Prototyping

First Prototype

Side Alignment Pins

The first iteration of the project was manufactured using FDM printers that were on-campus. The material of choice was Glow PLA for its radiodense properties and Natural ABS for its radiolucent properties when imaged via Computed Tomography (CT) or O-arm. NinjaFlex was also used as a more elastic material to allow for necessary motion of the spine. This material is readily available, inexpensive, and reproducible, making it an ideal candidate for prototyping.

Advantages

Inexpensive
Adequate imaging quality
Portable and lightweight
Reproducible
NinjaFlex allows for necessary motion of the spine phantom to meet the project constraint

Disadvantages

Limited access to the sides of the vertebrae
Difficult to re-align vertebrae
No attachment site for Medtronic's Starburst Pin
Very low quality
Inaccurate home positioning due to sagging of the vertebrae with no support material underneath

Glow PLA Filament

Radiodensity of Bone: 1000-3000 HU

Radiodensity: 418.83 HU

*Material used for the vertebrae. The material has very rigid properties, which will help in durability testing.

Natural ABS Filament

Radiodensity of Bone: 1000-3000 HU

Radiodensity: -62.40 HU

*Material used for the jig to hold the vertebrae in place for calibration.

NinjaFlex Filament

Radiodensity of IVD: 40 HU

Radiodensity: 175 HU

*Material used for the spinal cord and intervertebral discs (IVDs). The elasticity of the material was very promising in comparison to a human spine and allows for necessary motion.

Second Prototype

Bottom Alignment Pins

The second iteration of the project was manufactured using Polyjet printers that were on-site at Medtronic. The material of choice was VeroWhite for its radiodense and rigid properties and Tango Filament for its elastic properties. This material is readily available and reproducible during prototyping, and even after the senior project is finished because Medtronic can continue making these parts with the same material and equipment. This makes it an ideal candidate for prototyping.

Advantages

Easy vertebrae re-alignment
High quality
Reproducible both during and after senior project
Access to full vertebrae during demonstrations
Possesses all necessary parts to achieve project constraints

Disadvantages

More expensive
Imaging issues with VeroWhite making up both the vertebrae and jig. (In need of zinc paint to coat the vertebrae)
Durability issues with pins
Base contains too much material to print on current Polyjet printer

VeroWhite Filament

Radiodensity of Bone: 1000-3000 HU

Radiodensity: 150 HU

*Material used for vertebrae and entire jig holding the vertebrae. Zinc paint is needed to provide a higher radiodensity to the surface of the vertebrae in order to make them more distinguishable on O-arm imaging.

Tango Filament

Radiodensity of Bone: 1000-3000 HU

Radiodensity: 100 HU

*Material used for the intervertebral discs (IVDs), spinal cord, and support base. The elasticity of the material was very promising in comparison to a human spine and allows for necessary motion.

Durability Testing

Durability testing was performed as a way to ensure each vertebrae can handle any stress that could be put onto it. One stress that will be performed on the vertebrae is drilling the vertebrae at the location shown below, to simulate where a pedicle screw would be placed during an operation to attach a spine cage.

Marker Alignment for Pedicle Screw

Drill Alignment for Pedicle Screw

Testing Setup for Drilling Procedure

20191211_234027.mp4

Video: Drilling Procedure

Stress tests were also performed on the dual rod pins by applying continuous cycles. Compressive load cycles were performed at the top of the dual rod pins, and shear load cycles were performed to the side of the dual rod pins. After deciding on the elliptical dual rod alignment pins, they were manufactured to be tested. Some issues of failure were discovered for both top and bottom pins, which prompted a design change for the dual rod alignment pins.

Compressive Load Tests

Due to continuous loading cycles to the top pins that interact with the vertebrae interface, it caused the top of the pins to break off into the alignment holes within the vertebrae.

Shear Load Tests

Due to continuous shear loading cycles, by moving the dual rod pins into and out of the calibration location, it caused the side pins to fail at the interface of the dual rod pin face.

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