Comparison of two cable configurations in 3D printed steerable instruments for minimally invasive surgery

Abstract

In laparoscopy, a small incision size improves the surgical outcome but increases at the same time the rigidity of the instrument, with consequent impairment of the surgeon's maneuverability. Such reduction introduces new challenges, such as the loss of wrist articulation or the impossibility of overcoming obstacles. A possible approach is using multi-steerable cable-driven instruments fully mechanical actuated, which allow great maneuverability while keeping the wound small. In this work, we compared the usability of the two most promising cable configurations in 3D printed multi-steerable instruments: a parallel configuration with all cables running straight from the steerable shaft to the handle; and a multi configuration with straight cables in combination with helical cables. Twelve participants were divided into two groups and asked to orient the instrument shaft and randomly hit six targets following the instructions in a laparoscopic simulator. Each participant carried out four trials (two trials for each instrument) with 12 runs per trial. The average task performance time showed a significant decrease over the first trial for both configurations. The decrease was 48% for the parallel and 41% for the multi configuration. Improvement of task performance times reached a plateau in the second trial with both instruments. The participants filled out a TLX questionnaire after each trial. The questionnaire showed a lower burden score for the parallel compared to multi configuration (23% VS 30%). Even though the task performance time for both configurations was comparable, a final questionnaire showed that 10 out of 12 participants preferred the parallel configuration due to a more intuitive hand movement and the possibility of individually orienting the distal end of the steerable shaft.

Fig 1. Multi-steerable strategies to control surgical instruments.

a) Cable-ring mechanism with its cross-section. Cables are placed concentrically to actuate the segments and guide each other along the shaft, adapted from [23]. b) Parallel configuration of a multi-steerable instrument, adapted from [25]. c) Multi configuration of a multi-steerable instrument, adapted from [17].

Fig 2. The three cable configurations presented in this work.

Left: 2D representation of the segment, center: Corresponding bending moment diagram, right: Segment deforming under the applied pulling force. a) Parallel configuration, b) diagonal configuration, c) multi configuration. For the explanation of the used symbols, see the text.

Fig 3. 3D model of the parallel and multi configurations.

a) The parallel cable configuration with a close-up on the shaft and the final design of the device. Each color corresponds to a segment mirrored on the control side. b) Multi configuration of the cables and the final design of the device. In this configuration, all cables are connected at the ends. The close-up shows the cable configuration in the steerable shaft.

Fig 4. Cross-section of the shaft for the parallel and the multi configurations.

a) In the parallel configuration all cables are equally distant from the central backbone Segments S are numbered from 1 to 6. In the multi configuration, cables are concentrically placed at three different radii to avoid overlapping. S represents the steerable shaft.

Fig 5. Steerable segment with parallel cables.

A) Four helicoids concentrically placed around the central backbone. b) Cross-Section of the steerable segments with the T-shape of the helicoids highlighted in green and cables in black. c) 3D model of the steerable segment with d) cross-section A-A showing the looped cables in the fixation point. Adapted from [24].

Fig 6

Instrument prototypes employing the parallel configuration (top) and multi configuration (bottom). The close-ups show the steerable shafts. Notice that the fixation points in the shafts differ, depending on the configuration. In the parallel configuration, each segment has two fixation points whereas in the multi configuration all cables are fixed at the distal end of the shaft.

Fig 7. Experimental setup.

a) Setup and its components: 1. simulator, 2. instruments, 3. participant information letter, 4. general instruction, 5. informed consent and questionnaires, 6. user interface. b) A participant during the test. c) The user’s interface during the experiment. Each green circle represents a target. The light green circles are the targets already hit, and the dark green circles are the targets still to be hit. The status bar displays the next target that the participant has to hit. d) The instrument into one of the targets. d) A participant during the test. e) CAD model of the target with back view and cross-section. The two steel plates are represented in grey.

Fig 8. Box and whisker plots of the task performance time for the two cable configurations.

Yellow represents the parallel configuration (PC), and blue the multi configuration (MC). For each instrument, the participants performed two trials. The red line in the box represents the median and the red crosses, the outliers.

Fig 9. Box and whisker plots of the average time per run in the two trials performed by each participant for each instrument.

Yellow represents the parallel configuration (PC), and blue the multi configuration (MC). Each box and whisker plot represents the median, the upper and the bottom quartile of the average time for 12 participants. The outliers above 350 s have been cut off in the figure. The full picture can be found in the supplementary material.

Fig 10. Box and whisker plots of the average time per run in the two trials performed by each participant for each instrument in the two different groups.

Orange represents Group A and purple Group B. Each box and whisker plot represents the median, the upper, and the bottom quartile of the average time for the six participants of Group A and the six of Group B.

Fig 11. Average and standard deviation of the Raw TLX score for the six subscales (mental demand, physical demand, temporal demand, performance, effort, and frustration) in Trials 1 and 2.

The average was calculated over the score given by the 12 participants. Yellow represents the parallel, and blue the multi configuration.

Fig 12. Results of the final questionnaire on personal preference.