MISquito Helical Hook

Background

The MISquito Helical Hook is used to thread a titanium rod through a series of pedicle screws below the fascia in lumbar spine fixation. Stainless steel wiring is thread through the titanium rod and the eye of the hook tip, and together they pull the rod into placement. Below is a depiction of the MISquito Pedicle Screw System.

Problem

The MISquito Pedicle Screw System needed a hook that would accommodate patients of all sizes. Larger patients tend to have more fatty tissue above the fascia (thick layer of skin below fat). The original design of the hook was not able to protrude 45mm below the skin, which was determined to be the minimum depth the hook can be below the skin for a larger patient per the new customer requirement.

Designed 3 iterations of the hook in order to satisfy customer requirements and surgeon needs / requests. The three iterations below are a representation of the critical thinking performed in order to continuously improve the device. Problems were analyzed and design modifications were executed. Below is the original hook that was initially released to manufacturing.

MISquito Gen II Helical Hook - Original

Iteration 1

Increased diameter from 30mm to 55mm, to accommodate patients of all sizes.
Flattened the bottom of the handle so that when in use surgeons could rest the device parallel to the skin.
Increased the size of the handle and thumb / finger features to allow surgeons to have increased grip with performing surgery.

1

2

3

In order to ensure the hook was able to reach a depth of 45mm under the skin, a prototype of the Hook was 3D printed and tested by an orthopedic spine surgeon using sawbones. The sawbones were placed within a mold and saran wrap was used to mimic the different layers of the skin (top layer and fascia). In addition a tolerance analysis was performed to verify this customer requirement as well as ensure no interference occurred with the pedicle screws. To evaluate the part interactions, the Maximum Material Condition (MMC), shown below, and Least Material Condition (LMC) was used in SolidWorks to determine these tolerances.

Ensuring no Interference with Pedicle Screws (MMC)

Ensuring Hook can reach a depth of 45mm (MMC)

Although the hook was able to protrude the minimum 45mm, surgeons did not believe the thumb pad was needed because it reduced the amount of material that could enter the skin. They also thought the angle of the hook was to large, creating an awkward angle of approach when performing surgery. The hook tip was also not long enough to protrude up through the skin when the device was fully submerged below all layers of the skin.

Iteration 2

Removed the front thumb pad and extended the neck of the device to increase the amount of material that would enter the body.
Increased the length of the hook tip by 15mm to allow ample protrusion through the skin when the hook was fully inserted.
Decreased the angle of the neck to 20 degrees to allow a more comfortable position for the surgeon to perform this technique and avoid interference with the pedicle screws.

2

3

1

Similar to iteration 1, the prototype of the hook was 3D printed and tested in sawbones, to ensure the surgeons were satisfied with the removal and addition of the features stated above. The tolerance analysis was also updated and all measurements were in check.

Unfortunately, the surgeons were still not fond of the angle of the hook and preferred to work in line with the incisions made in the spine.

Iteration 3

Designed a job feature, adjacent to the handle, to allow the surgeon to work parallel to the incisions created in the spine while avoiding interference with the pedicle screws. It also increased the amount of material that could enter the skin, thus increasing the maximum depth the hook could achieve.
Increased the handle space to allow ample grip of the hook during surgery.

1

2

Similar to iteration 2, the prototype of the hook was 3D printed and tested in sawbones, to ensure the surgeons were satisfied with the addition of the jog feature and the increased handle space. The tolerance analysis was also updated and all measurements were in check. This was the final iteration of the design before it was initially released to manufacturing.

Hook Configurations

Designed two additional configurations to accommodate small and medium sized patients.

Small Hook: 37mm diameter
Medium Hook: 45mm diameter
Large Hook (shown): 55mm diameter.

Each configuration had identical features, the only difference was diameter of the hook to allow each hook to reach different depths below the skin.

Similar to iterations 1, 2, &3, the tolerance analysis was updated to ensure the hook did not interfere with the pedicle screws and each hook achieved a minimum depth as defined per the customer requirements.

Small Hook: 20mm depth
Medium Hook: 32.5mm depth
Large Hook: 45mm depth