Von Mises stress analysis of surgery chair designed for laparoscopic surgeon with lifting mechanism Open Access

Laparoscopic surgeons spend most of their time standing while doing surgical operations on patients, as shown in Fig. 1(a), leaning down to the patient, making quick motions, and rotating his body from side to side.¹ The laparoscopic surgeon will repeatedly do each of these actions until the patient’s surgery is over. This may result in musculoskeletal disorders,² which may make the laparoscopic surgeon uncomfortable, suffer excessive tiredness,³ and loss of focus during the surgical operation,⁴ all of which may cause the laparoscopic surgeon to make technical mistakes that might be dangerous to the patient.⁵ 

These issues highlight the necessity for ergonomic systems to be implemented in the healthcare industry, especially for laparoscopic surgeons performing surgical operations. It is possible to lessen or eliminate the likelihood of musculoskeletal disorders occurring by paying attention to the laparoscopic surgeon’s posture⁶ and ergonomic positioning,⁷ but this is ineffective given the length of time required for the procedure.⁸ Meanwhile, taking enough time to rest for laparoscopic surgeons when experiencing musculoskeletal disorders is also considered impossible considering that procedures for handling surgical operations must be completed as soon as possible because they involve the lives and safety of patients.⁹ 

The medical technology industry has advanced significantly in response to the ergonomic issues faced by laparoscopic surgeons by offering a variety of instruments for their use during surgical operations, including chairless chair,¹⁰ surgical robot assistance,¹¹ and surgery chair.¹² To help laparoscopic surgeons with the first option, modern technology has developed surgical robot assistance. However, using these tools requires technical know-how for laparoscopic surgeons to operate these devices, and the high costs are a hindrance.¹³ For the second option, the use of a chairless chair is a more cost-friendly alternative compared to the first option. Unfortunately, the use of a chairless chair is considered not to eliminate musculoskeletal disorders better than sitting in general.¹⁴ The last option with a surgery chair as presented in Fig. 1(b) is the wisest option considering the relatively affordable cost and effectiveness in dealing with musculoskeletal disorders for laparoscopic surgeons.

Surgery chairs are now widely available in the market with various types of commercial products. However, there are no products specifically designed for Indonesian laparoscopic surgeons, and the need for medical devices in Indonesia is still dominated by imported products by more than 90% as reported by the EU—Indonesia Business Network.¹⁵ The development of the latest design of surgery chairs for Indonesian laparoscopic surgeons is important considering that the commercial surgery chair products on the market today refer to the anthropometry of Europeans who are generally larger than Indonesians.^16–18 For lifting mechanisms, there are several existing mechanisms, starting from hydraulics,¹⁹ screw gears,²⁰ and worm gears.²¹ The use of worm gears is seen as a wise option for self-locking,²² space efficiency,²³ and minimizes sound when lifting mechanism.²⁴ 

In designing a mechanical component, including a surgery chair, it is necessary to ensure its strength. This is intended so that the mechanical components that are made do not fail during long-term use.²⁵ Experimental testing has been widely used for testing the safety factor of a mechanical component as performed by Markani et al.,²⁶ Cemiloglu et al.,²⁷ and Buchroithner et al.²⁸ Unfortunately, the long time and high cost are the main obstacles in this regard. Computational simulation using the finite element method can respond well to problems from the constraints found in experimental testing,²⁹ which has been widely applied to the world of engineering,³⁰ biomedicine,³¹ and mathematical physics³² in studying various parameters, such as materials,³³ geometries,³⁴ conditions,³⁵ to designs.³⁶ The finite element method can facilitate the safe construction of the surgery chair design by performing von Mises stress analysis.³⁷ 

This study contributes to the development of the latest design of a surgery chair designed for Indonesian laparoscopic surgeons. The worm gear is used for the lifting mechanism of the proposed surgery chair design. Computational simulation using the finite element method for surgery chairs has been carried out to ensure the safety of the worm gear lifting mechanism.

The surgical chair designed in this study was adopted from a commercial product of surgery chair that has been available in the market with adjustments in Indonesian anthropometry³⁸ so that it can provide an ergonomic system advantage because it is intended for Indonesian laparoscopic surgeons in carrying out surgical procedures. Apart from the size aspect, the surgery chair that is available in the market uses a hand-controlled pedal for lifting mechanisms,³⁹ which can distract the laparoscopic surgeon from using surgical equipment during surgical procedures.⁴⁰ The use of a foot-controlled pedal for lifting mechanism⁴¹ can overcome this problem considering that the position of the laparoscopic surgeon’s feet during a surgical procedure tends to be silent and does not interfere with the concentration of the use of the hands⁴² as presented in Fig. 2(a).

A lifting mechanism is a mechanical mechanism to make a mechanical component move from a lower position to a higher position, and vice versa.⁴³ The design of the surgery chair proposed in the current study uses a worm gear as shown in Fig. 2(b). This mechanism consists of a seat base, crankshaft, stepped shaft, worm gear device, frame, direct current motor, and battery. The working procedure of the proposed lifting mechanism is to step on the pedal on the surgical chair according to the position required in carrying out the surgical procedure. To ensure that the present worm gear lifting mechanism design is safe, this study focuses on researching the worm gear lifting mechanism of the proposed surgery chair design.

The worm gear lifting mechanism studied in the current computational simulation is represented by four main components, there are the seat base, vertical bar, crankshaft, and stepped shaft. The geometry of the worm gear lifting mechanism is determined based on the proposed surgery chair design considerations described in Subsection II A. The geometric details of the worm gear lifting mechanism components are shown in Fig. 3.

Components in the worm gear lifting mechanism of the proposed surgery chair designed for Indonesian laparoscopic surgeons using AISI 1020 for the seat base and vertical bar, mild steel for the crankshaft, and AISI 1045 for stepped shaft are explained in Table I. Material properties used in this study were adopted from default material libraries in SolidWorks Premium.⁴⁴ All of the materials used in the worm gear lifting mechanism are assumed to be homogeneous, isotropic, and linear elastic as the common assumption in the finite element analysis.⁴⁵ 

The load was applied with 1200 N (≈120 Kg) in the worm gear lifting mechanism of the proposed surgery chair design. The load given was evenly distributed on the surface of the seat base.⁴⁶ Loading was defined as a static load during computational simulation.⁴⁷ The load magnitude was adopted based on the assumed average body weight of Indonesians.⁴⁸ 

The worm gear lifting mechanism component of the proposed surgery chair designed for Indonesian laparoscopic surgeons, including seat base, vertical bar, crankshaft, and stepped shaft, has been computationally simulated via the finite element method. Three positions of the lifting mechanism, bottom, middle, and top, are investigated, as shown in Fig. 4. Von Mises stress investigation has been performed using SolidWorks Premium with static structural analysis feature to efficiency computational load.⁴⁹ The friction that occurs during the lifting mechanism process is assumed as frictionless.⁵⁰ Also, changes in temperature during the simulation are not taken into account.⁵¹ The discretization of the finite element model is carried out using the automatic meshing feature from SolidWorks Premium with the dominant tetrahedral-element type.⁵² The number of elements used in the finite element model is determined through a convergence study with the h-refinement method, to select elements with not too many numbers, but still provide accurate simulation results.⁵³ 

The distribution contour of the von Mises stress of the worm gear lifting mechanism is shown in Fig. 5 for the seat base and vertical bar, Fig. 6 for the crankshaft, and Fig. 7 for the stepped shaft. Overall, the contours of the von Mises stress distribution from the four components of the three-worm gear lifting mechanism positions do not show a significant difference, where the distribution contours tend to be similar. This shows that changing the position of the worm gear lifting mechanism does not give a significant change in stress.⁵⁴ From the distribution of the von Mises contour, it can provide an understanding of the location of the area on the worm gear lifting mechanism that is experiencing critical stress⁵⁵ so that it can be a reference for the future development of the proposed surgery chair design.

Table II presents the maximum von Mises stress for the components of the worm gear lifting mechanism for three different positions. There are similarities in the maximum von Mises stress for the seat base and vertical bar at three different positions indicating position change during the lifting mechanism of the surgery chair does not affect von Mises stress on these components.⁵⁶ However, changes in the maximum von Mises stress were found on the crankshaft and stepped shaft, although not large. This is because the position of the crankshafts and the stepped shaft has changed to accommodate the lifting mechanism of the surgery chair.⁵⁷ In addition, the maximum von Mises stress on the seat base and vertical bar for every position is only 1.7 and 0.5 MPa, respectively, which is very low compared to that of the crankshaft and stepped shaft, which are more than 40 MPa. The maximum von Mises stress is considerable in the crankshaft and stepped shaft due to the occurrence of stress concentrations in several areas.⁵⁸ The different phenomena found in the seat base and vertical bar are more distributed von Mises stress evenly in the whole component’s area.

Ensuring that the worm gear lifting mechanism as an important part of the proposed surgery chair design does not fail is crucial. Referring to the von Mises failure theory, where the maximum von Mises stress is below the material’s yield strength, it can be ensured that it is safe based on von Mises failure theory.⁵⁹ Seat base and vertical bar use a material with a yield strength of 205 MPa, which only experiences a maximum von Mises stress of not up to 2 MPa. Crankshafts use a mild steel material with a yield strength of 250 MPa, which experiences the highest maximum von Mises stress of 56.97 MPa in the bottom position. Meanwhile, the stepped shaft uses AISI 1045 material with a yield strength of 310 MPa, which experiences the highest maximum von Mises stress of 57.55 MPa in the top position. Comparison between the maximum von Mises stress and the material’s yield strength shows that all components of the worm gear lifting mechanism are stated to be safe because there is no maximum von Mises stress of any worm gear lifting mechanism’s component that exceeds the material’s Yield strength.⁶⁰ Even so, the crankshaft and stepped shaft are components that are prone to failure.

The computational simulation results of von Mises stress from the worm gear lifting mechanism of the proposed surgery chair design that is obtained via the finite element analysis can become the basis for future design development. By knowing the areas experiencing critical stress and the ratio between the maximum von Mises stress and the material’s yield strength, minimizing failure can be achieved by avoiding stress concentrations⁶¹ and increasing the gap between the maximum von Mises stress and the material’s yield strength.⁶² The strategic steps for further development of the proposed surgery chair design can be carried out by design optimization,⁶³ studying the geometric parameters,⁶⁴ and choosing materials that are better from a mechanical point of view on the present worm gear lifting mechanism.⁶⁵ 

The current investigation of the von Mises stress of the worm gear lifting mechanism from the proposed surgery chair design has several shortcomings that need to be conveyed as a form of research integrity. First, the finite element analysis in this work covered static structural analysis, which is less realistic. The worm gear lifting mechanism should undergo dynamic structural analysis to provide more feasible results.⁶⁶ Second, the applied load used in the investigation of the worm gear lifting mechanism only considers a static load of 1200 N. The load received by the surgery chair can change over time according to the laparoscopic surgeon’s condition so that the worm gear lifting mechanism needs to be analyzed by dynamic load.⁶⁷ Furthermore, the effect due to friction is neglected with the assumption that it is frictionless along with temperature changes, which is not considered. Since the worm gear lifting mechanism has a different position, in the process of the mechanism, there should consider friction and heat generation triggering von Mises stress increases.⁶⁸ Investigation in the computational simulation of the worm gear lifting mechanism to ensure that the proposed design surgery chair is safe needs to be continued with a more comprehensive study in the future to complement this study’s deficiencies.

Finite element analysis has been successfully performed to investigate the von Mises stress of the worm gear lifting mechanism from the proposed surgery chair design in the present computational simulation study. The obtained results show that the lifting mechanism using worm gear is safe to use based on the von Mises failure theory. Changes in position, whether bottom, middle, or top, did not have a significant impact on changes in von Mises stress on the components of the worm gear lifting mechanism. However, it was found that the crankshaft and stepped shaft components had a relatively greater probability of failure due to stress concentration. Further development in the future can be done by studying design optimization, geometric parameters, and choosing materials that are better from a mechanical point of view.

The authors gratefully thanked the author’s respective Institution for their strong support in this study. This research was funded by a research grant from Universitas Pasundan.

The authors have no conflicts to disclose.

All authors listed have significantly contributed to the development and the writing of this article.

Gatot Santoso: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Muhammad Imam Ammarullah: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). S. Sugiharto: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Randy Media Rachayu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Abdul Mughni: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Athanasius Priharyoto Bayuseno: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). J. Jamari: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.