Design Solution

Platform to Investigate Novel Surgical Tools

There is a lack of spine holders customized for performing surgery on excised spine specimens. Current systems in the market are specialized to conduct range-of-motion testing and typically position the spine upright to mimic the spine's natural movement; however, these methods limit access to the spine. Each surgical technique utilizes a different angle of entry to approach the spine, which would be greatly restricted by current systems as they secure the spine in one position.

Our device aims to address these problems. We developed a bracket-pegboard fixation system that comes as three separate parts: two attachment bases and an axial correction apparatus (Fig.1A). The use of separate, individual parts allows for more freedom and flexibility that accommodates spine specimens with varying lengths (Fig.2). The attachment bases can further be broken down into the rotation mechanism (Fig.1B) and spine anchoring instrument subprojects.

Fig.1. Finalized CAD model of design solution.

Fig.2. Finalized manufactured model of design solution.

Initial Design

Fig.3. Initial sketch of design solution.

The initial design features an attachment base anchored to a table using C-clamps attached underneath. This iteration features a prebuilt sprocket that acts as a gear for rotation. The spine anchoring instrument, which is modeled after a bench vice clamp, fixes the spine specimen. The axial correction apparatus is a retractable bar that ensures proper alignment of the separate attachment bases (Fig. 3).

Fig.4. Initial CAD model of spine anchoring instrument.

Fig.5. Initial CAD model of rotation mechanism.

This initial design was revised due to two main concerns: misalignment and weight. The bench vice clamp works by rotating the vice screw, which pushes the sliding jaw across the clamp base (Fig.4). This works well to secure the potted ends; however, the vice screws might be rotated unequally, causing the sliding jaw to be moved varied distances on the two bases. This acts as a source of misalignment, causing the spine specimen to be unleveled and introducing more validation errors. The clamp also causes the device to be top-heavy, adding the risk of tipping over. Additionally, the gear design was too complex and would be too difficult to manufacture (Fig. 5).

Final Design: Project Breakdown

Fig.6. Finalized CAD models of attachment base.

Attachment Base

Each attachment base is joined to a platform using triangular metal sheets and screws (Fig.6A), enabling them to be secured to the surgical table using C-clamps. To allow the spine to rotate, we assembled a rotation shaft that goes through the attachment base. Bearings were added between the attachment bases and the capsules for smoother revolutions. Rotation plates were hollowed to allow space for the rotation shaft on either side (Fig.6B). Capsules were developed with symmetrically distanced pairs of screw holes that would secure the spine anchoring instrument to the attachment bases. We developed a dowel locking system to cease rotation; there are a total of 12 inner holes and 12 outer holes to ensure there is no movement when dowels are inserted (Fig.6A).

Fig.7. CAD and manufactured models of spine anchoring instrument.

Spine Anchoring Instrument

To fix the potted ends in place, we created a spine anchoring instrument that operates with a similar mechanism as a pegboard (Fig.7A). We designed brackets with three screw holes through the top (Fig.7B). These holes were made for set screws to go through in order to pin the potted ends for fixation. Additionally, two screw holes on the face of the brackets were designed as a way to secure the brackets to the attachment bases (Fig.7).

Fig.8. Finalized CAD models of axial correction apparatus.

Axial Correction Apparatus

Because there are two separate attachment bases, we designed an axial correction apparatus (Fig.8A) that fits into the side of each base to align the pieces straight. This is because misalignment of the spine can reduce its integrity and/or impact the results of surgery. The axial correction apparatus uses nested bars that can extend/retract to accommodate spines one FSU to 8” in length. Sliders can move across the length of the bar depending on the distance between the attachment bases (Fig.8B).

Conclusions

Although the spine holder meets all of our design specifications, it has room for improvement to increase its effectiveness. One limitation is that the brackets are thick and add extra weight as well as visual impedance. Additionally, the attachment of the brackets and securement of the spine specimen relies on screws, which can require repetitive motions over a long period of time during usage. Another limitation is the lack of visual cues on our holder for the locking mechanism. The user may have to test multiple holes or move around to visualize the pinhole locations in order to lock the rotation, which slows down workflow. These limitations don’t render the spine holder unusable, but they may require workarounds to compensate for those failings.

Future Work

Our team has made significant progress in the development of the surgical holder. However, there are potential changes to the design and implementation of the holder. Additional tests—such as autoclaving and repeated surgical use—can be performed to confirm the long-term use of the device. Improvement to radiolucency and reduction of the device weight can be achieved using different materials, such as titanium and carbon fiber polyetheretherketone (CF-PEEK). However, the cost of these materials requires additional funding. Finally, the functionality of the device can be evaluated through experimentation with cadaveric spine specimens.

Subproject 1: Spine Anchoring Instrument

The spine anchoring instrument is responsible for fixing the potted ends of the spine; improvements to this subproject will primarily focus on improving the screw workflow. To adjust for different potted ends, the screws must be twisted for eight set screws and sixteen bracket screws. The process is tedious and ergonomically taxing for the user. Future studies can focus on developing a new design or procedure to streamline the adjustment process, such as substituting the set screws with magnets.

Subproject 2: Rotation Mechanism

The rotation mechanism currently consists of bearings within the capsules attached to the base pieces; however the device requires directly spinning the disks to perform rotation. Future designs can focus on implementing a handle to improve ease of manual rotation and reducing the spread of bloodborne pathogens or biohazards.

Subproject 3: Axial Correction Apparatus

The axial correction apparatus is currently made of 6061 aluminum. Because this is an ancillary component to the attachment bases and is not in use when surgery is performed, the axial correction apparatus could be made from a lighter weight material to prioritize portability over rigidity. Polypropylene is a favorable option because it is rigid, lightweight, and radiolucent.

Page Leader: Tammy Phan

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