of view of the volunteer and is considered a positive angle. Negative angle is considered for the supination motion when the rotation of the wrist is in the clockwise sense. For the measurement of the abduction and adduction of the shoulder, the acromioclavicular joint is used together with the humeroradial joint.
      Figure thumbnail gr6
      Fig. 6Markers used for the goniometry measurements of the wrist.
      The head, trunk, and lower extremities were considered in a symmetric posture, and they were not included in the study because their ergonomic influence is negligible when the monitor of the endoscopic system is aligned correctly with the Frankfurt plane. Something similar occurs with the flexion and extension of the forearm, which is always between the limits considered acceptable (from 60° to 100°).
      No time limit was established for completing the tasks, but the time needed for testing (trial time, TT) was recorded for each volunteer. This TT can be considered as a measurement of the efficiency, because it is related to resources expended to achieve the objective of the task. The effectiveness of the task completion was also measured considering 100% when the task is fully completed and a percentage of the achievement when it is not finished. In task T3, 100% of the effectiveness was considered when all the figures were placed in their correct position. A reduction of the efficiency is considered when the volunteer dropped one or more figures outside the work area.

      3.3 Statistical analysis

      For the interpretation of the results obtained from the experiment, a statistical analysis was done for both subjective and objective surveys. For a description of the qualitative variables obtained from the questionnaires, the percentages and their confidence interval were used based on the Wilson method. The Shaphiro–Wilk test was used to verify whether the qualitative variables follow a normal distribution. The variables are described by using the mean and standard deviation (SD) when they have a normal distribution or by using the median and interquartile range (IQR) when statistical variables do not follow this kind of distribution. The Student or the Man–Whitney tests were used to check independent means when the application conditions (normality and homoscedasticity) were not fulfilled. The paired Student t-test was used when the means were paired, and the Wilcoxon test was used when the conditions of applicability were not fulfilled.
      A multiple regression model was used to measure the difference between the two instruments and to establish the relationship with other parameters involved in the study. This regression model enables the elimination of those variables that were not significant in the experiment. The difference between instruments was also transformed into a variable with two categories: PH is equal or better than RH and PH is worse than RH, establishing the relationship with the characteristics of the volunteers. The importance of these factors is expressed by means of the odds ratio. To evaluate the muscular effort, the maximum and mean values of the EMG were used. Significant values are considered when P < 0.05.

      4. Results

      4.1 Subjective survey: results from the questionnaires

      All individuals were asked question Q1 after each task. The statistical results of the answers to this question provide the opinion of the volunteers about the degree of difficulty in completing the tasks. These statistical results are shown in Figure 7, whereas Figure 7A shows the overall results for the complete test, and Figure 7B–D show the results obtained from the volunteers' answers after each task.
      Figure thumbnail gr7
      Fig. 7Statistical results from question Q1.
      The overall results show that the degree of difficulty of completing the tasks reported by the volunteers when using the RH has a mean value of 11.1 and a SD of 3.11, whereas the mean value of the same parameter reported when using the PH is 9.5 with a SD of 3.74 (see Fig. 7A). This difference is significant with P = 0.017, (paired Student t-test with t = 2.54) and strongly favorable to the new PH. This tendency is the same when each task is analyzed individually. In all tasks and in the overall assessment, the mean values are higher for the RH than for the new PH. In tasks 1 and 2, this difference is significant (P = 0.042 and P = 0.028, respectively), however, the difference in task 3 is not significant (P = 0.573).
      The answers to question Q2 reported the pain experimented by the volunteers during the performance of the three tasks. At the end of the experiment, 19 of the 28 volunteers reported more pain with the RH than with the new PH (67.8%, CI 95%: 49.3%–82.1%). This result is not significant (P = 0.087).
      The intensity of the pain for each instrument is reflected by the answers to the question Q3. In this case, the mean value for the RH is 3.9 with a SD of 2.40, whereas the mean value for the new PH is 2.5 with a SD of 2.16. Thus, the new PH produces less pain for the users with a difference that is highly significant (P = 0.008).
      The answers to question Q4 addressing the preferences of the volunteers for using one or another instrument revealed that 18 of the 28 volunteers preferred the new PH, if a repetition of the test was necessary (64.2%, CI 95%: 45.8%–79.3%). This difference is not significant with P = 0.185.

      4.2 Objective survey: results from the measurement of variables

      All data from electromyography, goniometry, and TT were recorded, and their results were statistically analyzed. In the next paragraph, these results are presented graphically and numerically.
      The EMG results are shown in Figure 8 for each task, and Table 2 shows the overall outcomes for each muscular group. There are no significant differences in the muscular activity in the Thenar muscle, Flexor digitorum superficialis, and Extensor digitorum communis. However, the muscular activity is slightly higher when the PH is used. This difference is significant in the case of the Trapezius muscle. These differences can be appreciated in Figure 8, where the muscular activity is shown for each task. The differences between the two instruments are not very important, but there is a tendency in the required muscular effort to be somewhat higher with the new PH than with the RH.
      Table 2Overall results obtained from the EMG measurements.
      Muscular groupRHPHWilcoxon P value
      MeanSDMeanSD
      EMG139.121.8246.122.000.431
      EMG238.918.0246.618.510.167
      EMG317.211.7021.315.990.500
      EMG417.08.5225.314.280.024
      The overall goniometric outcomes are shown in Table 3 where they are compared with the ISO 11226 and REBA limits when it is applicable. The differences obtained between the two instruments are highly significant. As is shown in this table for the G1 angle, the RH has a tendency to require a rather large flexion of the wrist, whereas the new PH provides a hand posture with a smaller angle of extension. Considering the REBA method, which is the most restrictive in this case, this angle should be within the range ±15 to be considered acceptable. Although in both cases this angle is surpassed, the new PH is very close to this bound.
      Table 3Overall results from the goniometric angles.
      Angles (degree)RHPHStudent t-valueP valueLimits
      MeanSDMeanSDREBAISO 11226
      G125.16.29−15.84.3339.30<0.001±15±90
      G2−3.19.42−12.35.944.33<0.001+20/−30
      G3−2.17.8150.79.7631.69<0.001+90/−60
      G428.14.8532.45.694.13<0.001+60
      At the angle G2, Ulnar abduction is present in both instruments. The value of this angle is higher for the new PH than for the RH. Nevertheless, this difference was due to the platform height where the volunteer stands during the performance of the test and could be modified with the variation of the height. In both cases, the wrist and hand posture are considered acceptable by ISO 11226.
      The RH provides a slight supination of the wrist (angle G3 in Table 3), whereas the new PH provides a pronation with a mean value of 50.7°. In these cases, both postures are acceptable according to ISO 11226.
      The abduction/adduction motion of the shoulder is shown in row G4 in Table 3. There is a difference of 4.3° in the mean values, and this difference is significant. Both angles are acceptable following the recommendations of the ISO 11226 because their values are below 60°. However, the ISO standard establishes that when the values exceed 20°, the maximum acceptable holding time must be considered. In this case, the maximum holding time is about 4 min, which is considerably higher than the holding time obtained during the tests.
      Statistically, the total trial time necessary to finish the tests was higher for the RH instrument (median = 800 s; IQR = 362.8) than with the PH (median = 637 s; IQR = 190.8). This difference was quite significant in the overall results (Man–Whitney, V = 304.5, P = 0.021). These results are shown in Figure 9A and Table 4. Figure 9B–D show the results obtained for each task. In the three tasks, the median was higher for the RH than for the PH. The differences were significant for tasks 1 and 2 and nonsignificant for task 3 as is shown in Table 4.
      Figure thumbnail gr9
      Fig. 9Trial time necessary to complete the tests.
      Table 4TT necessary to complete the tasks.
      TaskRHPHV valueP value
      MedianSDMedianSD
      T1348.9230.8266.3248.6296.00.035
      T2229.0133.1176.068.0276.50.037
      T3247.0100.6225.054.0212.00.847
      Overall800362.8637190.8304.50.021
      The effectiveness of tasks 1 and 2 was 100% with the two handles, whereas task 3 had an effectiveness of 0.25% with the two handles, which means that the number of figures dropped was the same with each instrument.

      5. Discussion and conclusions

      The main goal of this work was to determine the ergonomic advantages of a new design of a handle compared with the typical RH used in MIS. The new handle was designed using ergonomic principles based on the use of intuitive motion in the activation of the end-effector and reducing the high-pressure zones in contact with the hand. The experimental phantom trials have demonstrated the differences between the two handles and they are discussed in the following paragraphs.
      One of the most significant results is that 64.2% of the volunteers expressed their preference for the new handle. In the comments registered after each test, many of the volunteers remarked that the new handle fitted well in the hand, reducing the pain, and providing secure handling and a comfortable instrument. Indeed, the values reported by the volunteers in the pain scale were lower with the new PH, and the intensity of the pain is also less significant.
      The pain in the RH appears when the volunteer squeezes the handle to exert force in the grasper. This pain is caused in the reduced area of contact between the proximal part of the thumb and the part of the ring where the finger is placed. This fact was proved by the photographic register, where the marks provoked on the skin of the hand are clearly visible in all individuals after working with the RH instrument. Although the other fingers (i.e., middle, ring, and little) also withstood high pressure during squeezing, the pain reported is lower, and the marks left were inappreciable. Thus, the shape of the new design reduces the pain because the handle corresponds better to the anatomic features of the hand. The new design avoids both edges and contact in small parts, allowing the instrument to be manipulated with the maximum surface area of the hand. When the user squeezes the levers, the pressure is distributed over the fingers reducing the pain to insignificant values. The photographic register of the PH did not show any marks on the skin.
      Another important difference appears in the results of the goniometric study and refers to the flexion and extension of the wrist. In fact, the extreme wrist movements required for the manipulation of endoscopic instruments is strongly reduced with the new PH. The flexion of the wrist when the volunteer is working with the RH greatly exceeds the upper limit considered as acceptable, whereas the new PH is around this limit. This reduction of the wrist motion is not surprising given the different configuration of the two handles, and the arm postures required when performing the tasks. In the RH instrument, the position of the fingers leads to an excessive flexion of the wrist even when the volunteer's arm is in a rest posture. On the other hand, the pistol handle provides a neutral position when the arm is resting because the thumb and index fingers are practically aligned with the arm. It is expected that this decrease will result in reduced joint strain in the wrist during surgery and reduced surgeon fatigue.
      Although there are differences in the goniometric study of the other angles (i.e., radial and ulnar deviation, pronation and supination of the forearm, and shoulder abduction and adduction), the values of these angles are within the acceptable range of motion in both instruments. A positive aspect of the new PH is that the pronation and supination rotation are easier because the rest position of the surgeon's forearm is centered in the range of motion. This allows the surgeon to rotate the instrument both clockwise and counterclockwise, reducing the use of the rotation wheel. The RH reduces the capability of rotation in the outer sense of the forearm (i.e., clockwise sense for the right forearm and counterclockwise sense for the left) because the rest position of the forearm is close to one of the limits of comfort. Of course, the lack of this motion can be compensated by turning the rotation wheel, but this implies the use of the index finger and increases the time necessary to perform the task. A positive aspect of the RH is that it produces a lower ulnar deviation compared with the new PH. In the new PH, the ulnar deviation depends on the inclination of the pistol handle with respect to the main shaft of the instrument.
      No extreme muscle strain was observed for any of the two handles during the experiments. In both instruments, the thenar muscle and flexor digitorum superficialis were the most activated muscles in opening and closing the instruments. The strain was <50% MVC of full activity. For the rotation of the instrument, the main muscles activated were the trapezius and the extensor digitorum communis in both instruments. In this case, the strain necessary was lower, <30% MVC of full activity. The new PH fails to reduce the muscular activity and the values obtained are even slightly higher than those obtained with the RH. This can be seen as a negative aspect of the new handle, although the differences are not significant for the majority of the muscles. Only the trapezius presents a significant difference, but in this case the muscle strain is low enough to not to cause fatigue.
      The statistical results do not show significant differences between medical students and individuals from other nonsurgical professions. Goniometric and electromiographic analyses do not reflect significant differences between experienced and inexperienced individuals related to the motion of different parts of the body (i.e., arms, hands, wrist, and so forth) or the muscular effort for controlling and operating the instrument. However, there are significant differences between surgeons and the other individuals in the time necessary to complete the tasks. Table 5 shows these differences and it can be highlighted that the new handle reduces the time to complete the task for the inexperienced volunteers. In fact, in terms of the median value the inexperienced individuals need about 2 min more to complete the task with the RH, whereas surgeons can complete the tasks in approximately the same time with both instruments. This means that the new users can adapt faster to the use of the new PH and they have fewer difficulties when performing the tasks. Surgeons with experience in using the RH can adapt their skills to the new handle without losing efficiency. Thus, the positive aspect of the new handle is that its use is easier for newcomers although the differences may fade over the time. Further research work would be necessary to state whether any of the two handles provides an enhancement of the efficiency when the long-term experience is the same.
      Table 5Statistical values of the trial time for volunteers with and without previous experience.
      InstrumentInexperiencedExperiencedP value
      MedianIQRMedianIQR
      RH838.5308.75370.5118.50<0.05
      PH700.0172.00307.0160.50<0.05
      It is worth pointing out the possibility of including additional functions in the handle. Although the work presented in this article only permits the authors to ensure the ergonomic features of the new handle for two basic functions (i.e., opening–closing and rotation), other additional functions could be included. For safety and to prevent undesired touches, dissectors only include the two basic functions in the handle, and the control of the power for tissue division is activated with a foot pedal. However, other instruments such as vessel sealing instruments include the activation of the power in the handle. In the application of the new handle to this kind of instruments, the additional function could be activated with the middle finger in a similar way that is used for the activation of the rotation wheel. The location of these buttons could be situated in the gripping part of the handle behind the rotation wheel and close to the rest position of the middle finger. On the other hand, the use of articulated graspers has grown in interest among researchers in Laparo-Endoscopic Single-Site [
      • Trejo A.E.
      • Done K.N.
      • DiMartino A.A.
      • Oleynikov D.
      • Hallbeck M.S.
      Articulating vs. conventional laparoscopic grasping tools - surgeons' opinion.
      ,
      • Herring S.R.
      • Trejo A.E.
      • Hallbeck M.S.
      Evaluation of four cursor control devices during a target acquisition task for laparoscopic tool control.
      ]. From the point of view of ergonomic design, this is a challenging problem because it is necessary to introduce an additional function for the articulating system. The handle presented here can incorporate new functions by introducing additional wheels located behind the levers and next to the rotation wheel but further research work would be necessary to solve these problems.
      The findings of this study reveal that the new topological structure of the handle provides important advantages and improves the usability of laparoscopic instruments by avoiding high pressure areas of contact, reducing extreme motions, and providing a neutral position of the hand, wrist, and forearm. The study demonstrates that ergonomically designed handles enhance the comfort of surgeons. These handles can be manufactured with a reasonable price and used in both disposable and reusable instruments with different types of end-effectors.

      Acknowledgment

      This article has been developed in the framework of the project DPI2010-18316 funded by the Spanish Ministry of Economy and Competitiveness.
      Author contributions: S. R. was responsible for conception and mechanical design of the new handle, design of the experiments, analysis and interpretation of the results, and writing the article. G. M. C. was responsible for designing the experiments and questionnaires, analysis and interpretation of the results, and critical vision of the article. T. C. was responsible for designing of the measurement system, data collection and analysis, and interpretation of the results. B. M. A. was responsible for design and performance of the experiments, analysis and interpretation of the results, and critical vision of the article. R. C. was responsible for data collection and statistical analysis of the results, analysis and interpretation of the results, and writing the article. M. J. C. was responsible for conception and design of the new handle, design of the experiments, analysis of the results, and critical vision of the article.

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