Low Cost Haptic Simulation Using Material Fracture

I. Introduction

DELICATE hand motion control is a necessary skill for completing invasive medical procedures such as peripheral nerve blocks (PNB), epidural anesthesia, central venous catheterization, and laparoscopic procedures. These procedures involve the precise insertion of needles and controlled cutting punctures of tissue. For example, an ultrasound-guided PNB is a regional anesthesia procedure where an anesthetic drug is injected in close proximity to a peripheral nerve, such as the sciatic nerve using real-time image feedback from ultrasound [1]. PNB serves to interrupt pain signals allowing for effective pain relief or even completion of surgical procedures without the need for general anesthesia. It can also reduce the use of opioid based medications [2–4]. The effectiveness of the PNB depends on factors such as the precise placement of the local anesthetic around the target nerve without causing damage to the nerve or adjacent vital structures, and the amount of anesthetic injected. Failure to properly place the needle can result in a wide variety of adverse effects such as, neurological complications caused by nerve injury such as permanent neuropathy, transient neuropraxia hematoma due to vascular injury, ischemic injury caused by pressure and volume of the injected anesthetic, and in severe cases local anesthetic systemic toxicity can be caused by injection of anesthetic into the blood circulatory system [5–7]. Improved outcomes of ultrasound-guided PNB versus opioid based pain relief and general anesthesia have led to a significant increase in the use of ultrasound guided PNB over the past two decades, however there is currently no standard objective method for teaching ultrasound guided regional anesthesia procedures [8, 9].

Over the past 20 years, there has been a dramatic increase in the use of competency based, or mastery learning, medical education compared to the traditional Halstedian apprenticeship model of “see one, do one, teach one” as the primary method of resident training [10]. Competency based medical education is centered on achieving a set of core skill competencies and positive clinical outcomes as opposed to time-based training. This allows learners to progress at different rates and focus on their individual training needs. Recently this has been used in medical schools to train doctors before they start their training in a particular specialty as residents. Numerous studies have shown the highly effective nature of competency based education and the importance of a well-structured training program involving consistent high standards and objective measures of performance [11–13]. This paradigm shift in resident education along with other factors including ethical concerns about practicing on patients, increasing standards for operating room efficiency, restrictions on resident work hours, and decreased length of surgical clerkship has led to an increased use of simulators in resident training [14–17].

Simulators offer an effective way to train a wide variety of competencies and often allow residents to focus their training on a specific task. One of the most important competencies for an anesthesiologist to obtain is the ability to accurately place a needle with ultrasound guidance while causing minimal harm to surrounding tissue and structures [18, 19]. Evaluation of needle skills has been conducted using both direct observation by a trained expert and quantitative methods using computer based motion tracking [20, 21]. Through the use of motion tracking, it has been found that one of the key differentiators in needle insertion skills is in the difference in needle motion between experts and novices [20, 22–25]. Experts have reduced movement, needle path length, and insertion time when compared to novices. Few anesthesia simulators for ultrasound guided procedures have been designed to address the training of these fine motor skills despite this understood difference between experts and novices. One possible method of improving fine motor skills is through the use of haptic training. Research has shown that early exposure to haptic training can enhance performance in surgical simulator training suggesting that haptic training can improve surgical skills [26].

This paper presents the development of the Low Cost Haptic Force Needle Insertion Simulator (LCNIS) which provides objective learning feedback, is inexpensive, and presents diverse patient training scenarios. Current medical training methods often use evaluation from a trained observer as their primary form of feedback. Unfortunately, this type of learning feedback can be subjective and observers have been found to suffer from “examiner burnout” over time [23, 27]. The LCNIS uses motion tracking to provide a more objective and real-time assessment of performance. State of the art medical training simulators such as central line manikins can cost from $400 to thousands of dollars, yet lack real time feedback, requiring a trained person to observe practice sessions to give feedback [28]. The LCNIS is inexpensive, around $119, making it an affordable alternative to commercial simulators. Finally, the LCNIS uses an innovative haptic cartridge to simulate needle insertion into a variety of human tissues. Traditional manikin simulators do not have this ability to train for a wide variety of patient scenarios. This leaves trainees unprepared for more difficult patients such as the obese or severely injured [29].

The following sections detail the design, development, and validation of the LCNIS. The materials and methods section will discuss the design of the LCNIS, as well as the cadaver study conducted to validate the forces in the haptic cartridge. Next, the results of the cadaver study and cartridge force validation will be presented. Finally, the discussions and conclusions section will evaluate the effectiveness of the haptic capabilities of the device, as well as future work to be conducted.

II. Materials and Methods

III. Results

The forces measured from one of the cadaver needle insertions in Experiment 1 are shown in Fig. 5. The results of the insertions showed a wide variety of different needle force profiles. The force profile shown in Fig. 5 was chosen to be mimicked by the LCNIS haptic cartridge because of its stable insertion velocity and minimal needle retraction. In the force profile, there is an early force peak of 8.56 N at a depth of 4.47 mm. This early peak occurs at the moment the needle breaks through the skin. The needle force then gradually increases over time to a maximum force of 15.5 N at a depth of 57.9 mm. While the force increases over time, there are a few sudden drops in force. This is typical of a needle insertion in biological tissue due to the lack of homogeneity of tissue and the periodic buildup and release of needle forces cutting into tissue [33].

For Experiment 2, the maximum needle forces and residual needle friction measured using the linear motor and thin plastic materials are shown in Table 4. Needle friction is defined as the average needle force measured after the needle tip has passed completely through the plastic material. The materials tested provide a wide variety of forces with the polycarbonate providing the strongest maximum puncture force at 9.85 N and the FEP providing the weakest maximum puncture force at 0.84 N. The different materials also had a low standard deviation of puncture force with the maximum standard deviation being 0.14 N in the polycarbonate and the lowest being 0.011 N in the 0.003” thick ABS. The PVC plastisol is a thicker and more flexible material than the other plastics tested. Because of this, the maximum insertion force occurs much deeper at 13.1 mm. The PVC plastisol was designed to be placed between thin disks of other materials in the cartridges.

Using the experimental data from Table 4, a piecewise linear equation was fit to approximate the needle cutting force and friction force of the PVC plastisol based on the thickness of the material. Using the experimental data from Experiment 2, Equation (3) was developed to predict forces from the LCNIS haptic cartridge based on the placement of the different plastics in the syringe and the depth of the insertion. The variables used in (3) are detailed in Table 5. For each peak in force in the piecewise equation, there are three main components. The first component involves the rise of forces while the needle is puncturing material as well as any previous residual needle friction. The second component is the rapid decrease in force after puncture occurs. This involves a rapid reduction in force from the peak insertion force to the final residual friction force in the material disk. Through testing, it was found that this reduction in force typically takes place over 0.5 mm. The third component of the equation is the residual friction from the material disk rubbing against the body of the needle.

The results of Experiment 3, in Fig. 6, show the predicted forces using (3) for a haptic cartridge plotted against the actual forces recorded from insertions haptic cartridge Experiment 2. The five major peaks occur where the needle breaks through each of the 5 disks of material in the cartridge. A gradual increase in force also occurs between the first and second forces peaks due to the needle puncturing through the PVC plastisol. The error between the predicted force measurement and the average of the measured forces for each of the five force peaks were 0.27 N, −1.76 N, 0.13 N, −0.33 N, and −0.50 N. The standard deviation of each of the five force peaks were 0.04 N, 0.70 N, 0.47 N, 0.56 N, and 0.97 N showing consistency between insertions.

Experiment 4 used the data from the cadaver and plastic needle puncture experiments to create a custom cadaver haptic cartridge to mimic the forces of a PNB into the thigh of a cadaver. Table 6 shows the details of the materials used in the custom cadaver haptic cartridge. Fig. 7 shows the results of Experiment 4. The predicted needle insertion force is shown with the force measured from the cadaver mimicking haptic cartridge and the cadaver needle insertion experiment. Only one of the haptic cartridge insertions is shown in Fig. 7 for clarity. The error between the cadaver cartridge and the three main peaks was 0.01 N, 1.00 N, and 1.54 N. The error between the maximum force of the cartridge and the cadaver was 0.40 N. Again, there was consistency with the cartridge force peaks with the standard deviation of the three peaks being 0.60 N, 0.55 N, and 0.41 N. The standard deviation of the maximum force was 1.50 N.

IV. Discussion

The results of the cadaver experiment show that the initial puncture of the skin around the thigh provides a large amount of resisting force compared to the puncture of subcutaneous tissue. The skin puncture occurred at a depth of 4.47 mm, and the force transducer did not measure a force of this magnitude again until the needle reached a depth of 39 mm. This large skin puncture force is typical of needle punctures and poses one of the greatest challenges in accurately positioning a PNB needle [32]. Additional peaks in force occur throughout the insertion. During a needle insertion, an anesthesiologist should insert the needle at a controlled rate to prevent overshooting the target position or unintentionally harming nearby anatomical structures.

The results from the thin plastic material testing, Experiment 2, show a wide variety of needle puncture forces depending on the material. This variety makes these materials ideal for use in the LCNIS haptic cartridge as it allows for diverse haptic cartridges to be developed. Testing showed that using (3) in conjunction with the thin plastic material puncture data enables the accurate prediction of forces obtained from the haptic cartridges. The cartridge was also shown to be consistent for all four insertions into the material with insertion forces remaining within 1 N of each other between insertions. This consistency aided by the precision bearing placed at the front of the syringe accurately guiding the needle through 4 distinct puncture locations on the disks of materials as seen in Fig. 8. Through various testing, it was found that it is important for the plastic films in the disks to resist tearing between puncture holes. If tearing between holes occurs, the puncture force will not be accurately predicted.

The final validation testing, Experiment 4, showed that the custom cadaver haptic cartridge was able to successfully emulate the forces experienced during a needle insertion into a cadaver thigh. For this experiment, two identical cartridges were made. The low standard deviation of the force measurement results show that the cartridges can provide reliable repeatable force feedback. This repeatability is vital to the consistency of the system. The PVC plastisol placed between disks of other plastic materials enabled the cartridge to effectively emulate the gradual, continuous increase in force during a needle insertion. The third force peak in the custom cadaver cartridge was slightly larger than predicted. This is possibly due to the PVC plastisol pressing tightly into the PTFE disk. This may have negated the decrease in force that would be normally expected to occur when the needle tip exits the PVC since the tip immediately began pressing into the PTFE.

The LCNIS achieved its goal of being able to accurately simulate tissue forces. Equation (3) enables the accurate prediction of needle cartridge forces. Using (3) and the data from Experiment 2, all that would be required to create cartridges mimicking needle insertions into other parts of the body would be axial needle force insertion data based on depth such as that found in Fig. 5.

The LCNIS also achieved its goal of being low cost. The total cost of the electronic components is $47. Currently, the 3D printing cost of the syringe bodies and haptic cartridge cylinders is $58 and $14 respectively. Production for the LCNIS would use injection molding to significantly reduce this cost compared to 3D printing, with estimates for the cartridge cost being around $1. As currently designed, the device’s low cost cartridges would be disposed of after four insertions and replaced with new cartridges. The LCNIS provides an inexpensive alternative to robotic haptic training systems based on devices such as the 3D Systems (Rock Hill, SC) Geomagic Touch, which cost over $1000 before being integrated into a haptic training system.

V. Conclusions

The LCNIS is able to successfully emulate the forces experienced during a needle insertion at a fraction of the cost of traditional haptic training devices. The design of the LCNIS allows for easy exchange of its haptic cartridge and measurement of syringe movements. Equations have also been developed that allow for the creation of custom haptic cartridges for use in the LCNIS. These cartridges are then able to recreate the forces experienced during a needle insertion into a variety of materials including needle insertions into cadaver tissue. Future work will include the creation of a graphical user interface for the LCNIS, and the development of a trainee performance assessment method utilizing the LCNIS’s IMU capabilities to provide training of motor skills during needle insertion.

Biographies