Group 2, Senior Design, Spring 2020

Prototype Overview

Figure 6: Component diagram of ultrasound compatible hip injection model

The main components are the bulk material (shown as the transparent hip), the internal structures (which includes the labeled bones, femoral artery, femoral vein, and nerve), and circuitry (which includes the needle, nickel mesh, and processor).

Figure 7: Photograph of final prototype of the hip injection model

Note that the right of the photo is towards the head of the model, whereas the left of the photo is towards the feet.

The model was made based on an adult male anatomy, and contains only representative anatomical features, specifically the femoral artery, femoral vein, sciatic nerve, femur (top portion), pelvic bone, and joint capsule. The femoral artery and vein were modeled using silicone tubing. The nerve was modeled using a thin wire.

Materials

The texture of the model surface and bulk material should feel similar to human skin and muscle tissue, respectively. Also, because the use of ultrasound is crucial to the function of this model, several materials with different acoustic impatiences are used. Acrylonitrile butadiene styrene (ABS) was used for 3D-printed bones; silicone rubber was used for blood vessels; and a mix of polyvinyl chloride (PVC), chalk powder, and mineral oil was used for bulk tissue. Together, these materials create an image similar to that seen when ultrasounding human tissue.

The PVC bulk material can be repaired by applying heat higher than its melting temperature. As a result, the ultrasound image and continuity of the gel material can be restored to how they were before insertions were performed.

Circuitry

Internal circuitry in the model detect if the needle contacts any structures representing vessels, nerves, bones, etc. that should not be touched. It is also able to detect if the user had reached the correct target at the end of insertion.

In the final model, the circuitry is composed of conductive nylon meshes, resistors, conductive needle, and wiring, that connect the model to an Arduino microcontoller. The meshes are used to represent the different anatomical structures. Each conductive mesh is raised to a known voltage by placing each mesh between two resistors of known values. This forms a voltage divider, and the voltage of each mesh can be calculated using a voltage divider equation. For the prototype, the circuitry was built on a breadboard.

Figure 3: Arduino circuit diagram

GUI (Graphical User Interface)

The communication to the user is done through the GUI, which was developed using Python. The GUI allows users to initiate a training session and immediately receive feedback on whether the injection was correct or not. The users then have the opportunity to see the results of all their attempts.

From the main page, the user will have access to the model's Instructions for Use (IFU) (which can be found here), as well as the ability to load results from past attempts. Figure 8 below shows the main page of the GUI. Once the user begins the training session, the GUI obtains data from the Arduino and prints the corresponding feedback in the Results window, shown in Figure 9 below. Figure 10 below shows a flowchart summarizing the GUI function. In the future, storing the results will be implemented into the GUI.

Figure 8: The main page of the GUI consists of three buttons, “Begin Procedure!”, “Instructions” and “Import Data File”.

Figure 9: Example of GUI output

Figure 10: Flowchart summarizing GUI function

Prototype Construction

Construction of the prototype was done in small components and ultimately assembled together to form the overall model.

A plaster mold was constructed in the shape of an average human hip, into which layers of melted PVC plastic were poured. Figures 11 through 14 are photographs that were taken during the building of the final prototype. Figure 11 shows that the joint capsule and bones were held together using hot glue. Figure 12 shows how the vessels and nerves were suspended prior to being set in the mold, using the femoral artery as an example. Figure 13 depicts how the femoral artery, vein, and nerve were suspended in relation to each other. Additionally, Figure 14 demonstrates a partially submerged femoral head, pelvis, and joint capsule.

Anatomically, the submerged vasculature and nerves are ordered from lateral to medial in the following manner: femoral nerve, femoral artery, and femoral vein. Moreover, the femur and pelvis are posterior to these structures, with a joint capsule wrapped around the junction between the two bones [16]. Finally, care had to be taken to ensure that the conductive meshes surrounding each structure remained electrically insulated from each other. In practice, this limited each structure to a narrow region of 3D space in which they could be feasibly placed.

Figure 11: The joint capsule being hot glued to the bones

Figure 12: The femoral artery attached to a wire and fishing line prior to being set in the mold

Figure 13: (From left to right) The femoral vein, femoral artery, and sciatic nerve partially submerged in the PVC

Figure 14: The bones partially submerged in the PVC

Overall Function and Goals

The first goal of obtaining user feedback was met. Feedback informs the user if the syringe has been placed in the correct location (the femoral head-neck junction), has hit a different negative structure (nerve or vasculature), or is in tissue that would have neither a negative or positive response.
The second goal of creating an ultrasound-compatible hip injection model was met. The PVC plastic selected for the bulk material of the model allows ultrasound to pass to a degree that provides an image acceptably similar to that seen when using ultrasound on a patient. Additionally, the included anatomical structures, such as, nerves, vasculature or bone, resemble a similar contrast under ultrasound as a real ultrasound image. Future testing can reveal if the internal structures of the hip are in the correct placement.
The third goal of anatomical accuracy is still in progress. Due to not having access to ultrasound, it is unknown if the relative distances of the anatomical features within the hip model are correct. Although, the bone, vein, artery, nerve, and joint capsule were placed in relatively the correct locations when the prototype was built, it is unknown if they shifted as the bulk material solidified. The exterior of the hip is anatomically accurate since a 3D CAD model of an average male hip was used. CAD models were also used for the femur and pelvis. In addition, the diameters of the silicone tubes are correct for the artery, vein and nerve.

The fourth goal of durability was met. The model is able to withstand more than one injection and still provides the same level of training. Heating the material to PVC's melting temperature, with a heat gun, allows the holes created by each injection to close. As a result, the effectiveness of the model will be maintained for subsequent users. While other training models made from self-healing materials exist (such as those by Blue Phantom), our device is much more inexpensive, allowing hospitals to purchase multiple training models [10].

Conclusion

The prototype successfully integrated a feedback system, ultrasound compatibility, and durability. Four tests were performed: an Ultrasound Test, Circuitry Test, GUI Test and a Repairability Test. The results show:

Anatomical features within the hip (bone, nerve, vein, artery) can be seen under ultrasound.
Internal circuitry in the model detects if the needle is in the correct or incorrect site of injection.
The results of injection able to be printed with the GUI.
The surface can be remelted after injections using a heat gun.

The anatomical accuracy of the model has yet to be verified under ultrasound. However, the feedback system and material repairability set our device apart by being inexpensive and allowing independent use by medical trainees.

Future Directions

Further testing will be performed to verify the model meets the needs indicated by the physicians.

The current model will be analyzed under ultrasound, and images will be taken for the IFU (which can be found here ).
Usability testing will be performed with the help of medical professionals and trainees to receive feedback on the device. The usability testing protocol can be found here. Feedback will be implemented in later iterations of this model, and testing will be repeated as needed.
The correct storage of the device will be determined for handling.
The model will be analyzed after multiple injections under ultrasound to verify its durability. A heat gun will be used to remelt the plastic.

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