Dynamic Extension Splint

— About this Project —


Splinting for orthopedic injuries and illnesses in the hand can be unwieldy, uncomfortable and inconvenient. It is also limited in its effectiveness, due to a combination of design issues with the splints and adherence issues that result from the usability challenges. This splint was designed to improve patients’ experiences with finger splinting and improve its effectiveness by using soft materials instead of hardware to encourage extension.

— Background —

Contracture of the PIP joint, a common condition where the tendons in the hands shorten and become stiff, is painful can lead to functional limitation. The typical treatment is surgery followed by splinting, but current splints are painful and ineffective and therefore usually patients do not improve after surgery. As part of a collaboration between the Human Engineering Research Lab and the Department of Veterans Affairs I worked with a surgeon, occupational therapists, and engineers to design a splint for the treatment of these contractures.

— Design —

The final design abandoned any rigid supports, and relies on an elastic tube that is formed with a curve in the opposite direction of the contracture to apply a straightening force. The final design was finished by my collaborators after my internship ended. I helped them finish up the packaging when I returned to work for them again. There is a U.S. Patent filed for the splint, and I am a co-inventor.

— Process —

To research the project I spent several days a week in the VA hand clinic observing and interviewing therapists, patients and surgeons.
I also did a literature review and detailed study of splints available on the market, looking at their designs and functional problems.

Issues Raised by Research:

  1. Adherence. If the splint is uncomfortable or difficult to put on, it will not be worn.
  2. Straightening. Splint needs to apply enough corrective force to straighten the contracture.
  3. Fit. Custom splints are difficult to fabricate, off-the-shelf splints are difficult to fit
  4. Comfort, Health. Splints cause pain and discomfort because pressure is uneven

Design Criteria Responding to these issues:

  1. Flexible Material combined w/ effective extension moment (Straightening)
  2. Low profile, not interfere with activities of daily living (Adherence)
  3. Comfortable, held in place by circumferential tension (Adherence, Comfort)
  4. Form fitting for variety of finger sizes and still easy to don (Adherence, Fit)
  5. Pressure diffused over the whole finger (Adherence,Comfort)
  6. Permit visual monitoring of skin health and circulation (Adherence, Comfort)

Based on these criteria I developed sketches and models to test out lots of ideas:

These are some of the many sketches and models I made to explore the form and function of the splint. At first the lab wasn’t expecting me to want to make sketch models so I made them out of post-it notes. Eventually I got some craft supplies and silicone tubes and things really took off.

We decided it was best to use cast urethane rubber, so when it was time for “real” prototypes I worked in CAD. I made the splints I wanted to try out then made them into negatives.

I used the SLA machine (a rapid prototyper) to make the CAD models into molds to cast rubber into, then poured the splints and tested them out.
As you can see, there were lots of issues to work out, such as fit and wall thickness. Eventually the molds got more complicated with more features to help the splints come out consistently.
The design we settled on was one where the splint rolls over itself to slide onto the finger easily and painlessly. After it is on stiff batons could be inserted to straighten the joint. It was my belief that by shaping the splint in a curve the plastic alone would be enough and we could do without the batons. After I left, that’s what they decided to do.