Force measurement platform for training and assessment of laparoscopic skills

Software was written in C++ to record rotation and translation vectors at a rate of 60 Hz (Fig. 3). The data were saved in arbitrary units together with a time vector. To compute the force in Newton and torque in newton millimeter for further analysis, the relationship between the measurements and the applied forces was determined by calibrating the force platform.

To use the SpaceNavigator as a 6D force platform in box trainers, the allowable range of forces needs to be increased. Increased stiffness in all directions is required to measure forces over 2 N without limiting the movement of the cap. This is accomplished by adding three springs around the SpaceNavigator (Fig. 2). On one side the springs are connected to the table (i.e., the upper plate) that is mounted on the cap of the SpaceNavigator. On the other side, the springs are connected to a base plate fixed on the housing of the SpaceNavigator. Small adjustments in the position and orientation of all individual springs, with respect to the base plate and table, are possible by repositioning of the spring holders with the three star screws at the top and three Allen screws at the base plate (Fig. 3). If springs with stiffness of 14 N/mm are used, a force range of 12 N is easily reached. For the first needle-driving tests a lower force range of 6 N is sufficient. Therefore, springs with 4 N/mm stiffness were used to maximize resolution.

Calibration was accomplished with standardized weights of 50, 100, 250, and 500 g. A frame was built from mechanical components to exert well-defined forces and torques, in all directions, at the center of the platform table. During the force calibration of each axis, the load on the platform was increased from −650 to 650 g in steps of 50 g. The torque on the platform was increased from −1.08 to 1.08 Nm in steps of 98.1 × 10⁻³ Nm. Each axis was calibrated three times. After calibration, regression lines were added to the platform output data of each individual axis.

The forces applied during calibration result in force and torque vectors with components in three orthogonal axes (X, Y, and Z). During calibration, the output error is determined for every individual axis. However, if the force or torque vector is spanned between two or three axes, each individual translation along, or rotation around, one axis can influence the force–output or torque–output relation of the other axis. To determine a general value for the accuracy of the platform, a series of tests were conducted. During the first test series, three different forces of 0.981, 1.962, and 2.943 N were exerted in line with the eight direction vectors (Fig. 4, Q1 to Q8). During the second test series, three different torque values of 0.384, 0.256, and 0.28 Nm were exerted around the eight direction vectors (Q1 to Q8). During both tests, each measurement was repeated three times.

A pilot study in which subjects performed a needle-driving task was undertaken to investigate the potential of the force platform. The task was conducted inside a training box (Fig. 5, right) equipped with two 5-mm and one 11-mm trocars (Endopath XCEL; Johnson & Johnson), two needle drivers (B Braun), and one laparoscopic camera. Artificial tissue, imitating skin and fat layers (Professional Skin Pad, Mk 2; Limbs & Things, Bristol, UK), was fixed on the force platform. On top of the artificial tissue, the point of insertion and direction were marked by two lines (Fig. 5, left). The line thickness was 2 mm, and the distance between the two lines was 9 mm. The test group (n = 10) consisted of five surgeons who had performed at least 50 laparoscopic sutures during surgery and five novices without hands-on experience in laparoscopic surgery or training. All subjects were asked to pick up a needle (Vicryl 3–0 SH plus 26 mm; Ethicon, Johnson & Johnson) with the needle driver and to insert it at the right line on the tissue. Secondly, the subjects were asked to drive the needle, in the desired direction, through the tissue and to remove it completely at the location of the left line. If a subject was not able to insert the needle at the right line or to remove it at the left line, the measurement was removed from the database and the subject was asked to try it again. All subjects were asked to complete the needle-driving task two times. During the test, no feedback was given to the subjects. For each subject we determined the maximum absolute force and the mean absolute nonzero force. We defined the mean absolute nonzero force as the force averaged across all samples during which force was exerted so that the resulting measure is based only on the periods of time when interaction took place. To determine whether the results obtained for the experienced surgeons differed from the data from the novices we performed Student’s t-tests (SPSS 17.0) to compare group means. Also, striking differences in force signatures were further investigated. In addition, we asked one novice and one expert to perform the needle-driving task four times instead of two. This was done to see if learning effects occurred within a small number of repetitions.

The maximal force range per axis is determined by the linearity of the force platform output. As soon as the moving parts are out of boundary and motion is restricted, the force platform output becomes highly nonlinear and unpredictable. Within the working range, the output of the force platform is linear. For torque applied around the X and Y axes, the output is linear. However, if torque is applied around the Z axis, the output is quadratic. Table 1 presents the regression lines and R ² values for the fitted data of each axis. The positive mean sensor output and output errors (±standard deviation, SD), together with linear fitted regression lines are presented in Fig. 6. The force and torque graphs of the negative sensor output range are similar to the graphs of the positive sensor output range of Fig. 6.

In Fig. 7 the results of the force and torque accuracy tests are presented. Three horizontal lines indicate the desired value.

A threshold, below which all data are discarded, prevents the untouched 3Dconnexion mouse from drifting. Until this threshold displacement is reached, the output values are zero. The threshold displacement together with the stiffness of the installed spring determines the threshold force and torque. Therefore, a stiffer set of springs increases the measurement range as well as the threshold value. In the force platform with a suitable measurement range for suture tasks, threshold values of 0.7 N for the Z axis and 0.5 N for the X and Y axes were found. Furthermore, the torque threshold values were determined as 0.02 Nm for the Z axis and 0.03 Nm for the X and Y axes.

It took the surgeons 17.8 s (SD 2.1 s) and the novices 29.4 (SD 3.7 s) to complete the task. Before the surgeons and novices inserted the needle into the artificial tissue, a clear difference between orientation and position of the needle inside the needle driver was visible. After inserting the needle tip, the two types of subjects used different strategies to drive the needle through the tissue. The surgeons used mostly rotation (R) of the needle around an imaginary rotation point (Fig. 8A), whereas the novices used rotation (R) as well as translation (X, Y) (Fig. 8B). Furthermore, unlike most surgeons, all novices pressed the needle driver against the tissue during the task. A force graph and 3D force signature of the best-performing surgeon and novice are presented in Fig. 9A, B. The absolute nonzero mean force and maximal force of all subjects, measured during the needle-driving task, are presented in Fig. 10. The force graphs of the novice and surgeon who performed the needle-driving task four times are presented in Fig. 11. The maximum and mean absolute nonzero force used by the novices was on average 4.7 N (SD 1.3 N) and 2.1 N (SD 0.6 N), respectively. For surgeons, the average maximum force (2.6 N, SD 0.4 N) and the average mean force (0.9 N, SD 0.3 N) were much lower. Student t-tests showed that there was significant difference between the two groups of subjects for both depend variables (mean nonzero force: t = 4.3, p < 0.005, maximum force: t = 3.6, p < 0.017).

In the present study we evaluated performance in a needle-driving task. However, potentially any training task used to practice laparoscopic skills can be mounted on the force platform just like the suture task used in the pilot study. Box trainers equipped with the force platform can provide students and instructors with objective information about interaction forces and torques for more effective training and assessment.

With respect to training, an important question that remains is how to present torque and force data to the student in real time (Fig. 12). When tasks are performed inside a laparoscopic box trainer, the resident’s attention is directed to the monitor. Furthermore, the complexity of the task may make it difficult to detect whether the proper amount of force is applied. If the platform is used for well-defined simple tasks, it should be possible to find an effective method of providing force feedback during training. One option is to use this same monitor to display torque and force information. Another option is to use sounds to indicate, for example, that the exerted torque or force exceeds a stored maximum value.