Persistent In-Line Sensor for Real-World Loading Characterization of Osseointegrated Implants and Prosthetic Devices

CERSI Collaborators: Jonathan Thornhill (JHUAPL), Courtney Moran C.P. MS (JHUAPL); Luke Osborn, PhD (JHUAPL); Jared Wormley (JHUAPL) Jill Curran (JHU)

FDA Collaborators: Kimberly Kontson, PhD (CDRH); Snehal Shetye, PhD (CDRH);

CERSI “In Kind” Collaborators: Michelle Nordstom, MS, OTR/L (USUHS)

Project Start Date: September 1, 2022

Project End Date: August 31, 2023

Regulatory Science Challenge

Osseoanchored prosthetics use a permanent bone anchored osseointegrated (OI) implant that penetrates and locks into the long bone inside the body and passes permanently through the skin to the outside to facilitate direct skeletal attachment of external arm and leg prosthetic devices. Such OI implants provide an alternative to traditional socket-based attachment systems and can eliminate issues with skin irritation, ease of applying and removing prosthetics, and improve range of motion. Although certain OI devices have been used internationally for the last decade, the emergence of OI use in the US market is more recent but steadily increasing. While there have been studies that have explored the loads placed on the implant abutment, or support, and consequently the bone and soft tissue to which it is implanted, few of these studies have quantified the loads and forces created during real world use. To date, most measurements of forces between the attached prosthetic and the implant have been interpreted from externally measured forces between the ground and prosthetic foot. Others have used sensing technology in a controlled laboratory and clinical setting whose size and bulk can impact the accuracy of some prosthetic limb movement. This means that the loads experienced by wearers during daily use have yet to be directly determined. Early adopters of OI devices report activities of daily living and recreational activities that are likely creating loads well above what has been measurable to date. Smaller, lighter sensors which can be integrated as part of arm and leg prosthetic systems, attached to the implant, and worn daily can provide critical information to understand the actual implant loads being experienced.

Project Description and Goals

The goal of this project was to design and test a prototype sensor for amputees implanted with OI implants which is small, light, accurate and safe enough to be used day-to-day as part of the total OI prosthetic limb system. The results informed whether a small light load sensor could accurately measure the range of loads placed on an OI implant by a prosthetic limb during real world use and demonstrate a sensor with form and function that can be included as part of the daily prosthetic system. To achieve this, investigators 1) Designed a low profile multi-axial load sensing prototype compatible with real world use; 2) Quantified load sensing ranges relative to overall device size and 3) Validated load sensing responsiveness, accuracy and repeatability in bench top model.

Research Outcomes/Results

JHU CERSI will continue research, development, test and evaluation of the prototype sensor for persistent OI sensor. To date, JHU CERSI has achieved significant progress toward specific aims.

1. Design and fabrication of a low-profile load sensing prototype that measures4.5 cm diameter X 2.5 cm high and weighs 3 grams (not including boards and sensors) which is 1.5 and .5 cm smaller than our target and 161 grams lighter.
2. Through bench testing, demonstrated the ability to measure multidirectional bending loads up to the maximum load of 70 Nm measured during take home tests with an experimental arm. Please note that experimental features included integrated load sensing capability.
3. Validated the persistent load sensor prototype ability to sense loads in two axes: bending loads and torsional(rotational) loads. Sensor prototype accuracy has also been maintained over repeat loading and does so for bending loads up to 35 Nm of torque and 32 Nm of bending load. Both values exceed the max loads expected to be seen in real world upper limb use based on our past research experience.

Overall, investigators, to date, have demonstrated proof-of-concept for a compact lightweight dual axial load sensor prototype capable of measuring loads consistently at load limits higher than expected during real world use ensuring the full range of loads both expected and unexpected are measurable.

Research Impacts

Demonstration of a prototype sensor small and light enough to be integrated as a persistent prosthetic component for amputees with OI implants is the first step in realizing FDA’s regulatory science priority of understanding real world use in general, and specifically to the use of OI implants. This is especially true of upper limb OI implant users. Our prototype demonstrates that the load limits up to those expected in real world upper limb OI users can be captured accurately in a bench top model and in a light, small enough package to be used with upper limb amputees.

The bench top demonstration is the ‘catalyst for future research’ as the foundation required to realize the next steps which include real time communication of measured loads in a closed loop information delivery application. The delivery of load information directly to the patient/wearer of the prosthetic device and capturing information from the patient about the activities being performed. Investigators expect ongoing development to lead to a final design that can inform the goal of establishing targeted rehabilitation protocols, development of methods to encourage rehabilitation exercise compliance, and empowerment of the end user to understand the impact of their activities on device loading.