A modular attachment system for thumb UCL splints

The ulnar collateral ligament stabilizes the thumb metacarpophalangeal joint during lateral pinch, the grip you use to hold a key or a fork. It tears when the thumb is forced into abduction and hyperextension, which is why the two classic presentations are skier’s thumb, from a ski pole, and gamekeeper’s thumb, from repeated twisting loads. Roughly one in four high school athletes will injure a UCL before graduating, and a national claims database recorded close to 20,000 injuries between 2010 and 2019.

The non-surgical treatment is a splint that immobilizes the thumb until the ligament heals, and it is the preferred route because it avoids an operation. The catch is that the splint does exactly what it is supposed to. With the thumb locked out, eating, brushing teeth, pinching, and dressing all become difficult, so patients take the splint off. Compliance drops, and the injury drags on longer than it should.

That gap became the team’s need statement: a way to increase splint compliance in UCL patients by developing a modular assistive device for activities of daily living.

Requirements came out of a patient interview that identified the tasks causing the most trouble: feeding, brushing teeth, pinching, and bathroom use. Rather than design one tool that compromised across all four, the team built a mounting system and let the attachments specialize.

Three patents anchored the prior-art search: a powered straightening device (JP5557529B2), a low-profile dynamic extension splint (US 2009/0062708 A1), and a hand orthosis (US7837641B2). The orthosis came closest to the platform concept, but it leaves the thumb free to participate in the grip, which is precisely what a healing UCL cannot do. Keeping the thumb fully out of the load path is the distinction this design is built around.

The rail bonds permanently to the thermoplastic splint; the attachments come and go. Each one replaces a different function the thumb used to provide.

Platform. An 80A rubber block that gives the forefingers something to press against, standing in for the opposing thumb. Soft enough to grip, firm enough not to deform under load. It handles the broadest range of tasks: pinning a toothbrush against the pad and wrapping the fingers around it, holding a can, working a spray bottle trigger.

J-hook. Bent from 1/16 in low-carbon steel, for pulling rather than gripping. It threads through a belt loop or catches a waistband, which is what makes independent bathroom use possible, and it carries bags and anything else with a loop. It mounts in either orientation so the user can pull up or down.

Utensil holder. An FDM-printed base and hollow quarter-cylinder packed with foam. Push a fork, knife, or spoon into the foam and it holds; pull it out and swap it. The utensil sits below the thumb line so the remaining MP joints do the gripping while the thumb stays locked.

Every attachment hangs off the same small bonded joint, so the rail interface is the part that decides whether the system holds. Each attachment got a free-body model at its rated 2 lb load to size the reaction forces and moment the joint has to survive.

Kian ran the platform analysis. Treating the finger pressure as a point load at the pad center, the joint sees 2 lb of shear and a resisting moment of 0.125 lb·ft over the 3/4 in offset, the couple that would otherwise peel the attachment out of the rail. The J-hook is the harder case: the same 2 lb acting 1.2 in out from the mount produces 2.2 lb·in at the joint. The utensil holder, loaded by the weight of a fork rather than a pulled object, barely registers by comparison at roughly 0.1 lb.

The device classifies as Class I under both FDA and EU MDR. Working through Annex VIII of the MDR, implementing rule 1 puts all non-invasive devices in Class I unless a later rule pulls them out, and none apply here: the device does not alter biological or chemical composition, does not contact injured skin or mucous membrane, is non-invasive, non-active, and is not software. The FDA classification follows the same reasoning and the precedent of comparable marketed orthoses.

The report also carries a full standard operating procedure, covering surface preparation and the 24-hour cure before first use, plus per-attachment instructions and a split of responsibility between manufacturer and end user.

The rails are the weak point. Each one is currently three laser-cut acrylic pieces glued into a dovetail, and squeeze-out from the adhesive binds the track so attachments do not slide cleanly. Machining them as single metal pieces would fix the tolerance problem outright. Beyond that, fastening the rail into the attachment rather than relying on adhesive, and adding a positive lock so an attachment cannot slide off under load.

The market is also wider than the need statement assumed. The rail bonds to any rigid splint or cast, so wrist fractures, ligament tears, dislocations, severe sprains, and ganglion cysts all fit the same system, as do longer-term conditions like arthritis, carpal tunnel, and tendonitis where grip strength is the limiting factor rather than immobilization.