Inside the Build: Redesigning a Surgical Navigation Tracker

Featuring Aaron Harpster, Product Development Engineer; Mike Cook, Program Manager.

“This isn’t like a hypothetical, ‘Oh yeah, just trust us, it’s going to be good, it’ll work.’ We were able to show that we can hold all the angular tolerances even with these 3D printed components.”

Welcome to Inside the Build, a podcast series that dives deep into how Forj Medical solves complex manufacturing challenges, told by the people whose knowledge, expertise, and ingenuity make Forj Medical the partner medtech companies trust when the technology is complex and the stakes are high.

Where the project started

The segmental tracker is a bone screw used during spinal procedures to track the location of individual spinal segments. Previous surgical navigation systems assume the spine moves as a single rigid body, which it doesn’t. This product gives the surgeon segment-by-segment tracking. The OEM brought the concept to Forj originally as a request for help on the EM sensor coils. Three coils per assembly, very small, with 58-gauge winding wire on a pair of round cores.

The earliest version of the design tried to reuse a bone screw from the OEM’s previous generation tracking product. The carrier that held the coils had to wrap around that legacy screw, which meant a complicated geometry and a difficult assembly path. Coil wires would have to be routed around the top edge of the carrier and bonded to the structure after the coils were glued into place. The molding would have been buildable but not clean. The assembly would have been worse.

As the conversation widened, the scope grew. The OEM asked Forj to take the full design, from the EM sensors up through the molded carrier, the cap, and final assembly through sterile pack. That created room to rethink the architecture from the bottom up.

The coil package

The biggest manufacturability problem in the original design was the bonding step. The coils, once wound, have extremely delicate lead wires. After gluing each coil into the carrier, an operator would then have to make either solder or thermal-compression bonds with that delicate wire and somehow secure the joints to the outside of the carrier. Three coils per assembly meant three opportunities to scrap an in-progress part with three coils already attached. No rework path.

Forj had been running an internal tech development project around surface-mount coils for navigation. That work didn’t ship as a product, but it proved that the winding wire could be thermally compression-bonded directly to a PCB. The segmental program reused that concept to create a small coil-on-PCB package. The lead wires bond to solder pads on the board. The board then becomes the handling unit. It can be panelized in sheets of 25 and laser-cut to size. The coil itself never has to be touched again after winding.

That single change reframed the rest of the assembly. Instead of three high-risk bonds happening on a carrier with three coils already in place, the bonds happen at the PCB level, before anything is assembled. A failed bond scraps one coil, not three. There’s rework headroom. And because the package is a defined component, it can be staged, inspected, and positioned on the carrier as a known good part.

The coil geometry

The dual-core shape was a customer requirement tied to noise mitigation in their tracking algorithm. The original approach used two round cores held together, which required either an adhesive step or a heat-shrink step before winding. Neither scaled. The team prototyped a single-piece core with the same non-circular cross-section, initially by reforming existing wire. Once that worked, the team brought a supplier in to produce the shaped core directly. The cost dropped to about half per sensor, with three sensors in each finished assembly.

The carrier and the cap

The original carrier mold needed four cams and pulled away from the part in six different directions to release the geometry. The redesigned carrier is a two-part mold. No cams, no side actions, straight pull. The cap that closes over the assembly still uses one cam, but the rest of the geometry simplified enough that the cap could accommodate the new coil-on-PCB packages without driving the tool back toward complexity. Iteration speed mattered here. The team had access to quick-turn PCB houses, the laser depaneler in-house, and high resolution resin 3D printing for the molded parts. New design rounds went out roughly weekly. The angular tolerances on coil placement, which were a hard requirement for the tracking function, were validated on 3D printed parts before the molds were cut. By the time the molded parts arrived, the geometry was already proven.

Vertical integration as a design tool

The case for vertical integration usually gets framed as a sourcing and lead time argument. On this program it was a design argument first. Because PCB assembly, plastics molding, coil winding, and final assembly all live in the same building, the team could push design changes through prototype cycles without waiting on external suppliers for cores, boards, or molded components. Core material doesn’t have to come from a cut-core supplier, which has the longest external lead times in this category. When the OEM asks for a variant length, the team can turn it around in days.

On the production side, the simpler design and the panelized coil packages reduced the skill threshold for assembly. The original architecture would have required the company’s most experienced fine-wire operators. The new architecture widens the pool, which made line transfer to the Costa Rica facility a realistic plan from the start of the program rather than a retrofit after launch.

What didn’t change

Not every design improvement made it through. The team pushed for differently colored cables to distinguish the five trackers in a pack. The legacy cable stayed, and color identification ended up being handled by heat shrink marker bands. That’s the trade-off in a redesign that lives inside an existing system. Some legacy components carry forward because they’re already validated, and the program timeline doesn’t have room to revalidate them.

This has been Inside the Build, a podcast series by Forj Medical, where we’re shaping the future of life-saving devices, one build at a time. Thank you for listening. For more information, visit us at forjmedical.com.