Effects of a chair for ophthalmic microsurgery on pressure distribution and pelvic tilt in surgeons

2.1 Prototyping

An iterative approach was used for prototyping, which involved multiple rounds of design and testing to incorporate each ophthalmic surgeon’s feedback into the design specifications. Production of a prototype chair was conducted by a team that included ergonomists, ophthalmic surgeons, clinical laboratory technologists, designers, and manufacturers. Figure 2 shows a comparison of a conventional chair normally used in an operating room and the prototype chair. For the prototype chair, both the seat and the backrest were changed. The conventional chair had a flat seat surface and no lower support for the backrest (Fig. 2A), whereas the prototype chair featured a concave three-dimensional seat surface and a backrest with two roll supports (Fig. 2B).

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Fig. 2

(A) Conventional chair used in the operating room. (B) Prototype chair.

The seat surface of the prototype chair was based on the concept of a high-performance seat pan [2, 29]. The first study model reflects these functions: The wooden seat surface is divided into pelvic, ischial, and thigh zones [43]. The structure of the study model and a pre-scanned model of the adult human male buttocks (close to the 50th- percentile stature) (Fig. 3A) were emulated to create a three-dimensional model of the seat shell (Fig. 3B) using CAD software (Vectorworks, Vectorworks, Inc., Columbia, USA). The size of the seat shell was arranged to fit adult body sizes ranging from the 5th to 95th percentile based on data for the Japanese population [44]: The seat width was 400 mm to satisfy the hip breadth of the 95th-percentile adult male, while the seat depth was 395 mm to satisfy the buttock-popliteal length in the sitting position of the 5th-percentile adult female. The pelvic support, which has a wedged shape with an angle of approximately 20°, was placed at the seat’s rear end to prevent backward tilting of the pelvis, as reported by Wu et al. [37]. Side supports were installed to stabilize the surgeon’s body and distribute pressure. The front edge of the seat was also tilted downward to avoid pressure behind the thighs/knees. The seat surface was molded from fiber-reinforced plastic (Fig. 3C). Plastic entwined in a thread-like structure, which can be processed easily, was used as the main cushioning material instead of urethane (Fig. 3D).

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Fig. 3

Prototyping of the seat surface. (A) Overlaid image of the first study model and a model of the buttocks shape. (B) 3D CAD seat shell. (C) Fiber-reinforced plastic seat shell. (D) Plastic coil cushion.

The backrest of the prototype chair consists of two roll supports with different diameters: The upper part supports the surgeon’s lumbar region, while the lower part supports the sacral and pelvic regions. The upper and lower rolls can also be inverted to adapt to both conventional microscopic and heads-up surgery. The functions of the backrest and the results of relevant tests have been reported in the literature [20, 21]. The novelty of the prototype chair is the incorporation of a sliding adjustment mechanism for the backrest (Fig. 4A), which enhances support in the sacral and pelvic regions. The sliding mechanism can fill the gap that tends to occur between the buttocks and the joint of the seat surface and backrest (Fig. 4B). When the backrest slides to fill the gap, the two types of stress shown in Fig. 4C are generated, allowing the surgeon to maintain a neutral posture. A vertical force (Fig. 4C-a) pushes the pelvis upward on the pelvic support of the seat surface, whereas a horizontal force (Fig. 4C-b) pushes the sacral region forward on the backrest, thereby enhancing the effect of the vertical force and preventing backward tilting of the pelvis in the sitting position. The slide mechanism also addresses the issue of “shallow sitting,” which occurs when surgeons almost never use the backrest since the seat depth can be adjusted according to individual differences in thigh length. An important aspect of this design is the ability to adjust the shaft of the backrest mounted on the rear side of the seat surface. Depth adjustment can be found in other chairs, but it is less common in the backrest.

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Fig. 4

Sliding adjustment mechanism for the backrest. (A) View from below the seat surface. The backrest can be adjusted forward/backward and fixed with bolts below the seat surface. (B) Image showing a surgeon sitting in the prototype chair. (C) Image focusing on the area between the buttocks and the joint connecting the seat surface and backrest. The two types of stress generated by the slide mechanism for the backrest are shown. ‘a’ indicates the vertical force pushing the pelvis upward on the pelvic support in the seat surface, whereas ‘b’ indicates the horizontal force to pushing the sacrum region forward on the backrest, which enhances the effect of ‘a’ and thus prevents backward tilting of the pelvis.

In contrast to office chairs with reclining and complex dynamic mechanisms (Fig. 5A), the prototype chair has a simple structure, adjustment function, and material combination (Fig. 5B). Although the prototype chair has a fixed backrest, the shape and displacement of the cushions allow the surgeon to lean backward or make slight movements. Other requirements for the surgeon’s tasks were also addressed using features such as armrests, foot pedal for the adjustment of seat height, and caster lock functions. The seat fabric was upholstered in wipe-cleanable, antibacterial, and alcohol-resistant vinyl leather. Disposable sterilized covers were attached to the armrests to prevent infection duringsurgery.

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Fig. 5

Comparison of an ergonomic office chair and the prototype chair. (A) Dynamic backrest with a reclining mechanism found in ergonomic office chairs. (B) Fixed backrest found in the prototype chair. The adjustment mechanisms (vertical height, vertical inversion, and forward/backward) and two rolls aim to accommodate the requirements of static and dynamic seating.

Five male ophthalmic surgeons participated in the study. The mean height and body mass index among the five surgeons were 169±3 cm and 23.1±2.6 kg/m² (mean±standard deviation), respectively. The procedure involved vitreous surgery using a microscope, which is usually performed in the operating room (operation time: approximately 30–120 min). Two chairs were used during different surgeries: a conventional chair (A) and a prototype chair (B). The surgeons adjusted each part of the chair to fit the body before the operation. Each surgeon performed one surgery with each chair, and the order of exposure to each chair was randomized. Thus, ten observations were obtained.

The pressure distribution on the seat surface and pelvic tilt angle measurements, both of which have been used in previous studies [2, 19], were continuously recorded during surgery. The pressure distribution was measured using a pressure-sensing device (X-sensor X3, model PX100:36.36.02; XSENSOR Co., Calgary, Canada). The sensor sheet had a sensing area of 457×457 mm, a thickness of 0.8 mm, a sensor count of 36×36 (1,296), and a sensor pitch of 12.7 mm. The pressure ranged from 0 to 256 mmHg, and the measurement accuracy was approximately±10%.

Pelvic rotation is associated with lumbar alignment. Forward tilting of the pelvis causes lumbar lordosis, whereas backward tilting causes lumbar kyphosis. The pelvic tilt angle, which was used as an index to estimate lumbar alignment, was measured using a gyroscope sensor (VR-sensor, model GU-3013; Datatec Co., Tokyo, Japan) mounted on the iliac crest of the body surface, in accordance with Wu’s method [37]. The sensor system consisted of a gyro sensor and a control box containing an AD converter and a microcomputer. The sensor size was 35×35×29 mm, the sensor weight was 50 g, the detection angle (pitch angle) was±60°, and the measurement accuracy was±0.5°. The pelvic tilt angle was converted to a relative value based on the pre-measured value in the standing position (0°). Backward pelvic tilt was defined as negative. The sampling frequencies of the pressure and gyroscope sensors were both 1 Hz, and the start times were synchronized. The analyses were performed using the mean values of stable data collected over 30 s, starting from 30 min after the surgery began. This approach accounted for the differences in total operation time among surgeons.

Paired t-tests were used to analyze the following measurement items: average seat pressure, maximum seat pressure, contact area, and relative value of the pelvic tilt angle. Cohen’s d was used as a measure of the effect size. Cohen’s literature [45] describes as guide for use: small effect size is d = 0.2, medium effect size is d = 0.5, and large effect size is d = 0.8. If d is greater than 0.8, statistical power is generally considered sufficient. The correlation between the relative value of the pelvic tilt angle and each variable of the pressure distribution was examined, and Pearson’s correlation coefficient was calculated. The significance level was set at less than 5%. Statistical analyses were performed using IBM SPSS Statistics version 28 (IBM Corp., Armonk, N.Y., USA).

4.1 Comparison of chairs

In this study, we compared a prototype chair designed to reduce physical strain with a conventional chair during ophthalmic surgery. Pressure values were lower, and the contact area was larger for the prototype chair than for the conventional chair, indicating that the prototype succeeded in reducing the pressure on the thighs/buttocks. The average seat pressure level with prototype chair in all participants was less than 50 mmHg, which is referred to as the “sitting comfort zone” [28]. The side supports of the seat surface may contribute to stabilizing the surgeon’s body and distributing the pressure, while the downward-tilted front edge of the seat surface may aid in reducing pressure behind the thighs/knees, which was notable in surgeon 5. Similarly, the cushioning material may contribute to reduction and dispersion of pressure, which was notable in surgeons 4 and 5. Images of each seat pressure map can be found in the literature [43].

When compared with the conventional chair, the prototype chair also prevented backward tilting of the pelvis, suggesting that the surgeon could maintain a more neutral seated posture that achieves pelvic and spinal alignment similar to that observed in the standing position. This positive effect may be due to the pelvic support on the seat surface and the function of the backrest. According to previous studies [20, 21], the backrest of the same prototype chair is in contact with the surgeon’s lower back at a low pressure (<20 mmHg, contact ratio of >80%). The synergistic effect of the seat surface and backrest allows for a stable work posture while minimizing the reaction force on the backrest and the forward displacement on the seat, even when the surgeon’s back touches the backrest.

The correlation analysis also indicated that the seat pressure increased with backward tilting of the pelvis. This is presumed to have occurred due to the change in the position of the ischial tuberosities caused by the rotation of the pelvis. Chair design approaches targeting the pelvic region may promote more neutral postures and reduce seat pressure.

4.2 Study strength and limitations

The major strength of the present study was the use of a paired design to evaluate the effectiveness of a novel prototype chair for ophthalmic surgeons during actual microsurgery in the operating room. Although the chair was designed for surgical purposes, the findings may also be applicable to the development of general task chairs, such as office chairs. Based on these findings, we are also considering the development of a chair for daily use in an outpatient examination room.

A major limitation of this study was the small number of participating surgeons. A larger number of participants would have increased the validity of the findings; however, we were limited by the number of qualified microsurgeons who could participate at our institution. Another limitation was the relatively short duration of data collection/analysis to allow for detailed comparisons (30 s period after 30 min of surgery). Overall, the surgical procedure was performed with restrained body movements, thus even such a short duration analysis would be a sufficient substitute. However, a more detailed and longer experiment, including analysis, is required. Further studies are also required to elucidate the effects of long-term chair use on fatigue and performance in surgeons. Additionally, the measurement frequencies of both the pressure and gyroscope sensors were limited to 1 Hz because the microsurgery with the microscope was assumed to force the surgeon to a fixed posture. A higher sampling frequency would have been required for more detailed observation and analysis, but sitting is fairly static and therefore even at such a low sample rate there can still be confidence in the average angles measured (this would be similar to using radiographs to look at posture, i.e. just a single point in time measure, which still, however, gives an idea of the general posture). Lastly, our study may have been affected by intervention bias. The research group is the same group that designed the prototype chair. Although the multidisciplinary collaborative approach was useful for the development of the prototype chair, the potential for participant bias during evaluation cannot be excluded, even with objective measurements. Hence, blinded testing with more participants is therefore required.

Future work should include: a larger study with more participants, potentially a cluster randomized controlled trial with multiple sites if there are low numbers of surgeons at a single institution; longer data collection/analysis with higher sample rates; and potentially additional measures such as a low back angle (only one additional sensor at L1 that does not affect the surgery while minimizing interference with the chair backrest would be needed for this, e.g. a bodysuit type sensor).